Study On The Formation Characteristics And Separation Mechanism Of CO₂/CH₄ Gas Mixture

Due to the rapid growth of natural gas consumption in China,the output of conventional natural gas is no longer enough to meet the rapidly growing demand.The exploitation and utilization of unconventional natural gas,such as coalbed methane and shale gas,has become a hot topic in recent years.Supercritical CO₂ fracturing technology uses anhydrous fracturing,which can save water and store CO₂ gas.It is an effective method to extract unconventional natural gas nowadays.However,the unconventional natural gas extracted by supercritical CO₂ fracturing will be adulterated with part of CO₂ gas,so it is significant to choose a reasonable and efficient method to separate CO₂ from the gas mixture.Hydrate-based CO₂ removal form CO₂/CH₄ gas mixture has lots of advantages such as simple raw material,safe storage and transportation,and energy saving.Although it is prospective in the future,high phase equilibrium condition,slow growth rate and low separation factor need to be solved now.In this paper,the influences of different additives on hydrate phase equilibrium condition,hydrate formation and dynamic characteristics were discussed by experiments.At the same time,the visual morphology experimental apparatus was also be used to compare the influences of additives on hydrate growth form.The main contents and conclusions of this paper are as follows:?1?Firstly,phase equilibrium temperature of CO₂/CH₄ hydrates formed under the pressure of 2.8 MPa in TBAB solutions?0.29mol%,0.62mol%,1.38mol%,2.57mol%?was determined by experiments.The results showed that TBAB solutions could significantly increase the phase equilibrium temperature of CO₂/CH₄ hydrate.So TBAB was an effective thermodynamic promoter.?2?Secondly,kinetics researches on CO₂/CH₄ gas hydrate in different solutions were conducted:?1?Kinetic characteristics of gas consumption,CO₂ recovery and separation factor in different concentration of TBAB solutions?0.29mol%,0.62mol%,1.38mol%,2.57mol%?in CO₂/CH₄ gas mixture were studied under the condition of P_ex=2.8 MPa and?T=6K.The results showed that the gas consumption and CO₂recovery of 0.62mol%TBAB solution were both the highest,indicating the semiclathrate hydrates generated from TBAB could capture more CO₂ molecules and CH₄ molecules,but the separation factor was the lowest which indicated the selectivity of CO₂ was the worst.?2?This paper compared the effects of different SL solutions?300ppm,500ppm,1000ppm?on the consumption of CO₂/CH₄ hydrate gas and solubility through experiments.It was been found that 500 ppm SL solution had the best impact on hydrate formation and SL of different concentration could effectively improve the solubility.?3?Then,the hydrate growth characteristics of 500 ppm SL solution,1.0 mol%THF/500 ppm SL solution and liquid water under the same conditions were studied,and the effects of SL and SDS on hydrate kinetic characteristics were compared,too.The results showed that the hydrate nucleation stage in 500ppm SL solution was much longer than THF/SL solution and liquid water.On the other hand,compared with THF/SL and SDS solutions,CO₂ recovery in 500ppm SL solution was increased to 63.5±2.9%while the separation factor was 4.0±0.8,and the gas consumption was higher than that of liquid water,SDS solution,THF solution,TBAB solution and TBPB solutions.Therefore,SL has a broad prospect in promoting the formation of CO₂/CH₄ gas hydrate and improving the hydrate growth rate,and has room in increasing CO₂ selectivity.?3?In this paper,typical images of CO₂/CH₄ gas hydrate in different concentration of TBAB solutions and 500 ppm SL solution were taken by the visual morphology experimental apparatus.In TBAB solution,the hydrate nucleation was very fast.Hydrate only crystallized in the liquid phase and grew rapidly instead of growing along the wall.However,the nucleation of CO₂/CH₄ gas hydrate is slow in SL solution.Hydrate first crystallized from the gas-liquid interface and grew slowly upward along the wall.The microfilming results are consistent with kinetic experimental results.