DRYING: MICROWAVES WILL GUARANTEE MACRO-RESULTS
article in Russian by A.Yu. Voloshko, V.V. Lopatina, and O.V. Shishkin in International business journal EuroAsian Chemical Market (Evraziyskiy khimicheskiy rynok) http://www.chemmarket.info/
English adaptation by V.I. Shevtsova, T. Gurley and N.Yu. Gorobets
Drying is one of the most common engineering processes. It is used in the chemical, pharmaceutical and food industry. It's hard to find any chemical and pharmaceutical processes where drying is not used. Usually drying is the final step in any material treatment process.
Conventional drying methods utilize a fixed temperature to remove moisture from a material. The transfer of the thermal energy from the heater to the heated substance is realized by means of convection. The penetration of this thermal energy is dependent on the thermal conductivity of the material. While the material dries, the thermal and mass conductivity decrease. As a result, the penetration of the delivered heat is reduced progressively, and water is slower transferred to surface layers, where evaporation occurs. Consequently, the surface layers become overheated and it may even produce a crust, which will need to be mechanically broken up to transfer the dried material to shipping containers.
To increase the effectiveness of the drying process it is necessary to employ various tricks, such as stirring the substance, drying in fluidized bed etc. Normally this requires high power consumption.
There are other methods for heating materials. One of them we use in our kitchen at home. Everyone knows that one can heat up food very fast in a home microwave oven. Can this method be used in industrial plants?
The advantages of the effect of microwaves on dielectric materials are well known. These include:
- High conversion of microwave energy into thermal energy
- Independence on thermal conductivity
- Selectivity
- Self-regulation in the case of drying (see below)
Microwave irradiation can interact with substances, which are in the liquid, solid and gaseous states. When polar liquids and solutions are irradiated, one can observe the absorption of microwave energy. This absorption especially high in the case of water and aqueous solutions. When one irradiates solids with microwaves, the microwaves can be reflected, absorbed, or simply pass through the sample with no absorption occurring. The penetration of microwaves is dependent on the nature of the substance and specifically its dielectric properties.
During the microwave drying process in many cases the material being dried does not absorb or absorb a low level of microwave irradiation but the water molecules associated with it do. Thus the microwave drying process is caused by the property of water to absorb microwave irradiation. Consequently, removing of the water molecules from the drying materials eliminates the heating effect. In such a way the microwave drying appears to be self-regulative process. Finally selective heating of polar molecules of water results in a great time and energy saving.
It is also well known that the boiling point of liquids is lowered at lower pressures. Therefore lower drying temperatures can be utilized if the process is conducted in vacuum. Thus using microwave heating in vacuum is a great approach for effective drying even at low temperatures.
Based on these principles, the scientists and engineers at the Technopark ???Institute of Single Crystals??? have developed microwave/vacuum based technology for high quality drying of a variety of organic and inorganic substances, i.e. aminocapronic acid, cocarboxilase chloride, sodium and lithium iodide, etc. This technology allows significant time and energy saving in comparison with conventional drying method.
For instance the benefits realized in the vacuum/microwave drying of sodium iodide as compared to a conventional drying process includes:
- Reduction in electric power usage by a factor of 5
- Reduction in drying time by a factor of 3
- Final moisture content of the NaI of ?‰¤ 0.005% (0.05% for conventional drying).
The benefits for vacuum/microwave drying of carboxylase chloride is a 6-7 fold reduction in drying time, and for ??·-aminocapronic acid the drying time is 15 times shorter i.e. from 2,9% to 0.19% moisture it is only 34 minutes.
Other potential applications are in the agricultural and pharmaceutical area for drying phytopreparations. Artichokes (Cynara scolymus, Cynara cardunculus), for example, contain thermally unstable bioactive compounds, that are decomposed under conventional drying conditions even under low heat. In this case such phyto-products can be dried at low temperatures to insure proper dryness without degrading their bioactivity.
Based on these results, scientists and engineers at the Technopark ???Institute for Single Crystals??? have developed industrial scale rotary vacuum microwave dryers called ???Pharma-Micro??? that are currently being used by several Ukrainian companies. These units have a control system for automatic loading and unloading of production materials and they can monitor and control both temperature and pressure during the drying process. This technology has been patented in Russia and Ukraine. Currently, the Technopark "Institute for Single Crystals" has begun construction of commercial dryers for chemical and pharmaceutical companies. So this technology is available to any company that would like to benefit from the efficient operation ??“ reduced electrical power usage with high quality production output.
5 April 2006
