Enhancing performance of AGMD by applying Vacuum

Author: Niels Brand

Summary: The application of vacuum is found to improve the performance of MD. There are some questions or misconceptions on the application and the actual effect of the applied vacuum on the process of MD.Why does it even improve performance? What improvement is to be expected? What are the risks of applying vacuum? Vacuum is applied to the distillate channel only, not the brine channel(s) to prevent issues resulting from brine boiling inside channels. Changing the pressure inside the airgap directly influences the mass diffusivity of the water vapour molecules. Applying vacuum on membrane and airgap via distillate channel increases mass transfer, evaporation rate and thermal efficiency. The application of vacuum has increased Flux, (membrane) efficiency and GOR. The amount of improvement is directly related to the vacuum pressure. Experimental results are shown to display the effect. High flux modules improves most on flux with application of vacuum where-as high GOR modules improve most on GOR. The application of vacuum is not without risk. Applying too strong vacuum for the situation will reduce salt rejection. Adjustable vacuum pressure according to application is desirable.

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