Pervaporation Membranes Selection And Preparation, And Hybrid Process Development For Recovery Of N, N-dimethylformamide From Synthetic Leather Waste Water

Dimethylformamide （DMF） is a kind of organic solvent and widely usedin the manufacturing of synthetic leather, medicine, etc., in whichwastewater with DMF mass fraction ranging from10%to25%wasproduced, and needed to recover the DMF before discharge. Theconventional method for the recovering of DMF from the wastewater isdistillation. The disadvantage of this process is large energy consumption.Pervaporation （PV）, as a newly developing and energy-saving technique,can be used to replace the distillation unit or couple with it in order to cutdown the energy consumption and cost of DMF recovery process.Three kinds of commercial PVA composite membranes with differentcrosslinking degrees (HPV^-1005, HPV^-1015and HPV^-1050) and one type ofNaA zeolite membrane were used in the separation of DMF/H₂O mixtures.In order to select the most suitable membrane and obtain its optimumoperating parameters, the influence of feed compositions, operationtemperatures and downstream pressures on the pervaporation performancewas investigated. The results showed that HPV^-1050membrane amongthese three polymeric membranes was the most suitable one for DMF/H₂Omixtures. It showed best separation performance when mass fraction ofDMF was20%in feed, operation temperature was60oC and downstreampressure was lower than6kPa. Flux reached0.59kg·m^-2·h^-1and separationfactor was33at optimum operation conditions. For feed of which the massfraction of DMF was20%, the fluxes and separation factors of NaA zeolitemembrane were0.41kg·m^-2·h^-1and19.7respectively when operationtemperature was70oC and downstream pressure was lower than6kPa.At the base of investigating commercial PVA polymeric membranes,Polyvinyl alcohol grafting polyacrylamide （PVA-g-PAAm） copolymers withdifferent degrees of grafting were synthesized from polyvinyl alcohol andpolyacrylamide. The synthesized copolymers were characterized byviscosity average molecular weight. Contact angle, infra-red sepectrometry（FT-IR） and thermogravimetry （TGA） methods were used to characterize themembrane. The results showed that grafting reaction was successful and the hydrophilic nature, mechanic property and thermal stability improvedsignificantly. Then three PVA-g-PAAm/α-Al₂O₃composite membranes wereprepared with0.2μm α-Al₂O₃as support. The pervaporation performancesof the membranes were also investigated with DMF/H₂O mixtures atdifferent feed compositions and operation temperatures. The results showedthat PVA^-2/α-Al₂O₃membrane had the best separation performance. Withthe increment of DMF content in feed from5%to50%, the flux ofPVA^-2/α-Al^-12O3reduced from0.71kg·m^-2·h^-1to0.28kg·m^-2·h. The bestseparation factors of them were obtained when mass fraction of DMF was30%in feed. The flux of three PVA-g-PAAm/α-Al₂O₃compositemembranes increased with the increment of feed temperature, while theseparation factor decreased.Aspen Plus software was applied to simulate PV-distillation hybridprocesses. The energy consumption and costs of different hybrid processeswas calculated. Comparing with batch distillation, the energy consumptionof hybrid processes could be decreased from34.5%to60.0%by raisingmass fraction of DMF from30%to70%in the feed for ditillation. Inaddition, the cost was lowest when mass fraction of DMF was50%in thefeed for distillation. When it came to PV and two-effect distillation hybridprocesses, the results showed that the energy consumption of hybridprocesses reduced from14.1%to54.4%by raising mass fraction of DMFfrom30%to70%in the feed for distillation comparing with two-effectdistillation. And the cost was lowest when mass fraction of DMF was60%in the feed for distillation.