Molecular Simulation Of The Effect Of Ring Compound-Accelerators On The Clathrate Hydrate

Clathrate hydrate is a kind of solid substance composed of water molecules and guest molecules(mainly small molecule gases).The holes formed by hydrogen bonding between water molecules have selectivity and high gas storage rate for gas molecules.Hydrate solidifying technology is a new type of gas solidification based on this,which is widely used in the transportation,storage and separation of gases.Under the natural conditions,hydrate stability is characterized by low temperature and high pressure.Meanwhile thermodynamic accelerators can make hydrate keep stability under mild conditions.Among them,the thermodynamic promoters represented by ring compound focus on the experimental researches.But it is hard to explore the mechanism of microcosmic action;Molecular simulation provides a way to look insight into the characteristics of stable hydrate,which has a certain guiding significance for the industrial application of hydrate solidification technology.Molecular simulation is an ideal way to study the molecular and structural changes of hydrate guest.we analyze the microstructure of hydrates according to the first principle of molecular dynamics.The lattice parameters of hydrate is obtained from X ray diffraction experiments.Discribing the hydrogen bond interaction between water molecules by Simple Point Charge potential energy model.The crystal structure model of type II cage hydrate is built by Material Studio(MS).And Monte Carlo method is used to place guest molecules in clathrate hydrate lattice.Then we describe the interaction forces between guest molecules and hydrate framework with Lennard-Jones potential.After that,a lot of Van der Waals models of gas molecule are built and optimized.we calculate different models in the NVT and NPT ensemble by MS and comparing the stability of hydrates under different temperatures and pressures with related experience function as evaluation index.There are the results:(1)We research the thermodynamic conditions for the existence of hydrate stability,which shows low temperature and high pressure are beneficial to hydrate temperature.it is in accordance with the experimental simulation results.(2)We study the stability of 3 kind of models at different temperatures and pressures,which contains ring compound-accelerators of tetrahydrofuran,tetrahydropyran and cyclopentane.The results show that cyclopentane and tetrahydrofuran have better promotive effects,and tetrahydropyrans have a poor promotion at high temperature and low pressure.We analyze the simulation results from the molecular point of view and explain the mechanism of action.The crystal lattice size of hydrate occupied by accelerator is similar,and the occupation ratio is between 0.76 ~1.The structure of hydrate is stable.(3)The effects of two kinds of accelerators on the stability of hydrate are simulated.it turns out that simulated system always could keep stability when the occupancy rate of the big hole is 100%.However all systems structure is failure with occupancy rate dropping to 50%.(4)We simulate the hydrate stability of low-concentration coalbed methane which add accelerators of cyclopentane and tetrahydrofuran respectively.The final simulation results provide a theoretical basis for solidifying low concentration coalbed methane by hydrate technology.