Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
41 Cards in this Set
- Front
- Back
|
What is the equation for relative humidity? |
|
|
|
How do you express the moisture content of a wet solid? |
As kg moisture associated with 1kg dry solid (Can be expressed as %) |
|
|
What is the total moisture content? |
The amount of liquid associated with a wet solid |
|
|
What is free moisture content? |
-Easily removable water -Unbound water |
|
|
What is the equilibrium moisture content? |
-More difficult to remove -Depends on environment/storage |
|
|
Loss of water from a drying sold |
|
|
|
What are the five methods of drying? |
-Convective drying -Conductive drying -Radiation drying -Spray drying -Freeze drying |
|
|
What are the two different types of Convective drying? |
-Static (fixed-bed) -Dynamic fluidised-bed) |
|
|
What is Static Convective Drying? |
-Slow drying process -Maintains flow at cetain temperature |
|
|
How can you speed up static convective drying? |
-Turbulent air flow over the surface of drying materials -Maintaining the air flow at certain temperature |
|
|
What are the two rate periods of static convective drying? |
-Constant-rate period -Falloing-rate period |
|
|
Drying Curve |
Where EMC = Equilibrium moisture content |
|
|
What are the features of the constant rate period? |
-Linear relationship -A-B -Drying occurs on the surface |
|
|
Why is the constant-rate period important? |
To control the rate of heat transfer and the rate of removal of the vapour |
|
|
What are the features of the first falling-rate period? |
-End of the constant rate period, CMC (critical moisture content) -Moisture on solid surface to be removed not enough to saturate the air in contact with the surface -Drying rate decreases -Capillary to pendular state -Heat should be reduced -Solute migration may occur |
|
|
What are the features of 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 the pores of the bed |
|
|
What are the advantages of Dynamic Convective Drying? |
– Good contact between the wet particlesand the air– Drying from the surface of each particle,not from the bed (vs. static bed)– Uniform temperature– 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 toseparation of wet particles |
|
|
What is Conductive Drying? |
-Materials in thermal contact with a hot surface; dried by conduction -Can take place at low temperatures |
|
|
What are the two methods used for Conductive Drying? |
-Vacuum oven -Vacuum tumbling drier |
|
|
What are the features of a Vacuum oven? |
-Static -Not commonly used for drying in production |
|
|
What are the features of a Vacuum Tumbling Drier? |
-Higher heat transfer than vacuum oven -More popular than vacuum oven |
|
|
What are the features of radiation drying? |
-No heat transfer medium -Heat energy transferred via radiation |
|
|
What are the two different types of radiation drying? |
-Infrared radiation -Microwave radiation |
|
|
What are the features of infrared radiation? |
-Absorbed rapidly -Shallow penetration -Not common in pharmaceutics |
|
|
What are the features of microwave radiation? |
-Better penetration -Rapid drying at lower temperatures -Uniform heta transfer -Solute migration minimised -Static bed |
|
|
What are the features for spray drying? |
-Provides large area for heat and mass transfer -Atomises liquid to small droplets -Droplets sprayed into stream of hot air -Droplets dry into individual solid particle -Particle formation and drying occur in one step |
|
|
Spray drying |
|
|
|
Formation of a spray dried product |
|
|
|
What are the advantages of spray drying? |
• Millions of small droplets give large surface area so evaporation rapid• Rapid evaporation means droplets do not attain high temperatures• Uniform sized spherical particles• Characteristic particle form allows efficient packing• Increase dissolution rate and bioavailability of poorly water soluble drugs• Low labour costs• Can be used as continuous process |
|
|
What are the disadvantages of spray drying? |
• Equipment bulky and expensive• Thermal efficiency quite low |
|
|
What are the features of freeze drying? |
-Used for heat sensitive or extremely hygroscopic materials -Drying without excessive damage -Solvent undergoes solid-gas sublimation |
|
|
What are the four steps of freeze drying? |
1) Liquid freezes 2) Pressure reduction 3) Increasing heat 4) Sublimation occurs |
|
|
Phase diagram for water and freeze drying |
|
|
|
What are the advantages of freeze drying? |
• Low temperature drying – reduced product degradation• Product is light and porous• Porous product is readily soluble• No concentration of solution prior to drying• Oxidation minimized |
|
|
What are the disadvantages of freeze drying? |
-Product extremely hygroscopic: needs careful packaging-Slow complicated process |
|
|
What is solute migration? |
-Movement of soluble drugs or excipients to the surface of the particles during drying |
|
|
What happens during solute migration, and what does this result in? |
-Evaporated solvent move to the surface, bringing dissolved solutes with it -Results in localised concentrations of drugs/excipients at surface -Non-uniform tablets/granules |
|
|
What is intergranular migration? |
-Migration between granules -Solutes migrate granule to granule, non uniform distribution -Common in static convective dyring |
|
|
What is intragranular migration? |
-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 you prevent migration? |
-Water insoluble dyes -Increased viscosity of granulating fluid -Minimise/optimise volume of granulating fluid -Remixing of granules prior to tabletting |
