Jl b) Conventional Cross-flow Operation
Pressure Profiles out 0 1 2 3 4 bar
Figure 13. Transmembrane pressure profiles: (a) uniform transmembrane pressure operation, and (b) conventional cross-flow operation.
6.3 Effect of Operating Parameters on Filter Performance
A number of operating parameters need to be studied to optimize the overall filtration performance. Critical among these are the cross-flow velocity, transmembrane pressure, pore diameter, or MWCO and concentration of the retained species at the end of a batch operation or steady state concentration in continuous filtration. This latter parameter can be related to the recovery of product in the permeate or retentate. Other important operating variables are temperature (and hence viscosity), pH, backpulse or backwash, and pretreatment.
Membrane Pore Diameter or Molecular Weight Cutoff. The value of membrane pore diameter will have a major influence on the permeation and separation characteristics for most process filtration applications. The intrinsic membrane permeability is related to the pore diameter for many microfiltration membranes whereas, for ultrafiltration membranes, it is typically indicative of the solute retention properties. Tables 8 and 9 provide typical permeability and retention data for many common MF and UF membranes, respectively."
The permeability and retention characteristics listed in the tables, however, should only be used as a guide since the actual filter performance may be dependent on a number of other variables and operating conditions. In addition, for many MF/UF membranes, especially those made of polymeric materials, initial flux and retention properties may significantly alter with repeated use in aggressive conditions or over a longer (6 months to 1 year) period of operation.
It is evident that the smaller the pore diameter, the lower the pure solvent (in most cases water) flux, and the higher the ability to retain macrosolutes, colloidal and particulate matter. Users should also be aware that pure solvent flux values are seldom realized in practice and are often at least about an order of magnitude lower in most industrial applications due to effects of fouling and concentration polarization. As a rough rule of thumb, for maximum retention, the pore diameter should be at least about 40 to 50% lower than the smallest particle diameter under the operating conditions. This includes consideration of shear or particle agglomeration/ deagglomeration effects. The nominal MWCO on the other hand should be at least 20 to 30% of the smallest molecular weight of the species to be retained. This is due to the fact that for most membranes, particularly polymeric UF, the MWCO characteristics may be diffused'8' rather than sharp (see Sec. 4.1). Further, secondary layer formation on the membrane surface due to adsorption, fouling and gel polarization will also influence the retention of UF membranes.
306 Fermentation and Biochemical Engineering Handbook Table 9. Retention Characteristics of Cross-flow Ultrafilters
Molecular weight mtolTIMWCq
Molecular weight mtolTIMWCq
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