Mass liquid-solid systems drying

Baths have been used to apply ultrasound during the osmotic dehydration of apples in sugar solutions (Simal et al, 1998, 2006), cheese (Sanchez et al., 1999) or meat brining (Simal et al., 2006), and also to study its effects on mass transport kinetics. Ultrasound has also been applied in liquid-solid systems as a pretreatment prior to osmotic dehydration or hot-air drying of products such as banana (Fernandes and Rodrigues, 2007), pineapple (Fernandes et al., 2008) or malay apple (Oliveira et al., 2011). 

Semidry Scrubbers The advantage of semidry scrubbers is in that they remove contaminants by way of a solid waste that is easier to dispose of (less expensive). Initially, the scrubbing medium is wet (such as a lime or soda ash slurry). Then a spray dryer is used to atomize the slurry into the gas which evaporates the water in the droplets. As this takes place, the acid in the gas neutralizes the alkali material and forms a fine white solid. Most of the white solids are removed at the bottom of the scrubber while some are carried into the gas stream and have to be removed by a filter or electrostatic precipitator (discussed later). Although semidry systems cost 5-15% more than wet systems, when combined with a fabric filter, they can achieve 90-95% efficiencies. Dry scrubbers are sometimes used in a very similar fashion, but without the help of gas-liquid-solid mass transfer, these systems use much higher amounts of the solid alkali materials. 

The reactions are of either liquid-gas (wet) or solid-gas (dry) type. Dry processes show high reaction rates enabling compactness, but a poor heat and mass transfer behavior compared with wet processes [19]. The examples of the 2-propanol - acetone system [44, 56], the cyclohexane-benzene system [31], the hydride system [30], and the sponge iron system [54] are given in more detail in appendix C. 

One major disadvantage of the dry injection system is that without the aid of gas-liquid-solid mass transfer, as in the wet or semidry scrubbing systems, addition of alkali at a ratio much higher than stoichiometric is required for an equivalent removal. 

Gas—solids fluidization is the levitation of a bed of solid particles by a gas. Intense soflds mixing and good gas—soflds contact create an isothermal system having good mass transfer (qv). The gas-fluidized bed is ideal for many chemical reactions, drying (qv), mixing, and heat-transfer appHcations. Soflds can also be fluidized by a Hquid or by gas and Hquid combined. Liquid and gas—Hquid fluidization appHcations are growing in number, but gas—soHds fluidization appHcations dominate the fluidization field. This article discusses gas—soHds fluidization. 

An insoluble system is the first example—water placed on/in microcrystalline cellulose. Water present with insoluble materials is solute free. It behaves as associated liquid close to the solid surface. As more layers of water are added less and less surface association occurs until at the outer layers water behaves as unassociated free water. Water in capillaries can be considered bound with more heat needed than even associated water to remove water from capillaries. Water thus remains in location after drying in a very predictable manner based on drying temperature and drying duration. As moisture remains solute free, the mass transferring during drying is water only. 

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