Experimental And Theoretical Research Of The Effect Of TBAB And THF On Coal-bed Methane Hydrate Phase Equilibrium

As a high energy density, burning clean and valuable resource in ourcountry，Coal-bed methane (CBM) is very rich in reserves. However, the currentlow utilization of low concentrations of coal-bed methane, most of being vented,not only resulted in a waste of resources, but also led to the destruction of theenvironment. Rational use of low concentration coal-bed gas is of greatsignificance to protect the environment, reduce coal mine accidents, and achievesustainable energy development. Hydrate method separation of lowconcentration coal-bed gas has the advantages of mild reaction conditions、lowenergy consumption and storage stability, so it has been widely concerned andresearched by Domestic and foreign scholars.The main ingredients of CBM are CH4, N2, and O2. These ingredients canhydrate under different temperature and pressure conditions. Using this principle,we achieved a separation of CBM. Because hydrate formation needs harshconditions, so adding additives (such as Tetrahydrofuran (THF),tetra-n-butyl ammoniu bromide (TBAB), cyclopentane (CP) etc.) can improve the hydrateformation conditions. How much impact will have on the hydrate after addingmixture of TBAB and THF, which is studied relatively small for the domesticand abroad scholars. The domestic research on TBAB hydrate thermodynamicmodel is not deep enough. Therefore, this paper built a set of gashydrate experimental platform. Through the oxygen hydrate phase equilibrium,we verify the accuracy of the experimental system. On the basis of abovecontents, we add composite of TBAB and THF, studying the experimentalabout gas hydrate phase equilibrium. At the same time, using the fugacity modelcombined with the PRSV equation of state, we predict the phase equilibriumof gas hydrate in pure water system and TBAB system. These can providetheoretical basis for the separation of low concentration coal bed methane torealize industrialization. Specific research work includes:(1)For the purpose of separating out low concentration coal bed methane(30%CH4+65%N2+5%O2) by using the method of hydrate, We have establisheda set of experimental apparatus for gas hydrate formation, have done the oxygenhydrate phase equilibrium experiment under the existence of THF, and havecompared with the reference data to verify the accuracy and the reliability of thedevice.(2)According to the orthogonal experimental designing method, we havedesigned nine groups mixed systems of different concentrations of THF andTBAB, and have obtained the phase equilibrium data of the coal bed methanehydrate under the above device by the corresponding experiment. Though theanalysis of data, the results has been indicated that only small amount of themixture concentration can be reached to the10times concentration effect ofsingle-component and the phase equilibrium curve has been significantlyshifted to the right. Once the concentration of THF (or TBAB) fixed, the hydratephase equilibrium curve has not been shifted to the right with the gradual increasing concentration of the TBAB (orTHF). The reason is that theTHF generates the II hydrate, the TBAB generates semiclathrate hydrate, andthe two kinds of hydrate lattice should co-exist when the two additives havebeen mixed together. The primary and secondary relationship of the two typesof crystal lattice is related to the concentration ratio of each other. Through theanalysis of orthogonal test, the results have showed that: the order aboutthe influence of various factors on the phase equilibrium is THF THF×TBAB TBAB.(3)With the fugacity model and the PRSV equation of state, we havepredicted the gas (CH4, N2, and O2) hydrate phase equilibrium under thesystem of pure water. By the analysis of deviation, the result shows that thefugacity model is a reliable thermodynamic model for calculation of hydratephase equilibrium. Under this model, we have researched the thermodynamicmodel of the CH4and CBM with the presence of TBAB. Through thecalculation of deviation and the analysis of phase equilibrium image, the resultsshow that for the single component gas the predicted results are good in lowtemperature（281.67K～292.2K）and with the increasing of temperature, thepredictive values are gradually lower than the experimental values; But for thecoal-bed methane hydrate the predicted results of phase equilibrium are ideal.