Chemical Engineering June 2019 - 50

Treated gas
Treated gas
Untreated gas
Untreated gas
Makeup
chemicals
Makeup
water
Blowdown
FIGURE 2. A chemical oxidation system neutralizes H2S gas using a
base, typically sodium hydroxide
large fluctuations in inlet composition
and gas flow while maintaining
high H2S removal efficiencies
of greater than 99.9%. The systems
are, however, higher in capital
costs due to the increased control
sophistication that is required. Although
most liquid redox systems
have very similar overall chemistry,
different suppliers use different
chelating agents to keep the iron
in solution. Some of the chelating
agents, such as nitrilotriacetic
acid (NTA), are hazardous. Recent
advancements in this technology
have helped drive down the operating
costs through the development
of innovative chemical reagents
that reduce chemical consumption
rates. Similarly, the equipment and
installation costs are being reduced
due to modularization and skidpackaged
offerings.
Chemical oxidation scrubber
In a chemical oxidation scrubber,
a base, typically sodium hydroxide
(NaOH), is used to neutralize H2S
gas after absorption into the scrubbing
liquid. The absorbed H2S is
then oxidized using a chemical oxidizing
agent, typically hydrogen peroxide
(H2O2) or sodium hypochlorite
(NaOCl), to form soluble sodium
sulfate (Na2SO4), as shown in the
overall equation below. Na2SO4 is
removed from the system through a
blowdown stream. This sulfate-containing
wastewater (blowdown) must
be treated or sent for disposal.
50
Makeup
nutrients
Makeup
water
FIGURE 3. Biotrickling processes use autotrophic microbes to consume and
transform H2S
H2S + 2NaOH + 4H2O2
→ Na2SO4 + 2H2O
(Overall reaction with NaOH and H2O2)
Packed-bed scrubbers are the
most common process equipment
used for chemical oxidation (Figure
2). Vertical countercurrent packedbed
scrubbers are generally preferred
due to their high efficiency.
Compared to a spray tower, tower
packing provides additional mass
transfer and therefore smaller equipment
size and pumping costs for the
same performance.
In the vertical countercurrent design,
gas flows upward while scrubbing
liquid flows down through
spray nozzles or a liquid distributor
above the random packing section.
The absorption of gases occurs in
the packing section. A mist eliminator
inside the vessel removes entrained
liquids from the gas before
exiting the scrubber. A regulated
amount of the reagents is added to
the re-circulating liquid to maintain
setpoint pH and oxidation-reduction
potential (ORP) levels. Fiberreinforced
plastic (FRP) is typically
the material of choice for the process
vessels, particularly if sodium
hypochlorite is used as the oxidizing
agent.
One
of
the disadvantages
chemical oxidation is that caustic
is not selective to H2S in the presence
of other contaminants, such as
carbon dioxide (CO2). Not only does
CO2 increase chemical consumption,
but the reaction products can
cause scaling in the packing. Care
must be taken during the equipment
design and selection of process setpoints
to ensure CO2 interference
is minimized.
Chemical oxidation scrubbers
are compact and low in cost.
They are also capable of handling
large fluctuations in inlet composition
and gas flow while maintaining
high H2S removal efficiencies
of greater than 99.9%. The chemicals
that are used are commodity
chemicals, and therefore, availability
is rarely a concern. The rate of
chemical consumption can be high
relative to the amount of H2S that
is treated. For example, for every
1 mole of H2S, 2 moles of NaOH
and 4 moles of H2O2 are required,
as shown in the overall reaction.
Furthermore, handling the sulfatecontaining
wastewater stream adds
significantly to the operating cost of
the scrubber.
of
Biological processes
There are several types of H2S removal
technologies that use biological
or biochemical processes.
This article focuses on biotrickling
filters, which use autotrophic microbes
to consume H2S and convert
it into sulfuric acid. Biotrickling
filters consist of a vessel with a
packed or porous media section
(Figure 3). The media provides a
large surface area for the microbes
to grow on. Makeup water is required
to maintain the pH, and a
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Chemical Engineering June 2019

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