CONTINUOUS DISTILLATION WITH REFLUX (RECTIFICATION)

A binary mixture has two miscible liquid components. If the boiling points of them are widely different then they can be separated easily by flash distillation. In that case the low boiling point liquid will be evaporated quickly leaving the high boiling point liquid in the distillation apparatus.

If their boiling points are very close then it is not possible to separate by flash distillation method. In that case rectification method is adopted.

CONTINUOUS FRACTIONATING COLUMN WITH RECTIFYING AND STRIPPING SECTIONS

See Fig. 18.8

The feed is entered on a plate in the central position of the column. The liquid film flows down the column. Vapor rises from the boiler at the bottom of the column.

Rectification unit: At the plates above the feed plate forms the rectification unit. At these plates the reflux liquid is flowing down. Ideally it should be free of component A. But small amount of A remains. This small amount of A is also taken out by the vapor. Hence this unit is called rectification unit (some kinds of mistake is rectified).

Stripping unit: In the feed plate and the lower plates the feed liquid (full of component A) flows downward. On the way it passes through the vapor that extracts the component A, hence it is called stripping section.

Material Balances in Plates

Step-I : Over all material balance

Overall material balance:

Total material balance: F = D + B

Component A balance: Fx_F = Dx_D + Bx_B.

Eliminating B we get:

Eliminating D we get:

N.B. Why we are expressing in

and

format?

Because the D and B are expressed in terms of fraction of F (i.e. Feed flow rate).

Step-II Net flow rate in rectification section

* Material balance around condenser:

Material entering condenser = Material leaving condenser

or, V_a = L_a + D or, D = V_a – L_a.

* Material balance around upper section:

Material entering upper section = Material leaving upper section

L_a + V_n+1 = L_n + V_a or, V_n+1 – L_n = V_a – L_a = D.

And V_n+1 = D + L_n.

* Material balance with respect to component A

V_n+1 y_n+1 – L_n x_n = Dx_D.

or,

For convenience Vn+1 is exchanged with L_n + D

This equation is operating line-1 for rectifying section.

Step-III Net flow rate in stripping section

* Material balance around reboiler

Material entering the reboiler = material leaving the reboiler

or, L_b = V_b + B or, B = L_b – V_b.

* Material balance around lower section

Material entering the lower section = Material leaving the lower section

or,

Operating line-2 for rectifying section

Operating line-1 and operating line-2 shows that if Ln ¹ Lm then the operating lines will be curved and become difficult to draw unless the x_n, x_m, y_n and y_m of all the internal plates are known.

Assumption: Constant molal overflow

In order to simplify the equations for operating lines it is assumed that

The heat required to vaporize one mole of component A is nearly equal to the heat required to condense one mole of component B. In this case the enthalpy of component A and B is not required and the operating lines become linear.

Þ Subscripts n, n+1, n–1 m, m+1 and n–1, L and V may be ignored and the simplified equations thus obtained are as follows:

Operating line-1:

Operating line-2:

Now let us put x = x_D in operating line 1

We get

Or,

Similarly if we put

in operating line-2

then

x and y will be same at any point on diagonal. Thus operating line-1 and 2 are cutting the diagonal at x_D and x_B respectively.

Feed plate

At the feed plate the liquid flow rate or the vapor flow rate or both may be changed depending on the thermal condition of the feed. All conditions of feed flow can be expressed by a term q, which is defined as the moles of liquid flow in the stripping section that result from the introduction of each mole of feed. i.e.