Reactor-separator System Research Articles

Phenomena-based analysis of fixed points in reactive separation systems

Integrated systems for reaction and separation offer promising technology for less expensive and more energy-efficient chemical processes. In this paper, we show how to view reactive separation processes as a vector combination of reaction, separation and mixing. In composition space, vector directions are calculated from physical and chemical data only; thus they are independent of design. On the other hand, vector lengths reflect design decisions which are at our disposal, allowing us to manipulate resultant moves in composition space by how we design and operate the equipment. Of particular interest is the discovery of a reaction difference point. We show that this mathematical artifact is found outside the feasible composition space. It determines the underlying structure of linear composition changes by reaction in mole fraction space. Using the proposed vector notation and the reaction difference point, we provide a simple and visual explanation for the term `reactive azeotrope', herein referred to as a reactive fixed point. The potential for such points arises from collinearity among reaction, separation and mixing as individual phenomena and is independent of operating conditions. We discuss the possible impact of such points on limiting behaviour in systems featuring simultaneous reaction and separation.

On the dynamics of integrated plants—non-minimum phase behavior

Abstract Recycling of material and or energy in chemical plants imposes a feedback mechanism which is known to move the poles of the plant. In this paper the effect of recycling on the zero dynamics is considered, and it is shown that the feedback effect imposed by recycling also may move the zeros of control relevant transfer-functions. Sufficient conditions for the existence of non-minimum phase transmission zeros caused by the presence of recycling are derived. The conditions may be evaluated based on steady-state information about the individual units only, and are hence particularly well suited for incorporation in a process design environment. The derived results are illustrated through application to the design of a reactor-separator system.

Environmentally friendly processes

Environmental concerns have raised public awareness of environmental issues and are driving forces for regulation. The impact of regulation on the cost of production is expected to become important in determining the international competitiveness of the US chemical industry. In response to cost pressures, industry has launched a number of initiatives aimed at improving efficiency and reducing environmental impact. Some of these environmental success stories are receiving increased national attention due to programs such as the Presidential Green Chemistry Challenge Awards Program. In addition to traditional metrics for evaluating process performance, such as productivity, environmental considerations increasingly are important in process development. Chemical processes evolve through life cycle phases, beginning with research, and then moving to process engineering, plant operation, and eventually, decommissioning. The number of technology options available for reducing environmental impact are highest early on in the life cycle and then decrease drastically. In contrast, costs associated with resolving an environmental problem typically increase exponentially as the process matures and the scale of equipment gets larger. There is, therefore, a considerable incentive to address and resolve environmental issues early in the life cycle. Chemical reactions responsible for producing high value-added products are, in most cases, also responsible for generating by-products and pollutants. New chemical and biochemical approaches are providing new reaction concepts. As in the development of traditional chemical and petrochemical processes, reaction engineering, broadly defined as the field that quantifies the engineering aspects of chemically reactive systems, is providing enabling tools that accelerate the development of environmentally friendly processes. Core reaction engineering methods are being utilized for kinetic modeling, reactor selection, scale-up, and design. Meanwhile, the research frontiers are providing new reaction engineering tools, from computational chemistry to probe the nature of catalytic active sites to computational fluid dynamics modeling for designing the internals of reaction-separation systems. The long-term goal is to develop processes having 100% raw materials utilization, or zero waste. The near-term strategy for controlling emissions is to institute pollution prevention programs and install cost-effective end-of-pipe technologies. These technologies typically control generic classes of air pollutant emissions such as carbon monoxide, volatile organic compounds (VOCs), nitrogen oxides (NO x ) and sulfur oxides (SO x ). Technologies for treating wastewater are also available. Over time, a shift in focus is expected, from mere compliance to a point where environmentalism, like safety, is fully integrated into the corporate culture. The present paper discusses the role of reaction engineering in the development of environmentally-friendly processes. Select examples of processes, from the author’s experience, that benefit from reaction engineering will be presented.

A hybrid approach for reactive separation systems

In this paper, a general modelling framework employing a hybrid approach for modelling and simulation of reactive separation systems such as reactive distillation is presented. The general modelling framework allows rate-based as well as equilibrium-based models for steady state and dynamic simulation modes. The hybrid approach also provides tools for analysis of equilibrium for the rate-based model and for bifurcation analysis for all models. Simulation results validating the models and highlighting the hybrid approach are presented for two well-known reactive systems.

A Multiobjective Framework for the Integrated Process Design and Control

This paper presents a systematic framework for the integrated design of process and control systems. A detailed controllability analysis is performed over the entire frequency range of interest and a new method for control structure selection, based on the minimization of an overall interaction measure, is used. The trade-off between increased cost and improved controllability is resolved using the goal attainment method for solving multiobjective optimization problems. The proposed method is clarified through an application example, specifically the integrated design of a reactor-separator system with recycle.

Zeolitic coatings and their potential use in catalysis

The formation of zeolitic coatings and their properties, such as the thickness, continuity and orientation of the crystals, are related to the presence and macro-organization of a precursor phase. Based on this view, preshaped zeolitic coatings can be prepared which may be either active catalysts themselves or an inert thin membrane on an existing catalyst. They can be applied in low pressure drop reactors, in adsorption units, in catalytic distillation units and in integrated reaction separation systems as zeolitic membranes. Coating preparation and particular aspects of the performance of the zeolitic coatings in a membrane configuration, in selective catalytic conversion of NO x, as an inert membrane on a catalyst phase, and model structured catalyst systems are discussed.

Dynamic modelling of simultaneous reaction and distillation in a semibatch reactor system

A dynamic model for a system containing a semi-batchwise operating reaction vessel connected to a distillation unit was developed. The approach consisted of kinetic equations for the reactions as well as a dynamic model for the distillation unit. As an example reaction, the catalyzed polyesterification of maleic acid with propylene glycol was used. The kinetic and the mass transfer parameters at 170°C were estimated from experimental data. The fit of the model to the data indicated that the approach is suitable for the description of semibatch liquid phase reaction-separation systems.

Plant optimisation by retrofitting using a hierarchical method: entrainer selection, recycling and heat integration

Systematic procedures for reducing wastes in complex chemical plants are needed to allow efficient optimisation. A hierarchical procedure was applied to the optimisation of a real industrial plant to reduce wastes as well as energy and raw material consumption. In the case studied, the continuous production of methyl-butynol (MBI), acetylene reacts with acetone. The solvent ammonia and acetylene are recycled to the reactor. Unreacted substrates and by-products are separated from the product stream by distillation. Part of the unreacted acetone can be reused for other purposes after distillation. A substantial part of the unreacted substrates and by-products is delivered to a wastewater treatment plant. These waste streams constitute a substantial problem for the operation of this plant. First, waste streams were characterised and tracked back to their origin. Following the hierarchical design procedure, the overall input-output structure was fixed. The entrainer in the present process was critically examined and options were suggested. Then various recycle schemes were considered for later detailed study. The existing plant was simulated using ASPENPLUS. After adjusting the model to all important aspects of the real process scheme, excellent agreement between actual process performance data and simulation was obtained. The various process schemes were simulated and assessed for their economic and ecological performance. The objective functions used included utility, substrate and catalyst costs, as well as costs for wastewater treatment. Additionally, the environmental burden related to energy supply was accounted for by a carbon dioxide tax as suggested by the Nordic countries. The process changes included separation of unreacted acetone from the product stream and recycling to the reactor. By-products were converted back to substrates in an additional reactor separation system and recycled. In various simulated process configurations and operational schemes substantial economic and ecologic improvements were achieved. This study demonstrates the usefulness of hierarchical approaches combined with process simulation for plant optimisation.

Design targets of separator and reactor-separator systems using conceptual programming

In the absence of any systematic procedures, heuristics and hierarchical decomposition methods have been extensively used for screening and scoping of process flowsheets. This work introduces a new technology in this area in the form of Conceptual Programming. Conceptual Programming combines engineering insights and shortcut design models with mathematical programming technology. Using only basic information, Conceptual Programming targets the performance of the chemical process ahead of simulation and screens for the few promising layouts that need to be validated further with rigorous models. The basic idea of Conceptual Programming is first outlined with a sequencing problem where generic screening tool has been proposed as a mathematical programming model set up without the need for simulation, iterations or fine tuning of its parameters. Once constructed, the model is tested against an extensive list of examples without additional time or effort required in its development. The paper also illustrates applications of Conceptual Programming with complex separation system and reactor-separator problem. In all the cases Conceptual Programming outsmarts previous technology available in the form of heuristics, shortcut methods, pure mathematical programming or methods making use of expert systems and inference rules.

Effect of recycle on the plant zero dynamics

It is well known that material and energy recycle may have a profound influence on the plant dynamics. Previous studies have shown that the introduction of recycle typically increases the dominant time-constant and disturbance sensitivity of the plant. In this work we consider the effect of recycle on the zero dynamics and show that recycle may introduce severe overshoots and inverse responses. Sufficient conditions for the existence of nonminimum phase transmission zeros caused by the presence of recycle are derived. The conditions may be evaluated based on steady-state information about the individual units only. The results are illustrated through application to a reactor-separator system.

Reactive separation systems—I. Computation of physical and chemical equilibrium

New algorithms for the computation of the simultaneous chemical and physical equilibrium involved in simulations of reactive flash operations, calculations of phase diagrams, and the determination of reactive azeotropes are presented. These algorithms are based on the use of the elements balance approach where the mass balance equations and the Gibbs energy minimization problem are solved in terms of the elements balance variables and not in terms of component compositions. The total number of elements, which can be atoms, molecules or groups (radicals), is smaller than the number of components in the reactive system. The reduced number of variables allows the visualization of the phase diagrams for many multicomponent reactive systems in two- or three-dimensional figures. Element fractions which are by definition similar to the component mole fractions, are employed to determine the conditions for the existence of element azeotropes. These conditions are identical to those that apply for non-reactive (conventional) determination. Illustrative examples highlighting various features of the proposed algorithms are presented through four reactive systems.

Design Targets of Separator and Reactor-Separator Systems using Conceptual Programming

Design Targets of Separator and Reactor-Separator Systems using Conceptual Programming

Decomposed algorithmic synthesis of reactor-separation-recycle systems

A two-level algorithmic approach to synthesis of reaction-separation-recycle systems is presented. On the upper level, reactor conversion and purge fraction are optimized using a simple direct search method, and the flows among subsystems such as the reactor system and separation system are calculated. On the lower level, individual subsystems are optimized in every iteration of the upper level loop. On this level, the structure and operating conditions of the subsystems are determined, and the cost is returned to the upper level. This decomposed approach makes the synthesis of the overall system less complex and more robust than approaches addressing the entire problem simultaneously. The proposed approach is demonstrated on the well-known HDA process. The results are comparable to previously published results using the Hierarchical Decomposition Procedure, and the optimal results are demonstrated to be insensitive to initial estimates.

DYNAMIC BEHAVIOR OF REACTIVE DISTILLATION COLUMNS. EQUILIBRIUM SYSTEMS

This paper deals with the dynamic behavior of simultaneous reaction-separation systems which operate at or near the chemical equilibrium in the liquid phase. The process under study comprises a whole set of “instantaneous”, and very fast reversible reactions where the difference in volatilities favors both the progress of reaction and product separation. The main aim of our study is to gain a deeper understanding of the dynamic behavior of distillation columns by using a model that is simple and efficient, yet informative. This kind of model is outstanding for synthesis and design of control schemes which require a careful modelling and understanding of process response to different changes in the environment. We use a suitable transformation of variables (after Barbosa and Doherty, 1988b) in order to define a new set of state variables; as a result, the balance equations become identical to those for conventional distillation. Also, an efficient physicochemical algorithm that can handle both the original and new state variables is used. Thus, a composition-holdup dynamic model is simulated in the “transformed field” using a stage-by-stage approach. To further reduce computational time, the transformed problem has also been solved by means of a reduction procedure based on approximating by orthogonal polynomials the transformed composition and flow profiles in the column. The performance of the two proposed methods are compared by using the top section of a quaternary reactive column. The results obtained shown that reactive distillation dynamics has certain peculiarities derived from superimposing reaction and separation phenomena.

A unified approach for the simultaneous synthesis of reaction, energy, and separation systems

Previous studies on process integration have generally considered reaction and separation as processes that occur sequentially in a flowsheet. In this paper, a unified formalism is presented for the synthesis of reaction-separation systems, while ensuring optimal energy management. The synthesis model stems from a target-based approach for reactor networks due to an earlier study. It is shown that, by postulating a species-dependent residence time distribution function, one can arrive at a general representation for a reaction-separation network. Optimization of this distribution function leads to a separation profile as a function of time along the length of the reactor. The synthesis model is formulated as a mixed integer optimal control problem, where the integer variables account for the fixed costs of separation. The control profiles include the temperature, the separation profile, and residence time distribution defined for the network. Costs for maintaining a separation profile are handled through a separation index (defined to model the intensity of separation), and a fixed charge for any separation between two components in the reaction mixture. Also, using an energy targeting formulation, the maintenance of the optimal temperature profile is integrated to the energy flows within the flowsheet. Strategies based on simultaneous optimization and modelmore » solution are presented for the optimization problem and demonstrated for two case studies.« less

Bioreaction-separation on continuous chromatographic systems

Combined bioreaction and separation has been carried out successfully on a simulated moving column continuous countercurrent chromatographic reactor-separator (SCCR-S) system. The continuous inversion of sucrose to glucose and fructose using invertase and the biosynthesis of dextran from sucrose in the presence of the enzyme dextransucrase have been studied. 100% sucrose conversions were achieved and product purities of up to 95% w/v were obtained in the inversion studies. The operation was carried out on a preparative scale with throughputs in excess of 16 kg of dry sugar solids per cubic metre of resin per hour used. The simultaneous inversion and product separation was found to overcome problems associated with substrate inhibition. Complete reaction and separation was obtained at feed concentrations as high as 55% w/v. Previous studies on the biosynthesis of dextran on batch chromatographic systems showed significant improvements in product yields. The simultaneous removal of the acceptor by product fructose from the reaction mixture led to the formation of greater amounts of high molecular weight dextran (over 80% more than the conventional process at 20% w/v sucrose concentration). To carry out dextran biosynthesis on the SCCR-S system, it was necessary to repack it with larger-size resin to allow for the high pressures caused by the more viscous dextran. Complete conversions have been achieved at sucrose concentrations of 5% w/v with dextran and fructose product purities of up to 100% and 97% respectively obtained under certain conditions. Some levan formation occurred after about 50 h of operation, which is readily removed from the dextran in the normal dextran processing procedures.

In situ product separation in butanol fermentation by membrane-assisted extraction

Application of membrane-assisted extraction to butanol fermentation was investigated as a means of product separation and also as a way of alleviating the problems concerning the end-product inhibition. The coupled reactor-separator system was found to be stable enough to sustain continuous operation lasting several weeks. The data on continuous run reaffirmed most of the advantages found in our previous study on fed-batch system in that the reactor separator system rendered high productivity and yields due primarily to reduced product inhibition. The improvement in productivity was particularly notable, as a fourfold increase over straight batch operation was obtained. In normal continuous operation, spontaneous cell deactivation occurred after 200–400 h of operation despite the removal of inhibitory products. In further investigation, the presence of autolysin was found to be one of the probable causes of cell deactivation. The cell viability, however, was prolonged significantly when the bioreactor was operated under glucose-limited conditions.

Enzyme immobilization in an asymmetric charged membrane

An entrapped enzyme asymmetric membrane was prepared by treatment of a porous polypropylene or polyethylene film by plasma-induced graft polymerization of acrylic acid on one side and N-(2-methacryloyloxyethyl)- N,N,N-trimethylammonium chloride on the other, followed by enzyme absorption through the pores of the film. Such a single membrane is called an immobilized-enzyme bipolar membrane. In this system, the reaction of urea by urease is studied. Using Donnan equilibrium and electroneutrality conditions for 1-2-valent ions, the permeation behavior of the system is interpreted. Although this model is unsuitable for use when a current is flowing, it provides insight into membrane design because it depends on structural parameters such as fixed charge density in the equations rather than on nonequilibrium thermodynamic theory. This type of membrane is applicable for use in reaction-separation systems as a compact membrane reactor.

Optimal synthesis of reactor‐separator systems by nonlinear programming method

AIChE JournalVolume 34, Issue 1 p. 163-166 Article Optimal synthesis of reactor-separator systems by nonlinear programming method L. Pibouleau, L. Pibouleau E.N.S.I.G.C., UA C.N.R.S. 192, 31078 Toulouse, FranceSearch for more papers by this authorP. Floquet, P. Floquet E.N.S.I.G.C., UA C.N.R.S. 192, 31078 Toulouse, FranceSearch for more papers by this authorS. Domenech, S. Domenech E.N.S.I.G.C., UA C.N.R.S. 192, 31078 Toulouse, FranceSearch for more papers by this author L. Pibouleau, L. Pibouleau E.N.S.I.G.C., UA C.N.R.S. 192, 31078 Toulouse, FranceSearch for more papers by this authorP. Floquet, P. Floquet E.N.S.I.G.C., UA C.N.R.S. 192, 31078 Toulouse, FranceSearch for more papers by this authorS. Domenech, S. Domenech E.N.S.I.G.C., UA C.N.R.S. 192, 31078 Toulouse, FranceSearch for more papers by this author First published: January 1988 https://doi.org/10.1002/aic.690340123Citations: 13AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume34, Issue1January 1988Pages 163-166 RelatedInformation

Hydrogen and oxygen from water—V. the ROC system

A thermodynamic analysis of a two-membrane reactor-separator system is presented. The system can be made to operate at a reduced oxygen concentration (ROC) in the reactor-separator, thus greatly increasing the number and kinds of materials that may be considered candidates for separation membranes. The range of operation, thermal efficiencies and operating characteristics of an idealized model are presented. Candidate materials for hydrogen passing membranes are discussed. The unique characteristics of the ROC system make materials which might ordinarily be considered unsuitable in a reactor-separator environment worthy of experimental investigation.