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PRETREATMENT SYSTEM MODIFICATIONS FOR IMPROVING CO2 REMOVAL IN THE FEEDGAS FOR 3 GAS UTILITY PEAK-SHAVING PLANTS

James Goodchild Xcel Energy

Todd Lind Centerpoint Energy

Andrew Melville Metropolitan Utilities District

ABSTRACT

Natural gas transmission systems are being extended to new sources of natural gas to satisfy the additional needs of more gas-fueled power generation systems throughout the US. However, some of these new gas sources have gas compositions which stretch the pipeline tariff limits. Three major Utility-owned LNG Liquefaction/Peak shaving plants in Omaha, Nebraska and Minneapolis, Minnesota were notified of increased CO2 levels in the feedgas to these plants, which would exceed the design capabilities of these plants. Designed in the early 1970's for typical feedgas conditions, which included a CO2 content of less than 1%, and these facilities operated over 30 years without issues. Once notified that new gas supply CO2 concentrations would reach up to 3% in Omaha and up to 2% in Minneapolis, each utility decided to replace their original Molecular Sieve with a Sieve material capable of CO2 removal at these high concentrations. This presentation briefly outlines the situations and the efforts of these 3 LNG facilities to quickly modify their Pretreatment systems in order to continue to operate their existing Liquefaction plants without complete replacement of their Pretreatment system.

* * * * * Natural gas transmission systems are being extended to new sources of natural gas to satisfy the additional needs of more natural gas-fueled power generation systems throughout the US. However, some of these new gas supplies are stretching the CO2 allowable limits of the natural gas pipelines delivering gas to major cities in the Upper Midwest region of the United States. These areas are served by 3 major gas utilities, each operating their own LNG Liquefaction/gas peak shaving plant.

This presentation outlines how the higher CO2 levels in the feedgas affect the operability of these LNG Liquefaction facilities, and what considerations where needed in order to continue operating these plants. A brief review of the results and a few pictures of the process are also included.

LNG Liquefaction PlantLNG Liquefaction Plant

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All of these LNG facilities were designed and built in the mid 1970’s to liquefy pipeline quality natural gas, with a CO2 content of less than 1%. The Liquefied Natural Gas, LNG is then stored for use during the heating season when natural gas supplies are not in balance with the Gas Utility system’s demand.

During the Liquefaction process the temperature of the feedgas is reduced to approximately -260 degrees Fahrenheit, therefore the H2O and CO2 in the feedgas must be removed prior to liquefaction as their freezing will create blockages in the process piping at these cryogenic temperatures.

The 2 primary methods for the removal of CO2 and water from natural gas for liquefaction are:

• Amine gas treating systems

• Zeolite molecular sieve systems.

Amine systems utilize liquid removal of high concentrations of H2S and CO2 from the Sour Gas that typically may be found in raw gases at the well head. This illustration shows the process flow of the amine solution and “sour” gas being treated, as well as the waste stream of Sulfur and CO2.

Feedgas COFeedgas CO22 ContentContent

0

0.5

1

1.5

2

2.5

3

Before After

OmahaMSPDesign

Amine Gas treatment systemAmine Gas treatment system

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These Amine systems require the use of hazardous chemicals in the stripping process, which must be periodically refreshed. These systems must also have a means for disposing of the byproducts (sulfur and CO2) that has been stripped from the gas. This coupled with the high initial investment, and close management of the chemicals used for the system’s operation make these systems impractical for the gas utility-owned LNG facilities.

Temperature Swing Adsorber systems utilizing zeolite molecular sieve material are also used in large scale applications, however they are also well suited for the seasonal operations of the gas utility-operated LNG plants.

Amine SystemAmine System

Large capital investmentLarge capital investment Typically used to remove HTypically used to remove H22 S and COS and CO2 2

Require managing hazardous chemicals in the Require managing hazardous chemicals in the processprocess

Must have a disposal plan for the byproducts Must have a disposal plan for the byproducts (Sulfur and CO(Sulfur and CO22) )

Two bed Adsorber Two bed Adsorber sytemsytem

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Zeolite molecular sieves have the ability to adsorb water and CO2 when cool, and then release these elements once heated. This feature enables the gas processing system to remain idle for extended periods of time without any detrimental effects on the zeolite. This picture shows what some of the different types of molecular sieve materials look like.

Zeolite molecular sieves have the ability to adsorb water and CO2 when cool, and then release these elements once heated. This feature enables the gas processing system to remain idle for extended periods of time without any detrimental effects on the zeolite. This picture shows what some of the different types of molecular sieve materials look like.

Molecular Sieve Particle Molecular Sieve Particle StructuresStructures

Molecular Sieve Unit Cell Molecular Sieve Unit Cell StructureStructure NanoNano--scale crystalline unit cellsscale crystalline unit cells Multiple stacks of unit cells grown to Multiple stacks of unit cells grown to

form molecular sieve crystalsform molecular sieve crystals Crystals bound by clay binder, Crystals bound by clay binder,

formed into particles and kilnformed into particles and kiln--firedfired

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The common decision of these three gas utilities was to modify the adsorber pretreatment system by changing their operating procedures, and replacing their current molecular sieve with a newly developed zeolite sieve material capable of CO2 removal at 30% higher concentrations than before.

Adsorber principles of operation

Each adsorber system consists of:

• Several vertical, and internally insulated pressure vessels containing the zeolite molecular sieve

• associated flow control valves and gas piping to and from the vessels

• a gas heater for heating the regeneration “purge” gas that passes through the bed to heat the zeolite during regeneration

• a gas cooler to remove the heat from the hot regeneration gas after it passes through the adsorber beds, and before sending the gas back out as “tailgas” into the gas distribution pipeline system.

This chart above shows that the zeolite has a higher affinity for water than other elements. Therefore the water will be adsorbed first from the feed, and will displace the other elements which had already been

Molecular Sieve Unit Cell Molecular Sieve Unit Cell StructureStructure NanoNano--sized crystalline unit cellssized crystalline unit cells Uniform pore size and volumeUniform pore size and volume Multiple stacks of unit cells grown to Multiple stacks of unit cells grown to

form molecular sieve crystalsform molecular sieve crystals

AdsorptivityAdsorptivity & Selectivity& Selectivity

HH22OONHNH33

MethanolMethanolEthanolEthanolEthyl Ethyl MercaptanMercaptanMethyl Methyl MercaptanMercaptanTT--Butyl Butyl MercaptanMercaptanEthyl Methyl SulfideEthyl Methyl SulfideHH22SSCC99HH2020

CC88HH1818

CC77HH1616

BenzeneBenzeneThiopheneThiopheneCC66HH1414

COSCOSCOCO22

CC44HH1010

CC33HH88

CC22HH66

CHCH44

OxygenOxygen

SelectivitySelectivityType 3AType 3A:: HH22O, NHO, NH33

Type 4AType 4A--LNGLNG:: HH22O, NHO, NH33, Methanol, Ethanol, H, Methanol, Ethanol, H22S, COS, S, COS,

COCO22, C, C22HH66, CH, CH44, O, O22

Type LNGType LNG--IIIIII:: HH22O, NHO, NH33, Methanol, Ethanol, H, Methanol, Ethanol, H22S, COS, S, COS,

COCO22, C, C33HH88, C, C22HH66, CH, CH44, O, O22, , MercaptansMercaptans, , Sulfides, Sulfides, ““NormalNormal”” HydrocarbonsHydrocarbons

Type LNGType LNG--5:5: HH22O, NHO, NH33, Methanol, Ethanol, H, Methanol, Ethanol, H22S, COS, S, COS,

COCO22, C, C33HH88, C, C22HH66, CH, CH44, O, O22, , MercaptansMercaptans, , Sulfides, Sulfides, ““NormalNormal”” and and ““IsoIso””Hydrocarbons, Benzene, Hydrocarbons, Benzene, ThiopheneThiophene

AdsorptivityAdsorptivity

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adsorbed in the molecular sieve material as it passes through the zeolite material. These other elements will then be adsorbed at another area of the adsorber bed.

This creates a virtual “wave front” that will move through the adsorber bed during the adsorption process. The illustration shows how this moves down through the Adsorber vessel as the molecular sieve is “loaded” by the water and CO2

These adsorber systems require a substantial amount of purge gas, and heat for regenerating the molecular sieve material for continued use. While the design flow rate, and temperature of the “regeneration” gas used for purging the beds varies greatly, the result remains the same in removing the H2O and CO2 from the feedgas.

There are several designs for the Adsorber systems, but in these facilities there are just the “Two Bed” and “Three Bed” Temperature Swing Adsorber systems.

Equilibrium & Mass TransferEquilibrium & Mass Transfer

Typical Range of Design and Typical Range of Design and Operating ConditionsOperating Conditions

Adsorption (Feed) ConditionsAdsorption (Feed) Conditions Source: pipeline quality natural gasSource: pipeline quality natural gas Flow Rate: ~ 5 to 30 MMSCFDFlow Rate: ~ 5 to 30 MMSCFD Pressure: ~ 300 to 800 psigPressure: ~ 300 to 800 psig Temperature: 60 to 100 Temperature: 60 to 100 ººF F COCO22 Content: 0.5 to 1.5 (mole)%Content: 0.5 to 1.5 (mole)%

Regeneration (Purge) ConditionsRegeneration (Purge) Conditions Source: adsorption product or plant flash gasSource: adsorption product or plant flash gas Flow Rate: ~ 20% to 50% of feed rateFlow Rate: ~ 20% to 50% of feed rate Pressure: feed or lowerPressure: feed or lower Temperature: ~ 300 to 600 Temperature: ~ 300 to 600 ººF F

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The 2 bed Adsorber system illustration shows the process flow through the 4 components of the system. This system is designed so that the adsorption time required in the first Adsorber bed is the same as the heating and cooling times in the other Adsorber bed.

Two bed adsorber systemTwo bed adsorber system

Minneapolis, MinnesotaMinneapolis, Minnesota-- Centerpoint Energy Centerpoint Energy LNG plantLNG plant’’s detailss details

Built by CB&I and operational in 1976. Built by CB&I and operational in 1976. 1 BCF LNG storage tank1 BCF LNG storage tank Liquefaction designLiquefaction design-- 5 MMCF per day5 MMCF per day 2 bed adsorption system for feedgas pretreatment for 2 bed adsorption system for feedgas pretreatment for

liquefaction with a regeneration gas flow of 8.4 liquefaction with a regeneration gas flow of 8.4 MMCFD.MMCFD.

The typical feedgas processed by this plant had a CO2 The typical feedgas processed by this plant had a CO2 content of 0.5 to 0.6%. content of 0.5 to 0.6%.

g Two Bed Cycle, Open Loop RegenerationTwo Bed Cycle, Open Loop Regeneration

AdsorptionAdsorption

(H(H22O & COO & CO22Removal)Removal)

RegenerationRegeneration

(H(H22O & COO & CO22Removal)Removal)

CoolCoolK.OK.O..

HH22OO

Wet FeedWet Feed

Product (to LNG)Product (to LNG)

HeaterHeater

To Fuel/PipelineTo Fuel/Pipeline

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While the frequency of the changing feedgas content is now unpredictable, the results thus far have been satisfactory for the Centerpoint Energy facility.

• They have been able to operate the Liquefaction facility with CO2 levels up to 1.55% without any issues to their process.

• They have made some process changes, such as timing of the Adsorber bed switching

• They are considering the conversion to a Three Bed system if the CO2 content should increase further.

Results of Molecular Sieve Results of Molecular Sieve changeoutchangeout

Results;Results; The CO2 capacity of the The CO2 capacity of the adsorberadsorber system was system was

improved by 30% improved by 30% The maximum CO2 content successfully The maximum CO2 content successfully

processed had a CO2 content of 1.55% without processed had a CO2 content of 1.55% without breakthroughbreakthrough

Future considerations include; Future considerations include; lengthening bed switching times lengthening bed switching times adding a third bed if higher COadding a third bed if higher CO2 2 levels developlevels develop

Three bed adsorber systemThree bed adsorber system

St. Paul, MNSt. Paul, MN-- Xcel Energy LNG plantXcel Energy LNG plant’’s detailss details The Xcel Energy plant was built by CB&I and The Xcel Energy plant was built by CB&I and

operational in 1974. operational in 1974. The plant includes;The plant includes; 2 BCF LNG storage tank2 BCF LNG storage tank Liquefaction designLiquefaction design-- 10 MMCF per day10 MMCF per day 3 bed adsorption system to pretreat the feedgas for 3 bed adsorption system to pretreat the feedgas for

liquefaction with a regeneration gas flow of 23 liquefaction with a regeneration gas flow of 23 MMCFD.MMCFD.

The typical feedgas processed by this plant had a CO2 The typical feedgas processed by this plant had a CO2 content of 0.5 to 0.6%. content of 0.5 to 0.6%.

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The 3 bed Adsorber system illustration shows the process flow through the 4 components of the system. This system is designed so that the adsorption time required in the first Adsorber is the same as the Heating and cooling times in the other 2 adsorber beds.

While the frequency of the changing feedgas content is now unpredictable, the results thus far have been satisfactory for the Xcel Energy facility.

• They have been able to operate the Liquefaction facility with CO2 levels up to 1.2% without any issues to their process.

• They have made some process changes, and have extended their Adsorber bed switching time from 60 minutes to 120 minutes when operating at CO2 content levels below 1%.

• They have reconfigured the gas analyzing system for faster response to possible CO2 breakthroughs.

Prolonged usable life of the zeolite is anticipated by extending the Adsorber bed switching time and reducing the thermal cycles.

g g Three Bed Cycle, Open Loop RegenerationThree Bed Cycle, Open Loop Regeneration

AdsorptionAdsorption

(H(H22O & COO & CO22Removal)Removal)

HeatingHeating

(Open(OpenLoop)Loop)

CoolCoolK.OK.O..

HH22OO

Wet FeedWet Feed

ProductProduct(to LNG)(to LNG)

HeaterHeater

To Fuel/To Fuel/PipelinePipeline

CoolingCooling

(Open(OpenLoop)Loop)

Results of Molecular Sieve Results of Molecular Sieve changeoutchangeout

Results;Results; The CO2 capacity of the adsorber system was The CO2 capacity of the adsorber system was

improved, but testing has not been completed improved, but testing has not been completed The maximum CO2 content while on line was at The maximum CO2 content while on line was at

1.2% without an issue1.2% without an issue Experience operating the system with Experience operating the system with

lengthening bed switching times has been lengthening bed switching times has been successful with CO2 levels below 1%. successful with CO2 levels below 1%.

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Again, the 3 bed Adsorber system illustration (page 9) shows the process flow through the 4 components of the system. This system is designed so that the adsorption time required in the first Adsorber is the same as the Heating and cooling times in the other 2 adsorber beds.

While the frequency of the changing feedgas content is now unpredictable, the results thus far have been satisfactory for the Metropolitan Utilities District facility.

• Testing has shown that CO2 breakthrough has improved from 90 minutes at 0.58% CO2 to 150 minutes at 1.1% CO2

• They are able to maintain normal Liquefaction plant operations up to 1.6% CO2 content.

• They are reviewing other options to be able to operate at 2% CO2 content level.

SUMMARY

None of the Liquefaction plants have experienced feedgas containing the maximum CO2 content levels during liquefaction, and they will be required to occasionally stop their processes if these maximum levels develop.

Three bed adsorber systemThree bed adsorber system

Omaha, NebraskaOmaha, Nebraska--Metropolitan Utilities District Metropolitan Utilities District LNG plantLNG plant’’s detailss details

The Metropolitan Utilities District plant was built The Metropolitan Utilities District plant was built by CB&I and operational in 1976. by CB&I and operational in 1976.

The plant includes;The plant includes; 1 BCF LNG storage tank1 BCF LNG storage tank Liquefaction designLiquefaction design-- 5 MMCF per day5 MMCF per day 3 bed adsorption system to pretreat the feedgas for 3 bed adsorption system to pretreat the feedgas for

liquefaction with a regeneration gas flow of 5.52 liquefaction with a regeneration gas flow of 5.52 MMCFD.MMCFD.

Results of Molecular Sieve Results of Molecular Sieve changeoutchangeout

Results;Results; Molecular sieve loading design included the combination of 3 Molecular sieve loading design included the combination of 3

types of zeolite molecular sievetypes of zeolite molecular sieve adsorber bed switching time for adsorbing is 114 minutes, and adsorber bed switching time for adsorbing is 114 minutes, and

heating and cooling cycle timers are set at 110 minutesheating and cooling cycle timers are set at 110 minutes Initial testing indicated COInitial testing indicated CO22 breakthrough at 0.58% in 90 breakthrough at 0.58% in 90

minutesminutes Final testing indicated COFinal testing indicated CO22 breakthrough at 1.1% in 150 breakthrough at 1.1% in 150

minutes, and normal bed switch times at 1.6% COminutes, and normal bed switch times at 1.6% CO22 content. content. Reviewing other options to be able to continue Liquefaction up Reviewing other options to be able to continue Liquefaction up

to 2% to 2% COCO22 contentcontent

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However, all plants have been able to modify their systems which will greatly extend the number of days available for plant operations, and each have found this solution successful at this point. Only a few options exist for processing natural gas at such high CO2 content, and they will require major investment into the existing systems.

[Graphics and Zeolite Molecular Sieve specifications provided by UOP.]

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These pictures illustrate the change out process. The stainless steel liquid nitrogen tanks with the nitrogen used for purging the adsorber vessels, and a vacuum truck used to remove the molecular sieve from the Adsorber vessels are seen in the upper left picture. Loading the removed sieve material into containers for disposal is shown in the lower right picture.

Internal Inspection and Refilling Internal Inspection and Refilling Adsorber BedAdsorber Bed

Entry into the Adsorber for internal inspection is shown in the upper left picture. Loading the new molecular sieve from the transport sacks is shown in the lower left picture.

Removal and Disposal of Sieve Removal and Disposal of Sieve MaterialMaterial