I am currently working with a dead-end ultrafiltration system with constant flux rate. The schematic of the experimental system used for constant flux experiment is shown in the figure attached. The feed reservoir is open, and the peristaltic pump is set at 10 rpm to have the flow rate equal to 8 mL/min (DI water). The pore size of asymmetric commercial membranes is 100 kDa. Since the flow rate is always and must be constant, the pressure will increase as a function of time if there is a fouling on the membrane.
However, in my case the pressure always becomes constant at certain numbers when filtering an homogenous polymeric solution after around 5 minutes, and therefore, the flux decreases as a function of time due to fouling. It is possible that the pressure in the stirred cell is higher than in the feed which causes a pressure drop across the tube connecting the feed, peristaltic pump, and stirred cell. If that is the case, then the peristaltic pump does not pump 8 mL/min the polymeric solution after 5 minutes pumping.
If someone wants to share some experiences regarding this problem, I would really appreciate it.
Your system involves a peristaltic pump, which operates at a constant rotation speed (and not at a constant flow rate, nor at a constant pressure). So the observed behavior is simply related to the characteristics of your pump.
The flux rate decline could also be due to increased concentration polarization as well as osmotic pressure (Not only fouling). You can perform tests to identify which is dominant.
Olumoye Ajao Leander Boer thank you for the answers. I am also considering concentration polarization and osmotic pressure. However, what does not make sense to me is only the relationship between constant flow rate and constant pressure while fouling is happening at the same time. To my knowledge, the peristaltic pump always gives the same flow rate and prevents backflow. Thus, when there is a fouling, the pressure in the stirred cell increases for a certain period. Lukily, I spotted a backflow because the pressure in the system exceeded the recommended working pressure of the tubing.
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