Page 70 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 59

for 300 mm Hg total pressure system, reading the inter- Then, the distillate:
section point for benzene and steam at 46”C, gives 220
mm Hg for benzene and (300 - 220) or 80 mm Hg. Then
the mol ratio of benzene to water vapor 220/80 = 2.75; or
2.75 parts of benzene to 1 part of water.
When the composition of the compounds in the still or Operation of the open still with only a steam injection
bottoms changes significantly as the batch distillation pro- sparger to bring steam below the liquid level in the still
gresses, an unsteady state condition will exist as for differ- may not be totally efficient as for the same condition oper-
ential distillation (see discussion of this subject later). ating the unit with internal trays as a stripping column.
When nonvolatile material in the bottoms is significant, This should be examined for each situation, as the instal-
and no liquid water exists there-that is, ps is below satu- lation of trays can be expensive, particularly if they do not
ration of the steam pressure at the still temperature-then aid significantly in achieving the desired separation. Gen-
the Raoult’s Law steam efficiency is [ 1271 : Values of E are erally, when no liquid water is present, the best operations
found to range from 60% to 70% for many organics, but of an open still (with condenser) may be at the highest
values of 90% to 95% are reported [127] for good sparger working temperature suitable for the effects on the fluids
design for steam injection, and molecular weight of organ- (Le., no polymer formation, no breakdown, etc.) . Often,
ics < 100, and 50% for many lubricating oils. direct steam injection can be reduced for any operation by
careful heating of the still by an internal reboiler coil, or
possibly a steam jacket. The heat sensitivity of the com-
pounds involved must be recognized.

Steam Distillation-Continuous Flash, Multicomponent
where E = vaporization efficiency of steam distillation or Binary
This system requires direct steam injection into the still
Note that for this discussion now, pirn, just above and in
equations to follow, refers to the pure component vapor with the liquid, all the steam leaves overhead with the boiled-
pressure of the immiscible liquid being distilled [127]. up vapors (no internal condensation) in a steady-state oper-
ation, and system at its dew point. Steam is assumed immis-
When steam is added to the still [127]:
cible with the organics. Steam distillation is usually applied
in systems of high boiling organics, or heat sensitive materi-
(8 - 1 10) als which require separation at vacuum conditions.

and, for a constant distillation temperature, pim is con- (8- 115)
stant. Then for constant pressure:
b is more volatile reference component
i * s = components, i, are not to include steam, s
M, = total mols steam required
bo total mols hydrocarbons at start (not including the
=
As the organic or volatile material is reduced due to the steam)
batch distillation, the steam pressure rises during the = mols liquid in bottoms of still at time, T
progress of the operation due to the loss of the volatile Bi, = mols of component, i, at start
material, and the decrease of pirn. When the volatile mate- cq = relative volatility of more volatile to each of other
rial is stripped down to a low residual concentration, then components
ps approaches the total system pressure, x. When the Pb = vapor pressure of reference more volatile component, b
steam saturation pressure and temperature is greater than n = total system pressure, absolute
IC, no steam condensation will occur during the operation. Bb = mols of component, b, used as reference for vohtibty,
after a given time of distillation
When the non-volatile concentration is low to insignifi- Bo = mols of component, b, used as reference for volatility, at
cant in the still feed, then No is small relative to Ni,. Then start of distillation
pim is considered constant [127]:
Example 8-18 Multicomponent Steam Flash
Pim = E Pim (8 - 11 2)
A mixture of bottoms material of composition Bi, below
has accumulated in the run-down tank. It is necessary to