Moisture content critical

Despite these fundamental insight, today it is not possible to predict the behavior of the critical moisture content without experiments. 

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Critical moisture content is that obtained when the constant rate period ends and the falling rate periods begin. Second critical moisture content specifies that remaining in a porous material when capillary flow dominance is replaced by vapor diffusion. 

Particle size distribution determines surface-to-mass ratios and the distance internal moisture must travel to reach the surface. Large pieces thus have higher critical moisture contents than fine particles of the same material dried under the same conditions. Pneumatic-conveyor flash dryers work because very fine particles are produced during initial dispersion and these have low critical moisture contents. 

Critical moisture content is the average moisture content when the constant-rate period ends. 

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... 

TABLE 12-7 Approximate Critical Moisture Contents Obtained on the Air Drying of Various Materials/ Expressed as Percentage Water on the Dry Basis... 

Design Methods for Vactium-Shelf Dryers Heat is transferred to the wet material by conduction through the shelf and bottom of the tray and by radiation from the shelf above. The critical moisture content will not be necessarily the same as for atmospheric tray drying [Ernst, Ridgway, and Tiller, Jnd. Eng. Chem., 30, 1122 (1938)]. 

For materials of moderate to low porosity, a good starting vacuum level is 0.6 to 0.7 bar (18 to 21 in Hg), as the capacity of most vacuum pumps starts to fall off rapidly at vacuum levels higher than 0.67 bar (20 in Hg). Unless there is a critical moisture content which requires the use of higher vacuums, or unless the deposited cake is so impervious that the air rate is extremely low, process economics will favor operation at vacuums below this level. When test work is carried out at an elevation above sea level different than that of the plant, the elevation at the plant should be taken into account when determining the vacuum system capacity for high vacuum levels (>0.5 bar). 

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...

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. 

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. 

During the period of drying from the initial moisture content w i to the critical moisture content wc, the rate of drying is constant, and the time of drying /,. is given by ... 

Madhiyanon, T. and Soponronnarit, S., High temperature spouted bed paddy drying with varied downcomer air flows and moisture contents effects on drying kinetics, critical moisture content, and milling quality. Drying Tech., 23 (2005) 473-495. 

An approximation that may be justifiable is that the critical moisture content is roughly independent of the drying conditions and that the falling rate curve is linear. Then the rate equations may be written... 

The critical moisture content is assumed indpendent of the drying rate. Accordingly, under the proposed operating conditions, the rate of drying will be... 

The drying rate equations above and below the critical moisture content of 0.58 are... 

Agricultural products are often subjected to dehydration to improve storage stability grains, and seeds are typical examples. Critical moisture contents must be achieved so that these products can be stored safely. This is closely related to the economic value of crops... 

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