Improving flux in flat plate modules for membrane distillation
This work is concerned with a study of membrane distillation, through modelling and experimental work, in order to determine factors which enhance the permeate flux in this process. The driving force in membrane distillation is a temperature induced vapour pressure difference caused by having a hot feed and a cold permeate. Three theoretical models were developed in order to analyse the process of membrane distillation in a flat plate module. The first was a flow distribution model utilising the relationship between flow rate and pressure drop in rectangular channels. It was found that increasing the flow rate increased the pressure drop over the module. The second model used mass and heat transfer to predict the permeate flux for PTFE, PVDF and Versapor membranes. The flux was found to increase with increasing mean membrane temperature, temperature difference, and decreasing channel height. It was concluded that the Versapor membrane was unsuitable for membrane distillation. The final model utilised boundary layer theory to predict the development of the thermal boundary layers in a flat plate module. Increasing the region where the boundary layer was still growing, reduced the drop in the temperature difference driving force over the module. For a specific velocity, there was an optimum channel height which produced the maximum possible flux. An experimental program was carried out in order to investigate membrane distillation, to characterise the performance of the flat plate module used and to provide corroborating data for the theoretical models described. A new module design was developed incorporating boiling and condensing heat transfer to overcome the decline in temperature driving force along a module channel. The heat transfer through the channel walls was found to stop the decline in driving force and introduce equilibrium.