Process for Sustainably Sourced p-Xylene
Demand for para-xylene, a feedstock used to produce polyethylene terephthalate (PET) plastics like those found in water bottles or textile fibers, has risen steadily.14 In the PET market, there is a new sustainable focus, pushing producers to use environmentally-friendly processes to create plastic consumer products.3 The current process to create para-xylene relies on fossil fuel cracking and reforming - a decidedly non-green process. This project follows a patent by GEVO, titled Integrated Process to Selectively Convert Renewable Isobutanol to p-Xylene, that explores a green process to convert isobutanol, created from biomass, to para-xylene.20 The raw material for this project is 500,000 tons of isobutylene per year, which is sourced from biomass and has already been converted from isobutanol. The design first feeds the fresh isobutylene, as well as the diluent isooctane, into the process. Isobutylene then oligomerizes over a zeolite catalyst, with a separation following to remove the undesired byproducts. While they are undesired in the process, these byproducts are trimers which can be a valuable coproduct, similar to GEVO’s alcohol-to-jet fuel (ATJ).4 The desired intermediate, 2,4,4-trimethylpentene, proceeds to another reactor and dehydrocyclizes over a chromium oxide doped alumina catalyst. Another separation occurs to isolate para-xylene and remove hydrogen. The final product stream is 89 wt% para-xylene. According to a financial analysis over a 10-year period, the process is currently unprofitable with a negative IRR. To be competitive in para-xylene markets and reach a 15% IRR, a 79% premium is necessary. This is unattractive for a standalone production facility, leading us to recommend that this plant should not be built. However, in the future as the push toward sustainable projects increase, this design may serve as a feasible and economic process for green para-xylene.