When high-quality products are dried, contamination with foreign particles, germs or deposits of matured products must be avoided. The MT mixer-dryer fulfils this requirement.
Dipl.-Ing. Jürgen Oess
For the drying of high-quality products, equipment is required which does not impair or effect the quality of the product. Gentle thermal and mechanical treatment of the product in the dryer ensures a high product quality and also avoids contamination with foreign particles, germs or deposits of matured products. When pharmaceutical products or special products of particularly high value are processed, a batch-operated vacuum dryer is generally preferred, especially a unit which can be sterilised as well as easily and thoroughly cleaned. Due to the high expenditure involved and the small production quantities, multi-purpose dryers are used.
Multi-purpose vacuum dryer
The MT mixer-dryer fulfils the above requirements. It basically comprises a conical vessel, a mixing screw and a mixer. The conical vessel is designed as a vacuum/pressure vessel. The welded-on double shell or half-round coil can be used for heating or cooling of the product. The connections for the product inlet, manhole, vapour and temperature measurement are integral and are normally on the upper part of the vessel. The product outlet is on the side, at the lowest point of the conical vessel, to ease and maximise product discharge.
The mixing screw provides thorough, convective product mixing and reduces the build-up of material deposits on the vessel wall. The screw rotates about its own axis and conveys the material in an upward direction as well as about the conical vessel axis, in order to prevent deposits of material on the vessel wall. The mixer-dryer screw protrudes into the conical vessel from below. Thanks to this design, all drive and bearing parts can be arranged outside of the process area. This rules out any contamination of the product through lubricants for bearing and gearing parts. The process area is freely accessible from above and can therefore be easily inspected and cleaned. At the same time it is possible to insert auxiliary equipment and measuring probes directly into the product without having to stop the screw mixer. The screw can be heated using a liquid heat-transfer medium and superheated steam. Due to the over-mounted design of the screw, the condensation of the superheated steam can be discharged without difficulty. The bearing, drive and sealing of the screw are all inside the mixer, which is flanged onto the conical vessel. Double-acting mechanical seals are preferred for sealing the screw and the rotary drive. Minimal leakage, minimal wear and long life characterise this form of sealing. The part of the screw in contact with the product is of tapered design and fitted with wipers in order to ensure complete discharge, even of products with poor flow properties. The mixing screw and the rotary drive are driven by electric motors via reducing gears. The speed can be altered by means of frequency converters according to the particular product properties.
A discharge valve seals the product outlet vacuum and pressure-tight. The discharge valve is designed as a plunger valve, whereby the sealing action can be either via sealing rings or metal-to-metal.
The equipment can be fitted with appropriate auxiliary equipment corresponding to the type of use. The product temperature can be measured without edge effects by means of a temperature probe which is attached to the upper part of the vessel and projects into the centre of the bulk material. The product temperature is a pointer to the condition of the product and is an optimum control variable for process control. For processing products which form lumps, agglomerate annihilators can be introduced into the dryer from above. By using deodorising fittings on the lower part of the vessel it is possible to feed the carrier gas through the bulk product, which can reduce drying time by up to 50% with inherently moist products. The equipment can be cleaned via fixed CIP connection fittings without having to open the dryer.
Various peripheral equipment is required for operating the MT mixer-dryer. The most important devices are the filling system, the vapour filter, the condenser, the vacuum system, the discharge system, the heating and cooling circuit and the measuring and control equipment.
Dust filters are essential in the vacuum drying system
When drying free-flowing bulk material, fine material is entrained by the vapour flow. In order to avoid loss of material and contamination of equipment connected down-stream, vapour filters are used to separate the solid particles from the vapour flow.
Textile fabrics, plastic diaphragms or sintered metal fabrics are used as filter material. Filter elements made of sintered multi-layered fabric or metal-fibre fabric are particularly suitable for the production of high-quality products, as these materials are practically wear-free. There is consequently no contamination of the product as a result of fibre wear. The filter elements are cleaned by means of an air blast. The solids that are removed drop straight back into the dryer.
Integrated vapour filter
Conventional vapour filters comprise heat-able, cylindrical housings with various connections, mounted on the dryer cover. With the MT mixer-dryer the vapour filter is integrated in the dryer cover. Owing to the conical shape of the dryer cover, it is possible to insert filter elements horizontally into the equipment from the outside. A separate filter housing is not required.
Thanks to this design, the installation height of the drying system and the area required to fit the filter elements are reduced considerably. There is no need for additional heating-agent piping to the filter. Inspection win-dows on the dryer cover permit the filter element to be observed, also during the process. All the process steps in the dryer and filter area can be monitored from the outside. The filter elements can be inspected directly without removing flanges through the inspection window on the dryer. Due to the conical design of the cover, it is also possible to obtain a direct view of the cover bottom with all the connections and fittings.
The MT mixer-dryer is a batch-operated vacuum contact dryer, which has been specially optimised for processing high-quality products. The drying process takes place without impairing the product quality (purity, particle shape and particle size). The possibility of contamination with foreign particles, germs and material deposits from previous batches is avoided. Thanks to the geometrical shape, complete emptying and cleaning is possible. These features mean that the MT mixer-dryer is also suitable for discontinuous drying of high-quality products. It has proved equally successful for drying processes with a low foreign-particle content, for pharmaceuticals that are parentally administered (through the injection of an infusion).
Further information cpp-248