The filtration is a Procédé of separation making it possible to separate the components from a mixture which has a phase Liquide and a solid phase through a medium Poreux. Filtration is a technique very much used that it is in the field of the Agro-alimentaire or of the Pharmacie.

The use of a filter makes it possible to retain the particles of the heterogeneous Mélange which are larger than the holes of the filter (Porosité). The liquid having undergone filtration names filtrate , and what the filter retains names a residue (also commonly called " Gâteau" or rétentat).

Classification of filtrations

Mode of passage of the fluid

There exist two principal techniques of filtration:

  • frontal filtration , the most known, consists in making pass the fluid to be filtered perpendicular to the surface of the filter. It is the technique employed for example for the coffee filters. The particles being retained by the filter, this technique is limited by the accumulation of the particles to its surface, which finish little by little by the butcher.
  • tangential filtration , on the contrary, consists in making tangentially pass the fluid on the surface of the filter. It is the pressure of the fluid which makes it possible this one to cross the filter. The particles, in this case, remain in the tangential flow of circulation, and stopping is carried out thus much less quickly. However, this technique is reserved for the filtration of the very small particles, of a size going of the nanometer to the micrometer.

Dimension of the particles

One can also differently name the operation of filtration according to the size of the pores of the filter:
  • clarifying Filtration: when the diameter of the pores ranges between 10 and 450 micrometers
  • Microfiltration: when the diameter of the pores is between 10 Nm and 10 micrometers
  • Ultrafiltration: when the diameter of the pores ranges between 1 and 10 Nm
  • Osmose reverses: when the diameter of the pores ranges between 0,1 and 1 Nm
  • sterilizing Filtration: when the diameter of the pores is higher than 0,22 micrometers (the retention of Micro-organisme allows)

Mechanisms of filtrations

The sifting

It is a mechanical phenomenon , otherwise called filtration surfaces some The filter is a membrane perforated by gauged pores and nearby diameters. The filter retains all the particles whose diameter is higher than the diameter of the pores. One speaks about filter screen or filter membrane.

The advantage of this technique is that it does not retain the liquids The disadvantages are:

  • Possibility of filling of the filter. To mitigate it, one increases the diameter of the filter and/or one uses a prefilter of higher diameter.
  • Low holding capacity

Adsorption

Otherwise called filtration in-depth This mechanism consists in retaining inside the porous network of the filter of the particles whose size can be lower than the diameter of the pores.

It is a physical phenomenon , with 2 pricipaux factors:

  • porous Network electrically charged
  • Consisted the long ones and fine strongly circumvented canaliculi

Filters made up of cellulose, teases, cotton

The principal advantage is the great holding capacity.

The disadvantages are:

  • Possibility of slackening the particles (salting out or desorption)
  • Adsorption of liquids
  • Difficulty in defining porosity.

Physical characteristics of the filters

Holding capacity

It corresponds to the diameter of the largest solid particle which passes through the filter. According to the mechanism, one will speak about average diameter of the pores (for the sifting) or about threshold of retention (for adsorption).

Average diameter of the pores

Porosity is the maximum diameter of the particles retained by the filter. Porosity is determined by the measurement of a pressure, according to the following formula:

d=\frac{K.4.\ alpha} {P} \ qquad \ begin {boxes} D, & \ text {Diameter of the pores} \ \ K, & \ text {Constant of the operating conditions} \ \ \ alpha, & \ text {surface Tension of the liquid} \ \ P, & \ text {Pressure} \ end {boxes}

There is an arrival of compressed air in a tube hermetically closed which contains the filter to be studied. Water is brought to moisten the upper part of the filter. Then, one gradually increases the pressure of the air and one notes the pressure necessary to reveal the first bubble of air, it is the point of bubble, allowing to determine the size of the largest particles being able to pass through the filter and thus, his specificity. After having further increased the pressure, bubbles appear on the whole of surface, and one obtains the average diameter of the pores.

Thanks to the formula, one calculates two values of porosity:

  1. the diameter of the largest pores
  2. the porosity itself of the filter

Threshold of retention

It is the diameter of the largest solid spherical particle which passes through the filter under conditions given. It corresponds to 1% of the particles of a given diameter retained by the filter.

Flow of filtration

It corresponds to the quantity of filtrate receuillie during a unit of time. The formula of one tenth of a poise theoretically makes it possible to determine it:

V= \ frac {N.dP.R^4} {8. \ eta. L} \ qquad \ begin {boxes} V, & \ text {Flow in mL/min} \ \ NR, & \ text {Many channels (proportional to surface)} \ \ dP, & \ text {Difference in pressure between the two faces of the filter} \ \ R, & \ text {Average radius of the pores} \ \ L, & \ text {thickness of the filter} \ end {boxes} 3

The flow increases with surface, the pressure and the diameter of the pores. It decreases with the viscosity of the fluid and the length of the filter.

This flow is not constant, because it occurs a phenomenon of filling. Filling slows down filtration by increase thickness of the filter, but also by reduction of the diameter of the pores.

Characteristics of the performances of the filters

To be able of separation

It must be homogeneous and stable in time. It depends on the structure of the filter with homogeneous distribution of the pore, and there should not be evolution of the diameter with time.

Nominal effectiveness

It is the relative arbitrary value based on the percentage of retention (in million particles) compared to the value of reference given by manufacturing. Unfortunately various manufacturing filters define the nominal effectiveness in a different way. Some define the effectiveness on the basis of percentage of retention of the particles of size equalizes with the threshold of filtration, whereas others define it on the basis of percentage of retention of the particles of size equal or higher than the threshold of filtration, which gives obviously a higher value. This variety of definitions make the comparison between filters very difficult for the users.

Absolute effectiveness

It corresponds to the diameter of the largest spherical and indeformable particle which crosses the filter under the specified test conditions.

Materials of filtration

Wood or cellulose fibers

The organic additives of filtration constitute a replacement solution of the ground of diatoms and pearlite, by offering to the user new technical and economic advantages.

In addition to their excellent capacity of separation liquid-solid, the organic additives of filtration are particularly economic, ecological, inoffensive, reliable and powerful.

It is possible to practically treat all filtrations liquid solid in the following sectors: Chemical industry and pharmaceutical, Agro industry, Drinks, fruit juice, spirits, Metallurgy, work of metals - Environment: treatment of waste water, conditioning of muds.

See too

  • hydraulic Filter

Heat-welded polypropylene fibers

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