Published December 2012
This Review presents a technoeconomic evaluation of a methyl tertiary butyl ether (MTBE) production process from raffinate-1, a C4-hydrocarbon stream coming after butadiene extraction from a steam-cracking-based olefins plant. MTBE production in this Review is targeted as an intermediate product that would subsequently be converted to high-purity (polymer-grade) isobutylene in an integrated MTBE cracking plant (analyzed in PEP Review 2012-6). This two-step (commercialized) route for production of high-purity isobutylene from C4 streams is an important commercial step/option among others, aimed at enhancing the value of C4 streams produced in refineries, olefins plants, and natural gas liquids plants. Other commercial options used needed involve chemical conversions of C4 components (1-butene, 2-butenes, butanes, and butadiene) employing processes such as metathesis, selective hydrogenation, total hydrogenation, dehydrogenation, extraction, isomerization, skeletal isomerization, etc. Products from C4 feeds can include propylene (via metathesis of butenes with ethylene), 1-hexene (via self-metathesis of butenes), MTBE, isobutane, isobutylene, butadiene, C4-based liquefied petroleum gas, maleic anhydride, etc.
In the conventional type of MTBE plants, two catalytic fixed-bed reactors are generally used with attached external coolers that remove heat of reaction from the reactors. Because the etherification reaction is equilibrium-controlled and a low reaction temperature favors isobutylene conversion, the presence of MTBE in the reaction system and potential temperature gradients in the catalyst beds tend to limit the conversion rate of reactants. Therefore, despite the fact that a two-reactor system provides a higher catalytic area for reaction, the overall conversion rate is generally limited within a range of 90–95%.
More recent etherification systems are comprised of a primary reactor followed by a reactive distillation column in which MTBE product is removed from the reaction system as soon as it is formed. As a result, overall isobutylene conversion of up to 99% is possible. This Review is based on that design. The catalyst consists of a strongly acidic ion-exchange resin having sulfonic acid groups (preferably of styrene-divinylbenzene type). Details about the catalyst are given in the description section. Another feature of the new system (used in our design) is that the cooling system for the reactor is not external, but rather, the reaction is carried out at or close to the boiling temperature of the reactants mixture, allowing a portion of the liquid to vaporize, and thus, removing the heat of reaction from the reactor. This partially vaporized reactor stream then goes to the reactive distillation column for additional reaction in addition to providing an advantage of reduced reboiler duty. Separation of MTBE product, unreacted methanol, and non-reactive C4 components is done in a conventional way by means of extraction and distillation columns.
Based on our cost analysis, the economics of a standalone MTBE plant with an installed capacity of approximately 284 thousand metric t/yr of MTBE (designed to operate at a stream factor of 0.9) are provided in this Review. In addition, the economics of an integrated MTBE-isobutylene plant producing 150 thousand metric t/yr of isobutylene (the installed capacity of the plant being 167 thousand metric t/yr) are also given (see Tables 9, 10, and 11).