VibraFilter

surface filtration in oscillatory shear

 
 
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Micropore Technologies Ltd., UK
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microfilter under scanning electron microscope and pore size distribution What's wrong with conventional microfiltration media?

To the right is a picture of well-known metal microfilter, taken using an electron microscope - note the 100 micron scale bar at the top, below it is the same filter's pore size distribution obtained from a Coulter Porometer and based on an old ASTM technique for pore size analysis:
according to the analysis the modal pore size is 3.5 microns and there are no pores bigger than 5.5 microns, but the scale bar clearly shows surface pore openings up to 40 microns in diameter; much bigger than the pore size distribution suggests.

At low solids concentration, the microfilter can only achieve its rated pore size capacity by capturing particles within the matrix of the filter; i.e. it is really a depth filter.

Deposition of particles within the matrix leads to long-term membrane fouling; i.e. low permeate flow rates. Even when clean, the pressure drop required to pass the fluid through the filter is high, especially with viscous fluids, because of the tortuous pore channel required to capture the particles.

Hence, the pore size of a tortuous membrane is the 'equivalent' pore size of the tortuous pores passing through the microfilter compared to a true surface filter and is, typically, provided by a Coulter Porometer calibrated against a track etched (true surface) microfilter.

How is Micropore Filtration media different?
one example is: Slotted surface microfilters


array of filtration slots Micropore Technology produces filtration media that passes straight through, from one side of the filter to the other. Our filtration media does not rely on depth filtration mechanisms to remove particles, it really does sieve suspended solids, and drops, from a fluid. The picture shows an example of our slotted filter media, where we can provide media with slot widths of as low as 2 microns. For filtration to sizes less than this, we can supply an alternative media, pr a system using a slotted filter followed by a polishing filter.

Many different types of filter geometry are possible, but the filter tube is the most common and it is possible to provide surface finishes, such as gold - as shown on the home page.



principle of oscillatory shear - click for a shockwave flash animation High shear at the membrane surface - without crossflow

The image on the right represents the motion of the vibrafilter surface in linear oscillatory motion; i.e. the entire surface moves backwards and forwards. The advantage of linear oscillation is that high shear exists at the membrane surface over the entire surface. In circular geometry oscillating systems high shear exists at the outer radii, but very low shear exists at the inner radii - as shear is proportional to radius.

Rigid microfilters can be used in linear oscillation by dipping in to the process suspension. The Micropore Technologies membrane has such a low pressure drop the process can operate under vacuum - hence no pressurised seals are needed and no high shear crossflow pumping. Shear sensitive materials are often damaged by the high shear pumps and fittings in a crosslfow system, with the MIcropore Technologies VibraFilter these are absent and shear sensitive material are not damaged. All the shear is at the membrane surface - where it is needed and all that surface is used in the filtration.

The oscillation can be represented by the sine wave shown in yellow. The mathematics of the oscillation and the resulting shear are below.

Analysis of linear shear oscillation

shear stress at the surface The equation shows the shear stress at the membrane surface as a function of operating conditions of frequency, amplitude of vibration liquid phase density and viscosity. Shear stress is proportional to frequency to the power 1.5 and linearly proportional to amplitude. So, a small increase in frequency has a big increase in shear stress at the membrane surface.

The frequency is related to the oscillations. Typically, oscillations are given in Hertz (Hz). To convert into frequency for use in the equation the oscillation in Hertz has to multiplied by 2 PI. For conversion in to shear rate, for Newtonian liquids, the shear rate is: shear rate at the surface

Mild operating conditions with a VibraFilter are 25 Hz, with an amplitude of 12 mm (24 mm peak to peak), giving a shear rate of 20 000 inverse seconds over the entire surface of the membrane. A comparison with crossflow in a 6 mm internal diameter tube shows that a velocity of over 5 m s-1 is needed to match this shear. For very high shear applications oscillations up to 150 Hz have been used with the VibraFilter - giving shear far higher than is practical by crossflow and without the shear damage caused by pumps and fittings.

Nomenclature:

frequency at the surface

amplitude at the surface

frequency at the surface

amplitude at the surface

shear stress at the surface


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