Moisture equilibrium

Use the molecular weight ratio to calculate the apparent extent of reaction of the caprolactam in these systems. Is the variation in p qualitatively consistent with your expectations of the effect of increased water content in the system Plot p versus moisture content and estimate by extrapolation the equilibrium moisture content of nylon-6 at 255 C. Does the apparent equilibrium moisture content of this polymer seem consistent with the value given in Sec. 5.6 for nylon-6,6 at 290°C ... 

Equilibrium moisture content is that which a material retains after prolonged exposure to a specific ambient temperature and humidity. 

Fig. 6. Equilibrium moisture content profiles vs relative humidity for a hygroscopic material.

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 ... 

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

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. 

Equilibrium moisture content of a hygroscopic material may be determined in a number of ways, the only requirement being a source of constant-temperature and constant-humidity air. Determination may be made under static or dynamic conditions, although the latter case is preferred. A simple static procedure is to place a number of samples in ordinaiy laboratoiy desiccators containing sulfuric acid solutions of known concentrations which produce atmospheres of known relative humidity. The sample in each desiccator is weighed periodically until a constant weight is obtained. Moisture content at this final weight represents the equilibrium moisture content for the particular conditions. 

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... 

X = equilibrium moisture eontent at turbine diseharge (% of total weight flow)... 

Free moisture This is solvent in excess of the equilibrium moisture content and is relatively easily removed. 

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...

Essentially Complete Removal of Water from Sample. Though this assumption must be made in the case of all direct methods (oven, distillation, Fischer, etc.) its correctness is most difficult to prove and in many cases it turns out to be one of the greatest sources of error. Two factors are involved the equilibrium moisture content and the rate of removal of moisture. 

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... 

Fig. 5. First (open circles) and second (full circles) equilibrium moisture sorptions in the DGEBA-TETA neat resin. The dotted line is eq. (1)...

The diffusivity Dq and equilibrium moisture content Wg are parameters which describe a specimen s moisture diffusion properties. 

In many products it seems highly probable that there exists a narrow range of optimum moisture contents that should be maintained. More specifically, the effect of moisture on MCC-containing tablets has been the subject of an investigation that demonstrates the sensitivity of this important excipient to moisture content [10]. These researchers found that differences exist in both the cohesive nature and the moisture content to two commercial brands of MCC. A very useful report on the equilibrium moisture content of some 30 excipients has been compiled by a collaborative group of workers from several pharmaceutical companies and appears in the Handbook of Pharmaceutical Excipients [11,12],... 

Finished hard gelatin capsules normally contain an equilibrium moisture content of 13-16%. This moist-... 

Equilibrium moisture content, Moisture, Ash, Volatile Matter, Fixed carbon, Volatile carbon. 

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. 

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