Freeze Drying

Vacuum Drying & Freeze Drying 

Redline Chambers can provide you with the long cylindrical, Square, mailbox shape chamber designed to operate under vacuum. Industrial Freeze Drying chambers can be manufactured to specific sizes and specific needs for particular freeze dryer operations. Computer control systems can be added to monitor and document during the freeze-drying process. Drying temperature and Vacuum pressure can be also be adjusted during the drying period. Redline chambers offer chillers to freeze product to -40 and cold traps that reach -100 Fahrenheit to trap the moisture and reach the desired vacuum to dry your product. 

RLC 745 Dryer
 Four stages of the complete vacuum drying process

Pretreatment 

Freezing 

   Pretreatment includes any method of treating the product prior to freezing. This may include concentrating the product, formulation revision (i.e., the addition of components to increase stability, preserve the appearance, and/or improve processing), decreasing a high-vapor-pressure solvent, or increasing the surface area.

     In this step, it is important to cool the material below its triple point, the lowest temperature at which the solid and liquid phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following steps. Larger crystals are easier to freeze-dry. To produce larger crystals, the product should be frozen slowly or can be cycled up and down in temperature. This cycling process is called annealing. However, in the case of food, or objects with formerly-living cells, large ice crystals will break the cell walls (a problem discovered, and solved, by Clarence Birdseye), resulting in the destruction of more cells, which can result in increasingly poor texture and nutritive content. In this case, the freezing is done rapidly, in order to lower the material to below its eutectic point quickly, thus avoiding the formation of ice crystals. Usually, the freezing temperatures are between −50 °C and −80 °C (-58 °F and -112 °F). The freezing phase is the most critical in the whole freeze-drying process because the product can be spoiled if improperly done.

Primary Drying 

Secondary Drying 

During the primary drying phase, the pressure is lowered (to the range of a few millibars), and enough heat is supplied to the material for the ice to sublime. The amount of heat necessary can be calculated using the sublimating molecules’ latent heat of sublimation. In this initial drying phase, about 95% of the water in the material is sublimated. This phase may be slow (can be several days in the industry), because, if too much heat is added, the material’s structure could be altered.

In this phase, the pressure is controlled through the application of partial vacuum. The vacuum speeds up the sublimation, making it useful as a deliberate drying process. Furthermore, a cold condenser chamber and/or condenser plates provide a surface(s) for the water vapor to re-solidify on. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump’s performance. Condenser temperatures are typically below −40C.

    The secondary drying phase aims to remove unfrozen water molecules since the ice was removed in the primary drying phase. This part of the freeze-drying process is governed by the material’s adsorption isotherms. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0 °C, to break any chemical interactions that have formed between the water molecules and the frozen material. Usually, the pressure is also lowered at this stage to encourage desorption (typically in the range of microbars, or fractions of a pascal). However, there are products that benefit from increased pressure as well. After the freeze-drying process is complete, the vacuum is usually broken with inert gas, such as nitrogen or filtered air.

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