Drying Flashcards
What is drying?
Removal of moisture
What is the importance of drying?
Purification of drug
Granulation for tabletting
What two factors does moisture content affect?
Product stability Physical properties (flowability)
How do you remove liquid component to leave behind a solid?
Convert liquid component to vapour (gas)*.
• Remove vapour from remaining solid.
What is vaporisation?
Liquid to Gas
What happens in evaporation?
ONLY LIQUID surface
BELOW boiling point
What happens during boiling?
Throughout the body of liquid
Boiling point ONLY
Which is more useful for pharmaceutical drying: evaporation or boiling?
Evaporation as boiling can cause thermal degradation
What are the principles of effective drying?
Heat transfer
Mass transfer
What is convection?
Mass transfer of hot fluid (gas/liquid). ~heating up water
What is conduction?
Heat transfer through contact with
medium. ~holding handle of pot
What is radiation?
Electromagnetic waves, no medium
What is free moisture content?
Unbound water- readily removed by evaporation
What is equilibrium moisture content?
Equilibrium moisture content
• ‘Bound water’—not removed by evaporation
• In equilibrium with ambient moisture content
What does the equilibrium moisture depend on?
temperature, relative humidity, and nature of solid (e.g. porosity, hygroscopicity)
What is a bone-dry solid?
Removing equilibrium mixture
What is an air dry solid?
Removing free moisture content
What is relative humidity (RH)?
Water content in air
Affects equilibrium mositure content in solids
What is the equation for RH?
RH(%) = current/saturation
What are the 6 drying techniques?
1) Tray drying oven
2) Vacuum oven
3) Microwave
4) Fluidised bed
5) Spray drying
6) Freeze drying
What is oven drying?
Fan-assisted ventillation and vapour removal
Drying rate depends on layer thickness
What is a vacuum oven?
Low pressure promotes evaporation at lower temp
low risk of oxidative degradation
What is solute migration?
Solvent diffuses from centre of particle to particle surface during drying, bringing dissolved solute with it.
Reduces content uniformity
What is a fluidised bed dryer?
Large drying surface area- rapid drying
Particle movement prevents aggregation
Uniform heating reduces solute migration
Particle attrition rounds particles- more fines
What is a microwave dryer?
Magnetron generated
Unifrom heating reduces solute migration
High thermal efficiency:rapid drying with low thermal stress
What are the three steps of spray drying?
Drying, atomisation and collection
Explain each step of spray drying?
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
What are the three stages of the freeze-drying cycle?
Freezing
Primary drying
Secondary drying
What does each stage mean?
1) Pressure lowered to nelow solid-gas phase
2) Warming under sustained low pressure
What are the practical considerations?
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
What materials can be used for vacuum?
Thermolabile
