| In industrial process,terephthalic acid is produced through oxidation of paraxylene at high-temperature.Nevertheless,this process can yield many residues.The main compositions of oxidation residues are mixed aromatic carboxylic acids,such as terephthalic acid(PTA),4-carboxybenzaldehyde(4-CBA)and p-toluic acid(p-TA),and so on.It is particularly important to purify and recover the mixed aromatic carboxylic acids.Many researchers have performed study on the separation and recovery of mixed aromatic carboxylic acids,where the used methods include extraction,water washing,hydrogenation,etc.But these methods can not achieve the desired results.Therefore,how to efficiently and economically use the mixed aromatic carboxylic acids has become an urgent problem.Solvent crystallization is an effective separation method with low energy cost to separate mixed aromatic carboxylic acid.It is mainly based on solubility difference of mixed aromatic carboxylic acid in different solvents to achieve crystallization separation.The solvent used in separation process can be recycled and reused.Many researchers have determined the solublity of aromatic carboxylic acids in neat solvents and solvent mixtures.Obviously the basis of the purification of terephthalic acid through solvent crystallization is the mutual solubility for quaternary system of 4-carboxybenzaldehyde+terephthalic acid+p-toluic acid+solvent.The solubilities of neat p-toluic acid,terephthalic acid and 4-carboxybenzaldehyde in many mixed solvent and neat solvents can be obtainable from many previous works,nevertheless the accessible solubility data is insufficient for the purification process of terephthalic acid.The mutual solubility data for 4-carboxybenzaldehyde+terephthalic acid+p-toluic acid+solvent quaternary system and its corresponding ternary sub-systems of 4-carboxybenzaldehyde+terephthalic acid+solvent,p-toluic acid+terephthalic acid+solvent and p-toluic acid+4-carboxybenzaldehyde+solvent cannot be obtainable up to yet in previous publications.It should be noted that the terephthalic acid can be crystallized from the N,N-dimethylformamide(DMF)or N-methylpyrrolidone(NMP)solutions in the form of adduct with solvent molecules of NMP/DMF.In addition,DMF and NMP are green solvents used in industry.Therefore,in this paper,mixed aromatic carboxylic acids(terephthalic acid,p-methylbenzoic acid and 4-carboxybenzaldehyde)were used as research objects,and DMF and NMP were selected as solvents to study the solid-liquid equilibrium of quaternary system and its sub-ternary systems formed by mixed aromatic carboxylic acids in the two solvents.The solid-liquid equilibrium data was calculated and correlatedThe mutual solubility for terephthalic acid+p-toluic acid+NMP/DMF,terephthalic acid+4-carboxybenzaldehyde+ NMP/DMF and p-toluic acid+4-carboxybenzaldehyde+NMP/DMF systems at three termperatures of 298.15 K,323.15 K and 353.15 K were experimentally determined by means of static technique under local atmospheric pressure(p-101.2 kPa).Nine isothermal phase diagrams were constructed in terms of the solubility data determined by Schreinemakers' wet residue method.Two neat solids were formed in each isothermal system,which were adduct 1:2 terephthalic acid-NMP/DMF(298.15 K and 323.15 K)or terephthalic acid(353.15 K)and p-toluic acid for the ternary terephthalic acid+p-toluic acid+NMP/DMF systems,4-carboxybenzaldehyde andp-toluic acid for the ternary 4-carboxybenzaldehyde+p-toluic acid+NMP/DMF systems,and adduct 1:2 terephthalic acid-NMP/DMF(298.15 K and 323.15 K)or terephthalic acid(353.15 K)and 4-carboxybenzaldehyde for the terephthalic acid+4-carboxybenzaldehyde+NMP/DMF systems.Each diagram consisted of two cosaturated curves,one cosaturated point and three crystalline areas.The solubility of terephthalic acid,4-carboxybenzaldehyde and p-toluic acid in the solvents of NMP/DMF increased with an increase in temperature.The crystallization region of pure terephthalic acid or adduct of terephthalic acid with NMP/DMF(mole ratio,1:2)quickly increased with the decrease in temperature.At the identical temperatures,the crystallization regions of pure terephthalic acid and adduct of terephthalic acid with NMP/DMF(mole ratio,1:2)were bigger than that of 4-carboxybenzaldehyde and p-toluic acid.It should be noted that the adduct compound terephthalic acid-NMP/DMF(mole ratio 1:2)appears below 323.15 K for the terephthalic acid dissolved in NMP/DMF;while at 353.15 K,it dissociates to neat terephthalic acid and neat NMP in solvents.Based on the determined mutual solubility for terephthalic acid+p-toluic acid+NMP/DMF,terephthalic acid+4-carboxybenzaldehyde+NMP/DMF and p-toluic acid+4-carboxybenzaldehyde+NMP/DMF systems,the solubility of quaternary systems of 4-carboxybenzaldehyde+terephthalic acid+p-toluic acid+NMP/DMF was measured at temperature of 298.15 K,and the corresponding quaternary phase diagrams and their Janeche projections were constructed by Schreinemakers' wet residue method.Each phase diagram included three crystalline regions,three co-saturation lines,and one co-saturation point.In each quaternary phase diagram,when a third solute is added,the solubility of the other two solutes in the solvents decreases.Finally,the solubility data of the quaternary system of 4-carboxybenzaldehyde+terephthalic acid+p-toluic acid+NMP as well as its ternary systems was mathematically described by the NRTL model.The binary interaction parameters of terephthalic acid-p-toluic acid,terephthalic acid-4-carboxybenzaldehyde and p-toluic acid-4-carboxybenzaldehyde,and ternary interaction parameters of terephthalic acid-p-toluic acid-4-carboxybenzaldehyde were acquired through regression in terms of the determined mutual solutiliby data,obtaining relative average deviation values smaller than 9.65%and RMSD values smaller than 5.723.The calculation results showed that the experimental values and back-calculated ones can be in good agreement. |
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