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Application of Reactive Distillation and Strategies in Process Design

T. Frey, F. Nierlich, T. Popken, D. Reusch, J. Stichlmair, and A. Tuchlenski 3.1

Introduction

Mixed C4 streams, containing butadiene, butenes, and butanes, are co-produced in steam-cracking processes. These streams contain valuable components, which can be either processed in situ or extracted purely. A typical composition of a steam-cracker C4 cut is shown in Fig. 3.1. Maximizing its value is a major objective for most petrochemical companies. Therefore, a variety of techniques exists for upgrading the C4 streams by removal of pure C4 components and conversion of low-value streams to higher-value products.

Over the last decade reactive distillation (RD) has become a key technology for meeting increased productivity demands. The best-known example in C4 chemistry is given by MTBE (methyl tert butyl ether) synthesis. Both the CD Tech process and the Ethermax process by UOP consist of fixed-bed reactors followed by an RD col-

n-Butane 7%

Inerts 5%

n-Butane 7%

Mtbe Process Ethermax

1,3-Butadiene 40%

rans-Butene 4%

Butene-1 15%

Fig. 3.1. Typical composition of steam-cracker based C4 cut ([18], reprinted from Chem. Eng. Sei., Vol 57, Beckmann et al., Pages 1525-1530, Copyright 2002, with permission from Elsevier Science)

1,3-Butadiene 40%

rans-Butene 4%

Butene-1 15%

Fig. 3.1. Typical composition of steam-cracker based C4 cut ([18], reprinted from Chem. Eng. Sei., Vol 57, Beckmann et al., Pages 1525-1530, Copyright 2002, with permission from Elsevier Science)

umn to reach high conversions. More than 90% conversion, limited by chemical equilibrium, occurs in the fixed-bed reactors, the rest in the RD column.

Besides MTBE synthesis, other attractive applications of RD can be found in C4 processing, for example in the production of isobutene. Pure isobutene demand is forecast to reach 0.8 million metric tons in Europe by 2010. Production of polybu-tenes will probably dominate the demand for isobutene, with an average annual growth rate of 3.9%.

Pure isobutene can be obtained from a number of sources: raffinate I, pseudoraffinate (the product stream from selectively hydrogenated mixed C4s), FCC (Fluid Catalytic Cracking) C4 raffinate, isobutane dehydrogenation, etc. In general, conventional distillation fails because of the very close boiling points of C4 components. In the case of streams like raffinate I or hydroraffinate, isobutene can be extracted either by cold acid extraction with sulfuric acid or by conversion to an oxygenated intermediate, for example MTBE, and subsequent back-cracking of this stream. The production cost of pure isobutene depends mainly on the C4 source. However, MTBE decomposition is becoming the preferred way of producing isobutene since it can be integrated easily into refinery processes.

Fig. 3.2 shows the flow diagram of such a reactive-separation process [1]. An iso-butene-containing stream, for example raffinate I, is fed to a standard MTBE synthesis plant, where the reactive component isobutene is converted to the intermediate MTBE while the inert C4 components are separated as distillate. Pure MTBE enters a second RD column and is decomposed. Isobutene, as the low-boiling component, is recovered at the top, while the bottom product methanol, acting as a reactive entrainer, is recycled to MTBE synthesis.

This promising way of producing isobutene will be discussed below. A further objective is to illustrate our understanding of process design for RD from the industrial point of view.

Fig. 3.2. Process scheme of reactive separation for the production of isobutene by MTBE synthesis and subsequent decomposition of MTBE ([18], reprinted from Chem. Eng. Sei., Vol 57, Beckmann et al., Pages 1525-1530, Copyright 2002, with permission from Elsevier Science)

Challenges in Process Design for Reactive Distillation

A strategy for designing and developing reactive distillation processes involves three stages: feasibility analysis, catalyst and hardware selection, and column scale-up.

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