New Technology For Removing Acid Gas From Natural Gas

The importance of natural gas in our daily life and industry is increasing recently.In order to take the responsibility of the environment and comply with the policy of our nation,natural gas should get more application.It is essential to remove hydrogen sulfide during the process of purification of natural gas.Most of the methods of purification of natural gas may cause environmental problems and usually these methods are companied with serious corrosion of the equipment.A new method for the purification of natural gas is urgently in need.In the past decade,two kinds of new "green solvents" have been reported.Namely,ionic liquids(ILs)and deep eutectic solvents(DESs).They have extremely low vapor pressure,causing few environmental problems.At the same time,they are excellent at gas absor:ption.In a word,these solvents have a promising future to take the place of traditional solvents.In this work,5 phosphonium-carboxylic acid ILs(P4444C2H3O2,P4444C5H9O2,P4444C6H11O2,P4444C7H13O2 and P4444C11H21O2)and 3 tetrabutylammonium bromide-carboxylic acid DESs(1:2TBAB/Val,1:2TBAB/Hex and 1:1,1:2,1:3 TBAB/Hep)were synthesized.A constant-volume method was employed to measure the solubility of hydrogen sulfide in these solvents at 298 K,308 K,318 K and 0.1 MPa～0.5 MPa.Solubility of H2S in P4444C2H3O2,P4444C5H9O2,P4444C6H11O2,P4444C7H13O2 and P4444C11H21O2(molar fraction)was 0.657,0.673,0.685,0.696 and 0.750 respectively,while solubility of H2S in 1:2 TBAB/Val,1:2 TBAB/Hex,1:1 TBAB/Hep,1:2 TBAB/Hep and 1:3 TBAB/Hep was 0.669,0.700,0.670,0.700,and 0.716 respectively.Analysis was done according to the results of experiments under different conditions such as temperature,pressure and structures.Increasing temperature,decreasing pressure and less carbon atoms on anion showed negative effect on the solubility of H2S in phosphonium-carboxylic acid ILs,while Increasing temperature,decreasing pressure,less carbon atoms on HBD and higher molar ratio of HBD showed negative effect on the solubility of H2S in tetrabutylammonium bromide-carboxylic acid DESs.Krichevsky-Kasamovsky equation was employed as the thermodynamics model to do the fitting of absorption for both two types of solvents.Henry ’s law constants can be obtained from the fitting results.Then △sG∞、ΔsH∞ and △sS∞can be calculated through the Henry’s law constants at different temperature of the solvent.These two types of solvents both showed low capacity of CH4.P4444C2H3O2,P4444C5H9O2,P4444C6H11O2,P4444C7H13O2 and P4444C11H21O2 showed a selectivity of H2S and CO2 as 3.01:1,3.21:1,3.91:1,3.13:1 and 4.42:1 and a selectivity of H2S and CH4 as 43.51:1,57.06:1,32.91:1,40.29:1 and 47.56:1 at 298 K and 0.5 MPa,while 1:2 TBAB/Val,1:2 TBAB/Hex,and 1:2 TBAB/Hep showed a selectivity of H2S and CO2 as 6.58:1,5.96:1 and 5.82:1 and a selectivity of H2S and CH4 as 38.03:1,36.09:1 and 36.85:1.In this work,the recycling efficiency of both types of solvents was tested.After 5 times of recycling,P4444C2H3O2,P4444C5H9O2,P4444C6H11O2,P4444C7H13O2 and P4444C11H21O2 showed efficiency of 81.6%,81.4%,81.4%,80.0%and 79.3%respectively,while 1:2TBAB/Val,1:2TBAB/Hex,1:1TBAB/Hep,1:2TBAB/Hep and 1:3TBAB/Hep showed efficiency of 85.0%,84.1%,79.4%,84.9% and 79.8% respectively.