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
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
LC
1008
FC
1021
29-PA-101A/B
Fra T-100
Fra T-200
Fra T-300
FC
1004
FC
1036
FC
1016
FC
1025
FF
1016
Til salgsgass
sugedrum
FC
1037 LC
1046
FC
1068
Etan fra C2 kompressor
46-system
Etan rundown
CO2 stripper
refluks seperator
29-HG-103
Etan rundown
kjøler
29-HG-101
Koker
29-HG-102
Kondenser
CO2 kompressor
29-KA-101
CO2
stripper
LC
0071
HT seperator
25-VA-011
LC
0078
Akkumulator
25-VA-013
HT dampturbin
Propan kondenser
PC
0027
Antisurge
FFIC 028
Antisurge
FFIC 039
LT seperator
25-VA-012
25
25
64
57
1
34,5 bar / 5,5 C / 119 t/h
PC
1038
17,2 C
34,0 bar / -3,3 C
2,6 bar
-8,6 C
25
0,32 bar
-34,7 C
11,4 bar / 52,9 C / 216 t/h
Sjøvann Sjøvann
MM 11.00
12,5 t/h
34,7 bar / 17,2 C / 73 t/h
106,5 t/h
Til fakkel
B
A
PC
0031
25
Til turtalls
regulering av
dampturbinen
85,5 t/h
73 t/h
25-HA-011
29-HV
-1028
25-HV-0063
Til fakkel
25-HV-0064
PC
1038
Til fakkel
25-HV-0081
Til fakkel29-HV-1065
29-QSV-1022
29-QSV-1003
29-HV-1055
29-HV-1042
29-HV-1066
29-HV-1045
24-HV
-1060
29-HV
-1095
29-HV-1096
29-HV-1097
29-HV-1098
PC
1017
HC
1017 B
Table 1: Product specifications high purity CO2 applied
Components Ethane product CO2 product
Methane Max 1,5 wt% Max 1 ppbV
Ethane Min 95 wt% Max 1000 ppmV
Propane + Max 4,5 wt% Max 1 ppbV
Carbon dioxide Max 100 wt ppm 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
Composition, Mole Fraction CO2, (P = 34.000 BAR)
0 0.2 0.4 0.6 0.8 1.0
Temperature, C
-8.0
-4.0
0
4.0
8.0
12.0
16.0
T-X-Y Plot for CO2 and C2
B Bubble Point
B
B
B
B
B
B
B
B
B
B
BB
B B B BB
B
B
B
B
D Dew Point
D
D
D
D
D
D
D
D
D
D
DD
D D D DD
D
D
D
D
Distil 1 (CO2-splitter) Distil 2 (CO2-purification)
feedoverheadfeed bottomsbottoms overhead
Membrane separator
azeotrope
CO2/(CO2+C2)=0.7
Pure CO2Pure C2
permeatefeed
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 Ethane product CO2 product Methane Max 1.5 wt% Ethane Min 95 wt% Propane + Max 4.5 wt% Carbon dioxide Max 100 wt ppm 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.)