Page 191 - Adsorption Technology & Design, Elsevier (1998)

Design procedures 177

energy losses. In order to promote intermixing of the flow streams in the two
velocity regions (wall and centre) it is desirable that the bed length should be
at least 100 times the adsorbent particle diameter. The correction factor
which may need to be applied to the pressure drop in order to take into
account flow maldistributions is shown in Figure 6.11.
In low pressure gas phase applications such as vent stream cleaning and
solvent recovery, high pressure drops are liable to occur due to the low
density and high velocity of the fluid. To overcome this problem very
shallow adsorption beds are sometimes used. Adsorption beds with low
aspect ratios, however, may result in flow distribution problems, and
therefore some of the pressure drop that is saved must be sacrificed by
adding flow distribution systems of manifolds, baffles and screens. Other
ways of keeping the pressure drop to a minimum in low pressure gas phase
applications include the use of specially shaped adsorbents such as the
trilobe and monolithic materials shown in Figure 2.7 in Chapter 2.
Two other factors deserve consideration. First, all the correlations
described above for pressure drop were obtained from steady state flow
measurements, and therefore may not be applicable to situations in which
velocities are rapidly changing with time, e.g. in the pressurization and
depressurization steps of PSA processes. Secondly, the changes in super-
ficial velocity along the axis of an adsorption bed which is used for a bulk
separation process, such as air separation, may be sufficiently large to
require the summation of the pressure gradient across incremental lengths
of the bed.

6.8.2 Fluidization

At high flowrates the potential exists for an adsorption bed to expand, lift or
fluidize. At the point of expansion, the net weight of the bed is fully
supported by the drag forces due to the flow of fluid:
AP
L- = (1 - e) (p, - p)g (6.70)

where p~ is the density of the solid, and e is the voidage at the point of
expansion.
Without a retaining device, the bed will start to expand if flow in the
upwards direction is sufficiently high. Even if a bed retaining device is fitted
the particles will still tend to move and rub against each other and cause
attrition to occur. In order to avoid this problem it is normal practice to limit
the allowable upwards velocity for packed beds to less than about 80% of the
minimum fluidization velocity calculated from equation (6.70).