4. Introduction of Feed
The plate on which feed is introduced is called the feed plate.
The condition of the feed introduced on the feed plate alters the phase flow rates
The feed to the column may be introduced as a
1. Cool liquid – its increases liquid flow rate in stripping section
2. Liquid at bubble point (saturated liquid)- its increases liquid flow rate in
stripping section
3. Partially vaporised ( partialy vapour and partialy liquid) - its increases liquid
flow rate in stripping section and vapour flow rate in rectifying section
4. Satureted vapour at its dew point - its increases vapour flow rate in
rectifying section
5. Superheated vapour - its increases vapour flow rate in rectifying section
5. To calculate the change in phase flow rates by the introduction of feed a factor ‘q’ is introduced
The q is measure of thermal condition of the feed.
‘q’ is defined as the number of moles of saturated liquid resulting in the stripping section for each mole of feed
introduced.
Thus for a feed F
L’ = L + q F
V = V’ + (1- q) F
The values of q for various thermal conditions of the feed
1. Cold fluid : q > 1
2. Saturated liquid : q= 1
3. Feed partially vapour and liquid : 0< q < 1
4. Saturated vapour feed : q = 0
5. Superheated vapour feed : q < 0
The q line equation for any particular feed condition can be calculated from the following relation
The q line or feed line equation
6. Total reflux ratio
It is one of the limiting Values of the reflux ratio
The total reflux operation is necessary only to know the minimum number of plates
required.
At the total reflux, the product withdrawal rate is zero
This can be done by returning all the overhead product back to the column as reflux
(total reflux) and all the liquid going to the reboiler is vaporised and is fed to the
column.
Since at total reflux
F = 0, D= 0 and W= 0
Vn+1 = Ln
7. Total reflux ratio
Hence The slope of the operating
line of both section becomes unity
and hence the operating lines of both
sections coincide with the diagonal
and the minimum number of stages
are then required for desired degree
of the separation (xD-xW) can be
calculated.
At total reflux operating line coincide
with diagonal and to effect a desired
separation, number of stages
required are minimum
8. Minimum reflux ratio
The minimum reflux ratio Rm is the
maximum ratio which will require an infinite
number of trays for the separation desired,
and it corresponds to the minimum reboiler
heat load and condenser cooling load for
the separation.
As the reflux ratio reduced, operating lines
move towards the equilibrium curve along
the feed line and the number of stages
increases.
Further reduction in reflux ratio results into
the top operating line touches the
equilibrium curve at (“pinch point”) at y’
and x’ (number of steps required becomes
infinite). The line passes through the
points x’, y’ and xD (y=xD):
The point of intersection of the operating
line on the equilibrium curve and at this
point step become very close together, so
that a zone of nearly constant composition
is formed near the feed plate and an
infinite number of plates are then required
At minimum reflux ratio, required heat
supply of the reboiler and coolant supply
for the condenser are minimum.
9. Optimum reflux ratio
The optimum reflux ratio is
defined as that reflux ratio at
which the total cost of
operation is minimum
At minimum reflux ratio as
infinite number of plates are
required, the fixed cost is
also infinite while the cost of
heat supply to the reboiler
and condense coolant is
minimum
As the reflux ratio is
increased, the number of
plates decreases and the
fixed cost decreases at first,
passes through a minimum
and then increases as with a
higher reflux ratio the
diameter of the column and
sizes of the reboier and
condenser increases
The optimum reflux occurs
at a point where the sum of
the fixed cost and operating
cost is minimum.
The optimum reflux ratio
usually lies in the range of
1.1 to 1.5 times the minimum
