Pertraction Properties And Mass Transfer Of Phenol From Aqueous Solution Using Silicone Rubber Membranes

Pertraction is a new membrane-based separation process to remove azeotropic compounds, volatile organic compounds, and thermosensitive and low boiling point materials from aqueous solution. By means of hydrolytic equilibrium of aromatic acid or base, combined nonporous membrane (permeable for the aromatic compounds but impermeable for the ionic species) and caustic or acidic stripping solution, the new pertraction process could be provided with high efficiency, low energy consumption, simpleness and conveniency and so on. Presently, this technology is being at the initial industrialization application stage, still many problems need to be investigated. In this study, phenol was chosen as model compound which is the representation of phenolic compounds to study and caustic stripping solution (sodium hydroxide) was as stripping solution. Two kinds of pertraction system were constructed by nonporous membrane (Poly (dimethylsiloxane), i.e. PDMS and Poly (methyl vinyl) siloxane, i.e. PVMS) and flat sheet composite membrane (PDMS/PVDF, Polyvinylidene Fluoride, i.e. PVDF). The characteristic, mass transfer process and mechanism of the systems were investigated, and the following studies were carried out:1) Membrane pertraction process to the removal of phenol from aqueous solution was investigated, where the apparatus with spiral wound silicone rubber membrane was applied. The pertraction ability and stability of PDMS and PVMS water-phenol mixtures were compared. PVMS performed better than PDMS but was not suitable for long-term operation under strong base condition. The role of operative conditions on the process performance was widely investigated, and flow rate of liquid feed, composition (phenol, stripping solution and salt), temperature and pH in the stripping solution were the main operative variables. Among these, the pH in the stripping solution was the major factor since it greatly affected the separation efficiency. The phenol removal efficiency was over 95% under the conditions of flow rate 2.0 L/d, phenol concentration 5.0-20.0 g/L, 323.2 K and pH 12.5-13.0, which was independent of initial stripping concentration. Phenol concentration in the outlet was lower than 500 mg/L. The overall mass transfer coefficient (OMTC, K_ov) increased as salt concentration increased. That indicated the system was especially fit for high phenol concentration and high salinity (0-300 g/L) wastewater. The feasible range of flow rate was from 1.0 to 3.0 L/d and hydraulic retention time was about 2 minutes.2) The phenolic wastewater from double benzene factory of Jilin Petrochemistry Corporation had been disposed with pertraction technique under PDMS system. The technical feasibility of the process was obviously: Over 97% of phenol could be removed from the wastewater under the conditions of flow rate 2.0 L/d, phenol 4.3-10.7 g/L, 323.2 K and pH 12.5-13.0, and phenol concentration in the outlet was lower than 150 mg/L, the system ran steadily.3) As for the mass transfer character of liquid film boundary layer, membrane and support layer, pertraction mass transfer mechanism and process of phenol through membrane of nonporous and composite silicone rubber were investigated. Mathematical models descriptions of the two kinds of processes were developed respectively. Based upon the resistance-in-series model, OMTC was measured.4) Using spiral wound and single tubular PDMS nonporous silicone rubber membrane modules respectively, mass transfer process of pertraction was researched at steady state and non-steady state. At steady state, the effects of liquid flow status, initial phenol concentration and system temperature on the surface of membrane on OMTC and the permeability for phenol through the membrane were discussed. A correlation between OMTC and Reynolds number (Re) as well as system temperature were obtained, and the results were compared with the experiments. The effects of stripping solution pH value, stripping solution flow status on the surface of membrane and liquid phase pressure difference between opposite membrane sides on OMTC were analyzed at the non-steady state. The experiment results indicated that chemical reaction enhancement was not remarkable, and mass transfer was dominated by membrane resistance (K_ov=3.5×10^-7 m/s). Liquid film resistance of stripping solution (pH>13) could be negligible under turbulent condition. In the initial phenol concentration ranges (5.0-20.0 g/L), OMTC was nearly, constant, and mass flux of phenol displayed linearly with initial concentration (1.6-7.7×10^-6 kg/m²Â·s). The presence of liquid phase pressure difference between opposite membrane sides went against permeability of phenol. K_ov was in direct proportion to the temperature of the process. And the experimental data conformed to Arrhenius relationship for the temperature dependence of the permeability of phenol through the polymer. Especially, the effects of liquid film boundary layer resistance and membrane resistance on OMTC were analyzed.5) At non-steady state, pertraction mass transfer characteristic of phenol from aqueous solution through a novel flat sheet composite membrane (PDMS/PVDF) with radial flow in both feed and stripping sides was investigated. OMTC was split into liquid film and membrane mass transfer coefficient, by conducting a calculation from the experimental data with different nonporous selective layer thickness of membranes. The experiments were carried out to investigate the effects of composition and flow rate (feed and stripping solution) temperature, pH value and liquid phase pressure difference between opposite membrane sides on the pertraction performance. The parameters according to different operation conditions were regressed. It appeared that feed-side boundary layer mass transfer coefficient was proportional to Re^0.46, and mass flux increased exponentially with increasing temperature, that is, Arrhenius relationship. A correlation between the thickness of selective layer and the mass transfer resistance was established. On the basis of above results, a pertraction mass transfer model of composite membrane was achieved, and the results were compared with the experiments. The experimental results showed that OMTC was independent of stripping solution flux and concentration (pH>13). The support layer and stripping side resistance to mass transfer could be eliminated completely due to chemical reaction enhancement. OMTC was independent of initial phenol concentration (5.0-15.0 g/L). The presence of liquid phase pressure difference between opposite membrane side counteracted permeability of phenol. Membrane diffusive mass transfer coefficients of selective layer (thickness 4, 6 and 8μm) were 15.0, 9.9 and 7.5×10^-7 m/s respectively(323.2 K), which exhibited 2-4 times higher than that of nonporous silicone rubber. Mass transfer resistance in membrane was also dominant for composite memebrane system. In a continuous experimental period of four months, the membrane had been behaving well. However, after being used for three weeks, composite membrane was compressed obviously and short life-span.6) The new pertraction technology offers an excellent combination of Simplicity, high efficiency, sufficient final product purity, low energy consumption, mild operation conditions and less pollution compared with traditional ones. As a Consequence, it is an advanced, effective, economical and environment amity for recovery of aromatic acids and bases from wastewater streams.