Pharmaceutics: Drying

Question 1

What is drying?

Answer 1

• the removal of moisture from the substance.

- involves the transfer of a volatile component out of a mixture, leaving a solid/sem-solid residue.

Question 2

Why is drying important?

Answer 2

• dry materials are widely used in pharmaceutical manufacture
• moisture affects product stability and physical properties.

Question 3

What is it called when you convert from solid to gas?

Answer 3

sublimation

Question 4

What is it called when you convert from liquid to gas?

Answer 4

vaporisation

Question 5

What is it called when you convert from solid to liquid?

Answer 5

melting

Question 6

What is it called when you convert from gas to solid?

Answer 6

deposition

Question 7

What is it called when you convert from gas to liquid?

Answer 7

condensation

Question 8

What is it called when you convert from liquid to solid?

Answer 8

freezing

Question 9

What are the two ways to vaporise a liquid?

Answer 9

by evaporation and boiling

Question 10

What’s the difference between evaporation and boiling?

Answer 10

evaporation is at the liquid surface only and occurs below the boiling point of the substance.
boiling is throughout the whole liquid body and requires the liquid to reach boiling point.

Question 11

Which is more useful for pharmaceutical drying, evaporation or boiling?

Answer 11

evaporation as boiling would affect the stability of the desired substance.

Question 12

What is required for effective drying?

Answer 12

1. Effective heat transfer to accelerate phase transition to vapour phase.
2. Effective mass transfer of drying material and vapour.
3. Large SA promotes effective heat and mass transfer

Question 13

What are the modes of heat transfer?

Answer 13

1. convection - mass transfer of hot fluid
2. conduction - heat transfer through contact with medium
3. radiation - as EM waves. No medium required (vacuum)

Question 14

What equation is used to work out the heat transfer rate?

Answer 14

Q=hA∆T
Q = Heat transfer rate (Js-1)
h = Heat transfer coefficient(Js−1m−2K−1)
A = surface area (SA) (m2)
∆T = Temperature difference between heat source and receiver(K).

Question 15

What conditions favour evaporation?

Answer 15

1. higher source temp
2. large SA
3. higher vapour removal rate (shifts equilibrium towards producing more vapour)
4. lower overall ambient pressure

Question 16

What is total moisture content?

Answer 16

the free moisture content + the equilibrium moisture content

Question 17

what is free moisture content?

Answer 17

the unbound water which is readily removed by evaporation.

Question 18

what is equilibrium moisture content?

Answer 18

bound water that remains after the free moisture content is removed. it is difficult to remove by evporation and exists in equilibrium with the moisture content in ambient environment.

Question 19

what happens when you go from wet solid to air-dry solid?

Answer 19

free moisture content is removed

Question 20

what happens when you go from air-dry solid to bone-dry solid?

Answer 20

equilibrium moisture content is removed

Question 21

what is relative humidity and what is the equation used to calculate it?

Answer 21

-it is an indicator of moisture content in air.
-equation:
Relative humidity(%)= (ρ/ρs) ×100
ρ = Instantaneous vapour pressure of water in air (Pa).
ρs = Vapour pressure of water in air (Pa) at saturation, measured at the same temperature as ρ

Question 22

what is required to be able to remove equilibrium moisture content?

Answer 22

relative humidity needs to be lowered to shift equilibrium so that the equilibrium moisture content can be removed.

Question 23

How can you reduce relative humidity?

Answer 23

remove vapour to reduce vapour pressure and relative humidity. This disrupts the equlibrium so more solvent molecules evaporate to restore equilibrium?

Question 24

What are the 6 drying techniques?

Answer 24

1. tray drying
2. vacuum oven
3. microwave
4. fluidised bed drying
5. spray-drying
6. freeze-drying

Question 25

how does spray drying work?

Answer 25

- the substance is placed on a tray in a thermal oven. - a fan in the oven is used to ventilate the oven and vapour is removed. - a convection current is set up by this process. - faster drying rate with thinner layer to dry

Question 26

what is solute migration?

Answer 26

- when solvent diffuses from the centre of the particle to the particle surface during drying, it brings dissolved solute with it. - the solvent then evaporates leaving the solute on the particle surface as concentrated. - this reduces content uniformity.

Question 27

how does vacuum oven work?

Answer 27

- uses conduction to transfer heat to the substance. | - low pressure in the oven increases vapour pressure of the liquid.

Question 28

how does microwave drying work?

Answer 28

- uses radiation to dry the substance in a microwave dryer. | - the solvent molecules absorb the microwaves and evaporate

Question 29

how do you calculate the loss factor in microwave drying/

Answer 29

microwave energy absorbed by molecule divided by microwave energy supplied

Question 30

how does fluidised bed drying work?

Answer 30

- uses convection to dry the substance. - hot air streams from below to suspend particles in air - suitable for discrete particles

Question 31

What is the advantage to using a microwave dryer?

Answer 31

the stationary bed avoids dust and product attrition

Question 32

what are the advantages to using fluidised bed drying?

Answer 32

- efficient mass transfer - efficient heat transfer - rapid drying so reduces cost and instability - constant movement of particles hence prevents aggregation - particles collide making them more spherical and causes attrition (however excessive attrition generates dust)

Question 33

how does spray-drying work?

Answer 33

- uses convection to dry the sample - produces spherical particles - three steps: 1. atomisation (sample sprayed into hot air stream as fine droplets) 2. drying (solvent evaporates rapidly leaves solid behind) 3. collection (particles collected in a cyclone)

Question 34

How does freeze drying work?

Answer 34

- uses conduction and radiation to dry the sample in a freeze dryer (aka a lyophiliser) - produces porous solids - three step process: 1. freezing (cryoprotectants often used) 2. primary drying (sublimation, bound moisture removed) 3. secondary drying (warmed under sustained low pressure and bound moisture is desorped)

Question 35

Practical consideration when drying are ....?

Answer 35

1. properties of sample 2. properties of liquid to remove 3. heat sensitivity of starting material and dry product 4. extent of drying required 5. scale of operation 6. advantages versus disadvantages (solute migration, attrition, cost)

Question 36

What type of materials is freeze drying suitable for?

Answer 36

thermolabile materials as freeze drying uses less heat