Table 2 Cross Flow Filtration Key Advantages Process Goal Crossflow Filtration Deadend Filtration

Ability to handle wide variations in particle size

Ability lo handle wide variations in solids concentration

Continuous concentration witli rccycle

Waste minimization

High product purity or yield






but may require diaHltration to overcome excessive (lux loss at higher recovery

Generally poor

Poor or unacceptable

Poor or unacceptable

Can minimize waste if handling low solids feed where cartridge disposal Is Infrequent

Performance Is generally acceptable except in situations involving high solids or adsorptive fouling



Dead End And Crossflow Filtration
Figure 2. Cross-flow versus dead end filtration.

On the other hand, in dead end filtration the retention is achieved by particle or gel layer buildup on the membrane and in the pores of the medium such as when a depth type filter is used. This condition is analogous to that encountered in packed-bed geometries.

In dead end filtration, the applied pressure drives the entire feed through the membrane filter producing a filtrate which is typically particle-free while the separated particles form a filter cake. The feed and filtrate travel concurrently along the length ofthe filter generating one product stream for every feed. In CFF, one feed generates two product streams, retentate and permeate. Per pass recovery in through-flow mode is almost 100% (since only the solids are removed) whereas in the cross-flow mode the per pass recovery typically does not exceed 20% and is often in the 1 to 5% range. Recirculation of retentate is thus necessary to increase the total recovery at the expense of higher energy costs.

As the filtration progresses, the filter cake becomes increasingly thicker which results in a reduced filtration rate (at a constant transmembrane pressure). When the flow or transmembrane pressure (depending on the control strategy) approaches a limiting value, the filtration must be interrupted in order to clean or replace the membrane filter. This discontinuous mode of operation can be a major disadvantage when handling process streams with a relatively high solid content.

Cross-flow filtration can overcome this handicap by efficient fluid management to control the thickness of the concentration-polarization layer. Thus, feed streams with solid loading higher than 1 wt. % may be better suited for CFF whereas feed streams containing less than 0.5 wt.% solids may be adequately served by dead end filtration. However, if the retained solids constitute the product to be recovered or when the nature of solids is the cause of increased fouling, cross-flow filtration should be considered. CFF is also the preferred mode when particle size or molecular weight distribution is an important consideration, such as in the separation of enzymes, antibiotics, proteins and polysaccharides from microbial cell mass, colloidal matter and oily emulsions. Tubular cross-flow filters are being used to continuously concentrate relatively rigid solids up to 70 wt.% and up to 20 wt.% with gelatinous materials.

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