Freeze drying is mainly used to preserve or store temperature-sensitive products: For the part of this method, the product would lose essential and desired properties if preserved or dried by other methods.
The application of freeze drying is mostly in industries such as pharmaceuticals,
biology and medicine. But freeze-dried food is also available.
Perhaps a little odd application might be the conservators at the museum who used it
and maybe even now apply / they freeze drying to their booths by i.a. reptiles.
Now, in recent years, florists have also discovered freeze-drying. They offer freeze-drying of wedding bouquets.
From a purely process-technical point of view, it is characteristic of freeze-drying, that water is removed from
a frozen solution or preparation without thawing, in other words by sublimation. Sublimation, which is the process where the solid phase is transferred directly to a vapor phase, e.g. skip the liquid phase. The liquid-to-vapor phase transition that usually occurs in normal drying is replaced by conversion first from the liquid to the solid phase (freezing)
followed by the solid-to-steam phase (sublimation or freeze drying in the strict sense of the term).
Then a so-called. after drying where you evacuate further to
evacuate away or degass the so-called non-condensable gases.
The most important phase of the process is actually the freezing phase.
It determines what the crystal structure of the finished dried product should look like
or have for function. The crystalline structure, solubility, activity, vitality
and the color of the product is affected by the freezing process.
As can be seen in the water phase diagram (Swedish text above), all three phases are in equilibrium at the so-called “triple point”. When an aqueous solution is cooled, crystallization of water begins in most cases below the freezing point (0 ° C). At the same time, one should know that all substances have a triple point or eutectic punk and that the values for this triple point differ considerably. This can be seen most clearly if you use “plain water” and add a little salt.
The most common freezing method used in laboratories is a common freezer
or freezer. The requirement is about -60 degrees C, but most preferably -80 degrees C.
The lower the temp the smaller the ice crystals the better
end result on the entire freeze-drying process.
The two main freezing methods used for pharmaceutical products:
1. Freezing on contact with a cooled surface, the product plate
2. Rotary freezing in cooling water bath; spin freezing and shell freezing technology.
There is no general rule for the large number of products that can be freeze-dried today.
Some of them require slow freezing and others faster
freezing to achieve a high quality final product.
We will only treat so-called. laboratory freeze dryers in this section. Mainly addicted
We have limited ourselves in our program of large and fully automatic freezer dryers. However, we are happy to convey requests.
The freeze-drying system consists of a chamber connected to a condenser
and a vacuum pump. Usually, the condenser is cooled via a cooling machine, which in laboratory facilities usually has a final temperature of -53 degrees C. Distinguish between single-chamber and two-chamber freeze dryers. This is because you want to be able to determine the end point of the drying process. This is done with the help of a valve, which separates the chamber from the cooling condenser. With the help of a pressure sensor of the Pirani type, the “water-phase diagram” can then be determined using barometric methods the temperature of the ice core. That way, set their own dryness limits.
If you do not have products of this kind where the final drying level is important, then a single-chamber freezer dryer can be advantageously selected. It is both cheaper and easier to handle.
At the beginning of the drying, the ice is sublimated at the surface.
As the process proceeds, the sublimating excess draws into the product, and the developing steam must be passed through the previously dried outer layers. Therefore, the drying depends on the speed of steam transfer and removal and the necessary sublimation heat.
The heat required for sublimation is delivered to the product through convection and heat conduction, and to a small extent through heat treatment. In addition to heat transfer through heat conduction and radiation, it is most important that the heat transfer through convection is optimized. However, it must also be taken into account that due to reduced pressure in the drying chamber, convection will practically cease at a pressure below 10-2 mbar.
The product dries on surfaces that can absorb heat. As the drying continues,
pulls the ice core into the center of the product.
In order to avoid damage to the product, obtain the maximum permissible temperature for it dried product is not exceeded. On the other hand, make sure the product is not damaged by thawing.
Therefore, in order to achieve quick and efficient freeze drying, it is necessary to ensure
precise temperature control on the product shelves, as well as partial pressure and total pressure control in the freeze-drying apparatus.
After the drying- process
In the final drying phase, the water bound by adsorption must be moved.
To achieve this, it is often necessary to overcome the capillary forces of the water and a freeze-drying plant must therefore be designed to provide a high-pressure gradient during the final drying phase.
This can be done by lowering the condenser temperature below -70 ° C when the pressure conditions are produced which allow secondary drying of the products to a low residual moisture content.
There are many good reasons for a close observation of the drying process, especially for determining the end point of the drying process. It may be considered that weighing the product may be a possible method for determining the loss of water during the drying cycle. However, in the ultimate drying phase, this method would be of little use, since the amount of water vapor that is still sublime will be too low for measurement. Although the method can be used in a laboratory, it is not suitable for industrial production.
Another method is to monitor the temperature, which, however, requires extremely precise arrangement of the sensing probes and virtually identical conditions at all measurement points.
Normally, the temperature probes are too large and thus give measurement errors that increase with size. The usual drawback, however, is that the product temperature can only be measured at one or two points. The most reliable and accurate technique is the pressure rise method (barometric temperature control) developed and patented by LEYBOLD-HERAEUS. Due to the pumping effect of the ice condenser, the partial pressure of water vapor during the drying process always remains below the saturation vapor pressure corresponding to the sublimation temperature.
When the condenser is insulated from the chamber, which of course must be vacuum tight, the vapor flow to the condenser is stopped and the partial pressure of water vapor rises within a very short time to the equilibrium pressure, as indicated by the vacuum pressure sensor. Towards the end of the drying process, the pressure increase gradually becomes smaller, due to the smaller amount of steam released by sublimation. When the pressure rise (pressure increase) stops, the drying process can be completed. There are no ice cores left in the product. This of course applies, provided the apparatus is vacuum tight.
The final dry products are very hygroscopic. The dried products must be protected after drying. If the product is dried in bottles, it seems logical to close these containers immediately after drying prior to removal from the plant.
For this purpose, special ribbed / grooved rubber plugs are placed in the neck of the bottle or bottle, before drying and, at the end of drying, are pressed hard into the bottle neck with a sealing unit, / (“stopper device”).
The containers can be sealed under vacuum or under a protective gas atmosphere – nitrogen or other inert gas. The method used depends on the type of product and the quality of the rubber plugs.
Products for Freeze Drying Applications
We are happy to assist with application tips in choosing the right dryer that can be expandable and modifiable for future use.
The main questions we will ask at the first contact are:
- What solution / preparation will be available?
- Is it just water?
- How much water / ice will need to be dried per batch?
- What method will there be Bulk / flat barrels / bottles / flasks?
- One or more shelves?
Continue to our freeze drying prospectus to get an idea of the different models available.