Drying Flashcards
Relative humidity (RH)
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
Moisture content (MC)
• 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
Free moisture content (Free MC)
– Easily removable water
– Unbound water
Equilibrium moisture content (EMC)
– More difficult to remove
– Depends on environment / storage (temp, humidity and nature of solid material)
Methods of drying
• Convective drying – Static (fixed-bed) – Dynamic (fluidised-bed) • Conductive drying • Radiation drying • Spray drying • Freeze drying
Static convective drying
• 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
Constant-rate period
- 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
First falling-rate period
- 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
Second falling-rate period
- 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
Dynamic convective drying - Advantages
– 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
Dynamic convective drying - Disadvantages
– Influence of air velocity can lead to separation of wet particles
Conductive drying
• 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
Radiation drying
- 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
Spray Drying
• 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)
Uses of spray drying
– Almost any substance in
solution or suspension
• E.g.citric acid,gelatin,starch etc.
