Drying temperatures

It was noted earlier that dryers are quite difierent in character from both distillation and evaporation. However, heat is still taken in at a high temperature to be rejected in the dryer exhaust. The appropriate placement principle as applied to distillation columns and evaporators also applies to dryers. The plus/minus principle from Chap. 12 provides a general tool that can be used to understand the integration of dryers in the overall process context. If the designer has the freedom to manipulate drying temperature and gas flow rates, then these can be changed in accordance with the plus/minus principle in order to reduce overall utility costs. 

The ratio of stress to strain in the initial linear portion of the stress—strain curve indicates the abiUty of a material to resist deformation and return to its original form. This modulus of elasticity, or Young s modulus, is related to many of the mechanical performance characteristics of textile products. The modulus of elasticity can be affected by drawing, ie, elongating the fiber environment, ie, wet or dry, temperature or other procedures. Values for commercial acetate and triacetate fibers are generally in the 2.2—4.0 N/tex (25—45 gf/den) range. 

Good effects can be obtained only in special processes with high washing and/or drying temperatures. 

Reoxidation occurs when the metallic iron in hot DRI reacts with oxygen in the air to form either Ee O or Ee202. The reaction continues as long as the DRI remains hot and sufficient oxygen is avadable. Because reoxidation reactions are exothermic and DRI is a good insulator, it is possible that once reoxidation begins inside a pde, the DRI temperature increases and accelerates the reoxidation rate. Although the inner core of the pde may reach temperatures up to the fusion point of iron, the maximum temperature of the outer parts of the pde will be much lower because of heat dissipation. 

Fig. 3. Phase-separation temperatures for 50 50 PVC—a-MSAN blends, where (-------) represents the blend drying temperature, ( ) are experimentally...

Drying of the poly(vinyl alcohol) is critical to both the color and solubiHty of the final product. Excessive drying temperatures result in high product color and an increase in the crystallinity, which in turn reduces the solubiHty of the product. Drying is initially subjected to a flash regime, where the solvent not contained within the particles is flashed off. This first phase is foUowed by a period where the rate is controUed by the diffusion rate of solvent from the poly(vinyl alcohol) particles. Because the diffusion rate falls as the material dries, complete drying is not practical. The polymer is therefore generally sold at a specification of 95% soHds. 

Volatiles such as residual methanol, methyl acetate, and water are determined as the loss in mass when the polymer is dried at 105 2° C until constant mass is attained. Higher drying temperatures may cause decomposition and related additional weight loss. 

Drying temperatures may range up to 1000 K, the limiting temperature for most common structure metals. At the higher temperatures, radiation becomes an important heat-transfer mechanism. 

Use isothermal aging tests to monitor stability at desired drying temperature... 

Below dris temperature, mainly GaCls transports gallium, but above dris temperature, the monochloride will begin to predominate. 

Other materials subject to self-heating (depending upon composition, method of drying, temperature, moisture content)... 

Thermogravimetry is a valuable technique for the assessment of the purity of materials. Analytical reagents, especially those used in titrimetric analysis as primary standards, e.g. sodium carbonate, sodium tetraborate, and potassium hydrogenphthalate, have been examined. Many primary standards absorb appreciable amounts of water when exposed to moist atmospheres. TG data can show the extent of this absorption and hence the most suitable drying temperature for a given reagent may be determined. 

Material Drying time, hours Drying temperature. ° C. Loss in weight, % HaO ... 

Godin et al. (10), working on the effect of insecticidal activity of pyrethrum flowers from fresh and dried flowers, showed that extracts from fresh flowers had little or no greater effect on pyrethroid content and insecticidal activity than extracts from dried flowers with drying temperatures up to 80 °C. However, some of all the pyrethrin was lost at 120°C. The principal loss was in pyrethin I and not pyrethin II. 

Sinnecker, P. et al., Mechanisms of soybean (Glycine max L. Merrill) degreening related to maturity stage and postharvest drying temperature, Postharvest Biol. Tech-nol, 38, 369, 2005. 

Representative emissivity data at freeze-drying temperatures are given in Table 5 for some materials of relevance to freeze-drying problems. Emissivities... 

---