Page 113 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 109

of the vessel. The simple mixed tank shown in Figure 5.2 is an example of the
former.

5.4.1 Solids in plug flow
The ideal countercurrent steady state configuration can be achieved
conceptually by allowing the adsorbent particles to fall in plug flow through
the rising stream of gas or liquid which is to be separated or purified. This
process arrangement would lead to a minimum adsorbent inventory and
would allow for good heat transfer performance. While conceptually simple,
there are several practical problems to be overcome. First, it is necessary to
have an adsorbent which is sufficiently rugged to withstand attrition.
Secondly it is necessary to devise a flow system to transport the adsorbent
back to the top of the adsorption column. Thirdly, it is necessary to
incorporate an adsorbent regeneration section somewhere in the process
scheme. This might be at the bottom of the adsorption column or in the
transportation section. In either case it is necessary to regenerate the
adsorbent to such an extent that the overall process can operate at steady
state.
Such moving bed processes were proposed in the 1930s and around 1950
the erstwhile Hypersorption process shown schematically in Figure 5.7 was
commercialized for separating various light hydrocarbons on activated
carbon. An example application was the recovery of ethene and propene
from cracked gas consisting otherwise of hydrogen, methane, ethane,
propane and butane. The adsorption column, which was about 1.5 rn
diameter and 25 rn tall, allowed the adsorbent to be transported downwards
in the rising gas flow. A heated stripping section was located at the bottom
and a cooler was located at the top. Between these two heat exchangers were
located four trays with the feed being introduced through a distribution tray
near the middle of the column. The lean overhead product gas was
disengaged through a tray which was located immediately below the cooling
section. The adsorbates were removed in two streams according to their
volatilities. The ethene was released on a disengaging tray which was located
immediately below the feed point. The propene which was more strongly
adsorbed was released with the carrier stream on a disengaging tray near the
bottom of the column. Heavier products could also be removed below the
feed point.
Typical processing conditions for a Hypersorption unit were 0.6 m3/s of
feed gas at 5 bar with a maximum carbon recirculation rate of 15 000 kg/h.
The stripping and cooling section temperatures were 265 and 210~
respectively. The literature contains very little performance data on the
process but the principal problems of inadequate carbon regeneration and of