We all know that dead-end cell has vertical pressure and cross flow has horizontal pressure on the membrane surface. What is the correlation between these two pressures? I tested my membranes with both of these UF cells at 1 bar. In all of cases, cross flow flux is higher than dead-end flux for same membrane sample. But when I plot Pure Water Flux vs Membrane type, the trend of PWF changes in different membranes is same for both of dead-end and cross flow curves. what is the reason of this result? and How can I create a correlation between vertical and horizontal pressures and PWFs?
You need to explain your problem a little better.
Pressure does not depend on direction.
One problem with membrane cells is that pressure changes from point to point. So, when you say that your cell is at 1 Pa in fact different regions of the cell have a pressure around this value, but the variation can be significant because pressure drops along the cell. If the pressure is being measure at the inlet the effective pressure inside the cells may be completely different. You need to post your results for a more accurate interpretation.
Dear Joao Miranda,
Thank you so much for your answer. Can you explain more about the reason of different pressures in different spots of the Dead-end or cross flow cells?
Regards
Dear Foroogh,
It is a concept from fluid dynamics. There is a pressure loss along a tube or any equipment due to friction:
https://en.wikipedia.org/wiki/Friction_loss
So, the real pressure is probabbly smaller in the regions downstream from your point of measurement. Due to the flow configuration, the effect is probably more significant in the cross flow cell.
Dear Foroogh,
One thing you can try is to determine the Pure Water Flux as a function of the pressure. The plot should be a straight line. Ideally it should cross the origin of the axes. However, since the pressure you measure is not the effective pressure, it will cross the xx axis at a distance to the origin. This distance is the correction you need to apply to your pressure. Lines obtained in different cells should have the same slope but not necessarily the same crossing for DP=0. From the slope you can calculate the membrane permeability or membrane resistance.
Dear Joao,
Many thanks for your time and help. The attached diagram is the PWF vs. Membrane code. I did not get your point perfectly. I have to consider the PWF (cross flow) vs time and PWF (dead-end) vs. time for every membrane sample and then check the regression to find a function? Can you explain more please?
Pure water flux @ 1 bar at dead-end cell for membrane 1= 101 LMH
Pure water flux @ 1 bar at cross flow cell for membrane 1= 125 LMH
If the suggestions above still show that the flux for the dead-ended membrane are lower, look at the membrane support surface in the cell and determine how much of the area is solid. Compare that to the support surface of the cross flow cell. At a 1 bar TMP, the area where the membrane touches the support may be not flowing. If these membrane samples are cast on a non-woven scrim, then this would not likely be such a factor.
Dear Craig,
Thanks for your support. Actually the robber O-ring in dead-end cell has much smaller surface area than the metal mesh of the cross flow which they fix the membrane sample. But the cross flow pure water flux is more than dead-end.
The membranes were cast on non-woven polyester support.
Foroogh
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