The Treatment Of Wastewater With High Concentration Of Phenol Using Membrane Extraction

Membrane extraction is a new separation technology which combines membrane process with liquid-liquid extraction. It is a new membrane-based separation process for the removal of azeotropic compounds, volatile organic compounds, 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 extraction process is characterized as high efficiency, low energy consumption, simpleness and conveniency etc. Presently, the technology is at its initial stage of industrialization application. There are still many problems that need to be investigated. In this study, phenol was chosen as model compound of phenolic compounds and sodium hydroxide was used as stripping solution. Two kinds of membrane process system were constructed by nonporous membrane (Poly Dimethylsiloxane, PDMS) and flat sheet composite membrane (PDMS/Polyvinylidene Fluoride, PVDF). The characteristics, mass transfer process and mechanism of the systems were investigated. The following studies were carried out:(1) At non-steady state, characteristics of mass transfer of phenol from aqueous solution through a novel flat sheet composite membrane (PDMS/PVDF) were investigated. The experiments were carried out to investigate the effects of composition and flow rate (feed and stripping solution), pH value, initial concentration, temperature, salt ion strength. It was demonstrated that mass transfer resistance in membrane was dominant for composite membrane system. The augmentation of the flow rate of the feeding solution is beneficial to mass transfer process. The enhancement of the chemical reaction had little effect on mass transfer process. The overall mass transfer coefficient (OMTC, K_OV) was independent of stripping solution flux and the initial stripping concentration (pH>12.5). When the phenol concentration was above 5.0 g/L, the OMTC was independent of initial phenol concentration. The flux increased exponentially with increasing temperature, i.e. it followed the Arrhenius relationship. The OMTC increased with the enhancement of the salt ion concentration. Thus this system was especially suitable for the treatment of the wastewater with high phenol concentration and high salt concentration.(2) Some improvements were taken out to the system mentioned above. Characteristics of the mass transfer of phenol from aqueous solution through a novel flat sheet composite membrane (PDMS/PVDF) at non-steady state were studied. Experiments were carried out to investigate the effects of different factors including Reynolds's number of the feed solution, pH value, initial concentration, temperature, salt ion strength, liquid phase pressure difference between opposite membrane sides on the extraction performance, and the effects of the thickness of the membrane, on the mass transfer process. It appeared that mass transfer resistance in membrane was dominant for composite membrane system. The enhancement of the chemical reaction had little effect on mass transfer process. OMTC was independent of stripping solution flux and concentration (pH>12.5). When the phenol concentration was above 8.0 g/L, OMTC was independent of initial phenol concentration. OMTC increased with the enhancement of the salt ion strength. Thus this system was especially suitable for the treatment of the wastewater with high phenol concentration and high salt concentration. The OMTC was independent of trans-membrane pressure when it was below 0.1 MPa. However, when it was above 0.1 MPa the enhancement of mass transfer occurred beside the densification. Thus a suitable trans-membrane pressure should be controlled. There is a linear relation between the OMTC and reciprocals of membrane thicknesses. However, after being used for three weeks, composite membrane was compressed obviously. Its life-span was shortened. The water flux of this process was 0.22 L/m2s, which was not favorable for the recovery of phenol in practice.(3) Using spiral wound silicone rubber membrane, membrane extraction process for the removal of phenol from aqueous solution was investigated. Phenolic wastewater from cholrophenols factory of Shandong was treated with MARS technique under PDMS system. The technical feasibility of the process was obviously: Over 98% of phenol could be removed from the wastewater under the conditions of flow rate 3.0 L/d, phenol 22 g/L, 323.2 K and pH 12.5-13.0, Phenol concentration in the outlet was lower than 500 mg/L. The system ran steadily. It shows that this new membrane extraction technique has good technical feasibility for the separation and recovery of phenol from concentrated phenol wastewater.(4) The phenolic coking wastewater from Shandong was treated with membrane extraction technique under PDMS system. The technical feasibility of the process was obviously: Over 45% of phenol could be removed from the wastewater under the conditions of flow rate 2.0 L/d, phenol 1.1 g/L, 323.2 K and pH 12.5-13.0. The phenol concentration in the outlet was lower than 700 mg/L. Compared to the data attained from the factory mentioned above, the effects were too small. Thus this process is more effective for wastewater containing higher phenol concentration.(5) The new membrane extraction 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. Thus it is an advanced, effective, economical and environment-friendly for recovery of aromatic acids and bases from wastewater.