Chapter 13 - Mathematical Analysis of Absorption/Stripping Columns

These problems ask you to calculate the amount of a chemical absorbed from a gas by a liquid or stripped from a liquid by a gas in a multi-stage separator. Assume that only the solute transfers between phases, that the separator consists of equilibrium stages, and that the temperature and pressure are constant.

Other than the number of stages, these are some of the same problems as in Chapter 12. For each system, use a "brute force" method on a two-stage separator and the Lewis method (stage-by-stage) method on a five-stage separator. Compare the effectiveness of multi-stage contact with single-stage.

13.1 100 mol/s of a water stream with 0.1 mol% chloroform is fed to a single-stage stripper operating at 25^{o}C and 1 atm. Enough pure air is fed to remove 99.9 mol% of the chloroform. The two streams mix, equilibrate, and then separate into liquid and gas products. Determine the flow rate and composition of each stream.

13.2 100 mol/s of an air stream with 5 mol% ammonia is fed to a 5-stage absorption column operating at 10^{o}C and 5 atm. Enough pure water is fed to absorb 90 mol% of the ammonia. Determine the flow rate and composition of each stream.

Equilibrium data in the form of g NH_{3}/100 g H_{2}O vs P_{NH3} (mm Hg) is found in Perry's Handbook and on the spreadsheet Excel "Ammonia Data" that I included for your convenience in the Chapter 12 problems. I developed polynomial equations to relate liquid phase mole fraction to partial pressure at several temperatures.

13.3 100 mol/s of a water stream with 5 mol% ammonia is fed to a 5-stage stripping column operating at 60^{o}C and 1 atm. Pure air at is fed at a rate such that 90 mol% of the ammonia is stripped. Determine the flow rate and composition of each stream.

13.4 100 mol/s of a water stream with 1 mol% methanol is fed to a stripper operating at 30^{o}C and 760 mm Hg. Enough air is fed to remove 90 mol% of the methanol. Determine the flow rate and composition of each stream.

Equilibrium data for the methanol-water-air system in the form of Pxy diagrams were constructed from ChemCAD and are found on the spreadsheet Excel "Methanol Data" that I included for your convenience in the Chapter 12 problems. I fit the curves with polynomial functions to relate liquid phase mole fraction to partial pressure at several temperatures.

13.5 100 mol/s of an air stream with 1 mol% methanol is fed to an absorber operating at 10^{o}C and 3800 mm Hg. Enough water is fed to remove 90 mol% of the methanol. Determine the flow rate and composition of each stream.

Be awesome! For any of these problems, change the flow rates, temperature, pressure, or number of stages and observe the effects. Do the results make sense to you?

Be really awesome! Compare your results to a process simulator like ChemCAD or Aspen. What assumptions have you made that might explain differences? Note that you should use the Sour Water thermodynamics for the ammonia system in ChemCAD - not what the thermo wizard picks.