Abstract
Theoretical and experimental studies of direct contact membrane distillation (DCMD) using commercially flat sheet hydrophobic polyvinylidene fluoride (PVDF) membrane were investigated to remove boron from simulated and natural seawater. Mass transfer mechanism and theoretical flux were determined using the dusty-gas model (DGM). Mass transfer analysis showed that the combined Knudsen-molecular model is considered to be the dominant mechanism which describes the transport of water vapor through the membrane pores. Then, the effect of various operating parameters such as feed temperature, boron concentration, salt concentration and feed solution pH on DCMD performance was investigated. Experimental runs showed that the permeate flux was enhanced exponentially with feed temperature, and maximum permeate flux of 27.5kg/m2∙h was obtained at 74°C. The permeate flux decreases slightly with feed boron and salt concentration. Feed solution pH had no significant effect on DCMD performance. DCMD process can produce water with high boron rejection of more than 90% even at feed concentration as high as 200mg/L. Natural seawater, containing 5.37mg/L of boron, was treated by DCMD process. During 18h of working period, the permeate flux had showed a reduction of about 15.75%, and permeate boron was kept below 0.47mg/L. The decreasing of contact angle value can explain the performance decline of DCMD process.
•DCMD is a suitable process for boron removal from synthetic water and seawater.•Knudsen-molecular model dominates the transfer of water vapor through pores.•Evaluation of flux and boron rejection under different operating conditions•Feed pH solution had no marked influence on permeate and boron rejection.•DCMD application of seawater showed promising performance.