Innovative method to purify Terephthalic Acid
The CTA produced from the oxidation section contains a high level of impurities and requires purification to reach the qualified specification of PTA. Sulzer GTC has developed the GT-PTA purification technology which differs from all other purification processes and gives dramatically lower capital and energy costs.
The conventional PTA purification processes require hydrogenation units to hydrogenise 4-CBA to p-toluic acid because 4-CBA and TA cannot be separated in the water system. This hydrogenation is a high-temperature, high-pressure step that dissolves all TA and 4-CBA in water. It also uses an expensive Pd catalyst and a special alloy for the hydrogenation reactor. All of these contribute to the main utility consumption and high capital cost. GTC’s GT-PTA purification technology uses the proprietary solvent Techtiv℠-TA to efficiently extract the 4-CBA and other impurities from TA at low pressure and temperature conditions. Since 4-CBA can be easily extracted by Techtiv℠-TA, GT-PTA purification technology doesn’t require the hydrogenation step, and thus eliminates its high utility and capital costs. GT-PTA can reduce about half of both utility and capital costs compared to conventional PTA purification processes while giving equal or better PTA product quality.
The CTA from the oxidation section is sent to GTC’s proprietary “ISF” step where it is mixed with recycled Techtiv-TA solvent and 4-CBA while other impurities are selectively and efficiently extracted. The ISF is under very mild pressure and temperature operating conditions. The cake from ISF already has the capability to reach the PTA purity level, but still needs to be sent to a mild crystallization step to reach the required crystal size. The ISF step can also be tuned to leave minor impurities in the cake for the mild crystallization step to reject it all. The 4-CBA and other impurities are easily extracted by ISF so there is no need for a hydrogenation step. The crystallization is also under mild pressure and temperature conditions. The cake from the mild crystallization is then filtered and dried to get high-quality PTA. The mother liquor from ISF which contains almost all 4-CBA and other oxidation intermediates is sent to a solvent recovery step where the solvent is recovered and all of the 4-CBA and other oxidation intermediates are re-slurried by HAc and sent back to the oxidation section to increase the TA yield. The Techtiv-TA solvent recovered from solvent recovery and other places is fully recycled within the process.
The whole process is under mild pressure and temperature conditions and with full heat integration design. Thus GT-PTA purification technology requires much lower utility and capital costs than all other PTA purification processes that require hydrogenation.
The ISF step can take up to 5% 4-CBA in the CTA feed and still be able to extract it out to the PTA purity. That gives much larger oxidation flexibility so “relaxed oxidation” – part of GT-PTA oxidation technology – can be done to reduce PX and HAc burning losses.
GTC’s GT-PTA purification technology has numbers of key advantages to the conventional PTA purification processes:
- No hydrogenation step
- No expensive catalyst and special alloy
- No Sundyne pump; no compressor
- Low and mild temperature/pressure throughout the process
- Low-cost metallurgy (C.S./304SS/316SS)
- Proprietary ISF step and Techtiv solvent to extract 4-CBA and other impurities with no heat input
- 4-CBA in CTA can be up to 5% – allowing relaxed oxidation to reduce PX and HAc burning losses
- Full recycle of 4-CBA other oxidation intermediates to oxidation section – PTA yield maximization
- Full internal recycle of Techtiv solvent
- Very little waste water (10% of conventional)
- Capital cost 65% of conventional; operating cost 50% of conventional
- Easy to revamp existing conventional PTA purification to GT-PTA purification as most of equipments can be reused.
- Capable of taking mix-CTA/CIPA feed and co-producing Purified TA & Purified IPA