KINETICS, CATALYSIS, AND REACTION ENGINEERING Process Analysis for Dimerization of Isobutene by Reactive Distillation Ravindra S. Kamath, ²,‡ Zhiwen Qi, ² Kai Sundmacher,* ,²,§ Preeti Aghalayam, ‡ and Sanjay M. Mahajani ‡ Max-Planck-Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, D-39106 Magdeburg, Germany, Process Systems Engineering, Otto-Von-Guericke-UniVersity Magdeburg, UniVersita ¨tsplatz 2, D-39106 Magdeburg, Germany, and Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, 400076 Mumbai, India Alkylates are a class of probable replacements for MTBE as gasoline additives that can be produced by dimerization of isobutene (to isooctene) with subsequent hydrogenation. The characteristics of the dimerization reaction make it a potential candidate for reactive distillation. The dimer, being heavier than C 4 , can be maintained at a low concentration level in the reactive zone by simultaneous distillation, thereby suppressing the subsequent oligomer-producing reactions. In this work, the influence of important design and operating parameters on the performance of the reaction in a hybrid reactive distillation column is studied through process simulations. the results show that a high selectivity toward diisobutene can be achieved along with adequate temperature control in the presence as well as absence of polar components. Multiple steady states are observed in some cases that introduce additional complexities in the determination of the optimal windows for certain parameters. The process seems economically attractive, as it is capable of utilizing the existing reactive distillation assets and the feedstock for MTBE production by suitable revamping. Introduction As a widely used gasoline additive, methyl tert-butyl ether (MTBE) is being phased out after a suit against such a ban in California. 1 Therefore, the petrochemical industries are currently seeking replacements for MTBE. The leading candidates at this point appear to be alkylates that have an average octane number in the range of 93-96. A potential source of alkylates is isooctane formed by the dimerization of isobutene (to the dimer diisobutenes, DIB) with subsequent hydrogenation. 2 This process is especially attractive because it makes use of the existing MTBE feedstocks and plants following a simple and low-cost revamp. A successful world-scale plant is based on Fortum’s NExOCTANE technology. The isobutene dimerization reaction takes place in the liquid phase in a fixed-bed reactor with acidic ion-exchange resin as the catalyst. The produced dimers and byproducts [e.g., isobutene oligomers such as the trimer triisobutenes (TIB), tetramer, etc.] are separated from the unreacted isobutene and inert C 4 compounds. The dimerization of isobutene using ion-exchange resins has been investigated in the absence of polar components 3,4 and as a side reaction of the MTBE synthesis 5-7 where methanol is the polar component. The reaction, in the simplest sense, can be represented as a series-parallel network as shown in Figure 1. The reaction is highly exothermic (ΔH R )-19.8 kcal/mol) 8 and leads to the formation of undesired oligomers, which not only are unsuitable as gasoline additives but also promote catalyst deactivation. 6 The presence of polar solvents inhibits the accessibility of isobutene to the active centers on the catalyst, thereby reducing the reaction rates with a simultaneous decrease in oligomer formation. 6,9-11 Honkela and Krause 11 compared the effect of methanol, MTBE, and tert-butyl alcohol (TBA) and concluded that TBA is a better selectivity-enhancing agent. TBA undergoes dehydration to produce isobutene (the reactant for dimerization) and water, which is also polar and can improve the selectivity toward the dimer. Moreover, TBA has a high octane number and is currently also being used as a gasoline additive. 12 The nature of the isobutene dimerization reaction (i.e., fast and exothermic), the difference in the volatilities of the species involved (difference of normal boiling points between C 4 and dimer is more than 100 °C), and the matching temperatures for reaction and distillation have led to this reaction being consid- ered as a potential candidate for implementation in a reactive distillation column. The dimer, because of its higher boiling point, is separated by simultaneous distillation. Therefore, its concentration can be maintained at a low level in the reactive zone, thereby suppressing subsequent oligomer-producing reac- tions. If the reactive distillation (RD) technology is found * To whom correspondence should be addressed. Tel.: +49-391-6110351. Fax: +49-391-6110353. E-mail: sundmacher@ mpi-magdeburg.mpg.de. ² Max-Planck-Institute for Dynamics of Complex Technical Systems. ‡ Indian Institute of Technology Bombay. § Otto-von-Guericke-University Magdeburg. Figure 1. Reaction scheme for isobutene dimerization and oligomerization. 1575 Ind. Eng. Chem. Res. 2006, 45, 1575-1582 10.1021/ie0506522 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/01/2006