Adsorption - Principles of Chemical Separations with Environmental Applications - page 199

7.8.2 Pressure and temperature considerations

The temperature and pressure conditions are very important for column operation. If there
is a temperature choice within some range, the coldest temperature is usually selected to
obtain the highest sorption loading. The temperature of the fluid exit stream is the usual
point for evaluation. The sorption process is exothermic so that the exit temperature is
the highest in the system. One important exception is when the feed stream contains a
condensible vapor that is not to be separated out of the feed stream. The system temperature
must be maintained high enough so that condensation does not occur. If the condensible
vapors are to be separated, it is usually easier to do so by condensation (using an energy-
separating agent) prior to the sorption step.
For pressure considerations, the highest pressure is at the feed entrance since the pressure
will decrease due to flow through the system. For a system that contains condensible vapors,
the pressure must be maintained low enough so that condensation does not occur.

7.8.3 System orientation and flow direction

The system should normally be oriented in the vertical direction. The sorbent particles will
settle in the vessel with time and use. If the system were oriented horizontally, some open
space would develop at the top of the vessel and the fluid would tend to flow through this
section due to decreased flow resistance (channeling). This channeling effect is usually
very detrimental to separation performance and cycle lifetime.
The second factor to consider is the direction of the fluid flow, upward or downward.
Upward flow will cause lift of the particle bed at some critical velocity that causes the bed
to fluidize. This effect results in an increase in the fluid dispersion within the system and a
decrease in performance. Downward flow could cause the particles to be crushed at some
point. The allowable velocities for crushing are larger than those for lift, so downward
flow is the normal operating condition.

7.8.4 Flow dispersion

Axial dispersion (sometimes referred to as backmixing) is a spreading of the concentration
profile in the axial direction due to flow variations within the adsorbent bed (see the pulse
analysis section in Appendix C). This effect can also contribute to the spreading of the
mass transfer zone.

7.8.5 Non-isothermal effects

Many adsorption design approaches assume that adsorption is occurring isothermally. This
is a good assumption only when the adsorbable component concentration is low and/or
the heat of adsorption is low. There are two simple methods that can be used to determine