Dr Stephen Firth, Servomex, UK, examines the critical measurements for safety and control in PTA production processes.
P
urified terephthalic acid (PTA) – sometimes referred to as polymer-grade terephthalic acid – is a white crystalline substance that plays an important role as a precursor for plastic production. Its high melting point of 427˚C (801˚F) makes it particularly valuable in ensuring the thermal stability of the plastics created from it. It is most commonly used as the key component in the manufacture of polyethylene terephthalate (PET), a recyclable thermoplastic resin with US Food and Drug Administration (FDA) approval for use as food and drink containers and bottles. PET is also the most widely used of the polyester type of man-made fibres. Approximately 80 – 100 million tpy of PTA is produced, with the vast majority manufactured in Asia – eight of the ten largest PTA plants in the world are currently located in the region. There is a continuing and growing demand for PTA throughout the world, particularly in the fast-expanding Asian economies, which also make up the largest demand for PTA; China alone uses 60% of the PTA produced globally.
PTA production processes There are a number of licensed methods for the production of PTA. These all follow roughly the same basic process. PTA is manufactured from p-xylene by careful and specific air oxidation in a reactor at high pressure and elevated temperature. Liquid acetic acid, which is highly flammable, is used as a solvent for this reaction. The crystalline PTA product is separated from the reaction liquor in separate crystalliser vessels and then recovered and purified. Gas analysis plays a vital role in the PTA production process, delivering the measurements that support product quality, process efficiency, and safety. The two critical measurements within the process are made at the oxidation reactors and the crystallisers.
Measurements at the oxidiser Gas analysis at the oxidiser stage is focused on oxygen (O2), carbon monoxide (CO), and carbon dioxide (CO2). These measurements are primarily for safety, but they also focus on process control. HYDROCARBON 37
ENGINEERING
October 2021
