Hydrocarbon Processing - April 2022 - GP-14

SPECIAL FOCUS: GREEN TECHNOLOGIES
vides an excellent platform to reflect this
company image.
FGRS components. A flare gas recovery
installation at an existing plant will consist
of four main components. Brief descriptions
of these component are below:
1. Compression technology is
designed to recover flare gases
for reuse in the existing
processing facilities.
2. Staging devices safely divert
routine flared gases to the
FGRS, but not the emergency
or abnormal relief loads. FIG. 1
illustrates the integration of an
FGRS into the flare system.
3. A nitrogen-generation package is
included to supplement existing
nitrogen-generation capacity.
Nitrogen will be used as a purge gas
downstream of the staging device.
4. A control system within the
package interfaces with the plant's
distributed control system.
As shown in FIG. 1, the flare gas recovery
unit ties into the flare gas header beKnockout
drum
Flare gas
Recovered gas to process
FGRS 6
MMscfd
tween the knockout drum and the staging
device and pulls flare gas from the header
whenever flow is detected. Basically, the
staging devices enable safe operation of
the entire system through sealing the elevated
flares and providing backpressure
in the flare header, allowing the flare gases
to be routed to the FGRS.
Types of compression equipment.
The choice of compression equipment
will greatly influence the configuration
of an FGRS. A detailed survey of existing
technologies revealed that the compressor
types widely used for FGRS applications
include:
* Liquid-ring compressors with a
variety of service fluids (such as
water or diesel)
* Multi-staged screw compressors.
The major advantages of these compressor
types include:
* They can handle a wide range
of gases with varying molecular
weights, with no effect on their
performance.
* They can handle flow from zero
to full capacity with a robust
recycling system.
* They can tolerate liquid in the feed
vs. any other type of compressors.
N2
Staging
device
purge
Existing flares
HP/ACID or
INLET
FIG. 1. The integration of an FGRS into the
flare system.
Nozzle
Mixing chamber
Diffuser
Mixing fluid
outlet
Ejectors. An ejector is a well-known
technology for pressurizing gas by using
a high motive source. The operating principle
of the ejector is the pressure energy
in the motive fluid, which is converted to
velocity energy by an adiabatic expansion
in the converging/diverging nozzle. Due
to the pressure drop of the motive fluid,
it will create a low-pressure zone before
the mixing chamber. Due to this lowpressure
zone, the suction fluid will start
to move toward it and mix with motive
fluid in the mixing chamber. The mixed
fluid will enter the diverging portion of
the ejector, where its velocity energy is
converted into pressure energy (FIG. 2).
The authors' company has patented
this technology to be utilized in a flare
system to recover the wasted gas for
further treatment or to generate energy.
The team conducted several feasibility
studies that approved the applicability of
this technology to recover flare gas either
by using a gas/gas or liquid/gas ejector.
This technology was applied in oil-producing
facilities and refineries, and it has
successfully been used in gas-operation
facilities by using amine solution as a
motive liquid. When gas at sufficient
pressures is available onsite, gas-motive
ejectors could be an economical compression
solution. Eductors or liquidmotive
ejectors can also be considered
and may use a closed-loop liquid circulation
to provide the needed liquid-motive
pressure. Eductors can also be used for
gas sweetening purposes.
Recovering flare gas was not stopped
by these compression types. The feasibility
of flare gas recovery is considered the
process philosophy for each facility. For
example, each facility can recover flare gas
by using an atmospheric compressor. As
a result, hydraulic analysis was required
to install a jump-over line from the flare
header to the compressor suction pipe.
Challenges. Since Saudi Aramco started
recovering flare gas, there have been major
challenges. However, without these experiences,
the authors would not be able to list
all the lessons learned to overcome them.
One of the major challenges experienced
in FGRSs is the oxygen ingress
that resulted in the formation of solid
Motive fluid
Suction fluid
FIG. 2. An ejector schematic.
14 MARCH/APRIL 2022 | GasProcessingNews.com
particulates. As a result, these solid particulates
blocked the suction strainers of
the compressors. A sample was collected
and sent to the research and development
center for detailed analysis. It was discovered
that elemental sulfur composition
exceeded 99%.
Normally, elemental sulfur can be
found in natural gas processing plants
for two reasons: as part of the natural gas
mixture coming from the source reservoir,
or because of sulfur compounds reacting
with oxygen, resulting in sulfur in
its elemental form.
In natural gas, elemental sulfur is usu
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Hydrocarbon Processing - April 2022

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