Ch E 416 - Assignment 7 Solutions

---

(1) Minimum Reflux

 

  1. First find the intersection point (xi , yi) of the equilibrium line and the q-line.

 

Equilibrium line :

 

 

q – line:

 

 

 

 

 

  1. The slope for the op line:

 

 

 

 

 

  1. The operation reflux will be:

 

 

  1. McCabe – Thiele Method:

 

 

Equilibrium stages required, N=8

Number of trays = 8 – 1 = 7

 

 

The MATLAB file McCabeT.m was used to generate the above graph.

 

(2) Cooling and heating duties

 

  1. First convert the units of the heat of vaporizations supplied:

 

 

 

 

  1. Then, a mole balance around the reboiler:

 

 

 

Where, x1 and y1 are the compositions of the liquid and vapour in the reboiler (passing streams)

 

  1. The reboiler heat duty:

 

 

  1. A mole balance around the condenser:

 

 

 

  1. The condenser heat duty:

 

 

(3) Cooling and heating duties using HYSYS

 

The simulation case is COL1.HSC

 

  1. The reboiler heat duty:

 

 

  1. The condenser heat duty:

 

 

Note that the main difference is in the condenser duty. Remember that the graphical method was derived from assumptions as constant volatility model, constant molar flow rate and constant heat of vaporization.

 

 

From the figure, it is clear that to reach the purity specifications, the rigorous simulation produces a different reflux ratio, slightly higher than the 2.4424 calculated in part 1. As a consequence, more material is being condensed. Hence, the heat duty is higher.

 

(4) Modified column with a reboiler substitution

 

  1. In this case, a trial and error analysis is required since the column has 7 trays installed. McCabe – Thiele method is used until 7 stages are reached. The new composition is approximately:

 

 

 

(5) Murphree efficiency of 75%

 

 

 

Equilibrium stages required, N=11

Number of trays = 11 – 1 = 10

 

(6) HYSYS simulation

 

The simulation case is Col.HSC

 

 

 

Notice the differences from the CMO assumption especially in the enriching section. Also, notice the degree of separation water – methanol in the bottoms product.

 

(7) HYSYS simulation changing efficiencies:

 

 

As expected, the separation is less effective as the efficiency decreases.

 

(8) HYSYS simulation changing reflux ratio:

Taking the simulation from part 7, the reflux ratio was decreased from the value of 2. The following plots show the “pinch – like condition”.

 

R=2.0

 

 

R=0.9

 

 

Notice the plateau that starts to appear near the feed stage

 

R=0.8

 

 

R=0.5

 

 

At this point, no changes in composition take place after the feed stage. In distillation of near ideal binary mixtures, the pinch condition is always at the feed tray

 

 

Posted November 12, 2006

Return to top