Vapour-liquid equilibrium in the monoethylene glycol - methane system at elevated pressures
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A range of different chemicals are used in natural gas processing. The systems operate in closed loops, but a small amount of the chemicals are lost due to the solubility of the chemical in the gas phase. This leads to increased operational costs, it may cause HSE related problems, and it can lead to operational difficulties and contamination of downstream processes and products. A limited number of vapour-liquid equilibrium, VLE, data for processing chemicals in methane are available in the open literature. It is therefore important to obtain new experimental data to adjust and verify thermodynamic models used to calculate the concentration of processing chemicals in the gas phase. Monoethylene glycol, MEG, has been chosen for this work as there exist analytical methods and some literature data for the MEG-methane system. The plan is to continue the work started in this project to obtain experimental data for methyldiethanolamine, triethylene glycol and piperazine in addition to more experimental data for MEG. The purpose of this work was to develop laboratory equipment and to obtain experimental data for the solubility of processing chemicals in methane. Two experimental rigs using analytical techniques with sampling have been used; A static equipment that apply an isothermal method, and a dynamic equipment applying a semi-flow isobaric-isothermal method. The static equipment, where methane is saturated with MEG in a PVT-cell, has been built and tested as part of this work. An existing dynamic equipment, where the gas is saturated and condensed in a series arrangement, was modified for experiments with glycols. Experiments were conducted in the temperature range from 273.15 to 313.15 K, and for pressures up to 150 bar. The samples from the experiments were analysed using ATD-GC-FID. The results from the experiments were compared to an existing model based on the CPA-EoS. In addition a model based on the SRK-EoS was developed. The initial results from the SRK-EoS model were significantly higher compared to the CPA-EoS model and the experimental data. An interaction parameter, K12, of 0.35 in the mixing rule was found to improve the agreement between the developed model and the experimental data. The experimets showed that the concentration of MEG in methane measured increases at increasing temperatures. The modelled values show that the concentration of MEG in methane starts to decrease at increasing pressures, before it passes through a minimum and starts to increase. Further experiments to determine the pressure resulting in the minimum value will be conducted in the continuation of this work.