Flow channel spacers turbulence promoters

For tubular and flat sheet membrane modules, one of the commonly used hydrodynamic techniques for the concentration polarization control is turbulence promoters, such as spacers used in spiral wound membrane modules, helical insert used in tubular membrane modules, and the corrugated membrane for the flat sheet membrane. Research has been conducted to assess the effect of the membrane flow channel spacers and inserts on the membrane filtration and to optimize their design. 

Spiral-wound cartridges are inserted ia series into cylindrical pressure vessels. Feed flows parallel to the membrane surfaces ia the channel defined by the mesh spacer which acts as a turbulence promoter. Permeate flows into the center permeate-withdrawal tube which is sealed through the housing end caps. 

Figure 5. Gelman pleated crossflow filter cartridge. Cartridge components (A) a porous pleated support screen to provide mechanical support under applied pressure (B) the pleated microporous filtration element (C) the pleated spacer which creates the thin flow channel and promotes turbulent flow (D) the impermeable film which creates the flow channel (E) a porous support tube to provide an exit for permeate (F) open-end cap which provides for exit of product flow (G) closed-end cap completely which seals one end of module (H) outer seal ring which creates the seal between the impermeable film in the module and the interior of the housing. The back pressure support tube is not pictured. The ends of the cartridge are potted and sealed. A space between the ends of Film D and the end seals is provided to allow the entrance and exit of the flow-channel fluid.

When a liquid flows through a pipe or a channel a pressure drop is observed across the flow geometry due to friction with the wall. The presence of a spacer or turbulence promoter in the pipe or channel will increase the friction and consequently the pressure drop increases. The pressure for a well developed flow can be given by the foDowing correlation. 

Concentration and temperature polarization can be reduced by the presence of spacers that are mrbulence promoters, which enhance the mass flux by increasing the film heat transfer coefficient. Spacers also change the flow characteristics and promote regions of turbulence thus improving boundary layer transfer [106]. DCMD in spacer-filled channels have been shown to improve flux by 31% 1% than that without spacers. The temperamre polarization coefficients are substantially increased and approach unity when the spacers are used in the channels. 

Other authors (Martmez-Di ez et al., 1998 Phattaranawik et al., 2001) have considered the use of spacer-filled channels of plate-and-frame membrane modules used in DCMD. It was observed that DCMD permeate fluxes were higher when using a screen separator than when an open separator was used. The spacers changed the flow characteristics and promoted regions of turbulence, leading to a decrease of the temperature polarization. These effects were found to be higher for the coarse spacer than for the fine spacer. 

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