Page 353 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

12.1 Introduction 355

Table 12.1 Merits and demerits of fixed and moving beds.
Fixed bed Moving bed
Advantages
· Simple · Low adsorbent inventory (regeneration
· Relatively inexpensive immediately after adsorption possible)
· Minimal attrition of adsorbent particles · Better heat transfer
· Easier to design

Disadvantages
· Difficult to design accurately due to the unsteady nature of · More complex and expensive equipment
process · Attrition of adsorbent
· Large inventory of expensive adsorbent and
proportionately large pressure drop for the slow progress
of MTZ a
· Safety concern for highly porous adsorbents as poor
thermal conductivity may lead to hot spots and fire
hazards, e.g., in case of activated carbon bed with air.
Also, the equilibrium may be adversely affected.
· Multiple beds required for continuous operation.
a
Mass transfer zone (MTZ), elaborated later in the chapter.

For single component adsorption, the computations can be performed using Eq. 12.1

G s ðY i Y o Þ¼ A s ðX o X i Þ

which gives a linear operating line of slope A s=G s . The line lies below the equilibrium curve for
adsorption and above it for desorption. Minimum adsorbent requirement is given by the operating line
of maximum slope that touches the equilibrium curve at a point as discussed in Chapter 10 (Section
10.3). Although in practice, the operation is adiabatic and the temperature does not remain constant
except for solute collection from vey dilute liquid solutions, the calculations are usually done assuming
isothermal operation following the design procedure of packed bed discussed in Chapter 10. The
governing equations for gas adsorption are

A s dX ¼ G s dY ¼ K Y a P ðY Y Þdz (12.9)
Which on rearranging gives
Y h
Z 1 Z bed
dY K Y a P
N tOG ¼ ¼ dz (12.10a)
Y Y G s
0
Y 0