Unit Processes

Question 1

What are the 3 main mixing mechanisms?

Answer 1

Convective
Shear
Diffusive

Question 2

Describe convective mixing.

Answer 2

Large group of particles from one part of the powder bed to another. This is a macroscopic mix.
Uses planar and ribbon mixers.

Question 3

Describe shear mixing.

Answer 3

Occurs when a layer of material flows ver another layer so mix at layer interface. This is a semi-microscopic mix. Uses tumbling agitators

Question 4

Describe diffusive mixing

Answer 4

Movement of indivdual particles. Powder bed is forced to move or flow. Powder particles become less tightly packed and there is an incerase in air spaces or voids between them. Powder particles can pass through these spaces. This achieves a true random mix. This is a microscopic mix.

Question 5

What is percolation?

Answer 5

When smaller particles fall through voids between larger particles and move to the bottom of the mass.

Question 6

What is trajectory segregation?

Answer 6

Larger particles have greater kinetic energy and therefore move greater distances than smaller particles.

Question 7

What is dusting out?

Answer 7

When material is discharged from a container, small particles may be blown upwards by turbulent air currents. When mixer stopped, these small particles form a layer on top of coarser particles

Question 8

Describe granulation.

Answer 8

Primary powder particles are aggregated at high pressure. There are 2 main mechanisms: either the production of a large tablet (slug) in a heavy duty tabletting press (slugging) or the squeezing of powder between two rollers.

Question 9

Describe wet granulation.

Answer 9

Massing of dry primary powder particles using a granulating fluid. Typical liquids used are water, ethanol and 2-propanol.

Question 10

State the 5 bonding mechanisms in granulation.

Answer 10

1. adhesion and cohesion
2. interfacial forces in mobile liquid films
3. Solid bridges
4. Attractive forces between solid particles
5. Mechanical interlocking

Question 11

How are solid bridges formed?

Answer 11

Partial melting - melting of solid under pressure. When pressure relieved crystallisation will occur, binding particles together.
Hardening binders - use of binders in granulating solvent
Crystallisation of dissolved substances - when granules dried crystallisation of this material will occur due to the solvent used to mass the powder during wet granulation may partially dissolve one of the ingredients.

Question 12

Explain how granules are formed

Answer 12

Nucleation
Transition
Ball growth - Coalescence, Breakage, Abrasion transfer, Layering