Cooler & Safer Freeze Drying

Consider both performance and safety when selecting a heat transfer fluid for freeze-drying applications.

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 -40^oF (-40^oC).
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 122^oF (50^oC), 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.

Figure 1. The viscosity of the typically used silicone oil is higher than that of PFPE and HFPE fluids, which may lower heat transfer efficiency and decrease fluid pumpability.

Good thermal properties and low viscosities at operating temperatures result in high heat transfer efficiency as well as greater heat capacity with small volume flow and small temperature change. Viscosity is of great importance because it influences flow conditions and also determines pressure drop. Figure 1 shows that the viscosity of the typically used silicone oil is higher than that of PFPE and HFPE fluids, which may negatively affect heat transfer efficiency and fluid pumpability, especially at low temperatures.

Figure 2. HFPE and PFPE show volumetric heat capacity values higher than that of silicone oil, which requires greater volume flow and fluid velocity for a given refrigerating capacity (∆T) and pipe diameter. Silicone oil will require a greater volume flow.

Figure 2 shows volumetric heat capacity as function of temperature. Volumetric heat capacity represents fluid transport capability and can be plotted as a function of temperature. A high-volumetric heat capacity always corresponds to better heat transfer performance. Note that HFPEs and PFPEs show volumetric heat capacity values higher than that of silicone oil. Silicone oil will require a greater volume flow and fluid velocity for a given refrigeration capacity (∆T) and pipe diameter.

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).

Table 1. A relative heat transfer factor (HTf) can be calculated on silicone oil basis (silicone oil HTf = 1). PFPE heat transfer efficiency is approximately 30 percent higher than that of silicone oil at temperatures higher than -40^oF (-40^oC) while HFPE tends toward better performance at any temperature. HFPE heat transfer efficiency at -76^oF (-60^oC) is approximately 40 percent higher than that of silicone oil; it reaches 90 percent at 176^oF (80^oC).

Safety Profile

PFPEs 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 -112^oF (-80^oC). 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.

Figure 3. The heat transfer factor (HTf) can be plotted as a function of temperature and used to compare different fluids, given the same geometry of the heat exchanger and same flow conditions. A higher HTf corresponds to high heat transfer efficiency.

With their immiscibility and relatively low boiling points, PFPEs and HPFEs can be removed completely from materials being freeze dried. The processor using PFPE or HPFE fluids avoids the possibility of reprocessing contaminated material and can recover all valuable products without affecting the production cycle.

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.