Experimental Study On Multi-component Gas Hydrate Dissociation Via Depressurization

Due to the development of human society and economy relying on traditional fossil fuels for a long time,the problems such as the global climate change,resource scarcity and environmental pollution are increasing daily.Natural gas hydrate,known as a kind of clean and efficient unconventional energy,gradually becomes a hot research topic all over the world.Hydrates in nature can be divided into biogenic methane hydrate and pyrolysis multi-component hydrate according to the origin of formation gas.The dissociation characteristics of multi-component hydrate differ greatly with that of pure methane hydrate because of the complex component,various structures.In addition,depressurization is believed as the most economical and feasible method among the regular hydrate exploitation methods.Hence,it is necessary to study the characteristics of multi-component hydrate dissociation via depressurization.Firstly,we conducted experiments about formation of methane-ethane,methane-propane,ethane-propane and methane-ethane-propane hydrates with a transparent stirred reactor.In the stirred system,we have noticed that hydrate formed both from dissolved gas and gas-liquid interface during the formation process.The evolution of each component in gas showed obvious separation phenomenon during the formation process of methane-ethane,methane-propane and methane-ethane-propane hydrates.In contrary,the fraction of each component in gas became closer within ethane-propane hydrate formation process.as propane(ethane)entered the hydrate phase followed by methane.Propane,ethane and methane successively filled in cage of hydrate structure.Secondly,the dissociation experiments of binary hydrates(methane-ethane,methane-propane and ethane-propane)and ternary hydrates(methane-ethane-propane)were both conducted in transparent stirred system and statistic system to study the influences of temperature,back pressure and system type on hydrate dissociation,respectively.The stability of hydrate cage was different due to the filled gas,which generated several periods during the process of multi-component hydrate dissociation in high pressure.The phenomenon of “temporary retention” and “long-term retention” were noticed in ethane and protane,respectively,which were more obvious in statistic system,low temperature,high pressure and larger difference of stability for different hydrate cage.Due to the phenomenon above,the hydrate retented in dissociated zone would form into hydrate layer that wrapped around inner hydrate particle and impeded its dissociation.Finally,the system came into “metastable status” and behaved as part dissociation.Finally,some experiments were carried out below ice point to study the self-preservation effect on the multi-component hydrate.The experimental results showed that each kind of hydrate dissociated partly,which came from the ice film of self-preservation of hydrate below ice point.This phenomenon in-4? was more obvious than that in-1?.In addition,the rention of protane revealed gradually with the growth of ice film and that in-4? appeared earlier than that in-1?,while,there was no obvious ethane rention during the whole process in experimental conditions.