Drying

Question 1

What is drying?

Answer 1

Removal of moisture

Question 2

What is the importance of drying?

Answer 2

Purification of drug

Granulation for tabletting

Question 3

What two factors does moisture content affect?

Answer 3

Product stability
Physical properties (flowability)

Question 4

How do you remove liquid component to leave behind a solid?

Answer 4

Convert liquid component to vapour (gas)*.

• Remove vapour from remaining solid.

Question 5

What is vaporisation?

Answer 5

Liquid to Gas

Question 6

What happens in evaporation?

Answer 6

ONLY LIQUID surface

BELOW boiling point

Question 7

What happens during boiling?

Answer 7

Throughout the body of liquid

Boiling point ONLY

Question 8

Which is more useful for pharmaceutical drying: evaporation or boiling?

Answer 8

Evaporation as boiling can cause thermal degradation

Question 9

What are the principles of effective drying?

Answer 9

Heat transfer

Mass transfer

Question 10

What is convection?

Answer 10

Mass transfer of hot fluid (gas/liquid). ~heating up water

Question 11

What is conduction?

Answer 11

Heat transfer through contact with

medium. ~holding handle of pot

Question 12

What is radiation?

Answer 12

Electromagnetic waves, no medium

Question 13

What is free moisture content?

Answer 13

Unbound water- readily removed by evaporation

Question 14

What is equilibrium moisture content?

Answer 14

Equilibrium moisture content
• ‘Bound water’—not removed by evaporation
• In equilibrium with ambient moisture content

Question 15

What does the equilibrium moisture depend on?

Answer 15

temperature, relative humidity, and nature of solid (e.g. porosity, hygroscopicity)

Question 16

What is a bone-dry solid?

Answer 16

Removing equilibrium mixture

Question 17

What is an air dry solid?

Answer 17

Removing free moisture content

Question 18

What is relative humidity (RH)?

Answer 18

Water content in air

Affects equilibrium mositure content in solids

Question 19

What is the equation for RH?

Answer 19

RH(%) = current/saturation

Question 20

What are the 6 drying techniques?

Answer 20

1) Tray drying oven
2) Vacuum oven
3) Microwave
4) Fluidised bed
5) Spray drying
6) Freeze drying

Question 21

What is oven drying?

Answer 21

Fan-assisted ventillation and vapour removal

Drying rate depends on layer thickness

Question 22

What is a vacuum oven?

Answer 22

Low pressure promotes evaporation at lower temp

low risk of oxidative degradation

Question 23

What is solute migration?

Answer 23

Solvent diffuses from centre of particle to particle surface during drying, bringing dissolved solute with it.

Reduces content uniformity

Question 24

What is a fluidised bed dryer?

Answer 24

Large drying surface area- rapid drying
Particle movement prevents aggregation
Uniform heating reduces solute migration
Particle attrition rounds particles- more fines

Question 25

What is a microwave dryer?

Answer 25

Magnetron generated
Unifrom heating reduces solute migration
High thermal efficiency:rapid drying with low thermal stress

Question 26

What are the three steps of spray drying?

Answer 26

Drying, atomisation and collection

Question 27

Explain each step of spray drying?

Answer 27

Drying- solvent evaporates rapidly leaving behind solid particles
Atomisation- Sample solution/suspension sprayed into hot air stream as fine droplets
Collection- Particles collected in a cyclon
LIQUID SAMPLES

Question 28

What are the three stages of the freeze-drying cycle?

Answer 28

Freezing
Primary drying
Secondary drying

Question 29

What does each stage mean?

Answer 29

1) Pressure lowered to nelow solid-gas phase

2) Warming under sustained low pressure

Question 30

What are the practical considerations?

Answer 30

Sample type: solid, solution, suspension
Liquid to be removed: flammability
Thermal stability
Extent of drying: final moisture content
Scale of operation
Solute migration 
Particle attrition: fines
Cost

Question 31

What materials can be used for vacuum?

Answer 31

Thermolabile