drying_1
DESCRIPTION
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1. Drying principles: a. Water in solids & gases; b. Heat & mass transfer. 2. Dryers description & operations: a. Vacuum-shelf dryer; b. Rotary dryer; c. Spray dryer; d. Freeze dryer.
Lecture Outline
BTE3481 (Separation Processes 1)
Recall: Overview of bioseparations
What is “drying”?
1. Drying Principles
The process of removing all or most of liquids by supplying latent heat to
cause thermal vaporization, i.e. a liquid is converted into a vapour.
Reasons for drying?
1. To prevent degradation of product due to chemical (e.g. deamidation or
hydrolysis) and/or physical (e.g. aggregation or flocculation) reactions
during storage;
2. Help in the preservation of products by preventing fungal or bacterial
growth (their enzymes may lead to product degradation).usually under
10% wt, microb will not grow;
3. Convenience in the final use of the product - it is often desirable that
pharmaceutical drugs be in tablet form;
4. More economical and convenient to store them in dry form rather than
frozen;
5. To reduce bulk & weight, therefore reducing the cost of transportation;
Drug
synthesis Crystallization
Filtration
Drying
Milling Granulation
Excipients
Drying
Lubrication
Tabletting/
Encapsulation
Packaging/
Storage/
Transport
Typical pharmaceutical manufacturing process:
Suspension
Wet solid
Evaporative
dryer
Spray
dryer
- will not remove all moisture because
the solid equilibrates with the moisture
present in the air.
- removal of bound water.
- Also called free water;
- Mainly held in the voids of the solid;
- Exerts its full vapor pressure;
- Easily be removed by evaporation.
How water is held within biological solids?
Water contained
within solids
Unbound
water Bound
water
- Water that is adsorbed on surfaces
of the solid (to form a mono- or bi-
layer) or trapped in capillaries within
solid structure;
- Cannot exert its full vapor pressure;
- Not easily lost by evaporation.
Magnified cross-section of a wet solid
Moisture content of a wet solid can be expressed as g of water presents in 100
g of water-free or dry solid.
Similarly, moisture content of air can be expressed as g of water presents per
100 g of dry air.
Relative humidity (RH) of air
The percentage RH is defined as:
100% RH means that at a given temperature, air has taken up water vapor until
it is saturated.
Amount of water vapor in
air at a given temperature
Maximum amount of water vapor the
air could hold at the same temperature
x 100
The curves give valuable information about the water capacity of solids:
Water remained in solid at 100% RH = bound water
Unbound water = Water contained in wet crystal – bound water
The equilibrium moisture content of a solid exposed to moist air varies with the
RH, as exemplified by the equilibrium-moisture curves below:
Example 1:
The wet antibiotic cefazolin sodium crystals contain 30 g of water per 100 g of
dry antibiotic. Determine the percentages of bound and unbound water in the
wet crystals.
The equilibrium-moisture curve for the
antibiotic can be extrapolated to give a
water content of the solid at 100% RH:
Solution:
23
Since water remained in solid at
100% RH = bound water;
Bound water for the antibiotic crystals
= 23 g/100 g dry solid
23 g/100g
30 g/100g x 100% % bound water =
= 76.7%
% unbound water = (100.0 – 76.7)%
= 23.3%
The properties of air (dry and moist) are provided by the “humidity” or
“psychrometric” chart:
Moisture content of air
(g/kg dry air)
Relative
humidity (%)
Dry bulb
temperature
Example 2:
Air at 1 atm and 25°C with a relative humidity of 50% is to be heated to 50°C
and then to be used in drying wet crystals of the antibiotic cefazolin sodium.
Determine the moisture content of the crystals after drying.
In order to use the equilibrium-moisture
curve for the antibiotic to find the water
content of the crystals after drying (at
50°C), requires the RH value at the
same temperature. This can be found
using the humidity chart:
Solution:
25
Move from a point at a temp of 25°C &
50% RH to a point at a temp of 50°C,
keeping the moisture content constant.
By interpolation, the RH of the air is
read to be 13%.
From the equilibrium-moisture curve for the
antibiotic, the moisture content of the
antibiotic after drying (i.e. at 13% RH) =
Solution (continued):
13
8
8 g water/100g dry solids
Exercise 1:
Wet insulin crystals containing 32 g water per 100 g of dry insulin
need to be dried in air to a moisture level of 5 g water per 100 g of
dry insulin. Determine:
1. The percentage of bound and unbound water in the wet crystals
before drying;
2. The humidity of the air to accomplish the drying;
3. For drying with air at 20°C, what should be the moisture content
of the air (g moisture/g dry air)?
Relative humidity (%)
20 40 60 80 100 0
10
20
30
40
0
10
20
30
40
Wate
r conte
nt
(g/1
00 g
dry
solid
)
29
5
26
20
4