Retention screen

Screen filters are polymeric membranes that have uniform distribution of pore sizes. They are relatively thin so that there is a minimal amount of liquid retention. Screen filters clog more rapidly than depth filters. Table 2.2 lists the common screen filter materials and their solvent compatabili-ties. 

The conventional top spray method shown in Figure 2 has been used for layering and coaling for decades. It evolved from the fluidized bed dryers commercialized more than 40 years ago. The substrate is placed in the product container, which is typically an unbaffled. inverted, truncated cone with a fine retention screen and an air or gas distribution plate at its base. Perforated plates such as a Conidur or Gill plate may also achieve air distribution and product retention. These types of plates may have directed holes for guiding the airflow horizontally in the product container (for side discharge as an example). Process air is drawn through the distribution plate and into the product. 

The conventional filtrate screen, (4B), has openings complementary to those of the retentate screens. Blocking the retentate channels at one end is performed by silicone (4D). Incorporation of two such modified screens provides for serpentine-flow through the unit with the entire retentate volume sweeping each membrane pair. Table I compares these two techniques. 

Figure 6. Serpentine-flow pathway generated by modification of filtrate screens (retentate flow only shown). Key 1, retentate inlet distribution manifold 2, retentate outlet distribution manifold - — end gasket - - - -, filtrate screen - - —, membrane filter (Durapore) —, retentate screen on linear path retentate channel ----retentate serpentine-flow pathway.

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