Fluid/fluid filtration

Flavouring oils are sometimes added to liquid preparations in the form of a spirit, i.e. dissolved in alcohol. When these spirits are added to aqueous-based formulations some of the oil may come out of solution, giving the product a degree of turbidity. Removal of the oil droplets by passing them through an appropriate filter (a liquid/liquid filtration process) is used to produce the desired product appearance.

Compressed air is used in a number of pharmaceutical processes, e.g. film-coating spray guns (Chapter 32), bottle-cleaning equipment and fluid energy mills (Chapter 10). Before use, the compressed air needs to be filtered to ensure that any entrained oil or water droplets are removed. This is an example of a fluid/gas filtration process.

Straining/sieving

If the pores in the filter medium through which the fluid is flowing are smaller than the material that is required to be removed, the material will be retained. Filtration occurs on the surface of the filter in this case, and therefore the filter can be very thin (typically around 100 µm). Filter media of this type are referred to as membrane filters. Because filtration occurs on the surface there is a tendency for them to become blocked unless the filter is carefully designed (see later). Filters employing the straining mechanism are used where the contaminant level is low or small volumes need to be filtered. Examples of the use of membrane filters include the removal of bacteria and fibres from parenteral preparations.

Impingement

As a flowing fluid approaches and passes an object, for example a filter fibre, the fluid flow pattern is disturbed, as shown in Figure 25.1. Suspended solids may, however, have sufficient momentum so that they do not follow the fluid path but impinge on the filter fibre and are retained, owing to attractive forces between the particle and the fibre. Where the pores between filter fibres are larger than the material being removed, some particles may follow the fluid streamlines and miss the fibre, this being more likely if the particles are small (owing to their lower momentum) and as the distance from the centre of the fibre towards which they approach increases. To ensure the removal of all unwanted material, filter media using the impingement mechanism must be sufficiently thick so that material not trapped by the first fibre in its path is removed by a subsequent one. These types of filter are therefore referred to as depth filters. The fluid should flow through the filter medium in a streamlined manner to ensure the filter works effectively, as turbulent flow may carry the particles past the fibres. Depth filters are the main type of filter used for removing material from gases.

• the area available for filtration (A, m²), which in this case is the cross-sectional area of the funnel

• the pressure difference (ΔP, Pa) across the filter bed (filter medium and any cake formed). With the Buchner funnel apparatus, this difference is due to the ‘head’ of unfiltered suspension and therefore it decreases as filtration proceeds and the level drops. Note that it is the pressure difference across the filtration medium that is important, and this can be increased by drawing a vacuum in the collection flask. The difference between atmospheric pressure and the lower pressure in the flask is added to the pressure due to the unfiltered product to give the total pressure difference

• the viscosity of the fluid passing through the filter, i.e. the filtrate (µ, Pa s). A viscous fluid will filter more slowly than a mobile one owing to the greater resistance to movement offered by more viscous fluids (see Chapter 6)

• the thickness of the filter medium and any deposited cake (L, m). The cake will increase in thickness as filtration proceeds so if this is not removed, the rate of filtration will fall.