[show abstract][hide abstract] ABSTRACT: Spacers are introduced in membrane modules as turbulence promoter to enhance mass transfer while the detailed hydrodynamics inside the flow channel is hard to recognize. The situation becomes more complicated when air bubbles are incorporated as second phase, i.e. air sparging processes. In this work, transparent modules with precise slit accommodating for spacer were fabricated to visualize the behavior of single- and two-phase flows. For single-phase flow, dye injection method and pressure drop measurement were conducted in channel with and without net-type spacer. The results show that with the incorporation of the spacer the critical Reynolds number decreases from 1000 to 100 approximately which is lower than channel with ladder-type spacer always adopted by researchers. For two-phase flow consisted of liquid and air bubbles, the behavior of bubbles in coalescence system (water–air) and non-coalescence system (ethanol–water–air) were investigated respectively. With stationary liquid in tubular module, the bubble size increases accordingly with the increase of the gas flow rate while the size of the bubbles is obviously shrunken in non-coalescence systems. In empty rectangular channel, the smallest bubble we found (2 mm approximately) in non-coalescence system is larger than the channel height. Therefore bubbles of both systems could keep contact with channel wall which equips them with ability to clean fouling. Three different flow behaviors of two-phase flow are classified from the flow visualization in channel with diamond-like spacer D1. For both systems, working conditions in region (III) is optimal since the flow distribution is well managed. The threshold of region (III) is higher in non-coalescence system, because higher liquid and gas flow rate is required to sweep stagnant bubbles away. Higher porosity contributes less to the energy saving therefore porosity between 0.887 and 0.917 for the test spacer geometry is appropriate. Ladder-type spacers could avoid maldistribution of bubbles, which are suitable for conditions required lower gas and liquid flow rate.
Separation and Purification Technology 01/2014; 125:275–283. · 2.89 Impact Factor
[show abstract][hide abstract] ABSTRACT: The aromatic polyamide nanofiltration membrane was evaluated for the treatment of water-soluble dyestuff effluent. Simultaneously, NF270 commercial membrane was used as a reference. It was observed that the rejections of three dyes were all above 95% under 0.7 MPa at 25 ℃. The desalination-purification experiment of dye Direct Blue 71 and NaCl mixed solution showed that the membrane could concentrate DB71 with a high flux (＞50L•m-2•h-1), a high dye rejection (＞96%) and a salt retention lower than 5%. The dye rejections and high-temperature resistance was better than commercial NF270 membranes, but its membrane flux was slightly lower than that of commercial membranes.
Technology of Water Treatment. 03/2012; 38(3):67-70.
[show abstract][hide abstract] ABSTRACT: The removal of boron from aqueous solution by direct contact membrane distillation (DCMD) was studied with self-prepared polyvinylidene fluoride (PVDF) hollow fiber membranes in the present work. The effect of pH, boron concentration, temperature and salt concentration of the feed solution on the boron rejection was investigated. The experimental results indicated that boron rejection was less dependent on the feed pH and salt concentration. DCMD process had high boron removal efficiency (>99.8%) and the permeate boron was below the maximum permissible level even at feed concentration as high as 750 mg/L. Although the permeate flux was enhanced exponentially with the feed temperature increasing, the influence of feed temperature on the boron rejection could be neglected. Finally, the natural groundwater sample containing 12.7 mg/L of boron was treated by DCMD process. The permeate boron kept below 20 microg/L whether the feed was acidified or not, but pre-acidification was helpful to maintain the permeate flux stability. All the experimental results indicated that DCMD could be efficiently used for boron removal from aqueous solution.
Journal of hazardous materials 05/2010; 177(1-3):613-9. · 4.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: The feasibility of employing nanofiltration for the removal of chromium (VI) from wastewater was investigated. Poly (m-phenylene isophthalamide) (PMIA) was used to fabricate asymmetric nanofiltration membrane through the phase-inversion technique. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the obtained membrane, and the both confirmed a much smoother surface which could reduce membrane fouling. The PMIA membrane showed different rejections to electrolytes in a sequence of Na2SO4 > MgSO4 > NaCl > MgCl2, which was similar to the sequence of the negatively charged nanofiltration membranes. Separation experiments on chromium(VI) solution were conducted at various operating conditions, such as feed concentration, applied pressure and pH. It is concluded that chromium(VI) could be effectively removed from chromium-containing wastewater by the PMIA nanofiltration membranes while maintaining their pollution resistance under alkaline condition.
Journal of Environmental Sciences 01/2010; 22(9):1335-41. · 1.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: Fabrication of polyvinylidene fluoride (PVDF) hydrophobic asymmetric hollow fiber membranes was studied by introducing inorganic salt LiCl and water soluble polymer polyethylene glycol (PEG) 1500, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. The membranes properties also were tested and characterized. It is found that the non-solvent additive can increase membranes porosity, ether LiCl or PEG 1500. Because of the addition of PEG 1500, the PVDF membranes obtained a rough topography on the membrane surface and the contact angle of the PVDF membranes increased to 113.50 degrees compared to 89.82 degrees of the PVDF membranes spun without an additive. During direct contact membrane distillation (DCMD) of 0.6 M sodium chloride solution, the PVDF membranes spun with PEG 1500 as a non-solvent additive achieved higher water permeation flux compared to the membranes spun from PVDF/DMAc and PVDF/DMAC/LiCl dopes, but the latter two membranes exhibited higher salt rejection rate.
Water Science & Technology 02/2009; 59(6):1219-26. · 1.10 Impact Factor
[show abstract][hide abstract] ABSTRACT: A novel membrane distillation concentration method was used to prepare high concentration polyaluminum chloride (PACl) with high content of Alb or Alc. 2.52 mol/L PACl1 with 88% Alb and 2.38 mol/L PACl2 with 61% Alc were successfully prepared. Three coagulants, AlCl3, PACl1 and PACl2 were investigated on their hydrolysis behavior and speciation under different conditions. The effects of pH and dilution ratio on Al species distribution were investigated by ferron assay. Experimental result showed that pH had a significant effect on Al species distribution for the three coagulants. Dilution ratio had little effects on Alb and Alc distribution in whole dilution process except the beginning for PACl1 and PACl2. The results indicated that transformation of Al depends largely on their original composition. AlCl3 was the most unstable coagulant among these three coagulants during hydrolysis process. PACl1 and PACl2 with significant amounts of highly charged and stable polynuclear aluminum hydrolysis products were less affected by the hydrolysis conditions and could maintain high speciation stability under various conditions.
Journal of Environmental Sciences 01/2009; 21(10):1342-6. · 1.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: The mixture of inorganic salt LiCl and soluble polymer polyethylene glycol (PEG) 1500 as non-solvent additive was introduced to fabricate hydrophobic hollow fiber membrane of polyvinylidene fluoride (PVDF) by phase inversion process, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. Compared with other three membranes from PVDF/DMAc, PVDF/DMAc/LiCl and PVDF/DMAc/PEG 1500 dope solution, it can be observed obviously by scanning electron microscope (SEM) that the membrane spun from PVDF/DMAc/LiCl/PEG 1500 dope had longer finger-like cavities, ultra-thin skins, narrow pore size distribution and porous network sponge-like structure owing to the synergistic effect of LiCl and PEG 1500. Besides, the membrane also exhibited high porosity and good hydrophobicity. During the desalination process of 3.5 wt% sodium chloride solution through direct contact membrane distillation (DCMD), the permeate flux achieved 40.5 kg/m2 h and the rejection of NaCl maintained 99.99% with the feed solution at 81.8 °C and the cold distillate water at 20.0 °C, this performance is comparable or even higher than most of the previous reports. Furthermore, a 200 h continuously desalination experiment showed that the membrane had stable permeate flux and solute rejection, indicating that the as-spun PVDF hollow fiber membrane may be of great potential to be utilized in the DCMD process.