Page 85 - Applied Process Design For Chemical And Petrochemical Plants Volume II

74 Applied Process Design for Chemical and Petrochemical Plants

($flcorrected = 0.584 - (-0.016) / (18.948) = 0.584 + 0.00084 Xhn = pinch composition of heavy key component, mol
= 0.58484 frac
XD = overhead composition of any light component,
NOW use this new value of $fl in Underwood's equa- mol frac
tions; x, = pinch composition of any light component, mol
frac

Calculate D, B, DXD~ and BXB~ from problem specifi-
cation.
Assume or set the operating pressure and overhead
For minimum reflux: Cpfl = Cpr = $s temperature (may be calculated).
From calculations of related problem* the value of DXD~ Calculate the liquid and vapor quantities and their
has been calculated respective compositions in the feed to the column.
Calculate estimated ratio of key components on feed
DXD~ 0.01072 for heavy key plate based on the liquid portion of the feed.
=
DXD~ 0.428 for light key Mol fraction light key
=
DXD~ 0.225 for lighter than light key. ' Mol fraction heavy key
=
(a) For all liquid feed at feed tray temperature (boil-
0.01072 0.428 0.225 ing point)
0.5848 + 0.5848 + 0.6848 -V, = 0
l-- 1-- l--
0.50 1 .o 2.0 rf = mol fraction ratios in feed.
(b) For a part or all vapor feed just at its dew point,
rf = ratio of key components in the equilibrium liquid
phase of feed.

(c) For all liquid feed below feed plate temperature
-ITr = -1.285 rf = ratio of key components at intersection point of
operating line (from a McCabe-Thiele diagram).
V, = 1.285 5. Determine approximate pinch zone liquid composi-
tion for light key component

(Lr)min = 1.285 - 0.6637 = 0.622 (8-142)

o'6622 0.94, Minimum Reflux Ratio 2% x~i sum of ah + 1 xfi + 1 + ah + 2 xfi +2 + . . . for
=
all components in liquid portion of feed heavier than
Minimum Reflux Colburn Method Pinch heavy key. Note that XF~ values are the mol fractions of
Temperatures [ 121 the component in the liquid portion of feed only and
the ZXF~ equal to 1.0.
This method has also found wide usage and might be
considered less tedious by some designers. It also yields an 6. Calculate approximate value for (L/D)min.
approximation of the rectifying and stripping section
pinch temperatures. For adjacent keys,
Rectifjmg:
The second term in the right hand parentheses can
-
(8 141) be omitted unless the mol fraction of the heavy key in
the distillate, XhD, is 0.1 or greater. Use Xhn = xln/rf.
where a = relative volatility of any component referenced to 7. Estimate stripping and rectifying pinch temperatures
the heavy key component at values one-third and two-thirds of the interval
XhD = overhead composition of heavy key component, between the column bottoms and overhead, respec-
mol frac tively.