July 14, 2014
C5 hydrocarbons are the key to a new ethylene–propylene process. Engineers at Mobil accidentally discovered methanol-to-olefins (MTO) technology during the startup of the company’s methanol-to-gasoline process in New Zealand in the mid-1980s. This process for producing olefins, however, was not commercialized until 2010 when the first so-called coal-to-olefins (CTO) plant began operation in China. (In CTO technology, methanol is made from coal and fed to an MTO unit.) Since that Chinese plant startup, several more MTO facilities have opened, and many more are under construction.
In a May 2, 2014, announcement, BASF indicated that it is seriously evaluating a methane-to-propylene (MTP) process that presumably would proceed through methanol. In an MTP plant, very little ethylene is coproduced. Other work in this area has focused on converting dimethyl ether to olefins; consequently, another process has emerged: oxygenate-to-olefins (OTO).
L. A. Chewter and colleagues disclose a version of OTO in which tertiary alkyl ethers are used as the feedstock. In particular, the inventors found that it is advantageous to etherify C5 hydrocarbon fractions from steam crackers or fluidized catalytic crackers with methanol to give the corresponding tertiary ethers. One example is the selective etherification of isoamylene with methanol to give tert-amyl ether (TAME).
Isoolefin-depleted C5 streams can be used as blending stocks in gasoline production, but the inventors found that TAME is a good feedstock for an OTO process. With ZSM-22 as a fixed-bed catalyst at 525 ºC, TAME is completely consumed, and ethylene and propylene yields of 19.5and 56.8%, respectively, are obtained. Side products include C4 (15.0%) and C5 (3.9%) olefins with <1% aromatics.
As more and more steam crackers in the United States use inexpensive shale gas–based ethane as a feedstock, less and less propylene will be coproduced. Although refinery-based propylene fills most of this supply gap, “on-purpose” propylene is already being made via propane dehydrogenation or ethylene–butene metathesis. It remains to be seen whether OTO technology will become commercial in the United States. (Shell Oil [Houston]. US Patent 8,742,188, June 3, 2014; Jeffrey S. Plotkin)