Hold on! Now we start another process: absorption and stripping. Isn’t that two? Well, yes. But they are so similar – it just depends on the direction that the solute travels. Gas to liquid – absorption. Liquid to gas – stripping.
These problems ask you to determine the outlet flow rates and compositions from a single-stage absorber or stripper. As always, follow the problem solving procedure. For a condensable solute like ethanol, use Raoult’s law (or a modified Raoult's Law if you have a non-ideal solution) for phase equilibrium. For a non-condensable solute like carbon dioxide, use Henry’s law. Assume that only the solute transfers between phases, that the separator is an equilibrium stage, and that the temperature and pressure are constant.
12.1 100 mol/s of CO2 is fed to a single-stage absorber operating at 5oC. Pure water is fed at a rate of 100 mol/s. The two streams mix, equilibrate, and then separate into liquid and gas products. The system pressure is high enough to carbonate the water to 3-4 volumes (soft drink industry speak), or 6-8 g/L, or around 0.003 mol CO2/mol. Determine the flow rate and composition of each stream.
Food (or maybe, drink) for thought: Does the CO2 flow rate matter? Would there be any improvement in the process by adding extra stages?
12.2 100 mol/s of a water stream with 0.1 mol% chloroform is fed to a single-stage stripper operating at 25oC 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.
12.3 100 mol/s of an air stream with 5 mol% ammonia is fed to a single-stage absorber operating at 10oC and 5 atm. Pure water is fed at a rate such that 90 mol% of the ammonia is absorbed. The two streams mix, equilibrate, and then separate into liquid and gas products. Determine the flow rate and composition of each stream.
Equilibrium data in the form of g NH3/100 g H2O vs PNH3 (mm Hg) is found in Perry's Handbook and on the accompanying spreadsheet Excel "Ammonia Data" that I included for your convenience. I developed polynomial equations to relate liquid phase mole fraction to partial pressure at several temperatures.
12.5 100 mol/s of a water stream with 1 mol% methanol is fed to a single-stage stripper operating at 30oC and 760 mm Hg. Enough air is fed to remove 90 mol% of the methanol. The two streams mix, equilibrate, and then separate into liquid and gas products. 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 accompanying spreadsheet Excel "Methanol Data" that I included for your convenience. I fit the curves with polynomial functions to relate liquid phase mole fraction to partial pressure at several temperatures.
Be awesome! For any of these problems, change the flow rates, temperature, or pressure 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.