An important parameter in the design of gas absorbers is the ratio of the flow rate of the feed liquid to that of the feed gas. The lower the value of this ratio, the lower the cost of the solvent required processing a given quantity of gas but the taller the absorber must be to achieve a specified separation. Propane is recovered from a 7 mole% propane?93% nitrogen mixture by contacting the mixture with liquid n-decane. An insignificant amount of decane is vaporized in the process, and 98.5% of the propane entering the unit is absorbed.

(a) The highest possible propane mole fraction in the exiting liquid would be that in equilibrium with the propane mole fraction in the entering gas (a condition requiring an infinitely tail column). Using the Cox chart (Figure 6.14) and Raoult?s law to relate the mole fractions of propane in the entering gas and exiting liquid, calculate the ratio (h/h0,) corresponding to this limiting condition.

(b) Suppose the actual feed ratio (n_L1/n_G2) is 1.2 times the value calculated in part (a) and the percentage of the entering propane absorbed is the same (98.5%). Calculate the mole fraction of propane in the exiting liquid.

(c) What are the costs and benefits associated with increasing (n_L1/n_G2) from its minimum value [the value calculated in part (a)]? What would you have to know to determine the most cost-effective value of this ratio?

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Feed liquid AL(Ib-mole/h) Feed gas ha₂(lb-mole/h) 0.07 lb-mole C3Hg/lb-mole Product gas AG, (Ib-mole/h) ABSORBER T= 80°F, P 1 atm Product liquid AL(Ib-mole/h)