co2 removal membranes for gas processing
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CO2 Removal Membranes for Gas Processing
Matt Henley
KCC ProcessEquipment, nc.
Houston, Texas
Abstract
CO2 reduction is a significant and growing part of gas processing due to equipment and pipline
corrosion. Membrane basedCO2 removal systemsprovide a cost effective, low maintenanceapproach
for removing CO2 rom gas streams. Operational experience n the gas field combined with thousands
of units in refineries and air processing has shown what parameters are important in design and
operation of units that have both long life and ow maintenance.
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CO2 Removal Membranes for Gas Processing
Introduction
Natural gas as it flows from a well typically requires a number of treatment steps before it can be put
into a pipeline for sale. The primary treatments nclude dehydration and liquids separation o meet the
specification required for the pipeline. Sour components such as Carbon Dioxide and Hydrogen
Sulfide which cause co1Tosion f pipelines and equipment may also require removal. CO2 removal
membranesare one method for meeting thesespecifications.
Amine type systemshave been traditionally used to remove sour components and work well for this
purpose. However, these systems ypically have high capital and operational costs. They also can
have high maintenanceand require a close watch by operations.
There are many caseswhere a large amine system s not required, for example: There are housandsof
gas plants that have little or no Hydrogen Sulfide but do have enough CO2 o cause concern for the
pipline companies. This has become a greater concern as pipeline owners inspect their piplines and
di~over the extent of damage hat CO2has calLc;ed.
Another area s the bulk removal of CO2 rom a gas streamwhere the amount of CO2would otherwise
require an extremely arge amine unit.
In either case,CO2 emoval membranescan provide a low maintenanceoption for the removal of CO2.
However, from operational experence n air separation, efinery and gas production areas,KCC and
Air Products have identified a number of operational ssues hat must be addressedn order to design
and operatea low maintenanceand ong life membranesystem.
Membrane Theory
Most membrane systems are semipermeable polymers such as cellulose acetate, poly sulfone and
polyimide and polyamides. Air Products PRISM Membranes are constructed of a proprietary
polyimidc formulation which is tolcrant to liquid water and hydrocarbons and allows opcration at
elevated emperatures o avoid more elaborateand costly pretreatment Membranes allow transport of
different molecules through the membrane based on the rate of solubilization into the membrane
material and diffusion through it [I]. Each molecule dissolves and diffuses through a membrane at
different rates that allow separation of the molecules. Partial pressure of that speciesprovides the
driving force for this transport.
As the gas mixture flows along the surface of the membrane, he components hat have a faster rate of
permeation pass tlu ough the membrane leaving belJiIld tIre components with a slower rate of
penneation. This phenomenoncan be expressed s
p
(p i2 Pil)A
i=-
(I)
Q;
p
I
p
= Molar Penneate Flow of Component i
= Pe11lleability of Component i
= Membrane Thickness
= Partial Pressure of i on Low Pressure Side
= Partial Pressure of i on High Pressure Side
= Membrane Area
Pil
Pi2
A
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This equation has to be evaluated along the length of the membrane as the partial pressme of the
component will change along that surface as more is transported through the membrane.
A number of things can be seen when evaluating the above equation. First, since the driving force of
the transport is partial pressures, the greater the difference in partial pressme between the low and high
pressure of the membrane, the greater the permeation rate of that component through the membrane.
Sccond, thc gas flux through thc mcmbranc incrcascs as thc thickncss dccrcascs. Lastly, as thc
required amount of CO2 removal goes up, smface area needs to be increased.
Selectivity of the membrane is another important factor. This is the difference in the permeability rates
of different components in the gas mixtw-es. The larger this difference, the greater the efficiency of the
separation and the lower the loss of salable gases are. Relative permeabilities of a number of
components for Polyimide membranes are shown in Figme I.
4 Fast Gases
Slow Gases~
Figure 1 Relative Permeability of Selected Gases hrough Gas Membranes
In the case of removal of CO2 rom natural gas, one of the important considerations s the amount of
CO2 removed verses he amount of methane ecovered. This is typically an inverse relationship, ie, as
the amount of CO2 removed goes up, the amount of methane ecovered goes down as shown n Figure
2. To compensate or this, many units are broken into two stage systems wherein the permeate s
recompressed nd separatedagain to improve the hydrocarbon recovery of the system. The different
systemconfigurations are shown n the next section.
100
90
~
0
~
...
GI
>
O
U
GI
Q: 80
C
0
.c
...
~
U
O...
j,
~ 70
601
0 10 20 30 40 50 60 70 00
CO2 Removal %
-5% CO2 in Feed --15% CO2 in Feed --30% CO2 in Feed
100
0
Figure 2: CO2 Removed verses Methane Recovered
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.
Membrane System Design
Design of a practical membrane system begins witil tile membrane unit itself. Several design criteria
should be included:
.High surface area to membrane volume
.Counter-current flow to keep tile greatest partial pressure difference across tile membranes
.Resistant to harsh conditions of Gas Processing
With a diameter of about 0.5 mm, Hollow fiber membranes ypically have a surface area of over 5000
m2 per cubic meter. This is the most compact ype membranecurrently in use. An example of hollow
fiber membranes s shown n Figure 3.
Figure 3: Hollow fiber membranes
Air Productsproduceswhat are called asymmetric membranes. Theseare membranes hat have several
layers that have different purposes. As shov.1l n Figure 4, these membraneshave a porous layer that
serves as a support structure to add strength and resistance o high pressures. This is the thickest
portion of the membrane. However, the pores are relatively large and therefore it does not hinder the
mass flux of the gas. The second ayer of the membrane s the actual membrane skin that does the
separationof the components.This layer is kept as thin as possible. The outside ayer of the membrane
is a defect repairing layer This layer prevents gas bypassof any gas hrough defects n the membrane
skin and allows the skin to be fabricated hinner
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Figure 4 Cross Section of Hollow Fiber Membrane
The membranesare packaged nto modules n a variety of sizeschosen o fit the application. As shown
in Figure 5, each module is designed o allow for counter current flow of the gases. This allows for the
maximum relative partial pressuresof the gasses o be removed which improves flow through the
membrane. The gas is evenly distributed through the membrane fibers by a gas distributor which is
built into the membranemodule.
Non-Permeate
r Product
c-:;
Membrane Filter Bundle
Feed
Gas
~I
:Feed
,
,Gas
,
,
I
I
i.,J
Permeate
Product
Figure 5 Hollow Fiber Separator Module
Figure 6 shows a typical flow scheme or a singe stagemembranesystemwhere the gas s coalesced o
remove any liquid droplets and then heated to keep the gas away from its hydrocarbon dewpont. It
then goes hrough the membranes o remove the CO2. The high CO2 offgas typically still has enough
calorific value to be used as fuel for the gas heater or other service n the plant. The advantageof this
flow scheme s its simplicity and low maintenance equirements.
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Non-Permeate
Product
~
Permeate
Product
ntrained
Liquids
Figure 6 Single Stage Membrane System Scheme
A two stage membrane system can be used to achieve high hydrocarbon recovery rates while still
removing large amounts of CO2. The off gas rom the secondstage s typically up to 60% CO2 but will
boost hydrocarbon recovery rates close o 99%.
Figure 7 Two Stage Membrane System Scheme
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Operational Experience
The use of membranes in the gas industry has been around for ahnost 20 years. However, there is a
growing amount of operational experience that shows which issues are unique to this technology that
should be taken into consideration when considering about their use.
Contamination and Pretreatment Requirements
The largest area of concern in designing a reliable membrane system is the area of contamination.
Contamination s the causeof most membrane ailures and lack of performance Becauseof this, it is
very important to carefully consider he required pretreatment equirementsof each system.
There are two distinct types of results from membranecontamination. If the membrane s coated with
a deposited ayer of hydrocarbon or oil that stays on its surface, he efficiency of the membranewill be
reduced. This is because he hydrocarbon forms a layer on top of the membrane hat the gas has to
traverse before it can start the process of moving through the membrane. Very heavy contamination
can effectively block off a portion of the active surfaceareaof the membrane.
Liquid hydrocarbon contamination can be effectively eliminated with a combination of a high
performance coalescerand a heater. The coalescer emoves entrained iquid in the gas and the heater
vaporizes any that might be left. In addition, the heater servesanother purpose. The gas entering a
membrane stagewould otherwise be close to saturation. As the COz is removed, the composition of
the gas changes. In cases such as the second stage of a two stage system or bulk COz removal, the
composition change may be enough o causehydrocarbons o condenseon the membrane tself. The
heatermoves he gas away from its saturationpoint and keeps hese iquids from forming.
A tubesheet separates he high pressure side of the membrane from the low pressure side. The
tubesheet s cast on one end of the fiber bundle using a specialized epoxy formulation. The tubesheet
is very strong and can withstand differential pressureswell in excess of the maximum differential
pressure across the membrane. However, certain contaminants can adsorb into the membrane and
epoxy materials causing them to swell. Methanol is one such swelling agent that is sometimes
encountered n natural gas streams due to upstream njection for prevention of hydrate formation.
Liquid methanol can causeswelling to an extent that tubesheet ailure can occur.
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Another issue n the use of membranesand the pretreatment equired is the membrane's compatability
with water and water vapor. Different membranesmaterials have varying abilities to tolerate water in
different forms. Before committing to a particular type of membrane, determine what the effects of
water are on the membranesparticular material and what pretreatment hat may entail. Air Products
mcmbranc matcrial was in part choscn for its cxccllcnt watcr handling abilitics. Whilc cfficicncy of
any wetted portions of the membrane will be temporarily reduced, the membrane will return to full
capacity as it dries. Water vapor travels through the membrane at a rate faster than that of even CO2
allowing the membranes o also be used in gas dehydration applications. This provides the added
benefit of dehydrating the gas while removing CO2. In addition, membranescan be provided for the
purposeof dehydration.
In two stageunits where compressionbetween stages s used, particular care should be taken with the
choice of cylinder lublicant. A low vapor pressuresynthetic lubricant should be chosenand engine oils
that contain additive packages hat may causeemulsions should be avoided. An oil with a high vapor
pressuremay cany over into the membranevia the vapor phase and be depositedonto the membrane.
We have seen caseswhere the additives in compressoroil have formed stable emulsions hat clogged
the coalescerand caused requent changoutof filter elements. This issue can be resolved by changing
the lubricant used for the compressor
Conclusions
Gas Membranes provide a cost effective way to reduce CO2 in natural gas. They can do this while
having low maintenance equirements hat are suitable for unmanned facilities and remote fields. As
long as attention is paid up front to pretreatment equirements, heseunits have both a long life and low
maintenance.
Acknowledgments
Bill Pope, Air Products for help with the technical details of membranes
John Branch for producing the diagrams
References Cited
Fundamentals of Gas Pemleation ,
acLean D.L., Stookey D.l., and Metzger T.R.,
Hydrocarbon Processing,August 1983.
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