Drying

Question 1

Relative humidity (RH)

Answer 1

Consider:
- The amount of moisture in the air
- The amount of water saturated in the air - temp dependent
- Expressed as a %
(Vapour pressure of water vapour in air/ vapour pressure of water vapour in air saturated at same temp) x 100

Question 2

Moisture content (MC)

Answer 2

• Moisture content of a wet solid
– Expressed as kg moisture associated with 1kg dry solid
– Can be expressed as a %
• Total moisture content
– The amount of liquid associated with a wet solid
• If the moisture content is said to be 0.5, what
does this mean?
– After complete drying, 0.5 kg of water will be present per kg of the moisture-free solid

Question 3

Free moisture content (Free MC)

Answer 3

– Easily removable water

– Unbound water

Question 4

Equilibrium moisture content (EMC)

Answer 4

– More difficult to remove

– Depends on environment / storage (temp, humidity and nature of solid material)

Question 5

Methods of drying

Answer 5

• Convective drying
– Static (fixed-bed)
– Dynamic (fluidised-bed)
• Conductive drying 
• Radiation drying
• Spray drying
• Freeze drying

Question 6

Static convective drying

Answer 6

• Slow drying process
• In order to speed up process:
– Turbulent air flow over the surface of drying materials (the rate at which vapour removed)
– Maintaining the air flow at certain temperature (the rate at which heat transferred)
• Rate ofd rying:
– Constant-rate period – Falling-rate period

Question 7

Constant-rate period

Answer 7

• Linear relationship
• A-B
• Drying occurs on the surface
• Important to control the rate of heat transfer and the rate of removal of the vapour

Question 8

First falling-rate period

Answer 8

• The end of the constant rate period = critical moisture content (CMC)
• The moisture on the solid surface to be removed not enough to saturate the air in contact with the surface
• Drying rate decreases
• Capillary to pendular state
• Heat should be reduced
• Beware of solute migration

Question 9

Second falling-rate period

Answer 9

• Drying can no longer take place on the surface
• Drying the solid body of material
• Rate dependant on removal of water vapour through the pores of the bed

Question 10

Dynamic convective drying - Advantages

Answer 10

– Good contact between the wet particles and the air
– Drying from the surface of each particle, not from the bed (vs. static bed)
– Uniform temperature
– Efficient drying rates with high output
– Risk of solute migration prevented

Question 11

Dynamic convective drying - Disadvantages

Answer 11

– Influence of air velocity can lead to separation of wet particles

Question 12

Conductive drying

Answer 12

• Materials in thermal contact with a hot surface
• Drying achieved by conduction
• Can take place at low temperatures
• Vacuum oven 
– Static 
- Not commonly used for drying in production
• Vacuum tumbling drier
– Higher heat transfer than vacuum oven 
– More popular than vacuum oven

Question 13

Radiation drying

Answer 13

• No heat transfer medium
• Heat energy transferred via radiation

Infrared radiation
• Absorbed rapidly
• Shallow penetration
• Not common in pharmaceutics

Microwave radiation
• Better penetration
• Rapid drying at lower temps
• Uniform heat transfer
• Solute migration minimised 
• Static bed

Question 14

Spray Drying

Answer 14

• Provides large area for heat and mass transfer
• Atomises liquid to small droplets
• Droplets sprayed into stream of hot air
• Droplets dry into individual solid particle
• Particle formation and drying occur in one step
• Particle size controlled by droplet size controlled by atomizer
– Important atomiser does not experience build up of solid
• Air rotates in chamber tangentially increasing droplet residence time (drying time)

Question 15

Uses of spray drying

Answer 15

– Almost any substance in
solution or suspension
• E.g.citric acid,gelatin,starch etc.

Question 16

Advantages of spray drying:

Answer 16

• Millions of small droplets give large surface area so evaporation rapid
• Rapid evaporation means droplets do not attain high temperatures
• Uniform sized spherical particles
• Characteristic particle form allows efficient packing
• Increase dissolution rate and bioavailability of poorly water soluble drugs
• Low labour costs
• Can be used as continuous process

Question 17

Disadvantages of spray drying:

Answer 17

• Equipment bulky and expensive

* Thermal efficiency quite low

Question 18

Freeze drying

Answer 18

• Used for heat sensitive materials or materials which are excessively hygroscopic
• Drying without excessive damage
• Solvent undergoes solid-gas sublimation
• Process:
1. Liquid freezes
2. Pressure reduction
3. Increasing heat
4. Sublimation occurs

Question 19

Advantages of freeze drying:

Answer 19

• Low temperature drying – reduced product degradation
• Product is light and porous
• Porous product is readily soluble
• No concentration of solution prior to drying
• Oxidation minimized

Question 20

Disadvantages of freeze drying:

Answer 20

• Product extremely hygroscopic: needs careful packaging

* Slow complicated process

Question 21

Solute migration

Answer 21

• Movement of soluble drugs or excipients to the surface of the particles during drying
– As solvent being evaporated moves to surface the dissolved solutes move with them
– Results in localised concentrations of drugs/excipients at surface
– Tablets / granules aren’t uniform
– Quality control problems

Question 22

Mechanism of granule drying • Intergranular migration

Answer 22

– Between granules
– Solutes (drug/excipients) migrate granule to granule resulting in non uniform distribution
– Common in static convective drying

Question 23

Mechanism of granule drying • Intragranular migration

Answer 23

– Within individual granules
– Solute moves towards pheriphery of granule leading to uneven distribution
– Common in dynamic convective drying and vacuum tumbling drying

Question 24

Preventing solute migration

Answer 24

• Water insoluble dyes
• Increased viscosity of granulating fluid
• Minimize / optimise volume of granulating fluid
• Remixing of granules prior to tabletting