DRY - Drying of a Solid

A thin slab of solid material dries first by evaporation from the top surface and then by diffusion from the interior of the solid. The water movement is approximated by the diffusion equation 

Water is removed from the top surface where X = 0. Drying rate 

At the instant that the depth of the water layer is reduced to zero, the drying of the solid begins. The solid is considered as 10 finite difference elements, each of length AX. 

The drying rate is obtained by approximating the concentration gradient at the surface 

Total water removed in both stages of drying 

Cn represents the concentration at the midpoint of element n, and the gradient dC/dX is approximated by AC/AX. 

5 Simulation Tools and Examples of Chemical Engineering Processes 

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The use of a ternary mixture in the drying of a liquid (ethyl alcohol) has been described in Section 1,5 the following is an example of its application to the drying of a solid. Laevulose (fructose) is dissolved in warm absolute ethyl alcohol, benzene is added, and the mixture is fractionated. A ternary mixture, alcohol-benzene-water, b.p. 64°, distils first, and then the binary mixture, benzene-alcohol, b.p. 68-3°. The residual, dry alcoholic solution is partially distilled and the concentrated solution is allowed to crystallise the anhydrous sugar separates. 

The study of internal moisture gradients estabhshes the particiilar mechanism which controls during the drying of a solid. The experimental determination of rehable moisture gradients is extremely difficult. 

Unsteady state diffusion processes are of considerable importance in chemical engineering problems such as the rate of drying of a solid (H14), the rate of absorption or desorption from a liquid, and the rate of diffusion into or out of a catalyst pellet. Most of these problems are attacked by means of Fick s second law [Eq. (52)] even though the latter may not be strictly applicable as mentioned previously, these problems may generally be solved simply by looking up the solution to the analogous heat-conduction problem in Carslaw and Jaeger (C2). Hence not much space is devoted to these problems here. 

Moreover, the isotherms determine the proper storage environment and the packaging conditions, especially for foods. Through the isotherms, the isosteric heat of sorption can be determined and, hence an accurate prediction can be made of the energy requirements for the drying of a solid. The utility of the isotherm is extended to the determination of the moisture sorption mechanism as well as to the degree of bound water. 

Simprosys is applied to the drying of a solid material containing ethanol. Due to the low flash (burning) point,... 

Under otherwise similar drying conditions, the drying rate depends on the surface area of the wet crystals and the air used in drying. This means that small crystals dry quickly in airflow, such as that produced by a pneumatic dryer. Instead, the drying of a solid bed of... 

Solubility in concentrated sulphuric acid. Place 3 0 ml. of pure concentrated sulphuric acid in a dry test-tube and add 0 -10 g. of a solid or 0 -20 ml. of a liquid. If the compound does not dissolve immediately, agitate for some time but do not heat. Observe any change in colour, charring, evolution of gaseous products, polymerisation accompanied by precipitation etc. 

While the rotary dryer shown is commonly used for grains and minerals, this system has been successfully applied to fluid-bed diying of plastic pellets, air-hft diying of wood fibers, and spray drying of milk solids. The air may be steam-heated as shown or heated By direct combustion of fuel, provided that a representative measurement of inlet air temperature can be made. If it cannot, then evaporative load can be inferred from a measurement of fuel flow, replacing AT in the set point calculation. 

Equilibrium moisture content of a solid is particularly important in drying because it represents the limiting moisture content for given conditions of humidity and temperature. If the material is dried to a moisture content less than it normally possesses in equilibrium with atmospheric air, it will return to its equilibrium value on storage unless special precautions are taken. 

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 dry gas has been made by direct reduction of a solid mixture of nitrite and nitrate with chromium(III) oxide (3KNO2 4- KNO3 4-Cr203 —4NO 4- 2K2Cr04) but is now more conveniently obtained from a cylinder. 

Zinc(II) 1,8,15.22-tetraazaphthalocyanine (116 mg, 0.2 mmol) was suspended in anhyd DMF (60 mL) and dimethyl sulfate (7.65 mL, 0.08 mol) was added. The mixture was healed to 120 C with stirring under argon for 12 h. A ten-fold quantity of acetone was added resulting in the precipitation of a solid. The crude product was treated with anhyd acetone in a Soxhlet apparatus and dried in vacuo at 60 C over CaCl2 yield 61 mg (30%). 

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