Shelf driers

Versatile driers that are easy to clean are preferred in MPPs for fine chemicals. Therefore, shelf driers that can also be operated under vacuum are still in common use. They are easy to clean and may be used for a variety of products. Shelf driers, however, have poor heat- and... 

Fluidized-bed driers are also widely used due to their large heat- and mass-transfer coefficients. However, materials of even moderate adherence and cohesiveness cannot be dried in a fluid bed. The same applies to materials that are sensitive to oxygen, especially at elevated temperatures. Vacuum drying is often necessary for oxygen sensitive materials and this is not easy to realize in fluid-bed driers, although there are systems to deal with this problem. Fluid-bed driers are not as easy to clean as shelf driers or rotary driers. 

The tetryl is then dried for 24 hr in a shelf drier at 55-60°C. This period, however, may be reduced to 12 hr if a vacuum drier is employed. 

The polished powder is dried at 50°C to 0.5-1.0% moisture content (in countries with a damp climate the permissible moisture content is higher and in countries with a dry climate—lower). Shelf driers with a natural stream of warm air are usually employed. The use of a pressurized air stream is permissible provided that it is... 

Cotton or wood pulp as delivered to nitrocellulose plants contains 6% or more of moisture. This moisture should be removed prior to nitration so that it does not bring about any dilution of the mixed acid and thereby change the course of the reaction. Cotton is usually transported in the form of compressed bales, which are opened and shredded before drying. Wood cellulose, in the form of pressed cellulose pulp or of crepe paper, is dealt with similarly. Dryers for dehydrating cotton or cellulose may be of various types. The simplest is the shelf drier, equipped with mechanical ventilation and heaters situated at the bottom, supplied with water or steam so as to maintain a temperature of 80-100°C inside the drier. 

Then the reaction mixture is cooled while stirring, the precipitated product is filtered off on a stoneware filter, washed with water until neutral and dried in a shelf drier at 50°C for 12-16 hr. The product is obtained in almost theoretical yield. 

The construction of such driers depends greatly upon the nature of the substance being dried. Tray driers, also called cabinet, compartment, or shelf driers, are used for drying solids which must be supported on trays, e.g., pasty materials such as wet filter cakes from filter presses, lumpy solids which must be spread upon trays, and similar materials. A typical device, shown schematically in Fig. 12.6, consists of a cabinet containing removable trays on which the solid to be... 

Drier, shelf, vacuum heat surface area, 1-100 0.53... 

Instantisation, in which the fluidized bed is responsible for a significant change to the structure and size of the granules, should not be confused with the use of an integrated fluidized bed drier immediately following spray drying. There are many descriptions in the literature of such processes a recent example of a novel application of this technique (Jha et al, 2002) is in the production of a shelf-stable powdered kheer mix, an Indian dessert which consists of a sweetened mixture of cooked partially concentrated milk and rice flour. This is spray dried, the particles are then dried in a fluidized bed and finally dry blended with sugar. 

The samples (ca. 2500 g for each sample) were spray dried in a Niro Utility drier with the inlet temperature at 200 C and outlet at 100 C. The drier temperatures were allowed to stabilize before samples were collected for analysis. The dried samples were analyzed for total oil, surface oil, moisture, emulsion size and emulsion stability. Samples were also stored at an elevated temperature for shelf-life determination. Sensory analysis of rehydrated powder from the coarse and Microfluidized emulsions was performed to determine if differences in emulsion size affects the perceived flavor intensity. 

Static solids bed Heated shelf tray drier... 

It must be emphasised that the actual numerical values used in the construction of Figure 6 are hypothetical and refer to a particular set of conditions for product composition, vial type, fill depth, model of freeze-drier and degree of shelf loading. However, similar diagrams could be constructed for any freeze-drier/vial/loading combination. Furthermore, Figure 6 takes no account of discontinuities in the thermophysical properties of the product at Fg. This discussion nevertheless serves to illustrate the general principle of interplay between shelf... 

Having estimated the optimum primary drying parameters, a pilot study should be performed. It is not good practice to carry out such a study either in an incompletely filled drier or by filling up shelves partly with empty vials. The reliability of pressure and temperature measurements should be checked at this stage. It has sometimes been found that the (apparent) product temperature exceeds the shelf temperature. Since this is an obvious physical impossibility, such an anomaly is due to faulty calibration of the thermocouple probe. 

Spillages during dispensing are often overlooked since liquid product is invariably transparent. However any spilled liquid on the freeze-drier shelf will dry to form a thin, friable layer of powder which will be readily disseminated from the chamber when the vacuum is released and the product batch removed. 

Vials loaded into the freeze-drier in trays with removable base plates, which enable direct vial to shelf contact, and which may fracture when the base plates are relocated into the trays after drying. 

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