Chapter 8 Problems

Now we return to mathematical analysis of distillation columns. But instead of assuming EMO, we will assume adiabatic trays, which requires an energy balance for each stage with Q = 0 kJ/s. This also provides the heat added to the reboiler and the heat removed from the total condenser. One additional unknown is added to the mix for each stage – Q (kJ/s) – and one additional equation is required – the energy balance.

These problems are identical to the ones for Chapter 6. Analyze the column as described above. Compare your solutions to the Lewis method with the EMO assumption. Note also the heat transferred to the reboiler and from the condenser. Be awesome by examining the effect of process parameters on heat requirements.

In addition to phase equilibrium data you will need liquid and vapor heat capacities and heats of vaporization.

8.1 50 mol/s of an equimolar mixture of benzene and toluene is fed to the fifth tray of a 10-tray distillation column operating at 1 atm. The feed is a saturated liquid at 1 atm such that it all contributes to the liquid flowing down the column. The distillate flow rate is 25 mol/s and the reflux ratio is 1. Determine all unknown flow rates, compositions, and temperatures.

8.2 100 mol/h of a liquid at containing 10.0 mol% ethanol and 95.0 mol% water is fed to the ninth tray of a 10-tray distillation column operating at 1 atm. The feed is a saturated liquid at 1 atm such that it all contributes to the liquid flowing down the column. The distillate is 80.0 mol% ethanol and the reflux ratio is 3.0. Determine all unknown flow rates, compositions, and temperatures.

8.3 1,000 kmol/h of a vapor containing 70 mol% water and 30 mol% isopropanol is fed to the fifth tray of a 10-tray distillation column operating at 1 atm. The feed is a saturated liquid at 1 atm such that it all contributes to the liquid flowing down the column. 80 mol% of the water fed to the process is in the liquid product and the reflux ratio is 3.0. Determine all unknown flow rates, compositions, and temperatures.

8.4 100 mol/s of an equimolar mixture of n-pentane (45 mol%) and n-hexane (55 mol%) is fed to the 4th tray of a 10-tray distillation column operating at 1000 mm Hg. The feed is a saturated liquid. The distillate flow rate is 50 mol/s and the reflux ratio is 2.0. Determine all unknown flow rates, compositions, and temperatures.

8.5 100 mol/s of an equimolar mixture of styrene and ethylbenzene is fed to the fifth tray of a 10-tray distillation column operating under vacuum at 100 mm Hg. The feed is slightly subcooled such that the liquid flow rate below the feed tray is 125 mol/s greater than the liquid flow rate above the feed tray. The flow rates of distillate and bottoms are equal. The reflux ratio is 2.0. Determine all unknown flow rates, compositions, and temperatures.

8.6 100 mol/s of a mixture containing water (60 mol%) and methanol (40 mol%) is fed to the ninth tray of a 10-tray distillation column operating at 1 atm. The feed is a saturated liquid at 1 atm such that it all contributes to the liquid flowing down the column. The bottoms is 90.0 mol% water and the reflux ratio is 1.25. Determine all unknown flow rates, compositions, and temperatures.

8.7 10 gal/h of a 20-proof ethanol in water mixture is to be concentrated to 90-proof in a 10-tray distillation column operating at 1 atm. The feed to Tray 8 is a slightly subcooled liquid so that the liquid flowing down from the feed stage is 5.0 mol% greater than expected because the feed condenses some of the vapor flowing up into the feed stage. The reflux ratio is 2.0. Determine all unknown flow rates, compositions, and temperatures.

8.8 1.0 m3 (STP)/s of 40 mol% n-butane and balance n-hexane is fed to the sixth tray of a 10-tray distillation column operating at 2 bar (absolute). The feed is a saturated vapor at 2 bar such that it all contributes to the vapor flowing up the column. The bottoms flow rate is 20.0 mol/s and 80% of the n-butane fed to the process is in the distillate. Determine all unknown flow rates, compositions, and temperatures.

8.9 100 mol/s of a vapor containing equimolar amounts of benzene and chlorobenzene is fed to the fifth tray of a 10-tray distillation column operating at 1 atm. The feed is a saturated vapor at 1 atm such that it all contributes to the vapor flowing up the column. The distillate flow rate is 50 mol/s and the reflux ratio is 2.0. Determine all unknown flow rates, compositions, and temperatures.

8.10 1000 kmol/day of an equimolar mixture of acetone and isopropanol is fed to the fourth tray of a 10-tray distillation column operating at atmospheric pressure. The thermodynamic state of the feed is such that 25 mol% enters the vapor flowing up the column and 75 mol% enters the liquid flowing down the column. 80 mol% of the acetone fed to the column is captured in the distillate and its flow rate is 600 kmol/day. Determine all unknown flow rates, compositions, and temperatures.