Study On The Formation Characteristics Of Carbon Dioxide Hydrate With Liquid Hydrocarbons

With the gradual improvement of people's requirements for indoor thermal comfort,the use of air conditioners is increasingly popular,which has aggravated the electricity load of urban power grids.Therefore,cold storage air-conditioning technology has been paid more attentions.The cold storage for air-conditioning system consumes power at low-valley time and supplies cold energy during peak hours.It effectively achieves the purpose of shifting the peak and filling the valley.Hydrates used as cold storage medium for cold storage systems have the advantages of large cold storage capacity and phase transition temperature range suitable for conventional refrigeration systems,so it has been widely studied.However,in practical applications,hydrates have a long induction time and a small amount of hydrate form.These problems are the key factors restricting the development of hydrates technology.For this reason,it is particularly important to study the promotion methods from the two aspects of phase equilibrium thermodynamics and formation kinetics of hydrates.From the above two perspectives,the effect of hydrophobic liquid hydrocarbons on the phase equilibrium conditions of CO₂ hydrate is studied in this work.The phase change latent heat values of hydrate with the liquid hydrocarbons are also calculated.Then,the formation processes of CO₂ hydrate with alkane additive are explored,and their optimal additive mass concentration are determined.The researching contents are as the following:(1)The effect of n-dodecane,n-tetradecane,n-tetradecane+n-heptadecane liquid alkane additives on the CO₂ hydrate phase equilibrium temperature-pressure conditions is studied using the constant volume stepwise heating method.The experimental results show that the phase equilibrium pressure of the CO₂ hydrate with n-dodecane is lower than that of CO₂+pure water system at higher temperatures.The addition of n-tetradecane or n-tetradecane+n-heptadecane also reduces the phase equilibrium pressure of carbon dioxide hydrate.As the temperature increases,the promoting effect of liquid hydrocarbons becomes more obvious.At higher temperatures,the promoting effect of n-tetradecane is stronger than that of the mixture of n-tetradecane and n-heptadecane.The quintuple point of CO₂ hydrate with the mixture of n-tetradecane and n-heptadecane(w=0.3363)is almost the same as the quadruple point of carbon dioxide hydrate in pure water.(2)Latent heat of gas hydrate of the above systems are calculated based on hydrate equilibrium temperature and pressure data using Clausius-Clapeyron equation.It is found that the addition of liquid hydrocarbons has little effect on the latent heat of CO₂ hydrate.(3)The effect of initial pressure,temperature,stirring rate,and the addition amount of liquid hydrocarbons(n-dodecane and n-tetradecane)on the induction time of CO₂ hydrate formation and gas storage was studied experimentally.The amount of CO₂ hydrate formation in pure water is not much,and hydrate formation rate is slow.The induction time of CO₂ hydrate formation is about 1339 min when the pressure is 2.4 MPa.The gas storage capacity of hydrate formed in pure water is only 38.48 V/V.It is beneficial to the formation of hydrates with initial pressure increase or temperature decrease.The addition of liquid hydrocarbons also effectively shortens the induction time of CO₂ hydrate formation and increases the storage capacity of gas in hydrates.Compared with the addition of n-dodecane,the formation induction time of CO₂ hydrate with n-tetradecane becomes longer,and the gas storage capacity in hydrates becomes smaller.