Separation Processes Engineering

Part 1: Plate distillation column with side streams
1. A mixture of ethanol and water containing 0.15 mole fraction ethanol, is separated in a fractionating column to give a top product containing 0.8 mole fraction ethanol and a side stream product containing 0.55 mole fraction ethanol. The aqueous waste is to contain 5.2 mole per cent of the alcohol fed to the column and has a concentration of
0.01 mole fraction ethanol. The feed is supplied as boiling liquid and a reflux ratio of
4.0 is used. Carry out a mass balance around the column and Determine:
a) The number of theoretical plates required in the column
b) The feed plate and side stream plate locations.
The ethanol equilibrium data is given below:

x 0.019 0.072 0.124 0.166 0.261 0.397 0.573 0.676 0.747 0.894
y 0.170 0.389 0.470 0.509 0.588 0.612 0.684 0.738 0.782 0.894

Details of how the operating line equations, for the three sections of the column, derived from first principles must be shown.
2. Use Aspen Plus to carry out the following simulation:
A distillation column is required to separate a liquid feed containing 25 mole % benzene and 75 mole % toluene to give a top product containing 90 mole % benzene and a bottoms product containing 4 mole % benzene. A reflux ratio of 2.50 is to be used and the feed enters at its boiling point. The feed, re-boiler and condenser pressures are 2.0 bar, 2.2 bar and 1.8 bar respectively.
Assuming the plates to be 100% efficient, calculate the composition of the liquid mixture on the third plate (counting from the top of the column). Assume a feed flow of 100 kmol/hr and feed temperature of 25 °C. You should provide printouts of the
Aspen Plus results in you report.

Part 2: Gas Absorption column
Ammonia is to be recovered from an air/ammonia mixture, containing 12% by volume ammonia, by scrubbing with clean water in a packed column. The aim is to recover at least
95% of the ammonia. The operation is to be done counter-currently at 1 bar pressure and
25 °C using randomly packed 1” rashing rings.
The mass flow rate of the gas mixture entering the column is 2 kg/s.
Under these operating conditions the following has been determined.
KGa = 0.05 kmol/m3.s.bar
The equilibrium relationship is given by:
p = 0.2x
where p = partial pressure of ammonia in gas phase (bar)
x = concentration of ammonia in liquid phase (mole fraction)
Determine
1) The diameter of the column. Assume a flooding condition of G = 0.6Gf
2) Concentration of ammonia leaving in the liquid phase at the bottom of the column.
3) The height of packing required using the following methods:
a) Graphical integration using the complete design expression
b) Using the design equation for a Logarithmic mean driving force.