Monitoring of DOTP production via esterification with inline analysis

Importance of the Topic

Polymers and plasticizers enable flexible and durable polymer products. Dioctyl terephthalate DOTP is a leading non-phthalate plasticizer with high performance and safety advantages over traditional phthalates. Ensuring consistent product quality and high reaction yield in DOTP production is critical for regulatory compliance and industrial efficiency.

Objectives and Study Overview

This application note outlines a real-time monitoring strategy for direct esterification in industrial DOTP synthesis. The key objective is to simultaneously track reactants and products including purified terephthalic acid TPA, 2-ethylhexanol 2-EH, water, and the DOTP ester. The approach leverages near-infrared spectroscopy NIRS to replace time-consuming offline lab methods and enhance process control.

Methodology and Instrumentation

Inline analysis is achieved using the NIRS XDS Process Analyzer MicroBundle 9 Channels. This system supports up to nine fiber-optic probes that can be positioned meters away and is certified for hazardous areas. A micro interactance immersion probe operates in transflectance mode, with adjustable pathlength defined by a high-energy mirror tip. The probe is installed directly in the reactor stream for continuous spectral acquisition.

Main Results and Discussion

Calibration models were developed by correlating NIR spectra with reference laboratory measurements of acid number, alcohol content, ester concentration, and moisture. The analyzer successfully quantified 2-ethylhexanol in a range of 20 to 68 weight percent, terephthalic acid as pellets from trace to 31 weight percent, DOTP up to 78 weight percent, and water between 0.1 and 0.5 weight percent. Real-time monitoring of TPA transfer rate from solid to liquid phase enabled precise control of the 1:2 reactant ratio.

Benefits and Practical Applications

• Enhanced product quality through immediate detection of process deviations
• Reduced batch and cycle times by eliminating offline sampling delays
• Reagent-free analysis that lowers operational costs and environmental impact
• Improved safety by removing manual sampling in hazardous zones

Future Trends and Potential Applications

Advancements in chemometric modeling are expected to further refine inline NIR accuracy and robustness. Integration with automated process control systems will enable closed-loop adjustments in real time. The technique may expand to other polyester syntheses and non-phthalate plasticizers, as well as multisite deployments across distributed production units.

Conclusion

Inline NIRS analysis with the XDS Process Analyzer provides a reliable, efficient, and safe solution for monitoring the direct esterification process in DOTP production. By delivering multiple critical parameters in real time, manufacturers can optimize yield, ensure consistent plasticizer quality, and respond promptly to process fluctuations.

Reference

• ASTM E1655 Standard Practices for Infrared Multivariate Quantitative Analysis
• ASTM D6122 Standard Practice for Validation of Multivariate Process Infrared Spectrophotometers
• AN-PAN-1041 Inline monitoring of free isocyanate content in polyurethane
• AB-414 Polymer analyses using near-infrared spectroscopy