Question

The things given : Distillation tower, reflux is present, 1atm, The feedstock is a liquid at the bubble point, XF= 0.5, XD=0.98

XB is not given.

In this case, is XB = 0.02 ?

Then, why?

Answer 1

Answer:

No! you can not take X_B = 0.02

• X_F is the mole/wt fraction of more volatile component in the feed stream.
• X_D is the mole/wt fraction of more volatile componetn in the distillate.
• X_B is the mole/wt fraction of more volatile component in the bottom stream.

X_D = 0.98 means 98 % of Distllate contains mole volatile component and rest 2 % other component. (if it is binary distillation)

Assume basis: per unit time.

For example, if the distllate (D) is 80 mol and X_D = 98 mol %. Then the amount of more volatile component in the distillate will be

• = (X_d x D)
• = 0.98 x 80 mol
• = 78.4 mol

The other component will be 2 mol %. i., the amount of other component in the distillate will be

• = 0.02 x D
• = 0.02 x 80 mol
• = 1.6 mol

Note: This 0.02 mol fraction or 1.6 mol refers to the composition in the distillate not in the bottom stream.

X_B refers to composition in bottom stream. To calculate the amount of X_B one shoud know the value of B.

To find unkonwn values. These two material balances very important.

Overall material balance:

Feed = Distillate + Bottoms

F = D + B

Overall component balance:

FX_F = DX_D + BX_B

You can even take material balance over the top of column containing Vapor stream (V) , Distillate (D) and some portion of distillate fed back to the column in liquid form (L).

V = L + D

Reflex ration R = L/D

Then, the above equation can further be modified to

V = (R+1)D

If you still have any doubt feel free to ask me.