Near-Infrared Analyzers for Polyester (PET) Synthesis

Flowchart of a polyester synthesis and where nir analyzers can be used

Polyesters are polymers formed from a dicarboxylic acid and a diol. The most common polyester is polyethylene terephthalate (PET). At its most basic level PET Synthesis requires the combination of ethylene glycol, terephthalic acid or dimethyl terephthalic Acid, and heat. The production of polyesters can often be improved from the real time information collected by continuous process monitoring of both component and product streams. Near infrared analyzers are a tool that can help process engineers and technicians to make informed process optimization decisions.

How Near-Infrared Analyzers are Beneficial to Ethylene Glycol Synthesis

The standard PET synthesis starts by combing ethylene oxide and water in a thermal hydration reactor (about 200°C). Near-infrared analyzers can be used to monitor reaction kinetics such as the concentration of excess water as well as the ratio of glycol reaction products. The ethylene oxide to ethylene glycol thermal conversion process typically yields about 90–92% monoethylene glycol (MEG) and 8–10% heavier glycol products, mainly diethylene glycol (DEG) and triethylene glycol (TEG). Near-infrared measurements can be made on the different glycols and also in ethylene carbonate to ethylene glycol reactors.

The resultant water–glycol mixture from the thermal hydration reactor is then fed to multiple evaporators where the excess water is recovered and recycled. Finally, the water-free glycol mixture is separated by vacuum distillation into MEG and the higher glycols. Near infrared analyzers can be used here to verify yield, and other useful parameters. Most MEG is reacted with para-terephthalic acid (PTA) to produce polyester fibers for textiles and garments and polyethylene terephthalate (PET) plastics for food and drink packaging. Additionally, the APHA color can also be monitored as off color (yellow) monomer is undesirable as it can lead to off color product.

How Near-Infrared Analyzers are Beneficial to Terephthalic Acid and Dimethyl Terephthalic Acid Synthesis

Terephthalic Acid and Dimethyl Terephthalic Acid can both be synthesized by reacting P-xylene with cobalt–manganese–bromide catalyst, acetic acid, and air.  Real-time quantitative monitoring of this caustic reaction is possible with the Guided Wave Clearview DB  analyzer. A Kalrez sealed or gold brazed single-sided insertion probe (SST) can be installed directly into the reaction vessel for continuous process monitoring and optimization. Additionally, in an extremely corrosive environment, the sample probe can be constructed out of Hastelloy C276.

How Near-Infrared Analyzers are Beneficial to PET Synthesis

In batch polymerization, dimethyl terephthalate (DMT) reacts with ethylene glycol in esterification to form a monomer alcohol, which then polymerizes with terephthalic acid (TPA). The continuous method, on the other hand, involves polymerization between TPA and ethylene glycol, so DMT is not necessary. Both methods involve heating either DMT or TPA with ethylene glycol to about 536º F for 30 minutes at atmospheric pressure, and then the mixture spends 10 hours under vacuum. Near-Infrared analyzers such as the Clearview DB and NIR-O Process analyzers can be used to provide in-situ monitoring of the cross-linking under vacuum conditions. Out of specification product can be detected with high-temperature shuttle probe and Near infrared monitoring during melt transfer. By monitoring the molten polymer stream for Acid Number, a cost savings can be realized by preventing off spec product from reaching the extrusion or pelletizer.

Interested In Learning More?

Like many petrochemical reactions, polyester (PET) synthesis, benefits from continuous monitoring using near infrared analyzers.  Please complete our application questionnaire, so that we can work together to determine how our analyzers can be used to optimize your process and reduce your operating costs. Download a PDF copy of the infographic Near-Infrared Process Analyzer Polyester (PET) Synthesis Measurement Solutions.