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Optimizing
ethane
recovery
in
turboexpander
processes
Laura Kherbeck, Rachid Chebbi*
Department of Chemical Engineering, American University of Sharjah, PO Box 26666, Sharjah, United Arab Emirates
1. Introduction
There are many extraction processes for natural gas liquids
which include Joule–Thompson (JT) expansion, refrigeration using
propane in a chiller, and turboexpansion. More often than not, all
three processes are used at once. Mixed refrigerants can also be
used [1] but the most popular process in the natural gas liquids
(NGL)-recovery industry is turboexpansion. A review of NGL
recovery can be found in Manning and Thompson [1], McKee [2],
Pitman et al. [3], GPSA [4], Chebbi et al. [5,6], Mehrpooya et al. [7]
and in the references therein. The optimized conventional process
for ethane recovery [5], to which the present results are compared,
is shown in Fig. 1. The cold residue recycle (CRR) process, examined
in this paper (Fig. 2), is claimed to provide very high ethane
recovery, above 98% [4]. The CRR process [3,4,8] is built upon the
gas subcooled process (GSP). In the GSP [4], the gas leaving the
separator is split, with one fraction subcooled by heat exchange
with the overhead stream from the demethanizer, and the otherfraction entering the turboexpander. The fraction subcooled by the
demethanizer overhead stream is flashed in a valve and fed to the
tower as reflux [4]. The GSP process is considered in the present
work (Fig. 3). The process in Fig. 4, referred to as GSP with cold
separator in the rest of the manuscript, has a cold separator
operating at a lower temperature than the chiller temperature. The
CRR process has one addition when compared to the GSP process
(Fig. 3): a reflux stream to rectify the vapors in the demethanizer
tower in order to minimize the amount of ethane and other heavier
hydrocarbons that leave with the overhead. A compressor is used
to boost part of the demethanizer overhead stream to a slightly
higher pressure so that a fraction of the methane could be liquefied
by the flashed stream and sent to the top stage of the demethanizer
(see Fig. 2). The flashed feed to the demethanizer would condense
some of the ethane from the turboexpander outlet vapor and the
liquid reflux stream would condense some of the remaining ethane
vapors at the top of the tower.
Maximum ethane recovery can be carried out by changing a
select number of design variables. Ethane and NGL recovery
problems are characterized by a large number of design variables
affecting ethane and NGL recovery that include, but are not limited
to demethanizer pressure and split ratio(s) if any.
The present paper considers the effect of demethanizer
pressure on maximum ethane recovery for the CRR process ascompared to a conventional turboexpander process [5]. Further-
more, GSP (without or with cold separator) is considered and its
performance compared to both the CRR process and the
conventional turboexpander process [5] for a lean and a rich feed
gas at different demethanizer pressures. Optimization is per-
formed by maximizing the percent ethane recovery. Ethane
recovery as a function of demethanizer pressure is then reported
and analyzed for the two types of feed. Feed composition, flow
rates, temperature and pressure are identical to the values used in
Bandoni
et
al.
[9],
and
Chebbi
et
al.
[5,6]
for
feeds
A
and
D.
Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
A R T I C L E I N F O
Article history:
Received 5 October 2013
Accepted 19 February 2014
Available online xxx
Keywords:
Simulation
Ethane recovery
Natural gas liquids (NGL)
Turboexpander
Cold residue recycle (CRR)
Gas subcooled process (GSP)
A B S T R A C T
Optimization of ethane recovery usingthe CRR process shows that, except for thecase of lean gas at low
demethanizer pressure, theCRRprocess reduces to GSP, in which there is no reflux streamand therefore
no added cryogenic compression and heat exchange equipment. Adding a second cold separator,
operating at lower temperature, in GSP is found to lead to more or less recovery depending on the NGL content of the feed gas and the demethanizer pressure. GSP is also compared with the conventional
turboexpander process. Optimization shows that adding more equipment or even flow splitting may
lead to less ethane recovery.
2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights
reserved.
* Corresponding author. Tel.: +971 65152983.
E-mail address: rchebbi@aus.edu (R. Chebbi).
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Contents
lists
available
at
ScienceDirect
Journal of Industrial and Engineering Chemistry
jou r n al h o mepag e: w ww.elsev ier .co m / locate / j iec
http://dx.doi.org/10.1016/j.jiec.2014.02.035
1226-086X/ 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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2.
Simulation
and
optimization
2.1. CRR process
The
study
was
conducted
by
first
simulating
the
CRR
process.
Fig. 2 demonstrates the process flow sheet for the CRR process. The
figure
does
not
depict
the
refrigeration
loop,
which
is
connected
to
the main process through the chiller. The feed is first cooled by
providing the duty necessary for the reboiler, and further cooling of
the
feed
is
achieved
by
heat
exchange
with
the
residue
gas.
The
four
demethanizer
pressures
considered
are
100,
215,
335,
and
450 psia as in [5,6]. The pressures are grouped as low (100 psia),
Fig. 1. Conventional ethane recovery process optimized for maximum ethane recovery in [5].
Fig. 2. CRR process flow sheet.
Fig. 3. Gas subcooled process (GSP) flow sheet.
L. Kherbeck, R. Chebbi / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx2
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intermediate
(215
psia),
and
high
(335
and
450
psia)
and
cover
the
typical
range
of
demethanizer
operating
pressures,
100–450
psia
[1]. The reboiler duty cannot be provided through heat integration
at high demethanizer pressures (335 and 450 psia) due to the fact
that
the
temperature
profile
in
the
column
is
shifted
up
and
the
feed
gas
temperature
ceases
to
be
enough
to
provide
the
reboiler
duty, as also indicated in [6].
The pre-cooled feed is senttoa chiller where propane is usedto
reduce its temperature to
31 8F.
This
temperature
was
selected
to maximizecooling; taking into account the lowest temperature
allowed in the chiller of 40
8F required to avoid air leakage into
the
system [1],
and temperature approach in
the
chiller. The cold
feed from the chiller enters a separator where thegas is separatedfrom the liquid. A portion of the separated gas is cooled by heat
exchange with a fraction of the overhead stream leaving the
demethanizer column. It is
then
expanded through
JT
expansion
and enters another
heat exchanger designed
to cool
a
portion of
the recycled overhead, following which the stream enters the
demethanizer. The other portion of the separated gas is expanded
in
a
turboexpander
andsent to thedemethanizerat
a
lower
stage.
A
fraction of
the
liquid
leaving
the
separator is
mixed with
the
separator gas outlet that goes into heat exchange, while the
remainder undergoes JT expansionto column pressureand enters
the
demethanizer at
a
lower
stage than
the
feed stream from the
turboexpander.
The
flashed
split-vapor
stream is
not cold
enough
to condense partially the overhead methane reflux stream at the
operating pressure of
the
demethanizer. Thus, a
cryogeniccompressor
is
used to
boost
part
of
the demethanizer
overhead
to
a
slightly
higher
pressure so that a
fraction
of
the methane
can
then be condensed [3]. The compressed overhead is cooled, and
then expanded through a valve before being suppliedto the topof
the
tower. The fractionof
the overhead
that is
notrefluxedback is
termed the
residue
gas. Part
of
the power
required
to
recompress
the residue gas is provided by the turboexpander, but a
recompressor is needed to bring the residue gas pressure up to
882
psia. Two different
feeds
are considered:
feed A
and feed D
as
in
[5,6,9].
The compositions
in
terms
of
mole
fractions
are given in
Table 1. FeedA is a lean gaswith 6% C2+ content, andD is a rich feed
with 30% C2+ content. Thefeedgasenters the NGLrecovery unit at
100 8F
and
882 psia,
the residue
gas is
recompressed to
882 psia,
and the
molar ratio
of
C1 to
C2 in
the NGL stream is
set to
0.02
in
consistency with the typical range in [1]. In all cases, the feed gas
flow rate is 10,980 lbmol/h. The maximum conventional tur-
boexpander ethane
recovery
values were
obtained from
[5].
2.2. GSP and GSP with cold separator
The
gas
subcooled
process
(GSP)
and
GSP
with
cold
separator
are
shown
in
Figs.
3
and
4, respectively.
3.
Results
and
discussion
The simulation was performed using Aspen HYSYS with the
Peng-Robinson
thermodynamic
package.
The
optimization
in-volved
changing
the
design
variables
affecting
ethane
and
NGL
recovery to maximize the objective function, defined as the ratio of
ethane in the NGL stream to the ethane in the feed. The design
variables
selected
were
the
split
ratios
in
all
of
the
splitters,
the
temperatures
at
the
outlet
(higher
temperature
side)
of
the
heat
exchangers following the mixer, and the cryogenic compressor
outlet pressure. The constraints were set so as to prevent
temperature
cross
in
all
the
heat
exchangers
and
to
ensure
that
the
reflux
stream
is
colder
than
the
flashed
split-vapor
stream
as
they enter the demethanizer. Optimization thus yielded the
optimum values of the design variables affecting ethane and
NGL
recovery
for
each
demethanizer
pressure
for
both
lean
and
rich
feeds,
i.e.
the
values
that
contributed
to
the
highest
ethane
recovery.
Fig. 4. GSP with cold separator.
Table 1
Feed gas composition.
Component Feed A Feed D
Nitrogen 0.01 0.01
Methane 0.93 0.69
Ethane 0.03 0.15
Propane 0.015 0.075
Butanes 0.009 0.045
Pentanes 0.003 0.015
Hexanes 0.003 0.015
C2+ (%) 6 30
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3.1.
Feed
A
The HYSYS optimizer tool indicated that the CRR process indeedmade recoveries of 99.9% possible only for a demethanizer
pressure
of
100
psia
and
for
the
lean
feed
gas
A.
For
all
the
higher
demethanizer
pressures,
results
indicated
that
for
feed
A,
the
optimum process would not include the reflux. Initially, these
results were suspected after a sensitivity analysis was carried out
on
the
split
ratio
in
the
overhead
splitter
and
the
cryogenic
compressor
outlet
pressure.
Later,
optimization
using
the
HYSYS
optimizer tool confirmed the results. The configuration in Fig. 2 has
hence been altered to discard the reflux splitter, the cryogenic
compressor,
heat
exchanger,
and
expansion
valve,
while
retaining
the two splitters at the top and bottom outlets of the first separator.
The results summarized in Fig. 5, show how ethane recovery with
the
CRR/GSP
compares
to
the
recovery
from
the
conventional
turboexpander process [5]. It can be seen that the CRR process, andhence the GSP, are not as effective at high demethanizer pressures,
while the conventional turboexpander process is not as effective as
the
other
two
processes
at
low
and
intermediate
demethanizer
pressures.Some of the differences between the GSP (Fig. 3), and the
conventional
turboexpander
process
in
[5]
(also
shown
in
Fig.
1),
lie
in
the
two
splitters
at
the
outlet
of
the
separator
operating
at
31
8F in the GSP case, and also in the additional cold separator in
the conventional turboexpander process [5]. However, since the
GSP
has
been
shown
to
give
superior
recovery
at
low
and
intermediate
pressures,
and
lower
recovery
at
high
pressures,
it
was postulated that the difference could lie in the additional cold
separator utilized in the conventional turboexpander process.
Thus,
the
GSP
was
modified
by
adding
a
cold
separator
operating
at
a temperature lower than the chiller temperature. At high
demethanizer pressures, GSP with cold separator provided higher
recoveries
than
GSP,
and
slightly
higher
ethane
recoveries
than
the
conventional turboexpander values [5]. The separator overheadstream splitter was found unnecessary, leading to the simplified
flow sheet in Fig. 6. At low and intermediate pressures, GSP with
Demethanizer pressure, psia
50 100 150 200 250 300 350 400 450 500
C 2
r e c o v e r y ,
%
0
10
20
30
40
50
60
70
80
90
100
CRR/GSP (CRR at 100 psia, GSP at higher P)
Conventional turboexpander
GSP with cold separator (GSP at 100 & 215 psia)
Feed A
Fig. 5. Effect of demethanizer pressure on ethane recovery for the lean gas feed A;
conventional turboexpander results from [5].
Fig. 6. GSP with cold separator process (feed A at high demethanizer pressure configuration or feed D at all demethanizer pressures).
Demethanizer pressure, psia
50 100 150 200 250 300 350 400 450 500
C 2
r e c o v e r y ,
%
50
60
70
80
90
100
GSP
Conventionel turboexpander
GSP with cold separator
Feed D
Fig. 7. Effect of demethanizer pressure on ethane recovery for the rich gas feed D;
conventional turboexpander results from [5].
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cold separator was found to reduce to GSP, with zero vapor flow
rate
leaving
the
cold
separator.
3.2. Feed D
In
the
optimization
for
rich
gas
D,
the
CRR
process
was
tested
first
and
found
to
reduce
to
GSP.
The
ethane
recovery
from
GSP
was
compared to the recovery from the conventional turboexpander
process
[5].
The
results
are
shown
in
Fig.
7. At
low
and
intermediate
demethanizer pressures, GSP and the conventional turboexpander
process were found to give close ethane recoveries. The deviations
in recovery were observed at high demethanizer pressures at
which
the
GSP
gave
better
recovery
than
the
conventional
turboexpander
process.
For
this
particular
feed,
the
GSP
with
cold
separator was also tested and found to give recovery values similar
to the other processes tested for low and intermediate pressures. In
the
high
pressure
range,
the
recovery
from
the
GSP
with
a
cold
separator
fell between
the
values
from
the
GSP
and
the
conventional
turboexpander process [5]. At all demethanizer pressures, GSP with
cold separator (Fig. 4) reduced to the configuration in Fig. 6, with no
need
for
the
separator
overhead
splitter.
3.3. Ethane recovery
Although
the
CRR
process
is
frequently
stated
to
allow
for
ethane
recoveries
of
99%
or
higher
values
[8],
the
process
optimization yielded significantly lower recovery values for
intermediate and high demethanizer pressures in the case of feed
A,
and
all
pressures
for
feed
D.
At
low
demethanizer
pressures
(100
psia),
the
reflux
stream
entering
the
tower
is
80.5
8F colder
than the turboexpander outlet stream entering the tower. Forintermediate pressure (215 psia), the temperature difference is
56.5
8F. It
is
speculated
that
it
is
the
large
temperature
gap
observed
at
low
demethanizer
pressure
that
is
responsible
for
the
superior ethane recovery as compression not only enhances
pressure but also temperature. The lowest recovery values for both
feeds
A
and
D
were
the
ones
at
the
highest
demethanizer
pressure
(450
psia).
Fig. 8 shows the effect of demethanizer pressure on ethane
recovery for feeds A and D. For all the processes, the impact of the
demethanizer
pressure
on
ethane
recovery
is
significantly
less
for
the rich gas D compared to the lean gas A over the range 215–
450 psia. On the other hand, higher ethane recoveries are obtained
for
the
lean
gas
A
at
low
demethanizer
pressure,
whereas
higher
C2
recoveries are attained for feed D at intermediate and higherpressures.
For the lean gas A, the GSP with cold separator was found to be
the
most
viable
process,
reaping
the
benefits
of
the
split-stream
configuration
at
low
and
intermediate
pressures,
and
those
of
the
conventional turboexpander process at high demethanizer pres-
sures. However, the CRR process remains the process of choice for
the
lean
gas
at
low
demethanizer
pressure.
With
the
exception
of
low
demethanizer
pressure,
the
process
that yields the highest recovery for feed D is the GSP, which
excludes additional cryogenic compressor and heat exchanger
from
the
CRR
process.
4. Conclusion
The
cold
residue
recycle
(CRR)
process
was
simulated
to
maximize
ethane
recovery
at
different
demethanizer
pressures.
The optimized design variables included all split ratios, the outlet
temperatures (higher temperature side) from the heat exchangers
(downstream
of
the
chiller)
and
the
cryogenic
compressor
outlet
pressure.
The
CRR
process
is
the
most
viable
option
for
low
demethanizer pressure with ethane recovery of 99.9% for the lean
gas considered. However, adding more complexity to the process
may
lead
to
lower
ethane
recovery.
This
result
concurs
with
the
finding
in
Chebbi
et
al.
[10]. In
particular
(i)
the
only
case
where
a
cryogenic compressor is needed is for feed A (lean gas) at low
demethanizer pressure (100 psia). In all other cases, the CRR
process
reduces
to
GSP
where
the
cryogenic
compressor
followed
by
the
heat
exchanger
and
JT
valve
following
it
are
all
not
needed
Demethanizer pressure, psia
50 100 150 200 250 300 350 400 450 500
C 2 r e c o v e r y ,
%
0
10
20
30
40
50
60
70
80
90
100
Feed A (GSP at 100 and 215 psia)
Feed D
GSP with cold separator
Demethanizer pressure, psia
50 100 150 200 250 300 350 400 450 500
C 2 r e c o v e r y ,
%
0
10
20
30
40
50
60
70
80
90
100
Feed A
Feed D
Conventional turboexpander (Chebbi et al., 2008)
Demethanizer pressure, psia
50 100 150 200 250 300 350 400 450 500
C 2 r e
c o v e r y ,
%
0
10
20
30
40
50
60
70
80
90
100
Feed A (CRR at 100 psia, GSP at higher P)
Feed D (GSP)
CRR/GSP
(a)
(b)
(c)
Fig. 8. Effect of demethanizer pressure on ethane recovery for (a) CRR/GSP, (b)
conventional turboexpander process [5] and (c) GSP with cold separator for feeds A
and D.
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for higher ethane recovery. On the other hand (ii) adding a cold
separator
to
GSP,
operating
at
a
lower
temperature
than
the
chiller
temperature (in addition to the separator operating at 31
8F),
yields less recovery in the case of the rich feed gas D, except at
100 psia. Also (iii) the GSP with cold separator reduces to GSP in the
case
of
the
lean
feed
gas
A
at
low
and
intermediate
pressures,
making
the
cold
separator
unnecessary.
Furthermore
(iv)
in
case
GSP with cold separator provides higher recovery (lean gas A at
high demethanizer pressures), only one splitter is required: the
separator
liquid
outlet
splitter,
with
the
separator
vapor
outlet
splitter
discarded.
For feeds containing CO2, care should be taken to make sure CO2
frost [1] will not occur; this point is not addressed in the present
investigation
since
the
two
feeds
considered
are
free
from
CO2.
On
the
other
hand,
the
objective
in
this
work
is
to
maximize
ethane
recovery; therefore costing is not required. Further investigations
could address the abovementioned two points.
References
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[2] R.L. McKee, Evolution in design, in:Proceedings of the56th Annual GPAConven-tion, Dallas, 1977.
[3] R.N.Pitman,H.M.Hudson, J.D.Wilkinson,K.T. Cuellar,Next generationprocessesforNGL/LPG recovery,in:Proceedingsofthe77th AnnualGPAConvention,Dallas, 1998.
[4] GPSA, Engineering Data Book, Sec. 16, twelfth ed., Gas Processors SuppliersAssociation, 2004.
[5] R.Chebbi,K.A.Al Mazroui,N.M.Abdel Jabbar, Oil& GasJournal106(46)(2008)50.
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[10] R. Chebbi, A.S. Al-Qaydi, A.O. Al-Amery,N.S. Al-Zaabi, H.A. Al-Mansoori, Oil& Gas Journal 102 (4) (2004) 64.
L. Kherbeck, R. Chebbi / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx6
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