Page 26 - Adsorption Technology & Design, Elsevier (1998)

Adsorbents 23

Table 2.3 Typical properties of adsorbent-grade silica gel*
Physical properties

Surface area (m2/g) 830
Density (kg/m 3) 720
Reactivation temperature (~ 130-280
Pore volume (% of total) 50-55
Pore size (nm) 1-40
Pore volume (cm3/g) 0.42
Adsorption properties Percent by weight
H20 capacity at 4.6 mm Hg, 25~ 11
H20 capacity at 17.5 mm Hg, 25~ 35
02 capacity at 100 mm Hg, -183~ 22
CO2 capacity at 250 mm Hg, 25~ 3
n-C4 capacity at 250 mm Hg, 25~ 17
i i iii i i |e
* From Keller et al. 1987, p. 652

2.7 ACTIVATED ALUMINA

Activated alumina is a porous high area form of aluminium oxide with the
formula AI203.nH20. Its surface is more polar than that of silica gel and,
reflecting the amphoteric nature of aluminium, has both acidic and basic
characteristics. Surface areas are in the range 250-350m2/g. Because
activated alumina has a higher capacity for water than silica gel at elevated
temperatures it is used mainly as a desiccant for warm gases including air
but in many commercial applications it has now been replaced by zeolitic
materials. Gases for which activated alumina is suitable include argon,
helium, hydrogen, low molecular weight alkanes (C1-C3), chlorine, hydro-
gen chloride, sulphur dioxide, ammonia and fluoroalkanes. Other uses for
activated alumina include chromatography and drying of liquids such as
kerosene, aromatics, gasoline fractions and chlorinated hydrocarbons.

2.8 CLAY MATERIALS

Like zeolites, clays can be synthesized or taken from natural deposits.
Unlike zeolites however, they comprise layer silicates which imbibe guest
molecules between their siliceous layers causing their crystals to swell.
Fuller's earth is an activated natural montmorillonite. Its pore size is altered
and its surface area increased by acid treatment to 150-250 m2/g. It is