Study On Removing Gas Impurities From Natural Gas With Ionic Liquids

As a relatively clean energy source,natural gas is playing an increasingly important role in the energy structure of China and even the world.However,the raw natural gas obtained from the gas field is often accompanied by some gaseous impurities such as condensable gases H2O and some volatile organic compounds(VOCs),as well as non-condensable gas CO2.The presence of these gaseous impurities can cause unavoidable defects in the delivery,utilization and subsequent processing of natural gas.Therefore,these gaseous impurities must be removed from natural gas.This work aims to propose a new method for capturing these gaseous impurities by using relaticely new green solvents,namely ionic liquids(ILs)as absorbents instead of the traditional organic olvents,and conduct in-depth technical research on dehydration,removal VOCs and capture CO2 from natural gas at the atomic,molecular and systematic scales.Firstly,the technology of natural gas dehydration with ILs was systematically investigated in this work.The IL[EMIM][Tf2N]was screened out as an appropriate absorbent by means of COSMO—RS model in the dehydration process.The CH4 solubility in pure[EMIM][Tf2N]and in the mixture of[EMIM][Tf2N]+H2O were experimentally measured,and compared with the predicted results by UNIFAC-Lei model to verify this model reliability.Moreover,the CH4 dehydration experiment was carried out,and the gas product with a low water content(down to 350 ppm in mole fraction)was obtained.Finally,the continuous dehydration process was simulated and optimized using the rigorous equilibrium stage model,into which the UNIFAC-Lei model parameters were input,and compared to the benchmark triethylene glycol(TEG)process.It was found that the CH4 dehydration process with[EMIM][Tf2N]demonstrates the better separation performance,i.e.,no solvent loss,plant miniaturization,and saving energy consumption.Moreover,the reaserch on the capturing VOCs with ILs was performed.In this process,considering simultaneously capturing VOCs and water,benzene,toluene and p-xylene(BTX)as three kinds of VOCs representatives were selected the research objectives.The IL[EMIM][Tf2N]was screened as a suitable absorbent from 255 ILs including 15 cations and 17 anions by the COSMO-RS model for simultaneous removal of BTX and water.The vapor-liquid equilibrium(VLE)of BTX+[EMIM][Tf2N]mixture systems was measured and compared with the predicted results by the UNIFAC-Lei model.The experiment of simultaneously capturing BTX and water with[EMIM][Tf2N]as absorbent was conducted.Moreover,the rigorous equilibrium stage model was built and compared with the experimental results.It revealed that simultaneous capture of BTX and water with ILs belongs to a typical process intensification technology.Subsequently,the effect of IL structures on the VLE behavior of the toluene-IL systems was investigated.The VLE of toluene-IL from short-to long-chain imidazolium-based ILs(i.e.,[C4MIM]+,[C8MIM]+,[C10MIM]+,and[C12MIM]+)with various anions(i.e.,[BF4]-,[PF6]-,and[Tf2N]-)was measured.The UNIFAC-Lei model was applied to describe the VLE and successfully extended from short-to long-chain imidazolium-based ILs.It is found that for the ILs with relatively short alkyl side-chains on cations(e.g.,[C4MIM]+ and[C8MIM]+),the vapor pressure of toluene-IL sytems depends on the types of both anions and cations,while for the ILs with long alkyl side chains on cations(e.g.,[C10MIM]+and[C12MIM]+),the vapor pressure of those systems is mainly dependent on the type of cations.Moreover,the COSMO-RS model and quantum chemistry(QC)calculations were used together to provide microscopic insights into the effect of IL structures on the VLE of toluene-IL systems.Furthermore,the microscopic mechanism at atomic and molecular level with respect to the dehydration and capture BTX with the IL[EMIM][Tf2N]as an absorbent was fully revealed by the QC calculation and molecular simulation method.The result of QC calculation indicates that in the process of-simultaneous dehydration and BTX removal with[EMIM][Tf2N],both.cation[EMIM]+and anion[Tf2N]-mainly remove BTX attributed to the C-H…? bond between[EMIM]+ and BTX along with the Van der Waals(vdW)interaction between[Tf2N]-and BTX,as well the anion[Tf2N]-dominate the removal of H2O resulted from hydrogen bond(HB)between[Tf2N]-and H2O.Molecular dynamics simulations revealed that the reason that water content in the initial IL at the inlet of the absorption column during the simultaneous dehydration and BTX removal with ILs does not affect the absorption effect of BTX is the C-H…?i interaction between cation[EMIM]+and benzene ring on the BTX molecules is much stronger than the interaction between cation[EMIM]+ and H2O,resulting in that the RDF of BTX-[EMIM]+can not be changed with increasing water content in mixed system of[EMIM][Tf2N]+H2O+BTX.In addition,H2O molecule may enter a three-dimensional cage hole composed of BTX and IL molecules with the large vdW volumes due to the small vdW volume of H2O and strong HB interaction between the anion and H2O.Thus,the water content in the initial IL does not affect the absorption effect of BTX.Finally,the thecnology of CO2 removal from natural gas with ILs was researched.The modified UNIFAC-Lei model(Mod.UNIFAC-Lei)was first proposed for predicting the gas-liquid phase equilibrium(GLE)of the IL-CH4 systems over wide temperature and pressure ranges.The experimental data of CH4 solubility in various common pure ILs and mixed ILs from 243.15 K to 353.15 K were determined and compared with the predicted values by the Mod.UNIFAC-Lei model to validate the capacity of UNIFAC-Lei model.The results indicated that there is good agreement between experimental data and prediceted values by the UNIFAC-Lei model.In addiotion,it was found that a low temperature is favorable for increasing both the solubility of CH4 and the selectivity of CH4 to CO2 in ILs.This provided the thermodynamical feasibility for removal CO2 from CH4 with ILs.Therefore,the technology for capturing CO2 from natural gas using ILs at low temperatures has been proposed,and the rigorous equilibrium stage model embeded in the Mod.UNIFAC-Lei model parameters was built to achieve the process simulation and optimization of separation CO2/CH4 with the IL[BMIM][Tf2N]at industrial scale during low temperatures.The results indicated that ILs have the greatly high efficiency of CO2 capture,in which the absorption ratio of CO2 is as high as 99.45%,and the loss ratio of CH4 gas in the process is only 0.62%.This further confirms that the separation of CO2/CH4 with ILs as an absorbent at low temperatures is a promising technology.