The Mass Transfer Behavior Study Of Cu(II) In The Hollow Fiber Renewal Liquid Membrane Process

Liquid membrane technique is a new separation method, which combines the characteristics of solvent extraction and solid membrane separation. It has several potential advantages compared to the conventional methods of solvent extraction technique.A new liquid membrane technology, Hollow Fiber Renewal Liquid Membrane (HFRLM), is proposed based on the surface renewal theory. In a HFRLM process, a very thin organic film is developed within the internal surface of fibers, which is used for separating of the feed phase and stripping phase, due to the wetting affinity of the hydrophobic hollow fibers and organic phase. The solute was selectively transported from feed phase to the stripping phase across the liquid membrane film. HFRLM not only has the advantages of non-equilibrium mass transfer and up-hill effect, but also aviods the disadvantages of supported liquid membrane and emulsion liquid membrane, such as loss of membrane liquid and difficult operation of emulsification and de-emulsification steps, etc. The mass transfer performance of HFRLM is systematically studied, and compared with other LM techniques. Based on the mass transfer mechanism, a mathematical model for the mass transfer of HFRLM process is developed. And this technique is applied to remove copper (II) from simulated industrial wastewater and recover citric acid in small laboratory scale.(1) The composition of the liquid membrane phase is confirmed by solvent extraction experiment. The solvent extraction experiments were studied for the determination of liquid membrane phase. The influences of pH and initial copper concentration in the feed phase, the carrier concentration, types of the stripping reagent, the H⁺ concentration in the stripping phase, the operation temperature, etc. on the distribution equilibrium of copper between CuSO4 aqueous solution and various organic solutions were studied. The extraction mechanism was also discussed. The results showed that the distribution coefficient and the extraction efficiency were higher with LIX984N/kerosene or D2EHPA/kenrosen as organic phase. When LIX984N and D2EHPA were used as the extractant, the optimal pH value of the feed phase is found to be >2.0 and 4.44, respectively. The distribution coefficient of stripping process increases with increasing H⁺ concentration in the stripping phase, and then decreases, mainly due to the strong oxidation of the high concentration sulfuric acid which leads to the degradation of the extractatnt.(2) The system of CuSO₄-LIX984N(D2EHPA)/kerosene-H₂SO₄(HCl) was used to study the effects of the hydrodynamic characteristic (flow rates, operation modes), interface chemical reaction (initial pH in feed phase, buffer solution concentration, stripping phase acidity, types of stripping reagent), composition of the liquid membrane (types of carrier and its concentration, aqueous/organic volume ratio), hollow fiber structure parameters (internal diameter of fibers, membrane thickness, module size), etc. on the mass transfer performance of HFRLM process. The comparison with other LM techniques is also carried out. In a HFRLM process, the overall mass transfer coefficient increases with increasing of the flow rates of two sides. The mass transfer resistance of aqueous boundary layer in feed phase is the main part of the overall mass transfer resistance. The coalescence and breakage of droplets are related to the relative size of the droplets and the internal diameter of hollow fibers. The results showed that the fiber with large internal diameter would benefit the mass transfer performance. Higher membrane thickness leads to higher diffusion resistance in membrane phase, which increases the overall mass transfer resistance. For the hollow fiber modules with the same fiber packing fraction, the individual mass transfer coefficient of the shell side increases with increasing of internal diameter of module, due to the higher diameter of module results in the greater non-ideal effect of shell side. Compared with HFSLM process, the stability and mass transfer coefficient are higher. The leakage of HFRLM is about 3-4 orders of magnitude lower than that of emulsion liquid membrane process.(3) Based on the mass transfer mechanism of HFRLM process, resistance-in-series model, and the surface renewal theory, the mass transfer correlations were developed. The modeled results are in good agreements with experimental values.(4)The wastewater contained Cu(II) was treated by using HFRLM, the removal efficiency was up to 99% and the Cu(II) concentration in the residue was less than 1.0 mg.L^-1, which gets to the national discharge standard. The Cu(II) concentration in the stripping phase reached to 1700 mg.L^-1 and the enrichment factor is up to 25. HFRLM process can avoid secondary pollution, and the concentrated Cu(II) solutions can be reused. For the recovery process of citric acid with HFRLM technique, the result shows that HFRLM process is very stable due to the continuous replenishment of membrane liquid; the extraction efficiency is up to 98%, which is higher than the recovery method of calcium salt. And the enrichment factor of citric acid in the stripping phase is bigger than 9. This technique can be successfully applied in recovery citric acid from dilute solution.The studies of this paper could provide more theoretical basis for the further investigation and the application of the HFRLM technique.