Effects Of Pipeline Spiral Flow And Promoters On The CO₂ Hydrate Formation Characteristics

Gas hydrates are ice-like crystalline substance that can form when water and light gases are contacted at low temperatures or high pressure.It has long been known that gas hydrate have been utilized in a number of technological applications,such as gas fractionation methane or hydrogen storage and transportation,carbon dioxide capture,transport,and sequestration,and desalination.However,slow formation rate of gas hydrates is one of the main problems of this process.How to improve the rate of gas hydrate formation and reduce phase equilibrium conditions have become one of the key scientific problems that need to be solved urgently.In order to solve the problem of low hydrate generation rate,the surfactants were used to reduce the gas-liquid interface tension and improve the gas-liquid mass transfer rate,nano-graphite particles with high specific surface area and high thermal conductivity characteristics were used to strengthen the hydrate formation and rapid removal of hydrate formation heat,spiral flow was used to strengthen the gas-liquid mass transfer and achieve the hydrate continuous formation.Based on this,the effects of pipeline spiral flow and promoters on the CO₂ hydrate formation characteristics were done.Using the K11-type automatic surface tension meter,the surface tensions of hydrate reaction solutions in the presence of sodium dodecyl benzene sulfonate?SDBS?,cetyltrimethyl ammonium bromide?CTAB?,sodium dodecyl sulfate?SDS?and nano-graphite were determined.It was found that the addition of surfactants could effective reduce the surface tension of the hydrate formation solutions,which will beneficial to the gas-liquid mass transfer during the hydrate formation,but the addition of nano-graphite particles has little effect on the surface tension of the hydrate formation solutions.The CMC values of SDBS,CTAB and SDS were 1.43mmol/L,0.82mmol/L,2.43mmol/L respectively.When the concentration of surfactant was lower than CMC concentration,the surface entropy and surface enthalpy of solution were greatly affected by concentration.Hydrate equilibrium conditions for carbon dioxide with water containing SDBS,CTAB,SDS and nano-graphitewere measured using high pressure visual reactor at different promoters concentration,pressure and temperature.The results show that,compared with deionized water at a given temperature,surfactants have a positive effects on the gas hydrate formation but graphite nanoparticles have a negative effect on the gas hydrate formation.In addition,an improved hydrate thermodynamic model based on the Chen-Guo model was proposed to predict the hydrate equilibrium conditions under the system of graphite nanoparticles suspensions.The results show that the maximum average deviation of pressure predicted by model is 6%.It indicates that the data of the model prediction was good agreements with the data obtained from experiments.The effect of GN solutions,SDSsolutions,SDBSsolutions,mixtures of GN and surfactants on gas consumption rate,total gas consumed and gas reaction ratio of hydrate were investigated at constant volume system using high pressure visual reactor.It was found that the addition of nano-graphite particles can significantly reduce the induction time of hydrate?up to 85%?and increase the formation rate of hydrate,but does not present significant effect on increasing storage capacity?except 0.4%GN?.SDS can reduce the induction time?maximum reduction is 54%?,the concentration of 0.07%,0.1%can significantly improve the hydrate storage capacity?up to 21%?.SDBS can increase the hydrate nucleation rate by 86%-98%,but only 0.08%SDBS can increase the gas consumption and gas reaction ratio by7.5%.However,the mixture of nano-graphite and surfactantsignificantly increase the storage capacity and gas reaction of carbondioxide.For example,the aqueous solution of0.8%GN+0.02%SDBS and 0.8%GN+0.07%SDS can increase the gas storage capacity by 59.6%and38.3%respectively,but the induction time of the hydrate would be prolonged in the presence of GN+SDS.Aiming at the need of research on the pipeline spiral flow on the hydrate generation characteristics,a high pressure pipeline hydrate flow experiment device was designed and constructed.The flow characteristics induced by spiral ties were analyzed and the size of the spiral ties for hydrate formation was optimized using CFD software.The effects of pipeline flow rate,gas-liquid ratio,experimental temperature and experimental pressure on hydrate induction time,gas consumption rate and reaction rate and pipeline flow pressure drop were studied.It was found that with the increase of hydration reaction pressure,the hydrate induction period will be prolonged,the degree of undercooling,the hydrate formation rate,the gas consumption,the gas reaction ratio and the critical volume fraction transformed into a heterogeneous distribution will be increased.With the increase of the flow rate,the hydrate induction period will be shortened,the degree of undercooling will be reduced,the hydrate formation rate will be reduced,the gas consumption will be reduced,the gas reaction ratio will be decreased,the critical volume fraction for the heterogeneous distribution will be decreased.With the decrease of outside control temperature,the hydrate induction period will be shortened,the degree of undercooling will increase,the hydrate formation rate will be increased,the gas consumption will be increased,the gas reaction ratio will be increased,the critical volume fraction of the heterogeneous distribution will be increased.With the increase of liquid loading,the hydrate induction period will be shortened,the degree of subcooling will be reduced,the hydrate formation rate will be reduced,the gas consumption will be reduced,the gas reaction ratio will be increased,the critical volume fraction for the heterogeneous distribution will be decreased.For the CO₂+water system,the hydrate formation in the flow system can be divided into droplet entrainment,hydrate growth and hydrate plug.With the increase hydrate volume fraction,the pressure drop of the pipeline will show three characteristics,such as steady change,sharp rise and large fluctuation.Based on the continuity equation,momentum equation and energy equation,the equation for multiphase flow including hydrate phase is established,which considering the phase transition caused by hydrate formation and the exothermic properties of hydrate formation.The mass transfer based hydrate generation model was improved with the effects of spiral flow,surfactant,and gas dissolution on the gas consumption were considered,the previous model overtaking deficiencies was fixed and the expression of the gas-liquid contact surface under the slug flow was given.The simulation results show that the improved multiphase flow hydrate generation prediction model can accurately predict the formation of CO₂ hydrate in constant volume flow system.