Experimental Investigation On The Co-Oxidation Of M-Xylene And Ethylbenzene

Purified isophthalic acid(PIA)which is a commercially important precursor of various polymers is produced by liquid-phase catalytic oxidation of m-xylene(MX)at present.The problem of the present PIA process is the obtaining of high-purity MX which is costive and limited.To create a widely applicable and effective process,a novel process was developed by our group.In the process,part of the raffinate from the PX adsorption units is used as the feedstock.The mixtures consist of MX,ortho-xylene(OX)and EB.OX can be removed by distillation,and the obtained MX-EB mixtures have to be co-oxidized for producing PIA with follow up purification operation.The advantage of the process is that the raw material is cheap and widely available,which could significantly reduce the production costs of PIA.To accomplish the industrialization of this process,the co-oxidation of the mixtures MX-EB process was experimentally investigated,this works covered the following aspects:1.The characteristics of the liquid-phase catalytic oxidation of EB to benzoic acid(BA)and MX to isophthalic acid(IA)in acetic acid were compared,and the reaction mechanism for oxidation of EB to BA was proposed accordingly.The effect of EB on the oxidation was evaluated.And the experimental results show that the introduction of EB accerates the oxidation of MX because of the influence of free radical concentration,but MX and EB co-oxidate follows their own pathway:MX to IA and EB to BA.BA and IA can be easily separated by crystallization,filtration and cake washing.By this way,co-oxidation of MX and EB for producing isophthalic acid is feasible.2.A fractional kinetic model for the liquid phase oxidation of the mixture MX-EB was developed,and the effects of initial MX-EB concentration,reaction temperature and catalyst concentration on the reaction rate were investigated.The model parameters were obtained by a nonlinear least-squares method and the calculated values show satisfactory agreement with the experimental data.This model can be used for the design and scale-up of the reactor and optimization of oxidation conditions for MX-EB oxidation process.3.The solubilities of isophthalic acid in acetic acid + water + BA ternary solvents were determined by the static analytical method.The influence of BA on the solid-liquid equilibrium was further evaluated.The experimental data were correlated by the NRTL equation,and the calculated experimental values could match the experimental results satisfactorily with the average relative deviation of only 1.78%.The solubility data can provide fundamental data for the crystallization process and reliable design of crystallizers.4.Cake-washing and recrystallization of crude isophthalic acid from oxidation was carried out for obtaining purified isophthalic acid.The effect of initial BA concentration and filter cake-washing operation on removing impurity BA residue was investigated systematically.The results show that although the mother liquor contains as high of 9.07%BA,multi-stage washing with acetic acid and recrystallization combined is still an effective method for obtaining high purity isophthalic acid with BA concentration as low as 25 ppm.