Page 298 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

300 Chapter 11 Distillation

T BPF ; T DPF ¼ bubble and dew point of feed having composition z F at column pressure
l ¼ molar heat of vaporisation (nearly same for both components)

q ð1 f Þ
(iii) Compute the feed line slope ¼ ¼ f and draw the feed line (also called q-line or
ð1 qÞ
f-line) passing through the point (z F , z F ). Equation of the line is
q z F
(11.4)
y ¼ x þ
ð1 qÞ ð1 qÞ

(iv) Construct operating lines for the rectification and the stripping sections. The operating line in
x D

the rectification section will have a y-intercept of , i.e., it passes through the points
R þ 1
x D
0; and ðx D ; x D Þ. The stripping section line is drawn through ðx B ; x B Þ and the
R þ 1
intersection of the rectification section operating line with the feed line.
x D
The easy way to draw the rectification section operating line is to locate the point 0; on
R þ 1
y-axis and join it with the point ðx D ; x D Þ on the 45 ðx ¼ yÞ line. Intersection point of this operating

line and the feed line marks the location of the feed tray. Stripping section operating line is drawn

by joining this intersection point with the point ðx B ; x B Þ on the 45 ðx ¼ yÞ line.
(v) Construction to find the number of stages is carried out by constructing triangles starting either
from the distillate or the bottoms composition as shown in Fig. 11.8. Usually the staircase is
stepped off between the operating line and the equilibrium curve from the point ðx D ; y D Þon the
45 degrees line and continues all the way to the bottom composition similar to the construction
described in Section 10.2. The construction step crossing the feed line locates the feed tray.
Counting the number of steps above and below the feed line gives the respective number of
rectification and stripping stages required. The number of stages in the rectification section is
obtained directly from the construction in case of a total condenser. A partial condenser provides
a stage of contacting and the number of rectification section stages to be provided within the
column in that case will be lower by one. Since the reboiler generally acts as an additional
equilibrium stage, the number of stages in the stripping section is less than the number of steps
below feed stage by one. In case of steam stripping in place of a reboiler, the number of stripping
stages obtained from the McCabeeThiele plot gives the number of ideal stages in the stripping
section of the column.
Usually an integral number of stages are not obtained and a fractional stage appears which is
rounded off to give the next higher number of stages.
Often close to the top and the bottom of the column the number of stages (steps of construction) is
more as the operating line and the equilibrium curve get closer to the 45 degrees line. A more
accurate estimate of the number of stages may even require constructions on these portions in
magnified scale. Improved accuracy is achieved by starting constructing of stages from the section
which has a smaller angle between the equilibrium line and the operating line, i.e., the construction
from the distillate as well as the bottoms end of the fractionator may be done simultaneously for
achieving better accuracy.