Measuring Purity of MEG (monoethylene glycol)

Introduction

MEG is an important raw material in production of polyester fibers. The quality of the fibers is dependent on the purity level of the MEG precursor. MEG is therefore purified of impurities (e.g. diethylene glycol) by passing the stream through an ion exchange column. Over time, the column performance deteriorates and impurities begin breaking through.

Fiber grade MEG must comply with analytical purity tests in order to be salable. Additionally, the ion exchange columns should be replaced precisely at the point of efficiency dropoff in order to avoid both column waste and impure product.

The traditional offline method for MEG quality control relies on pulling samples for lab analysis. The lab system compares the sample’s UV transmittance at a few wavelengths (220 nm, 240 nm, 275 nm, 350 nm) to that of demineralized water at the same wavelengths. Since pure MEG has zero absorbance in the UV range, a simple principle applies: higher transmittance = higher purity. Fiber-grade MEG is required to have at least 97% transmittance at 220 nm.

The OMA system applies the same principle, ASTM method ASTM E2193, but with enormous advantages, including: (1) continuous analysis updating every 1-5 seconds, as opposed to slow sample transport for discrete lab measurements; (2) fast response for immediately pinpointing failure point of ion exchange column; (3) totally automated sampling system normalizes measurement by Auto Zero on distilled water; (4) installation of system close to sampling point for convenience of process adjustment.

Transmittance Spectrum of Pure MEG

The spectra below visualize the transmittance spectrum of pure MEG, along with the corresponding absorbance values (AU) at each wavelength:

Required Accuracy Threshold

As demonstrated by the table below (see column A-A’), a photometric accuracy of ±0.001 AU (at 220 nm) is required to maintain a measurement accuracy of ±0.3% Transmittance:

Wavelength Accuracy and Reproducibility

Transmittance measurement requires extremely stable wavelength accuracy. If the diode drifts wavelength assignment, significant measurement error is introduced. Mechanical scanning instruments with filters have moving parts that wear over time and lose wavelength assignment. The solid state OMA uses a photodiode array for extremely stable accuracy.

Rich Trend Data

Since the OMA performs continuous analysis of MEG purity, trend data is available to the operator with easily configured data storage to view process history.

Application Data

The specifications below represent performance of the OMA-300 Process Analyzer in a typical MEG purity application.

For technical details about the OMA-300 Process Analyzer, see the data sheet:
DS-001A: OMA-300 Process Analyzer

All performance specifications are subject to the assumption that the sample conditioning system and unit installation are approved by Applied Analytics. For any other arrangement, please inquire directly with Sales.

Note: Subject to modifications. Specified product characteristics and technical data do not serve as guarantee declarations.

Installation and Product Photos

Further Reading