Capacity of Common Desiccants

Where large amounts of water are to be removed, a preliminary drying of liquids is often possible by shaking with concentrated solutions of calcium chloride or potassium carbonate, or by adding sodium chloride to salt out the organic phase (for example, in the drying of lower alcohols). 

Drying agents that combine irreversibly with water include the alkali metals, the metal hydrides (discussed in Chapter 2), and calcium carbide. 

P2O5 BaO Mg(C104)2, CaO, MgO, KOH (fused), cone H2SO4, CaS04, AI2O3 KOH (pellets), 

Ba(C104)2, ZnCl2, ZnBr2 CaCl2 (technical) CUSO4 Na2S04, K2CO3. 

---

Rough equilibrium vapor pressures for H20 above a variety of common desiccants are given in Table 3.1 along with comments on other properties such as H20 capacity. Additional factors that are particularly important in the drying of... 

The capacity to remove water using some of the desiccants varies widely, as Table 7.12 shows, and obtaining their full effectiveness often poses difficult problems of chemical engineering design. Of those listed in Table 7.12, only potassium carbonate is commonly regenerated, requiring temperatures of about 200 °C. The others are relatively cheap chemicals and, if they are used to remove only low levels of water often on a small batch basis, are imeconomic to process. 

As may be seen from Figure 1.3, at room temperature the affinity of activated alumina for water is comparable with that of silica gel but the capacity is lower. At elevated temperatures the capacity of activated alumina is higher than silica gel and it was therefore commonly used as a desiccant for drying warm air or gas streams. However, for this application it has been largely replaced by molecular sieve adsorbents which exhibit both a higher capacity and a lower equilibrium vapor pressure under most conditions of practical importance. 

Liquid water is particularly objectionable with high-capacity gel-type adsorbents because it can cause particle breakage. To minimize this effect, it is common practice to guard susceptible desiccants, such as Sorbead R, with a layer of liquid-water-resistant desiccant, such as Sorbead W or Activated Alumina. Another precaution is to insulate the tower surfaces and connecting pipelines so as to prevent the condensation of water on the cooled metal surfaces and avoid the possibility of such water reaching the desiccant. 

---