Page 352 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

354 Chapter 12 Adsorption

In Eqs. 12.2 e 12.5, the subscripts denote the exit concentration from the corresponding stage.
For isothermal operation with fresh adsorbent, the calculation for theoretical stages can be made
using Eq. 12.4. The same is also true for desorption/ regeneration. However, in practice the operation is
adiabatic and the equilibrium relationship changes with stage number due to considerable rise (fall) in
temperature during adsorption (desorption). Under this condition, the calculations are done stagewise
similar to nonisothermal gas absorption or any stage-wise operation.
For N stages of gasesolid contacting, solute balance in line with Eq. 12.4 is

G s ðY 0 Y N Þ¼ A s ðX 1 X Nþ1 Þ (12.6a)
where G s and A s are the respective flow rates of solute free gas and adsorbent. The enthalpy balance
gives

G s ðH G0 H GN Þ¼ A s H S1 H S;Nþ1 (12.6b)
where H G is the enthalpy of gas mixture in energy/mass

H G ¼ C p;g ðt G t 0 Þþ Y C p;adsorbate ðt G t 0 Þþ l vap;0 (12.7)
and H S is the enthalpy of the solid plus adsorbate in energy/mass adsorbent

(12.8)
H S ¼ C p;adsorbent ðt s t 0 Þþ X C p;adsorbent;liq ðt A t 0 Þþ DH ads
In Eqs. 12.8 and 12.9, enthalpies are expressed with respect to adsorbent, nonadsorbed gas and
adsorbate as liquid, all at base temperature t 0 , DH ads is the integral heat of adsorption at X and t 0 ; l vap;0
is the latent heat of vaporisation of adsorbate at t 0 and C p denotes specific heat of the component
denoted by subscript.

Continuous contact operation
Continuous contact devices can be operated in either continuous or semi continuous mode. In
continuous mode, fluid and solid move continuously at a constant rate (steady-state moving bed
adsorber) with either the solid moving relative to the walls of the container (moving beds) or the
particles remaining stationary relative to the walls (rotary beds). In semi continuous mode, fluid moves
through a stationary bed of solid and the composition changes with time (fixed bed adsorber). The
merits and demerits of fixed and moving bed processes are listed in Table 12.1.
In steady-state moving beds, the solid and the fluid phases move continuously through the
equipment at a constant rate. At any point in the equipment, the composition
of both the phases does not change with time. Use of moving bed is common
for collecting solutes and fractionation of gaseous and liquid mixtures through
Moving beds
adsorption and ion exchange. Since the purpose of these applications is to
achieve separation equivalent to many stages, only countercurrent operation is
considered. Parallel flow may be used when one theoretical stage of contacting
suffices. The major challenges of these equipment are (i) ensuring uniform flow of fluid and solid
particles, (ii) uniform distribution of solid particles in the fluid to minimise channelling and local
irregularities and (iii) continuous entry and removal of solids into the device. Attrition is unavoidable
in moving bed processes and high mechanical (attrition) strength of adsorbent particles in the bed is a
major requirement.