Hydrocarbon Processing - February 2021 - 50

Process Optimization
alloy above the liquid feed nozzle, where
corrosion is more significant.
Tray tower design for SWS. Most SWS
systems are designed with trayed towers.
Trays can be designed to be fouling resistant. However, even in trayed systems,
the selection of an inappropriate tray can
lead to poor performance of the SWS.
General recommendations for SWS tray
selection include:
* Trays should be a fixed-valve type
and should be designed for vapor
to flow horizontally out of the
valves to minimize bridging of
deposits on the fixed valves.
Tray designs like this are readily
available from major distillation
internals vendors. Sieve trays
can also be fouling resistant in
some services. For example, the
authors know of acceptable sieve
tray use in aqueous systems
with solid particles circulating
(i.e., in slurry service). However,
sieve trays have shown severe
fouling in SWS service, with vapor
flow area decreasing by as much
as 90%. This may be due to the
vertical direction of the vapor
leaving the tray deck, which allows
precipitation on the tray deck that
can foul the tray.12,13 FIG. 3 shows
an example of fouling that can
occur on sieve trays in SWS service.
This level of fouling occurred
over a typical SWS run between
maintenance intervals of 5 mos.13

* All trays should be constructed of
300-series stainless steel or better.
Depending on the sour water
processing demand, the tower
may be too small for personnel
to physically install the trays.
In this instance, cartridge trays
could be used.
* If a pumparound system is installed,
the trays used for the pumparound
loop should not be counted as
active mass transfer trays.
* In a fouling service like with an
SWS, the downcomers are potential
traps for fouling material and
can adversely affect the capacity
of a tray. Special designs that
are available from the internals
suppliers to address fouling material
in the downcomers should be used.
Tray efficiency is reported in several
different ranges for SWS service, but generally will vary from 15%-50% depending
on different factors. The number of trays
present in the SWS will then also vary
widely; a common range on the number of
actual trays installed may be 20-60. On a
24-in. spacing, this translates to 40 ft-120
ft of height for trays, which may mean an
SWS as tall as 150 ft in some applications.
From the authors' discussions with
a few refinery subject matter experts
(SMEs), a rough rule of thumb for design tray efficiency in an SWS is 3 actual
trays per 1 theoretical stage or 33% efficiency. This is probably a conservatively
low efficiency for most systems. For example, one SME acknowledged this rule

FIG. 3. Fouling of sieve tray in SWS service.13

FIG. 4. Example of tray fouling in SWS service.13

50 FEBRUARY 2021 | HydrocarbonProcessing.com

of thumb, but noted that actual tray efficiencies experienced in sour water service
(presumably well designed) were closer
to 50%. In designing a trayed system, one
could probably rely on the rule of thumb
to result in a system with significant overdesign built in. For a less conservative and
more economical design, careful engineering analysis and comparison with the
actual performance of other similar SWS
systems is needed.
Some factors that influence the efficiency of the trays include the following:
* Perhaps most importantly, tray
efficiency is a chemical engineering
factor that is applied to equilibriumbased designs to account for the
fact that operating trays do not
reach equilibrium conditions.
Hatcher and Weiland14 show that
component efficiencies for H2S
and NH3 will vary widely across
the stripper column, and could
depend heavily upon the stripped
water specification for the water
leaving the bottom of the stripper,
the steam rate to the stripper
or reboiler, etc. Therefore, the
efficiency of the tray is not a static
value throughout the stripper, varies
from one component to another
and may be different in the top of
the tower than it is in the bottom.
To reduce uncertainty, the designer
may need to do a more rigorous
simulation of the column.
* The most important consideration
for SWSs is that they work reliably.
As a result, designs for SWSs tend
to be conservative. One way of
introducing conservatism into the
SWS design is to specify a low tray
efficiency that, when installed,
will allow the stripper to operate
and meet specifications in a more
heavily fouled state and to meet
specifications if the impurities
present in the sour water exceed
the initial design values. If there is
access to an existing stripper in the
same service, then operating data
can be obtained to verify the
design parameters.
* In many instances, the actual
composition of the sour water
feeding the SWS system may be
uncertain. Crude oil slates in a
refinery can change frequently, with
the nitrogen and sulfur contents
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