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  • Chemistry  (42,112)
  • 1995-1999  (38,453)
  • 1945-1949  (3,659)
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  • 1
    ISSN: 1432-0975
    Keywords: Key words Otolith ; Chemistry ; ICP-MS ; Stock discrimination ; Epinephelus striatus
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences
    Notes: Abstract  We examined the utility of otolith minor and trace element chemistry, assayed with inductively coupled plasma mass spectrometry (ICP-MS), as a means of delineating population structure in the Nassau grouper (Epinephelus striatus). We characterized the elemental composition of otoliths collected in 1993 from three locations in Exuma Sound, Bahamas and from Glover Reef, Belize in 1995. A single location in Exuma Sound was sampled in 1994 to test temporal variability in otolith composition. Five elements (Ca, Zn, Sr, Ba and Pb) were routinely detected, at levels significantly above background, by solution-based ICP-MS. Results from analysis of variance of elemental data, expressed as a ratio to Ca, indicated that there were no significant differences among the Exuma locations for any element, but significant variability was found between Glover Reef and the pooled Exuma localities for Zn/Ca, Sr/Ca and Ba/Ca ratios. Significant inter-annual differences at one Exuma Sound location was restricted to Ba/Ca ratios. Discriminant function analysis correctly classified 86% and 95% of the Belize and pooled Exuma sites, respectively. Otoliths from Belize were characterized by low Zn/Ca and high Ba/Ca and Pb/Ca ratios compared to otoliths from fish collected in Exuma Sound. Although differences in Ba levels may be related to upwelling at Glover Reef, more data are needed to definitely link otolith composition with regional differences in water chemistry.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 224 (1995), S. 81-84 
    ISSN: 1572-946X
    Keywords: Chemistry ; Depletions ; Shocks ; IRAS 05338-0624 ; NGC 1333 IRAS 4
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract High resolution interferometer and single-dish observations of young, deeply embedded stellar systems reveal a complex chemistry in the circumstellar environments of low to intermediate mass stars. Depletions of gas-phase molecules, grain mantle evaporation, and shock interactions actively drive chemical processes in different regions around young stars. We present results for two systems, IRAS 05338-0624 and NGC 1333 IRAS 4, to illustrate the behavior found and to examine the physical processes at work.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 233 (1995), S. 125-137 
    ISSN: 1572-946X
    Keywords: Interstellar Medium ; Dark Clouds ; Chemistry ; Bistability ; C-shock Waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Shock waves perturb the chemical state of the interstellar gas. We consider the effects of C-shocks on the composition of molecular clouds, for a range of values of the pre-shock gas density and magnetic induction. The time required to re-establish equilibrium in the post-shock gas depends on the initial conditions and can become very large. The significance of the two known chemical phases of dark clouds and of bistability is considered in this context.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 233 (1995), S. 161-164 
    ISSN: 1572-946X
    Keywords: Stars ; Chemistry ; Diffusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have assessed the role of diffusion in determining chemical abundances in molecular interface regions. Chemical models have been developed which include the appropriate diffusion terms and that are appropriate to a narrow diffusion region (∼0.01pc) that may exist at the interface between a dark core and a hot, shocked T-Tauri wind. We have assumed pressure balance throughout and have calculated the chemical abundances as functions of time and position through the interface. The results show that significant enhancements of detectable molecules/transitions are expected (e.g. CO J=6→5, OH and CH). Using a realistic value of the diffusion coefficient a diffusive region of dimension 0.01pc may be established within about 104 years. In general it seems likely that diffusion processes are highly significant on these and smaller lengthscales.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Boundary layer meteorology 85 (1997), S. 197-222 
    ISSN: 1573-1472
    Keywords: Turbulence ; Chemistry ; Closure ; Convective boundary layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We study the interactions of chemistry and turbulent mixing of tracersin the convective boundary layer with a second-order closure model,including higher order chemistry terms. In order to limit the number of predictive equations we prescribe the profiles for ¯w¯Θ, ¯w¯θ ¯θ and the lengthscale l. However, for model validation we treat temperature and humidity asinert tracers, and compare the results with profiles observed during theAir Mass Transformation Experiment, and with similarity expressions for thesurface layer. We find good agreement of the mean profiles, but the (co-)variances are slightly underpredicted. Furthermore, the model usesdiagnostic equations expressing third moments of concentration in terms ofsecond moments and their vertical derivatives. They are compared withlarge-eddy model results, showing good agreement and, therefore, thesimplifications are justified. The model is applied to the transport of two gases subject to one bimolecular reaction. The importance of concentration correlations on themean transformation rate is studied. For two gases diffusing in oppositedirections we find for moderate and fast chemistry a 50% and90% decreased transformation rate due to the negatively correlatedconcentrations. These values are similar to large-eddy results of Schumannand Sykes et al. For two bottom-up tracers we find that the covariance ofboth reactive species is either positive or negative, increasing or reducingthe effective transformation rate depending on the Damköhler number (the ratio of the turbulent and the chemistry timescale). A significantdirect influence of chemistry on the flux divergence is found in bothcases. According to the model the effective transport to mid-levels of theboundary layer is increased when two reactive tracers diffuse in oppositedirections, and decreased in the case of two bottom-up tracers.
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  • 6
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    Electronic Resource
    Springer
    Journal of science education and technology 5 (1996), S. 193-201 
    ISSN: 1573-1839
    Keywords: Chemistry ; chemistry education ; multiple intelligences ; imagery ; visual-spatial thinking
    Source: Springer Online Journal Archives 1860-2000
    Topics: Natural Sciences in General , Technology
    Notes: Abstract Chemistry has evolved from a science dominated by mathematics into a science highly dependent on spatial-visual intelligence. Yet the chemical content of introductory courses remains taught essentially the same as 40–50 years ago. Chemistry, today, is recognized by chemists as the molecular science. Yet, school chemistry is alienated from that perception. Thanks to the computer, young people are more comfortable with visual imaging than their instructors were at the same age. Thus the time is rife to reinvigorate chemistry education by means of the visual-spatial approach, an approach wholly in conformance with the way modern chemistry is thought about and practiced.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2147-2156 
    ISSN: 1434-1948
    Keywords: Tetrapodal pentadentate ligand ; Ligand periphery ; Polydentate amine complex ; Nickel ; Podand ; Schiff base ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tetrapodal pentaamine ligand 2,6-bis(1′,3′-diamino-2′-methylprop-2′-yl)pyridine (1), which contains four equivalent primary amino groups, can be derivatised partly or completely by Schiff base condensation with suitable carbonyl compounds. The new ligands thus obtained are mononucleating, as shown by the X-ray crystal structures of their respective nickel(II) complexes. Reaction of 1 with 1 equiv. of salicylaldehyde and subsequent reduction allows the selective modification of one of the four sidearms. The resulting ligand 2 is hexadentate and uninegative in its nickel(II) complex {[(2)Ni]PF6} (3) with both the secondary amine and the phenoxide functionalities coordinated to the metal centre. The unreduced Schiff base form of the ligand, 4, does not form a complex with nickel(II) as readily, and only a small quantity of the mixed salt {[(4)Ni][(1)Ni(H2O)](Br)2(PF6)} (5)has been obtained. While the overall coordination of 4 resembles that of 2, there is considerably more strain in the appended chelate ring, due to the presence of the C=N double bond. Modification of one arm in 1 can also be achieved by condensation with 1 equiv. of acetylacetone, to give the new ligand 6 which, likewise, is hexadentate in its NiII complex {[(6)Ni](PF6)2} (7). In this case, however, the N/O-functional sidearm is not deprotonated. Rather, it is coordinated as the keto-imine tautomer, making 7 a rare example of a metal complex containing this structural fragment. Two-fold functionalisation of 1 is observed upon reaction with acetone, regardless of whether the ketone is present in stoichiometric amounts or in excess, to give the pentadentate ligand 8with two diagonally juxtaposed isopropylidene-imine units. The complex isolated with this ligand {[(8)Ni](PF6)2} (9) contains pentacoordinate NiII, the sixth coordination site being blocked by the rigidly positioned isopropylidene groups. When reacted with 4 equiv. of trans-cinnamaldehyde, all the primary amino groups in 1 condense to give the four-fold Schiff base 10, which acts as a pentadentate podand towards nickel(II). In this complex, {[(10)Ni(OH2)]Br2} (11), an aqua ligand completes the coordination octahedron. All ligands are stable towards hydrolysis when coordinated to the metal, despite the presence of alkyl-imine groups in some cases.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2157-2166 
    ISSN: 1434-1948
    Keywords: Septadentate N5S2-ligand ; Dinuclear complexes ; Nickel ; Reactivity ; Redox chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The coordination chemistry of a series of dinickel complexes of the new septadentate amine-thiolate ligand N,N′-bis[2-thio-3-aminomethyl-5-tert-butylbenzyl]diethylenetriamine, H29, has been investigated in the context of ligand binding and oxidation state changes. The complexes [Ni2(9)(L)][ClO4]2 (10), [Ni2(9)(Cl)][Cl] (11), [Ni2(9)(L)][BPh4]2 (12), and [Ni2(9)(NCS)][OH·OH2] (13) have central N2Ni(μ-SR)2NiN′3L cores [L = labile solvent molecule (10, 12), Cl- (11), and NCS- (13)] composed of dithiolate bridged planar NiN2S2 and six-coordinate NiN′3S2L units. This is demonstrated for 11 and 13 by crystal structure determinations and for 10 and 12 by UV/Vis spectroscopy and room temperature magnetic susceptibility measurements. Complexes 10, 11, or 12 readily add other co-ligands at the NiN′3S2L fragment by substitution of the solvent molecule L (10, 12) or the chloride substituent (11). The overall structure of the parent complexes is not affected by the substitution reactions. An electrochemical study has shown that complex 10 undergoes two successive one-electron oxidations at +0.88 and +0.41 V vs SCE. The oxidized species are not thermally stable, but electronic absorption spectra and EPR spectra are indicative of the presence of NiIII species.
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  • 9
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    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2167-2172 
    ISSN: 1434-1948
    Keywords: Macrocyclic compounds ; Amine-Thiolate Donors ; Polynuclear Complexes ; Nickel ; Condensation reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel route to macrocyclic amine-thiophenolate ligands is described. The new, air-stable thiophenolate precursor 1,2-bis(4-tert-butyl-2,6-diformyl-phenylsulfanyl)ethane (4) is readily condensed with two equivalents of 1,2-ethanediamine or 1,3-propanediamine under medium to high dilution conditions to give 2 × 4 condensation products. The smaller 1 × 2 macrocyclic compounds are not produced under these conditions. Subsequent reduction with NaBH4 (reduction of imine groups) and Na/NH3 (reductive cleavage of aryl-alkylsulfides) provides the 36- and 40-membered amine-thiophenolate ligands H46a and H46b. The macrocyclic compounds are versatile ligands for the preparation of polynuclear transition metal complexes. With divalent nickel H46a forms the di- and tetranuclear complexes [Ni2(6a)] (7) and [Ni4(6a)][ClO4]4 (8). Reaction of 8 with four equivalents of NH4SCN yields the novel isothiocyanate complex [Ni4II(6a)(NCS)4]·10MeCN (9). The structure consists of well-separated molecules of the tetranuclear complex [NiII4(6a)(NCS)4] (Ci symmetry). Two symmetry-related binuclear [N2Ni(μ2-SR)2NiN4] fragments composed of thiolate-bridged distorted planar {N2S2Ni}- and distorted cis-octahedral {(SCN)2N2S2Ni} units reside within the cavity of the macrocycle. The intramolecular distance between the two binuclear units is 6.144(1) Å.
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  • 10
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    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2173-2185 
    ISSN: 1434-1948
    Keywords: EPR spectroscopy ; Metallocenylboranes ; Organovanadium radicals ; Redox chemistry (CV) ; X-ray diffraction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The paramagnetic complexes di(mesityl)([5]trovacenyl)borane (5·), (mesityl)di([5]trovacenyl)borane (6··), and tri([5]trovacenyl)borane (7···) were prepared from [5]trovacenyllithium, (η5-C7H7)V(η5-C5H4Li), and (Mes)2BF, (Mes)BF2, and BF3, respectively. The propeller-shaped species 5·, 6··, and 7··· were subjected to X-ray diffraction with the aim of possibly correlating the twist angles with intramolecular intermetallic communication. Cyclic voltammetry points to successive vanadium-centered oxidation processes and boron-centered reduction, a small redox splitting δE1/2[(2+/+), (+/0)] being observed for 6··. According to EPR spectroscopy, performed in fluid solution, the exchange interaction J in the diradical 6·· approaches the fast-exchange region and is attenuated significantly by quaternization at boron in [6··-nBu]-. Although EPR spectroscopy of the triradical 7··· also indicates an extensive exchange interaction, the exchange parameters, derived from spectral simulation, follow the gradation J(7···) ≈ 1/3 J(6··). The magnetic susceptibility of 6·· and 7··· follows the same trend. As expected, compound 7··· exhibits spin frustration because it contains three antiferromagnetically coupled S = 1/2 systems that are arranged in an equilateral triangle.
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  • 11
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2187-2199 
    ISSN: 1434-1948
    Keywords: Ansa compounds ; Cp-derived chelate ligands ; Neopentane chemistry ; Spiro compounds ; Tripodal ligands ; Cyclopentadienyl compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The neopentane-derived functionalized oxetane O(CH2)2C(CH3)(CH2OMs), 1, reacts with indenyllithium (LiInd) or fluorenyllithium (LiFlu) to produce the derivatives O(CH2)2C(CH3)(CH2R) (R = indenyl, fluorenyl), 2. The oxetane ring of 2 undergoes nucleophilic ring-opening by reaction with LiPR′2 to give the chiral chelate ligands (HOCH2)(CH3)C(CH2R)(CH2PR′2) (R = indenyl, fluorenyl), 5. Nucleophilic ring-opening by LiInd or LiFlu is possible too, resulting in the functionalized ansa-Cp ligands (HOCH2)(CH3)C(CH2R)(CH2R′) (R, R′ = indenyl, fluorenyl), 12. Electrophilic ring-opening of 2 with HBr to give (HOCH2)(CH3)C(CH2R)(CH2Br) (R = indenyl, fluorenyl), 3, is also possible. The alcohol function of 3 may be activated directly, whereas activation of this group in 5 is only possible after BH3 protection of the phosphane function. The mesylates (MsOCH2)(CH3)C(CH2R)(CH2Br) (R = indenyl, fluorenyl), 4, undergo, under basic conditions, spiro cyclization to produce spirocyclobutane derivatives 9 with the α-carbons of the five-membered cycles acting as the spiro centres. Substitution of the mesylate group of 4 by PR2 nucleophiles is therefore not possible. Ansa-Cp derivative (MsOCH2)(CH3)C(CH2Ind)(CH2Flu), 12c also reacts with LiPPh2 with spiro cyclization to produce 9d, instead of giving the substitution product. Tripodal ligands (CH3)C(CH2R)(CHPPh2)2 (R = indenyl, fluorenyl), 11, are accessible by the reaction of (MsOCH2)(CH3)C(CH2PPh2)2 with LiInd or LiFlu. All compounds are fully characterized by the usual spectroscopic and analytical techniques including single-crystal X-ray analyses in several cases.
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  • 12
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2201-2207 
    ISSN: 1434-1948
    Keywords: Carboxylate syn-anti ; N-centered tripodal ligand ; Manganese ; Imidazole ; Helicoïdal inorganic chiral chain ; Magnetism ; π-π interaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structure of an infinite inorganic chain consisting of MnII and an N-centered tripodal ligand N,N-(2-pyridylmethyl)[(1-methylimidazol-2-yl)-methyl]glycinate is presented. It exhibits a chiral helical structure with a pitch of two monomeric units (each monomeric unit containing one Mn atom). Each manganese is connected to its neighbor through a carboxylate bridge in a syn-anti geometry. Around each manganese center, two carboxylates bind in a cis geometry. This peculiar bridging geometry (syn-anticis) provides a broken-line chain, running in a zig-zag manner along the b axis of the P21 space group. The magnetic properties have been investigated. They show a pseudo-2D magnetic structure, with one major pathway along the chain and an inter-chain minor one. The intrachain coupling is a weak antiferromagnetic interaction (J/k = -0.25). This low value is entirely consistent with the geometry of the bridge. The interchain coupling is a weaker antiferromagnetic coupling (J′/k = -0.11) and could be mediated through π-π interactions between pyridine and imidazole from two adjacent helixes.
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  • 13
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2221-2231 
    ISSN: 1434-1948
    Keywords: Calixarenes ; Inclusion compounds ; Imido Complexes ; Molybdenum ; Tungsten ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses, spectroscopic properties, molecular structures, and bonding of novel calix[4]arene imido compounds are described. Treatment of M(NtBu)2(NHtBu)2Ia (M = Mo), Ib (M = W) or M(NMes)2Cl2(dme) IIa (M = Mo), IIb (M = W) (Mes = 2,4,6-Me3-C6H2) with p-tBu-calix[4]arene LH4 affords calix[4]arene metal complexes LM(NR) 1a, b (M = Mo, W; R = tBu) and 2a, b (M = Mo, W; R = Mes). Analytical and spectroscopic data are consistent with monomeric structures for 1 and 2, retaining a local C4v symmetry for the calix[4]arene metal fragment. These complexes are well-suited to bind small molecules like acetonitrile, tBu-isonitrile, or water within their macrocyclic pockets. The spectroscopic data of some inclusion compounds and the crystal structures of LMo(NtBu)(NCMe) 1a(NCMe), LW(NtBu)(OH2) 1b(OH2), LW(NtBu)(CNtBu) 1b(CNt Bu), LMo(NMes)(NCMe) 2a(NCMe), and LW(NMes)(NCMe) 2b(NCMe) are reported. All complexes contain a group VI metal imido [M=NR] moiety mounted on the phenoxide rim of the calix[4]arene ligand as well as an incorporated guest molecule within the cavity. Some insights into the structures of complexes of the type L′W(NR′) (L′ = p-H-calix[4]arene; R′ = H, Me) and into bonding in these compounds are provided by density functional theory, applying the B-P86 density functional and an all SVP basis set within the RI-J-DFT approximation. At least one π bond is of importance for calix[4]arene-metal bonding in these compounds. The metal-imido bond can be described as a triple bond. A geometrically optimized minimum structure of L′W(NMe) 4 shows a calix[4]arene ligand only slightly distorted from a local C4v symmetry and an almost linear tungsten-imido moiety.
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  • 14
    ISSN: 1434-1948
    Keywords: Arsonium salts ; Superacidic systems ; Structure elicidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of trimethylarsane in the superacidic systems hydrogen/fluoride antimony pentafluoride and hydrogen fluoride/arsenic pentafluoride leads to trimethylarsonium fluorometallates. These salts are stable up to 60 °C and 5 °C, respectively. Structures were successfully elucidated in the cases of trimethylarsonium undecafluorodiarsenate and trimethylarsonium hexafluoroantimonate. (CH3)3AsH+As2F11- crystallizes in the monoclinic space group P21/n with four formula units per unit cell with the dimensions a = 7.362(1), b = 12.589(1), c = 13.598(1) Å and β = 95.37(1)°. (CH3)3AsH+SbF6- crystallizes in the monoclinic space group P21/m with four formula units per unit cell with the dimensions a = 8.313(1), b = 8.855(1), c = 13.285(1) Å and β = 94.358(1)°.
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  • 15
    ISSN: 1434-1948
    Keywords: Molybdenum ; μ-Oxo ; Conformational equilibrium ; X-ray diffraction ; NMR spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new thiocyanatomolybdenum(VI) dioxo-μ-oxo complex dimer bearing a 4,4′-di-tert-butyl-2,2′-bipyridine ligand (2) is shown to exist in the crystal cell unit as a meso (Mo-O-Mo angle of 180°) and d,l, pair (Mo-O-Mo angle of 155.7°). These conformers are in equilibrium in solution and have been observed clearly by 1H NMR spectroscopy.
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  • 16
    ISSN: 1434-1948
    Keywords: Macrocycles ; Lithium ; Fluorescence ; Cryptands ; Coordination ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three aza-cages with the anthracene-containing photoactive groups L1, L2, and L3 have been synthesized. All compounds are able to selectively encapsulate a lithium ion and solid complexes have been isolated. The formation equilibria have been investigated by UV/Vis and 1H, 13C and 7Li NMR spectroscopic techniques. The fluorescence emission of both free ligands and lithium complexes have been investigated. Results indicate that the CHEF (chelation enhancement of the fluorescence) effect obtained by lithium coordination exits although lower than that occurring upon full protonation.
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  • 17
    ISSN: 1434-1948
    Keywords: Copper ; Trinuclear complexes ; Formamidines ; Magnetism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three new copper(II) linear trinuclear compounds are reported, all of which were synthesized in-situ, with the general formula [Cu3(L)4](CF3SO3)2(Y)x, where L is the dehydronated ligand: N,N′-bis(pyridine-2-yl)formamidine (abbreviated as Hpdf), N,N′-bis(pyrimidine-2-yl)formamidine (abbreviated as Hpmf) and N,N′-bis(5-methylpyridine-2-yl)formamidine (abbreviated as Hmpdf), Y = EtOH or H2O and x = 0.5-1.5. The compounds were characterized by X-ray diffraction, IR, LF, and EPR spectroscopy and by magnetic susceptibility down to 4 K. The structure of the compound [Cu3(pmf)4](CF3SO3)2(H2O)1/2 was determined by X-ray crystallography; it was found to crystallise in the triclinic space group P-1 with a = 8.529(5), b = 15.760(5), c = 19.639(5) Å, α = 101.793(5), β = 101.263(5), γ = 102.389(5)°, Z = 2. The structure [Cu-Cu-Cu angle 174.96(11)°] consists of four nearly flat molecules of the ligand, which contribute to the propeller-type structure around the Cu-Cu-Cu axis. A strong antiferromagnetic interaction between the CuII ions is observed with calculated J values, based on the Hamiltonian H = -2J(S1·S2 + S2·S3) - 2J′·S1·S3, of -174(1) cm-1, -120(1) cm-1, and -167(1) cm-1 for the compounds studied with L = pdf, pmf, and mpdf, respectively. These values are in agreement with an S = 1/2 ground state below temperatures of 120-160 K.
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  • 18
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    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2277-2281 
    ISSN: 1434-1948
    Keywords: Boron ; Iron ; Nickel ; Carbyne complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The μ3-boryloxycarbyne complexes [{Fe(CO)3}3{μ3-COBCl{NtBu(SiMe3)}}2] (1) and [{(η5-C5H5)Ni}3{μ3-COBX(NR2)}μ3-CO] (2a: NR2 = NtBu(SiMe3), × = Cl; 2b: NR2 = N(SiMe3)2, × = Cl; 2c: NR2 = NMe2, × = BNMe2Cl) were obtained by reaction of the anionic complexes K2[{Fe(CO)3}3{μ3-CO}2] and K[{(η5-C5H5)Ni(CO)] with the corresponding chloroboranes Cl2BNR2 {NR2 = NtBu(SiMe3), N(SiMe3)2}, or 1,2-dichlorodiboranes(4) B2(NMe2)2Cl2, respectively. The products are formed by a nucleophilic attack of the CO oxygen atom at the boron centres with subsequent salt elimination. All compounds were characterized by IR and multinuclear NMR spectroscopy, and the structures of 1 and 2c in the solid state were determined by single-crystal X-ray diffraction studies.
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  • 19
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    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1271-1279 
    ISSN: 1434-1948
    Keywords: Chirality ; Dinuclear complexes ; Rhodium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dinuclear and polynuclear metal complexes with octahedral centers coordinated to di- or polydentate ligands are often obtained as complicated mixtures of various stereoisomers. Stereospecific synthesis of such species is therefore of high current interest. Chiral derivatives of pyridine can be used for this purpose. Dinuclear μ-chloro-bridged RhIII complexes with two didentate, cyclometalated thienylpyridine-type ligands at each metal center are formed stereoselectively when pinene groups are fused to the pyridine rings. The two octahedral RhIII centers have homochiral configurations, ΔΔ and ΛΛ. The heterochiral diastereomer ΔΛ is not observed. With (8R,10R)-2-(2′-thienyl)-4,5-pinenopyridine [Hth4,5-(R,R)ppy] the ΔΔ to ΛΛ ratio is 9:1 when the separation eluent contains NaCl. Modeling the ΛΛ and the ΔΛ isomers of the dinuclear species shows crowding of the pinene groups in both cases; however, the strain can be released by relatively small distortions only in the case of the ΛΛ isomer. NO3- cleaves the dichloro bridge, yielding the mononuclear species Δ[Rh(L2)2(NO3)] (2) in a completely stereoselective manner when NaCl is replaced by KNO3 in the eluent mixture. The molecular structure has been determined by X-ray structure analysis for both the ΔΔ and the mononuclear complex Δ[Rh(L2)2(NO3)] (2) in order to confirm the configuration at the metal center. 1H-NMR, 13C-NMR and CD spectra were measured and the latter shows that the CD activity is solely due to the chirality at the metal center.
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  • 20
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    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1281-1289 
    ISSN: 1434-1948
    Keywords: Phosphaalkynes ; Dimerization mechanism ; Diphosphacyclobutenes ; Density functional theory ; Ab initio calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The dimerization of phosphaalkynes (R-C≡P, R = H, Me, tBu) without and with the presence of transition metal fragments, including CpCo (Cp = cyclopentadienyl) and COT-Ti (COT = cyclooctatetraene), has been probed using density functional theory calculations (B3LYP with different basis sets). MP2 and CCSD(T) calculations were also performed for the [H2C2P2] systems. In an attempt to address the exciting controversy and uncertainty about phosphaalkyne dimerization, a number of dimer formation mechanisms proposed in the literature have been examined. Some new and plausible intermediates have also been identified.
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  • 21
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    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1315-1324 
    ISSN: 1434-1948
    Keywords: Ab initio calculations ; EHMO ; Vinylidene complexes ; Cobalt ; Rhodium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The transformation of acetylene into vinylidene, as promoted by the metal fragment [(pp3)Co]+ [pp3 = P(CH2CH2PPh2)3], is unimolecular and features the hydride-acetylide species as an intermediate. The paper describes a detailed ab initio study of the reaction, in particular with regard to the step involving 1,3-H shift. The best computational results are obtained by mimicking the pp3 ligand with actual ethylenic chains rather than with single PH3 molecules. The keypoints along the two-step reaction path (π-acetylene, hydride-acetylide, and vinylidene complexes, as well as intermediate transition states) have been optimized for CoI and RhI derivatives at the MP2 level. For the fragment [(pp3)Co]+, the barrier associated with transformation of the hydride-acetylide intermediate to vinylidene (20.6 kcal/mol) is easier to surmount compared to that for reversion to the reactants (28.6 kcal/mol). The situation is reversed for the analogous RhI system, with the initial π-acetylene adduct being slightly more stable. Although higher in energy, the hydride-acetylide species is the experimentally detected product of the reaction of acetylene with the fragment [(pp3)Rh]+. The salient chemical aspects of the 1,3-H shift are discussed in terms of perturbation theory arguments. Parallel EHMO calculations, which have provided a relatively good consistency with the ab initio results, allow the proposal of an orbital rationale for the mode of migration of the hydride ligand along the substantially linear Co-Cα-Cβ grouping.
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  • 22
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    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 1335-1342 
    ISSN: 1434-1948
    Keywords: Photolysis ; Alkynes ; Matrix isolation ; DFT ; Oxygenations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Photolytically-induced reactions of chromyl chloride with acetylene and but-2-yne (dimethylacetylene) in low-temperature argon matrices yield end-on ketene and dimethylketene complexes of O=CrCl2, respectively. The product formation probably results from an electrophilic attack of a Cr=O linkage on the C≡C triple bonds, leading to radical-like transition states or intermediates, which subsequently rearrange through 1,2-H or 1,2-methyl shifts. Consequently, allene is attacked at its central carbon and the allyl radical thus generated undergoes subsequent ring-closure to give a cyclopropanone complex of O=CrCl2.
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  • 23
    ISSN: 1434-1948
    Keywords: Copper chelates ; EPR spectroscopy ; Electronic structure ; Schiff base ligands ; Structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of tetrahedrally distorted copper(II) complexes with thiolate and imine coordination were synthesized. Schiff bases derived from 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-thione and various diamines were used as tetradentate ligands to obtain tetrahedrally distorted metal chelates with [CuN2S2] complex units. Crystal structures of the complexes 1, 2, 5 and 6 and of ligand H25 have been determined by means of single-crystal X-ray structure analysis. The structure data show a strong influence of the diamine building blocks on the tetrahedral distortion of the copper(II) complexes. Results of Extended Hückel LCAO calculations correlate strongly with structural, electrochemical, UV/Vis- and EPR-spectroscopic features obtained experimentally. The calculations confirm for the whole complex series a strong delocalization of the frontier orbitals. The highest fully occupied molecular orbital shows a weak contribution, resulting from thiolate donor atoms, whereas the antibonding singly occupied molecular orbitals (SOMOs) are distributed between the copper(II) centre (ca. 35-40%) and the N2S2 donor set. The SOMO energy significantly lowers with increasing tetrahedral distortion of the coordination sphere. The influence of the tetrahedral distortion of copper(II) complexes on redox potentials, UV/Vis and EPR spectra is discussed.
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  • 24
    ISSN: 1434-1948
    Keywords: Half-sandwich complexes ; Molybdenum ; Optically active complexes ; Rhenium ; Tungsten ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of (η5-PinCp*)Re(CO)3 [PinCp* = tetramethyl(pinanyl)cyclopentadienyl] is described. Successive substitution of two CO ligands by NO+ and PPh3 generates a 1:1 diastereomeric mixture of chiral-at-metal [(SRe)/(RRe)-(PinCp*)Re(CO)(NO)(PPh3)]BF4. The diastereomers are converted with sodium methoxide into the derivative “esters” (SRe)/(RRe)-(PinCp*)Re(COOCH3)(NO)(PPh3), and then with (+)-(R)-(1-naphthylethyl)amine to the “amides” (SRe)/(RRe)-(PinCp*)Re(NO)(PPh3)[CONHCH(CH3)C10H7] [(SRe)/(RRe) = 1:1]. Fractional crystallisation separates the (SRe) isomer with an optical purity of 〉 98%. The latter compound has been characterized by X-ray structure analysis. By treating the (SRe)-amide with CF3CO2H and NaBF4, (SRe)-(PinCp*)Re(CH3)(NO)(PPh3) can be generated. Protolysis of this compound with HBF4/Et2O in CD2Cl2 at -78 °C leads to the solvent-stabilized complex (SRe)-[(PinCp*)Re(NO)(PPh3)(ClCD2Cl)]+BF4-. The thermal and configurational stability of this chiral Lewis acid is investigated at various temperatures. The syntheses of [PinCp*RhCl2]2, PinCp*TiCl3 and PinCp*M(CO)2(NO) (M = Mo, W) are also described. Starting with PinCp*M(CO)2(NO), the relatively stable 16-VE complexes PinCp*MCl2(NO) and PinCp*W(CH2SiMe3)2(NO) are synthesized.
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  • 25
    ISSN: 1434-1948
    Keywords: 2,2′-Bipyridines ; Chelate complexes ; Hydrogen bonding ; π-π stacking ; Crystal structure ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new synthetic route for the synthesis of 5,5′-diamino-2,2-bipyridine (5) based on the coupling of 2-chloro-5-aminopyridine in the presence of NiCl2 × 6 H2O/PPh3/Zn in dimethylformamide is described. The reactions of the potentially ambidentate ligand 5 with salts of the transition metals Mn, Fe, Ni, Cu, Zn, Ag, and Cd gave a variety of 13 metal-ligand complexes depending on the anion, the crystallization conditions and the metal-to-ligand ratio. The complexes obtained were characterized by thermal analyses, NMR including 113Cd-NMR, IR, and for the iron complex 57Fe-Mößbauer spectroscopy. The structure of eight of the compounds was elucidated by X-ray crystallography. All of these metal complexes show a bipyridine-metal coordination. The amino functionality was never involved in metal coordination. The intermolecular arrangement is dictated by hydrogen bonding from the amino functionality and by π-π stacking of the bipyridine rings.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2005/1999/99078_s.pdf or from the author.
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  • 26
    ISSN: 1434-1948
    Keywords: Iron(III) ; Peroxo Complexes ; Kinetics ; Hydrogen Peroxide ; Catalase ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The new diiron complex [Fe2(tbpo){O2As(CH3)2}(CH3O)(CH3OH)](ClO4)3 · 5 CH3OH · 2 H2O (1) containing a (μ-alkoxo)(μ-dimethylarsinato)diiron(III) core was synthesized using the heptadentate ligand N,N,N′,N′-Tetrakis(2-benzimidazolylmethyl)-1,3-diamino-2-propanol (Htbpo). The complex was characterized structurally by X-ray crystallography. 1reproduces the coordination mode and the stoichiometry of the proposed purple acid phosphatase-arsenate inhibitor complex. More importantly, 1 is a good functional model for the activation of small molecules, since the solvent molecule in the coordination sphere of each iron ion can be substituted very easily by a small substrate molecule. This is confirmed by the comparatively high pH-dependent catalase-like activity of 1. In order to study the influence of the cacodylate bridge on the formation of the metastable adduct with hydrogen peroxide, the analogous hydroxo-bridged complex [Fe2(tbpo)(OH)(NO3)2](NO3)2 · CH3OH · 2 H2O (2) was employed. The reactions of 1and 2 with H2O2 were studied as a function of [H2O2], pH, temperature, and pressure, and the kinetic results including the activation parameters are reported. In the case of compound 2 the reaction proceeds in one step, and the observed first order rate constant, kobs, shows a linear dependence on the hydrogen peroxide concentration with a zero intercept. For complex 1 the kinetic traces could be fitted to two exponential functions. One of the observed pseudo-first-order rate constants, kobs1, exhibits a linear dependence on the hydrogen peroxide concentration with a zero intercept, whereas the other rate constant, kobs2, was independent of the hydrogen peroxide concentration. A mechanistic interpretation is presented.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2005/1999/99068_s.pdf or from the author.
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  • 27
    ISSN: 1434-1948
    Keywords: Ruthenium ; Coordination chemistry ; Paramagnetic complexes ; Magnetic properties ; Ion exchange ; Antitumor agents ; Colon tumors ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Indazolium trans-tetrachlorobis(indazole)ruthenate(III) exhibits excellent results against different tumor models in vitro and in vivo. To improve the water solubility necessary for the introduction of this tumor-inhibiting compound into clinical trials, we synthesized the corresponding sodium salt in a two-step ion exchange via the tetramethylammonium salt. The sodium salt shows a 35-fold higher solubility in water relative to the indazolium salt. We also synthesized the n-butylammonium, n-octylammonium, and tetraphenylphosphonium salts, all of which showed improved solubility in organic solvents. The X-ray crystal structure of the latter could be solved, proving the trans configuration of the complex anion. In spite of the paramagnetic RuIII center an assignement of the coordinated indazole protons could be made with the help of a COSY experiment.
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    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2355-2368 
    ISSN: 1434-1948
    Keywords: Phosphane complexes ; Lithium ; Aminophosphanes ; Hydrazides ; (N-lithioamino)diorganophosphanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Butyllithium (nBuLi) deprotonates Ph2P-NHtBu in ether to give (Ph2P-NLitBu)2·OEt2. There is no Li···P interaction in this molecule. Three compounds of the type R′P(NLiR)2 have been obtained by lithiation of R′P(NHR)2, isolated as [BuP(NLitBu)2·OEt2]2, [PhP(NLiPh)2·OEt2]2 and [PhP(NLiPh)2]2. Reaction of nBuLi with MeP(NHiPr)2 in hexane/THF leads to [MeP{N(Li)iPr]2·THF}4 with an asymmetric cluster structure comprising one LiP3, three LiPN2, three LiP2N and one LiN3 cluster units. The molecular structures of these compounds as determined by X-ray structure analysis show that they are best depicted as N-lithioaminophosphanes and not as the isomeric P-lithioiminophosphoranes.
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  • 29
    ISSN: 1434-1948
    Keywords: Alkylation ; Carbonyl complexes ; Lewis acids ; Phosphaalkenes ; Protonation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of the carbonyl-functionalized phosphaalkenes RC(O)P=C(NMe2)2 [R = tBu (2a), Ph (2b)] with protic acids and alkylating reagents occurred at the two-coordinate phosphorus atom to give the phosphanyl-substituted carbocations 3a,b and 4a,b. In contrast, treatment with Me3SiOSO2CF3 resulted in attack at the oxygen atom by the silyl group, and the formation of [RC(OSiMe3)=PC(NMe2)2]SO3CF3 (5a,b). Similarly, the Lewis acids B(C6F5)3, Al(tBu)2Cl and AlMe3 were ligated to the oxygen atom of the carbonyl group. Two equivalents of GaMe3 were added to the oxygen and phosphorus atom of the phosphaalkene to yield the thermolabile complexes [RC(OGaMe3)=P(GaMe3)C(NMe2)2] (10a,b). In contrast, one molecule of InMe3 was bound to the phosphorus center of the phosphorus compound. Reaction of the phosphaalkenes with [Ni(CO)4], [Fe2(CO)9] or [{(Z)-cyclooctene}Cr(CO)5] also took place at the pnictogen atom, resulting in complexes of the type [RC(O)P{M(CO)n}C(NMe2)2] (R = tBu, Ph; M = Ni, n = 3; Fe, n = 4; Cr, n = 5). The chemical transformations reported here underline the versatile chemistry of phosphaalkenes and emphasize a relationship between carbonyl-functionalized phosphaalkenes and the well-investigated class of phosphorus ylides. X-ray structures of compounds 6b, 7b*, 10a, 11a and 12a are reported.
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  • 30
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    AIChE Journal 41 (1995), S. 185-189 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 190-194 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 58-67 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Theoretical models of gas diffusion and permeation in microporous molecularsieve membranes are presented. The effect of the adsorbed diffusant on the total transmembrane flow is insignificant for permanent gases. For highly adsorbable gases the effect of the adsorbed molecules on the total transmembrane flux may be high at low temperatures. The activation energy of diffusion increases when the kinetic diameter of the diffusant increases. The activation energy of gas diffusion compares well with the values calculated based on the Lennard-Jones potential. Maximum possible permeability coefficients calculated for He in the molecular-sieve membranes do not exceed ˜30,000 Barrer at room temperature. The experimentally observed value for He permeability is ˜1,000 Barrer (T=30°C) because of the high tortuosity (τ≍ 25) and low porosity (θ = 0.22) of the membrane porous structure.
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  • 33
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    AIChE Journal 41 (1995), S. 68-77 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A pH-stat MSMPR crystallizer is used to obtain the nucleation rate equation of calcite formed in the Na2CO3-CaCl2 reacting system. The experiments are conducted for seeded and unseeded runs. Effects of breakage and agglomeration on the crystalsize distribution are demonstrated. In a continuous operation, the agglomeration of nuclei followed by the breakage of agglomerates occurs at the transient state, while the agglomeration is significant at the steady state. Taking crystal agglomeration into account, the corrected nucleation rate and agglomeration rate are obtained simultaneously. The agglomeration rate is a function of relative supersaturation and magma density, and the corrected nucleation rate is approximately two- to five-fold of the apparent nucleation rate, which is the nucleation rate without considering the agglomeration effect. Discussed also is the formation of calcium carbonate polymorphism.
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  • 34
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    AIChE Journal 41 (1995), S. 135-147 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Research on exothermic reactor operation has been based mostly on the classic two-state continuous stirred tank reactor model, implicitly assuming that the cooling jacket temperature dynamics are negligible. In this case, the cooling jacket temperature is the manipulated input instead of the cooling jacket flow rate for feedback control of reactor temperature. Adding a cooling jacket energy balance results in much more complex behavior than a simple lag effect. A stabilizing inner-loop cascade controller is assumed in the two-state CSTR model, because the three-state model incorporating cooling jacket temperature dynamics may be open-loop unstable when the two-state model is open-loop stable. The influence of design parameters on the multiplicity behavior of a three-state model is considered. Elementary catastrophe theory is used to study the effect of process parameters such as the cooling jacket flow rate, heat-transfer coefficient, heat of reaction, and cooling jacket feed temperature on the steady-state multiplicity of the three-state model. This multiplicity analysis is particularly relevant for control because the primary bifurcation parameter is the cooling jacket flow rate, the manipulated input for feedback control in the three-state model. This multiplicity analysis guides improvements in process design and/or operation to eliminate difficult operating regions associated with steady-state multiplicities; the presence of multiple steady states results in safety and operation problems due to ignition/extinction phenomena. Reactor scale-up affects the presence of these infeasible reactor operating regions. Certain design parameter changes that remove multiplicities in the two-state model cannot remove multiplicities in the three-state model.
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  • 35
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    AIChE Journal 41 (1995), S. 346-356 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Brownian and molecular dynamics simulations are used to study rapid bimolecular reactions at near-infinite dilution in near-critical and supercritical fluids. We probe the dynamics of both nonreactive and reactive collisions and measure rate constants for reaction and collision. Collision rate constants are nearly independent of bulk solvent density, but affected by local solute-solute density enhancements at a given density: their magnitudes depend on the length scale for molecular encounters (cybotactic radius) in the reaction through the equilibrium solute-solute radial distribution function. In contrast, reaction rate constants asymptotically approach the gas-kinetic limit at low densities and the Smoluchowski liquid-like limit at high densities. They also display the same radial dependence as collision rate constants at lower densities and a direct dependence on the cybotactic radius at higher densities (as in the Smoluchowski theory). Their behavior is explained in terms of a transition from a collision-limited regime at low densities to a diffusion-limited regime at higher densities. The transition between these regimes depends on the cybotactic radius and the density of the system, the interplay of which causes shifts in the transition region which depend not only on the properties of the near-critical solvent: they differ for different reactions, even at the same solvent density. This explains some of the apparent inconsistencies among previous experimental and computational studies of reactions in supercritical fluid media.
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    AIChE Journal 41 (1995), S. 402-414 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Many motile bacteria exhibit chemotaxis, the ability to bias their random motion to ward or away from increasing concentrations of chemical substances which benefit or inhibit their survival, respectively. Since bacteria encounter numerous chemical concentration gradients simulatneously in natural surroundings, it is necessary to know quantitatively how a bacterial population responds in the presence of more than one chemical stimulus to develop predictive mathematical models describing bacterial migration in natural systems. This work evaluates three hypothetical models describing the integration of chemical signals from multiple stimuli: high sensitivity, maximum signal, and simple additivity. An expression for the tumbling probability for individual stimuli (Brown and Berg, 1974) is modified according to the proposed models and incorporated into the cell balance equation for a 1-D attractant gradient (Ford and Cummings, 1992). Random motility and chemotactic sensitivity coefficients, required input parameters for the model, are measured for single stimulus responses. Theoretical predictions with the three signal integration models are compared to the net chemotactic response of Escherichia coli to co- and antidirectional gradients of D-fucose and α-methylaspartate in the stopped-flow diffusion chamber assay. Results eliminate the high-sensitivity model and favor the simple additivity over the maximum signal. None of the simple models, however, accurately predict the observed behavior, suggesting a more complex model with more steps in the signal processing mechanism is required to predict responses to multiple stimuli.
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    AIChE Journal 41 (1995), S. 426-429 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 439-443 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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  • 39
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    AIChE Journal 41 (1995), S. 446-455 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Studies of heat and mass transfer in packed beds, which disagree substantially in their findings, have nearly all been done with beds of regular particles of uniform size, whereas oil-shale retorting involves particles of diverse irregular shapes and sizes. We, in 349 runs, measured mass-transfer rates from naphthalene particles buried in packed beds by passing through air at room temperature. An exact analogy between convection of heat and mass makes it possible to infer heat-transfer coefficients from measured mass-trans-fer coefficients and fluid properties. Some beds consisted of spheres, naphthalene and inert, of the same, contrasting or distributed sizes. In some runs, naphthalene spheres were buried in beds of crushed shale, some in narrow screen ranges and others with a wide size range. In others, naphthalene lozenges of different shapes were buried in beds of crushed shale in various bed axis orientations. This technique permits calculation of the mass-transfer coefficient for each active particle in the bed rather than, as in most past studies, for the bed as a whole.The data are analyzed by the traditional correlation of Colburn jD vs. Reynolds number and by multiple regression of the mass-transfer coefficient on air rate, sizes of active and inert particles, void fraction, and temperature. Principal findings are: local Reynolds number should be based on the active-particle size, not the average for the whole bed; differences between shallow and deep beds are not appreciable; mass transfer is 26% faster for spheres and lozenges buried in shale than in all-sphere beds; orientation of lozenges in shale beds has little or no effect on mass-transfer rate; and for mass or heat transfer in shale beds, log(j·∊) = - 0.0747 - 0.6344logNRe + 0.0592log2 NRe.
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    AIChE Journal 41 (1995), S. 456-468 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The thermodynamics and transport properties of Lennard-Jones particles in pillared catalytic clays are studied by molecular dynamics simulation. The clays are represented by parallel sheets separated by a given distance and connected by pillars of a given size. Two different spatial distributions of the pillars are studied to determine their effect on the properties of the system. Calculations did not indicate a strong dependence of the diffusivity on the spatial distribution of the pillars, except at low porosities. The solvation force increases monotonically with decreasing porosity of the clays and increasing density of the molecules. The percolation threshold ϕc of the system is estimated from the diffusivity measurements in the limit of infinitely low sorbate densities. Near ϕc the diffusivity D vanishes according to the power law, D ˜ (ϕ - ϕc)n, where ϕ is the porosity of the system, and n is a universal constant. The simulations yield n ⋍ 1.7. Since 2- percolation systems require n ⋍ 1.3 and 3-D systems n ⋍ 2.0, pillared clays behave as a system with an effective dimensionality between 2 and 3.
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  • 41
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    AIChE Journal 41 (1995), S. 337-345 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The monolith froth reactor, involving two-phase flow and a monolith catalyst, is developed. The flow within monolith channels, consisting of trains of gas bubbles and liquid slugs, is produced by forming a two-phase froth in a chamber immediately below the bottom of the monolith. The froth then flows upward into the monolith channels through pressure forces, which differs from previous methods since it may be carried out for a commercial-scale reactor. Because the liquid film which develops between the gas phase and the surface of the catalyst is extremely thin, two-phase flow within a monolith can provide reaction rates which are near their intrinsic values. Catalytic oxidation of aqueous phenol over copper oxide supported on γ-Al2O3 is used as a model reaction for investigating reactor performance. Generation of a froth is confirmed by visual inspection; the average bubble size is approximately that predicted by a force balance. The effect of externally controllable process variables (liquid and gas flow rates, temperature, and pressure) on the rate of phenol oxidation was investigated. Reaction rate increases with temperature or pressure increase and decreases with gas flow rate increase, achieving a maximum with respect to liquid flow rate. The activation energy calculated from the apparent reaction rate measured in the monolith froth reactor is similar to that of intrinsic value, suggesting minimal mass-transfer limitations.
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  • 42
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    AIChE Journal 41 (1995), S. 377-388 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A thermogravimetric analyzer is used to study the synthesis of TiN from Ti powder over a wide range of temperature, conversion and heating rate, and for two Ti precursor powders with different morphologies. Conversions to TiN up to 99% are obtained with negligible oxygen contamination. Nonisothermal initial rate and isothermal data are used in a nonlinear least-squares minimization to determine the most appropriate rate law. The logarithmic rate law offers an excellent agreement between the experimental and calculated conversions to TiN and can predict afterburning, which is an important experimentally observed phenomenon. Due to the form of the logarithmic rate law, the observed activation energy is a function of effective particle size, extent of conversion, and temperature even when the intrinsic activation energy remains constant. This aspect explains discrepancies among activation energies obtained in previous studies. The frequently used sedimentation particle size is a poor measure of the powder reactivity. The BET surface area indicates the powder reactivity much better.
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  • 43
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    AIChE Journal 41 (1995), S. 430-434 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 435-438 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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  • 45
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    AIChE Journal 41 (1995), S. 501-508 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A primary method of reducing membrane fouling during cross-flow microfiltration is periodic reverse filtration. This in situ method of cleaning the membrane forces clear fluid in the reverse direction through the membrane and readjusts the particle or solute accumulation on the retentate side of the membrane. This work focuses on the design of a high-frequency, reverse-filtration strategy to maximize the flux for washed yeast suspensions through 0.2- μm cellulose acetate flat sheet membranes. Several experiments were conducted with reverse-filtration times ranging from 0.5-4 s and forward-filtration times ranging from 1-40 s. For every back-filtration time, there exists an optimum forward-filtration time that gives the maximum global average flux. The optimum average flux increases with decreasing back filtration times and feed concentrations, but shows little dependence on cross-flow velocity and reverse filtration transmembrane pressure. The optimum flux with rapid backflushing is 20 to 30 times higher than the long-term flux in the absence of backflushing. A theory presented assumes that cake formation during forward filtration follows dead-end filtration theory and the cake is instantly removed during reverse filtration. The measured average flux per cycle follows the trends predicted by the theory, but the measured values exceed the predictions, presumably due to brief delays in cake removal and cake formation at the start of reverse and forward filtration, respectively, during each cycle.
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  • 46
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    AIChE Journal 41 (1995), S. 868-879 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Fluidized-bed polyethylene reactors are prone to unstable behavior and temperature oscillations (Choi and Ray, 1985b). Their work is extended to show the effects of ethylene feed system operation, reactor cooling system design, catalyst properties, and gas composition on reactor stability and dynamics. The analysis is performed using a well-mixed model, because heat- and mass-transfer resistances between multiple phases are small and are not required to account for the observed bifurcation phenomena. The addition of a gas recycle and heat exchanger system to the model significantly affects dynamic performance, including the formation of limit cycles. The size and dynamics of the heat exchanger, however, have little effect on the overall stability. In contrast, automation of the ethylene feed system to replace the monomer in the reactor as it is consumed leads to substantially different dynamic behavior than if the ethylene feed is maintained at a constant rate. Catalyst properties (multiple sites, activation energy, and deactivation) significantly affect dynamics and stability, whereas comonomer and other gases affect them only mildly. The results confirm that without proper temperature control, gas-phase polyethylene reactors are prone to instability, limit cycles, and excursions towards unacceptable high-temperature steady states.
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  • 47
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    AIChE Journal 41 (1995), S. 924-927 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The Maxwell equal area rule is a basic construct for determining saturated phase volumes from equations of state. The fundamental idea is that for a pure substance a horizontal line drawn through the trace of an isotherm, which equalizes the van der Waals loops area above and below the line, intersects the isotherm at the saturated volumes.Another basic construct exists for determining the phase compositions in a binary mixture by establishing the tangent line to the total Gibbs energy curve plotted against composition. We demonstrate that finding the tangent also reduces to an equal area construction for the derivative of the total Gibbs energy plotted against composition. An efficient algorithm suggested effects such equal area determinations. The trial-and-error procedure normally converges in one to three steps to the correct solution given any resonable starting guess. Implementation using modern calculators with graphics capabilities is simple and straightforward.
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    AIChE Journal 41 (1995), S. 959-973 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The interaction between a winter flounder antifreeze polypeptide and an ice/water interface was studied using Molecular Dynamics computer simulation techniques to study the mechanism of action of this class of antifreeze molecules. Simple Point Charge models were used for the water molecules, and a molecular mechanics program (CHARMM) was used to construct the model for the polypeptide. A (2021) face was exposed on the ice surface, as this is believed to be the experimentally favored ice face for peptide binding. The polypeptide binds strongly to the ice surface even though it was placed with its four polar threonine (Thr) groups pointing away from the ice surface. This tested the previously advanced hypothesis that adsorption occurs primarily between these groups and the ice due to a matching of the spacing between oxygen atoms in the ice lattice and the polar Thr residues. As well as contacts with other polar groups on the peptide, the binding to the ice produces a good steric fit of the peptide with the corrugated ice interface. The presence of the peptide did not induce any melting of the ice at 200 K.
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    AIChE Journal 41 (1995), S. 985-990 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Computer simulation studies of refolding pathways and the formation of intermediates for a simple, 2-D lattice protein model are presented. The sequence of the 20-bead model protein chain is chosen so that hydrophobic beads will reside in the protein interior in the native state. Nonbonded hydrophobic beads attract each other with strength ∊ decreasing the |∊/kT| mimics increasing the concentration of the denaturant. Dynamic Monte Carlo simulations and exhaustive conformational searches have been performed on an isolated model protein sequence at different levels of |∊| (different denaturant concentrations). As the denaturant is withdrawn, the model protein exhibits a transition from a random coil state to a compact native state with a hydrophobic core. The refolding process is observed to be cooperative in that the chain does not start folding until the middle section has folded correctly, and proceeds along preferred pathways that are populated by distinct, partially folded intermediates.
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    AIChE Journal 41 (1995), S. 974-984 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A theoretical model for investigating physical phenomena underlying immune complex formation was developed, based on the statistical mechanical theory of associating fluids that identifies each molecule as a hard sphere with a nested point charge and vector dipole. The interaction between binding molecules (epitope-paratope binding) is represented as a cone truncated by two concentric spheres in which the potential energy is a modified square well with respect to particle separation and a square well with respect to mutual molecular orientation. Equilibrium binding results predicted by the model show good agreement with results obtained experimentally for a model system containing a single antigen and a single monoclonal antibody [bovine serum albumin (BSA) - anti-BSA antibody]. Moreover, values obtained for the system isothermal compressibility and the second virial coefficient by both the model and light scattering experiments also show good agreement with one another.
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    AIChE Journal 41 (1995), S. 855-867 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: For a triplet competitive-consecutive halogenation sequence forming mono-, di- and trihalogenated products of the form, A + B → R + B → S + B → T, under semibatch operation adding B to A, if perfect mixing could be assumed at all scales, the product distribution would be unchanged on scaling up. However, if the reaction rates are reasonably faster than the mixing rate, the semibatch addition of B to A will be imperfectly backmixed, exhibiting macroscale concentration gradients. This partial segregation of the primary reagents is capable of modifying the selectivity and corresponding appearance of R, S and T in the course of the batch. Imperfect mixing is quantified using the networks-of-zones model. The effect of scaling up at equal tip speed is examined for a lab-scale 0.3-dm3 reactor, a semitech 30-dm3 reactor, and a production-scale 3,000-dm3 vessel. The intensity of partial segregation is weak at the lab scale, but very severe at the production scale. The lab-scale reactor is therefore close to perfectly backmixed, and the primary, secondary and tertiary products appear in sequence. At the semitech scale the increased partial segregation causes the final product to initially precede the secondary product paradoxically but lag the initial product. At the large scale the more severe segregation between A and B gives an even greater paradox, whereby the final product appears ahead of both the primary and secondary ones. The segregated concentration fields of A and B are visualized as sectional image reconstructions for networks comprising on the order of 1,000 zones. Localized intensive plumes of B emanating from the addition point cause the paradoxical reversals of product sequences. The calculations are directly relevant to real industrial miscible liquid halogenations for which product distribution paradoxes have been observed (Haywood, 1990).
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    AIChE Journal 41 (1995), S. 907-914 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Microporous solids made by polymerizing the organic phase of bicontinuous microemulsions stabilized with didodecyldimethylammonium bromide show surface areas as high as 70 m2/g. Small-angle X-ray scattering measurements on microporous copolymers containing polymethylmethacrylate show that these areas result from structures larger than 250 Å that are generated during polymerization. These X-rays measurements also show that the 17-Å structures characteristics of the original microemulsion are retained during polymerization, but destroyed when the detergent is extracted and the solid is dried.
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    AIChE Journal 41 (1995), S. 938-947 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A method-based on evaporation of a constant-composition droplet containing two components that differ markedly in volatility accurately estimates activity coefficients of both components. This new technique is developed to simultaneously determine the evaporation rate and composition of a droplet from intensity peaks observed in the light scattered by the droplet. It has no upper or lower limits on the relative volatility of the system and is particularly suitable for systems containing one relatively nonvolatile component. The data on the glycerol-ethanol system, obtained from evaporation of glycerol droplets in ethanol vapor and correlated with Margules and Wilson equations, are thermodynamically consistent. The results of this study agree with ethanol activity coefficients calculated from the total pressure vs. solution composition data previously reported.
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    AIChE Journal 41 (1995), S. 1184-1193 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Mobile-phase additives, commonly used to modulate absorbate retention in gradient elution chromatography, are usually assumed to be either linearly retained or unretained. Previous theoretical work from our laboratory has shown that these modulators, such as salts in ion-exchange and hydrophobic interaction chromatography and organic modifiers in reversed-phase chromatography, can absorb nonlinearly, giving rise to gradient deformation. Consequently, adsorbate peaks that elute in the vicinity of the head of the deformed gradient may exhibit unusual shapes, form shoulders, and/or be concentrated. These effects for a reversed-phase sorbent with aqueous acetonitrile (ACN) as the modulator are verified experimentally. Gradient deformation is demonstrated experimentally and agrees with simulations based on ACN isotherm parameters that are independently determined from batch equilibrium studies using the layer model. Unusual adsorbate peak shapes were found experimentally for single-component injections of phenylalanine, similar to those calculated by the simulations. A binary mixture of tryptophan and phenylalanine is used to demonstrate simultaneous concentration and separation, again in agreement with simulations. The possibility of gradient deformation in ion-exchange and hydrophobic interaction chromatography is discussed.
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    AIChE Journal 41 (1995), S. 1550-1562 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A method for treating three-dimensional current and temperature distributions in large-scale battety modules is described. Simulations are presented for a module consisting of cells formed with a lithium metal anode, a polymer electrolyte, and a vanadium oxide cathode. The calculations illustrate the nonlinear dependence of power output on system temperature, which for the investigated lithium-polymer battery module is determined primarily by the influence of temperature on the electrochemical reaction rates and ionic conductivity. An appendix is devoted to the estimation of physicochemical parameters, some of which are not available from the current literature, but are nonetheless important model inputs.
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    AIChE Journal 41 (1995), S. 1581-1584 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 1602-1604 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 1629-1639 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A fundamental study of drop collision, coalescence and growth induced by combined effects of gravitational and electrostatic forces is presented. The focus is on the enhancement of rates of collision and growth of spherical, conducting drops bearing zero net charge in dilute, homogeneous dispersions by an external electric field. By completely accounting for hydrodynamic and electrostatic interactions, a trajectory analysis is used to follow the relative motion of two drops equation is then solved to predict the evolution in time of the size distribution and the average size of drops. The results show that the rates of drop collision and growth can be increased significantly by applying an electric field, in accord with fundamental experiments and patents on electrocoalescence. The enhancement of drop collision and coalescence is especially pronounced when the imposed electric field acts horizontally, that is, in a direction perpendicular to gravity.
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    AIChE Journal 41 (1995), S. 1324-1328 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 1341-1345 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    AIChE Journal 41 (1995), S. 1354-1355 
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    Keywords: Chemistry ; Chemical Engineering
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    AIChE Journal 41 (1995), S. 1389-1394 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The separation of air by pressure swing adsorption is improved by filling the interparticle voids in kineticaly selective adsorbent beds with fine particles of the adsorbent or inert material. He ratio of the average diameter of the coarse adsorbent particles to the average diameter of the fine particles, the size of the fine particles themselves, and the percent of volume of the fine particles in the bed are all critical to optimum PA performance.
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    AIChE Journal 41 (1995), S. 301-316 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Possibilities of predicting liquid holdup in rotating packed beds are examined using the film flow theory. A hydrodynamic model based on film flow on the particle scale accommodates both laminar and turbulent films in the entry and developed regime. Conductance measurement was used for experimental determination of liquid holdup and estimation of the degree of anisotropy of liquid distribution. Experimental results represent the first data for liquid holdup in the rotating packed bed as a function of operating conditions and liquid properties. They indicate an anisotropic liquid distribution dependent on the operating variables. While the film model can be fitted to the experimental data, such a fit lacks a theoretical basis and the classical theory of film flow on the particle scale cannot explain the liquid flow in rotating packed beds. An empirical expression correlates well the holdup data with the operating parameters.
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    AIChE Journal 41 (1995), S. 1926-1943 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Morphological aspects of the evolution of a gas - solid interface during typical CVD processes are presented, as well as a continuum model of CVD growth. A linear stability analysis used determines the effect of reactor conditions on the stability of planar growth. The main focus, however, is numerical solution of governing equations under a wide variety of conditions and with different initial interface shapes as starting point. Simplified solutions under specific deposition conditions and the numerical procedure for solving the complete system of equations are presented. The focuses are on the use of a parametrization that eliminates numerical problems encountered with steep interface gradients and the automatic generation of an adaptive mesh for the domain above the interface. Several examples illustrate the numerical solution procedure. To our knowledge, this is the first attempt to simulate interface evolution during CVD for long deposition times from various initial interface shapes. The simulation revealed several morphological phenomena observed experimentally in previous studies, including the formation of occlusions that contributes to film porosity and was clearly shown by the numerical results. Film uniformity strongly depends on the controlling mechanism of deposition. Severe nonuniformities develop under diffusional limitations, while deposition is very uniform under conditions of kinetics control. Film uniformity could be improved by choosing conditions for which a Damköhler number of deposition, Da, would have the lowest value.
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    AIChE Journal 41 (1995), S. 1723-1778 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Supercritical fluids posses properties that make them attractive as media for chemical reactions. Conducting chemical reactions at supercritical conditions affords opportunities to manipulate the reaction environment (solvent properties) by manipulating pressure, to enhance the solubilities of reactants and products, to eliminate interphase transport limitations on reaction rates, and to integrate reaction and separation unit operations. Supercritical conditions may be advantageous for reactions involved in fuels processing, biomass conversion, biocatalysis, homogeneous and heterogeneous catalysis, environmental control, polymerization, materials synthesis, and chemical synthesis. Moreover, supercritical fluids can be used profitably in fundamental chemical investigations of intermolecular interactions and their influence on chemical processes. Work on chemical reactions in and with supercritical fluids is reviewed. We discuss both fundamental studies and applications of reactions at supercritical conditions, with focus on work published after 1985.
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    AIChE Journal 41 (1995), S. 509-517 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: New adsorbents for olefin/paraffin separation are synthesized by effective dispersion of Ag(I) and Cu(I) cations on substrates with hydrocarbon-phobic surfaces. These cations bind olefin molecules by a π-complexation bond, a weak chemical bond. Ethane/ethylene and propane/propylene separations are considered. Cation exchange resins and CuCl/γ-Al2O3 are effective substrates. On the Ag(I) resin at 25°C and 1 atm, the equilibrium adsorption ratio for C2H4/C2H6 = 9.2 and C2H4 capacity = 1.15 mmol/g; the corresponding values for C3H6/C3H8 = 10.4 and C3H6 capacity = 1.29 mmol/g. The CuCl/γ-Al2O3 sorbent shows equally promising results. The sorption rates are pore-diffusion-controlled and rapid. The olefin selectivity; capacity, and rates are much higher than all previous attempts and are suitable for applications in cyclic adsorption processes.The equilibrium data are correlated with an isotherm equation that accounts for both physical adsorption and π-complexation with energy heterogeneity, using only two true fitting parameters. Molecular orbital calculations using a C6H5SO3- substrate indicate that the π-complexation bond is contributed mainly by the donation of olefin π-bond electrons to the empty s-orbital of the metal, while the d-π* back donation contributes only 16%. Moreover, the relative order of the heats of adsorption is correctly predicted.
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    AIChE Journal 41 (1995) 
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    Keywords: Chemistry ; Chemical Engineering
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    AIChE Journal 41 (1995), S. 1852-1863 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A nonequilibrium model for the dynamic simulation of distillation columns is described. The nonequilibrium model includes the direct calculation of the rates of mass and energy transfer and is better able to model the actual physical processes occurring on a real distillation tray than is the conventional equilibrium stage model. Example calculations show that heat-transfer limitations and the vapor holdup above the froth cannot be neglected at elevated pressures. Back-computed Murphree tray efficiencies are not constant over time, which implies that the equilibrium model should not be used for dynamic simulations.
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    AIChE Journal 41 (1995), S. 1898-1914 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Using asymptotic expansion and numerical analysis, we demonstrate how the step-response ignition time of an automobile catalytic converter depends on the ratio of the reaction rate to the interphase heat-transfer rate, as measured by a key Damköhler parameter χ and the degree of monolith subcooling ŋ.In the region of low reactionrate at smallχ, the normalized ignition time tig scaled bythe homogeneous ignition time from the inlet gas temperature is (tigtig∞) = 1 + 2 χ1/2∣ln( χ1/2/2 η)∣1/2, and the ignition takes place at a thermal front deep in themonolith. At large χ when the reaction rate is high, ignition occursat the leading edge of the monolith with (tig/tig∞) = 2.50 + χ(ln η - 0.34).The delay in ignition time with increasing χ is due to a Taylor-Aris dispersion mechanism induced by interphase heat transfer. Although the small- χ ignition mechanism is faster, its downstream ignition location leads to a very slow upstream propagation of the thermal front that follows ignition. An optimal converter system that ignites in 13 s, 25% of the current value in a standard step-response test, is then designed by placing a small igniter, which ignites by the small- χ mechanism, upstream to preheat the current converter which then ignites by the large- χ mechanism. The length of the igniter is kept small by bypassing 2/3 of the exhaust since, from our theory, tig∞ is independent of the gas velocity.
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    AIChE Journal 41 (1995), S. 1955-1963 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The study of mixed films has become of considerable interest both from theoretical and practical points of view. The π-A isotherms of monostearin-distearin (about 50% 1,3- and 50% 1,2-isomers) mixed films spread on water as a function of emulsifier ratio and temperature were studied in an automated Langmuir-type film balance. When molecules of both emulsifiers are spread on the air-water interface they are packed more closely together than when either emulsifier is present alone, and they enter into some form of association. This association can be demonstrated by surface pressure-area isotherms and by thermodynamic studies. The characteristics of the mixed films, their elasticity and miscibility, which are of interest in emulsion or foam stability are discussed. A molecular model established can explain the interactions between film and subphase molecules and their relationships with variables of the process (pressure and temperature). This model fits the calculated thermodynamic parameters.
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  • 71
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    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Conventional ozonation of recalcitrant organic compounds in wastewater suffers from low transfer rate of ozone into water. The low transfer rate can be enhanced by adding an inert fluorocarbon (FC) liquid immiscible with water but having a much higher solubility for ozone. A novel membrane reactor was studied for the destruction of organic pollutants by using PC liquid simultaneously as a reaction medium and a liquid membrane. The membrane reactor consisted of two sets of microporous and/or non-porous hollow-fiber membranes well-mixed in a cylindrical shell filled with the inert FC liquid. Wastewater was allowed to flow through the lumen of one set of microporous fibers; O3-containing gas flowed through the bore of the other set. Oxidative degradation products get partitioned back into the two mobile phases. Degradation of such pollutants as phenol, acrylonitrile, nitrobenzene, trichloroethylene, and toluene in such a reactor is presented. The kinetics of degradation of each pollutant in the two-phase (aqueous- FC) system were studied using batch and semibatch experiments. Simulation results based on a first-order model to predict the behavior of pollutant degradation in such a membrane reactor are compared with experimental data.
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    AIChE Journal 41 (1995), S. 2029-2039 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An analysis and applications of the wave model for longitudinal dispersion are pre-sented. Asymptotic forms of the wave model are considered and analytical solutions of typical linear stationary and nonstationary problems of chemical reactor engineering interest are obtained and compared to those for the Fickian dispersion model. The wave model leads to efficient analytical solutions for linear problems, which in principle differ from the solutions of the Fickian dispersion model; only for slowly varying concentration fields do the soluctions of both models approach each other. Spatial and time moments of the concentration distribution are obtained for pulse-dispersion problems; the first three spatial moments of the mean, variance, and skewness have exact, large-time asymptotic forms in the case of Taylor dispersion. Old experiments that could not be explained with the standard dispersion model are reconsidered and explained: the change with time of the variance of a concentration pulse when the flow direction is reversed and the difference in values of the apparent axial dispersion coefficient and the backmixing coefficient in a rotating disk contactor. The experimental determination of model parameters is discussed.
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    AIChE Journal 41 (1995), S. 2067-2082 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Phenomenologically based model for forth height was developed using the assumption of both vapor-and liquid-continuous zones within the total froth height. The model demonstrates the importance of liquid and vapor rates and determines that the Weber number (and therefore droplet size and surface tension) has little effect on froth height.A composite air/water database including entrainment data for both the spray and froth regimes is used to develop an entrainment correlation as a function of the ratio of tray spacing to froth height (Ts/h2φ) and the average froth density (φ2φ = hL).For conditions resulting in the ratio of liquid inventory to the perforation diameter (hL/dH) 〈4, the height, equation requires an empirical correction for froth height, and entrainment depends onφ2φ.This results from the predominantly vapor-continus nature of the froth. For large values of hL/dH, the theoretical value for h2φ appears adequate and entrainment is essentially independent of φ2φ, since most of the liquid is contained in the liquid-continuous region located far away from the underside of the tray above. In such cases, entrainment is from a light spray on top of the liquid-continuous region, and therefore entrainment depends much less on φ2φ.The differen dependency of entertainment on φ2φ can be used as a method to identify the spray-froth transition. For hL/dH〈2, a spraylikecondition exists; for hL/dH 〉 2, a frothlikecondition exists.The reported dependency of air/water system entrainment on major geometry and operating conditions is described well by the new correlation. Alternate correlations for the non-air/water entrainment data, however, are required. New methods for estimating the impact of entrainment on efficiency are given.
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    AIChE Journal 41 (1995), S. 2131-2139 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Composite metal membranes obtained by supporting thin metallic films on ceramic substrates have good thermal and mechanical stability. The use of a thin metal film increases transmembrane flux, while retaining high permselectivity that is characteristic of metallic membranes. Novel techniques have been developed for preparing metal-ceramic composite membranes. By the appropriate use of osmotic pressure, the microstructure, porosity, and thickness of the deposited metal can be systematically manipulated. Three new procedures are described for film densification and fabrication: (1) the osmotic pressure is used to densify an existing supported metal membrane; (2) densification and growth of the film is managed under the influence of osmotic pressure by using a dilute plating solution; and (3) films of varying porosity are deposited on the ceramic membrane by combining electroless deposition and osmotic pressure. Silver, palladium, and palladium-silver films prepared by these techniques on a commercial alumina membrane (Membralox) are thermally more stable than similar films deposited by conventional electroless plating.
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    AIChE Journal 41 (1995), S. 2140-2149 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Isothermal profiles of the extended meniscus in a quartz cuvette were measured in the earth's gravitational field using an image-analyzing intefferometer thai is based on computaer-enhanced video microscopy of the naturally occurring ineetference fringes. These profiles are a function of the stress field. Experimentally, the augmented Young-Laplace equation is an excellent model for the force field at the solid-liquid-vapor intrfeces for heptane and pentane menisci on quartz and tetradecane on SFL6. Effects of refractive indices of the solid and liquid on the measurement techniques were demonsrrauzd. Experimentally obtained values of the disjoining pressure and dispersion constantswere compared to those predicted from the Dzyaloshiniskii-Lifshitz-Pitaevskii theory for an ideal surface and reasonableagreements were obtained. A parameter introduced gives a quantitative measurement of the closeness of the system to equilibrium. The nonequilibrium behavior of this parameter is also presented.
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    AIChE Journal 41 (1995), S. 2177-2186 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An approximate steady-state method is devised for computing the flow field in a baffled, impeller-stirred tank reactor. The flow field in a cylindrical tank with a 45° pitched-blade impeller rotating at 100 rpm and four stationary rectangular side-wall baffles is simulated using a new approximate steady-state approach. The method provides an alternative to a full unsteady Navier-Stokes simulation. The new steady-state analysis involves accurately defining the geometry of the mixing tank using a multiblock grid technique. The flow is solved from a rotating frame of reference for a single position of the impeller relative to the side-wall baffles. The steady-state numerical results are then spatially averaged and compared with time-averaged data obtained experimentally using laser Dopler velocimetry (LDV). Spatially averaged numerical predictions obtained using this approximate steady-state method for the radial and axial velocity components agree well with the LDV data. The predicted magnitude of the tangential velocity component, however, is higher than the experimentally measured values. Closer agreement of the tangential velocities with experimental values is obtained using a finer grid and it is found that a relatively fine grid is needed for accurately predicting the tangential velocity magnitude. Use of this approximate steady-state method allows designers of mixing vessels to obtain flow-field results for baffled vessels much more efficiently than using full unsteady Navier-Stokes simulations.
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    AIChE Journal 41 (1995), S. 2227-2236 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The growth phenomena, mechanism, and kinetics of naphthalene crystal from supercritical CO2 solution were studies by using a single-crystal technique. Distinct growth features were observed, including the development of individuals from the seeds and the sprouting of plates from the individuals. The surface integration proceeded by two-dimensional nucleation mechanism at face corners and by subsequent spreading of the nuclei. The measured growth rates as functions of supersaturation and solubility were consistent with the derived growth-rate equations, based on the corner nucleation mechanism. In comparison with crystal growth from conventional media, the growth of naphthalene crystals from supercritical CO2 solution is similar to liquid-solution growth as far as growth mechanism and kinetics are concerned.
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    AIChE Journal 41 (1995), S. 2274-2281 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In this work the experimental observations are explained with the aid of a simple air-entrapment model based on the concept of two levels of porosity of fiber mats. A simple model that includes liquid bypassing with initial air trapping, subsequent capillary invasion of regular fiber bundles with air compression, and finally mobilization is proposed to explain air-entrapment phenomena. The simple model successfully rationalizes the observed air trapping and compression during initial liquid-fiber contact. An empirical equation for the mobilization efficiency is adapted to the model to estimate void content. The velocity dependence of the trapped void content predicted by the model is in good agreement with observations.
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    AIChE Journal 41 (1995), S. 1041-1043 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 1061-1070 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Numerical solutions were obtained for the field of velocity in angular forced flow through the annulus between two concentric cylinders of large and infinite aspect ratio with a gap-to-inner radius ratio of 0.05 using a finite-element representation and the FIDAP code. For an infinite aspect ratio, velocity vectors reveal a purely angular motion below a critical Dean number of 37.31 and a secondary motion in the form of pairs of counterrotating vortices above that value. The wavelength of these vortices and the friction factor are correlated in terms of the Dean number. For large but finite aspect ratios a weak secondary motion around the periphery is found to occur below the critical Dean number, while for greater values the vortex at each end of the channel is greatly extended. The computed patterns of flow are in good agreement with prior experimental visualizations as well as with those carried out as part of this investigation. The computed characteristics are also in good agreement with prior theoretical results for limiting cases. The adaptation of the results for flow through an Archimedean spiral is described.
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    AIChE Journal 41 (1995), S. 1113-1121 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Fluid and solid dynamics inside and outside a stable elongated gas bubble in operating conditions corresponding to those of the channeling regime in a fluidized bed are described. It considers the prolate ellipsoid of revolution as a model of the bubble. The results of the analytical model show a significant increase of the fluid velocity inside the bubble with the growth of its relative height. The maximum values of the fluid velocities exist along the vertical axis of the bubble. For the limiting case of a spherical bubble, the fluid and gas performance, such as the existence of a surface of penetration outside the bubble boundaries and circulation zone inside the bubble, are in good agreement with the existing theory.
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    AIChE Journal 41 (1995), S. 1153-1164 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Filtration on horizontal surfaces facing upward is accompanied by sedimentation. Material balances that are based solely on the volume of filtrate and neglect sedimentation flux lead to an understatement of the solids deposited in the cake and potentially large errors in calculated values of the average specific resistance αav. In a gravitational sedimentation experiment with kaolin, the value of αav neglecting sedimentation was 3.75 times greater than the value including the effect of sedimentation. In addition to errors due to neglect of sedimentation, CATSCAN studies show that the slurry concentration above the cake increases with time, contrary to usual assumptions. In a manner similar to batch sedimentation in a closed cylinder, characteristics of constant composition arose from the cake surface. Approximate predictions based on a combination of traditional sedimentation and filtration theory were in accord with the CATSCAN data. Existing filtration theory must be substantially modified to account for the effect of sedimentation.
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    AIChE Journal 41 (1995), S. 1194-1203 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The buildup of undesirable sulfur compounds(sulfates and thiosulfates) reduces the scrubbing effectiveness of the LO-CAT I autocirculation sulfur recovery process from acid-gas stream. Among various processes, withdrawing and disposing of a portion of the scrubber solution and replacing this “blowdown” with fresh solution have been the practice in the industry. The application of the electrodialysis system to recycle the blowdown is presented. Experiments were carried out using electrodialysis to separate salts (sulfates and thiosulfates) from the LO-CAT I autocirculation scrubber solution containing organic chelating agents, iron, and various alkali-metal inorganic salts. The results indicated that the electrodialysis was successful in removing 50% of the salts from the scrubber solution with less than 8% loss of organic and 8% loss of carbonates. The fluxes of the undesired salt species were high even at low current densities (200 to 400 A/m2).
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    AIChE Journal 41 (1995), S. 1235-1250 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The reaction rate in gas-solid systems can be affected by mechanical stresses that arise as the reaction proceeds. Stresses develop due to differences between precursor and product molar volumes and thermal expansion coefficients. Experimental evidence on the interaction of reaction rate and mechanical stress for the Ti/N2 and Ti/O2 systems is provided. A detailed and consistent mathematical model is developed for the reaction taking place at the constrained precursor/product interface. An elastic formulation for the stresses is adopted, and stress generation due to mismatches in linear thermal expansion coefficients and equivalent volume (Pilling-Bedworth ratio) for the precursor and product are considered. The effect of surface energy, which becomes significant for particle sizes below 1 μm, is also included in the model. Both experimentally and theoretically, conditions exist where the mechanical stresses exceed the strength of the material, leading to mechanical breakdown of the product layer, thus causing a discontinuity in the observed reaction rate. The entire processing history, including the reaction, temperature, and pressure profiles, plays an important role in determining the overall reaction kinetics of the powder.
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    AIChE Journal 41 (1995), S. 1281-1294 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The steady formation of fibers in the air-gap wet-spinning process is analyzed, in which solidification of the fiber material is brought about by the diffusional exchange of solvent and nonsolvent in a coagulation bath. The concentration profile within the fiber is determined, and a simple model is given for the change of the material behavior with changing concentration. The material behavior of the uncoagulated spinning solution is described by a constitutive model for viscoelastic liquids that allows the incorporation of a relaxation-time spectrum. The behavior of the solidified fiber is described by a model for rubberlike elastic solids. The effects of variable fiber temperature and skin friction on the fiber motion are also taken into account. The computed velocity profiles are compared to experimental data. These data were obtained by spinning three samples of nominally identical spinning solutions. Even though the theoretically predicted profiles agree well with those obtained with the first two samples, they completely fail to do so for the third. This failure is attributed to the unexpected extreme sensitivity of the model predictions to the width of the dynamic spectrum that is used to infer the relaxation-time spectrum. It is also found that the velocity profile depends rather strongly on the initial conditions for the stress.
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    AIChE Journal 41 (1995), S. 1321-1323 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 1333-1336 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 41 (1995), S. 1368-1374 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Effects of particle properties on the rise of single bubbles in a liquid-solid fluidized bed ared studied by focusing on the particle-shape effect at various bed voidages near incipient fluidization of glass beads and sand particles. Experiments, covering the bubble-size range 6-37 mm and solids holdups as high as 0.94 times that in the packed state, are conducted in a 2-D column to directly observe the possible relationship between the rate and tortuous path of bubble rise. The shape effect, being significant when the close-range (surface-to-surface) interactions between particles are predominant, is appreciable for the relative solids holdups exceeding 0.9. Marked reduction in the bubble rise velocity is observed as the bubble size is decreased below 8 and 12 mm for the spherical glass and irregular sand particles, respectively. This anomalous reduction stems partly from the tortuousness in the rise path and partly from the “hesitation” in the net vertical movement. The former, in particular, is quantified in terms of the tortuosity of the 2-D rise path. The results support the peculiar trend in the rise velocity observed, especially for the sand particles.
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    AIChE Journal 41 (1995), S. 1413-1425 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The adsorption of ethane from helium was measured in beds packed with 2.5 μm zeolite crystals and containing either a single hollow fiber or multiple fibers. THe single-fiber experiments indicate that the mass-transfer rate in bends containing zeolite 13X is limited by diffusion across the fiber wall and through macropores. FOr adsorption in single-fiber beds packed with zeolite 4A, mass transfer is limited by micropore diffusion within the particles. Breakthrough curves from beds containign 13X are adequately described with the linear driving force model, while curves from beds containing 4A are consistent with the Rosen model.Breakthrough curves from beds packed with zeolite 13X and containing multiple fibers can be predicted from the experiments with single-fiber beds when the fibers are evenly spaced. When fibers are unevenly spaced, the breakthrough curves are more disperse. Unevenly spaced fibers are the normal case. Even when fibers are evenly spaced, the productivity of hollow-fiber beds is expected to be no greater than that in conventional beds.
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    AIChE Journal 41 (1995), S. 1456-1470 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A systematic design method reported here uses extractive crystallization to separate a three-component mixture despite the presence of eutectics. Phase behavior can be classified into six basic types, and six flowsheet structures can handle systems with any of these solid-liquid-phase behaviors. Design equations are formulated for these flowsheet structures, and design variables and constraints are identified. In addition, design issues, such as the choice of solvent, the effect of design variables on recycle flows, and the magnitude of the costs, are discussed.
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    AIChE Journal 41 (1995), S. 1513-1520 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Continuum approximation is often used to treat problems involving reaction mixtures of a vast number of species. Previous studies predicted that in many situations the lumped kinetics of a continuum of reactions, regardless of their kinetics, are of the power law form after a long time. In this article we present experimental support for such asymptotic power law kinetics. The system considered is catalytic deep desulfurization of a petroleum distillate. In addition, a theoretical analysis is made to establish the condition under which the continuum approximation is valid in the long-time limit. The condition turns out not to be very stringent for practical purposes.
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    AIChE Journal 41 (1995), S. 1543-1549 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Special kinetic behaviors in isothermal reactions on heterogeneous catalysts composed of two components with different roles are studied with the focus on hi stabilities that could occur in stirred-tank flow reactors when control of catalytic sites on one component by mobile (spillover) species produced by the other operates. The comprehensive kinetic model involves: (1) generation of spillover species on the second phase, their transfer to the potentially catalytic phase, and their reaction with this acceptor to generate selective sites; (2) the kinetics of the catalytic reaction. We developed a model for selective catalytic oxidation, where catalyst sites change from nonselective to selective under the influence of the control by spillover oxygen. The model is based on the system of differential equations. By integration and iteration to the steady state for each value of the external control parameter under investigation, the oxygen and hydrocarbon reactant consumption, as well as the state of the catalyst, is calculated. These calculations predict bistable selectivity for certain ranges of concentration in the feed. Data on the boundaries of the hysteresis loops as a function of catalyst composition and oxygen partial pressure are discussed, as well as the variation of these hysteresis boundaries as a function of the internal parameters.
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    AIChE Journal 41 (1995), S. 1572-1580 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: One approach to demilitarize solid rocket propellants is treatment with ammonia. Ammonia extracts the oxidizers ammonium perchlorate and HMX, yielding a solid residue that is more suitable for incineration and less sensitive to impact and other modes of accidental initiation. Ammonia treatment of nitroglycerin-containing propellants is complicated by an exothermic reaction between ammonia an nitroglycerin. If not removed, the heat generated by this reaction can cause propellant ignition. To help design safe treatment processes, a model for the ammonia-propellant reaction was developed, which integrates transient energy and species conservation equations to simulate ammonia diffusion, heat generation, and heat flow in a propellant and in the solid residue resulting from ammonia treatment. It was calibrated using residue thickness and thermocouple data for one propellant. The calibrated model was used to predict conditions leading to ignition of thin propellant strips. The results agree well with experimental observations.
    Additional Material: 7 Ill.
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  • 95
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2345-2355 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In certain polymer-penetrant systems, nonlinear viscoelastic effects dominate those of Fickian diffusion. This behavior is often embodied in a memory integral incorporating nonlocal time effects into the dynamics; this integral can be derived from an augmented chemical potential. The mathematical framework presented is a moving boundary-value problem. The boundary separates the polymer into two distinct states: glassy and rubbery, where different physical processes dominate. The moving boundary condition that results is not solvable by similarity solutions, but can be solved by perturbation and integral equation techniques. Asymptotic solutions are obtained where sharp fronts move with constant speed. The resultant profiles are quite similar to experimental results in a dissolving polymer. It is then demonstrated that such a model has a limit on the allowable front speed and a self-regulating mass uptake.
    Additional Material: 6 Ill.
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  • 96
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2371-2382 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An experimental study of pulsing flow in trickle beds is presented, based on data from two measuring techniques. New evidence on pulse arrangement and propagation in the bed, and data on basic pulse characteristics (frequency, celerity, length, duration) as well as liquid holdup and pressure drop measurements are included. Some of these data, such as the length of the liquid-rich zone of pulses, are not currently available. Two flow regions exhibit different trends of pulse characteristics, “mild” and “wild” pulsing for relatively small and large liquid flow rates, respectively. New findings are compared with previously available data and correlations. An effort to develop new or modify existing generalized correlations is made for the aforementioned quantities.
    Additional Material: 15 Ill.
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  • 97
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2393-2398 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The Kinetics of diffusion in a spherical pellet saturated by a sorbate in the presence of an inert component are considered for the case of a step isotherm. A new convective flux inside the pores due to sorption effect has been taken into consideration, and an analytical expression for sorption kinetics is obtained. The convective flux may result in considerable increase in the sorption rate. It is also found that the formula obtained also gives a satisfactory description of sorption kinetics for the case of an S-shaped isotherm.
    Additional Material: 5 Ill.
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  • 98
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2427-2438 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Guildlines for the design of fractional crystallization processes to separate two-and three-solute mixtures are presented. By using solvent addition/removal, stream combination, and cooling/heating, these processes bypass regions of multiple saturation in the phase diagram and recover purse solutes. Design equations are formulated, and the constraints on the design variables are identified. also included is a discussion of the effect on recyle flows of changes in the design variables and an estimate of the cost of a fractional crystallization separation.
    Additional Material: 17 Ill.
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  • 99
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2465-2475 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Six algorithms following single particle trajectories are used to predict the axial dispersion in rolling or slumping flow in a continuous rotary kiln. Models incorporating different physical phenomena show that axial dispersion is affected by the Froude number, L/D ratio, solid fill level, and rolling or slumping layer thickness. The main cause of axial dispersion is segregation of the nonuniform particle size, density and shape. In a rolling bed with uniform particles the rolling layer thickness and the time of roll have to be accounted for in the prediction of the dispersion. The Peclet numbers computed for sold particles with uniform physical properties are of the order of 104. The sold segregated motion may lead to Peclet numbers of the order of 10 - 103, a much larger dispersion. The main obstacle for a-priori prediction of the axial dispersion is lack of a reliable relation between the segregated roll or slump distance and the variation in particles properties, as well as the change in segregation as the flow properties of kparticles are changed during calcination.
    Additional Material: 12 Ill.
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  • 100
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2513-2521 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The local time-averaged velocity, the mixed-mean velocity, and the friction factor for fully turbulent flow between parallel plates and in round tubes and concentric circular annuli can be expressed in terms of integrals of the turbulent shear stress, The pressure distribution across a channel can similarly be expressed in terms of an integral of normal stresses. These formulations, which are simple and exact, can be integrated numerically using experimental data, compyted values, or correlating equations for turbulent stresses. Their greatest merit, however, may arise from the insight they provide with respect to the contributions of the fluctuating components of the velocity. For example, for concentric circular annuli such a formulation identifies a difference between the locations of the maximum in the velocity and the zero in the total shear stress. This difference, which has been overlooked in most experimental and semitheoretical investigations, introduces an error of unknown but possibly significant magnitude into all of the results. It also precludes the application of the mixing-length, eddy viscosity and k - ∊ models.
    Additional Material: 3 Ill.
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