CO2 Removal Membranes for Gas Processing

8/16/2019 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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CO2 Removal Membranes for Gas Processing