Hydrocarbon Processing - October 2022 - 18

Plant Safety and Environment
tainer skid (FIG. 3). The container was designed
to be easily transported via truck
or ship for testing at different locations.
The steel mill gases were supplied to
the pilot unit from tie-ins made on the
steel plant's internal gas header system.
The BFG and COG from the pipelines
entered the unit at a slight overpressure
(TABLE 2). All treated gases and offgases
were routed back into the piping network
of the steel mill.
The gas from the steel mill first passed
through a set of coalescing filters-two
installed in parallel for each feed gas-for
high-efficiency removal of sub-micron
liquid droplets and solids. Mass flowmeters
installed downstream of the filters
were used to meter in the gases to the
compressor suction.
A process sketch of the acid-gas scrubbing
process used in the pilot plant is
shown in FIG. 4. The mixed gases were
compressed in a single-acting, three-stage
positive displacement compressor with
interstage cooling and liquid condensate
removal. The compressor can deliver
gas at a maximum discharge pressure of
32 barg. The compressed gases from the
compressor were cooled in a plate-andframe
heat exchanger to 40°C (104°F)
with chilled ethylene glycol (EG). A
flowmeter measured the flow of cooled
gas before it entered the absorber.
In the absorber, the steel mill gases
were contacted with the proprietary solventa
to
remove the acid gases by chemisorption.
The absorber-2.8 m in height
and 5 cm in diameter-had a single
packed bed loaded with stainless-steel
pall rings for enhanced gas-liquid contacting.
The column was fitted with a demister
pad above the packed bed for de-entrainment.
Pressure drop over the packed
bed was monitored using a pressure differential
cell to identify potential foaming
or fouling in the packed bed. The column
was also fitted with a resonating fork for
foam detection at the top of the column.
This was specified by Dow Industrial Solutions
based on successful commercial
scale applications. The bottom level in
the column was measured using a guided
wave radar and controlled via an air-actuated
level control valve. The treated gas
from the top of the absorber was measured
using a flowmeter. The inlet flow
measurements, along with the inlet and
outlet stream compositional analysis, enTABLE
2. Typical operational parameters for the CO/CO2
Parameter
Input pressure, bara
Compressor discharge pressure (normal), bara
Compressor mass flow, kg/hr
Liquid circulation rate, kg/hr
Proprietary solventa
in water, wt%
Absorber temperature liquid inlet, °C
Reboiler temperature, °C
separation pilot line
Value
1.05-1.3
29
10
45
> 45
40
120-125 (132 max)
abled the determination of the acid gas
removal efficiency in the absorber.
The treated gas left the absorption column
overhead and was routed to downstream
consumers at pressure. The solvent
left the absorption column through
the bottom nozzle. The solvent is fully
saturated (i.e., rich in CO2
) and is further
called " rich solvent. "
The rich solvent left the bottom of the
absorber and passed through a particulate
filter before being flashed in the horizontal
flash vessel. The flash gas was removed
from the top of the de-entrainment section
of the flash, and the pressure was regulated
between 4.5 barg-5.5 barg. The rich solvent
from the flash vessel was first routed
through an activated carbon filter before
entering the top of the stripping column.
The activated carbon filter removed foamcausing
impurities
(e.g., dissolved hydrocarbons,
degradation products and a
variety of surface-active agents) that had
entered or formed in the system. Heat
tracing was installed on the transfer line
from the flash vessel to preheat the solvent
to above 90°C (194°F) before entering the
stripper column. Typically, on a commercial
unit, preheating the solvent is done by
a heat exchanger that recovers heat from
the lean solvent after regeneration.
The rich solvent entered the top of
the regenerator column and contacted
stripped gases and water-amine vapors
generated in the reboiler over a single
packed bed loaded with stainless-steel pall
rings for enhanced gas-liquid contacting.
The power input to the electrical reboiler
was controlled by maintaining a desired
temperature at 120°C (248°F) of the solvent.
An overflow weir was installed in the
vessel to maintain a constant liquid level
to submerge the reboiler and ensure adequate
liquid residence time in the reboiler.
The overflow from the reboiler was routed
into a surge vessel that was located directly
below the vessel containing the reboiler.
The overhead vapors from the regenerator
column were cooled in a plate-andframe
exchanger to 20°C (68°F) with
chilled EG. The condensed liquid was
recovered from the acid gases in a knockout
vessel that had a demister for high-efficiency
de-entrainment. The liquid recovered
in the knockout vessel was returned
to the top of the regenerator column, using
a positive displacement pump.
The lean solvent exited the bottom
FIG. 4. Schematic representation of the CO2
removal process applied in this project.
18 OCTOBER 2022 | HydrocarbonProcessing.com
of the surge vessel and was cooled in a
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