Page 350 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

352 Chapter 12 Adsorption

liquid solution performed in batch, semicontinuous, or continuous mode can be treated analogous to
mixer-settler operations of liquid extraction (contact filtration). Gas adsorption for solute removal or
fractionation is usually performed in fluidised beds. The aforementioned analogies are often used to
simplify the design procedure. Fixed-bed adsorption is an unsteady state rate controlled process, not
similar to other mass transfer operations. Therefore, in this Chapter, we first outline the principles and
governing equations for stagewise and continuous contact adsorption and then discuss in detail the
fixed-bed adsorption process and its design.

Adsorption
operation

Stagewise Continuous
contacting contacting

Single stage Multistage Fixed bed Moving bed Fluidised bed Rotary bed

Cross-current Counter
flow current flow

FIGURE 12.1
Classification of adsorption operation based on the mode of contact.

Stagewise operation
Single-stage adsorption for liquids is used for extremely favourable distribution of solute towards
adsorbent. This is often employed to remove taste and odour from water
using powdered activated carbon. For greater economy of adsorbent, the
process is operated in multistage cross current or countercurrent mode
Contact filtration
accomplished by multiple tanks and filters connected in series. Counter-
current operation requires lower amount of adsorbent for the same sepa-
ration, which is particularly important for expensive adsorbents like
activated carbon. Although the savings are greater for larger number of stages, it is seldom economical
to use more than two stages. A higher number of stages increase the expense of filtration and other
handling costs. If the process needs to be operated in continuous mode as in decolorising petroleum
lubricating oils, the filter press can be substituted by a centrifuge or a continuous rotary filter or the
solid can be allowed to settle when the mixture is passed through a large vessel. Even if counterflow
operation is more efficient and ensures greater adsorbent economy, cross current flow seems to be the
more practical option in small-scale processing of liquids. This is particularly true when the amount of
solution treated in every batch may vary considerably. Cross flow is also preferred when long intervals
between batches may require partially spent adsorbents to be stored. Adsorbents like activated carbon