Cross-Flow Filtration

In many chemical engineering, biochemical engineering and protein purification fields, cross-flow filtration (also referred to as tangential flow filtration) is a commonly used filtration process. Crossflow filtration is different from other processes such as dead-end filtration in which the feed is passed through a membrane or filter bed, the solids getting 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 or through the filter. The principal advantage of this process is that the filter cake (which normally helps bind the filter) is substantially washed away during the filtration process, and this in turn greatly increases the life of the filter. This process is continuous, unlike batch-wise dead-end filtration.

How it Works

Cross-Flow filtration is typically employed for feeds containing a high proportion of small particle size solids and where the permeate is of high Cross Flow Filtrationvalue whereas in dead-end filtration solid material can quickly block  the filter surface. Industrial or commercial examples of this include the extraction of soluble antibiotics from fermentation liquors. In cross-flow 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. In the crossflow filtration process, the tangential motion of most 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 blocking.

The many benefits of cross-flow filtration include a higher overall liquid removal rate – achieved by the prevention of filter cake formation and the process feed remains in the form of a mobile slurry which is then suitable for further processing.

Applications

The cross-flow membrane filtration process is widely used throughout 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 for example, 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 filter membranes. The process can be used at different stages during purification, depending on the type of membrane selected.

Maintenance

Backwashing and Clean-in-place (CIP) techniques are commonly used to maintain and improve the performance of cross-flow systems. In backwashing, the transmembrane pressure is periodically inverted by the use of a secondary pump, so that the permeate flows back into the feed, lifting the fouling layer from the surface of the filter membrane. Backwashing is not used with spirally wound membranes and is not a general practice in most applications. 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 or alkalis such as citric acid and sodium hydroxide. Sodium hypochlorite (bleach) must be removed from the feed in some membrane plants as it oxidizes thin-film membranes. Oxidation will degrade the membranes to a point where they will no longer perform at rated rejection levels and have to be replaced. Bleach can be added to a NaOH sodium hydroxide CIP during an initial system start-up before spirally-wound membranes are loaded into the plant to help disinfect the system. Bleach is also used to CIP perforated stainless steel (Graver) membranes, as their tolerance for sodium hypochlorite is much higher than a spirally-wound membrane. Caustics and acids are most often used as primary CIP chemicals. Caustic removes organic fouling and acid removes minerals from the membrane. The pH and temperature are important to a CIP program. If pH and temperature are too high the membrane will degrade and flux performance will suffer. If pH and temperature are too low, the system simply will not be cleaned properly. Every application has a different CIP requirement. For example, a dairy RO (reverse osmosis) plant will usually require a more rigorous CIP program than a water purification RO plant. Different membrane manufacturers have their own guidelines for CIP procedures for their product.