Page 50 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 39

This is close enough to 1,533 mm; actual temperature
might be 265"F, although plotted data are probably not
that accurate. Because the feed enters at 158°F and its
bubble point is 266"F, the feed is considered sub-cooled.

Heat to vaporize one mol of@ed,

. - _. _. ___ ._ --- Solve first for YlF, assuming that the system follows the
-.
Latent Btu/Mol ideal (as it closely does in this instance).
Ht. @ (OF) (XF)
Component X, 266°F (XF) (&) @ 158°F (266-158)
Trichlor 0.436 12,280 5,600 30.9 1,523
Btu/mol
Perchlor 0.344 14,600 7,950 36.4 2,180
Btu/mol This takes the place of drawing the equilibrium curve
13,560 3,703 and solving graphically, and is only necessary since the "q"
is not 1.0 or zero.
The a should be for the feed tray. However, the value of
a = 2.8 should be accepted for feed tray conditions (not
heat required to vaporize one mol of feed
= 158°F). It would not be if this were predominantly a recti-
latent heat of one mol of feed fjmg or a stripping operation.
13,560 - 3,703 - 17,253
--I 1.272
= 13,550 1 3,550 0.456 (2.8) = o.70
'IF = 1 + (2.8 - 1) (0.456)
Minimum Number Tray at Total Reflux
Now, substituting to solve for (L/D)min.

(L/D) (0.456) + 1.272 (0.999)
(L/D) (1 - 0.456) + 1.272 (1 - 0.999)

- - 2.8{ [ (L/D) + 110.70 + (1.272 - 1) (0.999))
((L/D) + 1) (1 - 0.456) + (1.272 - 1) (1 - 0.999)
For a total condenser system:
(L/D) (0.456) i 1.271 - 2.8[ (L/D) (0.70) i 0.70 + 0.2711
(L/D) (0.544) + 0.00127 - (L/D) (0.344) + 0.544 + 0.000271

- log (.999/.001) (.99/.01) Solving this quadratic:
-
log 2.8
(L/D)min = 0.644
Nmin + 1 = 11.18
Nmin = 10.18 trays, not including reboiler Reading Figure 8-17 for (L/D)min assuming a liquid
feed at the boiling point, (L/D)min = 1.2. This demon-
Summary: strates the value of taking the thermal condition of the
feed into account.
Min. total phjsical trays in column = 10.18
Reboiler 1.0 Actually, any point on one of the curves represents a
For conservative design, add feed tray 1.0 condition of reflux and number of trays that will perform
Minimum total theoretical stages 12.18, say 12 the required separation.

Minimum Reflux Ratio Theoretical Trays at Actual Reflux
Because this is not feed at its boiling point, but sub- Assume actual reflux ratios of 1.2, 1.8, 2.25, 3.0 times
cooled liquid, the convenient charts cannot be used with the minimum and plot the effect on theoretical plates
accuracy. Using Underwood's general case: using Gilliland plot.