Hydrocarbon Processing - December 2021 - 61

Environment
and Safety
P. OTT and R. H. WEILAND,
Optimized Gas Treating Inc., Buda, Texas
Stack gas scrubbing to meet IMO's
0.5% sulfur bunkering requirement
The International Maritime Organization implemented the
IMO 2020 regulations on January 1, 2020. As they pertain to
sulfur emissions, the regulations require ships without exhaust
gas scrubbers to burn either 0.5-wt%-sulfur fuel (very low sulfur
fuel oil, or VLSFO) or 0.1-wt%-sulfur marine gasoil (MGO).
Ships outfitted with scrubbers can continue burning 3.5-wt%sulfur
heavy fuel (high-sulfur fuel oil, or HSFO). The economics
of installing scrubbers depends very much on the price differential
between VLSFO and HSFO.
One analysis1
co-absorption of CO2
. These aspects of shipboard exhaust gas
treating are discussed in a later case study.
reported a $100/t differential as the tipping
point above which installing scrubbers becomes cost-effective,
although the figure could be as low as $75/t. Of course, a decision
will depend on the specifics of the ship in question with regard
to the ease or difficulty of fitting scrubbers and associated
equipment, and therefore the capital cost component. As the
demand for VLSFO increases, the price differential is likely to
rise over the long term (the effects of COVID-19 notwithstanding),
not only from the rising value of VLSFO but also from the
falling value of HSFO. This will likely make the scrubber option
increasingly attractive.
Concern has been expressed by some port authorities about
the discharge of sulfur dioxide- (SO2
-) laden seawater from
open-loop (once-through) systems when ships are close to ports.
The alternative is a closed-loop system in which the solvent contains
caustic soda or another alkaline medium to absorb and
hold the captured SO2
. Closed-loop systems are more expensive
to operate. However, it is fairly straightforward to switch between
closed- and open-loop operation, so there is considerable
interest in both systems. In addition to monitoring SO2
from the
ship's stack, there are compliance requirements for nitrogen oxides
(NOX
); this means that monitoring exhaust gas from ships
is becoming more akin to monitoring emissions from refineries.
Some interesting learnings can be gained from the simulation
of packed absorbers for this service. One is the huge difference
between scrubbing with seawater only vs. seawater spiked
with caustic soda. Another is the effect of caustic concentration
on performance. Yet another is that scrubbing efficiency
is mostly a function of the area of the interface between the solvent
and the exhaust gas within the scrubber. A fourth is the
negative effect of CO2
optimization-maximize the SO2
co-absorption, which demands system
absorption and minimize the
General considerations. Gas-liquid contacting for the purpose
of removing one or more constituents in the gas by absorption
into a solvent is usually carried out in a column containing
trays or some form of packing (either structured or random).
Due to rocking motion (roll, pitch and yaw), treating aboard
ships presents special challenges. On land, every effort is made
to ensure that columns are closely vertical-a situation that cannot
be attained shipboard. Column motion disqualifies trays
from serious consideration as contacting devices. Between random
and structured packing, structured packing is usually preferred
in floating applications (such as FLNG and FPSO) because
lateral liquid and gas flows are discouraged by the fact that
the structured packing consists of vertically aligned corrugated
sheets stacked together. The sheets are an obstacle to the lateral
passage of liquid. This helps ensure that gas and liquid flows
stay fairly evenly distributed across the column cross-section. In
this article, attention is focused on the use of structured packing.
However, that is not to say that structured packing is the
only contacting device possible. Spray columns are an alternative,
but high-performance liquid distributors are still necessary.
A set of spray nozzles uniformly placed over a cross-section
near the top of an otherwise empty column can effectively produce
a spray of small droplets with large total interfacial surface
FIG. 1. Dependence of SO2
once-through seawater.
removal on bed depth and packing size-
Hydrocarbon Processing | DECEMBER 2021 61
Hydrocarbon Processing - December 2021

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