Page 255 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

256 Chapter 10 Absorption and stripping

• Operating pressure and maximum permissible pressure drop across the column
• The chosen solvent (in case of absorber)/inert gas (in case of stripper)

The last two are often decided based on economic considerations, and are, therefore, left to the
choice of the designer.
The design decisions are usually based on the following considerations:
1) In the case of absorbers, the solvent is selected, such that, it has a high solubility for the solute.
This reduces the amount of solvent. In addition, it should be relatively cheap, stable, nonvolatile,
noncorrosive, nontoxic, nonviscous, nonfoaming, and preferably nonflammable. Since the exit
gas is saturated with solvent, a part of the solvent is lost. A low-cost solvent is often preferred over
a costly one with higher solubility or lower volatility. The availability of suitable material of
construction also influences the choice to a large extent. Recovery of the solvent, usually by
distillation and at times by chemical means, is almost always necessary and needs to be included
in the design and cost analysis. The more effective the regeneration process, the less costly the
absorber, as the regenerated solvent is left with a lower concentration of residual dissolved solute.
Water is the most commonly used solvent, oils are used for hydrocarbons, and special chemical
solvents for acid gases like H 2 S, CO 2 , and SO 2 .
There is always a loss of solvent with the exiting streams, and the designer needs to specify
the equivalent make up rate. In case the solvent has several components, the loss of lighter
components from the solvent is more, and therefore, the makeup stream needs to be richer in
lighter component(s). A light, lean oil solvent for absorbing light hydrocarbons will have higher
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loss but has the advantage of high absorption capacity (moles/m circulated) compared to heavier
lean oil. The selection of a solvent close in carbon number to the components absorbed has a higher
absorption capacity, but the closeness of volatility makes the separation difficult. Therefore,
optimization of the absorber needs to be done by considering the absorber and the regenerator
together. As an example, lean oil with three carbon numbers heavier than the lightest component is
usually common in petrochemicals/refineries, whereas a lean oil heavier by about 10e14 carbon
numbers is adopted in natural gas processing units.
The considerations for deciding on the stripping gas (inert) are also similar and largely dictated by
the availability and economics. Steam and nitrogen are common industrial stripping agents, and as
in the case of solvent selection, the overall cost of the stripper operation and the cost of recovery
need to be considered for arriving at the optimum choice.
2) Height of the tower and its components, e.g., the depth of packing or the number of trays
3) The optimized liquid flow rate through the absorber and regenerator
4) Inlet and outlet temperatures of the stream(s) and amount of heat required to be removed to
account for heat of solution, etc.
5) Operating pressure of the column
6) Mechanical design