9. FILTRATION

FILTRATION

The process in which a heterogeneous mixture of a fluid and solid particles are separated by a filter medium which permits the passage of fluid but retains the solid particles is termed as filtration. The suspension of solid or liquid to be filtered is known as slurry. The porous medium used to retain the solids is known as filter medium. The accumulated solids on the filter media are referred to as filter cake, while the clear liquid passing through the filter is the filtrate.

OBJECTIVES OF FILTRATION

1. To clarify liquor purification.
2. To separate both the liquids and solids recovered.
3. To facilitate or improve other plant operations.

APPLICATIONS OF FILTRATION

1. Improving the appearance of pharmaceutical preparations (solutions, mouthwashes).
2. Removing potential irritants from eye drops or solutions used on mucous membrane.
3. Recovering desirable solid material from suspension or slurry.
4. Removing turbid products obtained after various unit operations.
5. Detecting microorganisms present in liquids by using a filter which retains bacteria.
6. Evaluating the efficiency of preservatives.
7. Production of Sterile Products: Sterile air obtained by HEPA filters; thermolabile substances passed through bacteria-proof filters.
8. Production of Bulk Drugs: Separating solids of intermediates and finished products from reaction mixture.
9. Production of Liquid Oral Formulations: Oil dewaxing, removing suspended oils, undesirable solid particles, clarifying potable water.
10. Affluent and Waste Water Treatment: Separating waste liquid and waste solids before disposal.

THEORIES OF FILTRATION

The liquid flowing through a filter follows the basic rules governing the flow of any liquid through a medium which offers resistance. The rate of flow is expressed as:

Rate of flow = Driving force / Resistance

Poiseuille’s Equation

V = (π ΔP r⁴) / (8ηL)

• V = Rate of flow (m³/s)
• ΔP = Pressure difference across the filter media (Pa)
• r = Radius of the capillary in filter bed (m)
• L = Thickness of the filter cake (m)
• η = Viscosity of the filtrate (Pa·S)

Darcy’s Equation

V = (K A ΔP) / (ηL)

• K = Permeability coefficient of the cake (m²)
• A = Surface area of the porous bed (m²)
• ΔP = Pressure difference across the filter (Pa)
• η = Viscosity of the filtrate (Pa·S)
• L = Thickness of the filter cake (m)

Kozeny-Carman Equation

V = (A ΔP ε³) / (k S² L (1-ε)²)

• S = Specific surface area of particles comprising the cake (m²/m³)
• k = Kozeny constant
• ε = Porosity of the cake (bed)

FILTER MEDIA

Characteristics

1. Retain solids, resulting in clear filtrate.
2. Should not plug or bind.
3. Chemically resistant and physically strong.
4. Allow clean and complete discharge of filter cake.
5. Should not be over-expensive.

Types of Filter Media

• Filter Papers: Retain very fine solids; available in various grades, shapes, sizes, and degrees of permeability.
• Membrane Filters: Pure cellulose/derivatives, nylon, teflon, PVC, or silver; used for micro-filtration (50-200 µm thick; pore size 0.05-14 µm).
• Cotton Filters: Small pledged absorbent cotton-wool inserted in funnel neck; removes large particles.
• Glass Wool: Used for highly reactive/corrosive solutions (strong acids, alkalis, oxidising agents).
• Asbestos: Prepared by compressing shredded asbestos; gives alkaline reaction; may release Ca²⁺/Mg²⁺ ions.
• Sintered Glass Filters: Borosilicate glass with flat/convex plate; consists of powdered Jena glass particles moulded together.
• Other Filters: Sand filters (municipal water purification), Berkefeld filters, Chamberland filters, Seitz filters.

FILTER AIDS

Characteristics

1. Light in weight and chemically inert.
2. Form high porosity filter cakes.
3. Particle size distribution tailored for desired flow rate and clarity.

Examples of Filter Aids

• Kieselguhr
• Talc
• Charcoal
• Asbestos
• Paper pulp
• Bentonite
• Fullers earth

Techniques

• Body-mix technique: Filter aids added directly to the liquid to be filtered.
• Pre-coating technique: Filter aids used as slurry in solvents to pre-coat the filter.

FACTORS INFLUENCING FILTRATION

1. Pressure: Filtration rate increases with increasing pressure difference.
2. Viscosity: Rate for highly viscous liquids is less than low viscosity liquids.
3. Surface Area of Filter Media: Increased by pleating filter paper or using fluted funnel.
4. Temperature: Reduced viscosity increases filtration rate of viscous oils, syrups.
5. Particle Size: Coarse particles filter easier than finely divided particles.
6. Pore Size of Filter Media: Coarse filter media increases filtration rate.
7. Thickness of Cake: Filtration rate decreases as cake thickness increases.
8. Porosity of Cake: Depends on nature of solid particles; filter aids added to make porous cake.
9. Characteristics of Slurry: Properties of liquid, solid, and amount of solids.

EQUIPMENT FOR FILTRATION

Filters used for filtration on an industrial scale are classified as:

• Based on application of external force: Pressure Filters (Plate and frame filter press, Metafilter), Vacuum Filters (Filter leaf), Centrifugal Filters
• Based on operation: Continuous Filtration, Discontinuous Filtration
• Based on nature of filtration: Cake Filters, Clarifying Filters, Cross-Flow Filters

Membrane Filters

Principle

The membrane functions as a barrier – larger particles are retained while smaller ones pass through based on pore size.

Construction

Membranes of cellulose acetate, cellulose nitrate, or mixed cellulose ester. Pore size ranges from 0.010 ± 0.002 µm to 5.0 ± 1.2 µm.

Applications

• Enhanced recovery of particular gram-positive organisms.
• Filtration of enzyme solution.
• Diagnostic cytology.
• Receptor binding studies.
• Clarifying filter.
• Sterilising and clarifying aqueous and organic solvents, buffers, microbiological and tissue culture solutions.

Sintered Filters

Construction

Filtering medium made of Jena or Pyrex ground glass particles fused into a disc by heating to sintering point. The disc is sealed into a Pyrex glass funnel (Buchner funnel shape).

Applications

• Coarse, fine, and bacterial filtration.
• Parenteral injection, ophthalmic solution, and solution of potent drugs.

SUMMARY TABLE: FILTRATION EQUATIONS

Equation	Formula	Key Parameters
Poiseuille’s	V = (πΔPr⁴)/(8ηL)	r (capillary radius), L (cake thickness)
Darcy’s	V = (KAΔP)/(ηL)	K (permeability coefficient), A (surface area)
Kozeny-Carman	V = (AΔP ε³)/(kS²L(1-ε)²)	ε (porosity), S (specific surface area)

COMPARISON: FILTER MEDIA TYPES

Type	Material	Best Suited For	Limitations
Filter Papers	Cellulose	Very fine solids, small amount of solids	–
Membrane Filters	Cellulose esters, nylon, teflon	Micro-filtration, sterile solutions	–
Glass Wool	Glass fibres	Corrosive solutions (acids, alkalis)	May contaminate with glass fibres
Asbestos	Shredded asbestos
Alkaline reaction, releases ions, absorbs alkaloids