Streamlined Terephthalic Acid Process lowers capital and operating costs

A simplified and streamlined new process for producing purified terephthalic acid (PTA), trademarked as Compress PTA and co-developed by The Dow Chemical Co. (Midland, Mich.; www.dow.com) and Davy Process Technology (DPT; London, U.K.; www.davyprotech.com), requires 15% lower capital spending, 20% lower utilities consumption and allows a 25% smaller processing space than conventiona processes.

While the process employs the same reaction chemistry as that found in conventional PTA plants, Dow and DPT have focused on technology improvements within the unit operations and
a less equipment-intensive flowsheet to realize operating and cost benefits.

Conventional PTA production is complicated, and the technology can be difficult to operate and can suffer from reliability issues, explain Hugo Gonzalez, Dow global commercial manager for technology licensing and Julian Gray, DPT principal process engineer.

Now available for licensing, the Dow/DPT process requires 20% less utilities consumption, half as many pieces of rotating equipment, one-third fewer control valves and one-quarter fewer agitators.

Less equipment means fewer constraints on plant layout, Gonzalez and Gray remark, which allows several areas of the plant to be integrated and improves water recycling and energy consumption.

In the first stage of the process (flowsheet), p-xylene (PX) is oxidized with air, using an acetic-acid based solvent and a cobalt-, manganese- and bromide-based catalyst to form crude terephthalic acid (CTA). The CTA then enters the purification stage, where an aqueous solution of the acid is subjected to a hydrogenation reaction to remove impurities. The PTA is recovered by crystallization, filtration and drying.

One key part of the process, used in both separation steps, is the use of pressure filtration, a technology that has demonstrated higher efficiency in filtering PTA crystals with less equipment.

Compress PTA reduces overall energy consumption by using an improved binary distillation system and by additional heat integration in the purification plant.

PTA is the raw material for the production of polyethylene terephthalate (PET), a common thermoplastic polymer used to construct synthetic fibers, packaging, bottles, engineering resins and in other applications.

Fuente: CHEMICAL ENGINEERING ♦ WWW.CHE.COM ♦ JANUARY 2011