Hydrocarbon Processing - December 2021 - GP-23

SPECIAL FOCUS: PLANT DESIGN AND PROCESS CONTROL
the expansion), the Exp. Out stream was
suggested as the second methanol injection
point. A concern was that the first
methanol inject point located upstream
(Sep. Out stream) may not be sufficient
to suppress the formation of hydrates at
this stream, due to its lower temperature.
It is important to ensure good operating
condition of Plate HE-02, as it will affect
its outlet stream (i.e., HE02-Out2
stream). This then affects the cooling of
the HE01-Out stream, and subsequently
the Sep. Out stream, in turn impacting
the recovery of hydrocarbons in the main
product stream.
The base case simulation model was
then used to determine the required
amount of methanol for both injection
points. Analysis showed that 0.35 tpd
of methanol is required for the Sep. Out
stream (Injection Point 1) and 0.39 tpd
for the Exp. Out stream (Injection Point
2). These flowrates should be optimized
by considering economic performance.
Prior to the construction of the optimization
model, important parameters
that affect the systems should be determined.
The simulation model revealed
that the operating temperature of the cold
separator had a significant effect on the
methanol requirement and the condensate
recovered, while the operating pressure
effect was not significant. Therefore,
the effect of operating temperature was
examined though the simulation model,
shown in FIGS. 2 and 3.
FIG. 2 shows the relationship of the
methanol flowrate vs. the operating temperature
for the temperature range of
-21.6°C to -1.6°C (with temperature
intervals of 5°C). The amount of methanol
required at the first injection point
decreased by only 0.07 tpd when the temperature
increases to -1.6°C. However,
the methanol required at the second injection
increased drastically from 0.39 tpd
to 1.19 tpd for the same temperature rise.
When the temperature increases, less water
is condensed to the bottom of the cold
separator (product stream); instead, more
water is vaporized to the Sep. Top stream
and enters the expander side of the turboexpander.
More water content is found in
the Exp. Out stream, which increases the
tendency of hydrates formation in this
stream, leading to an increased requirement
of methanol in this stream.
In FIG. 3, the amount of condensate
recovered against the temperature of the
cold separator is determined. The HE02Out1
stream is comprised of approximately
40% of hydrocarbons heavier than
C2
. Therefore, the operating temperature
of the cold separator greatly affects the
amount of its recovered condensate in
the main product stream. It was observed
that a higher amount of condensate was
recovered at a lower operating temperature
of the cold separator. The amount
of condensate recovered was observed to
drop by 80 bpd for every 5°C decrease in
temperature (FIG. 3).
Optimization model. The values obtained
from the simulation model were
incorporated into a linear regression
(FIGS. 2 and 3), and their equations were
used as optimization constraints. Eq.
1 shows the methanol required against
temperature (first injection point):
1.2
1.4
0.2
0.4
0.6
0.8
1
y = -0.003x + 0.2857
-25
-20
-15
Temperature, °C
FIG. 2. Graph of methanol flowrate against temperature.
50
100
150
200
250
300
350
400
-25
-20
-15
Temperature, °C
FIG. 3. Condensate recovered at the cold separator against temperature.
Gas Processing & LNG | NOVEMBER/DECEMBER 2021 23
-10
-5
-10
-5
First injection point, M1
Second injection point, M2
y = 0.0394x + 1.1985
FM1 = -0.003T + 0.2857
against
point):
FM2
(1)
Eq. 2 shows the methanol required
temperature (second injection
= 0.0394T + 1.1985
against temperature:
FCond
= -12.984T + 85.846
where FM1 (tpd) and FM2
(2)
Eq. 3 shows the condensate recovered
(3)
(tpd) represent
the methanol flowrate at injection Points
1 and 2 with respect to temperature; FCond
(bpd) is the condensate recovered with
respect to T (temperature in °C), which
represents the operating temperature of
the cold separator.
The optimization objective is given
by Eq. 4, which is essentially the difference
between condensate sales and the
methanol cost:
Flowrate, F (barrels/day)
y = -12.984x + 85.846
R2
= 0.9986
Flowrate, bbl/d
Minimum methanol required, t/d
Hydrocarbon Processing - December 2021

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