surface filtration in oscillatory shear


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Filter FAQ:
Answers to the above questions:
  • What's the difference between your filters and everyone elses?

    Most VibraFilters are supplied with true surface filters rather than the tortuous membrane type of filter. Coupled with this we provide high shear at the membrane surface by means of vibration rather than crossflow filtration, or rotary disc based systems. These two novel concepts in microfiltration provide a powerful new way to perform microfiltration.
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  • Why are surface filters better than tortuous ones?

    Surface filters have very small sieve type pores. The tortuous membrane pore channel relies on capturing particles within as well as on the membrane surface leading to long-term plugging of the membrane. By keeping the deposit on the surface of our filters we can ensure its complete removal by the high shear produced. A more detailed description with illustration is here. Other surface filters do exist, e.g. track etched polycarbonate, but in all cases the other surface filters are fragile, prone to greater blockage than ours (they don't use a long thin slot) and are much more expensive.
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  • What are the advantages of vibration over crossflow filtration?

    High shear, for a start. Also, the shear is where it matters - at the membrane surface and not within the rest of the process fluid, or suspension. When dealing with shear sensitive materials this is very important. For example, in the filtration of beer low temperatures may be better - in crossflow high shear is provided by the pump, leading to high heat input and possible damage of the cells. Thus, heat has to be taken out of the system and the filtration may become more difficult due to the finer particles formed in the pump. In vibration driven filtration both of these problems are removed!
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  • Is it noisy?

    No. Typical frequencies are less than 40 Hz and the laboratory VibraFilter can be driven off a standard compressed air supply. If no supply exists, a silent compressor providing 20 litres per minute at 2 bar is adequate. These have an audio level of less than 50 dB. There are no resonant surfaces or sonic masses in the VibraFilter.
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  • Does it scale-up?

    Multiple head systems, with tubes in parallel are available. Scale-up is simply a case of multiplying the throughput from one tube by the number of tubes producing - just like in crossflow filtration.
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  • How do your surface filters compare with the semiconductor type I've seen?

    We use very long slots and a variety of proprietary surface coatings tailored to the application. The semiconductor type have a much larger surface filtering area than ours, but their lack of long slots seems to make them block very easily. This is probably due to dissolved gasses attaching to their surface. In vibration our filters do not appear to suffer from attachment of dissolved gasses and the long slots do not become blocked. You cal also drop our filters on the ground and in most cases still use them!
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  • Can the filters be used on...?

    We have many years of research experience with this type of filter, surface treatment, coatings, engineering design, etc. We have also used the filters in many applications: soft squidgey drops, solid particles, yeasts, etc. Please do contact us to discuss your application.
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  • How is it possible to filter oil on a long thin slotted pore - doesn't it squeeze through?

    By careful control of permeate flow. The mechanism for drop passage in to the permeate for oil on a long thin slot is by liquid drag from the permeate over the surface of the oil drop. So long as the drag force is low, and the interfacial tension is high enough, the drop will be retained on the slotted pore and the surface shear will remove it. If the interfacial tension is very low, you have a stable microemulsion and microfiltration will not work. The models for drops deforming, and being dragged, into the permeate are well developed and we have a good understanding of this mechanism, see the microfiltration section.
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