Extraction Separation Of Aromatics/Alkanes With Nitrogen-containing Heterocyclic Ionic Liquids

Both naphtha and diesel oil,containing aromatics and alkanes,are important ethylene cracking raw materials.The alkanes are good feedstock for ethylene cracking,however the aromatics would coke in the ethylene cracking unit,which affects the ethylene yield.Therefore,removing the aromatics from ethylene cracking raw materials is conducive to improving the ethylene yield.However,the boiling points of aromatics and alkanes with similar carbon atom numbers are often rather close,making it difficult to achieve separation through traditional distillation.Currently,extraction separation commonly used also has unsatisfactory separation effects with organic solvents.Therefore,it is necessary to develop extractant with excellent separation performance.Introducing unsaturated bonds into the extractant can enhance theπ-πinteraction with aromatic hydrocarbons and improve the separation effect of the extractant on aromatic hydrocarbons.Based on this,protic ionic liquids were prepared using N-methylpyrrolidone with methanesulfonic acid,lactic acid and levulinic acid containing unsaturated bonds respectively through acid-base neutralization reaction.The experimental results indicate that due to the smaller volume structure and more heteroatoms,the extractant[NMP][MSA]has better separation performance.It was found that in the toluene/n-heptane and toluene/cyclohexane systems with 20 mol%toluene,the selectivity of the extractant for toluene is 61.71 and 15.10,respectively.The separation performance of the extractant on multi-component simulation system was researched,which indicated that the removal rate of aromatic hydrocarbons by the extractant reached 79.11%after five stage cross flow extraction.It was found that the interaction energy between extractant and aromatic was larger than those between extractant and alkanes,the type of interaction force between extractant and aromatic was van der Waals force,and electrostatic and dispersion force played a major role by quantum chemistry calculation.Coordinated ionic liquids with low cost and preparing easily were further developed in this work.Based on theπcomplexation between the empty orbitals of metal ions and aromatic molecules containingπelectrons,the introduction of metal elements into ionic liquids can enhance the interaction with aromatic hydrocarbons.So coordinated ionic liquids were synthesized by adding different inorganic metal salts into N-methylpyrrolidone.The optimal coordinated ionic liquid[NMPFe_0.1][（NO₃）₃]_0.1·9H₂O was determined by studying the separation performance of toluene/n-heptane and toluene/cyclohexane systems.And the effects of extraction temperature,extractant dosage,and initial concentration of feed on separation performance were systematically studied.The separation performance of the extractant on multi-component simulation system was also investigated,which suggested that the removal rate of aromatic hydrocarbons by the extractant reached 86.2%after five stage cross flow extraction.However,the separation performance of the extractant would decrease to some extent during the process of regeneration through vacuum distillation.The introduction of Lewis acidic structure in ionic liquids can polarize theπelectrons of aromatic hydrocarbons,which has a certain effect on promoting the dissolution of aromatic hydrocarbons.Therefore for the separation of high boiling aromatics/alkanes system in diesel oil,the extractant[EMIM][Fe Cl₄]containing short alkyl side chain was selected.The distribution coefficient and selectivity of tetralin in the tetralin/dodecane system with 50 mol%tetralin were0.6708 and 88.467,respectively.This extractant possessed good selectivity of aromatics at low temperatures or initial concentrations of aromatics.Besides,with the increasing alkyl side chain length of extractant,the distribution coefficients of solutes gradually increased and the selectivity of aromatics decreased gradually.It was found that methylnaphthalene could be completely removed from the feed and the total aromatic content could be reduced from 20wt%to 0.81 wt%with removal rate of 95.95%for total aromatic hydrocarbons after five stage cross flow extraction.It can be concluded that electrostatic and dispersion force play a major role in the separation process based on the analysis mechanism of quantum chemical calculations.Finally,in order to reduce the trial and error process of screening extractants and improve experimental efficiency,a model for predicting the solubility of aromatic hydrocarbons and alkanes in ionic liquids was established based on the group contribution method using BP neural network and support vector machine of machine learning by collecting 2061 experimental data on the solubility of aromatics and alkanes in ionic liquids.It was found that the BP neural network had better prediction effect through the comparison of the two models,which suggested that the values of mean absolute error（MAE）and correlation coefficient（R²）in the test set were 0.0138 and 0.9922,respectively.