Prediction of the equilibrium conditions of clathrate hydrates using updated pressure dependence of the langmuir adsorption constant in the van der Waals-Platteeuw model

Abstract

In this work, we generalized the previously developed model [Fluid Phase Equilibria 2012, 325 , 80-89] for the modelingof the phase boundary of clathrate hydrates with pressure- and temperature-dependent Langmuir adsorption constantby introducing guest-guest interactions among the encapsulated guests. Briefly, in this method, the fugacity of aspecies in the fluid phase is determined by the Peng-Robinson-Stryjek-Vera (PRSV) EOS combined with the predictiveCOSMO-SAC activity coefficient model through the 1 order modified Huron-Vidal (MHV1) mixing rule, i.e.,PRSV+MHV1+COSMOSAC method. The PRSV+MHV1+COSMOSAC allows for the prediction of fluid phasebehaviors (e.g., VLE) without input of any experimental data for the mixture fluids. In the solid hydrate phase, thefugacity is determined from the pressure- and temperature-dependent van der Waals-Platteeuw model, whichdescribes the deformation of the lattice with increasing pressures. This model successfully describes the various types of three-phase coexisting conditions of single and mixed-gashydrates from vapor-ice-hydrate equilibrium (VIHE) at low temperatures, to vapor-liquid-hydrate equilibrium (VLHE) athigher temperatures, and to liquid-liquid-hydrate equilibrium (LLHE) at high pressures, using a single set ofparameters. The updated model introduces universal parameters of guest-guest interaction of each chemical speciescontribute to the potential energy for stabilizing the encapsulated guests within different cavities, resulting in a highlyaccurate description of the hydrate formation. We demonstrate that this approach is capable of modeling CH4 , C2H6 ,C3H8 , iC4H10 , and CO2 of pure and their mixture gas hydrates upon 5 mixed gases. For single-gas hydrates, theaverage relative deviations in the equilibrium pressure are found to be 3.11 % in VIHE and VLHE regions, and theaverage relative deviations in the equilibrium temperature are found to be 0.33 % in LLHE region. For mixed-gashydrates, the average relative deviations in the equilibrium pressure are found to be 5.57% in VLHE region, and theaverage relative deviations in the equilibrium temperature are found to be 0.29% AARD-T in LLHE region. Thedescription of three-phase coexisting conditions of gas hydrate is over a large range of temperatures (148.8 K to 323.9K) and pressures (5.35x10 Pa to 4.79x108 Pa). ? 2020 American Institute of Chemical Engineers. All rights reserved.