Fluid Flow, Material Flow, Size Reduction and Size Separation
9 minute read
What you will learn
- Explain what a unit operation is
- Describe how fluids and solids are moved in a factory
- State the reasons for size reduction
- Describe the main size-reduction methods and the mills that use them
- Explain size separation and sieving
- Calculate the percentage of a powder passing through a sieve
Key points
- Industrial processing breaks manufacture into basic steps called unit operations, such as milling, mixing, filtering and drying.
- Fluid flow concerns moving liquids and gases through pipes with pumps. Thick liquids need more power.
- Material flow concerns moving powders and granules by gravity, conveyors, hoppers and chutes.
- Size reduction makes particles smaller to help dissolving, mixing, flow and absorption.
- Methods of size reduction are cutting, compression, impact and attrition.
- Size separation (sieving) sorts particles by size, using sieves of a stated mesh or aperture size.
- Smaller particles have a larger surface area for their weight, so they dissolve faster.
Industrial processing
A factory makes medicines in large batches. To control quality, manufacture is divided into basic steps called unit operations. Examples are milling, mixing, sieving, extraction, distillation, granulation, drying, evaporation, filtration and clarification. Each is studied on its own, and the same operation is used for many products.
The small-scale methods you met in the dispensing practicals are the same operations, done in a mortar, beaker or sieve. In industry the equipment is bigger, and it is controlled and recorded.
Fluid flow
Fluid flow is the movement of liquids and gases through pipes, pumps and valves.
- Pumps move liquids from tanks to mixers, fillers and storage.
- Viscosity is how thick a liquid is. Thick liquids such as syrups and creams flow slowly and need stronger pumps and wider pipes.
- Pipes and pumps must be of suitable material, such as stainless steel, and easy to clean, so that one product does not contaminate the next.
- Air bubbles and leaks must be avoided.
Material flow
Material flow is the movement of solids, such as powders, granules and tablets, and of containers and packs.
- Gravity feeds material from a hopper down to the next machine.
- Conveyors, chutes and scoops carry material between machines and rooms.
- Powders can cake, form bridges in hoppers, or separate by size and weight, so the flow must be controlled.
- Good layout keeps raw materials, in-process material and finished goods apart and prevents mix-ups.
Size reduction
Size reduction (milling, comminution) makes particles smaller.
Reasons
- Small particles dissolve faster, because they have more surface area for their weight.
- They are absorbed faster from the gut.
- They mix more evenly, and flow and pack better.
- They give smoother creams, ointments and suspensions.
- They help extraction of drugs from plant material.
Methods
| Method | How it works | Suits |
|---|---|---|
| Cutting | Blades cut the material | Fibrous plant material |
| Compression | Material is crushed between surfaces | Hard, brittle solids |
| Impact | Material is struck by hammers or balls | Many brittle materials |
| Attrition | Particles are rubbed against each other or a surface | Fine grinding |
Mills and equipment
| Equipment | Main principle |
|---|---|
| Mortar and pestle | Impact and attrition. Small scale |
| Cutter mill | Cutting |
| Roller mill | Compression |
| Hammer mill | Impact |
| Ball mill | Impact and attrition |
| Fluid energy mill | Particles collide in a jet of air, giving very fine powders (micronisation) |
Cautions: some drugs are damaged by heat produced in milling, and dust can be a health and explosion risk. Operators wear masks, and the equipment has dust control.
Size separation
Size separation sorts particles into groups by size. The usual method is sieving.
- A sieve is a mesh. Particles smaller than the openings pass through, and larger ones stay on top.
- Sieves are described by the size of the openings or by the mesh number, which is the number of openings per unit length. A higher mesh number means smaller openings, and a finer sieve.
- Sieves are stacked, with the coarsest on top, and shaken to separate a powder into several size groups.
- Sieving also removes lumps and foreign material and mixes powders.
Worked example
100 g of a powder is passed through a sieve. 82 g pass through and 18 g stay on the sieve.
- Percentage passed = 82 ÷ 100 × 100 = 82%
- Percentage retained = 100 − 82 = 18%
The retained powder may be milled again and re-sieved.
Powder grades
Pharmacopoeias describe powders by the sieves they pass, for example coarse, moderately coarse, fine and very fine. The more particles that pass a fine sieve, the finer the powder. Use the grades in your textbook.
Exam focus
- Define unit operation and give four examples.
- State why particle size is reduced, and name two mills with the principle each uses.
- Describe how a sieve separates particles.
- Common mistake: confusing a high mesh number (fine sieve) with a coarse sieve.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.A unit operation is:
2.Which is a reason for reducing particle size?
3.A hammer mill reduces size mainly by:
4.100 g of a powder is passed through a sieve. 82 g passes through. What percentage passed?
5.Which is moved mainly by fluid flow in a factory?