Drying Flashcards
What do you need to consider with regards to realtive humidity?
- The amount of moisture in the air
- The amount of water saturated in the air (which is temeprature dependent)
What is relative humidity expressed as?
It is expressed as the % of vapour pressure of water vapour in air/vapour pressure of water vapour in air saturated at same group multiplied by 100
What is the oisture content of a wet solid expressed as?
- Expressed as Kg moisture associated with 1Kg dry solid
- Can be expressed as %
What is 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?
-It means after complete drying, 0.5 Kg of water will be present per Kg of the moisture-free solid
What is free moisture content?
- Easily removable water
- Unbound water
Equilibrium moisture content is more difficult to remove and depends on environment/storage temperature, humidity and nature of solid material. TRUE OR FALSE?
TRUE
What do you need to consider before removing all water? (is it essential?)
- Moisture sensitive material
- Hygroscopic material
What do you need to consider before removing all water? (is it essential?)
- Moisture sensitive material
- Hygroscopic material
What are Hygroscopic material?
- They are materials that readily attracts moisture from its surrounding by either absorption or adsorption
What if he material are exposed to a humid condition?
- Regain moisture? check answer
What is free moisture lost through?
Evaporation
Name the methods of drying?
- Convective drying
- Conductive drying
- Radiation drying
- Spray drying
- Freeze drying
Name the two types of Convective drying?
- Static (fixed bed)
- Dynamic (fluidised bed)
Describe static convective drying?
-Slow drying process
What is doen i order to speed up the static convective drying process?
- Turbulent air flow over the surface of drying materials (which is the rate at which vapour is removed)
- Maintaining the air flow at certain temperature (the rate at which heat transferred)
What is the rate of drying of static convective drying dependent on?
- Constant rate period
- Falling rate period
Describe constant rate period?
- Linear relationship
- Drying occurs on the surface
It is important to control the rate of heat transfer and the rate of removal of the vapour in constant rate period. TRUE OR FALSE?
TRUE
The end of the constant rate period=critical moisture content. Is this TRUE for the first falling rate period?
yes
What occurs in first falling rate period?
- The moisture on the solid surafe to be removed not enough to saturate the air in contact with the surface
- Drying rate devreases
- Capillary to pendular state
- Heat should be reduced
- Beware of solute migration
What occurs in the 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 te pores of the bed
What are the advantages of dynamic convective drying?
- Good contact between the wet particles and the air
- Drying from the surface of each particle, not from the bed
- Uniform tempearture
- Efficient drying rates with high output
- Risk of solute migration prevented
What are the advantages of dynamic convective drying?
- Good contact between the wet particles and the air
- Drying from the surface of each particle, not from the bed
- Uniform tempearture
- Efficient drying rates with high output
- Risk of solute migration prevented
What are the disadvantages of dynamic convective drying?
-Influence of air velocity can lead to seperation of wet particles
Describe conductive drying?
- Materials in thermal contact with a hot surface
- Drying achieved by conduction
- Can take place at low temperatures
Describe the Vacuum oven in (Conductive drying)
- Static
- Not commonly used for drying in production
Describe the Vacuum oven in (Conductive drying)
- Static
- Not commonly used for drying in production
Describe the vacuum tumbling drier in (Conductive drying)
- Higher heat transfer than vacuum oven
- More popular than vacuum oven
Describe Radiation drying?
- No heat transfer medium
- Heat energy transferred via radiation
Name the two types of radiation drying?
- Infrared radiation
- Microwave radiation
Describe infrared radiation?
- Absorbed rapidly
- Shallow penetration
- Not common in pharmaceutics
Describe Microwave radiation?
- Better penetration
- Rapidly drying at lower temperatures
- Uniform heat transfer
- Solute migration minimised
- Static bed
Spray drying provides large area for heat and mass transfer. TRUE OR FALSE?
TRUE
Spray drying atomises liquid into stream of hot air. TRUE OR FALSE?
TRUE
Describe spray drying?
- Droplets sprayed into stream of hot air
- Droplets dry into individual solid particle
- Particle formation and drying occur in one step
In spray drying particle sixe is controlled by droplet size which is controlled by atomizer. TRUE OR FALSE?
TRUE
Atomiser experiences build up of solid. TRUE OR FALSE?
FALSE
In spray drying, AIr rotates in chamber tangentially increasing droplet residence time (drying time). TRUE OR FALSE?
TRUE
What is spray drying used for?
Almost any substance in solution or suspension e.g citric acid, strach
What are the advantages of spray drying?
- Millions of small droplets give large surface area so evaporation is rapid
- Rapid evaporation means droplets do not attain high temperatures
- Uniform sized spherical particles
- Characteristic particle form allows efficient packaging
- Increased dissolution rate and bioavialbility of poorly water soluble drugs
- Low labour costs
- Can be used as a continous process
What are the disdavantages of spray drying?
- Equipment bulky and expensive
- Thermal efficiency quite low
What is Freeze drying used for?
-Heat sensitive materials or materials which are excessively hygroscopic
Describe Freeze drying?
- Drying without excessive damage
- Solvent undergoes solid-gas sublimation
What are the 4 processes of sublimation?
- Liquid freezes
- Pressure reduction
- Increasing heat
- Sublimation occurs
What are the advantages of freeze drying?
- Low temperature drying reduces product degradation
- Product is light and porous
- Porous product is readily soluble
- No concentration of solution prior drying
- Oxidation minimized
What are the disdavantages of freeze drying?
- Product extremely hygroscopic - need careful packaging
- Slow compliacted process
What is solute migration?
The movement of soluble drugs or excipients to the surface of the particles during drying
How does solute migration occur?
- As solvent being evaporated moves to the surface the dissolved solutes move with them
- Results in localised concentrations of drugs/excipients at surface
What are the disadvantages of solute migration?
- Tablets/ Granules aren’t uniform
- Quality control problems
What are the two mechanisms of granule drying?
- Intergranular migration
- Intragranular migration
Describe intergranular migration
- Between granules
- Solutes (drug/excipients) migrate granule to granule
- Common in static convective drying
Describe intragranular migration
- within individual granules
- Solute moves towards pheriphery of granule leading to uneven distribution
- Common in dynamic convective drying and vacuum tumbling drying
How can migration be prevented?
- Water insoluble dyes
- Increased viscosity of granulating fluid
- Mninimize/optimise volume of granulating fluid
- Remixing of granules prior to tableting
