Page 127 - Adsorbents fundamentals and applications

112 ACTIVATED CARBON

2. Slightly carbonizing coals, especially anthracite (Walker et al., 1966; Met-
calfe et al., 1963; Mason and Eberly, 1965; Patel et al., 1972).
3. Coating of the pore mouths of the commercial activated carbon with a
carbonized or coked thermosetting polymer (Walker et al., 1966).

A variety of interesting molecular sieving properties were found for these sam-
◦
ples. For example, PVDC carbonized at 700 C was thought to have slit-shaped
pores because it adsorbed flat molecules such as benzene and naphthalene but
not spherical ones such as neopentane. Carbonized Saran had a pore entrance of
∼6 ˚ A and thus showed a striking separation between isobutane (with a kinetic
diameter of 5.0 ˚ A, which was admitted) and neopentane (with a kinetic diam-
eter of 6.2 ˚ A, which was rejected) (Lamond et al., 1965). With the carbonized
PVDC, molecular-sieve effects with regard to neopentane were not seen until
◦
the carbonization temperature reached 1200 C (Walker et al., 1966). Anthracite
◦
heat-treated in hydrogen at 650 C adsorbed n-butane at an amount about five
times larger than that of isobutane (Mason and Eberly, 1965). At carbon burnoff
◦
(by oxygen at 427 C) below 6.9%, anthracite admitted CO 2 ,lessN 2 ,and almost
no neopentane (Patel et al., 1972). CO 2 is a linear molecule and is thought to
have the smallest minimum diameter, 3.7 ˚ A, among the three adsorbates (Patel
et al., 1972). For the coke-coating technique, Walker et al. (1966) prepared sam-
ples by forming carbon on activated carbons from furfuryl alcohol, polymerized
with phosphoric acid. The pores were thought to be nearly 5 ˚ A in diameter since
the samples had a large capacity for n-butane (kinetic diameter = 4.3 ˚ A), a small
capacity for isobutane, and negligible capacity for neopentane. In a later work
(Kamishita et al., 1977), carbon was deposited into the pores of a lignite char
◦
by cracking methane at 855 C. With nearly 3% carbon deposited, the samples
showed significant molecular sieving between CO 2 (admitted) and N 2 (hindered).
The early development on CMS has been reviewed by Walker et al. (1966) and
Spencer (1967).
The patent literature on the processes for manufacturing CMS’s has been
reviewed elsewhere (Cabrera et al., 1993; Armor, 1994), including the Bergbau
patents (Munzner et al., 1974; Munzner et al., 1976) and Japanese patents (Eguchi
et al., 1974; Ohsaki and Abe, 1984). The general procedure is the same, that is,
carbonization/activation followed by carbon deposition, as described below.
At present, CMS’s are produced commercially by Bergbau-Forschung GmbH
in Germany and by Takeda Chemical Company in Japan, among others. The
detailed procedures of the manufacturing processes, although not revealed in
the open literature, are based on the carbonization of coal or nutshell followed
by coating of carbon on the char by using a variety of hydrocarbon vapors.
Carbonization of polymers such as PVDC is not used due to economic reasons.
The general procedure for the manufacture of CMS’s used by Bergbau-Fors-
chung is shown in Figure 5.22 (J¨ untgen et al., 1981). Two types of CMS’s are
produced, although that designated CMS N2 has O 2 /N 2 sieving properties and
is used for nitrogen production from air. The raw material is a bituminous coal,
ground to 90% passing 40 µm. The coal is first oxidized by air at temperatures