Membrane/Process Development FAQ

Crossflow velocity limits for commercially available spiral-wound membrane elements depend on several factors, including element construction, maximum allowable pressure drop, and feed stream characteristics (such as viscosity and solids content). Recommended operating ranges are typically provided by the element manufacturer. For application-specific guidance, please contact Sterlitech for assistance in selecting appropriate crossflow velocities and operating conditions.

Flow characteristics for a spiral element and flat sheet membrane are significantly different and are really only comparable on a qualitative basis. Variability in the membrane manufacturing process, differences in water composition, test procedures, and test equipment, used in a factory, laboratory, or elsewhere, will impact water flux results.

Reference: Understanding Variation of Experimental Flux and Rejection

lease refer to the following diagram to examine recommended feed flow rates for the crossflow cell you are working with.

Feed spacers are used to mimic the hydrodynamic conditions of large-scale membrane modules by changing the flow regime and creating turbulence inside the feed channel of our membrane test cells.   

For more information on feed spacers incorporated into our test cells, please visit here.

The “mil” measurement represents 1/1000th of an inch. Therefore, the thicknesses are 0.017, 0.031, 0.047, 0.065 inches (17, 31, 47, and 65 mil, accordingly). 

Permeate carriers, also known as “permeate water carriers” are aids that are installed between two layers of the flat sheet membranes in a spiral wound element. The main application of this collection layer is to prevent the membrane layers from closing in on each other under high-pressure operations and also facilitate the collection of permeate water. 

Product: Flow/pressure regulator valves: 1149421/1149428/1149427

Q: My pressure regulator valve is clogged and it fails to adjust the flow as expected.

A: The valve should be disassembled and cleaned with pressurized air.

Hydracell pump oil needs to be changed after the first 100 hours of operation. Then, pump oil should be changed according to the guidelines described in the pump user manual. For more information on the pump please see user manual.

The maximum feed water and the maximum solids size for the Hydracell pump are 150 cP and 200 µm, accordingly. 

Sterlitech recommends to install all plastic fittings by hand using PTFE tape. If a plastic NPT or compression fitting starts to leak during operational testing, tighten the fitting carefully until the leaking stops. Excessive tightening may break the fitting or damage other components.

If the fittings is broken and stuck in the cell top, use a spiral flute extractor bit or similar tool to release the portion of the fitting in the cell top. If the cell top becomes cracked from removal efforts, a replacement can be purchased.

Sterlitech crossflow test cells (Sepa® CF, CF042, and CF016) operate in true crossflow filtration mode, meaning the feed flows tangentially across the membrane and produces both a permeate stream and a concentrate (retentate) stream. These systems allow continuous operation, with user-controlled pressure and crossflow rate, and enable ongoing sampling from both streams during testing.

The HP4750 Stirred Cell, by comparison, is a sealed batch filtration device (up to 300 mL feed volume) typically pressurized with compressed gas. It runs in normal-flow (dead-end) mode and does not have a concentrate stream. A stir bar helps reduce concentration polarization and simulates crossflow-like mixing at the membrane surface, but it is not true crossflow.

During high-pressure filtration processes, the membrane sheet inside the cell may deflect outwards into the feed channel.  This occurs due to variations in pressure during operation. This movement can cause rubbing against O-rings which can result in abrasion, possibility rupture, in the membrane sheet. Therefore, a piece of material (shim) can also be placed between the membrane layer and the test cell to take up the free space and help to prevent movement.   

For more information on shims incorporated into our test cells, please visit here.