Published December 2012
This Review presents a technoeconomic evaluation of an isobutylene from methyl tertiary butyl ether (MTBE) production process based upon the technical information and data available in patents assigned to Evonik Oxeno GmbH on the subject. The design presented herein may differ from an exact construct of an actual commercial Evonik Oxeno process. However, we firmly believe that the process design and economics presented herein are a reasonably accurate representation of the actual process, and should be within the marginal boundary of errors.
The process primarily consists of vapor-phase cracking of MTBE-rich feed in a fixed-bed, shell-and-tube-type reactor over a proprietary magnesium aluminosilicate catalyst doped with an alkali metal oxide. The reaction is endothermic and reaction heat is supplied through the shell side via heating medium. The Evonik Oxeno process is carried out preferably at about 568°F (298°C) and 109–124 psia with a 98.3 wt% purity MTBE feedstock assumed to be available from an adjacent MTBE synthesis plant (analyzed in PEP Review 2012-07). Optimal weight hourly space velocity is equal to 1–3 hr-1. MTBE cracking is a reversible reaction with isobutylene production favored at higher temperatures and lower pressures. The catalyst and reaction conditions selected provide a 94% MTBE conversion to isobutylene with methanol being the major by-product. Side reactions, including dehydration of methanol and dimerization of isobutylene have been accounted for in the process design.
The cracking reaction is followed by a series of product purification steps, including distillation towers, water wash column, and molecular sieve-based dehydration to achieve a 99.95 wt% isobutylene purity. The excess MTBE-methanol stream is recycled back to the MTBE synthesis plant while the heavies and inerts are purged to avoid build up. Methanol tends to form azeotropes with several of the feed components such as: C4/C5 hydrocarbons, MTBE, 2-methoxy butane, and diisobutene, which complicates the separation procedure.
Our cost analysis is based on a plant producing 150,000 metric t/yr of high-purity isobutylene (HPI) at a 0.9 stream factor (equal to an installed capacity of 167,000 metric t/yr). The required installed capacity of an MTBE synthesis plant for the above isobutylene capacity is approximately 284,000 metric t/yr (at a 0.9 stream factor). Cost estimates, details thereof and relevant assumptions are provided in this Review.
The economics of an integrated MTBE-isobutylene plant (i.e., MTBE synthesis-dissociation-isobutylene production) are also provided (see Tables 9, 10, and 11).