# Cross-flow filtration

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Cross-flow filtration
Diagram of cross-flow filtration

In chemical engineering, biochemical engineering and protein purification, crossflow filtration[1] (also known as tangential flow filtration[2]) is a type of filtration (a particular unit operation). Crossflow filtration is different from dead-end filtration in which the feed is passed through a membrane or bed, the solids being trapped in the filter and the filtrate being released at the other end. Cross-flow filtration gets its name because the majority of the feed flow travels tangentially across the surface of the filter, rather than into the filter.[1] The principal advantage of this is that the filter cake (which can blind the filter) is substantially washed away during the filtration process, increasing the length of time that a filter unit can be operational. It can be a continuous process, unlike batch-wise dead-end filtration.

Diagram of cross-flow filtration

This type of filtration is typically selected for feeds containing a high proportion of small particle size solids (where the permeate is of most value) because solid material can quickly block (blind) the filter surface with dead-end filtration. Industrial examples of this include the extraction of soluble antibiotics from fermentation liquors.

## Operation

Ceramic membrane for industrial cross-flow filtration

In crossflow filtration, the feed is passed across the filter membrane (tangentially) at positive pressure relative to the permeate side. A proportion of the material which is smaller than the membrane pore size passes through the membrane as permeate or filtrate; everything else is retained on the feed side of the membrane as retentate.

With crossflow filtration the tangential motion of the bulk of the fluid across the membrane causes trapped particles on the filter surface to be rubbed off. This means that a crossflow filter can operate continuously at relatively high solids loads without blinding.

## Benefits over conventional filtration

• A higher overall liquid removal rate is achieved by the prevention of filter cake formation
• Process feed remains in the form of a mobile slurry, suitable for further processing
• Solids content of the product slurry may be varied over a wide range
• It is possible to fractionate particles by size [3][4]

## Industrial applications

Filtration unit for industrial cross-flow filtration

Cross flow membrane filtration technology has been used widely in industry globally. Filtration membranes can be polymeric or ceramic, depending upon the application. The principles of cross-flow filtration are used in reverse osmosis, nanofiltration, ultrafiltration and microfiltration. When purifying water, it can be very cost effective in comparison to the traditional evaporation methods.

In protein purification, the term Tangential Flow Filtration (TFF) is used to describe cross-flow filtration with membranes. The process can be used at different stages during purification, depending on the type of membrane selected.[2]

In the photograph of an industrial filtration unit (right), it is possible to see that the recycle pipework is considerably larger than either the feed pipework (vertical pipe on the right hand side) or the permeate pipework (small manifolds near to the rows of white clamps). These pipe sizes are directly related to the proportion of liquid flows through the unit. A dedicated pump is used to recycle the feed several times around the unit before the solids-rich retentate is transferred to the next part of the process.

## Techniques to improve performance of cross flow filtration

### Backwashing

In backwashing, the transmembrane pressure is periodically inverted by the use of a secondary pump, so that permeate flows back into the feed, lifting the fouling layer from the surface of the membrane.[5]

### Clean-in-place

Clean-in-place systems are typically used to remove fouling from membranes after extensive use. The CIP process may use detergents, reactive agents such as sodium hypochlorite and acids and alkalis such as citric acid and sodium hydroxide.

### Concentration

The volume of the fluid is reduced by allowing permeate flow to occur. Solvent, solutes, and particles smaller than the membrane pore size pass through the membrane, while particles larger than the pore size are retained, and thereby concentrated. In bioprocessing applications, concentration may be followed by diafiltration.

### Diafiltration

In order to effectively remove permeate components from the slurry, fresh solvent may be added to the feed to replace the permeate volume, at the same rate as the permeate flow rate, such that the volume in the system remains constant. This is analogous to the washing of filter cake to remove soluble components.[5]. Dilution and re-concentration is sometimes also referred to as "diafiltration."

### Process Flow Disruption (PFD)

A technically simpler approach than backwashing is to set the transmembrane pressure to zero by temporarily closing off the permeate outlet, which increases the attrition of the fouling layer without the need for a second pump. PFD is not as effective as backwashing in removing fouling, but can be advantageous.

## Flow rate calculation

The flux or flow rate in cross-flow filtration systems is given by the equation:

$J = \frac{\Delta p}{(R_m + R_c) \mu}$ [5]

in which:

• J: liquid flux
• ΔP: transmembrane pressure (should also include effects of osmotic pressure for reverse osmosis membranes)
• Rm: Resistance of the membrane (related to overall porosity)
• Rc: Resistance of the cake (variable; related to membrane fouling)
• μ: liquid viscosity

Note: Rm and Rc include the inverse of the membrane surface area in their derivation; thus, flux increases with increasing membrane area.

## References

1. ^ a b Koros WJ, Ma YH, Shimidzu T (June 1996). "Terminology for membranes and membrane processes (IUPAC)". Pure & Appl. Chem. 86 (7): 1479–1489.
2. ^ a b Millipore Technical Library: Protein Concentration and Diafiltration by Tangential Flow Filtration
3. ^ Bertera R, Steven H, Metcalfe M (June 1984). "Development Studies of crossflow filtration". The Chemical Engineer 401: 10.
4. ^ Novasep membranes
5. ^ a b c JF Richardson, JM Coulson, JH Harker, JR Backhurst (2002). Coulson and Richardson's chemical engineering (Volume 2) (5th ed.). Butterworth-Heinemann. ISBN 0750644451.

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