CO₂ Capture And H₂S Removal From Syngas Using Ionic Liquids

Carbon dioxide(CO2)and hydrogen sulfide(H2S)are the main industrial acid gases produced in the combustion processes of carbon-based fuels,coal and oil,and also are primarily found in natural gas and biogas.H2S content in fuel and flue gases should be lowered to ppb or ppm levels(e.g.,as low as 50 ppb in syngas for Fischer-Tropsch Synthesis catalysts)due to its detrimental impacts on equipment,health,and nature.Heat-trapping long-lived CO2 also is the main anthropogenic greenhouse gas which its concentration in the atmosphere reached 405.5 ppm in 2017,while surged at an annual record-breaking rate of 3.3 ppm in 2016.Emissions from combustion and purification of fuels and from industrial production make predominant contribution to the swift increase of concentration of the atmospheric CO2.After a growth of 1.6%in 2017,the rise in global CO2 emissions are subjected to reach an all-time high of 2.7%in 2018.Acid gas removal(AGR)processes based on absorptive solvents are currently the main industrially accustomed approaches to capture CO2 and to remove H2S.Aqueous alkanolamines,being able to effectively chemically absorb low-concentration CO2(<15 vol.%)along with deep H2S removal,are the most commonly used solvents in AGR processes.However,they suffer from an energy-intensive solvent regeneration step,along with being volatile,corrosive,and susceptible to oxidative degradation.Due to the Mreak interaction with the acid gases and thus easier regeneration,physical solvents are preferred to chemical ones for a feed gas stream having an elevated CO2 concentration..Rectisol process using-chilled methanol(MeOH)and Selexol process using dimethyl ether of polyethylene glycol(DEPG)are the most common physical absorption processes.They are both energy-intensive processes,while operate based on multi-column complex schemes and eco-unfriendly solvents.Synthesis gas(syngas),which can be virtually manufactured from any hydrocarbon feedstock,is a value-added supply of energy whereby CO2 and H2S should be separated from hydrogen(H2)and carbon monoxide(CO).After purification,H2S is sent to CLAUS unit,while CO2 can either be used in downstream processes,such as urea production,or can be utilized in enhanced oil recovery(EOR)or carbon capture and storage(CCS)units.A physical solvent process capable of achieving a high CO2 capture level(CO2CL,e.g.more than 97%on molar basis with less than 100 ppm H2S for EOR)in an energy-efficient environmentally benign way can be highly attractive for industrial application.In the past few years,ionic liquids(ILs)have been recognized by many researchers as propitious physical absorbents for CO2 and other acid gases as well,due to their non-flammability,designing versatility,non-corrosivity,and high acid gas uptake capacity.This work explores the practical exploitation of ILs in the AGR processes with regard to new experimental investigations,thermodynamic modeling,process design and simulation,and IL solvent design techniques.Herein,the focus has been on syngas purification;however,many techniques proposed here(specially the IL solvent design methodology)can be extended to CO2 capture and H2S removal from other sources such as coal gas,natural gas,and biogas.In this work,firstly,CO2 solubility in the allyl-substituted[AMIM][Tf2N]IL,MeOH,and their binary mixtures with different combinations(80 wt%[AMIM][Tf2N]+20 wt%MeOH,50 wt-%[AMIM][Tf2N]+50 wt%MeOH,and 20 wt%[AMIM][Tf2N]+80 wt%MeOH)at temperatures of 313.2,333.2,and 353.2 K and pressures up to 6.50 MPa were measured experimentally ILs were assumed to be non-volatile,and for the pure,MeOH and the mixed IL-MeOH solvents the gas phase composition above the solution mixtures were obtained by chromatography technique.Each constituent benefits from the thermophysical advantages of the other,and thus the new IL-MeOH solvents are proposed for the industrial usage.Moreover,the influence of addition of each to the other in VLEs were analyzed.Adding certain quantities of IL to MeOH brings about increase in CO2 solubility in the liquid phase and decrease in the volatile MeOH content in the gas phase.Mole fraction of CO2 in pure[AMIM][Tf2N]reaches around 0.6,0.5,and 0.4 at pressure of 6 MPa and temperatures of 313.2,333.2,and 353.2 K,respectively.Secondly,by means of correlation of the experimental and the literature data,new group binary interaction parameters(αmn and αnm)of the UNIFAC-Lei predictive thermodynamic model(which are required in the modeling of the syngas-involved processes)were introduced.The consistency between the experimental data and the predicted results in the UNIFAC-Lei model was investigated.The applicability of the UNIFAC-Lei model for the pure IL,the pure organic solvent,and the CO2-IL-organic integrated systems is justified since the predicted results agreed well with the experimental data with ARDs below 10%.Furthermore,the Henry’s law constants derived from the COSMO-RS model were utilized to investigate the solubility and selectivity of CO2 and H2S gases in various ILs.Meanwhile,CO2 and H2S solubility data collected from the literature was used to calculate the experimental Henry’s law constants and to validate the COSMO-RS predicted results.COSMO-RS gave satisfactory results for CO2-IL systems,while it systematically underestimated the H2S Henry’s constants.Hence,a modification correlation for COSMO-RS H2S Henry’s constants was introduced,by means of which the ARD between the experimental and a priori COSMO-RS results was reduced from 89%to 22%.Moreover,in-depth analysis of the COSMO-RS-derived σ-profile and σ-potential of CO2,H2S,H2,CO,and featured ILs were carried out to provide new insights into intermolecular interactions between the gases and ILs.Thirdly,the newly obtained UNIFAC-Lei parameters were incorporated into the UNIFAC property model of the Aspen Plus software,to optimize conceptual processes designed for purification of CO2-containing syngas streams.Four IL processes(pure[AMIM][Tf2N]at absorption temperature(Tabs)=243.2 K,pure[AMIM][Tf2N]at Tabs=265.7 K,pure[AMIM][Tf2N]at Tabs=288.2 K,and 50-50 wt%[AMIM][Tf2N]-MeOH)and a semi-Rectisol MeOH process with rigorous Radfrac equilibrium models were developed.The simulation results indicated that,in comparison with the benchmark semi-Rectisol MeOH process,using[AMIM][Tf2N]either mixed with MeOH or purely considerably lowers the processes power consumption,while improves the performance of the processes in terms of CO2CL and solvent loss.The pure[AMIM][Tf2N]process at Tabs=243.2 K was the most efficient one from the aspects of environmental benignity and process performance factors.Furthermore,using the UNIFAC-Lei parameters in the Aspen plus property model,a practical IL-based process for CO2 capture and H2S removal from syngas was designed,simulated and optimized in Aspen plus.[EMIM][Tf2N]solvent was utilized,and the newly developed IL-based process was named IL-Emitsol.Based on the Literature,a complete Rectisol process was also simulated and optimized and was taken as the reference AGR process,to be compared with the IL-Emitsol process.The IL-Emitsol process yields three product streams as:(1)a clean syngas with 0.27 mole%CO2 and less than 100 ppb H2S,(2)a CO2 stream with 96.68%CO2CL and 99.37 mole%purity with less than 100 ppm H2S,and(3)a H2S stream containing 63.66 mole%H2S In comparison with the benchmark Rectisol,the IL-Emitsol is more efficient in terms of clean syngas production,CO2 capture and production,H2S production,solvent loss,and electricity consumption;However,it requires nearly three times higher direct capital investment.Finally,A systematic ionic liquid(IL)screening/design methodology for AGR processes,exemplified by the syngas purification case,was established by taking into account a wide range of process performance-determining factors.Making use of the COSMO-RS and other fit-for-purpose IL-systems predictive models,process thermal efficiency(absorption enthalpy and sensible heat)and absorption-desorption working capacity were included in the design of ILs along with the essential factors such as solubilities and selectivities.Melting points of 5980 ILs formed from 130 cations and 46 anions were calculated using a group contribution(GC)model.After that,viscosities of the ILs that have liquid state at operating temperatures of low-and high-temperature processes(assumed to be 265.7 and 288.2 K,respectively)were predicted by the in silico artificial neural network and COSMO-based models.Based on the Henry’s law constants,absorption selectivity indices were defined to investigate the selective and non-selective removal of CO2 and H2S from syngas in conceptual low-and high-temperature absorption process configurations,and to single out the optimal IL solvents for each process.Meanwhile,based on the defined selectivity indices,ILs possessing higher selectivities than traditional solvents,i.e.,MeOH and DEPG,were identified.Afterwards,the efficiency of ILs in uptake and release of the acid gases was evaluated by working capacity.Then,thermal efficiency of the processes ascribed to exothermic absorption enthalpy and required sensible heat(for temperature-swing)of ILs was addressed.Finally,stability,toxicity,and cost of the pre-selected ILs were taken into account,and the top 2 ILs for four conceptual low-and high-temperature syngas purification processes were introduced.