Combination of a photoacoustic detector with a diffusion sampler for the measurement of water vapor concentration in ethylene glycols for the natural gas industry

A diffusion sampling based photoacoustic (PA) system for measuring the concentration of water as an impurity in different glycol samples (mono-, di- and triethylene glycol) is presented. Water from the glycol samples diffuses through a sampling membrane into the gaseous phase, where its concentration is determined by a PA system based on a telecommunication type diode laser operating at room temperature. The system was calibrated by measuring the PA signal as a function of the water content of the glycol samples. The sensitivity of the system was found to be dependent on the different glycol types, but in all cases the minimum detectable water concentration was in the low mg/l range. The selectivity of the method was proven by the fact that within the wavelength tuning range of the laser diode, other components, which may diffuse through the membrane, do not generate measurable PA signals. The sensitivity of the system was found to increase exponentially with the temperature of the glycol sample. Altogether, the system has a good potential for industrial applications.