Hydrocarbon Processing - December 2021 - 63

Environment and Safety
vent. If the unspent NaOH concentration
is high enough, then solvent pH will remain
quite high throughout the column.
A potential (and as it turns out, quite real)
downside is the fact of significant CO2
coabsorption.
In the engine exhaust, CO2
is present at approximately 65 times the
concentration of SO2
physical solubility, CO2
, so despite its lower
may absorb at a
significant rate and parasitically consume a
large share of the caustic soda. The higher
the caustic concentration, the larger the
amount of co-absorbed CO2
the fraction of caustic used up by CO2
and the larger
. It
is, therefore, important to use as low of a
caustic concentration as possible.
SO2
removal using seawater spiked
with NaOH. Simulations were again run
over a range of 1 m-10 m of total packed
bed depth using 1 wt% and 15 wt% NaOH
in seawater as solvent. FIG. 2 and FIG. 3
show the dependence of SO2
removal on
the depth of three beds of 170.X, 2.X and
250.X packing using 1 wt% and 15 wt%
NaOH, respectively. It is apparent that SO2
removal can be described in terms of total wetted area per unit of
tower cross-section, regardless of packing size. It is also apparent
that SO2
itself. Removing exactly 85% of the SO2
removal is, at best, a weak function of packed-bed depth
in the engine exhaust
can be a challenge using caustic soda because of the extreme
sensitivity of removal to bed depth (and wetted area) when substantially
less than complete removal is desired. Exceeding SO2
removal requirements is, of course, a waste of caustic soda; however,
in the interest of meeting the removal goal, a ship's crew
inexperienced in operating a small chemical process plant can be
assured of compliance by exceeding the required removal.
Perhaps the more costly consequence is excessive CO2
removal
when no such removal is required. As FIG. 4 shows, CO2
removal rises rapidly with the caustic level in the solvent. A
15 wt% NaOH strength will remove nearly 50% of the CO2
in
the stack gas, and even just 1% strength will remove nearly 30%
of the CO2
. Given an exhaust of 14% CO2
than in recovering SO2
.
and only 0.216% SO2
,
between 20 and 30 times more NaOH will be used in removing
CO2
Optimizing the system. When seawater is used alone as the
is a fairly simple matter using simulation.b
The key paramesolvent,
determining the flowrate to achieve a given removal of
SO2
ter is the total wetted area in the contacting device. A chart such
as that shown in FIG. 1 is very easy to generate. Although wetted
area is a function of liquid rate, under most commercial conditions
wetted area and the physical, or dry, area of the packing
are similar enough for practical purposes to allow the dry packing
area to be used as a reasonable approximation. A large-size
packing is preferred because its lower pressure drop results in
the least backpressure on the engine. Once the packing has been
selected, the needed bed depth can be readily calculated.
The situation is somewhat complicated when the seawater
is spiked with caustic soda. Several factors are at play that make
caustic addition attractive, but also more complex:
* NaOH provides high concentrations of hydroxyl ion
that react with SO2
and greatly increase its absorption
rate (the pH of seawater is typically 8.1, so the hydroxyl
ion concentration is only about 2 ppbw). Therefore,
with NaOH, much less packing area (i.e., a much shorter
column) is needed to remove most of the SO2
.
* CO2 is also absorbed by caustic soda, and since there is
roughly 65 times more CO2
than SO2
, CO2 absorption
will be a much heavier consumer of caustic unless the
packed depth is minimized.
* Keeping the caustic concentration as low as possible will
minimize CO2
concentration below which the effectiveness of caustic to
remove SO2
co-absorption, but there is a minimum
falls off rapidly (FIG. 5).
* There is nothing to be gained by having the caustic level
any higher than necessary. High NaOH just makes the
solution more corrosive, more hazardous to handle, and
Hydrocarbon Processing | DECEMBER 2021 63
Sr++
F-
Cl-
NaOH
FIG. 5. Types of absorption at various NaOH concentrations. Calculations are for a 1.5-m deep
bed of 2.X packing.
TABLE 1. Solvent and gas composition
Seawater, ppmw
CO2
Na+
SO4
=
Mg++
Ca++
K+
Br+
90
10,800
2,710
1,280
410
399
67
80
1.3
19,350
Various
H2
Exhaust gas, vol%
O
CO2
N2
O2
SO2
CH4
CO
NO2
11.08
14.052
74.478
0.141
0.216
0.00164
0.0046
0.0256

Hydrocarbon Processing - December 2021

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