1970 1975 1980 1985 1990 1995 2000 Year
Fig. 1. Journal publications on reactive and catalytic distillation over the last three decades according to the Science Citation Index and patents in these fields according to the Deutsches Patent- und Markenamt (www.depatis.net)
Moreover, since RD is going to become a more established technology, it has found its way into classic chemical engineering encyclopedias [11, 12],
However, a comprehensive volume covering all aspects of application, design, analysis, and control of RD processes is still missing, To fill this gap, the present book was prepared, Its chapters are written by leading international experts from both academic institutions and industrial companies, They summarize the present state of knowledge and give an outlook on challenging issues in the future,
The book is divided into four parts: Part I surveys various industrial applications and covers both established large-scale processes as well as new chemical reaction schemes with high future potential, Part II provides the vital details for analysis of reactive phase equilibria, and discusses the importance of chemical reaction kinetics, while Part III focuses on identifying feasible column configurations and the design of their internal structure, Analysis and control of the complex dynamic and steady-state behavior of RD processes are described in Part IV
Chapters 1-3 give a survey of chemical reaction schemes that are performed successfully in RD columns and present ideas for new applications, Sharma and Mahajani (Chapter 1) point out that RD has acquired its new status only recently in spite of the fact that the concept has been used in various processes since 1860s, Over the last two decades, especially after the commissioning of large-scale plants for MTBE and methyl acetate production, RD has been seen as a promising reactor/separator that can fulfill multiple objectives simultaneously, With respect to applications, engineers and chemists have started looking beyond the classic esterification and etherification reactions, Hydrogenation, hydrodesulfurization, isomerization, and oligomerization are some of the unconventional examples to which RD has been successfully applied on a commercial scale, Moreover, hydrolysis, alkylation, acetalization, hydration, and transesterification have also been identified as potential candidates for RD, Another important area of application is the removal of small amounts of impurities to obtain high quality product (e, g,, phenol), RD can also be used for the recovery of valuable products like acetic acid, glycols, lactic acid, and so on from waste streams,
Schoenmakers and Bessling (Chapter 2) give an overview of the tools that are available today and the methods that are now introduced in the industrial practice of chemical companies, A process synthesis procedure gives good qualitative reference points, Simulation tools have been developed that are mainly based on equilibrium models, But there are further steps to go on the way to the realization of an industrial plant, The scale-up from the miniplant scale used for the experimental validation of a new process is well known for conventional distillation, but complicated by several facts for RD especially in the case of heterogeneous catalysis, To overcome these problems either reference plant experience on an industrial scale or (if not available) further research is required, Other options both for homogeneous and heterogeneous catalysis are possible and are discussed in the contribution. The authors emphasize that the combination of reaction and distillation not necessarily has to be operated in a counter-current column. For slower reactions a broad variety of equipment not necessarily containing columns can be used.
In Chapter 3, Tuchlenski and colleagues illustrate a procedure for process design on an industrial scale using the decomposition of the fuel ether MTBE into methanol and isobutene as an important example. Based solely on thermodynamic considerations, a plausible column configuration is derived. In order to study the scale-up of structured packing, experiments were performed on the lab scale as well as on the pilot scale. While lab scale experiments could be described satisfactorily with a simple equilibrium stage model, the same approach failed in the case of pilot plant experiments. Hydrodynamics, maldistribution and/or mass-transfer limitations might be a reasonable explanation and are worth more thorough investigation. The authors conclude that pilot plant operation is indispensable to establish a heterogeneously catalyzed RD process.
Chapters 4 and 5 are dedicated to the thermodynamic and kinetic fundamentals of RD processes. In Chapter 4, Hasse reviews the fundamentals of thermodynamic modeling of simultaneous phase and reaction equilibria. The author emphasizes the importance of consistency of phase equilibrium models. Thermodynamic consistency provides a sound basis for developing predictive reaction models for RDs, which are valid over a wide range of concentrations. To develop phase equilibrium models, reliable experimental data of phase equilibria in reactive systems have to be available. For successful measurements, suitable experimental techniques are needed, which are briefly summarized in this chapter. Criteria for their selection are also given.
Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfacial mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples.
Chapters 6-8 focus on process design, i. e. determining suitable column configurations, suitable operating conditions, and suitable column internals. Chapter 6 by Doherty and co-workers is concerned with conceptual process design. Geometrical methods are provided, which allow at the initial stages of process development to decide quickly whether RD is likely to be a good process concept. The attainable region approach for reaction-mixing systems is applied to systems with simultaneous reaction and separation in order to assess the possible selectivity-yield-conversion benefits of this technology. Feasible direct and indirect sharp splits are predicted with a model in which each column section is represented by a series of co-current isobaric flashes. In the limits of no reaction, or of chemical equilibrium, the model reduces to conventional models for distillation lines, and each column section can be represented by the same equations. However, at intermediate rates of reaction the models for the column sections are different, and new results are obtained. A bifurcation study shows the limits of feasibility including the influence of flow rate, catalyst level and holdup. Unlike distillation without reaction, limited ranges of feasibility in all of these variables are found.
Chapter 7 by Krishna is concerned with hardware selection and design for RD columns. An overview on available hardware for homogeneously as well as hetero-geneously catalyzed RD processes is given. Criteria for suitable hardware selection are discussed and illustrated by different case studies. It is shown that the requirements for hardware selection are different from conventional non-RD. The author concludes that especially for heterogeneously catalyzed RD processes it is almost impossible to reconcile the conflicting requirements and introduces the side reactor concept as a promising alternative to overcome many of these conflicting hardware issues.
Chapter 8 by Kunz and Hoffmann introduces a special catalyst technology developed by the authors. State of the art in industrial catalyst technology is to use catalyst particles with a size in the millimeter range and to sew these particles into a wire mesh or glass-fiber clothing to form structured packing. The manufacturing process makes this type of packing expensive. In contrast to this, unstructured packing materials like Raschig rings are much cheaper. However, so far, Raschig rings with comparable catalytic activity are not commercially available. In the present contribution the authors introduce various methods for the preparation of cat-alytically active rings by polymerization of ion-exchange resin into the pores of a carrier material. Application is tested for MTBE synthesis. Based on this technology monolithic polymer/carrier materials were developed, which can be used for other reactive separation processes like reactive chromatography and polymerassisted solution-phase organic synthesis.
Chapters 9 and 10 are on modeling, dynamics, and control of RD processes. The contribution by Taylor and Krishna (Chapter 9) deals with the modeling of homogeneously and heterogeneously catalyzed RD processes. The focus of this contribution is on steady-state behavior. First, the equilibrium stage model is introduced, which is readily obtained from the non-reactive case by adding reaction terms. Afterwards, non-equilibrium stage models are introduced as a more rigorous approach. It is shown that different types of non-equilibrium models apply to homogeneously and heterogeneously catalyzed processes. In the homogeneous case, further distinction has to be made between slow and fast liquid-phase reactions. In the heterogeneous case distinction between negligible and finite intrapar-ticle diffusion inside the catalyst is essential. Finite intraparticle diffusion can be modeled with the dusty fluid model, which is an extension by the authors of the well-known dusty gas model. Finally, cell models are introduced to account for non-ideal flow patterns on distillation trays and maldistribution in packed columns. Equilibrium and non-equilibrium models are compared for different process applications including MTBE, TAME, and ethylene glycol synthesis and a perspective on the use of non-equilibrium models in RD process design is given.
The final contribution by Kienle and Marquardt (Chapter 10) gives an overview of the present knowledge of non-linear dynamics and control of RD columns. First, focus is on open-loop dynamics. It is shown that RD processes can sometimes show an intricate non-linear dynamic behavior, a profound understanding of which is not only of scientific interest but also very important for improved process design and operation. Basic terminology, methods, and tools are introduced for analyzing and understanding non-linear dynamics. Three different types of reaction systems are introduced including esterification, etherification, and the ethy-lene glycol system. Different patterns of behavior are identified depending on the reaction systems and the operating conditions. In the equilibrium regime of the chemical reaction the dynamic behavior of a RD column is qualitatively similar to a non-RD column, whereas in the kinetic regime the chemical reaction rate is dominating. For closely boiling mixtures, like in many etherification processes, the behavior in the kinetic regime is very similar to a single phase isothermal reactor. Additional effects arise for mixtures of components with completely different boiling points as for the ethylene glycol system, for example. In the second part, available guidelines for control structure selection and control system design are summarized. Emphasis is on the equilibrium regime. Here, similar methods as in non-RD apply. Additional complexity is introduced in inferential control schemes, where temperature is used as a cheap, fast, and reliable measurement instead of concentration. The authors conclude that control studies for kinetically controlled processes are missing to a large extent.
The present book is the outcome of the 1st International Workshop on Reactive Distillation held at the Max Planck Institute for Dynamics of Complex Technical Systems in Magdeburg, Germany, on 2-3 July 2001. The goal of this workshop was to bring together world-renowned pioneering scientists and leading industrial experts in the field of RD on a common platform. The workshop was organized by the editors and their colleagues with financial support from the Kompetenznetz Verfahrenstechnik Pro3 e. V., Germany, which is gratefully acknowledged.
The editors would like to thank their colleagues Nancy Eckstein, Erik Stein, and Frank Steyer for their support in organizing the Max Planck workshop. We are also very grateful to Erik Stein, Martin Häfele, Henning Schramm, and Zhiwen Qi for their support in collecting the manuscripts and in standardizing formats. Last but not least, we are very thankful to Karin Sora of Wiley-VCH for her helpful assistance during the preparation of the book.
Max-Planck-Institut für Dynamik komplexer technischer Systeme Magdeburg, Germany April 2002
Kai Sundmacher and Achim Kienle
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