Published November 1998
In recent years, interest has been growing in using dilute ethylene from fluidized catalytic cracking (FCC) off-gas to produce ethylbenzene (EB). Although efforts for economically conventing the off-gas ethylene into a more valuable petrochemical product (instead of using it as a fuel in refinery operations) were initiated in 1950s, it was not until May 1991 that the first commercial EB unit using dilute ethylene as feedstock came on stream at Shell's Stanlow, United Kingdom plant. The plant, based on the vapor-phase benzene alkylation technology of Mobil/Badger was soon followed by Dow's unit at Ternuezen, the Netherlands. The Dow plant, built in late 1991, is also based on Mobil/Badger technology.
Recently, Dow has patented a different approach for EB production (WO 96/34843). In its new process version, ethylene is generated by the catalytic dehydrogenation of ethane instead of using cat-cracker off-gas as its source or polymer-grade ethylene as the feedstock. Gallium- or zinc-promoted mordenite catalyzes ethane dehydrogenation to ethylene at a milder temperature of 700°C (1292°F) that the higher temperature range of 800-875°C (1472-1607°F) employed in steam cracking of ethane. Conversion of ethane is about 14 to 50 wt%, depending on the promoter used, and selectivity to ethylene is 85 wt%. Dow states that the products from the dehydrogenation reactor are free of acetylene and butadiene, and can be directly fed into the alkylation reactor for EB production without purification of the dilute ethylene stream. The benzene alkylation reaction is catalyzed by dealuminated mordenite, and the reaction products are separated by conventional means such as distillation to obtain EB. Unreacted ethane and benzene are recycled. Lower per-pass conversion rates in the alkylation phase depress formation of aromatic by-products, whose presence adversely affects product purity.
We foresee that the Dow process, when targeted to EP production, is unlikely to provide any economic advantage in terms of production costs in comparison with the currently widely used Mobil/Badger technology. The Dow process is probably appropriate only for a region where ethane is abundantly available, and where FCC off-gas or ethylene are not. Alternatively, the Dow process may be a useful, innovative technology for the large-scale catalytic production of ethylene because its process economics are comparable to those of conventional ethane cracking units.
This Review provides a technical and economic evaluation of Dow's process for a capacity of 551 million lb/yr (250,000 t/yr) of EB with a 0.9 stream factor at a US Gulf Coast location.