Week 5 - Drying Flashcards
What is relative humidity
The amount of water vapour in the air expressed as a % of the amount needed for saturation at the same temp.
- Air can take up a specific amount of water at a specific temp. = saturation vapour pressure
- if try to get more water in it will precipitate out = cloud / fog form - Can measure amount of water in air
- Warmer the air the more water it can take up
Boiling Point - is point when water vapour pressure is same as surrounding environmental pressure
How to measure Relative Humidity (RH)
- Dew Point Hygrometer
- most commonly used method + ONLY accurate method
1. shine light onto mirror
2. light is reflected off the mirror and is detached by a detector
3. light causes mirror to heat up
4. as mirror becomes cooler than the surrounding + air becomes saturated = can’t hold no more water = condensation occurs
5. condensation disrupts the light path = identify RH - Hair Tension Hygrometer
- based on how much human hair extends when wet
1. attach strand of hair to dial
2. when hair extends dial goes up = ↑ RH
3. when hair is drier dial goes down = ↓ RH
4. hair get brittle + snaps = need to use new strand - Psychrometer
- quicker to calibrate than hair tension
1. have 2 thermometers swinging in a room
- 1 thermometer is covered by wet fabric (100% H2O)
2. fabric will cool down + evaporate
3. Plot values on mollie diagram
What is the role of moisture (RH) in the drying process
- Hard to remove moisture / water from manufacturing process as it is all round us
3 Phases of Drying
1st Phase:
- when free water (water between particles) evaporates
- water evaporates quickly
2nd Phase:
- have porous particles, pores filled with water
- water in pores take longer time to evaporate
3rd Phase:
- removal of surface attached water
- slowest phase as this is the hardest to remove
- can’t get all of the water out depends on RH
- can heat up air to increase water uptake = ↓ RH
- need to change equilibrium (between environment + drug)
What is the role of moisture (RH) in the stability of pharmaceutical products
- Need to be aware of what moisture can do to product e.g. ca cause hydrolysis reactions in some drugs
- hydrolysis leads to ↓ potency, altered activity, new product could be formed, toxicity
What are the drying principles
- Before choosing drying method need to know if product can withstand heat, is it safe, sterile etc.
3 Drying Mechanism
Conduction:
- environment does NOT heat up sample
- directly warm up the sample itself + as it heats up solvent evaporates
Convection:
- have warmer environment, energy from environment heats up sample = solvent evaporates
Radiation:
- dry sample in direct sunlight
- energy is put directly into sample without heating environment
List the drying methods for pharmaceuticals
- Large capacity drying ovens
- Fluidised bed dryer
- Freeze Dryers / Lyophilisation
What are the advantages and disadvantages of the “large capacity drying oven” drying method
- The drying goods are put into the oven + dried to the required moisture content
- Hot air blows through goods
- Can have shelves (dry multiple goods)
What are the advantages and disadvantages of “fluidised bed dryer” drying method
- hot air flow through the powder bed + flows through particles
- hot air breaks powder + liquid bridges
- dries quicker than oven but lower capacity
As air cools down it wakes water from the drying goods + releases energy into to drying goods
What are the advantages and disadvantages of “freeze dryers / lyophilisation” drying method
- freeze sample + alter vapour pressure of sample
- sublime water out (by ↓ pressure)
- water can also crystallises out as ice (to avoid ice melting back into water sample has to cooled lower)
What are the advantages and disadvantages of “spray dryer” drying method
- nozzle sprays solution into hot tower + droplets generated
- nozzle produces fine mist due to high pressure solution travels under
- nozzle can also generate formulations i.e. diff. particles - as droplets fall they cool
- droplets have ↑ SA = solvent evaporate quickly - as droplets dry will be collected as small spherical particles (a powder)
What are the advantages and disadvantages of … drying method
List packaging and stability considerations
