Chemical Engineering January 2016 - 43

Inert padding
Instability due to surface disturbances or
agitation may be alleviated, even mitigated,
by padding the drum with non-condensable
inert gases. A source of inerts, such
as nitrogen or fuel gas, is connected to the
vapor space of the drum. The drum pressure
is controlled by adding or venting the
inerts. The drum pressure is no longer the
vapor pressure of the liquid, but now equals
the sum of the vapor pressure (VP) of the
drum liquid and the inerts partial pressure,
as shown in equation (8). The box on p. 44
presents a practical calculation example of
the effects of inerts in a tower.
P2 = VPdrum liquid + Pinerts
(8)
with total condensers, inert padding is
usually implemented during operation as
a temporary solution to alleviate instability,
especially since the inerts can be quite expensive.
The vented inerts contain vaporized
product. In the calculation example on
p. 44, assuming ideality and equilibrium,
10% (3.1 psia/30 psia) of the vent gas at
80°F on a molar basis will be hexane. on
a weight basis, the hexane fraction of the
vent gas is even higher due to the low molecular
weight of the inerts - 26% hexane
on a weight basis for nitrogen padding.
This vented product is likely to be lost, and
may increase flaring or emissions. The inerts,
even nitrogen and fuel gas, can be
absorbed into the product, and can later
increase pressure in downstream equipment,
resulting in more product loss and
flaring downstream. To avoid inconsistent
transitions from the inert-addition mode to
the venting mode, there is often a pressure
range in which inerts are added simultaneously
with venting, which compounds the
previously described issues [15]. In order
to maximize product recovery and minimize
emissions and flaring, some experts recommend
against using inert padding with
total condensers, other than as a temporary
solution [15]. diagnosing the cause of, and
eliminating the surface instability, is usually
a preferred longterm solution, especially
with volatile products.
Decanting water
If the reflux drum is used to decant small
quantities of free water from condensed
hydrocarbons or other water-insoluble organics,
the entry point of the condensate
liquid (and other subcooled liquid streams
that may contain free water) should be located
within the drum opposite to the end at
Water
which the liquid product and reflux are withdrawn.
The water-removal boot should be
just upstream of the point where the reflux
and product streams are withdrawn [16],
as illustrated in Figure 5 for the control system
from Figure 2a. In many cases, a short
standpipe (about 6 to 12 in. tall) or judicious
baffling are used as additional measures to
keep water out of the reflux and product
draw [16], but these additional measures
may lead to water accumulation in the drum.
Also, corrosion is possible when the interface
level controller in the boot malfunctions, and
potentially acidic water is not adequately removed
from the drum. ref. 17 describes a
related experience.
Non-condensable gases
Flooded condenser schemes are suitable
only for total condensers, although some
less satisfactory variations are also available
for partial condensers [5]. The schemes in
Figures 2 and 3 can handle small amounts
non-condensable gases, such as those introduced
during startups or upstream upsets.
To handle these non-condensables,
vents are required on the condenser and the
drum. The condenser vents can be directed
to the vapor space of the drum, to an upstream
unit or elsewhere. The drum vents
should be board-operated, and if frequent
venting is anticipated, the condenser vents
should also be board-operated. In one case,
a debutanizer flooded-condenser system
experienced frequent high pressure, instability
and flaring due to the breakthrough of
non-condensables from an upstream tower
that had control issues [18]. The problem
ChemiCal engineering www.Chemengonline.Com january 2016
43
Product plus
reflux
FIGURE 5. The floodedcondenser
control scheme
from Figure 2a is illustrated
here with a water decanting
configuration
PC
Overhead
from tower
Vent
~~
LC
ILC
Interface
Signal to
product
or reflux
valve
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Chemical Engineering January 2016

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