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recent trends in distillation
Abstract
Last few years have seen a dramatic rise in the number of applications of reactive distillation (RD). This useful technology is now being applied for any scale of operation- from manufacture of fine chemicals to that of bulk chemicals. This article reviews the very recent applications and serves as a supplement for the exhaustive review on this subject by Sharma and Mahajani (2003). It not only furnishes the information on the ongoing research in the existing applications but also elaborates the newly discovered applications such as manufacture of phenol, linear alkyl benzene, carbonates, chlorosilane derivatives and chiral chemicals. Multiple reactions in RD, failure of RD in some cases and new RD configurations to increase the overall yields are some of the additional aspects being covered.
INTRODUCTION
Distillation is one of the most commonly used separation techniques in the chemical industry spanning from refineries and petrochemicals to fine chemicals. Though distillation is one of the mature technologies, there still remains a lot to be explored with the advent of its variants like Reactive distillation, thermally coupled columns etc. There are growing contributions even on the theoretical front to give better insight and enable easier design of the distillation columns. This course aims to give an overview of all these developments through theory and experiments.Reactive Distillation (RD) is a combination of reaction and distillation in a single vessel owing to which it enjoys a number of specific advantages over conventional sequential approach of reaction followed by distillation or other separation techniques. Improved selectivity, increased conversion, better heat control, effective utilization of reaction heat, scope for difficult separations and the avoidance of azeotropes are a few of the advantages that reactive distillation offers. The introduction of an in situ separation in the reaction zone or vice versa leads to complex interactions between vapor–liquid equilibrium, mass transfer rates, diffusion and chemical kinetics, which poses a great challenge for design and synthesis of these systems. RD being a relatively new field, the research on various aspects such as modeling and simulation, process synthesis, column hardware, non-linear dynamics and control etc. is in progress. The suitability of RD for a particular reaction depends on various factors such as volatilities of reactants and products, reaction and distillation temperatures etc. and hence, the use of RD for every reaction may not be feasible.
Exploring the candidate reactions for RD, itself is an area that needs considerable attention to expand the domain of RD processes. RD has been successfully used and investigated in the past for several reactions such as etherification, esterification, hydrogenation, hydrodesulfurisation and polymerization. Various reviews have been published on this aspect (Doherty and Buzad, 1992; Podrebarac et al., 1997; Taylor and Krishna, 2000; Mahajani and Chopade, 2001; Sharma and Mahajani, 2003 and Ng and Rempel, 2003). However, even after the recent review by Sharma and Mahajani (2003) that was presented in the first international workshop on reactive distillation held at Magdeburg, Germany in 2001, an increasing number of articles have appeared in the last couple of years. Figure 1, which is an updated version of the recently published statistics (Malone and Doherty, 2000), shows that there are around 180 papers and 100 patents published in the last two years on reactive distillation alone. This clearly reveals the increasing interest in this area. Apart from the theoretical aspects of RD, we see the research being performed mainly in two different directions, firstly, in improving the performance of RD for the existing applications and secondly, in exploring new applications. Hence, considering the growing rate of publications, we felt it necessary to review this voluminous information shortly after the gap of about 2-3 years. The present article is intended to serve as a supplement to the review by Sharma and Mahajani (2003) which we feel the reader to go through as well, in order to get an idea of the entire spectrum of reactions that comes under the umbrella of reactive distillation.
Membrane distillation (MD) has gained significant regard from industrial and academic perspective in recent years, thus the frequency of publications related to the field has greatly accelerated. New perspectives have boosted the research activities related to deeper understanding of heat and mass transport phenomenon, novel applications and fabrication of the membranes specifically designed for MD. New efforts for module fabrication and understanding and control of non-traditional fouling in MD have also been highlighted in the recent literature. The current review summarizes the important and interesting recent developments in MD from the perspectives of membrane fabrication, heat and mass transport phenomenon, nontraditional fouling, module fabrication and applications. The future research directions of interest have also been pointed out.
Objectives
The course starts by refreshing the basics of distillation and phase equilibrium, and then concentrates on each of the different distillation types. The conventional distillation systems with their unique features are dealt with in the beginning. Focus then shifts to azeotropic distillation (both homogeneous and heterogeneous), extractive distillation and Reactive distillation, which are both challenging as well as have novel theoretical tools being developed to understand them better. A few lectures also address other related issues that are of relevance like Heat-Integrated Distillation Columns (HIDC), some features of process control of distillation columns, etc.Experiments to generate VLE data and obtain the interaction parameters using a commercial simulator, and distillation experiments on a lab-scale plant, are a part of the course. The ability of a simulator to predict this experimental data using an appropriate model is demonstrated in the tutorials. Use of tools like conceptual design in obtaining a column sequence, deciding the product purity for a given feed composition, etc. are also dealt with. Talks by experts from the industry on relevant issues are also included.
