Drying Flashcards
Define drying
Removal of moisture by; evaporation, vaporisation and sublimation
List the aims of drying powders and granules
- Improve product stability
- Improve resuspension/ dissolution
- Ensure process-ability - powder flow, reduced stickiness, tablet-ability
Describe what is meant by the dew point and what is the result?
Cooling air until partial pressure of vapour (Pv) exceeds saturation vapour pressure of water (Ps) for given degree - liquid water will be deposited.
Give one methods of measuring relative humidity and the result.
Psychrometer - the difference increases with decrease in relative humidity
List 2 types of convective drying methods of wet solids
- Dynamic convective drying e.g. fluid bed drying
- Static convective drying e.g. tray drier
What restricts convective drying of wet solids?
Insufficient fluidisation of particles below 100 or above 1000 micro moles
What are the advantages of convective drying of wet solids?
effective heat and mass transfer
drying of individual particles
precise control of bed temp
What are the disadvantages of convecting drying of wet solids?
Attrition due to turbulent air flow
Fine particle elutriation
Electrostatic charging
List two types of conductive drying methods of wet solids
Vacuum oven
Microwave drying
List 2 advantages of vacuum oven
- Drying at low temp
- Minimum risk of oxidation
What are the advantages of microwave drying?
uniform heating of wet mass
rapid drying at low temp
What are the disadvantages of microwave drying
Batch size is smaller compared to fluid bed drier
Microwave radiation can harm operators
Define lyophilisation
Freeze drying - removal of moisture by sublimation
Describe sublimation
Products are frozen subject to vacuum, allowing the ice to change from solid to vapour without passing through a liquid state
What is the aim of lyophilisation?
To dry heat sensitive materials without damage e.g. proteins
What is the minimum freezing temp for biological and pharmaceutical materials?
Biological -40 to -50
Pharmaceutical -10 to -30
What are the 2 main stages of the freeze-drying process?
Freezing stage - eutectic and glass formation
Vacuum stage - pressure below triple point and vapour removal
Summarise the freeze-drying cycle
- During freezing, water crystallises as ice leading to conc of solutes increases and changes solution from viscous liquid to gas
- Glass transition degree (Tg) represents optimum degree for 1 degree drying
- Secondary drying removes non frozen water to elevated temp
- The self life of product depends on the residual moisture content
List the critical temps for freeze drying
Tg
Tcol - collapse
Tcry - crystallisation
Teut - eutectic
List the 3 parameters for freeze drying
Cooling rate
Drying rate
Residual moisture content
State the importance of additives in freeze dried products
Stabilise the formulation
What are the advantages of freeze dried products?
Drying at very low temp
Enhancement of product stability
Preparation os stable pharmaceutical products
What are the disadvantages of freeze dried products?
Freeze dried products are very hygroscopic
Very low process
Difficult to control particle size of solids
