Question

Separation Process:

Consider the in-class demonstration problem involving the MeOH/water separation. Suppose that a saturated steam feed was injected into the bottom of column (it is sparged to get good contact with the liquid that is falling down from the tray above its entrance). Take the specifications for the feed to be identical as in the McCabe-Thiele demonstration done in class

(zF = 0.36, F = 216.8 kmol/h). Further, the compositions of the distillate and the bottoms

should be the same as in the demonstration: mole fraction of MeOH is xD = 0.915 and xW = 0.00565. The saturated steam feed is entering at 200 kmol/h.

2.

What are the internal vapour and liquid flow rates now? Are the distillate or bottoms flow rates significantly different compared to the previous case?

What is the actual reflux ratio under these conditions?

What is the equation of the lower operating line now?

Further, determine the number of ideal stages and compare to the demonstration in class. Describe any advantages or disadvantages of using steam injection compared to the previous case

Answer 1

Given feed is saturated with feed flowrate = 200 kmole/h

Mole fraction of more volatile MeOH in feed = 0.36 = x_F

Mole fraction of more volatile MeOH in distillate = 0.915=x_d

Mole fraction of more volatile MeOH in bottom product = 0.00565=x_w

Doing overall mass balance

As feed is saturated therefore,q=0 and q line will be parallel to x axis passing through (0.36,0.36), such that it cutting equilibrium curve at (0.065.0.36)

Minimum reflux ratio is given by

Actual reflux ratio R

Where 'L' is internal liquid flowrate

Where, 'V' is gas flowrate inside the system

Now,

Equation of top operating line is given by

Putting all values,we get top operating line

OR

..................................(1)

As we know that q line is parallel to x axis , therefore equation of q line

..................................(2)

Solving 1 and 2,we get

As we know that bottom operating line must pass through point of intersection of q line and top operating line, therefore bottom operating line must pass through (0.1649,0.36) and (0.00565,0.00565), therefore equation of bottom operating line

Now,

Relative volatality of MeOH water mixture is = 4.02

Using Fenske method

  

So, the number of stages with re boiler is 6