Freeze dryers

Silica gel filter for the protection of refrigerant oils

The high temperature fluctuations of a freeze-drying system pose great challenges for the refrigerant oil it contains, often silicone oil. In order to protect this from humidity and thus condensation water, a GIEBEL Adsorber® can be used as an alternative to nitrogen. This is quick and easy to install, efficient to use and simple to maintain.

Process

Freeze drying is a process for gentle drying of products. Freeze-drying is based on the physical process of sublimation. In this process, the ice crystals sublimate directly into the gaseous state without the intermediate appearance of a liquid phase. For cooling, either a refrigerant (e.g. silicone oil) is used or a refrigerant (typically nitrogen or carbon dioxide) is applied directly. The temperatures of the condenser are typically -60 to -80 °C.

Figure: Martin Christ Freeze Dryer; Source: https://www.martinchrist.de

Silicone oil and water

Figure: Water with silicone oil

Silicone oils are clear, colorless, non-toxic, neutral, odorless, tasteless, chemically inert, hydrophobic fluids that are temperature-stable over a wide range. Silicone fluids exhibit good lubricating properties between -60 °C and up to 200 °C. The hydrophobic property of silicone oil causes it to repel water. In this way, any condensation that occurs remains on the surface and causes damage if it crystallizes during the refrigeration process. The water or humidity is sucked into the tank by temperature fluctuations. During the freeze-drying process, the water condenses and freezes.

Protection of the freeze dryer

On the one hand, protection of the freeze-drying system can be achieved by the admission of nitrogen and the exclusion of air. However, this variant is very costly and requires a constant availability of nitrogen. A simpler and less expensive option is to use an adsorber to dry the drawn-in air. Since the air pendulum volume is very small, this solution is suitable.

An adsorber with valves designed for the particular plant, for example of the VV-DV series, offers a long maintenance interval and provides the silicone oil with lasting protection against atmospheric moisture. Condensate and the resulting damage are avoided.

Moreover, maintenance is very simple. If the the silica gel, which is orange at the beginning, turns into a dark green (see illustration on the right), the entire cartridge must be replaced. There are no specific guidelines to be observed and the disposal can be done in the normal household waste.

Figure: Adsorber on a freeze dryer

Conclusion

The use of an adsorber ensures that water from the air does not negatively influence the freeze-drying process. In addition, the complex and expensive nitrogen compound can be dispensed with. In addition to the cost aspect, simple assembly and fast maintenance with the visually highly visible silica gel speak in favor of the use of an aeration dryer.