Filtration, Clarification and Granulation
8 minute read
What you will learn
- Define filtration and name the common filter media
- Explain the factors that affect the rate of filtration
- Describe clarification methods
- Explain why sterile filtration needs a membrane filter
- Describe granulation and its methods
- Calculate filtration rate and yield
Key points
- Filtration separates solid particles from a liquid by passing the liquid through a porous filter medium.
- Filter media include filter paper, cloth, sintered glass, and membrane filters.
- Filtration is faster with a larger filter area, a thinner liquid, a higher pressure difference and a more open filter.
- Clarification removes small amounts of fine suspended matter to make a liquid clear. Methods are settling, decanting, centrifuging and filtering.
- Sterile filtration uses membrane filters with very small pores to remove germs.
- Granulation makes powders into larger granules that flow and compress well.
- Methods are wet granulation, dry granulation (slugging or roller compaction) and fluid bed granulation.
Filtration
Filtration separates solid particles from a liquid by passing the liquid through a porous filter medium. The clear liquid that passes through is the filtrate. The solid left behind is the residue.
Filter media
| Medium | Notes |
|---|---|
| Filter paper | Common for small amounts. Different grades have different pore sizes |
| Cloth or muslin | Coarse filtering, such as straining mixtures |
| Cotton wool and glass wool | Plugs in funnels for rough clearing |
| Sintered glass | Fine, reusable, resists chemicals |
| Membrane filters | Thin sheets with very small, exact pore sizes. Used for sterile filtration |
A filter aid, such as kieselguhr, is sometimes added to stop slimy particles blocking the filter.
Rate of filtration
Filtration is faster when:
- the filter area is larger;
- the liquid is thinner (less viscous), for example when it is warm;
- the pressure difference across the filter is greater, for example by using vacuum or pressure;
- the filter is more open, with larger pores;
- the layer of residue (cake) on the filter is thin.
Example. 500 mL is filtered in 25 minutes.
- Rate = 500 ÷ 25 = 20 mL per minute
Sterile filtration
Germs are very small. To remove them, a membrane filter with pores of about 0.22 micrometre is used, with aseptic technique. It is used for heat-sensitive solutions, such as some eye drops and injections, that cannot be sterilised by heat.
Clarification
Clarification makes a liquid clear by removing small amounts of fine suspended matter.
| Method | How it works |
|---|---|
| Settling and decanting | The solid sinks, and the clear liquid is poured off |
| Centrifugation | Spinning forces the solids to the bottom |
| Filtration | Passing through a filter |
| Using a fining agent | Something is added that carries the particles down, such as in clarifying juices |
Clarified liquids look clear but are not necessarily sterile.
Granulation
Granulation turns powders into larger granules. Granules flow better, do not separate into their separate ingredients, are less dusty, and compress well into tablets. See the lesson on powders and granules in BDT 152.
| Method | How it works |
|---|---|
| Wet granulation | The powders are mixed with a liquid binder to a damp mass, which is passed through a screen, dried and screened again |
| Dry granulation | The powder is compressed into large tablets (slugs) or sheets, then broken up and screened. No liquid is used. Suits drugs that are damaged by moisture or heat |
| Fluid bed granulation | Powder is kept suspended in a stream of warm air and sprayed with a binder, so it granulates and dries in one machine |
Yield
yield (%) = actual amount obtained ÷ theoretical amount × 100
Example. A batch should give 50 kg of granules. 47 kg is obtained.
- Yield = 47 ÷ 50 × 100 = 94%
Some loss is normal (dust, material on equipment). A low yield is investigated.
Exam focus
- Define filtration and name three filter media.
- State the factors that affect filtration rate.
- Describe wet and dry granulation.
- Common mistake: using filtration alone to make a solution sterile without a membrane filter and proper aseptic handling.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.Filtration separates:
2.Which of these makes filtration faster?
3.500 mL is filtered in 25 minutes. What is the average rate?
4.Which method of granulation uses a liquid binder?
5.A batch needs 50 kg of granules. Granulating gives 47 kg. What is the yield?