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LRGCC-03 CO2 from Ethane

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HYBRID SEPARATION OF CO 2 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 CO 2 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 CO 2 and ethane from a CO 2 stripper overhead stream has been successfully separated with cellulose acetate membranes to produce CO 2 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 CO 2 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 CO 2 have therefore been studied. There has also been interest in a CO 2 feed stream from the ethane plant, for further processing to a commercial CO 2 product. For this reason, a process providing a high purity CO 2 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 CO 2 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 CO 2 / C 2 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.
Transcript
Page 1: LRGCC-03 CO2 from Ethane

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.

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Page 2: LRGCC-03 CO2 from Ethane

Figure 1: Picture of Kårstø gas plant in Norway

Figure 2: Block Flow Diagram of Kårstø gas processing plant including the Ethane treatment plant

Page 3: LRGCC-03 CO2 from Ethane

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

Page 4: LRGCC-03 CO2 from Ethane

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

Page 5: LRGCC-03 CO2 from Ethane

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.

Page 6: LRGCC-03 CO2 from Ethane

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.

Page 7: LRGCC-03 CO2 from Ethane

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.

Page 8: LRGCC-03 CO2 from Ethane

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.)


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