Monitoring of DOTP production via esterification with inline analysis

Importance of the topic

Polyvinyl chloride (PVC) is widely used across industries, and its flexibility relies on plasticizers. Dioctyl terephthalate (DOTP) offers a non-phthalate alternative with strong performance and safety advantages. Continuous monitoring of key reaction parameters during DOTP production is essential to maintain high yield, product consistency and operational efficiency. Inline analytical techniques address limitations of offline sampling and provide timely process insights.

Objectives and Study Overview

This application note illustrates the implementation of near-infrared spectroscopy (NIRS) for real-time monitoring of the esterification reaction producing DOTP. The study focuses on simultaneous measurement of terephthalic acid, 2-ethylhexanol, DOTP and water to replace multiple offline assays and accelerate process decision-making.

Methodology and Instrumentation

The DOTP synthesis involves direct esterification of purified terephthalic acid (TPA) with 2-ethylhexanol (2-EH) in a 1:2 ratio at 160–235 °C under catalytic conditions. Inline analysis employs a NIRS XDS Process Analyzer – MicroBundle with up to nine channels connected to a micro-interactance immersion probe. The probe’s adjustable high-energy mirror tip defines the effective pathlength, enabling precise transflectance measurements in a high-viscosity reaction mixture.

Main Results and Discussion

Chemometric models were calibrated against standard reference methods: acid number titration for TPA, alcohol content assay for 2-EH, ester titration for DOTP and Karl Fischer titration for water. Calibration covered typical process ranges:

• 2-Ethylhexanol: 20.4–67.9 % wt
• Terephthalic acid pellets: 0.025–31.3 % wt
• DOTP: 0–78.4 % wt
• Water: 0.1–0.5 % wt

The transfer rate of solid TPA into the liquid phase was also tracked (0–100 %). Inline NIR measurements matched laboratory accuracy, enabling immediate detection of deviations and tighter reaction control.

Benefits and Practical Applications

• Concurrent monitoring of multiple analytes reduces analysis time and streamlines workflow.
• Eliminating sample preparation errors improves data reliability.
• Rapid response to process changes enhances yield and quality consistency.
• Non-destructive, reagent-free analysis promotes a safer operating environment.

Future Trends and Potential Applications

Inline NIRS approaches can be extended to other polymerization and esterification processes in chemical manufacturing. Advances in probe design and data analytics will boost sensitivity and robustness. Integration with digital process control and Industry 4.0 infrastructures will enable fully automated optimization, predictive maintenance and digital twins of production lines.

Conclusion

Near-infrared inline spectroscopy provides a fast, accurate and reagent-free alternative to traditional laboratory methods for monitoring DOTP production. The NIRS XDS Process Analyzer with immersion probe delivers reliable real-time data on reactants, product and moisture, supporting improved process control and product quality.

Instrumentation Used

NIRS XDS Process Analyzer – MicroBundle 9-channel system
Micro-interactance immersion probe with adjustable pathlength high-energy mirror tip

References

• ASTM E1655 Standard Practices for Infrared Multivariate Quantitative Analysis
• ASTM D6122 Standard Practice for Validation of Multivariate Process Infrared Spectrophotometers