On-line measurement of cobalt (Co), bromine (Br) and manganese (Mn) in terephthalic acid (TPA)

Scope

On-line measurement of cobalt (Co), bromine (Br) and manganese (Mn) in terephthalic acid (TPA) is demonstrated.

Background

Terephthalic acid (TPA) and purified terephthalic acid (PTA) are precursors in the making of polyester PET for polyester films, PET bottle resin, textile fabrics, and specialty chemicals. During production and use of TPA and PTA the catalysts Co, Br and Mn must be closely monitored to ensure optimum product quality. Since TPA and PTA are aggressive solutions, an on-line measurement is ideal. Continuous monitoring minimises lab testing requirements, allows for process optimisation, and is a critical step in insuring the end product meets specifications. The Rigaku NEX OL offers a simple and low maintenance on-line analytical technique for trending your process streams. Results are communicated to your plant DCS (distributed control system) via 4-20 mA current loops or MODBUS over Ethernet connection allowing for real time closed loop control.

Instrumentation

Model: Rigaku NEX OL Analyser 
Excitation: Direct 
X-ray tube: 50 kV 4 W Ag-anode 
Detector: Silicon Drift Detector 
Atmosphere: Air 
Measurement Time: 300 sec 

Sample Presentation

No sample preparation treatment is required. To demonstrate performance all samples were measured in a static position using the auxiliary sample input loop.

Calibration

An empirical calibration was built using a set of 8 standards with a measurement time of 300 seconds per sample. To demonstrate performance water-based standards were introduced through the auxiliary sample input loop and analysed in a static position.

Repeatability (Precision)

To demonstrate repeatability (precision), samples containing low and high amounts of Co, Br and Mn were chosen from the set of calibration standards. Each sample was measured in a static position for ten repeat analyses using a measurement time of 300 seconds per measurement.

(see tables below)

Typical Detection Limits

To determine the Lower Limit of Detection (LLD) using the empirical method, ten repeat analyses of a blank sample (DI water) is measured and the standard deviation calculated. The LLD is then defined as three times the standard deviation. The following detection limits are shown using a measurement time of 300 seconds per measurement.

Element
Co
Br 
Mn

LLD 
8 ppm 
14 ppm 
12 ppm 

Nex OL Features & Benefits

• Real-time process control
• Trend analysis charting
• Capable of measuring elements Al to U, depending on application
• Robust Rigaku NEX QC+ optical kernel with SDD detector
• Industrial touch screen user interface
• Unique toolless flow cell design
• No dangerous radioisotopes

Conclusion

The NEX OL offers real time on-line trend analysis in a simple yet powerful and versatile system for quantifying the elemental composition of a process stream. The results of this study indicate that given stable samples, proper sample handling and proper calibration technique, the Rigaku NEX OL EDXRF can achieve excellent results in monitoring the concentration of Co, Br and Mn in TPA and PTA solutions.

More information about EDXRF / XRF solutions for Petroleum & Petrochem applications is available here.