An introduction to stirred freeze drying

Tag : Vacuum Dryers

Typical vacuum levels are between 1mbar and 0.01mbar.
For example when water is sublimated such deep vacuum levels ensuresublimation temperatures of -20C or lower.
By freeze drying a product can be obtained with good shelfstability and which will remain unchanged after reconstitution withthe solvent.
In other words, by using the freeze drying process the material tobe dried is frozen in its original structure and the solvent isremoved from this structure without changing this structure.
A disadvantage of the conventional tray-dryer type of freeze dryeris the lump formation when the material is dried on a larger scale.
Despite the optimal structure of the individual product particles,the layer on the plates will usually form one piece of hard bakedmaterial.
Another disadvantage could be the relative low heat transfer ratedue to the quiescent state of the material.
Often the product has to be crushed after freeze drying, which maylead to damage of the product structure.
All these individual steps in the production process involve labourintensive handling of material in trays, making this uniquetechnology one of the most expensive unit operations in thepharmaceutical industries.
Why a stirred freeze dryer? Early experiments with a conventionalvacuum dryer operated at low temperatures and low pressures showedthat it is possible to operate a freeze dryer under stirredconditions.
The result is a lump-free, free-flowing product.
Stirred freeze dryers also exhibit a better heat transfer rate dueto the continuous mixing of the product, which shortens the dryingprocess.
Finally the freezing step is simplified because all steps can bedone in a single processing unit instead of handling trays filledwith product between freezing units, drying chambers and crushers.
This results in easy handling of the product especially whencompared to the traditional tray dryer equipment.
The positive results of these initial experiments have led to thedevelopment of a commercial stirred freeze drying technology.
In a dedicated designed stirred drying chamber the material to bedried is frozen.
Due to stirring motion the material, being either a liquid, pasteor solid, will be transformed into solid granules.
The granules can have sizes and shapes which are controlled by themixing characteristics of the machine.
Once the freezing step is completed the drying chamber is closedand vacuum is applied.
After evacuation of the freezing agent the sublimation process willstart.
From this stage the product temperature is dictated by the vacuumlevel.
During sublimation the heat is supplied through the jacket andefficiently distributed throughout the product by the stirrer.
The initially coarse granules will gradually reduce in size due tothe sublimation of the connecting ice structure in between thefrozen material.
The released dried particles will make up a loose powder.
Towards the end of the drying process when most of the frozensolvent is sublimated the product temperature will start to rise.
Finally the product temperature will equal the wall temperature,indicating that the drying process is finished.
By then all material is transformed into a fine and loose powder,After breaking the vacuum the dryer can be discharged easily fromthe dryer vessel, assisted by the transporting characteristics ofthe mixing element.
Applications.
The main application of stirred freeze drying is found inpharmaceuticals, e.g 30% of all antibiotics, 90% of themacromolecules and 50% of the electrolytes are produced byfreeze-drying.
Other products typically produced by freeze-drying are: proteins,hormones, viruses, vaccines, bacteria, yeasts, blood serum,liposomes and transplant materials like collagen sponge.
Especially the drying of probiotics offers an excellent applicationof the stirred freeze drying technology.
For all these products the decisive factor to use freeze-drying isthe preservation of the product structure, particle size and theminimal temperature load.
Another fast growing market for the application of stirred freezedrying on a larger scale is the materials business, in particularfor nano-materials.
By using stirred freeze drying for these wet base processedmaterials special advantages are obtained.
The suspended particles remain separated during the freezing aswell as the drying process.
During the progress in sublimation single particles will becomeseparated, but the continuous mixing of the material will inducethe formation of weak agglomerates.
The final product will consist of loosely bound single particlesforming a fine cohesive powder.
From lab-scale to bulk quantities.
With the introduction of the stirred freeze drying technology a newtype of production technology has been made available.
Stirred freeze dryer batch volumes can range from a few litres forlab-scale and small scale production applications to bulk drying ofa several cubic metres.
In all sizes the advantages are obvious: rapid drying, simpleproduct handling and unique product quality.
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