Study On Phase Equilibrium Theoretical Predictions And Dynamic Characteristics Of Gas Hydrate In The Complex System Containing Thermodynamic Additives

Gas hydrates,which has non-stoichiometric solid inclusion bodies,are crystalline solid structures consisting of water and small gas molecules,water molecules act as host and small gas molecules are guest.Most gas hydrates exist in permafrost land and seabed sediments,similar to natural gas,gas hydrates mainly use methane as guest molecules and are often referred to natural gas hydrate.From 1980’s,nations around the world began to carry out the investigation and research on the basis application of gas hydrate.Nowadays,hydrate technology has been widely used in sea water desalination,solution concentration,gas separation,gas storage and transportation,CO₂ deep sea storage,refrigeration and cold storage,automobile fuel and biological engineering,and other fields.With the further development of new energy in different fields of science,the hydrate technology are more widely used,such as recycle hydrogen ammonia,hydrogen manufacturing and hydrogen storage,hydrogen fuel cell and solar energy utilization,etc.However,many issues impede the extensive use of hydrate technology,such as severe formation conditions,long induction period and low hydrate conversion rate etc.,Adding additives to the hydrate system can effectively reduce formation pressure and increase the generating temperature,shorten the induction time,improve conversion rate,which have remarkable effect on kinetics and thermodynamics conditions during hydrate formation.At present,the study of hydrate basic properties in the additive system mainly includes experimental and simulation studies.Experimental study is a long and complicated process,the result of the experiment is susceptible to human factors and uncontrollable environmental factors,such as measurement accuracy,the airtightness of experimental device,etc.,the deviation of data is difficult to be noticed.While simulation study is less time consuming,high universality and theoretical,not affected by environmental and human factors,etc.,which can deep analysis heat transfer and mass transfer during the hydrate growth process,provide reasonable explanations for the change of the phase equilibrium state,and can also make up for the inadequacy of experimental research.It is of great significance to further reveal and understand the hydrate growth mechanisms.Therefore,through simulation method,this work studied kinetics and thermodynamics performance of hydrate formation by pure gas/gas mixture components in pure water /water + Tetra-n-butyl Ammonium Bromide（TBAB）,pure water + cyclopentane（CP）/cyclohexane（CH）and emulsified oil systems,respectively.First,through analyzing the phase equilibrium experiment data of pure gas（CH₄,N₂,CO₂）hydrate systems in the presence of TBAB,the activity of aqueous phase is associated with additive concentration and the system temperature,the change characteristics of the aqueous phase activity are obtained,and a correlation for calculating the aqueous phase activity of pure gas system thermodynamic model is proposed.Compared with experimental data,predicted results of the correlation in H2 O + TBAB + CH₄ system and H2 O + TBAB + N₂ system are fairly satisfactory,the pressure scopes are 0 20 MPa and 0 70 MPa,respectively;As for H2 O + TBAB + CO₂ system,because the hydrolysis reaction happens when CO₂ soluble in water,and leads the aqueous PH value turning to weak acid,which has a great influence on both the additive performance and the aqueous activity,so predicted results has relatively large deviation with the experimental data.Then,systematically studied hydrate phase equilibrium thermodynamic model in pure gas（CH₄,CO₂,N₂）and mixed gases（CH₄ + N₂,CO₂ + N₂,low concentration coal-bed methane）systems in the presence of CP or CH.An assumption that the volatility of additives will affect gas phase components and their mole fractions,thus change the system equilibrium conditions;Considering the gas solubility and the influence of additives on the aqueous phase activity,Henry’s law and UNIFAC group contribution method are used to calculate gas solubility and activity coefficients of the gas phase and aqueous phase components,respectively,combined the PSRK group distribution equation coupling GE rules and Chen-Guo model,established a high precision,wide applicable hydrate phase equilibrium thermodynamic model.Analyse the model results:（1）When adding a parameter of additive concentration in the secondary calculation of gas phase mole fractions,though the calculations of CP and CH volatility in aqueous phase lead simulation results of binary mixed gases systems seem not conform to the degree of freedom,they improve the accuracy of the prediction model;（2）In CP + binary mixed gases systems,the volatility of additive has an impact on gas phase components and their mole fractions,then influence the phase equilibrium conditions of the system;（3）Though the variation of the CP concentration indeed effects the equilibrium state of the system,it turns out to be very small or non-existent,especially in CH₄ + N₂ system.So the model results are considered as obeying the degree of freedom,i.e.,when the components are determined,phase equilibrium pressure only relate to the system temperature;（4）For most non-polar gases and additives that have functional parameters,the UNIFAC activity coefficient method has high precision and the model results are excellent,this method has feasibility and widespread applicability in research of phase equilibrium properties;（5）Some gases dissolving in water will affect PH value of the aqueous phase,it needs to modify parameters of the UNIFAC model or add parameters that associated with gas physical property in the phase equilibrium model.Finally,contrapose the inaccuracy regression parameters in the existing dynamic model for the W/O emulsified oil system,the solubility of methane in oil phase cannot be calculated and measured,the prediction moldel is difficult to widely used in industrial production,this work improved the dynamic model of Turner et al.by introduces industrial ground oil dissolving capacity model,coupling the methane solubility model in crude oil,and studied dynamics mechanism and features of the water-in-oil emulsion droplet during the methane hydrate formation.The results showed that:（1）Gas hydrate shell blocking the mass transfer process is the main factor to lower the reaction rate;（2）Smaller droplets need shorter time and lower methane consumption for completely conversion to hydrate;（3）The methane solubility in oil-liquid phase increase with the rise of system pressure,methane concentration become the driving force for the methane consumption;（4）The droplet of same size under 4.92 MPa requires less completely conversion time than 4.35 MPa,the conversion rate is higher than 4.35 MPa as well.In this work,theoretical study on reaction kinetics mechanism of gas hydrate formation process and phase equilibrium thermodynamics characteristics in emulsified oil,pure water + TBAB and pure water + CP / CH system are carried out,inward shell growth mechanism and kinetics characteristics of methane hydrate formation from the water-in-oil emulsion droplet are obtained,a correlation of aqueous activity for the TBAB + pure gas systems is proposed,the influence of the additives volatility on phase equilibrium state is affirmed,the hydrolysis reaction of CO₂ would cause deviation to the prediction model,the widespread applicability of UNIFAC method in simulation study of hydrate thermodynamics is proved.Researches of this work not only improve the hydrate reaction kinetics model in emulsified oil system,optimize the phase equilibrium thermodynamic model of hydrate systems that contain additives,but also have important academic value and engineering significance for the development and industrial application of gas hydrate technology.