Water content assessment in acid gas

Natural gas is well known as the cleanest fossil fuel. Today, it is estimated that more than 40% of the remaining conventional natural gas reserves are deemed to be acid, i.e., containing significant quantities of CO2 and H2S. As the global consumption of natural gas is expected to steadily grow, the demand will be met by sources such as sour/acid gas fields. In some specific applications that require cryogenic processes (LNG, NGL recovery), this issue is commonly addressed upstream of the gas dehydration unit, so that the gas is already sweet when arriving the drying section. In the other cases, the effect of the acidic species on the gas water content is often not properly accounted for, whereas an accurate appraisal of the water content is paramount for the sizing of dehydration units.
There are several methods to estimate the water content: some of them are based on empirical correlations or on thermodynamic models. This paper first explains why it is crucial to address the problem in terms of dehydration unit design and plant reliability. Herein, we will focus on CO2 rich gas mixtures, for which a comprehensive literature review from the public domain, and discussion of mathematical methods or thermodynamic models used for water content estimation, are presented. Finally, a new correlation, based on thermodynamic assumptions was developed and was tested by comparison with experimental data obtained with a new laboratory experimental set-up. This equipment uses a Tunable Diode Laser Spectroscopy (TDLS), with an accuracy of +/- 1%.