Page 130 - Adsorbents fundamentals and applications

CARBON MOLECULAR SIEVES 115

A two-step carbon deposition process using isobutylene was developed by
Cabrera and Armor (1991). Numerous optimized deposition processes have been
described by Cabrera et al. (1993). In the two-step scheme, the carbon support
with pore sizes of about 4.5 to 20 ˚ A is contacted with two different concentrations
of a hydrocarbon. The concentration in the first step is larger than that in the
second step. In this fashion, the pore openings of the support micropores are
narrowed successively in two distinct steps without excessive pore filling.
For rational design of the carbon deposition step, one needs to have a Thiele
modulus for the cracking reaction to be within an optimal range for which the
effectiveness factor is kept low while still so allowing reasonably high reaction
rates. The range for the Thiele modules of between 10 to 100 seems to be an
optimal range. For a first-order cracking reaction, the effectiveness factor is given
by (Levenspiel, 1972):

tanh(mL) k
E = and mL = L (5.8)
mL D
where E is the effectiveness factor, L is the pore length, k is the first-order
rate constant for cracking, D is the diffusivity of the cracking molecule in the
pore, and mL is the Thiele modulus. At effectiveness factor = 1, uniform carbon
deposition throughout the pore is expected. For mL = 10–100, E = 0.1–0.01.
Cracking in this range would ensure deposition at the pore mouths rather than
throughout the pore walls.

5.7.2. Kinetic Separation: Isotherms and Diffusivities
Because the finishing step in producing CMS’s is carbon deposition in an inert
atmosphere at a moderately high temperature, the surface of CMS’s is quite
uniformly covered by carbon. Unlike activated carbon that has a considerable
amount of surface functionality, CMS’s not have detectable surface functionality
(Armor, 1994). Moreover, they should have fewer exposed inorganic compounds
than activated carbon and not have cations. Consequently, adsorption of gas
molecules on CMS’s involves only nonspecific dispersion forces (see Chapter 2).
For these reasons, CMS’s should also be more hydrophobic than activated carbon.
The main use for CMS’s is nitrogen production from air and CH 4 /CO 2 sep-
aration, both by PSA. The latter is applied for: (1) landfill gas that contains
approximately 50% each of CH 4 /CO 2 , and (2) tertiary oil recovery where the
effluent gas contains ∼80% CO 2 and 20% of CH 4 plus other light hydrocar-
bons. The PSA separation of CH 4 /CO 2 with Bergbau Forschung CMS has been
discussed in detail by Kapoor and Yang (1989) and by Baron (1994), who also
discussed several other possible applications.
The difference in the kinetic diameters of N 2 and O 2 is ∼0.2 ˚ A. That between
CH 4 and N 2 is also ∼0.2 ˚ A. Given the importance of CH 4 /N 2 separation in the
field of natural gas upgrading, it is surprising that a suitable CMS has not yet
been developed. Attention is certainly warranted for developing such a CMS.