Drying equilibrium moisture

Equilibrium moisture content is the limiting moisture to which a given material can be dried under specific conditions of air temperature and humidity. 

The diffusion equation for the falling-rate drying period for a slab can be derived from the diffusion equation if one assumes that the surface is diy or at an equilibrium moisture content and that the initial moisture distribution is uniform. For these conditions, the following equation is obtained ... 

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. 

The value of equilibrium moisture content, for many materials, depends on the direction in which equilibrium is approached. A different value is reached when a wet material loses moisture by desorption, as in drying, from that obtained when a diy material gains it by adsorption. For diying calculations the desorption values are preferred. In the general case, the equilibrum moisture content reached by losing moisture is higher than tnat reached by adsorbing it. 

Strength increase (because the cellulose fibrils pack more closely). To prevent movement, wood should be dried to the value which is in equilibrium with the humidity where it will be used. In a centrally heated house (20°C, 65% humidity), for example, the equilibrium moisture content is 12%. Wood shows ordinary thermal expansion, of... 

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...

The fact that the equilibrium moisture content may be considerable at low humidities is of especial importance in the oven methods. Under ideal conditions no water vapor should be present in the oven, but this is impossible to attain in practice. It is difficult to maintain a dry atmosphere in an air oven, and most commercial vacuum ovens are not air-tight. Thus, the discrepancies in results of different investigators have frequently been traced to different humidities in their ovens. Any attempt to reduce the relative humidity by increasing the oven temperature introduces the danger of error from thermal decomposition. 

A sample dried at 110° C. was remoistened and then redried at a low temperature and a controlled low humidity to determine its equilibrium moisture content (regain). This value was compared with the regain of a remoistened control sample which was dried originally at a low temperature where decomposition was negligible (as proved by the... 

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 ... 

A wet solid is dried from 40 to 8 per cent moisture in 20 ks. If the critical and the equilibrium moisture contents are 15 and 4 per cent respectively, how long will it take to dry the solid to 5 per cent moisture under the some drying conditions All moisture contents are on a dry basis. 

An example of typical equipment used to make PET fibers is shown in this figure. Prior to melting, the polymer chips must be thoroughly dried, typically under vacuum at elevated temperatures for several hours. The equilibrium moisture content of PET under ambient conditions is about 0.4 % [3] and this much moisture would cause unacceptable levels of degradation. The dried polymer chips are fed to an extruder which melts the chips and forwards them to a... 

The moisture content of a material is usually expressed in terms of its water content as a percentage of the mass of the dry material, though moisture content is sometimes expressed on a wet basis, as in Example 16.3. If a material is exposed to air at a given temperature and humidity, the material will either lose or gain water until an equilibrium condition is established. This equilibrium moisture content varies widely with the moisture content and the temperature of the air, as shown in Figure 16.1. A non-porous insoluble solid, such as sand or china clay, has an equilibrium moisture content approaching zero... 

Wood is a hygroscopic material, due to the fact that the cell wall polymers contain hydroxyl groups. In an environment containing moisture, dry wood will absorb moisture until it is in equilibrium with the surrounding atmosphere. Similarly, saturated wood, when placed in an atmosphere of lower relative humidity (RH), will lose moisture until equilibrium is attained. If the wood is placed in an environment where the RH is stable, it will attain a constant moisture content (MC), known as the equilibrium moisture content (EMC). At this point, the flux of water molecules into the cell wall is exactly balanced by the outward flux into the atmosphere. 

Reay and Allen (1982) assumed that the drying rate is dependent upon the capacity of fhe gas in the dense phase to absorb moisture and therefore that the drying rate is proportional to the difference between the partial pressure of wafer vapour in equilibrium wifh a moisture content X and the partial pressure p in the inlet gas which is in equilibrium with the equilibrium moisture content of the particles X. Therefore the drying rate at a moisture content X is given by... 

Relative humidity method. Water absorption is defined as the water absorbed by a dried protein powder with equilibration against water vapor at a known relative humidity. This method, also known as the equilibrium moisture content (EMC) method, was described first by Mellon et al. (10). Huffman et al. (11) used... 

Figure 22 is a plot of the initial tensile modulus of the epoxy matrix after equilibrium moisture exposure and dehydration. At both 20 °C and 70 °C, the effect of moisture absorption on the matrix is reversible as evidenced by the reattainment of dry properties. The exposure at 125 °C is not completely reversible as shown by the data. 

In our work, we calculate the equilibrium moisture from the dry and saturated mass of samples. The method was described by Thomson. We use more hydrophilic polyols, for example, hydrophiUc polyurethanes and polyethylene glycol homopolymers are used. More commonly, however, we will be using polyols from the Pluronic series of block copolymers discussed in Chapter 2. 

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