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PMY2006 > Drying > Flashcards

Drying Flashcards

1
Q

Drying

A

removal of some or all of a solvent from a system - usually water for pharmaceuticals

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2
Q

Drying importance for pharmaceuticals (4 factors)

A
  • Chemical stability
  • Processing and handling
  • Toxicity
  • Dosing
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3
Q

Applications of Pharmaceutical Drying

A
  • Preservation of biologicals
  • Inhalable formulations
  • Transdermal delivery
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4
Q

What kind of heat is involved in drying?

A

Latent heat. Applied heat may cause degradation of the drug if it is biological

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5
Q

Moisture content

A
  • expressed as % w/w
  • 3% w/w means 3g of water removed from 100g wet material
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6
Q

Types of water in a solid sample

A
  • Water of crystallisation (hydrates
  • ‘Free’ water (water present as liquid)
  • ‘Bound’ water (hydrogen bonded to substrate)
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7
Q

Equilibrium Moisture Content

A
  • Equilibrium will be established between water content of material and moisture in atmosphere
  • Value will depend on ambient conditions
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8
Q

What is the typical equilibrium moisture content for drugs in terms of % w/w?

A

0.2-2.5%

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9
Q

Relative Humidity

A
  • Measure of water content in air
  • At 100% RH, maximum solubility of water vapour in air has been reached
  • Air can hold more water at higher temperatures
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10
Q

What is normal RH?

A

54%

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11
Q

Relative humidity after rain

A

75%

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12
Q

What types of sample have a greater Moisture uptake

A
  • Organic material takes up much more water than inorganics
  • Hygroscopic samples will take up water readily
  • Non-hydroscopic samples take up less water
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13
Q

Example of organic material that takes up water?

A

Kaolin

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14
Q

Example of hygroscopic materials that take up water readily?

A

sucrose, lactose

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15
Q

Examples of non-hygroscopic materials that take up water less readily?

A

inorganics, metals, non-polar materials

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16
Q

Water loss profile

A

See lecture graph

https://quizlet.com/cdn-cgi/image/f=auto,fit=cover,h=200,onerror=redirect,w=240/https://o.quizlet.com/chk2MdfnvlBJvFOD1fLTKw.png

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17
Q

Constant Rate Period (A-B)

A
  • Sample loses water from surface of bed and saturates air immediately above surface
  • Water is replaced immediately
  • Rate will be constant
  • Controlled by rate at which water vapour can be removed
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18
Q

First Falling Rate Period (B-C)
(3)

A
  • Rate of vaporisation not sufficient to saturate air above surface
  • Drying rate determined by rate of transfer to surface
  • As drying proceeds, this becomes increasingly difficult hence rate decreases
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19
Q

What is rate of transfer determined by?

A
  • Diffusion
  • Suction potential of porous material
  • Capillary force
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20
Q

Second falling rate period (3)

A
  • water loss is from within bed itself (no more free water)
  • as solid becomes dryer, thermal conductivity decreases
  • may need to increase bed temperature (problematic for thermolabile materials)
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21
Q

Methods of drying

A
  • Convection drying
  • Conduction drying
  • Radiation drying
  • Spray drying
  • Freeze drying
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22
Q

Convection drying

A
  • drying takes place via latent heat of evaporation provided via a hot air steam
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23
Q

Types of convection drying

A

Tray/Fixed bed drying, fluid bed drying

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24
Q

Fixed bed/tray drying

A
  • Tray of wet material placed in oven with hot air circulated around material via fan or baffle system
  • used in small scale experimentation, not efficient
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25
Q

What causes Caking

A
  • Rapid water loss from inappropriate choice of drying conditions which leads to rapid water loss from surface of material to form a hard dry crust
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26
Q

Problem with caking

A
  • caked material prevents further water loss from deeper in the bed while the now dry crust becomes hot
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27
Q

Types of solute migration

A
  • intergranular migration
  • intragranular migration
28
Q

Intergranular migration

A

Drug and water soluble excipients dissolve in aqueous phase and be transported to top of the bed. Evaporation then leads to deposit of material on bed surface

29
Q

Intragranular migration

A

Water soluble materials migrate to surface of specific granules and are then deposited

30
Q

Effects of intragranular migration

A
  • Intragranular migration of dyes may lead to higher concentration on granule surface
  • On compression, granules fractures and colourless interior exposed, leading to mottled appearance
  • This reduces patient confidence in the product
31
Q

Fluidized bed drier

A
  • hot air steam through powder bed gives buoyancy to particles
  • particles form a fluidised suspension in air
  • water loss takes place from individual particles
32
Q

Advantages of fluid bed drying

A
  • efficient process with short drying times
  • negligible intergranular migration
  • uniform and controllable temperature
  • small size and simple equipment
  • ensures stability of thermolabile drugs
33
Q

disadvantages of fluid bed drying

A
  • intragranular migration may take place
  • may be some attrition of surface of particles, generates fines and may also lead to drug loss
  • risk of explosion if not properly earthed (problem with organic solvents)
34
Q

Conductive drying

A
  • wet solid is on direct contact with a hot surface
  • heat transfer occurs via conduction
  • example is vacuum oven
35
Q

advantages of conductive drying

A
  • useful for low temperature drying of thermolabile materials
  • less chance of oxidation during drying
36
Q

disadvantages of conductive drying

A
  • tends to be fairly slow
  • small capacity
  • expensive
  • temperature must be controlled to avoid caking
37
Q

where is conductive drying most used?

A

in research labs

38
Q

Radiation drying

A
  • molecule sensitive, polar solvents will resonate as their dipole moments vibrate at microwave frequencies. This movement causes friction which generates heat leading to evaporation
39
Q

Key points for radiation drying

A
  • dry solids do not resonate as much so do not heat to same extent
  • good heat penetration leads to uniform drying throughout solid
  • very effective when combined with vacuum
40
Q

When is radiation drying useful?

A
  • useful for granulation
  • useful for drugs where containment is an ussue
  • efficient since casing remains cool, since wet material absorbs most of the energy
  • solvents are fully recoverable
41
Q

disadvantages of radiation drying

A
  • batch size small
  • danger from radiation
42
Q

Spray drying

A
  • used to produce dried product from aqueous solutions or suspensions
  • solution is atomised and sprayed into stream of hot air
  • results in free-flowing powder sample
  • spray dried products may be recognised by hollow spherical shape
  • outer surface dried very rapidly
  • particles may be intact or broken spheres
43
Q

3 steps of spray drying

A
  • formation of droplets after the liquid is atomised at the top of the drying chamber
  • evaporation of water from droplets to form product in drying chamber
  • collection of feedstock in collecting flask
44
Q

advantages of spray drying

A
  • products dried very rapidly, a whole batch can be dried in seconds
  • rapid and efficient drying results in low temperatures, as heat input goes into evaporation rather than temperature increase
  • product is uniform size and free flowing
  • liquid is transformed into a powder in one step
45
Q

When is spray drying useful?

A
  • useful for tablet and capsule formulations
  • examples include paracetamol
46
Q

Spray Drying for Dry Powder Inhalers

A
  • Considerable interest
  • due to good flow properties and well controlled characteristics
47
Q

Disadvantages of spray trying

A
  • higher cost
  • bulky equipment
  • products are partially or completely amorphous
  • means that material can recrystallize on storage
48
Q

Freeze Drying / Lyophilisation

A
  • used to produce dried product from aqueous solutions or suspensions in vials
  • important because it involves use of low temperatures
  • results in porous solid
  • used for preparation of proteins
49
Q

Freeze drying in general terms

A
  • solution is frozen
  • pressure is reduced
  • ice then sublimes (solid directly into gas)
  • heat is applied to remove remaining water
50
Q

Initial freezing

A
  • takes place via nucleation as temperature drops below 0
  • water may undergo considerable supercooling
  • removal of water as ice leads to concentration of remaining solutes
51
Q

Further Cooling

A

Two things can occur:
1. Eutectic mixture is formed
2. Amorphous matrix (glass) formed

52
Q

Eutectic mixture

A

Formed when two components both crystallize; system is now an intimate crystalline mixture of one component in the other

53
Q

Amorphous Matrix

A
  • more usually formed
  • system forms a mix of a glass with ice crystals distributed throughout
  • glassy matrix has characteristic glass transition value (Tg)
54
Q

Tg

A
  • materials are rigid below Tg and rubbery above Tg
  • Presence of water lowers Tg of dry material considerably
55
Q

Saquinavir as an example of water affecting Tg

A

Tg reduced from 120 degrees to 70 by presence of 5% w/w water

56
Q

Primary Drying

A
  • Pressure reduced to near-vacuum and heat applied to sublime ice
  • Typical 1o drying temperature is around -35oC
57
Q

Primary Drying Continued

A
  • primary drying conducted at temperature close to Tg of glassy matrix
  • if it takes place above Tg, matrix is in liquid state and product will collapse
  • Removal of ice gives porous structure
58
Q

Implications of primary drying

A

On basis that reduction of primary drying temperature of 1oC will increase drying time by 13%, cost implications of drying at too low a temperature are considerable

59
Q

Secondary Drying

A
  • drives off remaining water trapped in matrix
  • maximum temperature 25-60
  • left with dry porous product which can be easily reconstituted
  • system is amorphous and will have its own Tg
  • important to store below Tg to prevent collapse on storage
60
Q

Freeze drying of proteins

A
  • Large problem, as many proteinaceous drugs lose activity on freezing and drying
  • Cryoprotectants and/or lyoprotectants are necessary
61
Q

Cryoprotectant

A

Protects substrate during freezing

62
Q

Lyoprotectant

A

Material which protects protein through whole freeze Drying process

63
Q

Most commonly used cryoprotectants and lyoprotectants

A

Sucrose, trehalose, polymers, amino acids

64
Q

Advantages of freeze drying

A
  • low temperature is suitable for thermolabile materials
  • high dissolution rate due to amorphous structure
65
Q

Disadvantages of freeze drying

A
  • expensive
  • time consuming
  • complicated process
  • stability issues due to amorphous structure

PMY2006 (12 decks)

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