Study of membrane fouling in cross-flow vacuum membrane distillation

Abstract

Membrane fouling is one of the main obstacles to membrane distillation (MD) applications in desalination and other fields. Membrane scaling influences the efficiency and performance of vacuum membrane distillation (VMD). In this study, the process of a periodic membrane scaling experiment was investigated in detail by using a cross-flow hollow fiber membrane module. In the experiment, water flux significantly decreases from 42.5kg/(m2h) to 4.1kg/(m2h) because of membrane scaling; by comparison, water flux decreases from 42.7kg/(m2h) to 30.4kg/(m2h) in the blank experiment which is only affected by concentration and polarization. The blank experiment is performed under the same operating conditions as the periodic membrane scaling experiments, in which pure NaCl solution is used as feed. Scaling deposits form on/in membrane pores. Gas permeability experiments indicate that the membranes exhibit poorer transport properties as a result of aggravation of fouling and have smaller yield of MD. The scaling layer negatively affects heat and mass transfer in the feed near the evaporation surface. Membrane scaling further enhances temperature and concentration polarization, thereby decreasing the permeate flux. Experimental results further indicate that the overall heat transfer coefficient is reduced from 4569.4W/(m2K) to 2346.6W/(m2K) and the mass transfer coefficient in the membrane is reduced from 3.88kg/(m2Pah) to 2.18kg/(m2Pah) as scaling increases. Scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS) was employed to analyze the morphologies and compositions of the deposits formed on/in the membrane. The major component of the deposits is Ca; small amounts of Mg and S are also detected. The different morphologies of the deposits are significantly determined by various feed compositions.Using Bohai seawater, we investigated membrane fouling on the basis of VMD. The results are important for VMD application in seawater desalination.