Cross-flow filtration
Filtration method where feed flows tangentially across the filter surface.
Cross-flow filtration, also known as tangential flow filtration, is a unit operation in chemical and biochemical engineering, as well as protein purification. Unlike dead-end filtration, where the feed is forced directly into a membrane or bed, trapping solids and releasing filtrate, cross-flow filtration directs the majority of the feed flow tangentially across the filter surface. This tangential motion continuously rubs off trapped particles, preventing the formation of a blinding filter cake and allowing the system to operate continuously rather than in batches. The process is driven by transmembrane pressure, the pressure difference across the membrane. However, as permeate viscosity increases, this pressure can drop, reducing efficiency; this can be mitigated by diluting the permeate or increasing the flow rate.
This method is particularly suited for feeds with a high proportion of small solids, where the permeate is valuable, because dead-end filtration would quickly blind. Industrial examples include extracting soluble antibiotics from fermentation broths. In operation, the feed passes at positive pressure across the membrane; material smaller than the pore size passes through as permeate, while the rest is retained as retentate. Benefits include higher liquid removal rates, a mobile slurry for further processing, wide variability in solids content, and the ability to fractionate particles by size. Applications span reverse osmosis, nanofiltration, ultrafiltration, and microfiltration, using polymeric or ceramic membranes. In protein purification, it is called tangential flow filtration (TFF). Performance can be improved by backwashing (inverting pressure to lift fouling), alternating tangential flow (ATF) using a diaphragm pump, or clean-in-place (CIP) systems using detergents, acids, alkalis, or enzymes, though bleach must be avoided with thin-film membranes as it causes oxidation.
- field
- Chemical engineering, biochemical engineering, protein purification
- known_for
- Tangential flow filtration, continuous filtration with reduced filter blinding
- type
- Unit operation
- driving_force
- Transmembrane pressure
- applications
- Reverse osmosis, nanofiltration, ultrafiltration, microfiltration, protein purification
Lore & Background
In cross-flow filtration, the feed is passed across the filter membrane at positive pressure relative to the permeate side. Material smaller than the membrane pore size passes through as permeate, while everything else is retained as retentate. The tangential motion of the fluid causes trapped particles on the filter surface to be rubbed off, allowing continuous operation at high solids loads without blinding. The main driving force is transmembrane pressure, which may decrease due to increased permeate viscosity, reducing efficiency; this can be prevented by diluting permeate or increasing flow rate.
Reader's Guide
Cross-flow filtration is significant because it enables continuous filtration of feeds with high proportions of small particle solids, where the permeate is most valuable, such as in extracting soluble antibiotics from fermentation liquors. Its advantages include higher overall liquid removal rates, the ability to keep the feed as a mobile slurry, and the possibility of fractionating particles by size. The technology is used widely in industry with polymeric or ceramic membranes, and its principles apply to reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. In protein purification, it is called tangential flow filtration (TFF). Performance can be improved through techniques like backwashing, alternating tangential flow (ATF), clean-in-place (CIP), process flow disruption (PFD), and model-based fouling control. The flux or flow rate is given by the equation J = ΔP / (R_m + R_c), where ΔP is transmembrane pressure and R_m and R_c are membrane and cake resistances.
Did You Know?
- Cross-flow filtration is also known as tangential flow filtration.
- The principal advantage is that the filter cake is substantially washed away during filtration, increasing operational time.
- It can be a continuous process, unlike batch-wise dead-end filtration.
- In protein purification, the term tangential flow filtration (TFF) is used to describe cross-flow filtration with membranes.
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