drying Flashcards
what is drying?
removal of moisture, in particular water by: evapouration/ vaporisation / sublimation
what are the aims of drying?
To improve product stability To improve resuspensionor dissolution To ensure processability powder flow reduced stickiness Tabletability
what does the equilibrium pressure- temperature diagram for water show?
a graph of pressure vs temp and the requirements for freezing/ evapouration/ sublimination
in relative humidity how do air and water behave?
Air and water vapourbehave like ideal gases
Air and water are completely miscible
what is the relationship like between the total pressure of air-water vapor mixture and partial pressures of air and vapor?
p=pa+pv
why is there a restriction for relative humidity at a given temp?
at given temperature Pv≤ Ps(Ps: saturation vapour pressure of water)
what is the equation for relative humidity?
=pv/px X 100%
what is the dew point?
it is the cooling of air until pv exceeds ps for a given temp
liquid water will be deposited
what are the requirements of the psychrometer to measure relative humidity?
- wick must be kept moist
- min air flow velocity of 3m/s
how do you measure the relative humidity using the psychrometer?
measure the difference between the two thermometers
what does an increase in the difference in the psychrometer indicate?
The difference increases with decrease in relative humidity
what drying method would you use for a wet particle?
convective drying
what would be an example of convective drying?
tray drier
what would you use for convective drying of wet solids?
dynamic fluid bed dryer
which is better a tray drier or a fluid bed dryer?
fluid bed drier-5% moisture content vs 10% for tray drying
what occurs in convective drying of wet solids heat transfered during fluid bed drying?
gas to particle- convective particle
chamber wall to fluid bed- conductive aspect
particle to particle- smaller to coarser particles
what are some of the advantages of fluid bed drying?
Eeffective heat and mass transfer
Temperature exposure of temperature labilematerial to heat is minimal
Drying of individual particles
Precise control of bed temperature
what are some of the disadvantages of fluid bed drying?
Attritiondue to turbulent air flow
Fine particle elutriation
Electrostatic charging
restriction- insufficient fluidisation of particles below 100 or above 1000um
what is an example of conductive drying for wet solids?
vaccum oven
what are some of the advantages of vaccum oven?
Drying at low temperatures
Minimum risk of oxidation
what are some of the advantages of microwave drying?
Rapid drying at low temperatures
High thermal efficiency
Uniform heating of the wet mass
Granulation end point: microwave energy rises when there is little solvent in the wet mass
what are some of the disadvantages of microwave drying?
Batch size is smaller compared with that of fluid-bed driers
Care: microwave radiation can harm operators
what are the applications of drying using fluid bed dryer and vaccuum oven ?
analyse drying conditions/ dissolution time/ disintegration time na dtensile strength
what affects does drying of pelets have?
improve drug dissolution