Experimental Study And Process Simulation Of Methane Separation From Low-concentration Coalbed Methane By Hydrate Formation In The Presence Of TBAB

Gas hydrates are ice-like crystalline compounds formed when small-sized gasmolecules （e.g., N₂, CH₄, and CO2） are encaged in the cavities constructed byhydrogen-bonded water molecules. To separate gases from gas mixtures using hydrateformation has received much attention in recently years, however, the stiff synthesisconditions of gas hydrates have prevented the development of this novel technology.Therefore, it is necessary to study how to increase the rate of gas hydrate formation andseparation efficiency. This work focuses on the purification of low-concentrationcoalbed methane, which will be a breakthrough point in the effort to prevent energywaste, mitigate greenhouse effect and also optimize energy structure. In this work,Tetra-n-butyl Ammonium Bromide （TBAB） was used as an additive for the recovery ofCH₄from low-concentration coalbed methane by using gas hydrate formation.Purifying CH₄from low-concentration coalbed methane by hydrate formation hasbeen studied using experiment and theoretical methods respectively, and thethermodynamic and kinetic data have been obtained. The results obtained have laid afoundation for the separation of CH₄from CBM in the scale of industry and providedguidance for the development of this technology as well. This study was mainly carriedout in three aspects:Firstly, according to the purpose of this study, a set of visual experimentalapparatus was constructed for hydrate formation. The reliability of this apparatus wasvalidated using pure CH₄as a feed gas. A thermodynamic model was built based on theChen-Guo model and P-T gas equation of state to predict the phase equilibrium for gashydrates formed with the low-concentration CBM. The reliability of this model wasvalidated by the comparison between prediction and experiment phase equilibrium dataof gas hydrates formed from the50.25%CH₄+49.75%N₂gas mixture. Thus, themodel constructed was proved to be accurate in predicting phase equilibrium data of gashydrates. In the pure water system, equilibrium conditions of low-concentration CBMwas calculated by process simulation software HYSYS. A series of thermodynamicexperiments were carried out using the experiment apparatus and the hydrate phaseequilibrium curves were obtained. The thermodynamic effect of TBAB on the phaseequilibrium conditions of hydrate formed in the CBM system was investigated. Thecomparison of prediction data and experiment data indicated that the presence of TBAB in the solution has remarkably reduced the phase equilibrium pressure.Secondly, kinetic experiments were carried out for the separation of CH₄from thelow-concentration CBM. Three parameters （gas consumption, CH₄recovery andseparation factor） were calculated to evaluate the efficiency of separating gas mixturesby hydrate formation. The results showed that memory solution can greatly acceleratethe hydrate nucleation process. Subcoolings were used as the driving force for hydrateformation.（ΔT=5.0,7.0and9.0K）. It was found that the experiment conditions of1.38mol%TBAB, ΔT=5.0K and, P=4MPa is preferred conditions for theseparation of CH₄from the low-concentration CBM. The corresponding CH₄recoveryis about27%and the CH₄concentration is41%.