Comparison Of Crossflow 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.

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.

Proccss Conditions

Cross-flow Filtration

Deadend Filtration

Low .solids 0-1% by volume

Medium solids

Ijy volume

High solids (10 lo 70% by volume)

Ptecoat Filtration

_ Ce.ptrlfugation

Can handle efficiently but needs high flux to be cost effective

Can adequately handle ; economics depends on flux

May not be economical at > 25% solids (with few exceptions) for continuous process

Can handle effectively; Can handle effectively;

low cost

Not well suited

Not well suited low cost

Can handle but may be expensive

Can handle adequately Can handle adequately and economically and economically

May handle the solids high operating cost

Can handle high solids high capital and maintenance

.Emulsified liquids

Small density différences or line partlclcs

Can handle efficiently

Well suited due to wide range of pore diameters UF/MF

Not well suited

Can handle adequately

Not well suited

Not well suited

Not well suited Cannot handle efficiently

Separation of macromolccular solutes

Can handle very efficiently; cost effective altenatlve

Not well suited

Not well suited; low throughput

Not feasible

Solvents and/or high temperature

Can handle adequately using chemically/thermally resistant membranes

Not well suited

Not well suited In open system

May be difficult to handle

Continuous fractionation of solids

Not well suited

Can handle but Not feasible performance sensitive to operating conditions

Can handle adequately

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.

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