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
what is freeze drying also known as?
lyophilisation
what is freeze drying?
removal of moistire in particular water by sublimination ( ice- gas without passing through the liquid state)
what are the aims of freeze drying?
to dry heat sensitive materials without damage e.g.- proteins or blood products
what pressure is used in freeze drying?
Solid ice maintained below triple point pressure (610 Pa)
what occurs in freeze drying?
sublimination
what are the min freezing temp’s in sublimination?
pharmaceutical material :-40–50
biological material -10 to -30
what are the 4 stages of the freeze drying process?
freezing stage-
vacuum stage
primary drying
secondary drying
what happens in the freezing stage?
Eutectic formation- not as good freezing
Glass formation- good freezing
what is the differnce in primary and secondary drying in the freeze drying process?
1= sublimination of ice and vapour removal 2= for bound water desorption
what is the residual moisture content for primary freeze drying?
about 6%
Hence, secondary drying is required to reduce the residual moisture content to optimum value (<1%
what size particles can lyophilisation analyse?what else does it analyse?
particles down to nano meters
particle size, zeta potential, and dynamic viscosity
what is the cycle summary of freeze drying?
During freezing, water crystallises as ice, therefore the concentration of solutes increases and changes the solution from a viscous liquid to a glass
The temperature for this transition is called glass transition temperature (Tg)of the freeze concentrate
(Tg) represents the optimal temperaturefor primary drying Secondary drying removes the non-frozenor bound water at elevated temperature
The shelf life of the product depends on the residual moisture content
what happens if you go above the critical parameters?
T`g
Tcol (collapse temperature)
Tcry (crystallisation temperature)
Teut (eutectic temperature)
what kind of stabilising additives do you add to freeze dried products?
Sugars: sucrose, trehalose, and lactose
Polyols: glycerol Amino acids: glycine and alanine
Salts: organic or inorganic
Polymers: dextrane, poly vinyl alcoholand carboxymethyl cellulose
Surfactants: Tween 80
what are some advantages of freeze drying products?
Drying at very low temperatures
Enhancement of product solubility
Preparation of stable pharmaceutical products
No contact with air
what are some disadvantages of freeze dry products?
Freeze dried products are very hygroscopic
Very slow process
Difficult to control the particle size of the solid
what are some of the pharmaceutical applications of freeze drying?
Used for drying of heat sensitive products for example: antibiotics, blood products and vaccines
Development of solid protein pharmaceuticals (for long term storage)
Instantaneously dissolving tablets Lyophilised nasal inserts
Drying of micro-and nano-particles
what would not be ideal with a freeze dry product?
for it to collapse
