binary distillation - eth z · thermische verfahrenstechnik i binary distillation binary...

Prof M. MazzottiThermische Verfahrenstechnik I binary distillation

Binary distillation

A binary mixture is separated in an existing tray column with 10 trays. The more volatile compo-nent (A) has a feed stream concentration of xF =0.4; at the top of the column this component hasbeen enriched to xD=0.9. The reflux ratio is R =3. At the bottom of the column, component Ahas a concentration of xB=0.1. 71% of the liquid coming out of the column is sent back after beingvaporized into the reboiler.

a) What is thermodynamic state of the feed stream entering the column? (Hint: Draw carefullythe rectification and the stripping line into the composition diagram below.)

b) Estimate the overall efficiency E0 of the column.

For further separation processes, the column will be improved in order to increase the amount ofbinary mixture separated by the column. The new feed stream will have the same thermodynamicstate and the same composition as before. The composition of distillate and bottom stream willalso be the same. It is assumed that the column efficiency will decrease to E0,2=0.4

c) Is it possible to reach these specifications with the original column without any change incolumn design or equipment? Use the second composition diagram for your argumentation.

As an alternative, the trays of the column can be removed and the column will be used as a packedcolumn (height of the packing material: h = 3m, packing material: HETP≤50cm).

d) How many theoretical stages are available now to fulfil the separation task?

e) Is the reflux ratio, which is needed to fulfil the separation task, larger or smaller than in theoriginal column? Why?

Assumptions

- the column is adiabatic

- no effects of mixing enthalpies

- the molar evaporation enthalpies are identical

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Figure 1: Process Scheme

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Figure 2: Composition diagram

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Guideline for the solution

Question a):

Rectification line:

y =R

R + 1· x +

xP

R + 1(1)

The rectification line goes through point y=xD=0.9 and intersects with the y-axis at

xP

R + 1=

0.93 + 1

= 0.225 (2)

Stripping line:

y =L

V0

· x− B

V0

· xB (3)

The stripping line goes through point y=xB=0.1

Figure 3: Bottom of the column

It is known that 71% of the liquid is evaporated by the reboiler, this leads to V0/L=0.71. The slopeof the stripping line is equal

L

V0

=1

0.71= 1.41 ≈ 0.7

0.5(4)

Now, the two lines can be drawn in the composition diagram. The line connecting the intersectionpoint of these two lines and the point (xF , y) has an infinite slope (q=1), which means that thefeed is completely in liquid form at boiling temperature. Alternatively, the same result couldhave been achieved by equalizing the equations of rectification and stripping lines and solving thisequation system.

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Question b):

Step-by-step construction in the diagram used in part a) leads to a number of theoretical stages of7.5. The overall column efficiency E0 is calculated as follows:

E0 =nth

nreal=

7.510

= 0.75 (5)

Question c):

The reflux ratio at either top or the bottom of the column can be increased (for a given feed thesetwo variables ar not independent on each other). By increasing the reflux ratio, the stripping orrectification line will come closer to the diagonal of the diagram and less stages are needed to fulfilthe separation task.nth,min at R → ∞, this means that the operating lines fall on the diagonal of the compositiondiagram. Step-by-step construction in the second diagram gives nth,min=5.

nth,existing = E0,2 · norig = 0.4 · 10 = 4 (6)

nth,existing < nth,min: the separation task can not be fulfilled with this column.

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Question d):

nth =h

HETP=

30.5

= 6 (7)

Question e):

The reflux ratio must be higher than 3, since the separation task has to be fulfilled with 6 theoreticalstages. The original column had 7.5 theoretical stages.

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