Drying process from solids

The drying process from solids has been characterized by three drying regions, as shown in Fig. 

The purpose of this chapter is to review various forms of solids dryers and auxiliary components. It is intended to be a practical guide to dryer selection (as opposed to the theory of (hying, which is addressed in various technical manuals referenced in the bibliography). From a microscopic viewpoint, the process is simple water or solvent basically evaporates leaving the solid behind. When viewed macroscopically, it is apparent that the drying process is extremely complicated with many interdependent forces that combine in various dryers to achieve the end result. The information in this article can also help the reader become more familiar with the drying process from beginning to end. 

Drying refers to the removal of water from a substance through a whole range of processes, including distillation, evaporation, and even physical separations such as with centrifuges. Here, consideration is restricted to the removal of moisture from solids and liquids into a gas stream (usually air) by heat, namely, thermal drying. Some of the types of equipment for removal of water also can be used for removal of organic liquids from solids. 

Freeze-drying, like all drying processes, is a method to separate liquid water from a wet solid product or from a solution or dispersion of given concentration. However, the main difference is that the liquid water is separated by solidification (i.e., the formation of ice crystals) and subsequent vacuum sublimation instead of evaporation. This allows a drying at subzero temperatures which can be advantageous in case of heat-sensitive products. There are two general applications... 

Freeze drying or lyophilization is a drying process, in which the solvent and/or the medium of suspension is crystallized at low temperatures and thereafter sublimated from the solid state directly into the vapor phase. 

The spray drying process can be thought of as a method of preserving in the solid state the equilibrium between the two forms of dextrose which prevails in solution. The composition of the solid does not agree strictly with that of the solution, undoubtedly because some fractional crystallization takes place before all of the water has been removed from the solid. 

On the basis of proteinate yields (Table II) and their protein contents (Table I), the recoveries of protein during wet processing were about 73% for both legumes, which was only slightly below the efficiency of the dry process. However, the losses of starch in the whey and wash solids were substantial, and starch recoveries averaged 77.5%. The yields of refined fiber were about 8% of the raw materials. Almost 30% of the dry matter from wet processing would have to be recovered from whey and wash extracts to make the process economical. 

Solids can also be dried at room temperature on filter paper or porous tile. You should protect the substance from dirt and dust by covering with filter paper or a funnel. A vacuum desiccator will greatly speed up the drying process, and should be used on products that are destroyed by the small amount of water in the atmosphere. A vacuum desiccator is shown in Figure 4. 

The slow warm-up is accomplished conveniently by the removal of the solid dry ice from the cold bath. This process minimizes the formation of acetylenic by-products. The warm-up period can vary with the amount of coolant used. We have consistently obtained good results using sufficient coolant to match the liquid level of the reaction mixture. 

The polyimide is formed by the thermal polycyclocondensation of the poly(amide acid). For this purpose, 5 ml of poly(amide acid) solution are placed on a watch glass (diameter 10 cm) and kept in a vacuum oven at 50 °C for 24 h.The solvent evaporates and at the same time cyclization to the polyimide takes place the resulting film is insoluble in dimethylformamide.The formation of the polyimide can be followed by IR spectroscopy the NH-band at 3250 cm disappears while imide bands appear at 1775 and 720 cm" Once the initial drying process has raised the solid content to 65-75%, the polyimide formation can be accelerated by heating the poly(amide acid) film to 300 °C in a vacuum oven for about 45 min.The polyimide made from pyromellitic dianhydride and 4,4 -oxydianiline exhibits long-term stability in air above 200 °C. 

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