Hydrocarbon Processing - December 2021 - 62

Environment and Safety
(90,000 hp) running on fuel oil, plus two
gas turbines rated at 50 MW total. These
data correspond to a Queen Mary-type
cruise liner. Only the boiler engines are
considered in this example.
Two cases are considered: (1) onceFIG.
2. Dependence of SO2
removal on bed depth and packing size-1 wt% NaOH.
through flow of seawater, and (2) recirculating
flow of seawater containing some
level of caustic soda. To be specific, the absorber
column is assumed to contain one
of three sizes of proprietary structured
packinga
The X-seriesa
than the Y-seriesa
(M170.X, M2.X and M250.X).
has lower pressure drop
because of its 60° crimp
angle, which imposes less engine backpressure.
The absorber is sized for 85% of
flood, and the packed depth is calculated
to achieve the same discharge SO2
concentration
as would be in the stack exhaust
if VLSFO (0.5% sulfur) was burned. In
other words, about 85% of the SO2
is to be
FIG. 3. Dependence of SO2
removal on bed depth and packing size-15 wt% NaOH.
removed by the scrubbing system when
burning HSFO. Note that when using
caustic soda, much lower sulfur emissions
(> 95% removal) can be easily achieved.
A proprietary simulatorb
contains the
components needed to fully describe the composition of seawater.
TABLE 1 shows the typical compositions of seawater (100°F)
and hot engine exhaust gas (800°F) entering the system. The
exhaust gas flowrate was slightly over 42,360 standard cubic feet
per minute (SCFM), and the absorber was always sized for 75%
of vapor flood. Seawater flow was maintained at 2,000 m3
close to the value needed to remove 85% of the SO2
/hr,
in a reasonable
packing depth with just seawater. NaOH is listed separately
in TABLE 1 because it is added to seawater as though it were a molecular
entity. Seawater already has an inherent Na+
tion tied up with Cl-
concentraand
other ions as part of the charge-neutral
ionic soup comprising seawater. Of course, in the mass transfer
model, the solvent is treated as an aqueous ionic soup.
SO2
FIG. 4. Using NaOH for SO2
removal also removes much CO2
Calculations are for a 1.5-m deep bed of 2.X packing.
area for contacting the upward-flowing gas. However, droplets
tend to coalesce with each other and with the walls of the equipment,
so it may be necessary to place spray nozzle batteries at
more than one level in the column. The other concern with spray
contacting is significant carryover of liquid into the emerging
cleaned gas. This may not be a big issue if the solvent is seawater;
however, the presence of caustic soda is problematic. Carryover
can be controlled with mist pads; these would be mandatory using
seawater spiked with caustic. As will become apparent from
the case study, only a very short packed bed (or, alternately, a
short spray section) is needed when caustic soda is used to enhance
SO2
absorption and the upper part of the column can be
used to virtually eliminate mist when treating with seawater.
Case study. The specific case is a 76,000-metric-t ship driven
by four boiler engines with a combined rating of 67.2 MW
62 DECEMBER 2021 | HydrocarbonProcessing.com
.
removal using once-through seawater. Simulations
removed depends on both
, and the designation " X " coris
removed in a 2-m bed of 250.X
/m3
/m3) needs a 4-m depth,
were run over the range 2 m-10 m of total packing depth.
FIG. 1 shows how the fraction of SO2
packed bed depth and structured packing size. Note that the
packing size designation is roughly the specific surface areac
the dry packing in the units m2
responds to a 60° crimp angle, providing the minimum pressure
drop. About 85% of the SO2
packing. The M2X size (about 200 m2
and 170.X needs between 5 m and 6 m, which means that three
times the amount of 170.X packing is needed to achieve the same
SO2
removal as 250.X packing. Column diameter under these
conditions is typically 13 ft-15 ft with 170.X, 2.X and 250.X
packing, with the finest packing needing the largest diameter.
SO2
almost always exhibits a very small (usually negligible)
saturated. Adding caustic soda to seawater not
, but it also enhances the
equilibrium backpressure, except when the water starts to become
nearly SO2
only increases solvent capacity for SO2
absorption rate itself through acceleration by chemical reaction
with the high hydroxyl ion concentration of a very alkaline solof
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