PEP Review 2006-15
p-Xylene by Combined Crystallization/Adsorption Process
Published: December 2006
Adsorption has been the dominant technology for p-xylene recovery since the UOP Parex process was introduced in the early 1970s. However, modern crystallization units have significantly improved from the first generation units dating back to the 1960s. In addition to updated configuration and processing schemes, which result in higher unit efficiency, new crystallization plants use more reliable and larger-scale equipment. Crystallization technology can now be competitive with adsorption processes, particularly when the feed consists of a p-xylene enriched stream from para-selective toluene disproportionation (STDP). Recently, there has also been renewed interest in crystallization technology for use in combined or hybrid applications with adsorption for p-xylene recovery.
PEP Report 25D, Advances in p-Xylene Technologies (November 2006), presented preliminary process designs and economics for an aromatics complex using only adsorption, or only crystallization for the recovery of p-xylene. In this review, we develop a combined design case, where the Parex unit recovers p-xylene from reformate xylenes, while the crystallization section recovers product from STDP xylenes. Economics for the three cases are compared, assuming a grassroots aromatics complex with 400,000 t/yr (882 million lb/yr) of p-xylene capacity located on the U.S. Gulf Coast.
Our analysis indicates that the process combining crystallization and adsorption does not provide any economic advantage for new grassroots designs. The reduction in the feed to the Parex unit leads to a capital reduction of about 10% in that section, when compared the plant using only adsorption. However, this effect is not sufficient to offset the capital cost of the additional crystallization section. In revamp applications, however, the scheme can be an attractive option to increase throughput of an existing adsorption-based plant.