Studies On Recovery Of MTA Residues

Terephthalic acid(TA) is an important raw material for the synthesis of polyester fibers,which is mainly produced by p-Xylene(PX) liquid-phase oxidation.Medium purity terephthalic acid(MTA) refers to the product of second oxidation of TA,and is distinct from the purified terephthalic acid(PTA),which is obtained by hydrogenation of TA.TA residues,which are produced during the process of PX oxidation,are often thrown away.The main components of MTA residues is TA;it not only pollutes the environment,but also wastes the resource for treating the MTA residues as solid waste; So recovering TA residues is an important technology.In this thesis,a variety of analytic methods were carried to analyze the components of TA residues and the impurities formation mechanism were proposed according to the characteristic of free-radical reactions;several methods are exploited to recover the TA from TA residues,which provide an evidence for residues recovery technology industrialization.The work and results are outlined as follows:1.The composition of TA residues was analyzed in detail.High performance liquid chromatograph(HPLC),gas chromatograph-mass spectrometer(GC-MS),liquid chromatograph-mass spectrometer(LC-MS) were used to analyze the MTA residues,and the results showed that the main compontents of MTA residues is TA;OD340 is an important industrial indicator for evaluating the quality of TA.In all impurities,fluorenone dicarboxylic acid is the most sensitive to OD340,and then is benzophenone dicarboxylic acid;4-CBA is relatively insensitive.The concentration level of total impurities is 10~3ppm characterized by the representative of 9F-4CA.According to the free-radical reaction mechanism,a possible formation path of impurities during the oxidation of PX was proposed.2.The method of adsorption by use of activated carbons as adsorbent to separate impurities from TA residues was tested for different solvent systems at different temperature conditions.Experiments showed that TA can be purified from oxidation residues by activated carbon adsorption in both DMSO solvent and NaOH solution.The experimental data can be well fitted by Langmuir isotherm model.In DMSO solvent,both low temperature and certain water content are favorable for the adsorption of impurities and TA recovery.In NaOH-TA solution,pH=7.0 is an appropriate condition.Temperature has different effects for different activated carbons which exhibit different temperature sensitivity.3.The method of deep oxidation,crystallization from high temperature water and hydrogenation were tested.Oxidation experiments showed that the colored impurities are very stable and difficult to decompose in the oxidation process,and the deep oxidation is not an effective method for recovering TA from residues.Crystallization from water at high temperature can get high purity TA products,but it needs harsh process conditions.Hydrogenation is a feasible method for recovering TA from TA residues which can remove the colored impurities by using exsiting hydrogenation unit and conditions.4.The structures and properties of crystals formed by TA and solvents were studied.The experiments found that it was a special crystallization which is known as adductive crystallization when TA crystallized from certain solvents(such as DMAC and NMP).The solvent will precipitate together with TA and a new crystal will form. Dissolution and crystallization can preceed at constant temperature.The impurities can be removed with high selectivity by adductive crystallization.A variety of methods were adopted to investigate the structures and properties of adducts.According to the analysis of XRD,two DMAC/NMP molecules are linked to one TA molecule by strong O-H…O hydrogen bonds;every three crystal unit get together to form a trimerical layer;cross-stacking exist between the layers.According to the thermo gravimetirc analysis(TGA),adduct of TA·2DMAC(or TA·2NMP) is not stable,which will decompose at higher temperature or in the water.The hydrogen bonds energies were calculated by the program of Gaussian03, which are 83.01595KJ/mol(between TA and DMAC) and 87.82532KJ/mol(between TA and NMP).Similar methods were adopted to study the crystal structure and property of the adduct formed by one 1,3,5-Benzenetricarboxylic acid molecule and three NMP moledules.5.Thermodynamic data were measured and the knetic model of adductive crystallization was proposed.The solubility of TA in some amides was studied.The results showed that TA has the relatively large solubility among the DMF,DMAC and NMP.At higher temperature there will be higher solubility.The partition coefficient of colored impurities between the product and mother liquor were measured at different temperatures and it is shown that the partition coefficient reached minimum at 50℃. A knetic model of adductive crystallization was proposed which was used to describe the dissolution of oxidation residues,the crystallization of adduct and the enrichment of impurities in mother liquid.Model parameters were obtained by use of the dynamic experimental curve of impurities' concentration.6.The conditions of purifying TA residues by adductive crystallization were investigated and the appropriate conditions were determined.The solvent sieving experiments showed that DMAC and NMP were the suitable solvents for recovering TA from TA residues and DMAC showed better.Two ways of crystallization,cooling and isothermal crystallization,were studied.It showed that the closer of dissolution temperature to crystallization temperature,the better product purity will be for the process of cooling crystallization.The optimal crystallization temperature for the cooling crystallization is 60℃while optimal for isothermal crystallization is 50℃.The issue of solvent recycling was examined and the results showed that the mother liquid could be reused at least five times without significant effect on product quality.