Phase diagram of methanol and alkane binary systems in the field of natural gas fractionators: Experimental work and modeling

Natural gas is normally saturated with water inside reservoirs. The presence of water can lead to formation of gas hydrates, with high probability of serious operational, economic, and safety problems in production, transportation, and processing. In order to avoid gas hydrate formation, methanol is injected as a traditional inhibitor into natural gas lines. Therefore, down-stream of pipelines, natural gases contain non-negligible quantities of water and methanol. Water is classically removed from natural gases using dehydration units. However, methanol remains in natural gas streams, which may cause problems in optimizing the operating conditions of hydrocarbon fractionators. Accurate data and modelling of vapor-liquid equilibria for methanol-hydrocarbon mixtures are necessary to provide requested information for the design and optimization of natural gas processing units. In this communication, we first report the details of an experimental apparatus based on the ″static-analytic″ technique, which allows us to measure new vapour-liquid-equilibrium (VLE) data, including the propane + methanol binary system at 313.2 K, and n-butane + methanol binary system at six temperatures above and below the n-butane critical temperature, from 323.22 to 443.19 K. The new experimental results and some selected literature data of binary hydrocarbons-methanol VLE are modeled by a phi-phi approach, using the Peng-Robinson equation of state combined with the Wong-Sandler mixing rule. These data are also represented using the SAFT-VR and PC-SAFT models, which take into account self-association between methanol molecules. A comparison between the modeling approaches is presented herein.