Drying mass transfer process

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

For drying, the typical parallel heat and mass transfer process, the feature of impinging streams that significantly enhances transfer, should of course be fully utilized. 

Besides fluid mechanics, thermal processes also include mass transfer processes (e.g. absorption or desorption of a gas in a liquid, extraction between two liquid phases, dissolution of solids in liquids) and/or heat transfer processes (energy uptake, cooling, heating, drying). In the case of thermal separation processes, such as distillation, rectification, extraction, and so on, mass transfer between the respective phases is subject to thermodynamic laws (phase equilibria) which are obviously not scale dependent. Therefore, one should not be surprised if there are no scale-up rules for the pure rectification process, unless the hydrodynamics of the mass transfer in plate and packed columns are under consideration. If a separation operation (e.g. drying of hygroscopic materials, electrophoresis, etc.) involves simultaneous mass and heat transfer, both of which are scale-dependent, the scale-up is particularly difficult because these two processes obey different laws. 

Drying is the process by which volatile materials, usually water, are evaporated from a material to yield a sohd product. Drying is a heat-ancf mass-transfer process. Heat is necessary to evaporate water. The latent heat of vaporization of water is about 2500 J/g, which means that the drying process requires a significant amount of energy. Simultaneously, the evaporating material must leave the drying material by diffusion and/or convection. 

In fluidised beds the particles are mixed up by a flowing fluid and kept in suspension. They then have properties similar to that of a fluid. Chemical reactions, drying or other mass transfer processes take place rapidly in fluidised beds as a result of the brisk movement of the particles. 

Dehydration is basically a simultaneous heat and mass transfer operation as explained by Van Arsdel (1963). In applications where rates of drying are low, such as in IM meats, consideration of heat transfer alone is a quite satisfactory approach, and often a preferred one. However, in order to obtain a true understanding of drying and to develop a sound fundamental theory, one must better understand the mass transfer process, both internally and externally (Lubuza, 1976). In addition, one must develop a better... 

Drying is also a mass transfer process. In the vapor zone there is no driving force to evaporate solvent — because the driving force is a concentration difference, and there isn t any either. 

In recent years, several modified versions of the two-phase model were proposed for modeling fluidized bed reactors. They include a model proposed by Werther (1980) for catalytic oxidation of ammonia, in which the mass transfer process is expressed in terms of film theory, as described in Danckwerts (1970) a model proposed by Werther and Schoessler (1986) for catalytic reactions a model proposed by Borodulya et al. (1995) for the combustion of low-grade fuels a model proposed by Arnaldos et al. (1998) for vacuum drying and a model proposed by Srinivasan et al. (1998) for combustion of gases. The modifications include the consideration of axial mass transfer profile, the inclusion of a wake phase in addition to the bubble and emulsion phases, and the consideration of the growth of bubbles in the bubble phase. 

The conveyor dryer is conceptually very simple. Product is carried through the dryer on conveyors and hot air is forced through the bed of product. It is often described as simply a conveyor in a box with hot air. The reality, however, is that the conveyor dryer is one of the most versatile dryers available. Few drying technologies can match the conveyor dryer s ability to handle such a wide range of products. Products as varied in composition, shape, and size as coated breakfast cereals, nuts, animal feed, charcoal briquettes, and rubber can be dried in a conveyor dryer. Although it is simple in concept, an improper understanding of the heat and mass transfer processes in the conveyor dryer will surely lead to poor product... 

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