GB2483245A

GB2483245A - Polymer membrane for carbon capture

Info

Publication number: GB2483245A
Application number: GB1014475.6A
Authority: GB
Inventors: Jeremy Gordon Mansfield
Original assignee: Doosan Power Systems Ltd
Current assignee: Doosan Babcock Ltd
Priority date: 2010-09-01
Filing date: 2010-09-01
Publication date: 2012-03-07
Also published as: GB201014475D0

Abstract

A microporous gas absorber membrane has a polymer composition comprising a major part of a first component polymer comprising a melt-processable thermoplastic carrier of a substantially unfluorinated polyalkane (e.g. polypropylene), and a minor part of a second component polymer comprising a polymer melt-processable fluoropolymer. The membrane is preferably used in a gas/ liquid absorption/ reaction system comprising a gas/ liquid absorption/ reaction vessel having a vessel wall defining a volume containing a microporous gas absorber membrane such that liquid flows in use within the volume on a first side of the membrane, and a gas flows in use within the volume on a second side of the membrane.

Description

MEMBRANE

The invention relates to a membrane for gas! liquid exchange, in particular for use in a process system that makes use of an absorbent liquid reagent to effect the removal of a target gas from a gas phase. The invention additionally relates to a method of production of the same, and to a polymer composition for use in the method.

In the particular preferred case the invention relates to a column structure for an absorption column comprising a containment vessel for use with an absorbent liquid reagent to effect the removal of a target gas from a gas phase, the membrane serving as a gas! liquid exchange membrane therein.

The invention relates in particular to the removal of CO2 from a gas phase by means of absorption. The invention relates to a column structure for removing CO2 from a gas phase by means of absorption. The invention is particularly suitable for use in removing CO2 from the flue gases of thermal power plants fired by carbonaceous fossil fuels, both as new build and for retrofitting into existing thermal power plants.

Most of the energy used in the world today is derived from the combustion of carbonaceous fossil fuels, such as coal, oil, and natural gas. Post-combustion carbon capture (PCC) is a means of mitigating the effects of carbonaceous fuel combustion emissions by capturing CO2 from large sources of emission such as thermal power plants which use carbonaceous fuel combustion as the power source. The CO2 is not vented to atmosphere but instead is removed from flue gases by a suitable absorber and stored away from the atmosphere. Other industrial processes where similar principles might be applicable to capture post-process CO2 might include removal of CO2 generated in a process cycle, for example removal of CO2 from the process flow during production of ammonia, removal of CO2 from a natural gas supply etc. It is known that CO2 can be separated from a gas phase, for example being the flue gas of a thermal power plant, by means of absorption by passing the gas through a column where the gas flows in an opposite direction to an absorbent in liquid phase. Such a process is sometimes referred to as wet scrubbing. A well known absorbent reagent comprises one or more amines in water.

Packed tower absorber column technology is well established to exploit this. An absorption plant consists of at least one column where liquid absorber is run through the column as the gas that is to be scrubbed is passed in the other direction. Typical columns consist of multiple sections of structured packing consisting of multiple thin plates or like structures to maximize the surface area for mass transfer. These are stacked within a containment vessel of steel or other suitable structural material. Such structures can be cumbersome and heavy.

Microporous gas absorber membrane (CAM) contactors are known. These initially appear to be feasible in principle as an interface means for gas/ liquid systems for FCC and the like, and potentially far more efficient than packing columns, but in practice they tend to suffer from rapid performance deterioration due to capillary infiltration into the micropores. A microporous gas absorber membrane that could mitigate the effect of such capillary infiltration would be desirable.

In accordance with the invention in a first aspect, a microporous gas absorber membrane comprises: a polymer composition comprising a major part of a first component polymer comprising a melt-processable substantially unfluorinated polyalkane, and a minor part of a second component polymer comprising a polymer melt-processable fluoropolymer; the said composition comprising the microporous membrane, and in particular the said composition having been melt processed and formed into a microporous gas absorber membrane.

Such a membrane is found to mitigate the capillary infiltration problem of as a surprising consequence of the tendency of such a mixed composition to segregate with time as the first and second component polymers separate and for the minor component to migrate to the surface. This effect can be best understood by referring to the capillary infiltration problem in more detail with reference to figure 2, where the action of a membrane acting as a microporous gas absorber membrane (GAM) contactor in a typical CO2 with amine absorber chemistry known from existing systems for PCC from the combustion gases of carbonaceous fuel combustion is considered.

In Scenario 1 there is no significant capillary infiltration. Reaction of CO2 with amine takes place at the membrane surface. Reaction products (i.e. rich amine) are quickly swept away within the freely circulating amine stream, so that equilibrium conditions are not achieved.

In Scenario 2, there is significant capillary infiltration and reaction of CO2 with amine takes within the pore, due to capillary infiltration. Reaction products now have to diffuse along a lengthy corridor of stagnant amine solution, compromising the driving force for further reaction, and the material's ability to function as a membrane.

Resistance to capillary infiltration on the liquid side is thus of great significance to the effectiveness of the reaction process. However, a dense network of narrow pores remains desirable on the free gas flow side. Resistance to capillary infiltration is partly a question of pore size (liquids cannot penetrate in below a certain critical diameter) and partly a question of surface wetting (an aqueous liquid will not penetrate the sub-microscopic features of a sufficiently low energy surface. The ideal material would have a dense network of narrow pores and also have a low surface energy.

No one polymeric material combines these two properties, but in accordance with the invention it is possible to compound minor quantities of the second polymer to provide the low surface energy with major quantities of the first polymer selected for well understood bulk properties, in particular being readily formable into a structure with a dense network of narrow pores. The two components are powdered, then co-melted and formed into a final extruded/moulded shape.

As the skilled person will appreciate, the co-melt process is distinctively characteristic of the resultant product. it is necessary to produce the resultant product and distinctly characterises the structure of the resultant product.

The invention additionally relies upon a surprising exploitation of a usually undesired side effect of such a co-melted membrane product. Over time as the co-melted membrane product ages the first and second component polymers separate and the fluoropolymer component tends to migrate to the surface. This process is normally seen as a degradation, but in the present invention is exploited. With appropriate selection of materials for the compositions envisaged by the invention, this effect can be designed to be produced relatively quickly, in days for example. In the resultant product, the polyalkane component tends to have a dominant bulk effect and acts in effect as a thermoplastic carrier material. The fluoropolymer component tends to have a disproportionate surface effect without substantive detriment to the bulk carrier polymer properties, and in particular without substantive detriment to its ability to form a dense network of narrow pores. The segregated co-melted material can get nearer to the ideal of a dense network of narrow pores and a low surface energy.

The co-melted membrane product may be allowed to segregate prior to use by passage of time or by other control process such as heat-treating.

For the compositions envisaged by the present invention, a storage time of about a week may be sufficient. Thus, what is normally an undesirable behavioural property of co-melted compositions, leading to reduction in performance during use, becomes in the particular case of the invention a desirable behavioural property leading to improvement in performance before use.

In accordance with the invention in a second aspect there is provided a melt-processable polymer composition for a microporous membrane comprising a major part of a first component polymer comprising a melt-processable substantially unfluorinated polyalkane, and a minor part of a second component polymer comprising a polymer melt-processable fluoropolymer.

The polymer composition is suitable to be formed into a microporous gas absorber membrane in accordance with the invention in the first aspect.

In accordance with the invention in a further aspect there is provided a method of forming a microporous gas absorber membrane comprising the steps of: co-melting a polymer composition comprising a major part of a first component polymer comprising a melt-processable substantially unfluorinated polyalkane, and a minor part of a second component polymer comprising a polymer melt-processable fluoropolymer; forming the resultant co-melt into a microporous gas absorber membrane, for example by extrusion and! or moulding.

The method is suitable for fabrication of a microporous gas absorber membrane in accordance with the invention in the first aspect. In particular, it is an inherent property of the product of the method that the first and second component polymers tend to separate and the fluoropolymer component tends to migrate to the surlace. Preferably, the method comprises the further step of allowing or encouraging this separation effect after the microporous gas absorber membrane is fabricated.

In selecting materials suitable for the first and second aspects of the invention above, the first component polymer that forms the major part of the composition is selected to provide the predominant bulk properties in the melt co-processing stage of fabrication. The first component polymer is also selected for its ability to form membrane structures with a dense network of narrow pores.

The first component polymer comprises for example a melt processable thermoplastic polyalkane. The first component polymer may be comprise single or multiple constituent melt processable thermoplastic polyalkanes.

The first component polymer conveniently comprises a semi-crystalline (spherulitic crystallite structure) polyalkane.

The first component polymer is a substantially non-fluorinated polyalkane.

Preferably the first component polymer is entirely non-fluorinated.

Preferably the first component polymer is a substantially non-substituted polyalkane. Preferably, all (C-X) terminal atoms are Hydrogen.

Such a polymer is selected to be able to co-mingle with the fluorinated polymer species in an agitated melt but subsequently reject it from the solidified structure. PolyPropylene is particularly preferred.

The second component polymer is a substantially and preferably a fully fluorinated polymer (i.e. at least a substantial number and preferably all (C-X) terminal atoms are Fluorine), able to be melt-processed. The second component polymer may be selected from one or more fluoropolymer species such as a Poly-FluoroAlkoxy (PFA) species or a Fluorinated Ethylene Propylene (FEP) species. A Poly-FluoroAlkoxy (PFA) species from the "Telfon" family is a possible material example. A speciality PolyEther functionality (i.e. polymer chain containing integral (C-O-C) linkages) may be preferred.

The key principle underlying the material combination selection is that, after co-processing the two polymers to final shape the fluoropolymer behaves like an impurity to segregate from the majority species migrating to the surface and forming a bloom that would consist mostly of fluoropolymer. This surface would thus display the low energy properties required to resist wetting.

Thus, the effect of the fluoropolymer is mitigated in the initial co-processing but maximized in the final product by exploitation of a segregation effect hitherto usually seen as only detrimental in melt co-processed polymer compositions.

The first component polymer makes up a major part of the composition.

Preferably the first component polymer comprises at least 90% by weight of the composition. The second component polymer preferably comprises 2 to 8% by weight of the composition. Each of the first and second component polymers may comprise a single species or multiple constituent species with the desired properties in a processable mixture.

Optionally, additional minor components may be included in the composition.

The membrane of the invention is particularly suited for use as a microporous gas absorber membrane in a gas! liquid absorption! reaction system.

In accordance with the invention in a further aspect there is provided a gas! liquid absorption! reaction system comprising a rnicroporous gas absorber membrane in accordance with the first aspect of the invention.

In particular, a gas! liquid absorption! reaction vessel comprises a vessel wall defining a volume containing a microporous gas absorber membrane in accordance with the first aspect of the invention such that liquid flows in use within the volume on a first side of the membrane, a gas flows in use within the volume on a second side of the membrane.

In a preferred embodiment, the vessel comprises an absorption vessel for use with an absorbent liquid reagent to effect the removal of a target gas from agas phase.

The vessel preferably comprises in particular an elongate columnar structure with longitudinally extending walls defining a closed vessel perimeter, for example being generally cylindrical, rectangular or other polygonal columnar shape. The columnar structure is preferably orientated vertically. The vessel preferably comprises a columnar structure for removing CO2 from a gas phase by means of absorption.

This embodiment is particularly suitable for use in removing 002 from the gaseous byproducts of a carbonaceous combustion process, and for example the flue gases of thermal power plants fired by carbonaceous fossil fuels. The embodiment is suitable for application to such use both as new build and for retrofitting into existing systems such as existing thermal power plants.

Thus, in the preferred embodiment the vessel is a PCC absorber column in which the microporous gas absorber membrane in accordance with the first aspect of the invention substitutes at least in part for the structured packing more familiar in the prior art in providing a high surface area separation material for mass transport at the gas! liquid interface.

The vessel may be adapted for 002 separation in familiar manner, in that 002 is separated from a gas phase, for example being the flue gas of a thermal power plant, by means of absorption by passing the gas through the vessel where the gas flows in an opposite direction to an absorbent in liquid phase, the microporous gas absorber membrane in accordance with the first aspect of the invention being disposed to provide a high surface area for mass transport at the gas! liquid interface. The vessel thus comprise means to cause 002-rich gas to flow in a first direction through the vessel and means to cause absorbent in liquid phase to flow through the vessel countercurrently thereto, with the microporous gas absorber membrane in accordance with the first aspect of the invention being disposed such that the C02-rich gas flows across a first surface and the liquid phase flows across a second surface whereby the microporous gas absorber membrane in accordance with the first aspect of the invention provides a semi-permeable membrane at the gas! liquid interface.

The separation vessel may be divided into plural sections for example transversely (such that in a vertical column sections are arrayed vertically) with the microporous gas absorber membrane disposed to provide a high surface area separation material for mass transport at the gas! liquid interface in one or more such sections. Other sections may comprise other separation materials.

It is an advantage of the invention in this embodiment that the microporous gas absorber membrane may substitute for other mass transfer elements, such as structured packing, in a CO2 separation column system which can otherwise make use of known chemistry and structures.

In a more complete embodiment, the vessel is a CO2 separation column and the internal volume of the column preferably further comprises, typically for example disposed at the top of the or each section of separation material, one or more of a collector structure, a distributor structure, and a bed limiter in familiar manner. The column may further comprise in a washing stage a demister structure. All the foregoing will be of familiar design scaled up as applicable to the larger columns made possible in the present invention.

The column preferably further comprises a means to supply absorbent solution through one or more inlets in the vicinity of the top of the column.

In the preferred application the column is a CO2 wet scrubber, and the solution may comprise one or more aqueous amines, for example including but not limited to monoethanolamines or methyl-diethanol-amines.

In the preferred application the vessel is provided for use in a scrubber column for flue gases and is provided with a flue inlet towards the bottom of the vessel.

The invention will now be described by way of example only with reference to Figures ito 2 of the accompanying drawings, wherein: figure 1 is a schematic representation of a microporous gas absorber membrane such as might embody the principles of the invention suitable for application in a CO2 separation column system; figure 2 shows the processes occurring at a membrane being employed as a microporous gas absorber membrane in a CO2 separation column system such as one making use of the microporous gas absorber membrane of figure 1 to illustrate the advantages of a polymer composition in accordance with the invention.

Figure 1 is a general schematic of the circulation of amine and flue gas through a microporous gas absorber membrane such as might embody the principles of the invention illustrated, from left to right, as an end projection, as a longitudinal section, and with an enlarged view of a short section of microporous gas absorber membrane. The membrane is intended to comprise a CAM contactor in a CO2 separation column system, the other elements of which are not specifically shown but may be of any suitable standard design as will be readily available to the person skilled in the art.

Figure 2 illustrates the action of a microporous gas absorber membrane (CAM) contactor.

This action has been discussed briefly above as it would apply in a typical 002 with amine absorber chemistry known from existing systems for P00 from the combustion gases of carbonaceous fuel combustion.

In Scenario 1, with no capillary infiltration, reaction of CO2 with amine takes place at the membrane surface. Reaction products (i.e. rich amine) are quickly swept away within the freely circulating amine stream, so that equilibrium conditions are not achieved. Mechanical circulation of the amine solution can thus be used to drive the formation of a partial 002 vacuum within the pores, resulting in an efficient, selective CAM membrane.

Conversely, in Scenario 2, reaction of 002 with amine takes within the pore, due to capillary infiltration. Reaction products now have to diffuse along a lengthy corridor of stagnant amine solution, regardless of the external flow rate. There is no longer any driving force for further reaction, and the material's ability to function as a membrane is compromised.

To balance resistance to capillary infiltration with the desire for a dense network of narrow pores, the ideal material would be suitable for forming such a dense network of narrow pores and also have a low surface energy.

No one polymeric material combines these two properties, but it can be offered in a distinct and surprising way by the compositions in accordance with the invention. Minor quantities of a fluoropolymer providing the low surface energy in the finished product are combined with major quantities (for example the balance) of a medium-to-low cost polymer selected as suitable for forming such a dense pore structure with appropriate bulk strength characteristics (e.g. polypropylene: cheap, with well understood properties, and easily manipulated) which are co-melted via a powder melt process into the desired structure.

In the initial co-processing, it is primarily the bulk properties of the majority species polymer that are exploited in forming the co-melted composition into a final extruded/moulded shape (e.g. initially a cast plaque, then a microporous membrane, ideally in tube form). The quantity of fluoropolymer is sufficiently low to avoid excessive detriment to the bulk properties of the majority species.

The key principle is that, after co-processing the two polymers to final shape, the fluoropolymer, behaving like an impurity, would segregate from the majority species migrating to the surface and forming a bloom that would consist mostly of fluoropolymer. This is allowed to happen, for example over a timescale of days or weeks, before the final product is made available for use as a microporous gas absorber membrane contactor or the like. This surface would thus display enhanced low energy properties required to resist wetting.

EXAMPLES

Example 1

In a first example composition the main component material is a semi-crystalline polypropylene-random-copolymer such as Borealis RA13OE.

This exhibits good stability at the temperatures and conditions of an amine based carbon capture system. The composition has added 5% Du Pont Teflon PFA as the fluoropolymer component.

Example 2

In a second example composition the main component is instead selected as a more highly crystalline grade of Polypropylene (PP), allowing substitution of Fluorinated Ethylene Propylene (FEP) as the fluoropolymer component.

Claims

CLAIMS1. A microporous gas absorber membrane comprising: a polymer composition comprising a major part of a first component polymer comprising a melt-processable substantially unfluorinated polyalkane, and a minor part of a second component polymer comprising a melt-processable fluoropolymer; the said composition comprising the microporous membrane.
2. A microporous gas absorber membrane in accordance with claim 1 wherein the said composition has been melt processed and formed into the microporous gas absorber membrane.
3. A microporous gas absorber membrane in accordance with claim 2 wherein the said first and second components have been powdered, co-melted and formed into a final extruded/moulded shape.
4. A microporous gas absorber membrane in accordance with claim 3 wherein the co-melted membrane product has been allowed to segregate prior to use by passage of time or by other control process such as heat-treating.
5. A melt-processable polymer composition for a microporous membrane comprising a major part of a first component polymer comprising a melt-processable substantially unfluorinated polyalkane, and a minor part of a second component polymer comprising a polymer melt-processable fluoropolymer.
6. A method of forming a microporous gas absorber membrane comprising the steps of: co-melting a polymer composition comprising a major part of a first component polymer comprising a melt-processable substantially unfluorinated polyalkane, and a minor part of a second component polymer comprising a polymer melt-processable fluoropolymer; forming the resultant co-melt into a microporous gas absorber membrane, for example by extrusion and! or moulding.
7. A method in accordance with claim 6 comprising the further step of allowing or encouraging the fluoropolymer to migrate to the surface of the microporous gas absorber membrane after the microporous gas absorber membrane is fabricated.
8. A membrane, composition or method in accordance with any preceding claim wherein the first component polymer is selected for its ability to form membrane structures with a dense network of narrow pores.
9. A membrane, composition or method in accordance with any preceding claim wherein the first component polymer comprises a semi-crystalline melt processable thermoplastic polyalkane.
10. A membrane, composition or method in accordance with any preceding claim wherein all (C-X) terminal atoms of the first component polymer are hydrogen.
11. A membrane, composition or method in accordance with any preceding claim wherein the first component polymer comprises polypropylene (PP).
12. A membrane, composition or method in accordance with any preceding claim wherein the second component polymer is a substantially fluorinated polymer.
13. A membrane, composition or method in accordance with claim 12 wherein the second component polymer is selected from one or more fluoropolymer species such as a Poly-FluoroAlkoxy (PFA) species or a Fluorinated Ethylene Propylene (FEP) species.
14. A gas! liquid absorption! reaction system comprising a microporous gas absorber membrane in accordance with the membrane of one of claims I to 4 or one of claims 8 to 13 when dependent upon one of claims 1 to 4.
15. A gas! liquid absorption! reaction system in accordance with claim 14 comprising a gas! liquid absorption! reaction vessel having a vessel wall defining a volume containing a microporous gas absorber membrane such that liquid flows in use within the volume on a first side of the membrane, and a gas flows in use within the volume on a second side of the membrane.
16. A gas! liquid absorption! reaction system in accordance with claim wherein the vessel comprises an absorption vessel for use with an absorbent liquid reagent to effect the removal of a target gas from a gas phase.
17. A gas! liquid absorption! reaction system in accordance with claim 16 wherein the vessel comprises an elongate columnar structure with longitudinally extending walls defining a closed vessel perimeter, for example being generally cylindrical, rectangular or other polygonal columnar shape.
18. A gas/liquid absorption! reaction system in accordance with one of claims 15 to 17 wherein the vessel comprises a columnar structure for removing CO2 from a gas phase by means of absorption.