Hydrocarbon Processing - June 2022 - GP-38

ENVIRONMENT & SAFETY
discussed further in this paper. It is ideal
to reduce the amount of H2
S present in
the AD to minimize the toxic hazard and
the potential corrosion of equipment. A
common practice to reduce the production
of H2
to bleed air [0.3%-3% oxygen (O2
into the headspace of the AD.5
S and the O2
S is micro-aeration, which is
)]
Sulfuroxidizing
bacteria (SOB) are already
present in the AD and can easily convert
the H2
into elemental sulfur
and water vapor, so this method is cheap
and preferred by most facilities operating
an AD. While this is an effective method
to reduce the toxic hazard of H2
S, it increases
the risk of creating an explosive
atmosphere and resulting in an explosion
within the AD.
Air is added to the AD in a controlled
manner to ensure it remains below the
lower flammability limit; if the method
of adding air fails, the AD can quickly
contain an explosive mixture. It is recommended
to consider designing a mitigation
system to reduce the risk of an AD
explosion when incorporating air into
the AD with:
* Lower explosive limit (LEL)
detection of the AD and alarms
in the operator room
* An emergency shutdown protocol
to include safety actions to activate
appropriate mitigations, such as
shutdowns of electrical equipment
(ignition sources) or isolation of
the AD from oxygen sources
* Frequent testing intervals
of mitigation equipment
(transmitters, isolation valves, etc.)
* Proper maintenance of mitigation
equipment.
Safeguards and mitigations include:
* Preventing the creation of a
flammable/explosive environment
in the AD:
° Routine inspection and repair to
eliminate potential sources for an
influx of air into the AD; this not
only eliminates the possibility
of a flammable/explosive
environment but is
also operationally beneficial
for the AD.
FIG. 2. Vacuum causing the buckling of
AD tank.7
° Automatic shutoff of
downstream blowers to prevent
reverse flow of air into the AD,
if applicable.
° Safety controls to prevent the
over bleeding of air or oxygen
into the AD, if applicable.
* Reducing ignition potential by
determining hazardous area
classification (HAC) surrounding
the AD and using appropriately
classified equipment, especially if
using an electrically-driven mixer
for the AD.
AD rupture. Another concern is any
loss of biogas containment. The released
gases pose flammable and toxic hazards
to nearby onsite personnel. If ignited,
the flammable gas can lead to a flash or
jet fire, while the concentration of H2
S
FIG. 3. Corrosion causing material to leak from
the AD.9
38 MAY/JUNE 2022 | GasProcessingNews.com
gas can be lethal to personnel close to
the AD even without ignition. In cases
where chicken manure is used, ammonia
(NH3
) is produced as a byproduct,6
which is a potential toxic hazard even at
fairly low concentrations.
Specifically for animal waste, ADs are
constructed with floating roofs to account
for the growing volume of vapors.
However, if not properly monitored, the
pressure inside the vessel can increase
rapidly to cause the roof to " pop off, " or
the vessel to rupture. Based on three past
events in 2012 and 2013 reported by the
UK's Environment Agency,7
pressure relief
valves (PRVs) failed to mitigate overpressures
in the AD, causing the rupture
of the process vessel and the release of
flammable and toxic material into the atmosphere.
Overfeeding the AD and poor
temperature control can cause increased
foaming and liquid level in the vessel.
If high enough, this level can block the
PRVs and prevent them from operating
on demand. Alternatively, the pressure
inside the vessel can drop below design
pressure. Underfeeding the AD will cause
the vessel to vacuum (FIG. 2), buckle and
release biogas into the atmosphere.8,9
Safeguards and mitigations include:
* Properly inspect PRVs
* Closely monitor pressure, level and
temperature operations of the AD
* Use process hazard analysis
(PHA) studies to determine
the appropriate level of safety
interlocks needed to prevent
over-pressure and vacuum
conditions in the AD
* Perform safety instrumented
system (SIS) and safety integrity
level (SIL) evaluations to ensure
safety interlocks meet the desired
level of reliability.
Leaks from the AD. Leaks are expected
to pose smaller hazards than full-bore
ruptures. However, it is more likely in
normal operating conditions for smaller
leaks to occur with corrosion playing a
major factor, as seen in FIG. 3. Corrosion
of the AD5
occurs when H2
S created in
the process combines with moisture from
the AD process to produce sulfuric acid
(H2
SO4), eventually causing releases of
biogas but at a lower and more sustainable
rate than the rupture of the tank
(FIG. 4). With smaller rates of release, personnel
may experience low doses of toxic
gas (H2
S or NH3), which may be noticed
S.
by the pungent odor, especially with H2
The presence of odor can indicate a
leak and should be mitigated to prevent
additional hazards from occurring.10
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