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Pharmacy Stage 2 > Pharmaceutics: Drying > Flashcards

Pharmaceutics: Drying Flashcards

1
Q

What is drying?

A
  • the removal of moisture from the substance.

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

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2
Q

Why is drying important?

A
  • dry materials are widely used in pharmaceutical manufacture
  • moisture affects product stability and physical properties.
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3
Q

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

A

sublimation

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4
Q

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

A

vaporisation

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5
Q

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

A

melting

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6
Q

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

A

deposition

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7
Q

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

A

condensation

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8
Q

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

A

freezing

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9
Q

What are the two ways to vaporise a liquid?

A

by evaporation and boiling

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10
Q

What’s the difference between evaporation and boiling?

A

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.

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11
Q

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

A

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

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12
Q

What is required for effective drying?

A
  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
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13
Q

What are the modes of heat transfer?

A
  1. convection - mass transfer of hot fluid
  2. conduction - heat transfer through contact with medium
  3. radiation - as EM waves. No medium required (vacuum)
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14
Q

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

A 
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).
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15
Q

What conditions favour evaporation?

A
  1. higher source temp
  2. large SA
  3. higher vapour removal rate (shifts equilibrium towards producing more vapour)
  4. lower overall ambient pressure
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16
Q

What is total moisture content?

A

the free moisture content + the equilibrium moisture content

17
Q

what is free moisture content?

A

the unbound water which is readily removed by evaporation.

18
Q

what is equilibrium moisture content?

A

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.

19
Q

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

A

free moisture content is removed

20
Q

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

A

equilibrium moisture content is removed

21
Q

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

A

-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

22
Q

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

A

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

23
Q

How can you reduce relative humidity?

A

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

24
Q

What are the 6 drying techniques?

A
  1. tray drying
  2. vacuum oven
  3. microwave
  4. fluidised bed drying
  5. spray-drying
  6. freeze-drying
25
Q

how does spray drying work?

A
  • 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
26
Q

what is solute migration?

A
  • 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.
27
Q

how does vacuum oven work?

A
  • uses conduction to transfer heat to the substance.

- low pressure in the oven increases vapour pressure of the liquid.

28
Q

how does microwave drying work?

A
  • uses radiation to dry the substance in a microwave dryer.

- the solvent molecules absorb the microwaves and evaporate

29
Q

how do you calculate the loss factor in microwave drying/

A

microwave energy absorbed by molecule divided by microwave energy supplied

30
Q

how does fluidised bed drying work?

A
  • uses convection to dry the substance.
  • hot air streams from below to suspend particles in air
  • suitable for discrete particles
31
Q

What is the advantage to using a microwave dryer?

A

the stationary bed avoids dust and product attrition

32
Q

what are the advantages to using fluidised bed drying?

A
  • 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)
33
Q

how does spray-drying work?

A
  • 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)
34
Q

How does freeze drying work?

A
  • 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)
35
Q

Practical consideration when drying are ….?

A
  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)
36
Q

What type of materials is freeze drying suitable for?

A

thermolabile materials as freeze drying uses less heat

Pharmacy Stage 2 (85 decks)

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