Purification Of PTA Wastewater With PX Solvent By Hollow Fiber Membrane Extraction

Purified terephthalic acid?PTA?is a widely used chemical material.The PTA producing process generates a large volume of wastewater.The main impurity of PTA wastewater,p-toluic acid?PT acid?,has a low toxicity,and the wastewater therefore needs to be purified before discharge.Current PTA wastewater treatment methods mostly are bio-degradation ones,which have the problems of large space occupations,high costs and a waste of valuable water and aromatic materials.New methods are therefore of great importance.It is an effective way to use p-xylene?PX?as the extractant to purify PT acid in PTA wastewater;PX rich in PT acid can be recycled into the oxidation step of PTA process without further purification.The traditional extraction apparatus,however,results in an extractant emulsion inevitably.The use of hollow fiber membrane extraction?HFME?can avoid the emulsification of solvent effectively.At present,there is no report on the HFME application in PTA wastewater treatment,so it is necessary to investigate the HFME process on PTA wastewater treatment systematically to provide the basis knowledge for industrial implementation.This work focuses on the HFME of PTA wastewater with the use of PX as extractant.Systematic chemical engineering research was performed on this method.The content includes the following aspects:1.Phase equilibrium data of PT acid between PX-water were experimentally measured,the mole fraction based partition coefficient of PT acid between PX and water is 16-89.The non-random two liquid model was used to correlate the partition coefficient and solubility data simultaneously with relative errors less than 8%.The diffusivities of PT acid in PX and water were estimated.2.The linear polypropylene hollow fibers were measured for the feasibility of PTA wastewater treatment with HFME.The linear HFME can reduce the PT acid concentration from simulated PTA wastewater to lower than 100 gm^-3 within 50-60 s,which meets the re-use standard of PTA wastewater.The maximum pressure difference of the lumen/shell side is two orders of magnitude smaller than the breakthrough pressure,which implies HFME is stable for pressure control.The mass transfer rate of membrane module only reduced 0.6%after running of 100 h,showing HFME is steady for long-time running.Mathematical simulation shows the rate control step of mass transfer exists in the lumen side,and the optimal HFME conditions are extraction time 50-60 s,length of fiber 1.0 mm,ratio of aqueous/organic flow rate a/o 9.0,temperature 318 K,hollow fiber inner radius 200-250?m,ratio of porosity/tortuosity 0.20.3.The helical hollow fiber membranes instead of linear ones were used to promote the mass transfer bottleneck in the lumen side fluid.The results show that the mass transfer rate of helical HFME is two times of the linear one.An orthogonal helical coordinate was used to derive the mass transfer continuity equation of helical HFME.Optimal HFME conditions are extraction time 26-30 s,length of fiber 1.0 m,temperature 318 K,hollow fiber inner radius 200?m,radius of curvature 3.0 mm,pitch 4.5 mm.4.A new type of square membrane module was designed to overcome the decrease of shell side fluid mass transfer rate in the membrane module scale-up.The mass transfer process in the square membrane module was modeled with the boundary element method,which simplifies the coupled calculation of lumen and shell mass transfer of large amount of fibers.The result shows that the square membrane module can decrease the concentration of PT acid to lower than 100 gm^-3 within 50 s by simulation?length of total fiber 1.0 m?,but requires a smaller a/o 7.5.At last technical measures were proposed to separate other impurities in PTA wastewater.