Superhydrophobic-slip surface based heat and mass transfer mechanism in vacuum membrane distillation

Abstract

Superhydrophobic membrane is favorable for the membrane distillation technology in desalination applications although the theoretic reasons are still unsettled. In this study, some novel theories were first proposed to establish the bridges between the surface parameters (including water contact angle and surface roughness) and the heat and mass transfer process. Based on the hydrophobic PVDF membrane (with a water contact angle of 124°), a kind of superhydrophobic PVDF membrane with a water contact angle of 162° was prepared through the surface modification. Then both membranes were tested in the vacuum membrane distillation (VMD) process, and simulation models based on both kind of membranes were constructed to compare the membrane performances and the heat and mass transfer characteristics. The results show that the slip flow exists on the superhydrophobic surface, and the superhydrophobic membrane has higher permeate flux than the original PVDF membrane. The insight investigations show that the heat transfer resistance (HTR) of the VMD process is focused on the membrane layers, and the variation of water contact angle or roughness has little influence on the heat transfer coefficient of the membrane layers. However, when the water contact angle varies from 124° to 165°, the total HTR value decreases from 3.156 × 10-3 (m2 K)/W to 2.599 × 10-3 (m2 K)/W, which indicates that the feed side HTR value is largely decreased by the superhydrophobic surface. The further polarization investigations found that the temperature polarization coefficient (TPC) changes slightly when the membrane contact angle varies from 124° to 165°. Instead, the concentration polarization coefficient (CPC) decreases from 0.1455 to 0.0071. The vapor polarization coefficient (VPC), which comprehensively considers the influences of TPC and CPC, has a value of above 0.96 when the water contact angle is higher than 155°, indicating that the performance of the superhydrophobic membrane is not limited by the polarization problems.