DEHYDRATION- VACUUM FREEZE DRYING
Vacuum Freeze drying, also known as lyophilisation or cryodesiccation, is a low temperature dehydration process that involves freezing the product, lowering pressure, then removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat.
Because of the low temperature used in processing, the quality of the rehydrated product is excellent, and the original shape of the product is maintained. Primary applications of freeze drying include food processing (e.g., coffee, fruits & vegetables) and preservation.
Stages of freeze drying
There are four stages in the complete freeze-drying process: pre-treatment, freezing, primary drying, and secondary drying.
Pre-treatment
Pre-treatment includes any method of treating the product prior to freezing. This may include concentrating the product, formulation revision (i.e., addition of components to increase stability, preserve appearance, and/or improve processing), decreasing a high-vapor-pressure solvent, or increasing the surface area. Food pieces are often IQF treated to make them free flowing prior to freeze drying. In many instances the decision to pre-treat a product is based on theoretical knowledge of freeze-drying and its requirements, or is demanded by cycle time or product quality considerations.
Freezing and annealing
During the freezing stage, the material is cooled below its triple point, the lowest temperature at which the solid, liquid and gas phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following steps. To facilitate faster and more efficient freeze drying, larger ice crystals are preferable. The large ice crystals form a network within the product which promotes faster removal of water vapor during sublimation. To produce larger crystals, the product should be frozen slowly or can be cycled up and down in temperature in a process called annealing. The freezing phase is the most critical in the whole freeze-drying process, as the freezing method can impact the speed of reconstitution, duration of freeze-drying cycle, product stability, and appropriate crystallization.
Amorphous materials do not have a eutectic point, but they do have a critical point, below which the product must be maintained to prevent melt-back or collapse during primary and secondary drying.
Structurally sensitive goods
In the case of goods where preservation of structure is required, like food or objects with formerly-living cells, large ice crystals will break the cell walls which can result in increasingly poor texture and loss of 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 large ice crystals. Usually, the freezing temperatures are between −50 °C (−58 °F) and −80 °C (−112 °F).
Primary 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, 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 vapour to re-liquify and solidify on.
It is important to note that, in this range of pressure, the heat is brought mainly by conduction or radiation; the convection effect is negligible, due to the low air density.
Secondary drying
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 (32 °F), to break any physico-chemical interactions that have formed between the water molecules and the frozen material. Usually, the pressure is also lowered in 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 an inert gas, such as nitrogen, before the material is sealed.
At the end of the operation, the final residual water content in the product is extremely low, around 1% to 4%.
The process
The batch freeze drying plant is a cabinet with a door for loading and unloading. Inside the cabinet are a number of heating plates made of anodized aluminium. Hot water is circulated through the system to ensure efficient heat transfer by radiation of the product to be freeze dried. The water temperature can be regulated during the freeze-drying process in order to achieve optimal evaporation laps and avoid overheating of the products.
The product is placed on a wagon hanging on trolleys in a pre-weighed amount or by volume in frozen form. By correct loading of the cabinet, the product trays are placed between the heating plates for optimal heat transfer by radiation. Direct contact between the product trap and the heat plates must be avoided as heat can damage the product. The vapor condenser module made of stainless steel is built into the cabinet. Liquid NH3 is circulated in a pipe on which the sublimated water vapor will condense.
The larger cabinets are equipped with a duplex vapor condenser for Continuous De-Icing (CDI). When one of the vapor condensers has to be de-iced (typically after one hour of operation), the section is sealed off while the other takes over the condensation function.
To melt the accumulated ice, 25 ̊ C water vapor (vacuum steam) will be led into the room. The water vapor will now condense on the cold icy surface of the condenser and thus melt the ice. In order to restore the de-iced condenser to operating condition, any remaining vapor in the condenser chamber must be condensed by cooling it down until operating temperature and vacuum is reached. A direct switch-over when the next de-icing cycle is needed can now be done without loss of operating vacuum.
Depending on the size, the Freeze Dryers are either equipped with an internal vapor condenser with the built-in de-icing system, or a water-flushing de-icing system. Compared with external condenser systems, the first system not only saves space, it is more reliable, does not cause product loss, and it consumes less energy and thus ensures the best overall equipment efficiency. A water-flushing de-icing system, ensuring minimum investment costs and a simple operation, is the most feasible solution for small installations.
