MARITIME

The Author
Nicholas Confuorto is President and
COO of CR Ocean Engineering LLC (www.
croceanx.com). Confuorto is also Chairman
of the London-based Exhaust Gas
Cleaning Systems Association (EGCSA).
e: nconfuorto@croceanx.com
t: +1 (908) 209-9701
BY NICHOLAS CONFUORTO
owners to feel confident that they can
continue using the lower cost high sulfur
fuel oil even in the Environmental Controlled
Areas (ECA).
A properly designed marine scrubbing
system includes certain features. It
has to be light weight, relatively small
in both diameter and height, highly efficient
and it must be cost effective. It
also has to have low backpressure and all
metal construction. It shall be used as a
silencer when wet or dry and it shall not
require a bypass. The system shall also
include the required wash-water treatment
equipment and all required monitoring
and control equipment. An upflow
configuration seems to be preferred
by many ship owners because it requires
less space than the side-entry designs.
Scrubbing Systems are available in
Open Loop (a once trough design using
seawater to neutralize the collected
sulfur emissions), Closed Loop (using
a freshwater solution with an alkaline
solution to neutralize the collected sulfur
compounds) or Hybrid configurations
(deigned to be both Open Loop and
Closed loop and able to switch from one
configuration to the other on demand).
Specific selection is based on ship routing
or owner’s preference.
The Open Loop Design
An Open Loop system is one where
the scrubber uses seawater in and out
on a once-through basis, a design that
uses less equipment and fewer controls.
However, because seawater is not
as good a neutralizing agent as caustic
(NaOH), it will require larger piping and
larger pumps. For the same reason, it
is constructed using higher grade alloys
to allow for the lower pH environment.
The pH in the system is adjusted by the
amount of seawater used. The pumped
seawater is distributed within the scrubber
vessel by strategically located nozzles.
Each nozzle is designed to optimize
the droplet size, distribution and droplet
residence time to allow for the maximum
contact between seawater and flue
gas SO2. The SO2 is absorbed into the
droplets. The absorbed SO2 reacts with
water to form sulfurous acid. Sulfurous
acid dissociates into bisulfite. The bisulfite
further dissociates to sulfite and
sulfates. The sweater pH is then used to
neutralize the acidity of this discharge
water in order to meet the pH requirements
set up by IMO. CROE and various
other scrubber suppliers have provided
several of these open loop systems
to ship-owners globally.
The Closed Loop Design
For ship-owners who travel in fresh
or very low alkalinity waters, or wish
to keep the scrubber water discharge on
board (in a tank) for a period of time, the
Closed Loop scrubber design could be
the preferred configuration.
The scrubber features used in a closed
loop operation are the same as those
used in open loop or hybrid. However,
the pH in the system is controlled by the
amount of caustic added to the circulating
loop. A Closed Loop system uses a
solution of fresh water and caustic as the
reagent to remove SO2 from the exhaust
gas stream. This design earns its name
from the closed loop used to circulate
the caustic solution between tank and
scrubber. Because the caustic solution
is much more efficient in removing the
SO2 than seawater, much less water is
circulated a closed loop system than in
an open loop system. This means that
the piping, valves and the pumps used
for a closed loop system are smaller and
require less power.
Unfortunately the circulating solution
in a closed loop system cannot circulate
untouched for a long time because
the sulfates/sulfites/bisulfites and sludge
content in the circulating solution continue
to build up (due to continued capture
of SO2 and particulates). Should the
concentration of these collected pollutants
exceed a predetermined maximum
level, the scrubbing efficiency in the
scrubber will decrease significantly and
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