Vacuum drying
Vacuum drying is a thermal separation process at low temperatures with short drying times. The process is well suited for temperature-sensitive substances, since the boiling temperature of the liquid to be evaporated can be reduced according to the vapor pressure curve. The drying takes place under a system pressure that is lower than atmospheric pressure.
Areas of application
- chemistry
- pharmacy
- Organic and food industry
- Agriculture
- Raw materials and minerals
- recycling
- decontamination
- Bulk and waste processing
Development history of vacuum technology
In addition to Torricelli , the well-known inventor of the barometer (1643), it is primarily Otto von Guericke , the inventor of the air pump (1650), who gave the first insights into the vacuum with his demonstration of the Magdeburg hemispheres . The most important of his observations, which he summarized in his work "Experimenta nova, ut vocantur, Magdeburgica de vacio spatio" (Amsterdam 1672), are still recognized today as the basis in physics and technology. But it was only in the last century that the application of the evaporation found a broader entrance into technology and originally only for the field of vacuum evaporation and distillation. Eugen Hausbrand deserves the credit for having dealt with the theoretical relationships in detail in his works "Evaporation, Condensation and Cooling" and "Drying with Air and Steam".
Emil Paßburg , a pioneer in vacuum technology , then extended this knowledge to the field of drying in a vacuum and created the first basic types of vacuum dryers. Passburg, the technical manager of a sugar factory at the time, was able to thank for the first industrial application of vacuum drying, as he was already drying sugar breads industrially in 1881 with the help of vacuum technology.
Around 1908, Passburg founded a test facility for research and development in vacuum technology in Erfurt. Erwin Lothar Holland-Merten , an enthusiastic researcher, took over the management of this facility in 1922 and developed it into an independent research facility. This has been under the management of Holland-Merten without interruption since 1922. When the company moved to the former Vacuumtrocker GmbH Erfurt , today Deutsche Vacuumapparate GmbH , Erfurt, it was eventually expanded into an independent research facility for vacuum technology.
Holland-Merten describes in his work "Die Vakuumtechnik" from 1936 the successful use of vacuum-dried breast milk . At the initiative of Marie Elise Kayser , founder and head of the human milk collection point at the Landesfrauenklinik in Erfurt, an alternative to the atomization-drying process was sought, as the results obtained with the previous process were unsatisfactory. The series of tests carried out by the Holland-Merten research facility around 1931 confirmed the potential of vacuum drying. In cooperation with Dreyer, Director of Deutsche Vacuumapparate GmbH in Erfurt, the Landesfrauenklinik was given an apparatus for drying in 1932, which, after four years of operation, processed more than 2000 liters of breast milk into a very popular dry product. Owing to her success, Kayser published her scientific studies in 1936 in the Archive for Gynecology, Vol. 161. It is thanks to vacuum drying that breast milk was available as a dried product that was gentle on the product and retained its contents, storable and easy to process. A revolution in medicine and “lifesaver” for countless newborns.
Working principle
The vacuum dryers are hardly restricted in terms of their use in the various vacuum areas: rough, intermediate and fine vacuum. They can be used for all vacuum areas, whereby only the construction of the sealing elements, the respective vacuum level and the resistance to the product have to be adapted. Any withdrawal of liquid up to the occurrence of a dry residue that is practically free of moisture can be referred to as drying. However, in process engineering, only such processes for drying are included in which the residue is obtained in a more or less solid form. All other processes for removing moisture, especially those in which the residue remains liquid, are to be considered from the point of view of evaporation or distillation . Regardless of this, the processes should not be strictly separated from one another in practice, since the drying used by definition in combination with pre-evaporation stages opens up economically highly efficient application possibilities.
Systems for vacuum drying
Two-drum dryer
Although any thin liquid products can be processed by means of the two-drum dryer, for reasons of economy its use is limited to a highly pre-concentrated product that may be in a thick, thick pulpy to stiff paste form. Upstream systems for pre-evaporation are useful for this. The functional principle is based on two heated and counter-rotating rollers, which, as it were, roll the product against each other. The product is fed in from above. The product is evenly distributed in the so-called roller gap and is held in the appropriate position by a filling box with frogs. A product film is created on the rollers by the rotary movement. The variable roller gap, rotational speed, roller surface and temperature, as well as the degree of vacuum are directly dependent and have a direct influence on the drying speed and the drying result. The knives arranged on both sides remove the dried, mostly skin-shaped to lumpy product from the roller body. This is collected in receiving boxes directly under the roller bodies and then dries. Depending on the application of the discharge product or the drying can be continuously or batch, the latter via Abförderschnecken like, are designed or. The roller gap, which can be changed depending on the task at hand, is achieved by a displaceable mounting of a roller body. This technical property allows a roller body to evade and thus offers protection against mechanical damage to the roller surface, any foreign bodies and dry material crusts that have entered, etc. In addition, possible consequential damage caused by blocking rollers or the like can be prevented.
Areas of application
- For products with a thin to stiff consistency.
- For the recovery of solvents and solids.
advantages
- Fully automatic CIP / SIP possible.
- Low electrical power requirement.
- Compact design.
- For strongly sticking / sticking products.
- Drying is possible continuously and batchwise.
- Quick product change. Little cleaning effort because only the roller surface is in contact with the product.
disadvantage
- Slight knife wear in the solid.
Other designs
- Single drum dryer
- Atmospheric single / double drum dryers.
Practical examples
- Manufacture of milk powder and nutritional supplements .
- Solvent recovery.
- Drying of slaughterhouse waste .
Paddle dryer
The paddle dryer consists of a horizontal, cylindrical heating jacket , in which a built-in paddle system, which is suitably heated, ensures that the product is constantly mixed during the drying process. For the shoveling of the product to be dried by the horizontally arranged agitator shaft, the angle of repose (inner angle of friction) of the product and its change during the drying process play a certain role. Because the larger the angle of repose, the higher the product is lifted by the shovel. Due to the corresponding height of fall, the product also has a certain crushing effect, which in turn has a positive effect on drying by increasing the product surface. By adapting the shape and width of the shovel, the respective products with all their properties can be taken into account. For the drying process itself, the product should be brought into contact with as much heating surface as possible. Accordingly, the shape and size of the paddles, depending on the drying task, should be selected so that the shoveled product falls onto the heated paddle shaft, is continuously and evenly mixed and is optimally distributed in the dryer. In addition, by appropriately shaping them, the blades are suitable for discharging the dried product from the dryer by changing the direction of rotation. Depending on the application, the discharge or the product drying can be carried out batchwise or continuously, the latter via an adjustable weir with an attached rotary valve or screw conveyor or the like.
Areas of application
- For products with a high angle of repose (internal angle of friction).
- Products that do not tend to stick or agglomerate .
advantages
- Drying is possible continuously and batchwise.
disadvantage
- Mechanical effects on the product.
- Blade wear in the product.
- High electrical power requirement, since a lot of drive power is required and a lot of material has to be heated.
Practical examples
- Drying of salts .
- Making egg powder .
- Drying of sewage sludge and manure .
- Drying drilling mud .
Rotary drum dryer
Rotary vacuum drums are primarily used to subject coarse-grained, easily trickling wet goods with a particularly low angle of repose to drying. This process is primarily used for products in which the structure must not be destroyed, for example by abrasion or pressure. This applies, for example, to cocoa beans , legumes , grain , malt and schnitzel, etc. The rotating drum is a drum mounted in pegs with a heated jacket. With the aid of additionally built-in heating pipes, the sprinkled heating surface can be enlarged very effectively by means of a rotary movement. Usually helical blades are arranged on the heating jacket, which have a mixing effect on the product in one direction of rotation during drying. For emptying, the blades convey towards the emptying opening when turned in the opposite direction. In this form of vacuum drying, drying can only take place batchwise. The loading is done by hand or machine, depending on the purpose and design.
Areas of application
- For highly demanding and sensitive products.
- For products with a low angle of repose (internal angle of friction).
advantages
- Very gentle on the product, as there is no mechanical impact on the product from shovels etc.
disadvantage
- Drying is only possible in batches.
Practical examples
- Drying of fruits and vegetables .
literature
- ↑ Segebrecht, Udo .: Liquid ring vacuum pumps and liquid ring compressors : technology and practice . Expert-Verl, Renningen-Malmsheim 1994, ISBN 3-8169-1135-8 , p. 287 .
- ↑ a b E. L. Holland-Merten: Handbook of vacuum technology . Ed .: EL Holland-Merten. 3. Edition. VEB Wilhelm Knapp Verlag, Halle (Saale) 1963.
- ↑ Holland-Merten: THE VACUUM TECHNOLOGY . Ed .: Holland-Merten. May 1936, p. 222 .
- ↑ a b c Deutsche Vacuumtrocker GmbH - From the idea to production. Retrieved October 23, 2018 .