Hydrocarbon Processing - December 2021 - GP-22

SPECIAL FOCUS: PLANT DESIGN AND PROCESS CONTROL
scenario, while other models displayed
ice formation only under similar process
conditions at the respective highrisk
points. The inlet stream consisted
of associated gas with a volume of 8.0
MMft3
d, with a temperature and pressure
Component
Nitrogen
Carbon dioxide
Methane
Ethane
Propane
i-Butane
n-butane
i-Pentane
n-Pentane
n-Hexane
Water
TABLE 1. Composition of inlet stream
Mole fraction
0.01
0.03
0.64
0.08
0.09
0.03
0.02
0.01
0.01
0.01
0.07
TABLE 2. Specifications for process units
Equipment
Section
Turboexpander
Compressor inlet
Compressor outlet
Expander inlet
Expander outlet
Inlet
Air cooler
Outlet
Plant HE-01
In-line separator
Plant HE-02
Hot side inlet
Hot side outlet
Cold side inlet
Cold side outlet
-
Hot side inlet
Hot side outlet
Cold side inlet
Cold side outlet
45
-
0.2
10.46
-
0.2
-
-
-
0.2
-
-
0.2
-
-
of 81.6°C and 6 bar, respectively. The
composition of the gas stream is given
in TABLE 1, while the specifications of the
units are given in TABLE 2.
The associated gas entered the compressor
side of the turboexpander and
left at 9 barg. The compressed gas then
entered an air cooler, where it was cooled
to 45°C. The outlet stream was then fed
to the hot side of Plate Heat Exchanger 1
(HE-01), where it was cooled further to
10.5°C before passing through an inline
separator. The latter removed 98% of the
water present in the feed at its bottom
stream (Sep. Bottom), along with some
low-quantity heavy end hydrocarbons
(C4
s and C5+). This stream was discharged
from the system.
Next, the top product from the separator
was passed through the hot side
of plate HE-02, where it was cooled to
-21.6°C. This chilled stream was then
sent to the cold separator, where the
Specified temperature, °C
81.61
-
-
-
-
0.2
Pressure drop, barg
-
heavy end hydrocarbons were recovered
as the main product of the system at its
bottom stream. The top product of the
cold separator was passed into the expansion
section of the turboexpander, where
its pressure was reduced to 1.7 barg.
The energy recovered from the expander
was supplied to the compressor
section of the turboexpander for the compression
of feed. The compressed stream
was then passed on to Plate HE-02 to cool
the hot streams. This stream was then
combined with the bottom stream of the
suction scrubber (Scrub. Bottom), which
consists mainly of water. Finally, this
stream was used to cool the hot streams
in Plate HE-01 before it was sent to flare.
Methanol injection. Before determining
the methanol intake, the high-risk
points in the system were first identified.
These are locations where hydrate formations
were likely to occur. First, the outlet
stream of Plant HE-02 (i.e., HE02-Out1
stream) was found to exhibit Type II hydrate
formation, according to stream analysis
results of the simulation model. Note:
Type II hydrates will form in the case of
gas mixture,10
and they are relatively more
stable and form at a higher temperature
compared to Type I. Since stream HE02Out1
is the inlet stream to the cold separator,
it was assumed to have the same
condition as the separator outlet streams
(i.e., the Sep. Top and Product stream).
Therefore, methanol injected at the upstream
of HE02-Out1 is sufficient to suppress
hydrates formation at the separator
inlet and outlet streams. Another high-risk
point was found at the expander outlet
stream (i.e., Exp. Out) that also exhibits
Type II hydrate formation.
The next step was determining the location
for methanol injection points. The
first high-risk point (i.e., HE02-Out1) is
the outlet stream of Plate HE-02. Instead,
the inlet stream of the latter (i.e., Sep.
Out) was chosen as the first injection
point. Even though the temperature of
the Sep. Out stream was not low enough
to form hydrates, hydrates will form as
soon as the temperature is lowered to
-21.6°C in Plate HE-02.
Apart from preventing hydrates forFIG.
1. Simulation flowsheet of condensate recovery system.
22 NOVEMBER/DECEMBER 2021 | GasProcessingNews.com
mation in the HE01-Out1 stream, the
tubes within Plate HE-02 are also being
protected. Conversely, as the stream temperature
continues to drop from -21.6°C
(Sep. Top) to -57.0°C (Exp. Out, due to
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Hydrocarbon Processing - December 2021

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