Table of Contents
Filtration
It is defined as a process of separation of solid from fluids by passing mixture through a porous medium that retain the solid but allows the fluids to pass through. The mixture or suspension to be filtered is known as slurry. The porous medium used to retain the solids is known a filter medium. The accumulated solid on the filters are referred to as filter cake, while the color liquid passing through the filter is filtrate.
When solids are present in a very low concentration that is not exceeding 1.0% w/v the process of its separation from liquid is called clarification.
Applications of Filtration
- Production of sterile products- Air is filtered through HEPA filters (high efficiency particulate air filters) or laminar air bench to obtain sterile air, which maintain good environment prior to and during manufacturing of sterile products.
- A solution is passed through a bacteria proof filter in order to obtain sterile solution, particularly when heat sterilization is not suitable on account of the thermolabile nature of the contents. In case of sterile products particle as small as 0.2µm should be removed, which includes the bio- burden of fungi, bacteria etc.
- Production of bulk drugs-solids of intermediates and finished products are separated from the reaction mixture by filtration techniques by the method, impurities can be removed.
- Production of liquid oral formulation-Filtration is an essential steps in the production of liquid oral for obtaining clear solution.
- Affluent and waste water treatment-Waste solid must be separated from the waste liquid prior to its disposal. Sometimes, the soluble components are precipitated and the separated by filtration
Factors influencing filtration
A simple straining process does not provide a complete description of how particles are removed from s suspension.
The particles are exposed to a number of forces including gravity or electrical fields. Some of the factors influencing the filtration rate-
- Properties of the liquids- Density, viscosity, and corrosiveness.
- Properties of the solids- Particle shape, particle size, particle charge, particle density, rigidity or compressibility of the solid under pressure and tendency of particle to flocculate or adhere together.
- Temperature of the suspension.
- Filter cake formation rate.
- Surface area of the filter medium.
- Gravity forces.
- Appling pressure.
- Viscosity of filtrate.
- Centrifugal forces- Centrifugal force could replace the gravitational force and is used to increase the rate of filtration.
Theories of filtration
- The flow of a liquid through a filter follows the basic rules that govern the flow of any liquid through the medium offering resistance.
- The rate of flow may be expressed as–
Rate = driving force/resistance.
- The rate of filtration may be expressed as volume/time.
- The driving force is the pressure differential between the upstream and downstream of the filter.
- The resistance is not constant .it increase with an increase in the deposition of solids on the filter medium.
- Poiseuille’s Equation- Poiseuille considered that considered that filtration is similar to the streamline flow of a liquid under pressure through Capillaries.
V = ΔPπr4 / 8ηl
Where –
- V = rate of flow, that is volume of liquid flowing in unit time (m3/s).
- ΔP = Pressure difference across the filter. (Pascal)
- r = radius of the capillary in the filter bed. (Meter)
- L = Thickness of the filter cake (capillary length). (Meter)
- η = Viscosity of the filtrate. (Pascal/second).
- Darcy Equation-
V = KAΔP / ηl
- K = permeability coefficient of cake m2.
- A= Surface area of the porous bed (filter medium) m2.
- The term k depends on the characteristics of the cake, such as porosity, specific surface area and compressibility.
- Kozeny-Carman Equation-
V = A/ηs2 x ΔP/KL x ε3/(1-ε)2
Where –
- ε = porosity of the cake (bed)
- S = Specific surface area of the particles comprising the cake m2/m3.
- K = Kozeny constant (usually taken 5).