HYBRID SEPARATION OF CO2 FROM ETHANE USING MEMBRANES
Knut H. Nordstad and Tor K. Kristiansen, Statoil, Stavanger Norway
David Dortmundt, UOP Des Plaines US
Abstract
This paper presents hybrid concepts for the separation of CO2 from ethane involving the
combination of cryogenic distillation and UOP Separex™ membranes. Statoil and UOP have
together carried out pilot testing at Kårstø gas processing plant in Norway. A gas mixture of CO2
and ethane from a CO2 stripper overhead stream has been successfully separated with cellulose
acetate membranes to produce CO2 of specified purity. The pilot testing has been carried out in a
demonstration unit at approx. 38 barg (570 psia) pressure under varying temperatures.
Introduction
Ethane became a new product from the Kårstø plant in October 2000, with the start-up of
the Ethane plant. Ethane is exported from the plant by ship, and is used as a feedstock for
ethylene production. There is an incentive to maximize production in the plant, and studies have
been undertaken with this objective. With increasing CO2 content in the ethane feed stream to the
plant, ethane recovery from the existing plant has been a concern. Methods for more effective
separation of ethane and CO2 have therefore been studied.
There has also been interest in a CO2 feed stream from the ethane plant, for further
processing to a commercial CO2 product. For this reason, a process providing a high purity CO2
stream has also been studied.
Existing ethane plant at Kårstø
The Kårstø gas processing plant at Kårstø, in the Western part of Norway, was first put
into operation in 1985. The plant has since been extended several times, with the latest in
October 2000. The Kårstø ethane treatment plant was also put into operation in October 2000.
Figure 1 shows a picture of the Kårstø gas plant after expansion in 2000. Figure 2 shows how the
Ethane treatment plant is integrated into the total facilities at Kårstø.
The Kårstø Ethane treatment plant, built by Etanor DA, receives raw ethane from the
Statpipe processing trains 100 / 200 and Sleipner train 300. The Capacity of the plant is 620 mt/y
produced ethane. The raw ethane consists of some methane and carbon dioxide together with
ethane. The CO2 and the methane are stripped out of the ethane in a 64 tray cryogenic distillation
column operating at 34 barg (510 psia) and –3°C (27°F) reflux conditions. Due to the CO2 / C2
azeotrope, the ethane recovery is limited in this process. The heating and cooling duty is served
by a steam turbine-driven propane heat pump. In figure 3, a PFD for the existing Ethane plant is
presented.
Figure 1: Picture of Kårstø gas plant in Norway
Figure 2: Block Flow Diagram of Kårstø gas processing plant including the Ethane treatment plant
34,5 bar / 5,5 C / 119 t/h
PC
1017
29-HG-102
Kondenser
64
12,5 t/h Til salgsgass
Til fakkel
sugedrum
29-HV-1066
FC
1004
29-HV-1096
85,5 t/h
Fra T-100
FF
1016
FC
1036
29-HV-1097
2,6 bar
-8,6 C
PC
1038 B
HC
1017 B
57
HT seperator
25-VA-011
25
LC
0071
Fra T-200
FC
1025
29-HV-1098
29-HV-1045
CO2
stripper
Fra T-300
LC
1046
FC
1016
29-QSV-1003
34,0 bar / -3,3 C
29-HG-101
Koker
FC
1021
1
29-HV
-1028
25-HV-0081
25
LC
0078
CO2 kompressor
29-KA-101
FC
1068
PC
0031
Til fakkel
LT seperator
25-VA-012
29-HV-1055
29-PA-101A/B
PC
0027
0,32 bar
-34,7 C
24-HV
-1060
LC
1008
17,2 C
29-QSV-1022
25
PC
1038 A
29-HV-1042
106,5 t/h
25
FC
1037
CO2 stripper
refluks seperator
Til turtalls
regulering av
dampturbinen
34,7 bar / 17,2 C / 73 t/h
29-HG-103
Etan rundown
kjøler
Til fakkel
Propan kondenser
29-HV-1065
Akkumulator
25-VA-013
25-HA-011
Sjøvann
Til fakkel
25-HV-0064
Sjøvann
Etan fra C2
kompressor
46-system
29-HV
-1095
11,4 bar / 52,9 C / 216 t/h
73 t/h
Antisurge
FFIC 039
HT dampturbin
Etan rundown
25-HV-0063
Antisurge
FFIC 028
MM 11.00
Figure 3: Process Flow Diagram of the Kårstø Ethane treatment plant
Conceptual alternatives for enhanced ethane recovery and CO2 removal
The CO2 content in gas arriving at the Kårstø gas plant is expected to increase in the future.
The existing Ethane plant rejects the CO2 and lost ethane back into sales gas. The recovery of
ethane will be reduced as CO2 content increases due to the distillation process in the CO2-stripper
column being limited by the azeotropic mixture of ethane and carbon dioxide. The capability of
distilling close to the azeotrope in the overhead is determined by the number of separation stages
in the rectifying section of the column. With increasing CO2 content in the raw ethane feed, the
ethane recovery was predicted to drop below 80%. Hence the commercial need for removing
CO2 from the export sales gas and improve ethane recovery became obvious. For this reason, two
industrial concept applications were developed and studied:
1. Cryogenic–Membrane–Cryogenic CO2/C2 separation, producing high purity CO2 product
suitable for commercial sale
2. Cryogenic–Membrane CO2 /C2 separation, producing 95% CO2
Concept for increased ethane recovery and the production of high purity CO2
A Flow Diagram of the concept is shown in fig 4.
The product specifications applied to this concept are shown in table 1
Table 1: Product specifications high purity CO2 applied
Components
Methane
Ethane
Propane +
Carbon dioxide
Ethane product
Max 1,5 wt%
Min 95 wt%
Max 4,5 wt%
Max 100 wt ppm
CO2 product
Max 1 ppbV
Max 1000 ppmV
Max 1 ppbV
Min 99,98 mol%
The unit operations in the concept consist of:
•
The existing CO2 stripper column producing an overhead gas limited by the
CO2/C2 azeotrope of 0.7 and the number of separation stages in the rectifying
section.
•
The membrane separator receiving gas from the existing CO2 -stripping column at
approx 34 barg separates the gas into a low pressure permeate stream and a high
pressure residue stream. The membrane separator will break the C2/CO2 azeotrope
and produce a permeate stream with approximately 93% CO2. The permeate is
further compressed and passed to a CO2 purification column. The reject stream is
passed to a secondary CO2 stripper.
•
The CO2 purification column, with 50 theoretical trays, operating at 18 barg and
-30°C overhead temperature will produce a bottom CO2 product with less than
1,000 ppm hydrocarbons. Overhead gas from the column consisting of methane,
carbon dioxide and some ethane is used as low btu fuel. The CO2 purification
column separates the CO2/C2 mixture from the “other” side of the azeotrope than
the CO2 stripper. The separation principle is presented in graphically on a T-X-Y
Plot of CO2 and C2 mixture in figure 5.
In order to recover as much ethane as possible from the ethane rich residue gas,
the residue can either be re-circulated back to the existing CO2 stripper, or processed in a
new secondary CO2 stripper dependant on available capacity.
Figure 4: Flow Diagram for increased ethane recovery and the production of high purity CO2
Principles for Cryo/Membrane-hybride Process
T-X-Y Plot for CO2 and C2
16.0
B
D
C
B
Bubble Point
D
Dew Point
12.0
,
e
r
u
t
a
r
e
p
m
e
T
B
8.0
bottoms
D
Distil 2 (CO2-purification)
Distil 1 (CO2-splitter)
B
D
feed
overhead
B
D
4.0
B
feed
D
B
overhead
feed
Membrane separator
bottoms
permeate
D
0
B
D
B
B
D
-4.0
azeotrope
CO2/(CO2+C2)=0.7
D
B
D
B
D
B
B
D
D
B
D
D
D
D B D
DB
D
B
D
B
B
B
B
-8.0
0
0.2
Pure C2
0.4
0.6
0.8
Composition, Mole Fraction CO2, (P = 34.000 BAR)
1.0
Pure CO2
Figure 5: Separation principles for the cryogenic–membrane hybrid separation process
Concept for increased ethane recovery and production of low purity CO2
The product specifications applied to this concept are shown in table 2:
Table 2: Product specification for production of low purity CO2 applied
Components
Methane
Ethane
Propane +
Carbon dioxide
Ethane product
Max 1.5 wt%
Min 95 wt%
Max 4.5 wt%
Max 100 wt ppm
CO2 product
Min 95 mol%
The unit operations in the concept consists of:
•
The existing CO2 stripper column producing an overhead gas limited by the
CO2/C2 azeotrope of 0.7 and modified with additional separation stages in the
rectifying section.
•
The membrane separator receiving gas from the existing CO2-stripping column at
approx 34 barg separates the gas into a low pressure permeate stream and a high
pressure residue stream. The membrane separator will break the C2/CO2
azeotrope and produce a permeate stream with approximately 95% CO2. The
residue gas is used as low calorific fuel.
•
In order to recover as much ethane as possible from the ethane rich residue gas,
the residue can be re-circulated back to the existing CO2 stripper.
A flow diagram of the alternative concept is shown in figure 6.
Figure 6: Flow Diagram for increased ethane recovery and the production of low purity CO2
The hybrid concepts presented are comparable to the more traditional amine type
processes, and found favorable in several aspects. The hybrid concepts are lower in capital
expenditure and more environmental friendly as no chemicals are used.
Pilot demonstration tests
Industrial references for CO2 membranes are mainly for the separation of CO2 from a
natural gas, where methane is the dominant component. Our concept required a membrane
separating carbon dioxide from an ethane-rich gas, where methane is a minor component. In
order to demonstrate the membrane capability and performance in performing this task, a
demonstration program was set-up at the Kårstø Ethane plant in cooperation between Etanor DA,
Statoil and UOP’s Gas Processing Group.
An existing membrane separation rig was revamped, fitted with pilot-size Separex Spiral
Wound elements, and connected to the Ethane plant. The equipment lay-out and tie-ins to
existing facilities are shown in figure 7. Gas from the existing CO2-stripper overhead blower was
passed to the membrane pilot unit. Residue gas was returned to the suction side of the blower. In
this way, a real plant operation demonstration could be achieved.
Prior to starting the demonstration testing, acceptance criteria were established by Statoil
and its Etanor DA partners. The acceptance criteria are shown in figure 8. As the CO2 content in
the overhead gas from the CO2 stripper was fluctuating, two feed compositions were defined, and
are shown in table 3. The acceptance criteria are set by the calculated selectivity of the selected
membrane material to be used.
Figure 7: Pilot test rig and tie-in to existing plant
Table 3: Demonstration case definitions
Figure 8: Demonstration testing acceptance criteria
The testing involved varying feed temperature between 30 and 12°C (86 to 54°F), and
permeate pressure between 0.4 and 3 barg (5 to 45 psig). Due to operational fluctuations in the
CO2 stripper and in upstream processes, the feed CO2 content varied during testing between 15
and 23 mole percent. In the 5 weeks demonstration program, more than 140 data points were
collected.
After the demonstration was finished, the test data was collected, analyzed and compared
to the established acceptance criteria. The results are shown in figure 9. Because of operational
variations in feed temperature, feed composition and permeate pressure, the data had to be
adjusted for these variations in order to compare achieved performance to the projected
performance. Achieved performance lying on the “right hand” side of the acceptance criteria
curve, indicates better performance than predicted.
Figure 9: Achieved performance compared to acceptance criteria
Conclusion
The adjusted data shows that Cellulose Acetate Spiral Wound membranes were easily able to
meet expected performance for separating carbon dioxide from ethane, and a vital part of the
cryogenic-membrane hybrid separation concept was verified.
Acknowledgements
The authors gratefully acknowledge the contributions from Russel H. Oelfke with Exxon Mobil
in the planning and analyses of the pilot demonstration.
Acknowledgements also to Etanor DA, for allowing us to publish this paper. (Etanor DA is a
company owned by the Norwegian state, Statoil ASA, Norsk Hydro Produksjon a.s, A/S Norske
Shell, Mobil Exploration Norway Inc., and Norske Conoco AS.)