Kinetics And Mechanism Of Methane Hydrate Formation Pormoted By Extract Of Chinese Herbal Medicine

Gas hydrate technology shows great economic value in the field of gas storage and transportation,mixed gas separation,CO2 capture and storage,seawater desalination and so on.However,gas hydrate all suffurs from harsh formation condition,in which slow formation kinetics and low gas density cannot meet the requirement for industrial application.Surfactants,espcially SDS,are widely used in promoting the formation of methane hydrate.However,most of them come from the petrochemical industry as a by-product which is hard to degrade naturally,inevitably causing some environmental problems.Chinese herbal medicines(CHM),which are the crystallization of the Chinese wisdom during the last five thousand years,play a significant role in food,health care products,cosmetics,daily necessities and medicine industry,but the application in promoting the formation of gas hydrate has not yet been developed.Thus,based on 600 kinds of CHM extracts,we quickly screen the effective one through high-throughput experiments and study methane hydrate formation kinetics in each extracts,then explore the similarities of different promoter moleculars from the point of structure-property relationship.In addition,molecular dynamics simulation will be used to study the methane hydrate promoting mechanism in the existance of amino acid.It was found that the some of extracts can greatly enhance the formation kinetics of methane hydrate,especially Burnet and Rohdea japonica,the capacity can reach up to 148 mg/g and 145 mg/g,respectively,and t90(time to reach 90% of maximum capacity)was only 22 min and 20 min.Capacity of scutellaria baicalensis extract was only 123 mg/g.However,when the extract is dried to constant weight,aqueous solution of the solid extract at a concentration of 0.05 wt% shows similar performance compared to Burnet extract,which the capacity reaches up to 148 mg/g and t90 is only 25 min.That is,optimal concentration is existed in each of the effective CHM extracts.Further studies show that some of the amino acid,natural glycosides and their derivatives,extracted from CHM,are all efficient accelerator in the formation process.0.5 wt% of L-leucine,amygdalin and methyl gallate can achieve a high capacity of 143 mg/g,144 mg/g,144 mg/g,respectively,significantly better than SDS.It is worth mentioned that L-methionine can not only greatly promote methane hydrate formation kinetics,but also enhance the formation kinetics of CO2 hydrate.CO2 capacity of 0.2 wt% L-methionine reaches up to 356 mg/g,while t90 is only 15 min.The relationship between molecular structure and performance was studied and found that,for promoters with similar structure,special functional group have a significant impact on their performance.The promotion of amino acid molecules is derived from the synergistic effect of –NH2 and –COOH.However,compared to series of glucoside moleculars,structure with –phenyl shows the best promoting performance,while structure with –dodecyl and –methyl shows no promotion effect.In addition,Optimal carbon chain length is existed in promoter molecular structure.For instance,L-leucine homologues will achieve the best promotion effect only when carbon chain length contians 6 “C” atom.Similarly,the best performance of gallate homologues occurs when the number of “C” of ester reduces to 1.Combining the macroscopic phenomena with the microscopic molecular dynamics(MD)simulation,it is concluded that the hydrate formation mechanism based on the surface tension is only applicable to the surfactant system,unpersuasive for non-surface-active promoters such as amino acids.The promoting effect of amino acids in the formation of methane hydrate is mainly due to the adsorption on the gas/liquid interface,and also hydrate surface.Specifically speaking,in the process of adsorption and desorption,interfacial disturbance is enhanced so that methane hydrate can not be formed at the gas-liquid interface,but firstly nucleate at three-phase junction where the temperature is the lowest and the concentration of methane the highest.And then methane hydrate will continue to grow upwards along the reactor wall,driven by capillary effect.