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  • Chemical Engineering  (17,979)
  • 101
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 785-791 
    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 novel approach for stabilizing the intermediate steady state of a continuous stirred-tank reactor (CSTR) is proposed by using a special type of periodic forced operation, the so-called mediating operation. The mediating operation enables new additional steady states for which one of the new intermediate steady states is stable to be produced. Thus the proposed approach employs a periodic forcing for stabilization of a selected steady state by control of a steady-state multiplicity. The feed flow rate is considered to be a manipulated variable. Changing CSTR multiplicity is investigated for two-step control inputs. It is shown, analytically, that under two-step control CSTR can exhibit at most five steady states with a stable intermediate steady state. A constructive procedure is proposed for finding control parameters corresponding to maximal multiplicity, that is, for stabilizing an intermediate steady state by two-step control.
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  • 102
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    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 802-810 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Polymer degradation occurs when macromolecular chains are broken under the influence of thermal, mechanical or chemical energy. Chain-end depolymerization and random- and midpoint-chain scission are mechanisms that have been observed in liquid-phase polymer degradation. Here we develop mathematical models, unified by continuous-mixture kinetics, to show how these different mechanisms affect polymer degradation in solution. Rate expressions for the fragmentation of molecular-weight distributions (MWDs) govern the evolution of MWDs. The governing integrodifferential equations can be solved analytically for realistic conditions. Moment analysis for first-order continuous kinetics shows the temporal behavior of MWDs. Chain-end depolymerization yields monomer product and polymer molecular-weight moments that vary linearly with time. In contrast, random- and midpoint-chain scission models display exponential time behavior. The mathematical results reasonably describe experimental observations for polymer degradation. This approach, based on the time evolution of continuous distributions of chain length or molecular weight, provides a framework for interpreting several types of macromolecular degradation processes, particularly how bimodal MWDs can evolve during degradation.
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  • 103
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Transvascular and interstitial fluid movements are involved in many important biological processes such as convective macromolecular transport and contribute to the mechanical behavior of tissue. Although intimately coupled, there is a tendency in the literature to regard these two fluid-transport mechanisms separately; if the interaction is considered, the description is usually confined to the local level (e.g., transvascular or interstitial perivascular). A general framework presented here combines transvascular and interstitial fluid movement with the mechanics of soft tissue and integrates macro-and microscopic views of the phenomena. On the macroscopic level, interstitial fluid transport is described by adapting the field equations of the poroelastic theory using average field variables defined on a scale of several blood vessel diameters (∼ 1 mm), while transvascular transport is described by a generalized Starling's law. As an example, the model equations have been specialized for a spherical solid tumor and an analytical solution is presented for the transient redistribution of interstitial fluid following a rapid change in vascular pressure or flow. The model describes the overall average profiles of the interstitial fluid pressure and velocity, as well as the dilatation, displacement and stress of the solid matrix. Moreover, on a smaller length scale the model can describe the local fluid movement (perivascular) using the average field variables as boundary conditions. The basic theory provides new insight into understanding the fluid transport in biological tissues and a valuable tool for determining relevant fluid-transport parameters. Implications for improving drug delivery to solid tumors are also discussed.
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  • 104
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    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997) 
    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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  • 105
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    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 870-876 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Mathematical models developed describe the unfolding of polymeric crystals in the presence of a solvent followed by their subsequent disentanglement. A thermodynamic model considering the free energy changes during crystal unfolding was proposed to obtain an expression for the unfolding rate. A simplified version of this expression, assuming uniform crystal size, was incorporated into a continuum model to predict the dissolution kinetics of a semicrystalline polymer slab. The model yielded predictions of the crystalline and solvent volume fractions as a function of position within the slab, in addition to changes in the fraction of polymer dissolved as well as the degree of crystallinity of the polymer as a function of time. The degree of crystallinity of the polymer decreased with time and the plot of the fraction of the polymer dissolved as a function of time exhibited Case II behavior. The model predictions agreed will with experimental results obtained during dissolution of semicrystalline poly(vinyl alcohol) in water.
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  • 106
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    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 902-912 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Simple first-order closure models for covariances of concentration fluctuations, for use in modeling turbulent flow reactors, were tested by direct numerical simulations. Concentration covariances and other statistical functions were evaluated for a series parallel reaction scheme in decaying, homogeneous turbulent flow. The simulations involve solving the unsteady Navier-Stokes and mass conservation equations by a pseudo-spectral method in a 643 wavenumber domain, with initially segregated reactants, for an initial turbulence Reynolds number of 29.9. Simulation results show that covariances of concentration fluctuations normalized with respect to mean concentration values are almost constant and that the time dependence of concentration covariances can be estimated if the mean concentrations are known at any one time after the initial time. Predictions of the first-order closure models of Bourne and Toor, Brodkey and Lewalle, Li and Toor, and Dutta and Tarbell were compared to simulation results. While none of these closures are satisfactory for all the conditions tested, the Brodkey-Lewalle closure agrees best with the simulations.
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  • 107
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    AIChE Journal 43 (1997), S. 935-945 
    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 simulated moving bed (SMB) was operated for the separation of mixtures of 2-phenyl ethanol and 3-phenyl-1-propanol on columns packed with Zorbax C18 bonded silica, using a 60:40 (v/v) solution of methanol and water as the mobile phase. Series of four or eight columns were used. The experiments were carried out with low concentration mixtures, that is, under linear conditions. Band profiles of both compounds eluted from one of the columns during successive periods after steady state had been reached were recorded, as were the concentration histories at the extract and raffinate ports. These experimental results are compared to those predicted by two models: the linear ideal and the linear equilibrium-dispersive models of chromatography, applied to the SMB separator. These two models give excellent agreement between the experimental profiles and those calculated with the model. As expected, the profiles predicted by the ideal and the equilibrium-dispersive models differ only by the lack of dispersion in the profiles given by the former. The latter model is demonstrated to be a solid, reliable tool for further studies of the SMB design and optimization.
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  • 108
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    AIChE Journal 43 (1997), S. 971-978 
    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 multivessel batch column consists of a reboiler, several column sections, intermediate vessels and a condenser vessel. This configuration provides a generalization of previously proposed batch-distillation schemes, including the inverted column and the middle-vessel column. The total reflux operation of the multivessel batch-distillation column was presented recently, and the main contribution of this article is to propose a simple feedback control strategy for its operation. We propose to adjust the vessel holdups indirectly by manipulating the reflux flow out of each vessel to control the temperature at some location in the column section below. The feasibility of this strategy is demonstrated by simulations.
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  • 109
    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 the simulation and optimization of large-scale chemical processes, the overall computing time is often dominated by the time needed to solve a large sparse system of linear equations. A new parallel frontal solver that can significantly reduce the wallclock time required to solve these linear equation systems using parallel/vector supercomputers is presented. The algorithm exploits both multiprocessing and vector processing by using a multilevel approach in which frontal elimination is used for the partial factorization of each front. Results of several large-scale process simulation and optimization problems are presented.
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  • 110
    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, on-line calorimetry was used to estimate the conversion and the copolymer composition in emulsion polymerization systems. Real-time feedback control of the polymer composition during the semibatch emulsion copolymerization of vinyl acetate and butyl acrylate was carried out. Both homogeneous copolymers and copolymers with previous defined composition profiles were obtained. It was shown that the feedback control was able to avoid monomer accumulation in the reactor, and hence potentially dangerous thermal runaways, without any deleterious effect on the polymer composition, when a sudden inhibition was caused by deliberately adding a solution of hydroquinone. The use of feedback control for the maximization of the production rate under safe conditions in the unseeded emulsion copolymerization of vinyl acetate and VeoVa10 (an alkyl vinyl ester from Shell) is also presented.
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  • 111
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    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 1100-1107 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: High-pressure homogenization is a key unit operation used to disrupt cells containing intracellular bioproducts. Modeling and optimization of this unit are restrained by a lack of information on the flow conditions within a homogenizer valve. A numerical investigation of the impinging radial jet within a homogenizer valve is presented. Results for a laminar and turbulent (k - ∊ turbulent model) jet are obtained using the PHOENICS finite-volume code. Experimental measurement of the stagnation region width and correlation of the cell disruption efficiency with jet stagnation pressure both indicate that the impinging jet in the homogenizer system examined is likely to be laminar under normal operating conditions. Correlation of disruption data with laminar stagnation pressure provides a better description of experimental variability than existing correlations using total pressure drop or the grouping 1/Y2h2.
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  • 112
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    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 1117-1121 
    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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  • 113
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    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997) 
    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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  • 114
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    AIChE Journal 43 (1997), S. 1153-1162 
    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 behavior of droplet swarms in liquid-liquid dispersions, especially found in extraction columns, are analyzed by a new fluorescence measurement technique, which represents an extension to the capillary suction method. The measuring principle developd benefits from detectable-emission light intensity of a fluorescent dye even in a trace concentration. The addition of this nontransferring dye to the dispersed phase enables in situ determination of bivariate drop-size/drop-concentration distributions, which provides important information on droplet history for the evaluation of coalescence and breakup rates. Furthermore, the measuring method represents a new approach for the measurment of droplet-size-dependent residence times in extraction columns.
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  • 115
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    AIChE Journal 43 (1997), S. 761-776 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Experimental breakthrough results of methane, ethane and propane in activated carbon and silica gel obtained over a wide range of gas compositions, bed pressures, interstitial velocities, and column temperatures were analyzed using a dynamic, nonisothermal, nontrace column breakthrough model. A linear driving force (LDF) approximation is used for particle uptake, and the Langmuir-Freundlich isotherm represents adsorption equilibrium. The LDF mass-transfer-rate coefficient (and, hence, effective particle diffusivity) and column-wall heat-transfer coefficient were determined. The results show that hydrocarbon transport in the activated carbon particles used is essentially by Knudsen and surface flow, while for the silica gel used the transport is primarily by Knudsen flow. For activated carbon, the experimentally derived LDF coefficients for all three sorbates are well correlated using an average effective diffusivity value. With regard to heat transfer, the column-wall Nusselt number is approximately constant for the range of Reynolds numbers considered. Simulations of multicomponent breakthrough in the activated-carbon bed based on independently measured single-component kinetic parameters and the extended Langmuir-Freundlich isotherm agree very well with experimental results. The computational efficiency gained by adopting the simpler extended Langmuir isotherm model is also investigated.
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  • 116
    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 of the remaining challenges in application of heterogeneous photocatalysis for treatment of air streams containing dilute VOCs is to design a cost-effective photocatalytic reactor that simultaneously allows efficient contact of the contaminated air and solid catalyst while uniformly irradiating the solid catalyst with light. A pseudohomogeneous model was developed to study effects of system parameters on process performance for a gas-solid lamp-in-tube annular-photocatalytic-oxidation (PCO) reactor in which the annular space is filled with photocatalyst-coated packing. In this model the flow field is assumed to be uniform and radial diffusion negligible. Homogeneous reactions are neglected. Heterogeneous reaction rates follow Langmuir-Hinshelwood-Houghen-Watson kinetics with rate parameters extracted from independent experiments. A 1:D “two-flux” incidence submodel is used to account for the radial UV light distribution throughout the reactor annular space. This submodel requires knowledge of the UV lamp radiant emittance, the optical characterstics of the catalytic thin-film coating, and the UV irradiance at the outer wall of the reactor and contains only a single adjustable parameter - the mean free path between photon-catalyst interactions. The model was validated with experimental performance measurements for destruction of acetone and isopropyl alcohol in a bench-scale photoreactor. The validated model can be used to predict the optimum catalyst film thickness for given reactor dimensions, packing shape and size, and VOC abatement problem.
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  • 117
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    AIChE Journal 43 (1997), S. 835-836 
    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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  • 118
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    AIChE Journal 43 (1997), S. 847-850 
    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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  • 119
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    AIChE Journal 43 (1997), 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: Equilibrium partitioning of spherical solutes between slit pores and bulk solution is investigated by the Gibbs ensemble Monte Carlo method. Two types of perturbatins are performed in this simulation: a random displacement of solutes that ensures equilibrium within both bulk and pore regions, and random interchanges of solutes that equalize the interaction potentials between the two regions. To study the effects of electrostatic interactions, interaction energies between the solutes and pore walls and between pairs of solutes are evaluated by using a singularity method. Partition coefficients calculated for neutral solutes, which experience purely steric interactions, increase with increasing solute concentration and agree well with existing theoretical results. For pores and solutes of like charge, results for the limit of infinitely dilute solute concentration show a sharp decline in partition coefficient with decreasing ionic strength of solution. As the solute concentration increases, the interplay of solute-wall and solute-solute interactions becomes increasingly important, and the partition coefficient increases accordingly. The density profiles indicate unambiguously that, whether solutes and proes are uncharged or of like charge, solute-solute interaction promotes enhanced concentrations near the wall, causing the partition coefficient to increase. Even at solute concentrations as low as 5%, effects of solute-solute interactions caused by electrostatic charge can more than compensate for sphere-wall repulsive interactions, indicating that concentration effects should be considered at least as important as electrostatic effects in partitioning phenomena.
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  • 120
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    AIChE Journal 43 (1997), S. 1250-1264 
    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 method that quantitatively assesses property prediction uncertainty (imprecision) on optimal molecular design problems is introduced. Propety-structure relations are described with specific nonlinear functionalities based on group contribution methods. Property prediction uncertainty is explicitly quantified by using multivariate probability density distributions to model the likelihood of different realizations of the group contribution parameters. Assuming stability of these probability distributions, a novel approach is introduced for transforming the original nonlinear stochastic formulation into a deterministic MINLP problem with linear binary and convex continuour parts with separability. The resulting convex MINLP formulation is solved to global optimality for molecular design problems involving many uncertain group contribution parameters. Results indicate the computtional tractability of the method and the profound effect that property prediction uncertainty may have in optimal molecular design. Specifically, trade-off curves between performance objectives, probabilities of meeting the objectives, and chances of satisfying design specifications offer a concise and systematic way to guide optimal molecular design in the face of property prediction uncertainty.
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  • 121
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    AIChE Journal 43 (1997), S. 1300-1308 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Initiaton of incrustation by crystallization mechanisms is studied. An experimental technique was developed by which nucleation of a crystalline phase on a solid surface is investigated at controlled local supersaturation without the interference of particle deposition. Experiments were carried out with different substances and surface materials. The results show that the solid surface may catalyze primary nucleation and hence promote the initiation of incrustation, but this was recorded only at quite high supersaturation. Collision of crystals with the solid surface in a supersaturated solution may initiate incrustation at a reasonably low supersaturation and can be of substantial industrial importance.
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  • 122
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    AIChE Journal 43 (1997), S. 1325-1330 
    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 effectiveness of applying silent discharge plasmas (SDP) for destroying and removing volatile organic compounds (VOCs) from gas streams is experimentally evaluated with a laboratory-scale reactor. The VOCs selected for study include toluene and methyl ethyl ketone (MEK). Direct collision with energetic electrons and reaction with generated gas-phase radicals are two major mechanisms responsible for destruction and removal of VOCs from gas streams. Operating parameters investigated include applied voltage, gas residence time, and temperature and composition of the gas stream. Experimental results indicate that the removal efficiency of toluene and MEK achieved with SDP can be enhanced by operating the system at a higher gas temperature and applied voltage due to the generation of more energetic electrons and radicals. O2 is essential for removing VOCs from gas streams with SDP. More than 80% removal efficiencies were achieved with this system for both toluene and MEK. SDP can potentially serve as an alternative control technology for removing VOCs from gas streams.
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  • 123
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    AIChE Journal 43 (1997), S. 1357-1361 
    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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  • 124
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    AIChE Journal 43 (1997), S. 1369-1371 
    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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  • 125
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    AIChE Journal 43 (1997) 
    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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  • 126
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    AIChE Journal 43 (1997), S. 1426-1435 
    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 turbulence structure of the liquid phase near a wavy gas-liquid interface in stratified flow was experimentally investigated in a 50 mm (H) × 100 mm (W) rectangular duct. The characteristic parameters of the organized motion under the liquid waves such as the frequency of appearance and length scale were estimated by using the variable-interval time-averaging and the photochromic dye activation techniques. These characteristics were used in a hybrid surface renewal-eddy cell model to predict the interfacial heat- and mass-transfer coefficients in stratified two-phase flow. The predictions for cocurrent and countercurrent flows agreed reasonably well with experimental data available in the literature.
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  • 127
    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 cold model of a circulating fluidized bed with a 0.030-m-ID, 2.77-m-high riser was operated in a wide range of operating conditions. Several solids were tested, from 57 μm to 1,830 μm in size and from 1,100 kg/m3 to 2,540 kg/m3 in density. Pressure fluctuations were measured at several points along the riser, and time series were processed to evaluate chaotic invariants (Kolmogorov entropy and correlation dimesion). Axial profiles of average values of pressure and voidage were also evaluated. At fixed operating conditions, the Kolmogorov entropy changed along the riser, which appeared to be a fuction of the local voidage and showed a minimum when voidage decreases from 1.00 until about 0.90. Changes of the Kolmogorov entropy with local voidage were interpreted based on interactions among solids and gas turbulence structure. Three regions were identified in the voidage range investigated: particles-controlled region, clusters-controlled region, and bottom-bed-controlled region.
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  • 128
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    AIChE Journal 43 (1997), S. 1488-1493 
    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 experiments were performed to dewater bentonite slurries at different filtration pressures. The diffusivity of the cake was then determined as a function of void ratio using the measured filtration rates, the final void ratios of the cakes, and the theory of compressible filtercakes. Any filtration process can, in theory, be predicted, once the cake diffusivity is known as a function of void ratio and the void ratio as a function of applied pressure. In particular, the final stage of the filtration process can be modeled and offers an alternative route to determine the cake properties from the experimental results. The final slow approach to equilibrium is difficult to study experimentally; other techniques to determine cake properties are therefore preferable. Final stages of the cake compaction are also markedly slower than predictions based on the intial filtration rates. Possible explanations include a filtercake rheology that is more complicated than assumed in the model or time-dependent effects caused by exclusion of ions from the bentonite filtercake, but such explanations appear unsatisfactory. Filtration models in which the filtercake void ratio and permeability depend solely on the imposed stress, may therefore be too simple for some materials.
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  • 129
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    AIChE Journal 43 (1997), S. 1519-1528 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Back-and-forth-moving temperature fronts separating regions with high and low temperatures (amplitudes up to 100 K) formed on a Pd on alumina ring during the atmospheric oxidation of carbon monoxide. The back-and-forth front movement reflects the intrinsic nonuniformity of the catalytic system and the impact of global coupling. The ignition and extinction fronts usually moved at different velocities, in agreement with theoretical predictions. Complex spatiotemporal patterns were observed due to small-amplitude temperature fronts that moved on the high-temperature region at a much higher velocity than the high-amplitude fronts. The qualitative motion features were affected by changes in the excess oxygen concentration. The nonuniform states existed mainly for feed conditions for which the only uniform state was an extinguished one.
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  • 130
    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 recovery of the acidic structure and activity during the regeneration step by coke combustion of an HZSM-5 zeolite-based catalyst used in the methanol-to-gasoline (MTG) process in reaction-regeneration cycles was studied. The reactivation kinetics (activity us. coke combustion time) was determined as a function of temperature and time on stream of the reaction step. The validity of the reactivation kinetic model was proven by using it for the simulation of the cyclic operation under conditions of catalyst partial reactivation, together with the kinetic model for the MTG process.
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  • 131
    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 comprehensive composite database for distillation sieve-tray efficiency is used to develop point efficiency and entrainment correlations based on a model that considers the fluid on the distillation tray to be contained in a liquid-continuous region near the tray deck and a vapor-continuous region on top of the liquid-continuous region. This model allows estimates of the portion of the mass transfer that occurs in each region and the mass-transfer resistance that occurs on the liquid side and vapor side of the interface. For most cases, most of the mass transfer occurs within the liquid-continuous region. The liquid side resistance is often significant.The entrainment correlation is consistent with the work of Bennett et al., which relates entrainment to the ratios of the liquid to vapor density and the froth height to the tray spacing. A simple liquid continuous-only mass-transfer model containing only four empirical parameters correlates the point efficiency data to within 6.4%. Despite a twofold change in vapor Schmidt number, no dependency on vapor Schmidt number is seen. Important dimensionless groupings are the Reynolds number based on the hole velocity, effective froth density, ratio of the liquid inventory to the perforation diameter, and fraction of the tray area perforated. Mathematically simple and accurate methods allow the prediction of the section efficiency for trays operating in cross or parallel flow. They address vapor and liquid mixing, entrainment and a criterion to avoid significant degradation of the tray efficiency due to weeping.
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    AIChE Journal 43 (1997), S. 1641-1648 
    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 43 (1997), S. 1653-1661 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Pulsed magnetic field gradient stimulated echo NMR measurements are performed for diffusion and flow of an aqueous phase both within a sample of packed spherical beads and within a 25% porosity Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area q is used to derive the displacement probability distributions PΔ(X) for fixed observation times Δ. The shape of PΔ(X) as a function of Δ is simulated for computer-generated porous media, and a good agreement is obtained between the experimental NMR data and the simulations.
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  • 134
    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 study back-propagation, feed-forward neural networks are applied to estimate mass-transfer parameters in fast fluidized beds of fine solids. These networks are trained to predict mass-transfer rates using measurements of the sublimation rate of coarse naphthalene balls in fast fluidized beds of fine glass beads at several solid-to-gas mass flow rates within the relevant superficial gas-velocity range. When tested to predict the effective diffusivities from a coarse particle to the bulk of the fast bed of fine solids, trained neural networks calculated the Sherwood number with high accuracy. It is demonstrated that back-propagation, feed-forward neural networks provide a more accurate correlation for the mass-transfer coefficient compared to those obtained by the currently used heuristic models.
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  • 135
    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 electrochemically modulated complexation (EMC) process was designed to selectively extract and concentrate ethylene from a feed-phase mixture with ethane. Unlike previous EMC processes, where the separation was based on equilibrium solubility differences, this separation was based on differences in the rate of complexing with copper(I) solution between the two gases. The EMC process employed flowthrough electrolysis cells that electrochemically modulate aqueous-phase copper chlorides between high, copper(I), and low, copper(II), affinity states. The liquid phase in the high affinity form contracts a feed gas phase in a hollow-fiber membrane module for selective extraction and contacts the receiving gas phase in the low affinity form for concentration. Experimental results and modeling of the process are reported.
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  • 136
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Cylindrical internal reflectance infrared spectroscopy (CIR-FTIR) was used for the determination of the transport properties of hydrocarbons in zeolite L at high temperatures and pressures. This was the first time that the intracrystalline diffusivity of hydrocarbons in microporous media could be determined under supercritical conditions by using a dynamic technique. The study investigated the self-diffusion of n-heptane, the counterdiffusion of n-hexane and 1-hexene, and the codiffusion of n-heptane and pyxlene in zeolite L under subcritical and supercritical conditions. The diffusion time constants obtained for the single and binary systems showed that the diffusion within the pore was reduced significantly under supercritical conditions (i.e., liquidlike behavior) as the hydrocarbon density within the pore was increased.
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  • 137
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    AIChE Journal 43 (1997), S. 1751-1762 
    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 method to synthesize crystallization-distillation hybrid separation processes is presented. Two classes of hybrids are identified for binary mixtures. The first bypasses azeotropes and tangent pinches, while the second bypasses eutectics. Guidelines for flow-sheet selection are proposed based on an analysis of simple eutectic, constant relative volatility systems. In addition, the hybrids are compared to both extractive and adductive crystallization in order to determine the conditions under which solvent-based crystallization techniques outperform the proposed hybrid configurations. The method is extended to ternary mixture. The use of stream combination and complex distillation columns is also considered.
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  • 138
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    AIChE Journal 43 (1997), S. 1797-1812 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: ZSM-5 zeolite membranes with reproducible properties were prepared by in-situ synthesis on porous α- and γ-alumina tubular supports and characterized by XRD, SEM and electron microprobe analysis. Single-gas permeances for H2, CH4, N2, CO2, n-butane, and i-butane increase over some temperature range, but some gases exhibit maxima or minima. The highest ideal selectivities at room temperature are 299 for N2/SF6, 392 for H2/n-butane, and 2,820 for H2/i-butane. These membranes can separate n-butane/i-butane, H2/n-butane and H2/i-butane mixtures. All n-butane/i-butane separation selectivities have maxima as a function of temperature and are higher than ideal selectivities because n-butane inhibits i-butane permeation. Thus, separation is not by size selectivity, but is due to pore blocking. Temperature depenencies of single-gas permeances and separation selectivities depend strongly on the location of zeolite crystals and the location is determined by preparation procedure. Ideal selectivities also depend strongly on the preparation procedure. When the zeolite forms a continuous layer on the inside surface of the support tubes, pure i-butane permeates faster than pure n-butane so that the single-gas permeances are not determined just by molecular size. The i-butane permeance also increases much more with temperature than the n-butane permeance. The permeation behavior may be the result of permeation through nonzeolitic pores in parallel with zeolite pores. When zeolite crystals are dispersed throughout the pores of α-alumina supports, permeances are lower and gas permeation and separation properties are quite different. Ideal selectivities are lower, pure n-butane permeates faster than i-butane, and the permeances increase much less with temperature. Separation selectivities are lower but can be maintained to higher temperatures.
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  • 139
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    AIChE Journal 43 (1997), S. 1828-1837 
    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 of the challenges in building reaction mechanisms using algorithms for automated model construction is to describe the essential chemistry and enable prediction of experimental data over wide ranges of reaction conditions while maintaining a manageable model size. Two complementary methodologies for building compact reaction mechanisms were developed and combined with existing algorithms based on graph theory and bond-electron matrix operations. Each strategy was developed using pentadecylbenzene pyrolysis as an illustrative example. The first approach used a radical lumping strategy to group radicals according to their reactivity. The mechanism was reduced from 719 to 215 species and successfully predicted the experimentally observed initial reactivity. However, implementation of the radical lumping criteria alone was insufficient to allow for secondary reactions to higher-rank products. Therefore, on-the-fly sensitivity analysis was incorporated to identify the important and necessary species as the mechanism was generated to guide the mechanism building process. The generic algorithms developed can be applied to generate compact reaction mechanisms for a wide array of higher molecular-weight reactants.
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  • 140
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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: Volatile species are removed from polymers by devolatilization. Foaming experiments using corn syrup and low-density polyethylene were carried out to investigate effects of volatile content, operating vacuum level and stripping agent in devolatilization. When foaming and rupture occur, a limiting foam volume growth was observed for a polymer regardless of the volatile content and the addition of stripper bubbles. A mathematical model was also developed to study the effect of stripper bubbles in polymer devolatilization. Based on the cell model for bubble growth, a bimodal model for foam growth is developed as the first step to describe the initial formation and growth of the volatile and stripper bubbles in polymer during devolatilization. When foaming and breakup occur at a critical foam volume expansion, a film model for mass transfer is used to model the second step of devolatilization. A numerical factorial study shows that the devolatilization section should be designed and operated to provide the screw channel sufficient free volume and residence time for the desired foam expansion.
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  • 141
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    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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  • 142
    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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  • 143
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    AIChE Journal 43 (1997), S. 2002-2020 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Industrial continuous processes may have a large number of process variables and are usually operated for extended periods at fixed operating points under closed-loop control, yielding process measurements that are autocorrelated, cross-correlated, and collinear. A statistical process monitoring (SPM) method based on multivariate statistics and system theory is introduced to monitor the variability of such processes. The statistical model that describes the in-control variability is based on a canonical-variate (CV) state-space model that is an equivalent representation of a vector autoregressive moving-average time-series model. The CV state variables obtained from the state-space model are linear combinations of the past process measurements that explain the variability of the future measurements the most. Because of this distinctive feature, the CV state variables are regarded as the principal dynamic directions A T2 statistic based on the CV state variables is used for developing an SPM procedure. Simple examples based on simulated data and an experimental application based on a high-temperature short-time milk pasteurization process illustrate advantages of the proposed SPM method.
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  • 144
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    AIChE Journal 43 (1997), S. 2073-2082 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A one-dimensional nonequilibrium model for multicomponent condensation is used to simulate a vertical single-pass shell-and-tube heat exchanger in an industrial gas-phase polyethylene reactor system. Starting the calculation at the top of the exchanger, the model can predict temperatures at the bottom of the exchanger within an accuracy of ±5 K as compared to three sets of industrial data. Sensitivities of model predictions were analyzed, including uncertainties associated with physical and transport property estimates, step size, and convergence criterion. Model predictions are not particularly sensitive to the estimation errors of physical and transport properties if K values are calculated using an equation of state applicable to both liquid and vapor phases. Effects of operating conditions on heat removal from polyethylene reactors were investigated for an existing process. It was quantitatively demonstrated why and how severely noncondensable gases impede condensation heat transfer. The level of noncondensable gases and the cooling water temperature are the two most important factors influencing the heat-removal rate. Replacing a portion of noncondensable gas, such as N2, with a condensable fluid that is inert to polymerization reactions can substantially increase the heat-removal rate from the reactor, thereby allowing for an increase in polymer production rate.
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  • 145
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    AIChE Journal 43 (1997), S. 2096-2111 
    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 generalized approach to the use of pulsed-gradient spin echo (PGSE) NMR methods for the measurement of flow and diffusion in porous media is presented, in which the fluid dynamics is probed over well-defined temporal and spatial domains. Various NMR techniques based on PGSE encoding are described in the context of standard theories of dispersion, with reference to Eulerian and Lagrangian coordinate frames. This array of methods provides access not only to the dispersion coefficient and the mean local velocity but also to propagators relevant to spatial and temporal correlations. Methods investigated include flow imaging, average propagator analysis, dispersion measurement, velocity exchange spectroscopy, and flow and disffraction based on scattering analysis. We apply these to a study of flow and dispersion of water in a packed bed of 90.7-μm-dia. polystyrene latex spheres. Our measurements of the dependence on Peclet number of dispersion (parallel and perpendicular to the mean flow direction) are in excellent agreement with results reported in the literature. The scattering approach used here has potential for studying complex flow properties involving the interplay between hydrodynamic and structural characteristics of porous media.
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    AIChE Journal 43 (1997), S. 1989-2001 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Sheet and film processes pose a challenging identification and control problem due to high complexity, poor conditioning, and limited input-output data. The interaction between model accuracy and closed-loop performance is explored for sheet and film processes using a model decomposition in terms of two static orthogonal matrices in series with a diagonal transfer-function matrix. It is shown that the accuracy of the diagonal elements of the transfer-function matrix directly specifies the closed-loop performance achievable by a model-based controller. This motivates the development of a combined identification and control procedure in which the controller is designed to be robust to model inaccuracies quantified during identification. The resulting controller is compared to an industrially accepted controller design method for two examples, including a simulated blown-film process. Based on the theoretical results and simulations, it was concluded that the poor performance often reported for existing industrial sheet and film process-control systems is likely due to signs of the model gains being incorrectly identified.
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    AIChE Journal 43 (1997), S. 2048-2058 
    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 pathways, kinetics and mechanisms underlying the hydrolysis of aliphatic and aromatic nitriles in high-temperature water (HTW) were investigated. The reaction products were the associated amides and carboxylic acids. Autocatalytic kinetics were observed and confirmed by experiment and analysis of the physical chemistry of the HTW reaction environment. A model incorporating two autocatalytic steps captured the observed kinetics well, and the associated optimized rate constants highlighted the key differences in the reaction chemistry of aliphatic and aromatic nitriles. The rate behavior of nitrile hydrolysis at these conditions has tangible consequences regarding optimal processing strategies.
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    AIChE Journal 43 (1997), S. 2059-2072 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Inorganic membrane reactors combine reaction and separation operations in a single unit. Preferential permeation of a product species through the membrane enhances equilibrium-limited reactions beyond the thermodynamic limit. Proper catalyst formulation and spatial distribution also enhance membrane reactor performance. To minimize reactant loss due to high gas permeation, the thickness of the γ-alumina layer in a composite alumina membrane (Membralox) was increased to 17 μm by slip-casting with alumina sol. Among the catalysts investigated a formulation of 1.10 wt. % Pt and 1.36 wt. % Sn supported on γ-alumina pellets yielded the highest activity, selectivity, and stability for the dehydrogenation of ethane to ethylene. Using this catalyst composition, various nonuniform platinum distributions within the pellets were prepared by solution impregnation, while maintaining a uniform distribution of Sn. The effect of nonuniform catalyst distribution on dehydrogenation of ethane in a packed-bed membrane reactor (PBMR) under well-mixed, isothermal conditions was evaluated both experimentally and theoretically. Reactor performance is maximized when the catalyst is concentrated near the surface of the support. Supraequilibrium conversions, about 80% beyond equilibrium values, were obtained with the narrow surface-step catalyst pellets. Experimental results agree well with theoretical predictions, obtained without the use of any adjustable parameters. The effect of membrane thickness on reactor performance was also investigated, by comparing the 17-μm alumina membrane with a 1.2-mm-thick porous Vycor glass, using pellets with the narrow surface-step distribution. For the experimental conditions employed, relatively low permeation through the porous Vycor resulted in conversions near fixed-bed reactor values.
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    AIChE Journal 43 (1997), S. 2133-2136 
    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 43 (1997), S. 2123-2132 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Renaturation of protein expressed as inclusion bodies within Escherichia coli is a key step in many bioprocesses. Operating conditions for the refolding step dramatically affect the amount of protein product recoverd, and hence profoundly influence the process economics. The first systematic comparison of refolding conducted in batch, fed-batch and continuous stirred-tank reactors is provided. Refolding is modeled as kinetic competition between first-order refolding (equilibrium reaction) and irreversible aggregation (second-order). Simulations presented allow direct comparison between different flowsheets and refolding schemes using a dimensionless economic objective. As expected from examination of the reaction kinetics, batch operation is the most inefficient mode. For the base process considered, the overall cost of fed-batch and continuous refolding is virtually identical (less than half than of the batch process). Reactor selection and optimization of refolding using overall economics are demonstrated to be vitally important.
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    AIChE Journal 43 (1997), S. 2148-2152 
    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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  • 152
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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: A new concept for removing toxic gases with respirator applications based on inhalation and exhalation through absorbent polymer-lined tubes was studied. The two-way oscillatory flow through the respirator element enables capture and expulsion of toxic vapors, which is not feasible with activated charcoal because of the adverse effect of moist breath on the charcoal efficiency. To study the influence of various material, design and operating parameters on the performance of such a respirator, a numerical model that solves the governing equations of mass and heat transport in an oscillatory flow through an absorbent polymer-lined tube was developed and utilized with experimentally measured property data. Factors investigated include respirator element geometry, polymer and toxic vapor characteristics, and flowthrough speed. Results show a large delay in toxic gas penetration over that obtained by conventional one-way flow, and define the importance of partition coefficient, flow passage size, and length and velocity. Results are compared with data from simulated breathing experiments.
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    AIChE Journal 43 (1997), S. 2187-2193 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: After an aqueous solution containing Sr2+ was flowed through a bed packed with particles of activated charcoal for a very long time, the flow was abruptly switched to deionized water. The variation in ionic concentration at the outlet with time was characterized by the power law, Cout ∼ t-α, for sufficiently large t. This fractal-like behavior we reported previously has further been studied theoretically and experimentally. A major concern is to examine dependency of α on experimental conditions. Adsorption sites predominate over dead-end pores or spaces as “trappers” in our case where Sr2+ or Ba2+ is adsorbed on activated charcoal. The experimental results can well be explained by our theoretical model. It has been found, however, that the response curve for Sr2+ deviates from the power law after a very long time. This deviation can be explained by introducing the assumption that there exists a maximum value of the activation energy for the desorption process. The curve for Ba2+, on the other hand, exhibits no such deviation until Cout decreases to the detection limit of the analytical device used.
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  • 154
    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 one-component steady-state permeation of gases through a silicalite-1 zeolite composite membrane as a function of the temperature is studied from 190 to 680 K for light hydrocarbons, noble gases, and some inorganic gases. In general, with increasing temperature the permeance shows a maximum followed by a minimum. For gases weakly adsorbed the permeance has only a minimum and for gases strongly adsorbed only a maximum is observed in the permeance. The permeance for various gases, for a feed pressure of 101 kPa, span four orders of magnitude. The lowest permeation is for i-butane at 300 K: a permeance of 0.07 × 10-8 mol. m-2.s-1.Pa-1. The highest value is observed for methane: a permeance of 70 × 10-8 mol. m-2.s-1.Pa-1 at about 240 K. A comparison between the isobars and the temperature dependence of the steady-state permeance, both at 101 kPa, shows that at the temperature where the amount adsorbed vanishes the permeance starts to increase. The temperature dependence of the steady-state fluxes through the silicalite-1 membrane can be described only if two diffusion mechanisms are taken into account. For high occupancies the mass transport can be described by equilibrium adsorption followed by surface diffusion and for low occupancies the mass transport can be described by activated gaseous diffusion. With increasing temperature the mass-transport mechanism shifts from the surface diffusion regime to the activated gaseous diffusion regime. With these two diffusivities modeling results agree well with experimental results for the one-component flux through the silicalite-1 zeolite membrane.
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  • 155
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    AIChE Journal 43 (1997), S. 1925-1934 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Thin polymer coatings produced from water- and organic-solvent-based precursor liquids are ubiquitous components of industrial and consumer products. Design and optimization of dryers for these coatings require accurate predictions of their drying rates. Accurate drying predictions for many polymer/solvent coatings require a satisfactory description of the polymer/solvent mutual diffusion coefficients. A method is proposed ot determine a satisfactory description of the diffusion coefficients by effective free-volume theory parameters. The effective parameters are determined from gravimetric data measured in a bench-scale drying apparatus. The application of the method to a poly(vinyl acetate)/toluene solution shows that the effective free-volume parameters give diffusion coefficients that agree with published data. The method is also applied to a rubber-based adhesive in industrial-grade heptane by treating the complex adhesive as a pseudobinary solution. For both systems, the effective parameters lead to quantitatively accurate drying predictions in a pilot-scale oven.
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The use of optical fibers in a fluorescence-detection scheme permits the accurate determination of the radial distribution of the transit time, the column efficiency, and the analyte concentration at the exit of a chromatographic axial-compression column (50 mmID). The results obtained demonstrate that the column is not homogeneous, but suggest a nearly cylindrical distribution of the packing density. The average velocity close to the column wall is 7% lower than along its axis and the HETP 25% higher. The lack of homogeneity of the column packing is another source of band broadening not taken into account in chromatography so far. It causes the apparent HETP derived from the conventional elution chromatogram recorded on the bulk eluent to be larger than the local HETP and the band profile to be unsymmetrical with a slight tail reminiscent of kinetic tailing.
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    AIChE Journal 43 (1997), S. 2021-2033 
    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 problem of simultaneous control of composition and molecular weight of copolymers produced in semicontinuous emulsion polymerization is addressed. A first-principles model of emulsion copolymerization is used to produce optimal open-loop policies, which are then experimentally tested on the system methyl methacrylate/vinyl acetate. In contrast to earlier work in the control of semicontinuous emulsion copolymerization, the control problem addressed has multiple inputs and multiple outputs, and the experimental application is free of empirical correlations and performed on an unseeded system. Optimal open-loop policies are calculated efficiently by optimizing properties of the instantaneous polymer being formed. The experimental results confirm the feasibility of these techniques. A model-predictive control scheme, in which optimal policies are recomputed on-line to account for disturbances, is proposed. The scheme, which exploits the advantages of the efficient calculation of optimal policies, is tested by simulation showing good performance.
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  • 158
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    AIChE Journal 43 (1997), S. 2571-2578 
    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 new two-phase swirl-flow monolithic-type reactor was designed to study the kinetics of heterogeneous photocatalytic processes on immobilized semiconductor catalysts. True kinetic rate constants for destruction of a textile dye were measured as a function of wavelength of light intensity and angle of incidence, catalyst layer thickness, and the effect of absorption of light by liquid film on the overall rate of photocatalytic degradation. Photocatalytic activities of two commercially available TiO2 catalysts (Degussa P25 and Hombikat UV 100) were also compared for different light intensities and catalyst layer thickness. Residence time distribution and mass-transfer limitations were evaluated. This new reactor appears to be an attractive choice for kinetic studies of heterogeneous photocatalysis.
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  • 159
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    AIChE Journal 43 (1997), S. 2601-2604 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Additional Material: 4 Ill.
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  • 160
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    AIChE Journal 43 (1997) 
    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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  • 161
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    AIChE Journal 43 (1997), S. 2889-2902 
    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 model for a pneumatic-conveying dryer is presented, with the focus on the superheated steam drying of wood chips, although it can also be used for other porous materials and drying media. It includes a comprehensive 2-D model for the drying of single wood chips, which accounts for the main physical mechanisms occurring in wood during drying, including coupled transport of water, air, vapor and heat. This model allows for features such as initial condensation and flashing at the outlet, as well as the falling rate period when the drying is controlled by internal transport. External drying conditions in the dryer are calculated by applying mass, heat and momentum equations for each incremental step in dryer length. A plug-flow assumption is made for the dryer model, and single-particle and dryer models were solved iteratively. The irregular movement and nonspherical shape of wood chips are accounted for by measuring drag and heat-transfer coefficients.Model calculations illustrate the complex interactions among steam, particles, and walls that occur in a flash dryer. The drying rate, the slip velocity, and temperature vary in a complex manner through the dryer, necessitating the use of a comprehensive single-particle model, as in this case. Previous experimental data on the drying of bark chips in a pilot dryer was used to verify the model. The predicted temperature and pressure profiles, as well as the final moisture content of the material, agreed well with the measurements. Thus, the model provides a useful tool for the design and scale-up of pneumatic-conveying dryers. Effects of steam and material properties on the drying were investigated with different design parameters.
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  • 162
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    AIChE Journal 43 (1997), S. 2997-3006 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Based on the absolute-rate theory, a consistent interpretation was provided for the dependence of surface-diffusion coefficient, Ds, on temperature and the amount adsorbed in various liquid-phase adsorption systems, such as the Langmuir-, Freundlich-and Jossens-type adsorption. It was demonstrated that a restricted molecular diffusion model for surface diffusion was useful for the analysis of the characteristic features of Ds. A formulation of Ds was derived based on the model and was applied to the analysis of surface-diffusion phenomena in various adsorption systems. The temperature dependence could be interpreted by assuming surface diffusion as an activated process. By taking into account the change in both the logarithmic slope of an adsorption isotherm and an adsorption potential, the concentration dependence of Ds could be interpreted irrespective of the type of the adsorption isotherms.
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  • 163
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    AIChE Journal 43 (1997), S. 3021-3030 
    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 optimal alarm-system design method, which takes full advantage of the inherent hardware and spatial redundancies in a mass-flow network, is proposed. In particular, systematic procedures are developed to identify independent measurement methods for the flow rate of any stream in the process and to synthesize corresponding alarm generation logic. To implement the suggested alarm strategy, the formulas for evaluating conditional probabilities of type I and II mistakes are also derived. The results of applying the proposed alarm system to the application example show that the approach taken in this work is effective in suppressing the latter mistakes. Further, this system is superior to any of the existing alarm techniques in the sense that it can be appropriately tailored to minimize the expected loss due to misjudgments in generating alarms.
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  • 164
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    AIChE Journal 43 (1997), S. 2432-2445 
    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 phase holdups and the heat-transfer behavior were studied experimentally in three-phase fluidized beds over a pressure range of 0.1-15.6 MPa. Bubble characteristics in the bed are examined by direct flow visualization. Pressure effects on the bubble coalescence and breakup are analyzed mechanistically. The study indicates that the pressure affects the hydrodynamics and heat-transfer properties of a three-phase fluidized bed significantly. The average bubble size decreases and the bubble-size distribution becomes narrower with an increase in pressure. The bubble-size reduction leads to an increase in the transition gas velocity from the dispersed bubble regime to the coalesced bubble regime, an increase in the gas holdup, and a decrease in the liquid and solids holdups. The pressure effect is insignificant above 6 MPa. The heat-transfer coefficient between an immersed surface and the bed increases to a maximum at pressure 6-8 MPa and then decreases with an increase in pressure at a given gas and liquid flow rate. This variation is attributed to the pressure effects on phase holdups and physical properties of the gas and liquid phases. A mechanistic analysis revealed that the major heat-transfer resistance in high-pressure three-phase fluidized beds resides in a liquid film surrounding the heat-transfer surface. An empirical correlation is proposed to predict the heat-transfer coefficient under high-pressure conditions.
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  • 165
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    AIChE Journal 43 (1997), S. 2488-2508 
    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 concept of standing concentration waves is introduced to derive design equations for continuous moving bed (CMB) processes. For linear isotherm systems, simple equations are derived from the analysis to link product purity and recovery to zone lengths, bed movement velocity, flow rates, column capacity factors, and mass-transfer coefficients. Once product purity, recovery and feed flow rate are specified for a given system, the zone flow rates and bed movement velocity that provide the highest throughput and the lowest solvent consumption can be determined from the solutions. In a given system, there is a trade-off between product purity and throughput. If bed volume and product purities are fixed, the longer the zone lengths, the higher the throughput. Simulations based on a linear driving force model that considers axial dispersion and lumped film and intraparticle diffusion are used to compare the column profiles and effluent histories of CMB and simulated moving bed (SMB). A numerical algorithm is introduced to allow simulation of both CMB and SMB operations using the same program. The comparison shows that the design equations derived for CMB systems are applicable to SMB systems. Finally, the standing wave solutions are used to analyze an experimental SMB system from the literature (Ching et al., 1991). Simulations agree closely with the data and the predictions of the theoretical analysis.
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  • 166
    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 slide coating of photographic films, liquid layers flow down an inclined plane, across a small gap under a pressure differential (vacuum), and onto a fast-moving substrate. Process interruptions to allow splices of adjoining rolls of substrate to pass through the coater result in significant waste. Coating over splices fails due to air entrainment and/or two free-surface flow instabilities during the necessary gap widening: (1) standing waves (ribbing) or rupture of the liquid bridge when vacuum is kept high or (2) lateral contraction when vacuum is lowered. The problem of coating over splices is solved here by reducing it to a fluid-mechanical stability problem. First, air entrainment was eliminated by serrating the splice trailing edge and lateral contraction by prewetting the substrate with liquid streams. Avoiding rupture of the liquid bridge then reduces the problem to finding the vacuum at which the bridge becomes unstable to small disturbances. This “critical” vacuum is found by finite-element stability analysis of the quasi-steady 2-D free-surface flow to small 2-D and 3-D disturbances, direct tracking of critical points, and verification by solving the transient 2-D skip-out process. Three modes of failure are predicted if the vacuum is not sufficiently reduced with gap widening. Implementation of a computed vacuum strategy in a production line gave sufficient latitude for coating over imprecise splices.
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  • 167
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    AIChE Journal 43 (1997), S. 3205-3214 
    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 polymers and other homologous mixtures that span a molecular weight (MW) range, many reaction mechanisms involve the formation, addition and decomposition of free radicals. In the chain reactions of polymerization and depolymerization, radicals and polymer molecules interact by a variety of primary reactions. We postulate that these radicals and molecules can be described by continuous molecular weight distributions (MWDs). Their chemical interactions are developed from generic, reversible addition or bond scission reactions. Distribution balance equations are proposed for initiation-termination and propagation-depropagation reactions, such as hydrogen abstraction, disproportionation, chain cleavage, and radical coupling. A comprehensive model including all these events conforms to the special mechanisms for chain-end depolymer-ization and for random-scission polymer degradation. The evolution of an MWD from initial state to final equilibrium is described by the time dependence of MW moments. Irreversible degradation or polymerization relationships are obtained as limiting cases. First-order rate constants observed in pyrolysis and thermolysis reactions are composed of algebraic groups of primary reaction rate constants.
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  • 168
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    AIChE Journal 43 (1997), S. 3253-3256 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology