Solid critical moisture content

Total moisture content, kg water/kg dry solid Critical moisture content, kg water/kg dry solid Tree moisture content, kg water/kg dry solid Equilibrium moisture content, kg water/kg dry solid Absolute air humidity, kg water vapor/kg dry air... 

Figure 4.27 Transient drying rates during drying Xi, initial moisture content of wet solids, Xo, final moisture content, Xc, critical moisture content of wet solids and X, equilibrium moisture content of solids...

A wet solid is dried from 35 to 10 per cent moisture under constant drying conditions in 18 ks (5 h). If the equilibrium moisture content is 4 per cent and the critical moisture content is 14 per cent, how long will it take to dry to 6 per cent moisture under the same conditions ... 

Mg of dry mass of a non-porous solid is dried under constant drying conditions in an air stream flowing at 0.75 m/s. The area of surface drying is 55 m2. If the initial rate of drying is 0.3 g/m2s, how long will it take to dry the material from 0.15 to 0.025 kg water/kg dry solid The critical moisture content of the material may be taken as 0.125 kg water/kg dry solid. If the air velocity were increased to 4.0 m/s, what would be the anticipated saving in time if the process were surface-evaporation controlled ... 

A 100 kg batch of granular solids containing 30 per cent of moisture is to be dried in a tray dryer to 15.5 per cent moisture by passing a current of air at 350 K tangentially across its surface at the velocity of 1.8 m/s. If the constant rate of drying under these conditions is 0.7 g/sm2 and the critical moisture content is 15 per cent, calculate the approximate drying time. It may be assumed that the drying surface is 0.03 m2/kg dry mass. 

Toei et al. (T5, T6) believe that when the moisture content on the surface becomes less than the critical moisture content, the first falling-rate period starts and the evaporation occurs at the interior of the solid. The second falling-rate period starts when the moisture content at the surface reaches the equilibrium value. The evaporating plane retreats into the solid and dried-up zone begins to grow from the surface into the solid. The dried zone retains the equilibrium moisture content. 

For any material, the critical moisture content decreases as the particle size decreases. Eventually, moisture ceases to reach the surface which becomes dry. The plane of evaporation recedes into the solid, the vapor reaching the surface by diffusion through the pores of the bed. This section is called the second falling-rate period and is controlled by vapor diffusion, a factor which is largely independent of the conditions outside the bed but markedly affected by the particle... 

Yoshioka S, Cartensen JL. Nonlinear estimation of kinetic parameters for solid-state hydrolysis of water-soluble drugs. Part 2. Rational presentation mode below the critical moisture content. J Pharm Sci 1990 79 799-801. 

The critical moisture content is thus identified as the average moisture content of the solid at the instant the first increment of dry area appears on the surface of solid. 

The critical moisture content depends upon the ease of moisture movement through the solid, and hence, upon the pore structure of the solid, sample thickness and (hying rate. Segment BC is the constant-rate peri(xl. During this pericxl, the drying is controlled simultaneously by heat and mass transfer applied to a liquid-gas interface in dynamic equilibrium with a bulk gas phase. 

Critical moisture content—the average moisture in the solids when the constant rate drying period ends. 

To examine the developed model, 3D numerical simulations of silica slurry dryiug iu the adopted spray chamber have beeu carried out. The slurry consists of amorphous silica spherical particles dispersed homogeneously in water with initial average moisture content of 1.35 kg H20/kg dry solid. The size of silica particles is 272 mn and the density is 1950 kg/m [40]. The critical moisture content calculated using Equation 10.6 is equal to 0.342 kg HjO/kg solid, and the final moisture content of dried particles is assumed to be 0.05 kg H20/kg solid. 

A small batch experiment was performed using constant drying conditions, air velocity, and temperature of the solid approximately the same as in the continuous dryer. The equilibrium critical moisture content was found to be Xc = 0.0959 lb HjO/lb dry solid, and the experimental value of/Cj, Mg was found as 30.15 lb air/h-ft. In the falling-rate period, the drying rate was directly proportional to X. 

The mechanism of moisture-induced relaxation remains an open question. One hypothesis is that an amorphous solid contains a finite amount of hydrophilic sites that become saturated at some critical moismre content. Above this moisture content value, adsorbed/absorbed water molecules have a greater molecular mobility. Thus, water exhibits different type of impact on the solid dispersion below and above this critical moisture content. This water-binding-site saturation hypothesis is supported by calorimetric measurements which clearly show that the heat of water sorption approaches the enthalpy of condensation of water above RH threshold (Miller and Lechuga-Ballesteros 2006). 

Material Critical moisture content (kg water/kg dry solid)... 

When the average moisture content of the solid has reached a value X, the critical moisture content (Fig. 12.10), the surface film of moisture has been so reduced by evaporation that further drying causes dry spots to appear upon the surface these spots occupy increasingly larger proportions of the exposed surface as diying proceeds. Since, however, the rate is computed by means of the constant gross surface A, the value of N must fall even though the rate per... 

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