276 Fermentation and Biochemical engineering handbook DEADEND FILTRATION CROSS-FLOW FILTRATION FEED CAKE 单o MEMBRANE FILTRATE PERMEATE 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 oneproduct stream forevery feed. In CFF, one feed generates two product streams, retentate and permeate. Per pass recovery in through-flow mode is almost 100%(sinc 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 l to 5%range Recirculation of retentate is thus necessary to increase the total recovery at the expense of higher energy costs
276 Fermentation and Biochemical Engineering Handbook DEADEND FlLTnATlON CROSS-FLOW FILTRATION FlLl r J I IT nhTE 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 particlefree 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. PERMEAT E
Cross- Flow Filtration 277 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 inter rupted 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 3.0 COMPARISON OF CROSS-FLOW WITH OTHER COMPETING TECHNOLOGIES Cross-flow filtration as a processing alternative for separation and concentration of soluble or dissolved components competes with traditional equipment such as dead end cartridge filtration, pre-coat filtration and centrifugation. The specific merits and weaknesses of each of these filtration alternatives are summarized in Table 3. In addition to the ability to handle wide variations in processing conditions, other considerations may need to be addressed for economical viability of cross-flow filtration. These are briefly discussed below. A more detailed discussion on process design aspects capital and operating cost considerations is presented in Sec. 6.7 I. Energy Requirements. Centrifugal devices typically re- quire high maintenance. In contrast, cross-flow filtration requires minimal maintenance with low operating costs in most situations except for large bore(6 mm) tubular membrane products operating under high recirculation rates. The energy requirements in dead end filtration ar typically low
Cross-Flow Filtration 277 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. 3.0 COMPARISON OF CROSS-FLOW WITH OTHER COMPETING TECHNOLOGIES Cross-flow filtration as a processing alternative for separation and concentration of soluble or dissolved components competes with traditional equipment such as dead end cartridge filtration, pre-coat filtration and centrifugation. The specific merits and weaknesses of each ofthese filtration alternatives are summarized in Table 3. In addition to the ability to handle wide variations in processing conditions, other considerations may need to be addressed for economical viability of cross-flow filtration. These are briefly discussed below. A more detailed discussion on process design aspects, capital and operating cost considerations is presented in Sec. 6.7. 1. Energy Requirements. Centrifugal devices typically require high maintenance. In contrast, cross-flow filtration requires minimal maintenance with low operating costs in most situations except for large bore (>6 mm) tubular membrane products operating under high recirculation rates. The energy requirements in dead end filtration are typically low
Table 3. Comparison of Cross-Flow Filtration vs Competitive Technologies Pocess Conditlons Cross-[ow FUlrallon Deadend Filtraton PeroaL Filtration centrifugation ww solids Can handle cmdlently bu an handle clleculvely: Can handle effectively: Can handle but eeds hlgh nux lo be cost by volume economics depends Can handle adequately Can handle adequately gh solids (10 to 70 by at>25 st hlgh capltal and maintenance Emulsifed Can handle eliclently Not well sulled Not well sullen suited due to wide Can landle Cannot handle cfnclently UP/ME Can handle Not well sulled Not feasible cost elective allenatlv low throughput solvents and/or Can handle adequately ol well sulted Not well sulted In May be dificult to handle resistant mcmbrancs Not well sulted Can handle adequately
278 Fermentation and Biochemical Engineering Handbook
Cross-Flow Filtration 279 2. Waste Minimization and Disposal. CFF systems mir mize disposal costs(e.g, when ceramic filters are used) DE stantial waste disposal costs may beincurred, particularl if the DE is contaminated with toxic organics. Currently in many applications, DE is disposed of in landfills. In future, however, this option may become less available forcing the industry to use cross-flow microfiltration technology or adopt other waste minimization measures 3. Capital Cost. Many dead end and de based filtrate systems can have a relatively low capital cost basis. 2JOn the other hand, CFF systems may require relatively higher capital cost. Centrifuges can also be capital intensiv especially where large-scale continuous filtration is 4.0 GENERAL CHARACTERISTICS OF CROSS-FLOw FILTERS The performance of a cross-flow filter is primarily defined by its efficiency in permeating or retaining desired species and the rate of transport of desired species across the membrane barrier. Microscopic features of the membranes greatly influence the filtration and separation performance. 031 The nature of the membrane material Pore dimensions Surface properties such as zeta potential Hydrophobic/hydrophilic cha aracter Membrane thickness From an operational standpoint, the mechanical, thermal and chemical stability ofthe membrane structure is important to ensure long service life and liability table 4 summarizes the influence and significance of these features on the overall performance of a cross-flow filter The discussion on the general characteristics of polymeric and inorganic membranes is treated separately partly due to their differences in productio methods and also due to important differences in their operating characteristics
Cross-Flow Filtration 279 2. Waste Minimization and Disposal. CFF systems minimize disposal costs (e.g., when ceramic filters are used) whereas in diatomaceous (DE) pre-coat filtration substantial waste disposal costs may be incurred, particularly if the DE is contaminated with toxic organics. Currently, in many applications, DE is disposed of in landfills. In future, however, this option may become less available forcing the industry to use cross-flow microfiltration technology or adopt other waste minimization measures. 3. Capital Cost. Many dead end and DE based filtration systems can have a relatively low capital cost basis.[2] On the other hand, CFF systems may require relatively higher capital cost. Centrifuges can also be capital intensive especially where large-scale continuous filtration is required. 4.0 GENERAL CHARACTERISTICS OF CROSS-FLOW FILTERS The performance of a cross-flow filter is primarily defined by its efficiency in permeating or retaining desired species and the rate of transport of desired species across the membrane barrier. Microscopic features of the membranes greatly influence the filtration and separation The nature of the membrane material Pore dimensions Pore size distributions Porosity Surface properties such as zeta potential Hydrophobichydrophilic character Membrane thickness From an operational standpoint, the mechanical, thermal and chemical stability ofthe membrane structure is important to ensure long service life and reliability. Table 4 summarizes the influence and significance of these features on the overall performance of a cross-flow filter. The discussion on the general characteristics of polymeric and inorganic membranes is treated separately partly due to their differences in production methods and also due to important differences in their operating characteristics
Table 4. Influence of Membrane Characteristics on Filtration or Separation Performance Fogarty Influence or Significance symmetric high marginal Flux is hlgher compared with symmetric membranes ymmetric Particle retention In porous structure provides higher surface area per unlt volume. Thls also makes them susceptible to irreversible fouling Bubble point marginal Critical factor for membrane integrity. Porc dimensions marginal Must be carefully optimized to provide high flux combined Pore size marginal substantial Narrow pore size distribution often provides better separation efficienc Porosity hi marginal or none HIgher poroslty typically results In hlgher permeabilIty Zeta potential marginal to Relates to charge effects and can influence fouling due to adsorption/preclpl Hydrophobic margInal can be significant Rejection of water may be Important for sterility purposes Hydrophilic marginal can be significant Provides good wetting of membranes: can Increase transport lutlons; can minimize fouling due to organ substances
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