Chemical Engineering April 2022 - 52

mulation is infrequent, manual venting
from the top of the drum is often
sufficient. If non-condensable gases
accumulate frequently, or the column
is run unattended, automatic venting
is required.
Figure 11 illustrates an automatic
vent system that has worked well [1,
14, 15]. A second pressure controller
(PC 2), a level controller, and a control
valve in the vent line are added.
The set point of PC 2 is lower than
that of the normal pressure controller
(PC 1). When the drum is full, the
level controller keeps PC 2 tripped
off, and the vent valve is closed.
Drum unflooding (due to non-condensable
gas trapping) is sensed
by a drop in drum level. The lower
level activates PC 2. Since the setpoint
of PC 2 is lower than PC 1, it
opens the vent valve. As the pressure
falls, PC 1 closes, helping to
build up the drum liquid level. As
soon as the drum refills, the level
controller trips PC 2, and the vent
valve closes.
Excessive gas in the tower
Issues with an excessive quantity of
gas entering the tower are common,
potentially troublesome, and often
are unacknowledged. This is considered
in detail here. In vacuum towers,
there may also be issues with
deficient quantities of gas entering
the tower. These too are addressed.
The economic impact of non-condensable
gas venting. Other than
operating issues and instability in the
tower, the vented non-condensable
gas usually drags some product
with it, which is lost. These product
losses can be quite expensive, as is
illustrated in the following example.
Consider a tower whose overhead
product is hexane, with the reflux
drum at 30 psia and 80°F. There
is also some nitrogen in the drum,
whether coming from the process
or from a control system that adds
non-condensable gas to maintain
the drum (and tower) pressure. The
drum pressure (Pdrum, psia) is the
sum of the partial pressures of the
liquid and that of the gas, given by
Equation (1):
Pdrum = VPliquid + Pgas
52
(1)
Assuming ideality and equilibrium,
the partial pressure
of the hexane
liquid equals
its vapor pressure,
VPliquid, in
psia. Pgas
is the
partial pressure
of the gas in psia.
At 80ºF, the vapor
pressure of hexane
liquid is 3.1
psia. This means
that the gas partial
pressure is 30
psia - 3.1 psia =
26.9 psia. On a
molar basis, 10%
(equal to 3.1 psia
divided by 30
psia) of the vent
gas at 80°F will
be hexane. On
a weight basis,
the hexane fraction
of the vent
gas is higher
at 26%, due
contains
FIGURE 11. This configuration shows a flooded-drum automatic vent system
that worked well [14] (Reprinted courtesy of AIChE)
FIGURE 12. Here, nitrogen is absorbed by the liquid in a newly added debutanizer
feed drum
to the lower molecular weight of
the nitrogen. So, every ton of vent
gas
0.26
tons
of hexane.
This vented product is likely
to be lost, and may increase flaring
or emissions. The non-condensable
gas, even nitrogen, can be absorbed
into the product, and can later increase
pressure
in
downstream
equipment,
resulting in more product
loss and flaring downstream, as
discussed below.
To minimize the losses of product
in the vent, the vent gas is sometimes
chilled, often in a knock-back
condenser, and the condensate recovered.
Even cooling by as little
as 20°F (to 60°F), which can be
achieved with cold water, would reduce
the liquid hexane vapor pressure
to 1.8 psia. It would also would
reduce the hexane content of the
vent gas from 26 wt.% to 16 wt.%,
almost halving the product loss in
the gas.
Gas absorption by feed or reflux
drum liquid. This mechanism is one
of the most common, yet unappreciated,
sources of wasteful venting.
It is often inconceivable to think that
a gas like nitrogen or fuel gas can
be absorbed in significant quantities
by process liquids. Given enough
contact surface, the relatively small
quantity absorbed can become significant,
turn into a major economic
loss,
and
generate
environmental
issues and tower instability, as
described below.
Nitrogen was used to maintain
pressure in a newly added debutanizer
feed drum (Figure 12). Following
restart, it was observed that
much of the nitrogen ended up in
the downstream plant. The plant
needed to do considerable venting
from the downstream debutanizer,
deisobutanizer and depropanizer.
The nitrogen got into all of these
towers. The plant operators also noticed
that the nitrogen control valve
was opening almost fully, suggesting
that a lot of nitrogen was coming
in. This nitrogen had to get out, and
the only open route is with the drum
outlet liquid.
The high absorption rate was due
to the feed entry near the liquid surface.
This exposed extensive contact
area between the feed liquid
and the nitrogen in the drum vapor
space. The way to avoid this is to
pipe the feed liquid to discharge
near the bottom of the drum, so the
entering liquid has no contact with
the vapor space. The only contact
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