Study On Concentrating Aqueous Sulfuric Acid Solution By Multiple-Effect Membrane Distillation

Membrane distillation （MD） is a novel separation process which combinesmembrane and distillation technology, the driving force in the process is thedifference of the vapor pressure across the two sides of the membrane. Membranedistillation possesses many advantages, such as simple equipment, being operated atambient pressure and low temperature, high rejection of non-volatile solutes and so on.However, the energy utility efficiency of the conventional MD process is extremelylow when compared with the conventional multiple-effect distillation （MED） ormulti-effect flash （MSF） process, which is the main obstacle for the industrializationof MD process. Consequently, it is extremely important to study on the recovery ofthe latent heat of the permeate generated in the MD process. The concept ofmultiple-effect membrane distillation （MEMD） was raised in this paper. On the otherhand, there is large amount of industrial wastewater contains sulfuric acid. In thisstudy, the feasibility to use MEMD process to concentrating dilute sulfuric acidsolution was investigated. This study lays a necessary foundation for deepinvestigation of the MEMD process both in theory and industrialization.Suitable hollow fiber membrane was selected and air gap membrane distillation（AGMD） modules were manufactured. By used the AGMD modules, the possibilityof coupling heat recovery in the conventional MD process was studied and theconcept of multiple-effect membrane distillation was used. Dilute sulfuric acidsolution in2wt%was used as feed solution to investigate the possibility ofconcentrating dilute sulfuric acid solution by this MEMD process. When membranedistillation module made from hollow fiber membrane labeled PP150/330was used,dilute sulfuric acid solution in2wt%can be concentrated to40wt%or above, and theelectrical conductivity of the permeate was still less than200μm/cm. The experimentresults demonstrated that when compared with micro-porous hydrophobic membraneof low porosity, micro-porous hydrophobic membrane of high porosity was moresuitable for MEMD process, and the permeate flux and energy efficiency ofmembrane in high porosity is larger.The operating variations which would affect the performance of MEMD processincluded temperature at the inlet of membrane (T_h,i), temperature at the inlet of tube(T_c,i), feed volumetric flow rate （F_f） and the concentration of the feed solution （c_f）. Single factor experiments were conducted to find the effects of the operatingvariations to the performance of the MEMD process. The value of permeate flux andperformance ratio （PR） were used to evaluate the performance of the MEMD process.According to the experiments results, the value permeated flux and PR increased withthe increase of T_h,i; permeate flux decreased with the increase of T_c,i, while the valueof PR increased; however, compared with T_h,i, the effect of T_c,iwas relatively smaller;permeate flux increased with the increase of Ff, while the value of PR decreasedsharply as the increase of Ff; the viscosity of the feed solution increased with theincrease of the feed concentration, which led to the decrease of the value of permeateflux and PR. Based on the analysis of the results of the single factor experiments, T_h,ishould be as high as possible to improve the performance of the MEMD process, andthere must be a balance between the effects of operating variations to permeate fluxand PR when selecting the value of Ffor T_c,i. Also, long-term operational stabilities ofthe MEMD module and MEMD process were investigated used sulfuric acid solutionin10wt%.Two different modules made from different membranes were used in thestability experiments, and the stability experiments lasted for about30days. Both theMEMD module and MEMD process showed well stability in the experiments lastingfor30days, there were no evidently leakage through the whole experiments.