Page 98 - Adsorbents fundamentals and applications

PORE STRUCTURE AND STANDARD TESTS FOR ACTIVATED CARBON 83

sorbents. Commercial grades of activated carbon are designated for either gas
phase or liquid phase, depending on its application. A majority of the pore volume
is from pores near or larger than 30 ˚ A in diameter for liquid-phase carbons,
whereas the pores of gas-phase carbons are mostly in the range from 10 to 25 ˚ A
in diameter. The need for larger pores in liquid-phase carbons is due to the large
size of many dissolved adsorbates and the slower diffusion in liquid than in gas
for molecules of the same size.
A polymodal pore-size distribution is generally found in activated carbon. The
pore structure may be pictured as having many small pores branching off from
larger ones, which are open through the entire particle. The larger pores are called
feeder or transport pores; the smaller ones, which may be dead-end, are called
adsorption pores. The cumulative pore-volume distribution of the fine pores for
a typical gas-phase activated carbon is shown, along with four other sorbents,
in Figure 5.1. The larger pores are mostly submicrometer in size, and their total
volume amounts to a fraction of that found in the fine pores. The micropore
structures of activated carbons can be determined by a variety of adsorption
methods (Rodriquez–Reinoso and Linares–Solano, 1986).
Testing of activated carbons involves measurements of bulk density, absolute
density (or helium density), particle density, particle size distribution, pressure
drop across packed beds, and mechanical strength. Standardized ASTM tests are
available for these measurements. Details of these tests are available from ASTM.
The mechanical strength is defined by the abrasion number, which is a measure
of the change of the mean particle size, expressed as a percentage, of a sample
after 3 min of vigorous agitation with smooth steel balls. In addition, numerous
adsorption tests are used for the characterization of the adsorbent properties of
commercial carbons. The effectiveness of activated carbon is usually specified by
the amount of a certain test chemical it can adsorb under standardized conditions.
For gas-phase applications, CCl 4 is commonly used, whereas for aqueous-phase
or liquid-phase applications, iodine and molasses are used.
Carbon Tetrachloride Number or Butane Number: The carbon tetrachlo-
ride number, as defined by ASTM D3467-99, is the ultimate capacity for CCl 4
by adsorption of its vapor in air. The carbon tetrachloride number of a typical
commercial GAC is 60, meaning 60% (wt./wt.). Due to the adverse effects of
CCl 4 to the environment and human health, the butane number is adopted, which
measures the capacity for n-butane (ASTM D5228-92). Empirically, the CCl 4
number can be obtained by multiplying the butane number by 2.55.
Iodine Number: The iodine number is defined as the amount of iodine (in
milligrams) adsorbed by powdered carbon (per gram) from 0.02 N iodine aqueous
solution (ASTM D4607-94). Iodine is in the form of I 2 , with a very small amount
−
of I 3 anion. A typical iodine number for activated carbon is 900, with values
>1000 for better grades of carbon. The iodine number has been roughly correlated
to the surface area of pores >10 ˚ A diameter. It is regarded approximately as the
total pore volume.
Molasses Number: The molasses number is calculated from the ratio of the
optical densities of the filtrate of a molasses solution treated with a standard