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 Plant
1
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.
Feedgas CO2 Content
3
2.5
2
Omaha
1.5
MSP
1 Design
0.5
0
Before After
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 Gas treatment system
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.
2
Amine System
Large capital investment
Typically used to remove H2 S and CO2
Require managing hazardous chemicals in the
process
Must have a disposal plan for the byproducts
(Sulfur and CO2)
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.
Two bed Adsorber sytem
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.
3
Molecular Sieve Particle
Structures
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 Unit Cell
Structure
Nano-
Nano-scale crystalline unit cells
Multiple stacks of unit cells grown to
form molecular sieve crystals
Crystals bound by clay binder,
formed into particles and kiln-
kiln-fired
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.
4
Molecular Sieve Unit Cell
Structure
Nano-
Nano-sized crystalline unit cells
Uniform pore size and volume
Multiple stacks of unit cells grown to
form molecular sieve crystals
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.
Adsorptivity & Selectivity
Adsorptivity Selectivity
Type 3A:
3A:
H2O, NH3
H2O C7H16
NH3 Benzene Type 4A-
4A-LNG:
LNG:
Methanol Thiophene H2O, NH3, Methanol, Ethanol, H2S, COS,
Ethanol C6H14 CO2, C2H6, CH4, O2
Ethyl Mercaptan COS Type LNG-
LNG-III:
III:
Methyl Mercaptan CO2 H2O, NH3, Methanol, Ethanol, H2S, COS,
T-Butyl Mercaptan CO2, C3H8, C2H6, CH4, O2, Mercaptans,
Mercaptans,
C4H10
Sulfides, “Normal”
Normal” Hydrocarbons
Ethyl Methyl Sulfide C3H8
H2S C2H6 Type LNG-
LNG-5:
C9H20 H2O, NH3, Methanol, Ethanol, H2S, COS,
CH4
CO2, C3H8, C2H6, CH4, O2, Mercaptans,
Mercaptans,
C8H18 Oxygen Sulfides, “Normal”
Normal” and “Iso”
Iso”
Hydrocarbons, Benzene, Thiophene
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
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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.
Equilibrium & Mass Transfer
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.
Typical Range of Design and
Operating Conditions
Adsorption (Feed) Conditions
Source: pipeline quality natural gas
Flow Rate: ~ 5 to 30 MMSCFD
Pressure: ~ 300 to 800 psig
Temperature: 60 to 100 ºF
CO2 Content: 0.5 to 1.5 (mole)%
Regeneration (Purge) Conditions
Source: adsorption product or plant flash gas
Flow Rate: ~ 20% to 50% of feed rate
Pressure: feed or lower
Temperature: ~ 300 to 600 ºF
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.
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Two bed adsorber system
Minnesota- Centerpoint Energy
Minneapolis, Minnesota-
LNG plant’
plant’s details
Built by CB&I and operational in 1976.
1 BCF LNG storage tank
design- 5 MMCF per day
Liquefaction design-
2 bed adsorption system for feedgas pretreatment for
liquefaction with a regeneration gas flow of 8.4
MMCFD.
The typical feedgas processed by this plant had a CO2
content of 0.5 to 0.6%.
g
Two Bed Cycle, Open Loop Regeneration
Wet Feed Cool To Fuel/Pipeline
K.O.
H2O
Adsorption Regeneration
(H2O & CO2 (H2O & CO2
Removal) Removal)
Heater
Product (to LNG)
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.
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Results of Molecular Sieve
changeout
Results;
The CO2 capacity of the adsorber system was
improved by 30%
The maximum CO2 content successfully
processed had a CO2 content of 1.55% without
breakthrough
Future considerations include;
lengthening bed switching times
adding a third bed if higher CO2 levels develop
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.
Three bed adsorber system
St. Paul, MN-
MN- Xcel Energy LNG plant’plant’s details
The Xcel Energy plant was built by CB&I and
operational in 1974.
The plant includes;
2 BCF LNG storage tank
Liquefaction design-
design- 10 MMCF per day
3 bed adsorption system to pretreat the feedgas for
liquefaction with a regeneration gas flow of 23
MMCFD.
The typical feedgas processed by this plant had a CO2
content of 0.5 to 0.6%.
8
g
Three Bed Cycle, Open Loop Regeneration
To Fuel/
Wet Feed Cool Pipeline
K.O.
H2O
Adsorption Heating Cooling
(H2O & CO2 (Open (Open
Removal) Loop) Loop)
Heater
Product
(to LNG)
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.
Results of Molecular Sieve
changeout
Results;
The CO2 capacity of the adsorber system was
improved, but testing has not been completed
The maximum CO2 content while on line was at
1.2% without an issue
Experience operating the system with
lengthening bed switching times has been
successful with CO2 levels below 1%.
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 CO 2
breakthroughs.
Prolonged usable life of the zeolite is anticipated by extending the Adsorber bed switching time and reducing
the thermal cycles.
9
Three bed adsorber system
Omaha, Nebraska-
Nebraska-Metropolitan Utilities District
LNG plant’
plant’s details
The Metropolitan Utilities District plant was built
by CB&I and operational in 1976.
The plant includes;
1 BCF LNG storage tank
Liquefaction design-
design- 5 MMCF per day
3 bed adsorption system to pretreat the feedgas for
liquefaction with a regeneration gas flow of 5.52
MMCFD.
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.
Results of Molecular Sieve
changeout
Results;
Molecular sieve loading design included the combination of 3
types of zeolite molecular sieve
adsorber bed switching time for adsorbing is 114 minutes, and
heating and cooling cycle timers are set at 110 minutes
Initial testing indicated CO2 breakthrough at 0.58% in 90
minutes
Final testing indicated CO2 breakthrough at 1.1% in 150
minutes, and normal bed switch times at 1.6% CO2 content.
Reviewing other options to be able to continue Liquefaction up
to 2% CO2 content
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.
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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.]
*****
Removal and Disposal of Sieve
Material
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
Adsorber 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.
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