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janice0619

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Crossflow Membrane filtration
May 29^th, 2012, 11:26am

I'm facing 1 problem for my laboratory scale instrument. Below is the scenario of my problem.

I have a diaphragm pump to feed-in the feedwater (river water) to a crossflow membrane cell which placed with a RO membrane. The permeate water that flows out from crossflow membrane cell will be pumping out using a peristaltic pump. If the permeate flow rate is lower that the flow rate of pumping out, the pipeline should be in vacuum (suction) condition, therefore, the pressure sensor in that pipeline should be negative, am I right? However, the lowest negative pressure I get is only -0.80 bar. Should it be lower than -0.80 bar (say, -1, -2, -3 bar or lower, etc) over the filtration time? Typically, membrane fouling occurs when TMP increases over time is observed in a constant flux filtration (see formula below). However, I only manage to get a small decrease in Pp (-0.8 bar). Should it be due to the limitation of my pressure sensor (range from 0 to 25 bar) or the mis-installed of my crossflow set up system? Should I change to pressure sensor to special sensor so that it could accommodate vacuum pressure? I have totally no idea to solve. Please help!

TMP = [(Pi + Po) / 2] – Pp
Where
Pi = Inlet pressure (Feed pressure)
Po = Outlet pressure (Retentate pressure)
Pp = Permeate pressure (Filtrate pressure)

Many thanks!

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Jeff Naumann God Member Offline I love Water and Wastewater.com Posts: 1133 Torrance, California Gender: Company or Organization: Jeff Naumann & Associates	Re: Crossflow Membrane filtration Reply #1 - May 29^th, 2012, 11:43am I will preface my comments by stating that I have no experience with membranes. However, I don't think that using a diaphragm pump is the proper choice for feeding any device where you would want a constant pressure. Obviously, the diaphragm pump provides a "pulsating" flow and pressure. Therefore, "not good."
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janice0619 YaBB Newbies Offline I love Water and Wastewater.com Posts: 8 Gender: Company or Organization: UTAR	Re: Crossflow Membrane filtration Reply #2 - May 29^th, 2012, 11:46am Pressure varies over time. Diaphragm pump is only responsible to deliver feedwater to device that placed with membrane. It is not the main problem. My problem is in the permeate of device.
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grrun God Member Offline WaterandWastewa ter.Com is the best! Posts: 3526 Pekin, IL Gender: Company or Organization: Freelance Environmental Engine	Re: Crossflow Membrane filtration Reply #3 - May 29^th, 2012, 10:16pm I'm not sure, but negative pressures greater than 1 bar would result in cavitation and collapse of the peristaltic tubing. While inlet pressure may be high, I don't think you can pull a vacuum greater than 1 bar!
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janice0619

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Re: Crossflow Membrane filtration
Reply #4 - May 30^th, 2012, 12:57am

Thanks for all comments!

Hmm.. Do any know how can I measure the permeate pressure (Pp) in crossflow membrane cell under constant flow rate? I expect to use the formula as follow to calculate TMP. The TMP would be increased with increasing of filtration time due to membrane fouling. If I presume Pp to be constant, so I could only expect the Pi or P0 increase in order to get TMP increase with time. The Pi & P0 were found to be not very much increased with time even though I run the system for 14 hr (the initial applied pressure was 10 bar for RO membrane). Is my method correct? Anyone knows?

TMP = [(Pi + Po) / 2] – Pp
Where
Pi = Inlet pressure (Feed pressure)
Po = Outlet pressure (Retentate pressure)
Pp = Permeate pressure (Filtrate pressure)

Thanks again!

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sobisch God Member Offline Posts: 1777 Gender: Company or Organization: L.U.M. GmbH	Re: Crossflow Membrane filtration Reply #5 - May 30^th, 2012, 1:08am Maybe the vacuum conditions are limited by leaks?
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janice0619 YaBB Newbies Offline I love Water and Wastewater.com Posts: 8 Gender: Company or Organization: UTAR	Re: Crossflow Membrane filtration Reply #6 - May 30^th, 2012, 3:47am No leakage was observed in the pipeline.
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