Distillation 4

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Distillation 4
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q-line
1
2
3
4
5
B
xD
xB
xF
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Reflux Ratio R is a key variable in the operation
of a distillation column. R is the ratio of the
liquid flow in the rectifying section, L, to the
distillate flow, D. The gradient of the
rectifying section operating line is
Changing the reflux ratio for a given separation
will change the number of stages required.
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Original R 6 stages
q-line
steeper gradient smaller R 9 stages
shallower gradient - higher R 5 stages
xD
xB
xF
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The higher the reflux ratio, for a given
separation (i.e. values of xB and xD) the fewer
stages are needed. For a given number of stages
and a given feed (i.e. a given distillation
column) the higher the reflux ratio the higher
the purity of the products.
One measure of the effectiveness of a separation
process is Recovery, this is the fraction of the
desired material fed to the process which leaves
the process in the correct stream. This can be
used in any separation.
In distillation recovery is usually defined as
It is the fraction of mvc entering in the feed
that leaves in the distillate. However the value
of xD is lso important.
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Minimum Number of Stages

• For a given separation we can continue to
  increase R, reducing the number of stages
  required, until the distillate flow becomes zero
  and R ?

If R ? then
R ?
The rectifying operating line is then
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The Fenske Equation

• In this special case of total reflux the form of
  the operating line is so simple that we can
  derive an equation to calculate the number of
  stages required for a given separation.
• First we need an expression for the equilibrium
  line. From the Phase Equilibrium section, using
  ?, the relative volatility

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so we can substitute this into (1).
we know that with total reflux
So, at the top of the column
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The reboiler counts as an equilibrium stage,
hence N 1
Nmin, the minimum number of stages can be found
from this
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Minimum Reflux Ratio
As usual a pinch is required here it is at the
intersection of the rectifying operating line,
the q-line and the equilibrium line.
(xD, xD)
(xi , yi)
Slope of limiting operating line is
The intersection is at (xi, yi)
xi
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(xF, yfeed)
(xD, xD)
If q 1, i.e. the feed is a saturated liquid and
the q-line is vertical,
(from the equilibrium lecture)
xF
From the operating line coordinates the gradient
is
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Sometimes the shape of the equilibrium line
limits the reflux ratio, the feed condition is
irrelevant in these cases.
xF
xD
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Effect of R on costs of distillation
Choice of R can minimise running costs. Total
cost cost of steam cost of cooling capital
cost (interest)
total cost
Annual cost
running costs (steam cooling)
capital cost
Rmin
Ropt
R