Drying

Question 1

Define drying

Answer 1

Removal of moisture by; evaporation, vaporisation and sublimation

Question 2

List the aims of drying powders and granules

Answer 2

• Improve product stability
• Improve resuspension/ dissolution
• Ensure process-ability - powder flow, reduced stickiness, tablet-ability

Question 3

Describe what is meant by the dew point and what is the result?

Answer 3

Cooling air until partial pressure of vapour (Pv) exceeds saturation vapour pressure of water (Ps) for given degree - liquid water will be deposited.

Question 4

Give one methods of measuring relative humidity and the result.

Answer 4

Psychrometer - the difference increases with decrease in relative humidity

Question 5

List 2 types of convective drying methods of wet solids

Answer 5

1. Dynamic convective drying e.g. fluid bed drying
2. Static convective drying e.g. tray drier

Question 6

What restricts convective drying of wet solids?

Answer 6

Insufficient fluidisation of particles below 100 or above 1000 micro moles

Question 7

What are the advantages of convective drying of wet solids?

Answer 7

effective heat and mass transfer
drying of individual particles
precise control of bed temp

Question 8

What are the disadvantages of convecting drying of wet solids?

Answer 8

Attrition due to turbulent air flow
Fine particle elutriation
Electrostatic charging

Question 9

List two types of conductive drying methods of wet solids

Answer 9

Vacuum oven
Microwave drying

Question 10

List 2 advantages of vacuum oven

Answer 10

• Drying at low temp
• Minimum risk of oxidation

Question 11

What are the advantages of microwave drying?

Answer 11

uniform heating of wet mass
rapid drying at low temp

Question 12

What are the disadvantages of microwave drying

Answer 12

Batch size is smaller compared to fluid bed drier
Microwave radiation can harm operators

Question 13

Define lyophilisation

Answer 13

Freeze drying - removal of moisture by sublimation

Question 14

Describe sublimation

Answer 14

Products are frozen subject to vacuum, allowing the ice to change from solid to vapour without passing through a liquid state

Question 15

What is the aim of lyophilisation?

Answer 15

To dry heat sensitive materials without damage e.g. proteins

Question 16

What is the minimum freezing temp for biological and pharmaceutical materials?

Answer 16

Biological -40 to -50
Pharmaceutical -10 to -30

Question 17

What are the 2 main stages of the freeze-drying process?

Answer 17

Freezing stage - eutectic and glass formation
Vacuum stage - pressure below triple point and vapour removal

Question 18

Summarise the freeze-drying cycle

Answer 18

• During freezing, water crystallises as ice leading to conc of solutes increases and changes solution from viscous liquid to gas
• Glass transition degree (Tg) represents optimum degree for 1 degree drying
• Secondary drying removes non frozen water to elevated temp
• The self life of product depends on the residual moisture content

Question 19

List the critical temps for freeze drying

Answer 19

Tg
Tcol - collapse
Tcry - crystallisation
Teut - eutectic

Question 20

List the 3 parameters for freeze drying

Answer 20

Cooling rate
Drying rate
Residual moisture content

Question 21

State the importance of additives in freeze dried products

Answer 21

Stabilise the formulation

Question 22

What are the advantages of freeze dried products?

Answer 22

Drying at very low temp
Enhancement of product stability
Preparation os stable pharmaceutical products

Question 23

What are the disadvantages of freeze dried products?

Answer 23

Freeze dried products are very hygroscopic
Very low process
Difficult to control particle size of solids