Freeze-drying, or lyophilization, is a well-known process that removes a solvent (usually water). This process is for long-term preservation and purification. Scientists, pharmaceutical manufacturers, food producers, taxidermists and florists have long appreciated the advantages of freeze drying.
The basic process steps are:
- Cooling. The compound is frozen, a mandatory step for low temperature drying, usually at temperatures less than -40oF (-40oC).
- Vacuum. After cooling, a vacuum is applied to allow the sublimation (shift from a solid directly into a gas) of the frozen solvent.
- Heating. Heat, usually 122oF (50oC), is applied to speed up the sublimation.
- Condensation. Vaporized solvent is removed from the vacuum chamber by re-converting it into a solid.
The ideal heat transfer fluid for freeze-drying or lyophilization should cover the widest temperature range, provide good transport capability, good heat transfer efficiency and low pressure drop with limited pumping-power requirements.
When choosing a heat transfer fluid, attention ought to be given not only to its thermophysical properties but also to its safety profile. Most of the heat transfer fluids currently used in freeze dryers suffer from the disadvantage of having an autoignition temperature and they are flammable. Perfluoropolyethers (PFPEs) and hydrofluoropolyethers (HFPEs) often represent the balance needed for both performance and safety.
PFPEs and HFPEs are characterized by:
- Wide operating temperature range.
- Good thermal properties.
- Excellent chemical inertness.
- High thermal stability.
- No flash or fire point.
- No autoignition point.
- No explosion hazards.
- No toxicity.
Heat transfer performance can be judged by means of the heat transfer factor that is only a function of the fluid properties (figure 3). When flow rate, pipe diameter and ∆T are unknown, it is advisable to separate a fluid's properties from system parameters in order to compare the heat transfer performance of different fluids. The heat transfer factor can be plotted as a function of temperature and it can be used for comparing different fluids, given the same geometry of the heat exchanger and same flow conditions. A higher heat transfer factor corresponds to high heat transfer efficiency (table 1).
Safety ProfilePFPEs and HFPEs represent a safe choice for freeze-drying applications because they will not burn even if exposed to naked flame and will not form flammable degradation products.
In addition, PFPEs and HFPEs are NSF registered. They are also approved for incidental food contact.
PFPEs and HFPEs often are the right candidates for freeze-drying processes at temperature as low as -112oF (-80oC). Because fire hazards are not present, technicians can perform maintenance simply by isolating the section of interest, draining the fluid and carrying out the required tasks, including welding.
When selecting a heat transfer fluid for freeze-drying applications, a review of performance and safety factors should be considered. Heat transfer efficiency, processing temperature performance, non toxicity, non flammability, NSF registration, reduced maintenance expense and low production downtime have lead many processors to PFPE and HFPE fluids for the results and reliability they need.