Water content of liquid acid gas and liquid propane in the presence of a hydrate phase
Date
2020-12
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Abstract
Natural gas coexists with water in subsurface reservoirs. Other impurities such as hydrogen sulfide (H2S) and carbon dioxide (CO2) can also be present depending on location and source. Many issues are associated with the presence of water and the acid gas (H2S and CO2) impurities during production, processing and transportation of natural gas such as solid hydrate blockage, corrosion and safety concerns. Before sale to consumers, water and the acid gas impurities are removed or reduced to meet sales and pipeline specifications. One of the viable strategies for managing the removed acid gas is injection (AGI) into underground formations either for sequestration, pressure maintenance or enhanced oil recovery. After acid gas removal, in some cases, natural gas liquid (NGL) are separated from the treated natural gas streams for use as a fuel or chemical feedstock. NGL are separated from the methane (CH4) in a cryogenic separation process, where the presence of water is highly undesirable because it can cause the formation of solid clathrate hydrates. Propane (C3H8) is a principal component of NGL and a sII hydrate former; hence, conditions at which its hydrate will form in the presence of saturated and unsaturated water are important to avoid their formation or determine how much dehydration is required. Because of the toxicity of H2S (100 ppm is the immediate dangerous to life and health concentration), there are limited dissociation data for its hydrate in the presence of water reported in the literature. Also, prior to this work, there were no water content data in equilibrium with only hydrate reported in the literature for H2S. On the other hand, hydrate formation/dissociation conditions for pure CO2 are well studied; however, analysis of the literature water content data at hydrate forming regions shows some variation regarding the pressure dependence of the measurements. These data are necessary to accurately calculate and prevent hydrate formation conditions in sour natural gas production, as well as to define the dehydration requirements for acid gas during transportation to injection facilities. In this work, the dissociation conditions for pure CO2 and pure H2S hydrates in the presence of water rich phase was measured using the phase boundary dissociation method. Also, the water content of pure CO2, pure H2S and pure C3H8 in equilibrium with their respective hydrate were measured using a tunable diode laser spectroscopy technique. These results were modelled using the reference quality Helmholtz energy equations of state for the fluid phases, and the van der Waal and Platteeuw model for the hydrate phases. The calculated results were compared to few available literature, where a good agreement was mostly observed. A thermodynamic model capable of calculating the water content and three phase loci independently using the same optimized parameters, was successfully developed for CO2. However, a single equilibrium model was not successfully found for H2S and C3H8 fluids, hence two different models were recommended for both the water content and three phase loci calculation. In conclusion, the pressure dependence of water content of these gases in equilibrium with their respective hydrates are very weak, but the water content increases as the temperature increases.
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Keywords
Water content, hydrogen sulfide, acid gas, propane, clathrate hydrate, thermodynamic modelling
Citation
Adeniyi, K. I. (2020). Water content of liquid acid gas and liquid propane in the presence of a hydrate phase (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.