Separation Process Intensification Of Ionic Liquids/Deep Eutectic Solvents And Fundamentals Of Predictive Molecular Thermodynamics

Fluid catalytic cracking（FCC）diesel in petrochemical industry contains a large number of polycyclic aromatic hydrocarbons（PAHs）.Due to the limitation of solvent performance,traditional solvent extraction methods are difficult to achieve effective separation.At the same time,the downstream chemical industry has large condensable gas flow and high concentration.The use of traditional organic solvents as absorbents has problems such as volatility,difficulty in regeneration,and easy to cause secondary pollution.In order to give full play to the green and efficient separation performance of ionic liquids and their analogues,this paper proposes the new technologies of PAH extraction with ionic liquid and condensed gas capture with deep eutectic solvent to realize the effective utilization of PAHs resources and the efficient capture of condensed gas.In view of the diversity of the structural composition of ionic liquids and deep eutectic solvents and the lack of phase equilibrium data and thermodynamic model parameters caused by the large number of separation systems（FCC diesel components and types of condensate gases）.Under the guidance of predictive molecular thermodynamics theory,this paper systematically studied the practical application prospects of ionic liquids and DESs in two industrial backgrounds from molecular scale to process scale.The main work of this paper is as follows.（1）Based on the group contribution method,considering the separation performance indexes such as distribution coefficient（D）,separation factor（β）,solvent loss（SL）and solvent capacity（Cp）under infinite dilution,a computer-aided ionic liquid molecular design and screening program was developed,and a new multi-stage countercurrent-reflux PAH extraction technology with ionic liquid was proposed.For the two typical separation systems of n-hexadecane+1-methylnaphthalene and n-dodecane+biphenyl in FCC diesel,[BMIM][BF₄]was determined as the candidate extractant.At the molecular scale,quantum chemical calculations and molecular dynamics simulations were used to explore the aggregation characteristics of molecules during the separation process.Quantitative calculation analysis showed that the interaction between[BMIM][BF₄]and bicyclic aromatic hydrocarbons was mainlyπ-π,C-H···πand hydrogen bonding.On the single-stage equilibrium scale,the liquid-liquid equilibrium data of two binary systems were calculated based on COSMO-RS and UNIFAC-Lei models,and the phase equilibrium data were measured by a series of extraction experiments,which filled the gap of basic experimental data in this field and further confirmed the reliability of the predictive model.On the process scale,the UNIFAC-Lei model group parameters were embedded in Aspen Plus（V 11.0）to establish a new process of multi-stage countercurrent-reflux PAHs extraction with ionic liquid,which broadened the application scope of the UNIFAC-Lei model.（2）Under the guidance of predictive molecular thermodynamics theory,a simulation program of PAH extraction with ionic liquids considering the whole molecule（712 kinds of FFC diesel components）was constructed,and molecular refining was realized.The phase equilibrium data of aromatics with different ring numbers（toluene,1-methylnaphthalene and phenanthrene）and n-hexadecane were measured to improve the prediction accuracy of UNIFAC-Lei model by modifying the model parameters,and extend it to the ILs-FCC diesel system.[BMIM][BF₄]was selected as the extractant.A new technology of multi-stage countercurrent-reflux extraction of aromatics from real FCC diesel by ionic liquids was proposed,which realized the full molecular simulation of real fuel oil.The simulation comparison between real FCC diesel oil（including 712 components）and model oil（only n-hexadecane+1-methylnaphthalene）shows that the former is closer to the experimental data,and the average relative deviation（ARD）is only 6.71%,which proves the importance of considering the whole molecule in molecular refining.（3）A multi-functional（simultaneous desulfurization,denitrification and aromatics removal）integrated extraction technology using[EMIM][NTf₂]as extractant was proposed.The liquid-liquid equilibrium data of the system were measured by experiments,and the separation performance and extraction mechanism of ionic liquids were studied in depth by quantitative calculation.From the molecular scale,the quantitative calculation analysis shows that[EMIM]⁺plays a dominant role in the extraction process.The imidazole ring of[EMIM]⁺interacts with heterocyclic aromatic hydrocarbons mainly in the form ofπ···πstacking,and interacts with 1-methylnaphthalene mainly in the form of C-H···π.The anion is mainly formed by hydrogen bond interaction between the O atom of[NTf₂]^-and the H atom of the solute molecule.Inspired by this,the separation ability of extractants can be improved by designing anion and cation structures with functionalized hydrogen bond binding sites.On the single-stage equilibrium scale,the liquid-liquid equilibrium data demonstrate that[EMIM][NTf₂]is an efficient extractant,especially for aromatic hydrocarbons containing sulfur and nitrogen heterocycles.Based on the liquid-liquid equilibrium data,the parameters of the predictive UNIFAC-Lei model were modified and improved.（4）3 choline chloride-based deep eutectic solvents with low viscosity and high solubility were designed and synthesized for natural gas dehydration process,and a series of characterizations were carried out.The separation mechanism of deep eutectic solvents for natural gas dehydration was comprehensively explored from the perspective of molecular thermodynamics and molecular dynamics.FTIR and TGA showed that the related solvents were successfully prepared and showed good thermal stability.Based on the COSMO-RS model,the solubility,free volume（V_f）and free volume fraction（FFV）of the system were calculated.The calculation results confirmed that the deep eutectic solvent is a typical physical absorption of methane and conforms to Henry’s law.The free volume theory suggests that with the increase of the molar fraction of hydrogen bond donors,the intermolecular tightness of deep eutectic solvents increases,resulting in the decrease of V_f and FFV of the three deep eutectic solvents.Increasing the proportion of hydrogen bond donor is beneficial to improve the cost advantage of dehydrating agent.The dynamic methane gas dehydration experiments show that the water content in the product gas can be reduced to less than 200 ppm（the dehydration rate up to 98%）.The deep eutectic solvents have good regeneration performance,and the Henry’s constant of CH₄ is almost unchanged after 10 absorption-desorption cycles.The results of quantum chemistry calculations and molecular dynamics simulations show that the absorption performance of H₂O in deep eutectic solvents depends on the weak interaction between the Cl atom（or N or O atom）of Ch Cl and the H atom of H₂O.In particular,hydrogen bonding is the main intermolecular interaction between H₂O and hydrogen bond donor or acceptor.（5）Based on predictive molecular thermodynamics,the mechanism and separation performance of deep eutectic solvents for capturing condensable cyclohexane gas were systematically studied from the perspective of computational thermodynamics.At the molecular level,quantum chemical calculations show that strong hydrogen bonds can be formed between the hydrogen bond acceptor and the donor,which has good affinity.The van der Waals interaction is dominant between deep eutectic solvents and cyclohexane.At the system scale,the dynamic continuous gas absorption experiments show that the removal rate of cyclohexane is as high as 96%.After 10 cycles of absorption-desorption,the solubility of cyclohexane in deep eutectic solvents is almost unchanged.Deep eutectic solvents exhibit excellent absorption and regeneration properties.The solubility of cyclohexane gas in deep eutectic solvent proves that it is a typical physical absorption and conforms to Henry’s law.