Production Lyophilization
In considering a freeze dryer for production use,
the following general guidelines should be a part of your determinations:
- The product’s lyophilization profile.
(A vital stage in making a determination of the production
freeze dryer’s performance characteristics).
- This work should be carried out on scalable
pilot equipment.
Laboratory units, especially those equipped with
air cooled refrigeration systems cannot provide the depth of
data needed to determine a production cycle. If the user does
not have access to an industrial pilot unit, this work can be
accomplished on an outsource basis at relatively modest cost.
Determine the total length of the process cycle,
including all of the following factors:
- Preparation of the equipment.
- Dryer loading time.
- Freezing time.
- Freeze Drying time.
- Unloading time.
- Defrosting time.
- Clean up time required to make the area ready
for the next batch.
Heating Power, General Rules
A standard industrial freeze dryer should permit
sublimation of approximately one kilogram of water per square
meter of surface per hour, considering that the energy of sublimation
is about 800 kcal/kg of water sublimated. The heating value
of kw/hr=860 kcal/hr. Therefore, the freeze dryer should be
furnished with heating capacity equivalent to 1 kw/sq.meter
of surface area.
Refrigeration Power,
General Rules
The freeze dryer’s condensing system should
be capable of condensing the sublimated vapor liberated from
the product at a temperature of –55° C to –65°
C, at a rate of 800cal/kg of water, having a theoretical refrigeration
capacity of about 1000kcal/hr at -50 C/sq.meter of surface area.
It is important that at the end of secondary drying,
the refrigeration system must be capable of lowering the temperature
of the ice condenser to a temperature low enough to obtain the
necessary residual moisture in the product.
Production Cycle Duration,
General Rules
Assuming the optimum rate of flow, an estimate
for a dryer of approximately 100 square foot capacity would
be:
- 100 kg of product in bulk (liquid loaded on
trays).
- 1000 kg to 1500kg of stainless steel (product
shelves, hoses, etc).
- 300kg to 500 kg of thermal medium (heated
and cooled).
All of the above to be chilled or heated during
the drying cycle.
If the dryer is equipped with 10kw heating employed
80% of the cycle, the shelves and heat exchangers represent:
- 1000kg of stainless steel with a specific heat
of .12cal/kg per degree °C
- 500kg of thermal fluid with a specific heat
of 0.36kcal/kg per degree °C (Silicone)
- A set point freezing temperature of –40°
C and a terminal temperature of 40° C (Shelf)
The amount of energy required would be:
Q=100kg x 800kcal + 1000kg x 0.12kcal x 80°
C + 500kg x 0.36kcal x 80= 100,000kcal
The time of sublimation would be:
T= 100,000
--------------------------- = 15 hours
10kw x 860kcal x 0.8
The secondary drying time must be added to this
total to deduce the total cycle time.
Secondary drying time is dependent on the desired residual moisture
content.
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