Pharmaceutical drying Flashcards
importance of drying
primary manufacturing
-water is capable of degrading many types of drug
-hydrolytic degredation is to be discouraged
-dry product is free flowing
secondary manufacturing
-wet granulation is commonly used to prepare formulations prior to tablet compaction
-stability, flow properties and compactability are all influenced by residual water content
Total moisture content
-total water content of a wet solid
-not all of this water can be easily removed
-easily removable water is sometimes reffered to as free moisture content or unbound water
equilibrium moisture content
-all solids will equilibrate with moisture present in air
-this water is called the eqilibrium moisture content
the absolute amount is dynamic as it changed with chamges in temperature and humidity
-each type of solid has its own inherent hygroscopicity.
karl-fischer potentiometric titration
measures the amount of water by the electrical conductivity of a redox titration - most accurate
dynamic vapour sorption
measures the weight change as a function of both temperature and humidity
thermogravimetric analysis
measures the weight change as a funciton of temperature
relative humidity (RH)
increased temperature will increase solubility
maximum solubility at a particular temperature
precipitation of the solute on cooling
-air at a given temperature will take up water vapour
-when no more vapour can be taken up the humidity is considered to be saturated
Relative humidity formula
RH % = (vapour pressure of water vapour in air/vapour pressure of water vapour in air at saturated temperature) X100
considerations prior to drying
-heat sensitivity of the material to be dried
-physical characteristics of the material
-requirment of aseptic conditions
-nature of the liquid to be removed
-scale of the operations
available sources of heat eg.electrial or steam
3 classification of drying methods
conduction, convecton, radiation
states of matter
latent heat is associated with all these phase changs
solid to liquid - melting
liquid to solid - freezing
solid to gas - sublimation
gas to liquid - cindensation
liquid to gas - evaporation
endothermic processes (+Q)
melting
sublimation
evaporation
exothermic procceses (-Q)
condensation
freezing
evaporation
change from liquid to vapour
-the pressure of a vapour in equlibirium is called the vapour pressure
-when the vapour pressure is equal to the external pressure, the liquid boils (boiling point)
conduction
vibration of atoms/molecules with no appreciable movement of molecules
convection
macroscopic movement of molecules and their associated heat energy
radiation
absorption of electromagnetic rays resultiing in increase in temperature
practical heat transfer
-conductioin is the most important heat transfer process in industrial pharmacy
-convection in industrial processes is normally of a forced nature rather than being natural
sensible heat
-sensible heat is an appreciable rise in temperature
Q = mc(change in temp
Q - heat energy
c = specific heat capacity
Latent heat
-heat evolved or absorpbed by unit mass when it changes phases without a change in temperature
Latent heat formula
Q=mL
Q-heat energy
m-mass
L-heat absorbed/liberated in the change of phase of unit mass
fluidised-bed drier
equipment used for pharmaceutical drying
effect of air velocity on pressure drop
Describe all points on this graph
A - air velocity low / no disturbance on particles
B - frictional drag on particles is equal to force of gravity
C- rearrangement to offer least resistence / suspension
D - particle porosity produces small pressure drop
Advantages of fluidised bed granulator
-Efficient heat and mass transfer
-drying occurs from the surface of individual particles
-temperature of fbd is uniform throughout
-some attrition of particles cause particle spherity
-short drying times
disadvantages of fluidised bed drier
excessive attrition due to turbulent state
production of fines - must be filtered
generation of static electricity - explosion risk
Spray drier
-solution is atomised by high pressure gradient through a nozzle
-the small spherical droplets are sprayed into a steam of hot air
-evaporation of the solvent is extremely fast due to the very high surface area of the droplets
- most of the heat used as latent heat and as a degree of evaporative cooling takes place - this keeps the particles from overheating
-each droplet dries an individual solid particle
advantages of spray drying
-millions of small droplets give a massive surface area heat/mass transfer and hence fast evaporation of solvent
-the characteristic particle shape gives the product a high bul density - good flowability, fast dissolution
-uniform and controllable particle size
-labour costs are low - single operation
disadvantages of spray drying
equipment is bulky
heat trasnfer from air is relatively inefficient
applications of spray drying
-used for drying solutions and suspensions
-reasonably useful for thermolabile materials
-high outputs are possible
-capable of producing spherical particles suitable for dry powder inhalation products
-can be operated aseptically
-examples of materials that are spray dried: starch, lactose, calcium, citric acid
