Page 176 - Adsorption Technology & Design, Elsevier (1998)

164 Design procedures

Table 6.4 Summary of constant pattern solutions ]'or the
breakthrough curve with a Langmuir isotherm

Kinetic model Form of solution Source
, ,,, , ,
,i i f i ii | ,,
Linear, fluid film Algebraic Michaels (1952)
Linear, solid film Algebraic Hall et al. (1966)
Solid diffusion, Numerical Hall et al. (1966)
with constant tabulation
D
Solid diffusion, Graphical solution Garg and Ruthven
variable D (1973)
Pore diffusion Numerical Hall et al. (1966)
tabulation
Graphical solution Garg and Ruthven
Pore diffusion
(1973)
,,,

If axial dispersion is considered to be important reference should be made
to Acrivos (1960) and Garg and Ruthven (1975). Acrivos (1960) also
provides an analytical expression for constant pattern behaviour with a
rectangular (irreversible) isotherm. Garg et al. (1975) and Ruthven (1984)
provide information and solutions for the constant pattern situation under
non-isothermal conditions.

6.7 SHORT-CUT AND SCOPING METHODS

Empirical or short-cut methods are still used extensively for the design of
fixed beds. This is not only due to their simplicity and reliability but also
because of the formidable nature of the more rigorous alternatives. With
thermal swing processes, for which cycle times are generally long, it can be
argued that a reasonably accurate design can be made using short-cut
methods since much of the bed behind the MTZ will contain adsorbent
which is in equilibrium with the feedstock. Even so, special care should still
be exercised when determining the applicability of short-cut methods for a
particular design problem. Methods include the length or weight of unused
bed (LUB or WUB), the mass transfer zone length (MTZL), the empty bed
contact time (EBCT), the bed depth service time (BDST), the transfer unit
approach (NTU and HTU) and the capacity at breakpoint.