Page 72 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 61

dilution of the system with water in the lower portion. Of the total pressure. Use steam at 212°F and atmospheric
course, where steam is acceptable, it replaces the cost of a pressure.
reboiler and any cleaning associated with this equipment.
For most columns, quite a few trays can be purchased to 1. This is to be done by a continuous flash process.
offset this cost. 2. All the feed is to be flashed.
When one of the components of the binary is water, and 3. Steam does the heating.
steam is used, the following equation is used for the oper- 4. Some steam condenses.
ating stripping line (there is no rectifylng section): 5. Water is immiscible with the materials.
For component not including water:
Feed:

Benzene: 33.33 + mols
Slope of operating line (L/V), = B/S Toluene: 33.33 + mols
Xylene: 33.33 + mols
Operating line intersects the x-axis at %B. 100.00 mols feed

The stepoff of trays starts at X~B on the x-axis, y = 0. Because water will be present in liquid phase, it will only
exert its vapor pressure. Temperature of flash = 80°C.
Open steam is used for stripping of dissolved or
absorbed gases from an absorption oil, with all of the
steam going overhead, and the stripped oil leaving at the
bottom. This absorption coefficient of the oil for the com- where N2 = mols steam in vapor only
ponent must be known to construct the equilibrium P, = vapor pressure of steam
curve. The operating curve is constructed from several L0i = mols of each component at start
point material balances around the desired component, Pi = vapor pressure of each component at temperature
omitting the oil as long as its volatility is very low. The trays Mols at start Pi Lei/
can be stepped off from a plot of y vs. x as in other binary Component LOi at 80°C, mm Hg --
Pi
Mol
Wt
distillations, again using only the stripping section.
Benzene 33.33 760 0.043 78
Toluene 33.33 280 0.1190 92
Example 8-19: Continuous Steam Flash Separation Xylene 33.33 120 0.277 106
Process: Separation of Non-Volatile Component from 0.4397
organics
P, at 80°C (176°F) = 6.868 Ib/sq in. abs (from steam tables)
It is desired to separate a non-volatile material from an
equimolal mixture of benzene, toluene, and xylene at
80°C. Vapor pressure data for these compounds are shown =% (6.868)- 354mm Hgabs
14.7
in several physical property sources. The following
approximate values for the specific heats and latent heats N2 = P, z LOi/Pi
of vaporization may be used: = 354 (0.4397) = 155.7 mols steam in vapor per 100 mols of
feed (volatile) material
Benzene: cp = 0.419 cal/gm-"C
AHv = 97.47 cal/gm Steam required to heatfeed to 80°C:

Toluene: cp == 0.44 cal/gm-"C Benzene: Sensible heat
AH, = 86.33 cal/gm (78) (33.33) [ (0.419 cal/gm-"C) (1.8)] (80"-20")
= 11 7,800 Btu
Xylene: cp = 0.40 cal/gm-"C
AH, = 82.87 cal/gm Latent heat
(78) (33.33) (97.46 x 1.8) = 434,000
If the mixture is separated by a continuous flash Toluene: Sensible heat
process and the components are considered insoluble in
water (check references) and the feed enters at the flash (92) (33.33) [0.44 x 1.81 (80"-20") = 145,600
chamber at 20°C, calculate the mols of steam condensed, Latent heat
the total mols steam required per 100 mols of feed, and (92) (33.33) (86.53 x 1.8) = 477,000