Reactive Separation Processes Research Articles

Mesoporous Silica Composites Containing Multiple Regions with Distinct Pore Size and Complex Pore Organization

Porous ceramics are of great interest for filtration, catalysis, and reactive separation processes. Performance in these applications is highly dependent on features such as pore size distribution and connectivity and wall composition. Here, we describe a method allowing the rational design and synthesis of mesoporous silica composites with controlled heterogeneous pore architectures and demonstrate its validity by producing structures with predetermined placement of regions having different pore size and pore organization.

Effect of polymer nature and hydrodynamic conditions on a process of polymer enhanced ultrafiltration

Technical viability of two polymers, poly(ethyleneimine) and poly(acrylic acid), with different acid-basic behaviour, structure and molecular weight, in a reactive separation process for the recovery of nickel ions across zirconia ultrafiltration tubular membranes, has been studied in a laboratory-scale installation. Suitable experimental conditions (e.g., pH, feed flow rate, polymer and metal concentrations, transmembrane pressure) for the two steps of the process (metal recovery and polymer regeneration) have been optimised and justified with acid–base behaviour of each polymer and measures of polymer–nickel complex formation constants. For this reason, global stability constant of PAA–Ni and conditional complex formation constant of PEI–Ni were measured by potentiometric and spectrophotometric analyses, respectively. Furthermore, the effect of hydrodynamic conditions on two important factors limiting permeate flux (concentration polarization and fouling) have been analysed. The mass transfer coefficient in our system has been studied by means of stagnant layer model and dimensionless analysis.

A green and effective method to synthesize ionic liquids: supercritical CO2 route

Many room temperature ionic liquids (ILs) are nonvolatile solvents which have huge potential applications in various chemical processes. However, the synthesis of ILs usually uses different kinds of volatile organic solvents in the reaction and subsequent separation process. In this work, supercritical (sc) CO2, an environmentally benign solvent, has been utilized as the medium to synthesize ILs, 1-butyl-3-methylimidazolium bromide ([bmim]Br) and 1,3-dimethylimidazolium trifluoromethanesulfonate ([Me2Im]TfO). The results show that ILs can be synthesized in scCO2 with 100% yield and the excess reactants added can be extracted in situ by scCO2 without any cross-contamination. The whole process is green and very effective.

On the combination of CFD and rate-based modelling in the simulation of reactive separation processes

Reactive separations (RS) combining mass transfer with simultaneous chemical reactions inside one column unit provide an important synergistic effect and bring about several advantages. However, the design and modelling of the RS columns are more sophisticated than traditional operations and thus rigorous and reliable description methods are necessary. Besides, in RS, the influence of column internals increases significantly. These internals have to enhance both separation and reaction and maintain a sound balance between them. This is the reason why, frequently, the application of process-specific column internals, rather than internals currently available on the market, is desirable. In this paper, a new modelling methodology for RS is suggested which exploits a combination of modern CFD facilities and the rate-based process simulation approach. The task of CFD is to obtain hydrodynamic and mass transfer correlations necessary for the process description. These correlations are then used in a rate-based simulator to determine required column profiles of the process variables. Both CFD and process simulation are described and illustrated with industrially relevant examples. The sensitivity studies provide information on the influence of important hardware parameters and show optimisation tendencies. The CFD simulations can be regarded as virtual experiments carried out without really existing column units which help to predict the performance of the internals by varying their geometrical and structural properties. Supplemented by the rate-based simulations, the suggested method can be viewed as a principal way towards the virtual prototyping of internals.

Application of signed digraphs-based analysis for fault diagnosis of chemical process flowsheets

Recently, Maurya et al. (Ind. Eng. Chem. Res. 42 (2003b, c) 4789,4811) have presented a comprehensive framework for signed directed graph-based analysis of process systems where major theoretical results have been substantiated with simple examples or individual unit-based case studies. In this article, two case studies are presented to illustrate SDG-based analysis of process flowsheets containing many units and control loops. While the literature is replete with single unit examples, flowsheet level analysis as described in this paper is virtually non-existent. The first case study deals with prediction of initial response and its fault diagnostic application in the Tennessee Eastman (TE) flowsheet using a lumped parameter model of the process. The second case study deals with the steady-state analysis and fault diagnosis (FD) of a reaction–separation process. For this case study, the overall signed digraph for the process is developed from the digraphs for individual units and control loops in the flowsheet. It is shown that digraph-based steady-state analysis results in good diagnostic resolution.

Equilibrium theory and nonlinear waves for reactive distillation columns and chromatographic reactors

A general framework for analyzing and understanding the dynamics of reactive separation processes is developed. The theory is based on the assumption of simultaneous phase and reaction equilibrium. It makes use of transformed concentration variables, which were first introduced by Doherty and co-workers for the steady state design of reactive distillation processes (Proc. Roy. Soc. London A 413 (1987a) 459). It is shown that these transformed variables can be directly generalized to the dynamic problem considered here. Further, they can also be applied to other reactive separation processes like fixed bed as well as countercurrent chromatographic reactors, for example. They provide profound insight into the dynamic behavior of these processes and reveal bounds of feasible operation caused by reactive azeotropy. It is shown that reactive azeotropy, which is a well-known phenomenon in reactive distillation (Proc. Roy. Soc. London A 413 (1987b) 443) may also rise under very similar conditions in other reactive separation processes like chromatographic reactors for example.

Adaptive Control of an ETBE Reactive Distillation Column

Reactive distillation is a favourable alternative to conventional series of reaction-separation processes. It can reduce both operating costs as well as capital investments. However, its control is challenging due to its complex dynamics resulting from its integrated functionality of reaction and separation. Linear, e.g. PID, control with fixed parameters has been shown not to be satisfactory to handle its high nonlinearity. It needs to be re-tuned adequately over a wide range of operating conditions. In this work, the application of two adaptive PI control strategies, e.g. non-linear PI (NPI) and model gain-scheduling (MGS), is investigated for an ETBE reactive distillation column. Their control performance to handle the nonlinearity and reduce the unwanted variability is discussed. The simulation results show that the proposed control strategies outperform a standard PI controller in both set-point tracking and disturbance rejection.

Combined Gain-Scheduling and Multimodel Control of a Reactive Distillation Column

Reactive distillation (RD) is a favourable alternative to conventional series of reaction-separation processes. Control of RD is challenging due to its integrated functionality and complex dynamics. Linear PID algorithm is not satisfactory and needs because of the need for adequate retuning over a wide range of operating conditions. Combined gain-scheduling and multimodel control scheme is proposed to handle the nonlinearities of the process. Simulation results show the superior performance of the proposed method to that of a standard PI control.

The impact of interfacial mass transfer on the feasible products of countercurrent reactive separation processes

Abstract For the conceptual design of continuous countercurrent separative reactors, there is a need to predict the composition of the feasible top and bottom products. These products can be identified as stable singular points of the rectifying and stripping sections at infinite column height. General singular point equations are derived for the top and bottom products of separative reactors at countercurrent operation. They can be applied to phase equilibrium-controlled processes as well as to mass-transfer-controlled processes. For a given reactive mixture, all singular points at the top and at the bottom of a column are located on a unique potential singular point surface. It is demonstrated that interfacial vapour–liquid mass transfer has a strong impact on the feasible reactor products. Reactive distillation and reactive pervaporation of selected ideal and real reaction systems are used as illustrative examples.

Model Gain Scheduling Control of an Ethyl tert-Butyl Ether Reactive Distillation Column

Reactive distillation (RD) is a favorable alternative to the conventional series of reaction-separation processes. However, the control of RD is challenging because of its complex dynamics resulting from the integrated functionality of reaction and separation. A linear proportional-integral-derivative algorithm with fixed parameters has been shown not to be satisfactory to handle the high directionality of the process gain. It needs to be retuned adequately over a wide range of operating conditions. In this work, model gain scheduling is investigated for an ethyl tert-butyl ether reactive distillation column. It employs a set of simple local models, which cover relevant operating conditions and cope with the nonlinear characteristics of the process. The simple models are then integrated by using a proper switching scheme. Simulation results have shown that the proposed control strategy outperforms the standard proportional-integral control in both set-point tracking and disturbance rejection.

On the development of new column internals for reactive separations via integration of CFD and process simulation

Reactive separations combining mass transfer with simultaneous chemical reactions impose additional requirements on the applied column internals. Traditionally, reactive separation processes have been optimised via operational parameter adjustment. By this way, a direct influence of the internals geometry and structure cannot be taken into consideration. Therefore, the application of process-specific column internals, rather than of the internals currently available on the market, is desirable. Development of new internals is a major focus of interest in an European project entitled Intelligent Column Internals for Reactive Separations. In particular, such development is achieved by the application of modern CFD facilities combined with the rigorous, rate-based process simulation. An important challenge is to provide an opportunity of obtaining hydrodynamic and mass-transfer correlations, necessary for the process description, not only from the experimental measurements, but also directly from the CFD simulations. In this paper, a way to generate virtual correlations is demonstrated for a single-phase flow through structured packings. The rate-based simulation is illustrated with the heterogeneously catalysed synthesis of n-hexyl acetate via reactive distillation.

A general approach for the conceptual design of counter-current reactive separations

This paper is on a number of general aspects in the preliminary design of reactive separations. It provides a method for easy identification of reaction processes that can be improved by application of reactive separation. The method is based on analyzing chemical reactions by making a distinction between intensive and extensive concentration requirements for the involved chemical substances. Conflicting concentration requirements are indicators for a potentially attractive application of reactive separation. Here, the focus is mainly on improving the performance of the reaction process. Guidelines are given for the selection of a suitable separation technology on the basis of the intensive and extensive concentration assessment, together with a checklist on various practical issues. Further, a method for conceptual process design of a multi-stage counter-current reactive separation process is given, which is guided by the extraction factors of all relevant reaction ingredients. Thus, the technical feasibility of such a reactive separation process can be assessed in an early stage of the development. The resulting process concept is a good starting point for more detailed process development work.

Modelling of reactive separation processes: reactive absorption and reactive distillation

In the last years chemical process industries have shown permanently increasing interest in the development of reactive separation processes (RSP) combining reaction and separation mechanisms into a single, integrated unit. Such processes bring several important advantages among which are increase of reaction yield and selectivity, overcoming thermodynamic restrictions, e.g. azeotropes, and considerable reduction in energy, water and solvent consumption. Important examples of reactive separations are reactive distillation (RD) and reactive absorption (RA). Due to strong interactions of chemical reaction and heat and mass transfer, the process behaviour of RSP tends to be quite complex. This paper gives an overview of up-to-date reactive separation modelling and design approaches and covers both steady-state and dynamic issues. These approaches have been applied to several different RA and RD processes including the absorption of NO x , coke gas purification, methyl acetate synthesis and methyl tertiary butyl ether (MTBE) synthesis.

Process synthesis for reactive separations

The integration of reaction and separation in one single process unit proved to lead to enormous savings in capital as well as in operating costs. This paper provides a systematic framework to consider reactive separations in the early stages of process development. It highlights a process synthesis procedure guaranteeing that processes like reactive distillation, reactive crystallization, reactive extraction or reactive stripping will not be overlooked during conceptual flowsheet development. First step of this procedure is the analysis of the chemical reaction system. In case a separation of one or more products, by-products or solvents seems to be beneficial for the performance of the reactor the physical separation behavior of the components has to be checked. The chemical reaction, the physical separation and the design on the apparatus need a defined operation window. Only when an overlap of these windows can be identified a reactive separation process is feasible and might lead to advantages in the overall process performance. The paper finally demonstrates that the integration of reaction and separation in one unit does not necessarily lead to economic benefits as the overall performance of the total chemical process has to be considered.

Control of a reactive separation process

We discuss general aspects of the control of reactive separation processes and specifically the example of the control of a semi-batch reactive distillation process. It is pointed out that more complicated controller structures, more sophisticated controller design methods, and alternative, model-based nonlinear controllers are needed compared to conventional processes. For the example process, we discuss both conventional control structures and model-based predictive control using a neural net plant model.

Design and operation of an integrated membrane reactor for enzymatic cellulose hydrolysis

A purposely built membrane reactor with combined reaction and separation zone in a single piece of equipment was tested for heterogeneous enzymatic hydrolysis of a hardwood derived cellulosic material. The design and operation features a process of simultaneous reaction and in situ removal of products (reducing sugars) in order to reduce the effect of product inhibition on the cellulase enzyme. The integrated reactor system differs from the hybrid integration of reaction-separation system where the reaction takes place in a stirred reaction vessel and the separation of the product is carried out in a separate cross-flow membrane filtration unit. Various operating strategies of semi-continuous and continuous product removal and substrate feeding were examined and optimised in respect to their effect on reducing product inhibition and increasing reactor productivity and product yield. The effect of total enzyme concentration and possible enzyme deactivation by shear stress were also investigated. The findings from examining the performance of a well understood enzymatic system in a new integrated reactor system would facilitate the understanding and development of system integration and process intensification in combined reaction-separation process which may alleviate common problems (product inhibition, inefficient interfacial mass transfer, slow kinetics and low reactor productivity) in heterogeneous enzyme catalysis systems. The cellulose substrate conversion ratio was 53% in the continuous operation of reaction with simultaneous product removal. This compares favourably with 35% substrate conversion in traditional batch operations. The improvement in overall reaction rate through adopting an in situ product removal strategy was rather limited, mainly because of the inherent slow reaction kinetics of the heterogeneous cellulose hydrolysis and the complex interfacial interactions between the cellulase enzymes and the solid cellulose substrate.

Electrochemical chlorine absorption in cyclone membrane reactor: analysis of reaction mechanism and transport phenomena

A new reactive separation process is presented and analyzed which is based on simultaneous, dispersionless gas–liquid absorption and electrochemical reaction in the pore structure of electrically conductive membranes. As a model process of technical and environmental relevance, the electrochemical absorption of chlorine waste gases in hydrochloric acid is studied, both experimentally and theoretically. The membranes were manufactured from porous carbon black particles by rolling agglomeration. The reaction mechanisms and mass transport phenomena within these membranes were investigated in a novel cyclone flow reactor. With this membrane reactor, a series of experiments was carried out under control of membrane electrode potential using chlorine–nitrogen gas mixtures (1000 ppm Cl 2). A model-based analysis of experimental data reveals the electrochemical reaction microkinetics to follow the Volmer–Heyrovsky mechanism. Mass transport was found to be dominated by Knudsen diffusion in the membrane micropores.

ALTERNATIVE GENERATION OF H2, CO AND H2, CO2 MIXTURES FROM STEAM-CARBON DIOXIDE REFORMING OF METHANE AND THE WATER GAS SHIFT WITH PERMEABLE (MEMBRANE) REACTORS

A new process is proposed which converts CO2 and CH4 containing gas streams to synthesis gas, a mixture of CO and H2 via the catalytic reaction scheme of steam-carbon dioxide reforming of methane or the respective one of only carbon dioxide reforming of methane, in permeable (membrane) reactors. The membrane reformer (permreactor) can be made by reactive or inert materials such as metal alloys, microporous ceramics, glasses and composites which all are hydrogen permselective. The rejected CO reacts with steam and converted catalytically to CO2 and H2 via the water gas shift in a consecutive permreactor made by similar to the reformer materials and alternatively by high glass transition temperature polymers. Both permreactors can recover H2 in permeate by using metal membranes, and H2 rich mixtures by using ceramic, glass and composite type permselective membranes. H2 and CO2 can be recovered simultaneously in water gas shift step after steam condensation by using organic polymer membranes. Product yields are increased through permreactor equilibrium shift and reaction separation process integration. CO and H2 can be combined in first step to be used for chemical synthesis or as fuel in power generation cycles. Mixtures of CO2 and H2 in second step can be used for synthesis as well (e.g., alternative methanol synthesis) and as direct feed in molten carbonate fuel cells. Pure H2 from the above processes can be used also for synthesis or as fuel in power systems and fuel cells. The overall process can be considered environmentally benign because it offers an in-situ abatement of the greenhouse CO2 and CH4 gases and related hydrocarbon-CO2 feedstocks (e.g., coal, landfill, natural, flue gases), through chemical reactions, to the upgraded calorific value synthesis gas and H2, H2 mixture products.

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.

Synthesis of prepolymerization stage in polycondensation processes

AbstractA systematic design procedure of the prepolymerization stage of a polycondensation process is presented. This procedure treats polycondensation processes as reactive separation processes and offers an alternative to design through simulations. Feasible flowsheets and operating conditions are developed by analyzing the phase behavior of polycondensation systems at thermodynamic equilibrium. Functional end groups and apparent mole fractions are used to represent reaction kinetics and equilibrium, and real mole fractions are used to represent phase equilibrium. Reactive phase diagrams are plotted in transformed mole‐fraction space to facilitate visualization of the phase behavior. Three examples are presented: manufacture of polyethylene terephthalate (PET) from dimethyl terephthalate (DMT) and ethylene glycol (EG), from terephthalic acid (TPA) and EG, and manufacture of nylon‐6,6. For these processes, the results of the approach agree excellently with detailed iterative performance simulations. The approach also reveals additional feasible process alternatives and corresponding operating conditions.