Application of electrochemical oxidation for treatment of industrial wastewater — the influence of reactor hydrodynamics on direct and mediated processes

The influence of kinetic and hydrodynamic factors in electrochemical reactors used in the removal of pollutants from industrial wastewater is shown, distinguishing between the two main types of reactions, namely direct and mediated electro‐oxidation. The effect of stirring during treatment of four different types of wastewater is reported. Whilst for direct electro‐oxidation of pollutants, the influence of agitation on the performance of the reactor can be easily predicted from a mass transfer correlation, its effect during electro‐oxidation mediated in the homogeneous phase by a redox couple is not straightforward. The Hatta number can be a useful criterion to apply to electrochemical reactors performing mediated oxidation of compounds (in analogy to gas–liquid reactions), so as to define whether the reaction occurs in the bulk of the reactor or near the electrode, and thus can be affected differently by stirring. The hydrodynamic conditions in the reactor for treatment of industrial wastewater can affect the differential selectivity of the removal of pollutants and this can be used for optimising the performance of the reactor with respect to a target pollutant. Copyright © 2006 Society of Chemical Industry     

Polymerisationstechnik

Polymerization engineering. Polyreactions are complex chemical reactions which are generally conducted in liquid phase. The characteristic feature of polyreactions is the pronounced increase in viscosity of the reactants during the reaction. The increase in viscosity influences numerous reaction and process parameters, such as the reaction kinetics, the transfer of mass, heat, and momentum, the degree of mixing of the reactants, and the residence time distribution of continuous reactors. Thus the increase in viscosity influences the performance, selectivity, and safety of the reactor. The choice of reactor for performing polyreactions can be based on various aspects. Of decisive importance are the performance and selectivity of the reactor. For the cases of simple and complex polyreactors, various kinds of reactors are compared with respect to their performance and selectivity, with the molar mass distribution of the resulting polymers serving as example of the selectivity of polyreactions. In the case of simple polymerizations, the molar mass distribution of the polymers depends unequivocally on the type of reactor and the various reactor types can be arranged in appropriate order. This is no longer the case for more complex polymerizations because the concentration conditions in the reactors play an increasingly important role.     

Moderne Methoden der Meß- und Regelungstechnik und ihre Bedeutung für die Automatisierung verfahrenstechnischer Prozesse

Modern methods of measurement and control engineering and their significance for the automation of chemical engineering processes . This article demonstrates that modern system-theoretical methods open up new approaches in the solution of difficult measurement and control problems. These methods are frequently based on the assumption that a certain a priori knowledge of the process behaviour is available, which can be formulated as a mathematical process model. Model-supported measuring procedures are of particular significance and are discussed at length. These methods greatly enhance the information value of measured values, with regard to both extent and reliability. It is shown how the problem of early recognition of dangerous reaction states in reactors in the case of exothermic reaction can be overcome. In order to minimize the computational effort, methods of model reduction are considered.     

Multiple model-based controller design applied to an electrochemical batch reactor

This study demonstrates the control of an electrochemical batch reactor, that produces the desired product in a competing chemical/electrochemical reaction network, using a multiple model-based controller design. Since this type of control framework requires a process model to provide predictions of the controlled variables, and because batch reactors are highly nonlinear and nonstationary in nature, a bank of linear, dynamic, state-space models rather than a single, linear state-space or convolution model is developed to represent the nonstationary behaviour of the batch process. It is shown that the performance of this model/controller design can provide good reactor performance in the face of known disturbances.     

Vinyl chloride suspension polymerization with constant rate. A numerical study of batch reactors

Based on the analysis of a simple mathematical model, different temperature profiles are generated in order to provide vinyl chloride (VCM) suspension polymerization with constant reaction rates in batch reactors. In order to reproduce these temperature profiles in industrial-scale reactors, some process variables, such as coolant temperature, initiator concentration, and rate of water and monomer condensation, have to be manipulated. It is shown that those temperature practices can almost never be applied to large-scale reactors if the jacket temperature is the only variable that can be manipulated. It is also shown that developing an initiator feed procedure or using a reflux condenser may be advantageous.     

Katalytische Suspensions-Reaktoren

Catalytic suspension reactors . Reactions occurring in slurries are frequently encountered in industrial chemistry. The field of application of suspension reactors and their advantages and disadvantages compared with other catalytic reactors are underlined. It should be borne in mind when calculating space-time yields that several drag components (gas/liquid and liquid/solid mass transfer, as well as pore diffusion inhibition) precede the actual reaction. They are best accounted for by the concept of series switching of partial resistances. In the case of highly active and finely divided catalysts, however, absorption acceleration may take place. Methods are presented for determining reaction and transport parameters and correlations of mass and heat transfer are considered. Particular attention is devoted to the question whether the characteristic quantities of heat and mass transfer in a three-phase system can be estimated from the experimental data for two-phase systems (gas/liquid and liquid/solid). This is shown to be feasible in many cases with an accuracy adequate for practical purposes. Reactor and catalyst performance show opposing trends in suspension reactors. Guidelines for optimal operation can be derived on the basis of simplified model concepts.     

Age distribution and the degree of mixing in continuous flow stirred tank reactors

New development of mean age theory is discussed for quantitative analysis of mixing and age distribution in steady continuous flow stirred tank reactors. A new relationship between the moments of age and the moments of residence time are derived. With this new relationship the variance of residence time distribution can be computed much more efficiently and accurately. The relationships of three existing variances of age are described and a new set of variances and the degree of mixing are defined. The theory is used to characterize mixing performance in a CFSTR with different layouts of an inlet and an outlet. Mean age and higher moments of age in the reactors are obtained from CFD solutions of their steady transport equations. The spatial distribution of mean age reveals details of the spatial non-uniformity in mixing. Variances of age and the degree of mixing discussed by Danckwerts and Zwietering are computed for the first time in the literature for non-ideal stirred tank reactors. It is found that although these measures are useful, certain key features in non-uniform mixing are not reflected accurately. Results show that the new set of variances and the degree of mixing more accurately characterize the non-uniform mixing in the reactors.     

OPTIMUM REACTOR DESIGN IN METHANATION PROCESSES WITH NONUNIFORM CATALYSTS

A generic methodology is developed to design a heterogeneous catalytic reactor for methanation processes. For the optimization of a heterogeneous catalytic reactor, nonuniform catalyst pellets such as a layered catalyst are considered with respect to reaction type, reactor performance, and component distribution inside the catalyst. Heterogeneous uniform and nonuniform catalyst models were developed to analyze the effect of mass and heat transfer between both bulk phase and catalyst surface and inside a catalyst pellet. Then, concentration profiles of hydrogen and carbon monoxide in the catalyst pellet and along the reactor axis were obtained by analyzing simulation results. It was shown that the application of different types of nonuniform catalyst pellets at a certain number of separate zones within a reactor could produce higher catalyst performance than a reactor with uniform catalyst. Furthermore, it proved a significant decrease of catalyst deactivation behavior such as coking and sintering. Layered catalysts were optimized to maximize an overall reactor performance over the catalyst lifetime, achieving capital cost reduction characterized by reactor size, catalyst amount, and degree of catalyst deactivation. Last, temperature control throughout the reactor operating periods was strategically planned for a reactor operation with distribution of nonuniform catalyst pellets. This methodology can also be usefully applied to the design of heterogeneous catalytic reactors for other processes such as hydro-treating process and cracking process.     

Mathematical reactor modelling for coupled chemical and electrochemical processes: the ECE system

A procedure for modelling electrochemical reactions and reactors which involve heterogeneous reaction, homogeneous fast chemical reaction and diffusional mass transport is described. The procedure can be applied to any combination of first order reaction processes utilising numerical routines for the solution of initial value differential equations. By the use of collocation it can be extended to higher order processes. The reactor types considered are batch, plug flow and dynamic continuous stirred tanks and reactors with recycle. Operation with either potentiostatic, galvanostatic or constant cell voltage control is described and illustrated using the ECE reaction mechanism, involving successive electrochemical, chemical and electrochemical reaction.     

Modular Simulation of Fluidized Bed Reactors

Simulation of chemical processes involving nonideal reactors is essential for process design, optimization, control and scale‐up. Various industrial process simulation programs are available for chemical process simulation. Most of these programs are being developed based on the sequential modular approach. They contain only standard ideal reactors but provide no module for nonideal reactors, e.g., fluidized bed reactors. In this study, a new model is developed for the simulation of fluidized bed reactors by sequential modular approach. In the proposed model the bed is divided into several serial sections and the flow of the gas is considered as plug flow through the bubbles and perfectly mixed through the emulsion phase. In order to simulate the performance of these reactors, the hydrodynamic and reaction submodels should be integrated together in the medium and facilities provided by industrial simulators to obtain a simulation model. The performance of the proposed simulation model is tested against the experimental data reported in the literature for various gas‐solid systems and a wide range of superficial gas velocities. It is shown that this model provides acceptable results in predicting the performance of the fluidized bed reactors. The results of this study can easily be used by industrial simulators to enhance their abilities to simulate the fluidized bed reactor properly.     

The bifurcation behavior of tubular reactors

Methods for studying the bifurcation behavior of tubular reactors have been developed. This involves the application of static and Hopf bifurcation theory for PDE's and the very precise determination of steady state profiles. Practical computational methods for carrying out this analysis are discussed in some detail. For the special case of a first order, irreversible reaction in a tubular reactor with axial dispersion, the bifurcation behavior is classified and summarized in parameter space plots. In particular the influence of the Lewis and Peclet numbers is investigated. It is shown that oscillations due to interaction of dispersion and reaction effects should not exist in fixed bed reactors and moreover, should only occur in very short “empty” tubular reactors. The parameter study not only brings together previously published examples of multiple and periodic solutions but also reveals a hitherto undiscovered wealth of bifurcation structures. Sixteen of these structures, which come about by combinations of as many as four bifurcations to multiple steady states and four bifurcations to periodic solutions, are illustrated with numerical examples. Although the analysis is based on the pseudohomogeneous axial dispersion model, it can readily be applied to other reaction diffusion equations such as the general two phase models for fixed bed reactors.     

常压反应-减压精馏耦合生产氯化苄的工艺优化设计

采用常压反应-减压精馏耦合装置应用于甲苯氯化体系生产氯化苄。为了找到集成装置的最佳结构参数,建立了以独立反应量为基础的模型方程,并开发了基于Powell法的多层优化设计法。结果表明,在塔釜上升蒸汽量一定的情况下,增加精馏塔分离段塔板数、反应器台数、相邻反应器间间隔塔板数,反应能力有所增加,然而当其超过一定值后,反应能力增加的趋势不明显;在优化得到的结构操作参数下,反应能力与分离能力达到最佳匹配。     

IDEAS based synthesis of minimum volume reactor networks featuring residence time density/distribution models

This work addresses for the first time, the synthesis of globally minimum volume reactor networks, featuring segregated flow reactors (SFR) and/or maximum mixedness reactors (MMR), with the same normalized residence time density (NRTd) function. Global optimality is ascertained by demonstrating that the input–output information maps of SFR and MMR with general RTd/RTD models satisfy all properties required for the application of the infinite dimensional state-space (IDEAS) approach to the RTd/RTD reactor network synthesis problem. The resulting IDEAS formulation is shown to possess a number of novel properties, which can be used to facilitate its solution. The power of the proposed methodology is demonstrated on three case studies featuring segregated laminar flow reactors (SLFR) in which the Trambouze reaction scheme is carried out. In one of the case studies, the identified reactor network is shown to have volume that is as low as half the volume of a single reactor.     

Industrial monitoring of platinoid losses during the oxidation of ammonia in UKL-7 reactors

Platinoid losses are monitored during the oxidation of ammonia in industrial UKL-7 reactors with one- and two-stage catalytic systems. Twelve reactors at OAO Acron and eleven reactors at OAO Nevinnomysskii Azot are inspected. It is established that the losses of platinoids from the first five gauzes in reactors with diameters of 1.70 m and the first three platinoid gauzes in reactors with diameters of 1.93 m upstream of the gas flow are nearly equal with a considerable reduction in losses from subsequent gauzes. The concept of mechanochemical and reduced (to the amount of synthesized nitric acid) mechanochemical platinoid losses that depend on the intensity of the catalytic reaction and the hydrodynamic conditions in the contact reactor is introduced. It is established that the ratio of the mass fractions of platinoid losses from the gauzes of a catalytic system is determined by the diameter of the reactor and the location of gauzes in a stack. Previously unknown thermal losses are thus estimated. It is shown that the dependence of the specific losses of platinoids on the reduced mechanochemical losses obeys a linear law and allows calculations of mechanochemical losses for any UKL-7 reactor. Experimental dependences that relate the fraction of ammonia converted by a platinoid gauze to its location in a catalyst stack and indicate a need to correct the mathematical model used in calculations are revealed. Analysis of mechanochemical losses can reveal features of the service wear of platinoid gauzes in different reactors and help predict the degree and character of catalyst destruction for reactors equipped with efficient new catalytic systems.     

MODELING OF THREE PHASE NONCATALYTIC BUBBLE COLUMN REACTORS

Mathematical models have been developed for the design of bubble column slurry reactors wherein the particles with nonuniform distribution of solid reactant take part in the reaction and follow the shrinking core model. The cases of external mass transfer control, ash layer diffusion control and chemical reaction control have been analyzed using solid distribution functions presented by Dudukovic (1984). It was found that the nonuniformity of solid reactant can strongly affect the reactor performance. Correction factors for modifying the performance charts of Joshi el al. (1981) who considered only uniform solid reactant distribution are presented for design and scale-up purposes.     

MODELING OF THREE PHASE NONCATALYTIC BUBBLE COLUMN REACTORS

Mathematical models have been developed for the design of bubble column slurry reactors wherein the particles with nonuniform distribution of solid reactant take part in the reaction and follow the shrinking core model. The cases of external mass transfer control, ash layer diffusion control and chemical reaction control have been analyzed using solid distribution functions presented by Dudukovic (1984). It was found that the nonuniformity of solid reactant can strongly affect the reactor performance. Correction factors for modifying the performance charts of Joshi el al. (1981) who considered only uniform solid reactant distribution are presented for design and scale-up purposes.     

Monitoring of emulsion polymerisations: A study of reaction kinetics in the presence of secondary nucleation

A non‐linear state estimator that provides on‐line information on residual monomer concentration and radical concentration is experimentally validated under dynamic conditions, and in particular in the presence of secondary particle nucleation. The proposed estimator uses the heat of reaction as an input. Practical methods for determining the initial values of the parameters in the energy balance are also proposed and tested on laboratory and pilot scale installations. It is shown that this method provides a robust tool for the on‐line monitoring of batch (with and without shot additions of monomer, surfactant and initiator) and semi‐batch emulsion polymerisation reactors. The observer was shown to converge rapidly, even under conditions where the number of particles per litre of emulsion (N_p) changed rapidly.     

Monolithic catalysts as more efficient three-phase reactors

Monolithic reactors are an attractive alternative to conventional multi-phase reactors. Advantages are the low pressure drop, the absence of a need for a catalyst separation, and the large geometrical surface area. The main disadvantage is, however, the lack of practical experience with monolithic reactors. A pilot-scale study has been carried out to evaluate the performance of a monolithic reactor on a larger scale. The results of this study have demonstrated that a higher productivity can be obtained for a monolithic reactor compared to a trickle-bed reactor for a solid catalysed gas–liquid reaction that is mass transfer limited in the gas-phase reactant. A higher selectivity has been demonstrated for the selective hydrogenation of benzaldehyde towards benzylalcohol when a monolithic reactor is used instead of a trickle-bed reactor as a result of the narrower residence-time distribution.