Page 249 - Adsorption Technology & Design, Elsevier (1998)

226 Selected adsorption processes

Sorbex configuration) utilizes a 5A zeolite adsorbent and light naphtha as
desorbent for the separation of linear and branched chain paraffins. O|efins
may be separated from saturated hydrocarbon isomers by the Olex process
using CaX zeolite as adsorbent and heavy naphtha as desorbent. Separation
of fructose from glucose is achieved in the Sarex process using CaY zeolite as
adsorbent and water as desorbent. All of these processes are summarized in
Table 5.1.

7.8 CHROMATOGRAPHIC PROCESSES

The principles of chromatographic separation are widely used for gas or
liquid analysis. Chromatography has also been applied for the preparation
of pharmaceuticals on a scale of about 1 tonne per day. Chromatography
requires uniform packing of adsorbent and on a larger processing scale this is
difficult. Despite this, however, special packing devices have been designed
and a larger-scale chromatographic process is operated commercially by Elf
Aquitaine and Soci6t6 de R6cherches Techniques Industrielles (SRTI). The
Elf-SRTI process is designed to separate 100 tonnes per year of perfume
constituents. A plant to separate 105 tonnes per year of normal and iso-
paraffins has also been reported (Bernard et al. 1981). A flow representation
of the Elf-Aquitaine process is shown in Figure 7.18. Heated light naphtha is
distributed to a device which is capable of injecting pulses of the naphtha
feed into three specially packed chromatographic columns. Injection into
each column is arranged in sequence so that a continuous flow can be
maintained. Each column, however, is acting in a batchwise manner, the
components of the light naphtha separating into its constituents as the pulse
of feed traverses the column. The constituent with the least retention time in
the column emerges first and is collected in a receiving vessel. Constituent
components of the feed with longer retention times then follow and are
received in other fraction collectors.
A review of the principles involved in large-scale chromatography has
been presented by Conder (1973), LeGoff and Midoux (1981) and Valentin
and Midoux (1981). These articles should be consulted for fuller details.
Here we discuss, very briefly, what influences the efficiency of component
separation in a packed column. Separation of the components of a mixture
occurs preferentially according to the relative strengths of adsorption of
each component on the solid packing. Equilibration between the flowing gas
or vapour (the mobile phase) and the adsorbent (the stationary phase)
prevails during the continuous contact between the two phases in the
column, thus providing for a much superior efficiency of separation as
compared with the other adsorption processes described. Were it not for the