Experimental And Theoretical Study On The Effect Of Quaternary Ammonium Salts On Formation Of Coalbed Methane Hydrate

With the rapid development of our industry and increasing demand for energy,the contradiction between economic growth,energy supply and demand,the environment is becoming increasingly prominent.In response to the call of "energy-saving emission reduction,low-carbon economy",the proportion of natural gas utilization increased year by year.Coalbed methane?CBM?belongs to unconventional natural gas,the reserves are abundant in our country,but due to the irrational utilization and technical constraints,the CBM emission increased at a tremendous rate,not only caused the waste of resources but also the worse greenhouse effect.Using CBM reasonably,efficiently and safely becomes more imminent.Hydrate can separate and purify low-concentration coalbed methane by virtue of difference in equilibrium pressure of each component,which has unique advantages compared with the traditional separation method.There are many obstacles on the way to industrial application of hydrate,such as the high phase equilibrium pressure,low gas storage capacity and long induction time.To solve these problems,Scholars at home and abroad have carried out extensive research on the thermodynamics and kinetics promoters on hydrate formation,promoters like sodium dodecyl sulfate?SDS?,tetrahydrofuran?THF?were found to perform well.Quaternary ammonium salt?QAS?is a kind of surfactant which is common,tetrabutylammonium bromide?TBAB?,tetrabutylammonium chloride?TBAC?and tetrabutylammonium fluoride?TBAF?show excellent thermodynamic and kinetic promotion effect among lots of studies.In this paper,five kinds of quaternary ammonium salts including TBAC,TBAF,tetramethylammonium bromide?TMAB?,tetramethylammonium chloride?TMAC?and tetramethylammonium fluoride?TMAF?are used to explore the effects of quaternary ammonium salts on low concentration coalbed methane?30% CH₄ + 65% N2 + 5% O2?hydrate from thermodynamic and kinetic aspects,investigated variables including phase equilibrium condition,phase change latent heat,induction time and gas storage capacity of hydrate.A modified fugacity model was adopted to predict pure methane or CBM hydrate phase equilibrium conditions with TBAB,TBAC and TBAF.The specific works are as follows:?1?The thermodynamic and kinetic effects are researched in CBM hydrate formation experiments with five kinds of QASs including TMAF,TMAC,TMAB and TBAF,TBAC.The concentration of each quaternary ammonium salt was set to 0.17,0.29,0.62,1.38,and 3.5 mol%;an isochoric pressure-search method was used to obtain the phase equilibrium data,the phase equilibrium temperature ranged from 282.3 to 304.5 K,phase equilibrium pressure changed from 0.919 to 8.435 MPa,the relationship of the concentration of QASs and promotion are non-linear,there is a best concentration with an order : 3.5 mol% TBAF > 0.62 mol% TMAF > 3.5 mol% TBAC > 1.38 mol% TMAB > 1.38 mol% TMAC;Using Clausius-Clapeyron to abtain phase change latent heat of gas hydrate was as follows: 3.5 mol% TBAF > 0.62 mol% TMAF > 3.5 mol% TBAC > 1.38 mol% TMAB > 1.38 mol% TMAC;The induction time order as follows: 3.5 mol% TBAF < 3.5 mol% TBAC < 1.38 mol% TMAC ? 1.38 mol% TMAB < 0.62 mol% TMAF;gas storage rate of quaternary ammonium salt hydrate has been calculated,gas storage rate of TBAC is higher than TBAF.?2?A modified fugacity model of water is employed combining with PRSV2 equation of state for fugacity of gas and e-NRTL electrolyte activity coefficient model for activity coefficient in quaternary ammonium salt solution.The modified fugacity model is not in good agreement with the phase equilibrium of single-component gas hydrate,but works well in QASs and CBM hydrate.The average absolute relative deviations in pressure are less than 15% and 9% in single-component and CBM system respectively.The theoretical basis are provided for the application of quaternary ammonium salt hydrate in CBM separation.