Environmental Engineering Reference
In-Depth Information
y
Sat'd
liquid
( q =1)
y = x
Subcooled
liquid
( q >1)
Two-phase
mixture
(0 < q <1)
Sat'd vapor
( q =0)
Superheated
vapor ( q >1)
x F
x
Figure 4.6 Feed lines.
Solving the mass balance around the feed plate for V
V and substituting into this
feed-line equation gives:
L
L F
L
L
Fz F
y
=−
L
L
x
+
L
L =−
L
x
F
F
L
F
/
F
F
F Fz F
q
1
+
L
=
1 x
+
q z F .
(4.34)
q
1
L
F
/
F
F
So, the slope of the feed line is found as q
/
( q
1). If the feed is a saturated liquid, then
q
1.0, the slope of the line is infinity and the feed line is vertical. Conversely, if the
feed is a saturated vapor, the slope is zero and the feed line is horizontal. The feed can
also exist as a two-phase mixture, subcooled liquid, and superheated vapor. Figure 4.6
presents a number of representative feed lines and Figure 4.7 shows different qualities of
feed entering a distillation column.
The result above shows that the operating lines above and below the feed line, and the
feed line itself, all intersect at a common point.
As pointed out at the beginning of this chapter, the coolest temperature is at the top
of the column and increases as you proceed down to the reboiler. The slope of the feed
line reflects this as the cooler feeds point toward the top ( x D ) and the hottest feeds point
toward the bottom ( x B ).
=
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