Packed Height Requirement

The design of a tray tower for gas absorption and gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical trays needed to achieve a specified composition change (see Sec. 13). Distillation differs from absorption because it involves the separation of components based upon tne distribution of the various substances between a vapor phase and a liquid phase when all components are present in both phases. In distillation, the new phase is generated from the original phase by the vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

In gas absorption, the new phase consists of a relatively nonvolatile solvent (absorption) or a relatively insoluble gas (stripping), and normally no reflux is involved. This section discusses some of the considerations peculiar to gas absorption calculations for tray towers and some of the approximate design methods that can be applied (when simplifying assumptions are valid). 

Algebraic Method for Dilute Gases By assuming that the operating and equilibrium curves are straight lines and that heat effects are negligible, Souders and Brown [Ind. Eng. Chem., 24, 519 (1932)] developed the following equation  

14-8 Graphical method for a three-theoretical-plate gas-absorption tower with inlet-liquor composition y and inlet-gas composition t/j. 

Note that for the limiting case of A = 1, the solution is given by 

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The mass transfer equations discussed above are now combined with a material balance on the transferred component to calculate the column or packing height required for a given separation. The column cross-sectional area A is assumed known at this point although in a complete column design A must be determined based on pressure drop considerations. The column, which is in countercurrent flow with only liquid feed and vapor product at the top, and vapor feed and liquid product at the bottom (absorber, stripper, column section), is deflned as follows ... 

Consider the ethanol absorber of Example 4.4, packed with 50-mm metal Hiflow rings. Estimate the packed height required to recover 97% of the alcohol, using pure water at a rate 50% above the minimum and with a gas-pressure drop of 300 Pa/m. The system ethanol-C02-water at the given temperature and pressure obeys Henry s... 

Be able to determine the number of actual plates or packing height required, together with feed and product locations. 

Determine the number of actual trays or packing height required. 

Frequently, it is convenient to calculate the packed height required by means of theoretical stages or transfer units. This is due to the fact that the height equivalent to a theoretical stage for a particular system tends to be constant. Further, the height of a transfer unit varies less with flow rates than do the mass transfer coefficients. 

For a liquid-film-controlled system, the effect of gas rate on the overall Koa value is small and will not significandy alter graphical calculations. The plots in Figures 5-1, 5-2, and 5-3 permit rapid determination of the packed height required for fume scrubbers in such systems. As can be seen, the packed depth necessary for a given solute removal efficiency becomes greater with increasing gas rate. 

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