Experimental Study On The Characteristics Of Hydrate Occurrence And Memory Effect

As a special phase change material,gas hydrate has great development potential in natural gas storage and transportation,cold storage with phase change and resource exploitation.However,the complex formation and dissociation characteristics of gas hydrate are the key points that restrict the hydrate resource development and technical application.Based on the points,a set of visual high-pressure hydrate experiment apparatus was set up in this paper.Growth and dissociation forms of hydrate sediments,memory effects of hydrate reformation and the mechanism and internal correlation of the effect of depressurization on memory effects were studied in particular.The specific research contents are as follows.(1)Formation and dissociation forms and occurrence laws of THF hydrate and methane hydrate were obtained by experiments.Different growth laws between first formation and reformation of methane hydrate were clarified.In addition,an unstable flocculated hydrate was found in the reformation process,which exhibited the property of growing first and then shrinking.The flocculated hydrate was the first time to prove the intrinsic relationship between micro-nanobubbles and hydrate reformation using the way of experiments.Moreover,the channel of gas migration and expansion of porous media were observed in hydrate sediments.In particular,the expansion phenomenon was more obvious during the dissociation by depressurization,and the volume expansion rate of porous media reached 64.7% in the dissociation process.These occurrence characteristics had important influence on the heat transfer of the hydrate dissociation process.(2)Effects of pressure difference and temperature on the memory effect and its stability were studied by experiments.The experimental results showed that the induction time of hydrate reformation decreased and memory effects tended to remain stable in the closed and heating dissociation pattern.With the increase in dissociation temperature,the memory effect gradually weakened,and the memory effect disappeared when the temperature at 25°C above.The smaller the backpressure,the weaker memory effects of first hydrate formation and stable reformation were after dissociation by depressurization,in which the stable induced period and subcooling of reformation after hydrate dissociation at atmospheric pressure were 3 times and 2 times of those without depressurization,respectively.In addition,after the system pressure was compensated,the thickness of water layer above the porous media gradually decreased with the reduction in backpressure.When hydrates dissociated at atmospheric pressure,the water layer disappeared completely,which caused that the induction time of first hydrate formation was 1/3 shorter than that of reformation.(3)Growth characteristics and memory effects of methane hydrate with porous media unsaturated by water were studied by experiments.The results showed that when the formation of pure methane hydrate was concentrated over the porous media,its formation rate would increase significantly,which was about 18 times of the rate of first hydrate formation.In addition,the induction period of the first formation of pure methane hydrate after dissociation at 4.4 MPa was the shortest,which was 64.3% shorter than that without depressurization.(4)Based on the research results,experimental methods and an platform based on the boiling-condensation process of refrigerant were independently designed and developed to enhance hydrate formation.The preliminary studies about the formation and dissociation of R134 a refrigerant hydrate in the boiling-condensation pattern were carried out.This paper found that the induced period of hydrate formation disappeared and the formation rate increased by several orders of magnitude in this pattern.The experimental results showed that the increases in gas-liquid interface,mass transfer enhancement and disturbance phenomenon were the key factors restricting the growth rate of hydrates.