Fall 2008

ChE 545: Mass Transfer Operations I

Lecture 5: Binary Distillation

McCabe-Thiele graphical method for binary distillation, though superseded in

industrial practice by computer aided calculations, clarifies the principles and

operation of binary distillation column.

The simplest configuration of a continuous distillation column is comprised of: a

rectification (enrichment) section, a stripping (exhausting) section, a condenser

and a reboiler. The distillate and bottoms product streams are withdrawn from the

condenser and the reboiler respectively. The feed stream demarcates the interface

between the two sections.

The operating lines, relating the vapor and liquid phase concentrations (mole

fractions) of the light key (more volatile component) for the two sections are:

.. Rectification section

.. Stripping section

where, R is the reflux ratio (ratio of overhead stream returned to the distillation

column to distillate product), and VB is the boilup ratio (the ratio of stream

returned to column from the reboiler to bottoms product).

The feed line equation is:

.. Feed line,

Where q is the thermal condition of feed, defined as the ratio of the difference

between the enthalphy of saturated vapor and feed enthalpy to the difference

between enthalpies of the saturated vapor and liquid streams.

The McCabe-Thiele graphical method involves:

o Plotting the equilibrium curve (x-y) diagram and y = x line,

o Drawing the q-line from the feed concentration (on y = x line) till its

intersection with the equilibrium curve

o Drawing the operating line for the rectification section from the distillation

composition (on y = x line) to the intersection point; the slope of this line

is Rmin/(Rmin+1), where Rmin is the minimum reflux ratio

o Drawing the actual operating line for the rectification section by assuming

a higher reflux ratio than Rmin

o Drawing the operating line for the stripping section from the intersection

point of q-line and operating line for the rectification section to bottoms

composition (on y = x line)

o Drawing equilibrium stages from the distillate to bottoms composition to

determine the number of stages required to achieve the desired separation.

Infinite number of stages are required when the reflux ratio is minimum. As the

intersection of q-line and operating lines lies on the equilibrium curve, negligible

enrichment is achieved in each stage and the operating lines pinch the equilibrium

curve.

Increasing the reflux ratio reduces the number of stages required for the

separation. Minimum number of stages are needed at total reflux where operating

lines coincide with the y = x line. There is no feed, nor any product streams at

total reflux. Operating under total reflux allows the determination of the total

number of theoretical stages in a column.

The basis for the above analysis is the key simplifying assumption of constant

molar overflow, resulting in constant liquid and vapor flows within each section

(though these are different in the two sections).