Drying Flashcards
Define moisture content.
Amount of water contained in a material
Define moisture by volume.
The number of water molecules per unit by the total number of molecules
Define moisture in air.
The amount of water (kg) per kg of dry air
Define humidity.
Water vapour in air
Define % relative humidity.
Water vapour in air by total moisture that air can hold
What happens to maximum water content when temperature increases?
Also increases
Define dew point.
Temperature at which moisture in air both condenses and evaporates at the same rate
Why may moisture content change during drying?
Evapouration of water
Without air exchange, air may become saturated
Define a hygroscopic material.
Attract and retain water from their surrounding environment
What is the total moisture of a compound?
Amount of moisture associated with a wet solid
What is the free moisture of a compound?
Water easily removed from a wet solid (unbound)
What is the equilibrium moisture of a compound?
Proportion of bound water
What is solute migration?
Movement of solution in a wet system during drying
What is intergranular migration?
Movement of solvent between granules towards the surface
What is intragranular migration?
Separation of granules due to solvent movement
What factors should be considered when selecting a drying method?
Heat sensitivity Physical characteristics Solvent/liquid for removal Efficient mass transfer of evaporated liquid Efficient vapour removal from air Sterility
What are the three categories of drying for wet solids?
Convection
Conduction
Radiation
What are the features of a fluidised bed dryer?
Optimised contact between warm drying air and wet powder
Increased air velocity causes pressure drop with powder suspended in air
Powder free to move with further increase in velocity
Capacity 0.4-1.2kg
Drying time 20-40 minutes
What are the benefits of turbulence in drying?
Particles mix with air maximising surface area of powder bed allowing efficient heat transfer
What are the disadvantages of turbulence in drying?
May damage granules
Small particles may be stuck in fluidising air
Movement may induce charges of static
