Studies On Flux And Fouling In Direct Contact Membrane Distillation Of Seawater

With increasing demands for fresh water around the world, seawater desalination technology has been developing quickly in the past decades. Membrane distillation (MD) is a novel separating process combining membrane with distillation technologies, which can be proceeded in a large scale. Direct contact membrane distillation (DCMD) is one of MD configurations and best suited for the applications where water is the major permeate component such as producing fresh water from seawater and saline solutions. To resolve the problem of water shortage, the government advocates building a forehanded society and seawater desalination as the most effective solution to the water crisis. It is theoretically and practically significant to study the seawater desalination by DCMD.MD is an evaporation process of feed volatile components through porous hydrophobic membrane. For DCMD configuration, the surfaces of the membrane are in direct contact with two liquid phases, hot liquid and cold liquid, kept at different temperatures. Inside the pores, only a gaseous phase is present, through which vapor is transported as long as a partial pressure difference is maintained. The vapor flows through the membrane pores and condenses at the permeate-membrane interface. This process is the same as conventional distillation processes, evaporation-transportation -condensation. The driving forces for the separation are represented by the partial pressure gradient for the evaporating species, maintained by a suitable temperature difference between the evaporation and condensation surfaces.In this paper, a suitable brand of hollow fiber membrane was selected to study the desalination process in DCMD. The distillation processes of concentrated water and seawater were studied. Mass transfer model,membrane fouling and cleaning process were discussed. polypropylene hollow fiber membranes having suitable hydrophobic properties and pore structure were used for the DCMD researches. A cross-flow membrane module and the corresponding DCMD experimental procedure were designed to solve the problem of temperature polarization.First, four parameters: feed temperature, feed flow rate, NaCl concentration and condensed liquid temperature were changed to inspect the water flux, the efficiency of desalination and membrane process parameters. The effects of different feed concentrations (0%-25%NaCl) on water vapor flux were studied within varieties of proceeding parameters. The water fluxes change in a very small range when the salt concentration was in the range of 0-10%. When the concentration was higher than 12%, the water flux reduced rapidly, when the concentration up to 25%, the water flux decreased about 50%. With the increase of feed temperature water flux rose rapidly. This is mainly due to the relationship between temperature and water vapor pressure. T Increasing feed temperature and flow rate are the most effective ways to improve water flux.On the basis of exploration tests, L₁₆ (4³x2⁶) mixed-level orthogonal test was undertaken to obtained appropriate operating conditions for DCMD. All the parameters interacted and the higher were the parameters the more obviously they interacted. Appropriate conditions for the DCMD experiment were: feed temperature was 85℃, feed liquid flow rate was 150 cm/min and condensed liquid temperature was 25℃.Bohai sea water as raw material was used to a continuous direct contact membrane distillation desalination study. Experiments indicated that for the purpose of the production of fresh water membrane distillation desalination can maintain a relatively high flux (25 kg/m2h). MD can make higher proportion fresh water than conventional technologies under smaller losses in flux. Further experiments showed that MD can more easily get solid salt precipitation from the brine water. Membrane fouling is major factor to reduce the water flux. Membrane cleaning techniques were investigated; the method of hydrochloride washing and steam blowing is an effective method of renewable membrane. This study lays a necessary foundation for developing new MD desalination technology used in seawater distillation and chemical comprehensive utilization of resource from desalination concentrated drainage.