Page 192 - Adsorption Technology & Design, Elsevier (1998)

178 Design procedures

Large differential pressures can occur at the commencement of both the
pressurization and depressurization steps in PSA cycles. Flowrates there-
fore should be controlled to prevent fluid velocities from exceeding the
value which would cause significant attrition to occur. Continual pressure
cycling (perhaps of the order of a million times per year for rapid PSA
processes), external mechanical vibrations and compressor pulsations can
all exacerbate the problem. A practical solution to the attrition problem is to
immobilize the adsorbent particles by mixing a suitable adhesive binder
throughout the bed and interlocking the particles into a common rigid
structure. Immobilization is sometimes used in compact high throughput
PSA units for high technology applications. An alternative concept is to
manufacture the adsorbent bed as a monolith.
Equation (6.70) can be used for the design of fluidized bed adsorbers. It
should be noted, however, that the bed voidage will vary from the point of
bed expansion to full fluidization. Once fully fluidized the pressure drop
ceases to become a function of the flowrate.

6.8.3 Bed crushing

The possibility of crushing the adsorbent must be considered for those steps
in the process in which flow is downwards. The drag forces must be added to
the weight of the bed and compared with the crushing strength of the
adsorbent.

6.8.4 Axial dispersion

There is a tendency for both axial and radial dispersion of mass to occur
when a fluid flows through a packed bed. Since the bed diameter is normally
far greater than the particle diameter in an adsorption bed, it is common not
to have to consider the effects of radial dispersion. Hence the prevalence of
plug and axially dispersed plug flow models in rigorous design methods.
It is desirable to reduce axial dispersion because it can reduce the efficiency
of separation. The effects of axial dispersion differ for gas and liquid phase
systems, and special considerations may be needed for porous particles
when flowrates are low. Axial dispersion is caused by the twin effects of
molecular diffusion and turbulent mixing which arises from the splitting and
recombination of flows around particles. To a first approximation, these
effects are generally considered additive (Ruthven 1984):

DL = )'IDM + )'2 dpu (6.71)
where for non-porous particles )'1 and )'2 take values of approximately 0.7