Constant-rate drying

It can be seen in Figure 14.9 that, for oven drying, constant rate drying extends from a moisture content of 32.0 to 26.5% (by weight), characterizing the removal of moisture from the surface layers. The... 

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

Because many pesticides are appHed to the soil surface, the transport of pesticide during water infiltration is important. Water infiltration is characterized by high initial infiltration rates which decrease rapidly to a nearly constant rate. Dry soils have greater rates of infiltration than wet soils during the initial appHcation of water. Thus, perfluridone movement after appHcation of 3.8 cm of water was considerably greater in soil at a water content of <1% of field capacity than at 50% of field capacity (62). Fluometuron moved deeper into the soil in response to greater rainfall intensity or after rainfall onto a dry rather than a moist soil (63). 

Constant rate period is the drying period during which the Hquid vaporization rate remains constant per unit of drying surface. 

Capillarity. The outer surface of porous material has pore entrances of various sizes. As surface Hquid is evaporated during constant rate drying, a meniscus forms across each pore entrance and interfacial forces are set up between the Hquid and material. These forces may draw Hquid from the interior to the surface. The tendency of Hquid to rise in porous material is caused pardy by Hquid surface tension. Surface tension is defined as the work needed to increase a Hquid s surface area by one square meter and has the units J/m. The pressure increase caused by surface tension is related to pore size ... 

The principal appHcations of pneumatic conveyors are for materials that are nonsticky and readily dispersible in the gas stream as drying must be entirely constant rate. Many are employed as predryers ahead of longer residence time fluid-bed and rotary dryers in polymer drying operations. 

Constant-rate Period In the constant-rate period moisture movement within the sohd is rapid enough to maintain a saturated condition at the surface, and the rate of diying is controlled by the rate of heat transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the saturated surface of the material across a... 

Values of critical moisture contents for some representative materials are given in Table 12-7 for diying by cross circulation and in Table 12-15 for diying by through circulation. The tabulated values are only approximate, since critic moisture content depends on the drying histoiy. It appears that the constant-rate period ends when the moisture content at the surface reaches a specific value. Since the critical... 

Estimates of both the constant-rate and the falling-rate periods are needed to estimate the total drying time for a given diying operation. If estimates for these periods are available, the total diying time is estimated by summing as... 

The presence of the electrolyte is required to provide a pathway for the current and, in urban areas, this is commonly ironfll) sulfate formed as a result of attack by atmospheric SO2 but, in seaside areas, airborne particles of salt are important. Because of its electrochemical nature, rusting may continue for long periods at a more or less constant rate, in contrast to the formation of an anhydrous oxide coating which under dry conditions slows down rapidly as the coating thickens. 

During the first period of drying, the liquid that covers the particle external surface and is present in the macropores evaporates. The material structure does not affect the rate of evaporation. The liquid evaporates with the rate at which heat is supplied to the surface. The rate of drying is thus limited by heat transfer between the particles and their surroundings. The temperature at the particle surface remains constant. If heat is delivered by convection this temperature is the wet-bulb gas temperature. In case of radiation (e.g. microwave driers) or conduction (e.g. indirect contact driers) the surface temperature ranges between the wet-bulb gas temperature and the boiling point of the liquid. The moisture content at the end of the constant rate of drying period is called the critical moisture content. 

The critical moisture content is a complex function of material properties, particle size (the critical moisture content is higher for large granules), and rate of drying during the constant rate period (the lower the rate, the less the critical moisture content). Therefore, the critical moisture content is difficult to predict and should be determined experimentally for the selected drier and conditions of drying. 

The rate of drying during the constant rate period can be expressed by the following equation ... 

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