Drying Flashcards
What is drying?
Mass transfer of volatile component out of a solid-liquid mixture
What is drying important for?
Purification of drug + granulation for tabletting
What does moisture content effect?
Product stability
Physical properties
What can H2O do?
Cause microbial growth
Affect degradation = increase hydrolysis
Clump together = decrease flowability
What is a substance called that is normally a gas?
Gas
What is a substance called that is normally a liquid?
Vapour
What is it called when goes liquid to gas?
Vaporisation
Evaporation
Liquid surface only
= below boiling point
Boiling
Throughout body of liquid
= at boiling point only
Solid to gas
Sublimation
Gas to liquid
Condensation
Liquid to solid
Freezing
Gas to solid
Deposition
Solid to liquid
Melting
Liquid to gas
Vaporisation
What will it have if it has a large SA?
Effective heat transfer
Effective mass transfer
What is the equation for heat transfer?
Q = hAΔT
Q = heat transfer
h = heat transfer coefficient
A = SA
ΔT = Temp difference (k)
What is relative humidity (RH)?
H2O content in air
Affects eqm moisture content in solids
What RH equation?
RH (%) = Pcurrent
————-
Psaturation
p = partial pressure
What does it mean higher relative humidity?
Closer to saturation point
= harder liquid is to evaporate
= lower drying rate
What does it mean H2O evaporating off?
Increases surrounding humidity
= if vapour NOT cleared quickly
= drying decreases
What does it mean if sample is in high humidity environment?
Sample gains moisture
What is FMC?
Free moisture content
= unbound H2O
= readily removed by evaporation
What is EMC?
Eqm moisture content (EMC)
= bound H2O
= NOT removed by evaporation
How is bound H2O removed + problem?
Forcibly
= as soon as drying stops
= sample regains moisture
WET —-> AIR-DRY —-> BONE-DRY
WET —-> AIR-DRY = remove FMC
AIR-DRY —-> BONE-DRY = remove EMC BUT has to be stored in protective environment
What are the modes of heat transfer?
Convection
Conduction
Radiation
What is convection?
Mass transfer of hot fluid
What is conduction?
Heat transfer through contact with medium
What is radiation?
Electromagnetic waves, no medium
Describe static tray oven
Fan-assisted ventilation + vapour removal
Conduction
What does drying rate depend on in static tray oven?
Thickness
What is static tray oven susceptible to?
Solute migration
Describe vacuum oven
Low pressure = evaporation at lower temp
Convection
What does vacuum oven reduce risk of?
Oxidative degradation
Describe oven drying
Heat absorbs moisture + fan removes it
Describe solute migration
Solvent diffuses from centre of particle-to-particle surface
= brings dissolved solute with it
Solvent evaporates leaving behind solute concentrated at surface
What does solute migration do?
Reduce content uniformity
Describe microwave dryer
Uniform heating = reduces solute migration
High thermal efficiency
What is pro of high thermal efficiency?
Rapid drying with loss of thermal stress
Describe fluidised-bed dryer
Large drying SA = rapid drying
Particle movement prevents aggregation
Uniform heating
Particle attrition rounds particles = better flow
What is fluidised-bed dryer only suitable for?
Granules
Describe spray drying
Liquid samples
Suspension sprayed into hot air stream
Solvent evaporates, leaving behind solid
Particles collected
Describe freeze-drying cycle
Cytoprotectants used
Primary drying
Secondary drying
Describe primary drying
Freeze-drying cycle
Pressure lowered below solid-gas phase boundary
= sublimation
= bound moisture removed
Describe secondary drying
Freeze-drying cycle
Warming under sustained low pressure
= desorption of bound moisture
What are some particle considerations?
Sample type = solid/solution/suspension
Liquid to be removed = volatile/flammable
Extent of drying
Solute migration
Cost
