Question One 130 marksl A total feed of 200kmol/h, having an overall composition

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Question One 130 marksl A total feed of 200kmol/h, having an overall composition of 45 mol% n-heptane and the rest ethyl benzene is to be fractionated at 101.32kPa absolute pressure to give containing 97mol% n-heptane and a saturated bottoms product containing 2.0 mol% n-heptane. The feed enters the tower partially vapourised so that 55 mol% is vapour. The reflux ratio is 2.5. Equilibrium data is plotted below. Calculate the following: a saturated distillate [10 marks] a) Top and Bottom Product flows b) The number of theoretical of trays required [10 marks] c Minimum number of theoretical trays [3 marks] d) Minimum reflux ratio, Rmin [3 marks] e) Comment on the impacts on the McCabe-Thiele design of binary distillation columns, of: 1. switching from using the EOL to identify trays to the SOL before the "qline"- EOL intercept is reached; 2. persistence in the use of the EOL in identifying trays beyond the "q line"- EOL intercept for a few steps before making the appropriate switch. [4 marks] Question Two [20 marks] The Point Lisas Ammonia Products Company operatesa distillation column at 1000 kN/m2, which separates 2 kg/s of a solution of ammonia in water, containing 30 % by mass of ammonia, to give a top product of 99.5 wt/wt% purity and a weak solution containing 10 wt/wt% of ammonia. The reflux ratio is 0.450 a) Use the enthalpy composition diagram to determine the b) Find the condenser load; c) Find the reboiler heat duty d) Find the rate of flow of liquid and vapour from the second number of ideal stages. [6 marks] [5 marks ] marks] stage. [2 marks] [5 e What would happen to the outcome of a design if an operating line on the enthalpy composition diagram coincides with a tie line on the diagram? 2 marks]

Explanation / Answer

A1. No equilibrium data is mentioned.

A3. 1. Operating line is the equation obtained by performing material balance around the distillation column. By performing the mass balance around the rectification section, stripping section and feed section and plotting it on the graph along with the equilibrium line. The larger the distance between the operating line and equilibrium line, the larger the concentration difference of mass transfer, and thus, easier the separation.

2. The distribution curve can be calculated by obtaining the Vapour-Liquid Equilibrium(VLE) data and plotting the VLE on the graph.

3. Infinite Reflux Ratio refers to the total reflux ratio, that is when minimum theoretical plates are required to carry out the distillation process. The operating lines in the enthaly-concentration diagram represent the material balances involving the energy and the concentration properties of the system.

4. The purpose of reflux is to assist in the separation of the components for distillation. The more reflux provided for a given number of theoretical plates, the better is the column's separation of lower boiling materials from higher boiling materials. Conversely, for a given desired separation, the more reflux is provided, the fewer theoretical plates are required.

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