Hydrocarbon Processing - April 2021 - GP-15

SPECIAL FOCUS: GREEN TECHNOLOGIES

Reduce emissions and save energy with an
unconventional flare gas recovery system
A. H. AL-TIJANI, Saudi Aramco, Dhahran, Saudi Arabia; and
I. ASHIQ, Yanbu NGL Fractionation Plant, Saudi Aramco, Yanbu, Saudi Arabia

An unconventional flare gas recovery
system (FGRS) can be designed without
a gas compressor to collect the boiloff
gas from the ethane tank to the boilers at
the utility area. This innovative recovery
system will provide significant capital
and operating cost savings by eliminating the installation and operation of a gas
compressor as part of the conventional
FGRS. The FGRS scheme includes the
use of a gas ejector with high-pressure
motive gas to boost low-pressure ethane
flare gas to the intermediate pressure,
which is required to return the gas to the
boilers at the utility area.
The case study included here explains how the unconventional FGRS
was applied at Saudi Aramco's Yanbu
NGL fractionation plant to continuously recover approximately 1.1 MMsft3d
of valuable C2+, which is equivalent to
1,961 MMBtu/d in fuel energy savings.
The proposed FGRS scheme will also
minimize greenhouse gas emissions and
provide positive environmental benefits.
Project introduction. Flare systems are
essential parts of any oil and gas processing plant. These systems, which essentially consist of flare headers and laterals,
liquid knockout drums and flare stacks,
serve as one of the last layers of protection for the plant to safely relieve pressure from plant equipment during an
overpressure condition. As part of safety
requirements, flare headers are normally
provided with continuous purging to
prevent vacuums within the system, keep
air out of the system and prevent possible explosions.
The major component of any conventional FGRS is the gas compressor. It is
required to compress the low-pressure
flare boiloff gas to a pressure that can return the gas to the process. The recovery

gas compressors have recurring maintenance, operating spare and reliability issues, similar to any other rotating
equipment in a process plant. It would,
therefore, be an attractive and economic
venture if the use of a gas compressor can
be eliminated from the FGRS without
jeopardizing the performance and safety
of the system. To this end, a scheme was
evaluated using a gas ejector that can
provide additional operating flexibility
and reliability to the system.
At the Yanbu NGL fractionation
plant, the purge gas used is ethane.
To ensure no air ingress into the flare
headers, a minimum flowrate of purge
gas must be continuously maintained
for each flare system at the plant. One
100% ethane tank flare system is in use.
The system has a flare header of 12 in.-
16-in. diameter; therefore, the maximum continuous load of ethane boiloff
to flare system is 1.1 MMsft3d. The total
flared gas is available to be continuously
collected and routed back to the boiler
utility area.
The study revealed that it is feasible to
recover the continuous flared gas by connecting new piping from the flare system
upstream of the flare knockout drum to
the boiler utility area, to allow continuous boiloff and utilize it as fuel for the
boilers. FIG. 1 shows a schematic of the
ethane tank flare system arrangement at
the Yanbu NGL processing facility.
The unconventional FGRS scheme
was established and carefully evaluated
through hydraulic and process simulations, using both in-house programs and
proprietary software,a as described in the
following section.
Ejector-based

FGRS. The scheme

was developed by utilizing a suitable
ejector to collect the 1.15 MMsft3d of

continuous flare gas at 0.48 psig, using
0.95 MMsft3d of available high-pressure
gas as motive gas. The process scheme
was modeled, simulated and confirmed
possible, using proprietary software.a
To this end, a recovery system utilizing
high-pressure gas at 380 psig as motive
gas-in an ejector to transport the gas
from the flare site to the boiler utility
area-was established. FIG. 2 shows the
calculated flowrates and pressures for
the feeds and outlet streams of the ejector. FIG. 3 shows the flow scheme for the
ejector-based FGRS.
This flare gas recovery approach is
possible based on the fact that there is
enough room in the boiler utility area
to accommodate the flared gas volume.
Flare gas recovery at the plant requires

V85-F-196

Flare
stack

PCV-002
Knockout
drum
Ethane
tank (T-501)

FIG. 1. Schematic of ethane tank flare system
at the Yanbu NGL processing plant.
Collected gas
from ethane tank
HP motive gas

0.48 psig,
1.1 MMsft3d

380 psig, 0.95 MMsft3d

Gas to boilers
utility area
8 psig, 2.05 MMsft3d

Ejector

FIG. 2. Calculated flowrates and pressures
for the Yanbu FGRS ejector.
Gas Processing & LNG | MARCH/APRIL 2021

Hydrocarbon Processing - April 2021

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