738 Fermentation and Biochemical Engineering Handbook If one were to measure the temperature of the bed or individual particles of wet solids at this point, thetempera ture would be the wet cake temperature. After the free moisture has evaporated, the cake temperature rises, an indication of the end of the constant rate period. Several stages of this period can occur due to the existence of bound moisture. If bound moisture exists, the energy stream as heat input. As the bonds break, the bound moisture is released and is removed as surface moisture described above. The quantity of molecules of hydration and the temperature which the product must reach in order to break these bonds affect the overall constant drying rate period. One can generally observe a rise in the wet temperature, after the free moisture has evaporated temperature which is required to break the bonds. Thi emperature then becomes the next wet bulb level or isotherm. Several levels of bound moisture may exist during the drying process. (Note: In general, this bound moisture phenomenon occurs mostly with inorganic sal and therefore may not be a major concern of the pharma- ceutical or biochemical industry. 3. Diffusion, or falling rate periodthis is the stage where the rate at which the liquid leaves the solid decreases. The liquid which is trapped inside the particles diffuses to the outside surface of the particle through capillary action The random path which the liquid must take slows down the drying process at this stage. (See Fig 9 for typical graph. 5.0 FUNDAMENTAL ASPECTS OF DRYER SELECTION The starting point in determining how to dry certain products is first to ascertain whether the process will be a batch or continuous operation If the product is manufactured in relatively small quantities and identification of particular size lots is required, than batch mode is usually the route taken. Full accountability may be achieved when batch processing with the proper controls and procedures in place. The full batch of material to be dried must be enclosed in the dryer, a necessity for the equipment should be that the product dries uniformly738 Fermentation and Biochemical Engineering Handbook If one were to measure the temperature of the bed or individual particles ofwet solids at this point, the tempera￾ture would be the wet cake temperature. After the free moisture has evaporated, the cake temperature rises, an indication of the end of the constant rate period. Several stages of this period can occur due to the existence of bound moisture. If bound moisture exists, the energy required to break the bonds is absorbed from the gas stream as heat input. As the bonds break, the bound moisture is released and is removed as surface moisture described above. The quantity of molecules of hydration and the temperature which the product must reach in order to break these bonds affect the overall constant drying rate period. One can generally observe a rise in the wet cake temperature, after the free moisture has evaporated, to the temperature which is required to break the bonds. This temperature then becomes the next wet bulb level or isotherm. Several levels of bound moisture may exist during the drying process. (Note: In general, this bound moisture phenomenon occurs mostly with inorganic salts and therefore may not be a major concern of the pharma￾ceutical or biochemical industry.) 3. Diffusion, or falling rate period-this is the stage where the rate at which the liquid leaves the solid decreases. The liquid which is trapped inside the particles diffuses to the outside surface of the particle through capillary action. The random path which the liquid must take slows down the drying process at this stage. (See Fig. 9 for typical graph.) 5.0 FUNDAMENTAL ASPECTS OF DRYER SELECTION The starting point in determining how to dry certain products is first to ascertain whether the process will be a batch or continuous operation. If the product is manufactured in relatively small quantities and identification ofparticular size lots is required, than batch mode is usually the routetaken. Full accountability may be achieved when batch processing with the proper controls and procedures in place. The full batch of material to be dried must be enclosed in the dryer. A necessity for the equipment should be that the product dries uniformly