Computer-aided Screening And Design Of Ionic Liquids As Solvents For Deep Extractive Desulfurization Processes Of Fuel Oils

With the increasingly stringent sulfur-content limitations on fuel products and the deterioration of remaining crude oils,non-hydrotreating deep desulfurization approaches have become a very significant research topic.Among various explorations,ionic liquids?ILs?based extractive desulfurization?EDS?,which takes the advantages of mild operation conditions,easy operation options of extraction,and the unique physicochemical properties,easy designable ability of ILs,is widely regarded as one of the most promising alternatives for the conventional hydrodesulfurization methods.However,taking into account the huge number of possible cation-anion combinations,screening suitable ILs by experiments is time-consuming,expensive and even unrealistic to achieve the optimal solvent.Therefore,reliable theoretical methods,i.e.,computer-aid IL screening or design,are highly desired.In this contribution,theoretical screening and design of ILs for EDS process are carried out.This work aims not only to select ILs with high extractive performance and practical potential for EDS,but also to enrich and consummate previous methods for promoting the application of ILs in other separations.Based on the solvent-sensitive character of EDS process and the fact that most previous works only take into account the distribution coefficient and selectivity at infinite dilution??^?,S^??of ILs,a modified screening method is proposed.In this method,COSMO-RS is employed as the thermodynamic model and the IL solubility in the raffinate phase?Sol?is specially considered along with ?^? and S^? as the screening criteria.To evaluate the predictive performance of COSMO-RS for IL-based EDS system,the IL solubility in model fuel hydrocarbons,the effect of cation alkyl chain length on the ?^? and S^? of ILs,the effect of water content on the EDS performance of ILs,are systematically studied by COSMO-RS and experiments.It demonstrates that COSMO-RS can provide good qualitative and acceptable quantitative predictions for IL-based EDS system;various factors that affecting such thermodynamic properties can be reasonably interpreted by COSMO-RS based molecular descriptor ?-profile and energy analysis.After validating the reliability of COSMO-RS,a thermodynamic screening of ILs for EDS system is conducted.Through analyzing the effect of anion and cation type,cation alkyl chain number and chain length on the three criteria,respectively,[C₄MIm][H₂PO₄]is preselected as a suitable candidate.Experiments of IL solubility,liquid-liquid equilibria?LLE?and multistage exraction are carried out,which confirms the high EDS performance of[C₄MIm][H₂PO₄]and suggests the reliability of the proposed method.Considering the disadvantage that ?^? and S^??derived from the infinite dilution activity coefficient ?^??cannot reflect the effect of the multicomponent and concentration in extraction system as well as the molecular weights of ILs,the distribution coefficient???,selectivity?S?,solvent loss?SL?calculated from the mass-based LLE under practical extraction conditions are used as improved thermodynamic criteria.Correspondingly,a systematic screening method combining phase equilibrium calculation,physical property prediction,and process simulation is presented.In the EDS case study,the proposed thermodynamic criteria?,S and SL derived from mass-based LLE are demonstrated to be more reasonable and efficient for screening out practically promising ILs from a large number of IL candidates by comparing with the results based on ?^? and mole-based LLE,respectively.The estimation of melting point and viscosity by group contribution?GC?models also exclude a notable number of the prescreened ILs which cannot satisfy the physical property requirements on extractive solvents.The continuous EDS process with the remaining top-ranked ILs is simulated by Aspen Plus,which proves their higher process performance than the benchmark organic solvent sulfolane and identifies[C₁OC₃Py][For]and[C₂C₁Py][Lac]as the two most promising ILs.Moreover,the ranking based on the process performance differs from that based on the thermodynamic criteria,which verifies the importance of employing process performance as the final criterion for IL screening.In view of the fact that EDS is extensively studied by experiments and the quantitative performance of COSMO-RS prediction cannot be ensured,a systematic IL design method is put forward.The UNIFAC-IL model is extended by correlating to ?^? of EDS-involved solutes in ILs and validating with LLE of related systems.Combining the obtained UNIFAC-IL model with GC models for the melting point and viscosity of ILs,a mixed-integer nonlinear programming?MINLP?problem is then formulated.The MINLP problem optimizes the extraction performance??×S?derived from mass-based LLE calculation of a multi-component model EDS system,and subjects to the constraints on structural feasibility,thermodynamic properties,and physical properties.By solving the MINLP problem,44 satisfied candidates are preselected.The top-ranked 5 ILs are subsequently introduced to Aspen Plus for process simulation,where all ILs present higher process performance than sulfolane and[C₅MPy][C?CN?₃]is identified as the process-based optimal solvent.