摘要:

AbstractL‐Isoleucine and the isomorphic amino acids L‐leucine and L‐valine serve as a model system in the examination of the effects of impurities on the purity and morphology of crystals recovered from batch crystallizations. Factors are determined that influence crystal purity and, concomitantly, crystal size and shape. Characteristics of the L‐isoleucine crystal are determined by X‐ray analysis and compare favorably with the literature. These and literature characteristics of L‐leucine and L‐valine are used in simulations to evaluate possible mechanisms by which lattice substitutions with impurities lead to size and morpholo

ISSN:0001-1541    

DOI:10.1002/aic.690420112

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThis article is intended as a complement to a recent article in this journal, “Comparative Study of Flow Schemes for a Simulated Countercurrent Adsorption Separation Process” by Ching et al. (1992). These authors carried out experiments on continuous separation of three carbohydrate mixtures with both the usual four‐section scheme and a three‐section scheme: fructose dextran (MW ≌ 9,400), raffinose dextran (MW = 6,000), and fructose‐raffinose. These two schemes were modeled as equivalent true countercurrent systems. From the experiments and model results they conclude that the equivalent true countercurrent model is fully adapted to represent the four‐section scheme, but a major discrepancy arises with the three‐section scheme: the raffinate concentration is not correctly predicted.From these results it may be concluded that when a physically meaningful modeling of the equivalent true countercurrent is realized, the apparent discrepancy disappears. From another standpoint, such a mismatch between the experiments and model predictions would not have been observed with a model that considers the periodic port motion. Such a model foresees a cyclic steady state where extract and raffinate compositions vary throughout the period T; they can be considered as constant only when averaged over the period. It is worth comparing the results of these two models, especially when the equivalent countercurrent representation is not very close to the physical phenomenon it should represent.We agree with their major finding that a three‐zone scheme represents necessarily a higher desorbent demand than a

ISSN:0001-1541    

DOI:10.1002/aic.690420113

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA new approach to cation‐exchange kinetics views ion‐exchange resin as an aggregate of uniform cylindrical pores. Ion exchange occurs at the pore surface. The counterions dissociate only partially from the fixed sites, and the charge created on the pore surface is balanced by the counterions in the electrical double layer. Thus, any cross section of a pore consists of an annular double‐layer region and a central core region. Electroneutrality prevails in the core region through an influx of coions. Diffusion of ions through both regions is considered. The complete model incorporating these ideas for kinetics of monovalent cation exchange was tested with our experimental data as well as with the reported data. The pore diffusion coefficients used in the model are free ionic diffusion coefficients corrected for the tortuosity factor of the pores. Unknown parameters of the model are the degree of dissociation of counterions from the fixed sites of the pore (fσ), and the film thickness (δ). Both parameters are insensitive to the type of counterions, solution concentration, and the direction of exchange. Further, fσappears to be a characteristic property of resin alone. Superiority of the developed model over the previous models is est

ISSN:0001-1541    

DOI:10.1002/aic.690420114

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA stable adaptive control strategy is suggested for a class of continuous‐flow bioreactor processes described by Monod kinetics with two unknown parameters, one of which appears nonlinearly. Similarly, as in the case of the previously reported adaptive controllers, the parametrization of the process model, in conjuction with the adaptive exponential feeding strategy and corresponding adaptive algorithms, results in a stable system in which the convergence of the output errors to zero is guaranteed. In the former, however, two major problems are encountered: (1) both output errors were used to adjust the controller parameters, which may yield unacceptable performance of the resulting adaptive system; (2) conditions under which the process output can assume only positive values are difficult to derive. Hence, a design of a stable adaptive controller is suggested, whose parameters are adjusted using only one of the output errors and that yields acceptable performance of the control system. With this method, conditions under which the process outputs can assume only positive values can be readily derived. These conditions in turn guarantee that the control input saturation at value zero cannot occur. In this context, two adaptive controllers are suggested, such that the resulting adaptive systems are stable and the control objective is met. The adaptive controller design relies on a convenient coordinate transformation, while the proof of stability is based on suitably chosen Lyapunov functions. The performance of the adaptive system is evaluated through computer simulation

ISSN:0001-1541    

DOI:10.1002/aic.690420115

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractDiscrete‐time nonlinear feedback control laws are derived for multivariable nonlinear processes, whose “delay‐free” parts are minimum phase. These include mixed error‐and state‐feedback, error‐feedback, and mixed error‐ and output‐feedback laws, which can induce linear input–output closed‐loop response. This study is carried out within the framework of the discrete‐time globally linearizing control. The broader class of nonlinear processes, in which controlled outputs as well as some other process variables are measured, are also considered. A mixed error‐ and output‐feedback control law is derived for this class of processes. The conditions under which the mixed error‐ and output‐feedback can be applied to a process operating at or around an open‐loop unstable equilibrium point are determined. The application and performance of the derived control laws are illustrated by the numerical

ISSN:0001-1541    

DOI:10.1002/aic.690420116

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractSimultaneous effects of physisorption, diffusion, and reaction were characterized by dynamic experiments for propene metathesis over rhenium oxide/γ‐alumina catalyst in a Berty reactor. Single‐component sorption isotherms from steady‐state experiments for ethene, propene, and 2‐butene were used to predict multicomponent physisorption by the ideal adsorbed solution theory. Effective diffusivities were determined from molecular, Knudsen, and surface‐diffusion coefficients, including bimodal pore‐size effect and tortuosity factors. Transient sorption diffusion experiments were conducted with inactivated catalysts. The reaction rate expression, derived from elementary steps of carbene mechanism, was identified by solving the inverse problem with step test data. The direct problem was solved by orthogonal collocation within the method of lines. Model predictions agreed with responses from dynamic experiments carried out under conditions different from those used for estimating mode

ISSN:0001-1541    

DOI:10.1002/aic.690420117

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThe long‐time behavior of a reaction mixture containing infinitely many species in isothermal continuous stirred tank reactors (CSTRs) is analyzed and contrasted with that in a plug‐flow reactor (PFR). The reaction kinetics considered are irreversible first‐order, irreversible coupled pseudo first‐order, and reversible first‐order. Asymptotic lumped kinetics for the mixture as a whole are developed as a function of feed properties and reactor type. Such lumped kinetics in many cases are of the power law form, with the exponent for a CSTR being lower than that for a PFR. Conditions under which non‐power‐law kinetics arise are given. Due to the wide spread of reactor residence times, the mixture in CSTRs admits a much richer variety of asymptotic possibilities than in the PFR. The results uncover some behavioral differences and relationships between CSTRs and a PFR for converting complex mixtures such as petroleum distillates or

ISSN:0001-1541    

DOI:10.1002/aic.690420118

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractDissolution and crystal growth on the (010) cleavage surface of mineral gypsum have been studied by scanning force microscopy, which allows observation of surfaces while they are in contact with aqueous solutions. Etch pits formed on the (010) surface in contact with solutions undersaturated with respect to gypsum are oriented in relation to the [001] zone axis. Edges parallel to [001]are stable or move only slowly. Edges parallel to [100] and [101]move more rapidly. Crystal growth by migration of step edges is observed in contact with supersaturated solutions. The inhibition of crystal growth by phosphonate produces gross changes in the texture of steps and terraces.

ISSN:0001-1541    

DOI:10.1002/aic.690420119

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA thermodynamic framework is developed for calculating wax precipitation in petroleum mixtures over a wide temperature range. The framework uses the experimentally supported assumption that precipitated wax consists of several solid phases; each solid phase is described as a pure component or pseudocomponent that does not mix with other solid phases. Liquid‐phase properties are obtained from an equation of state. Calculated wax‐precipitation data are in excellent agreement with experimental results for binary and multicomponent hydrocarbon mixtures, including petrol

ISSN:0001-1541    

DOI:10.1002/aic.690420120

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractIon‐selective electrodes were used to measure the activity coefficients at 298.2 K of individual ions in aqueous solutions of NaCl and NaBr up to of 5 molal and of KCl up to 4 molal. The mean ionic activity coefficients of NaCl, NaBr, and KCl, obtained from the values of the activity coefficients of the individual ions, show good agreement with values reported in the literature. The experimental results show that the activity coefficients are different for the anion and the cation in an aqueous solution of a single electrolyte and that, as expected from the ion‐ion and ion‐solvent interactions, the activity coefficient of an ion depends on the nature of its counterion. A modified form of the Pitzer's model, which distinguishes between the activity coefficients of the anion and the cation, was used to correlate the experimental re

ISSN:0001-1541    

DOI:10.1002/aic.690420121

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY