Patent ID: 12201937

DETAILED DESCRIPTION OF THE INVENTION

FIG.1shows a simplified schematic design of a membrane gas separation process. The incoming feed stream1, is passed over a membrane2and separated into two components: permeate3and retentate4. The feed gas travels along the membrane and parts of the feed gas travel across the membrane2from the high pressure side5to the low pressure side6, where it is removed as a permeate3.FIG.2shows details of a typical gas separation membrane comprising a selective layer7supported by an asymmetric porous layer8and a woven support9.

The gas separation membrane according to the invention comprises a CO2selective polymer layer and a support layer. The CO2selective polymer layer is disposed, e.g. coated, on the support layer. The selective polymer layer comprises carbonic anhydrase (CA) enzymes fixed within the polymer layer and optionally also to a surface of the polymer layer.

The CO2selective polymer layer may in addition to CA enzymes, comprise amine groups acting as a CO2-philic carriers.

Carbonic anhydrase (CA) is a metalloenzyme generally containing a zinc ion in the active site. It exists in various classes and isoforms and has the fastest known reaction rate with CO2with a turnover higher than 106molecules of CO2per second. CO2dissolves in water producing a hydrated form according to the following equations
CO2+H2O=H2CO3;  (Eq. 1):
H2CO3=HCO3−+H+;  (Eq. 2):
HCO3−=CO32−+H+  (Eq. 3):

Among these reactions, the hydration of CO2(Eq. 1) is the rate-limiting step. The dissociation of bicarbonate to produce carbonate is slow, but faster than hydration of CO2. At pH>10, Eq. 3 dominates the carbonate formation, whereas this step is negligible at pH<8. CA catalyzes the reaction of Eq. 1. By increasing the reaction rate of reaction 1, through the addition of CA, a large amount of CO2can be fixed as carbonate at a low to moderate pH. This approach has been demonstrated by using carbonic anhydrases of bovine origin. CA isolated from thermophiles, which are bacteria living at temperatures ranging from 50° C. to 110° C., are thermo-stable and stable to the common enzyme denaturants such as O2, and thus, being suitable for example, for flue gas treatment (35 to 50° C.).

The CA enzymes are isolated/produced and reactive side groups of amino acid residues (e.g., amines, hydroxyls, thiols, or phenolic groups), not associated with the active site of the enzyme, will allow subsequent modification and integration/immobilization into a polymer matrix.

Several methods for immobilizing CA enzymes in a polymer layer are possible:CA enzymes may be dispersed within the polymer: a known amount of enzymes in an aqueous solution are mixed with a polymer solution consisting of a polymer such as polyvinyl alcohol (PVA), polyacrylamide (PAA, alginate, chitosan or any other suitable polymer and a suitable solvent, and a thin layer of this mixture is applied to the support layer and dried. There will be weak hydrogen bonds between the polymer and the dispersed enzymes.CA enzymes may first be chemically modified with e.g. vinyl groups, and then copolymerized together with monomers to form a biopolymer. Thus, the enzymes will be immobilized to the polymer chain in situ during polymerization. Various monomers, such as acrylamide, may be polymerized with CA enzymes.An existing polymer may by modified by coupling of CA enzymes to functional groups on the polymer. The polymer may have various functional groups, such as amine groups, in the polymer chain, which can be used to immobilize the enzymes. Thus, the CA enzymes become chemically (covalently) bound to the polymer chain.
Surprisingly, it is shown that the chemically immobilized CA enzymes maintain their activity in the polymer matrix.

The resulting hybrid membrane layer comprising a polymer and fixed carbonic anhydrase enzyme (CA) will combine the durability of a dense polymeric membrane with the selectivity of a supported liquid membrane (SLM), thus eliminating the drawback of SLM-washing out of the carrier over time.

FIG.3illustrates the transport mechanism of the hybrid polymer-enzyme membrane according to the present invention. The membrane comprises a porous support having a polymer-enzyme (CA) layer disposed thereon. The thickness of the polymer-enzyme layer may be in the range from 0.1 to 10 μm, preferably from 1 to 5 μm.

CO2molecules are transported selectively from the high pressure side (feed side) to low pressure side both by enzyme reaction (facilitated transport) and solution-diffusion. N2and CH4molecules, which do not react with CA enzymes, are transported only by solution-diffusion mechanism by dissolving and diffusing in the polymer matrix of the membrane. The reaction equilibrium is shifted towards CO2transport into the low pressure side (permeate side) and its desorption in the gas phase by continuously removing the permeate using sweep gas or vacuum. The CO2separation (absorption, reaction with water and enzyme and desorption), takes place in the selective polymer layer. Water in form of water vapours, provided by the target gas itself (flue gas, breathing, etc.), will permeate the thin selective layer and swell the polymer matrix, from feed side to permeate side. In one embodiment, the CO2concentration of the feed gas may be from 1-15%, and the permeate may then have a CO2concentration from 60-80%.

Preferably, the CO2selective polymer is a hydrophilic and/or a water vapour permeable polymer. Examples of suitable hydrophilic polymers are polyvinyl alcohol, chitosan, alginate, polyamide, polyacrylamide and polyvinyl amine.

Examples of suitable water vapour permeable polymers are polydimethylsiloxane (PDMS) and poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and perfluoro polymers such as poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene].

Gas separating membranes can typically take two forms, supported or unsupported. The present membrane is carried on a support to provide mechanical strength to the membrane. As noted below, the support can be in the form of a flat sheet or a hollow fibre support. Both these support types may be used in this invention.

Suitable supports giving mechanical strength are known in the art and most are porous. Suitable supports include polyether sulfone (PES), polytetrafluoroethylene (PTFE), polypropylene, sulphonated polysulfone, polyvinylidene fluoride, polyacrylonitrile (PAN) and related block copolymers, cellulose acetate polyimide, polyether imide (PEI), aliphatic polyamides, polyether ether ketone (PEEK), polyphenylene oxide (PPO) and polysulfone (PSF). In a preferred embodiment, the support is PSF. The support can be either flat sheet or hollow fibre support.

Most of these supports have pore size between 0.0001 and 1 μm or expressed more commonly in Daltons and MWCO (Molecular Weight Cut Off): reverse osmosis RO (1-100 Daltons), nanofiltration NF (200-400 Daltons), ultrafiltration UF (1000-200000 Daltons) and microfiltration MF (0.1 to 10 μm).

In some embodiments of the invention, microporous support structures are employed. Such supports have much bigger pores sizes, e.g. from 0.10 to 10 μm making gas transport therethrough very rapid. The pore size of these supports is not generally expressed in MWCO terms and microporous supports are considered to have MWCO values of greater than 100,000.

Microporous supports can be formed from any suitable material including those mentioned above in connection with ultrafiltration membranes and inorganic materials such as ceramics (alumina, zirconium oxide), glass membranes such as silica and the like. These supports can be prepared by sintering, sol gel or leaching techniques known in the art.

The supports providing mechanical strength to the gas separation membrane may as well be a high gas permeable dense support. Examples of suitable materials are: PDMS (polydimethylsiloxane), PTMSP (poly(l-trimethylsilyl-1-propyne), PMP (polymethylpentene) and amorphous fluoroplastics, such as 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene such as AF2400 or AF1600 (Teflon).

The gas separation membrane may further comprise a protective layer made of high gas and water vapour permeability material coated on top of the CO2selective polymer layer.

The CO2selective polymer layer may have a multilayer structure formed by at least two layers of CA enzyme modified polymer layers, seeFIG.4. Alternatively, the layers may be selected from CA-enzyme—and amine—containing polymer layers; seeFIG.5.

The gas separation membrane according to the invention enables a one-step gas separation process. There is only one phase (gas-gas), and no liquid present for absorption. This result in reduced complexity, reduced amount of enzyme, reduced size and weight of installation, and easy scalability compared with gas separation systems using enzymes such as Liquid Membranes (SLM) and membrane contactors (MC).

The gas separation membrane according to the invention is to be used for separation of CO2from a gas mixture. Examples of gas mixtures are flue gas, natural gas, biogas, CO2and H2; fermentation gases and anaesthetic gases. The membranes are especially useful for separating CO2from blood, or from water in aquaculture and pisciculture and other industrial applications which needs water or a liquid degassing.

Especially preferred are gas mixtures containing a low concentration of CO2, such as mixtures with less than 5% by volume of CO2.

In a method of separating CO2from a gas mixture, the gas mixture is being supplied along a feed side of a gas separation membrane according to the invention. At least part of the gas mixture will diffuse through the membrane due to an applied partial pressure difference, i.e. high pressure at the feed side and low pressure (atmospheric) on the permeate side. CO2molecules will be selectively transported from the feed side to a permeate side by enzyme reaction (facilitated transport) and solution-diffusion. CO2molecules will be continuously removed from the permeate side to maintain the partial pressure difference.

Applications of the gas separation membrane of the present invention include separation of CO2from gas mixtures including CO2with various components such as nitrogen, methane, carbon monoxide (CO), oxygen, volatile organic compounds or hydrogen. Separation of mixtures involving hydrogen is also envisaged. These gases can occur in any circumstance such as in industrial and domestic gas streams. In use, the gas mixture to be separated will typically flow across the membrane under pressure. The temperatures employed may vary, typically temperatures are in the range of 0 to 90° C., preferably at 20 to 65° C.

Preferably, the membrane is used to separate carbon dioxide from nitrogen and/or methane. In this latter regard, the membranes of the invention may therefore have applications in the field where these gases are present in mixtures such as flue gas, natural gas, biogas, air, fermentation processes and anaesthetic gases.

The gas separation membrane of the present invention is especially useful at low concentrations of CO2, i.e. less than 40%. In aquaculture CO2dissolved in water (60 ppm), from 400 ppm in clean air, 1% various industries such as aluminium industry, 4-5% breath, flue gas 3.5% to 15% depending on the fuel burned and biogas and natural gas which can contain from 10% to 40% CO2.

EXPERIMENTS

Carbonic Anhydrase Production and Purification

The carbonic anhydrase enzymes used in the experiments were produced recombinantly inEscherichia coliand most of the host proteins were then removed by heat precipitation at 65° C., followed by centrifugation as described in patent application WO2014/090327 A1. The strain used in the following examples was SCA 11 as referred in WO2014/090327 A1, but any other strain or other CA can be used.

Preparation of Flat Sheet Membranes

Composite membranes consisting of support, dense or porous, and a dense CO2separation layer were prepared by solution casting of the separation layer onto the support followed by drying at room temperature or at elevated temperature in an oven.

The porous supports used were either commercially available ultrafiltration polysulfone (PSF) 50 000 MWCO or fluoro membranes (PVDF) 10 000 MWCO.

The dense supports were prepared onto a Teflon dish by solution casting of PTMSP, PDMS in hexane or cyclohexane solvent and AF 2400 in FC-72 solvent followed by drying at room temperature or elevated temperature in an oven. Some commercial microporous (0.2 microns) PVDF membranes were also used as separate mechanical support underneath these membranes during gas permeation testing.

The dense CO2separation layer on top of the support was made of polyvinyl alcohol (PVA) or polydimethylsiloxane (PDMS) or polyacrylamide (PAA) containing CA enzymes introduced/fixed according to different methods described below:

Method 1: Chemical Coupling of CA Enzymes to CO2Separation Layer

CA enzymes were coupled to PVA membrane coated on a support by using a bifunctional linker (glutaraldehyde). One functional group reacts with hydroxyl groups and the second functional group reacts with residual amino groups of CA enzymes.

A sequential approach was used to reduce undesired side reactions:

1. The PVA membrane was prepared by coating on porous support.2. The PVA membrane was activated with a glutaraldehyde solution 10 mL 0.5 mmol/mL.3. The glutaraldehyde solution was removed and 10 mg enzyme was added in buffer solution pH=7.4.4. The multilayer membranes were prepared as following: The excess enzymes were removed, a new glutaraldehyde solution 10 mL, 0.5 mmol/mL was added for the activation followed by enzyme buffer solution addition.
Method 2: CA Enzyme Dispersed in Polymer Matrix of Separation Layer (PVA)

The CA enzymes become weakly bound to the polymer chains by hydrogen bonds when dispersed in polymer matrix. 2% aqueous solution of PVA was mixed with an enzyme solution. The concentration of CA enzymes was 52.9 mg/g PVA. The blend was coated on PSF 50000 MWCO support.

Method 3: Copolymerisation of Chemically Modified CA Enzymes and Acryl Based Monomers

CA enzymes were modified with vinyl groups and copolymerized with acrylamide in a buffer solution pH 7.4. Two different ratios of CA enzymes/Aam (acrylamide) were used; 40 mg CA enzymes/g Aam and 100 mg CA enzymes/g Aam. To introduce vinyl groups, the CA enzymes were treated with N-hydroxy succinimide acrylate (NSA). Two molar ratios NSA/CA enzymes were used; 8.89 and respective 5.56 (less vinyl groups). The resulting polymer solution was coated on porous and dense supports. Three different biopolymers were obtained: CA enzyme-PAA 0: NSA/CA enzyme ratio 8.89 and 40 mg CA/g Aam; CA enzyme-PAA 1: NSA/CA enzyme ratio 5.56 and 40 mg CA/g Aam, and CA enzyme-PAA 2: NSA/CA enzyme ratio 5.56 and 100 mg CA/g Aam.

When a dense support was used, it consisted of self-standing membranes of PTMSP, PDMS and AF2400 and following methods were also used:

Method 4: UV Grafting with Glycidyl Methacrylate (GMA) on Dense Support Followed by CA Enzyme Coupling.

The membrane surface was modified by UV grafting using a sequential approach. The sequential approach has the advantage that it reduces undesired side reactions.1. To create the grafting points where the monomer grafting will start, the membrane was soaked in an initiator (benzophenone) 1% solution in methanol and exposed to UV radiation.2. The initiator solution was removed and the membrane was washed gently with methanol.3. The membrane was then soaked in 10% monomer solution (glycidyl methacrylate (GMA) solution) and exposed to UV radiation to promote the polymerization.4. The monomer solution was then removed and the membrane was washed several times with water to remove the unreacted monomer as well as the polymer unbound to the membrane surface.5. The enzymes were coupled with the polyglycidyl methacrylate via epoxy groups.
Method 5: UV Grafting with AEMA Followed by Coating of the Biopolymer Solution Obtained According to Method 3

The membranes were prepared using the sequential described at Method 4 using aminoethyl methacrylate (AEMA) instead of GMA monomer. The grafted membranes were then coated with the biopolymer solution prepared by the Method 3.

Method 6: UV Grafting with AEMA Followed by Coupling of CA Enzymes by Activation with Glutaraldehyde.

The membranes were prepared using the sequential described at Method 4 using AEMA monomer instead of GMA monomer.

The grafted membranes were then coupled with CA enzymes using glutaraldehyde (GA) as linker.

Mixed Gas Permeation Testing

Gas separation properties such as CO2permeance or CO2permeability (permeance/membrane thickness) and CO2/N2selectivity (ratio of CO2and N2permeances) were measured for the prepared membranes by using gas mixtures of CO2and N2fully humidified, similar to real gas compositions: flue gas (5 to 15% CO2), breathing (5% CO2) etc. The experiments were conducted at 25° C. and feed pressure was between 1.2 bar and 5 bar absolute pressure. A sweep gas, helium, was used in the permeate side of the membrane as mean of creating a driving force. The feed gas had 5% or 15% CO2content. The permeate flow and its composition was measured continuously by flow meter and a gas chromatograph and used to calculate CO2permeance and CO2/N2selectivity.

Results

Example 1

PVA on 50 000 MWCO PSF support modified with glutaraldehyde and CA enzymes (600 μl of 17.6 mg/ml CA enzyme solution) according to Method 1.

The effect of CA enzyme addition to membrane performances at 1.2 bar, 25° C., and 15% CO2in N2humidified feed gas is shown in Table 1.

TABLE 1CO2permeanceCO2/N2Membrane/modification(m3(STP)/(m2bar h)selectivityPVA/PSF reference0.2041PVA/PSF + glutaraldehyde +0.0657CA enzymesPVA/PSF 2 + glutaraldehyde +0.1154CA enzymes

Example 2

CA enzymes were dispersed in PVA polymer solution and supported on 50 000 MWCO PSF according to Method 2. The resulting membranes were tested with 15% CO2in N2fully humidified at 25° C.

The effect of CA enzymes dispersed in PVA on membrane performance at 1.2 bar, 25° C., and 15% CO2in N2humidified feed gas is shown in Table 2.

TABLE 2CO2permeance (m3CO2/N2Membrane/modification(STP)/(m2bar h)selectivityPVA/PSF reference0.2041PVA + CA enzyme dispersed/PSF0.1358

The CO2permeance and CO2/N2selectivity variation with increasing pressure for PVA/PSF and PVA/PSF modified with CA enzymes are shown in the plot diagrams ofFIGS.6and7. The plot diagram (FIG.6) shows that the CO2permeance decreases for the PVA/PSF with CA enzymes. The plot diagram (FIG.7) shows that CO2/N2selectivity increases for the PVA/PSF with CA enzymes. Increasing feed pressure, the results are relatively constant for CO2/N2selectivity and slightly decreasing for CO2permeance.

Example 3

Copolymerisation of chemically modified CA enzymes and acryl based monomer according to Method 3.

The results obtained using CA enzyme-PAA biopolymer on various porous supports at 1.2 bar, 25° C., and using 5% CO2in N2humidified feed gas are shown in Table 3 andFIG.8.

TABLE 3CO2permeanceCO2/N2Membrane/modification(m3(STP)/(m2bar h)selectivityPAA/PSF reference0.117CA enzyme-PAA 1/PSF support0.0873CA enzyme-PAA 2/PSF support0.1474CA enzyme-PAA 1/PVDF support0.1173CA enzyme-PAA1/washed PSF support0.4243

All the membranes prepared using CA enzymes-PAA 1 and CA enzymes-PAA 2, both on porous and dense supports showed a substantial increase in CO2/N2selectivity compared with references.

FIG.8shows a plot diagram summarizing the results obtained using the membranes on porous supports, PSF and PVDF. The membranes according to present invention have increased CO2/N2selectivity 6 to 11-fold compared with the reference. The CA enzymes-PAA 2 membrane also showed higher CO2permeance than the reference membrane.

FIG.9shows a plot diagram summarizing the results of the CA-PAA biopolymer membranes on dense supports, PTMSP and PDMS at 1.2 bar, 25° C., and using 5% CO2in N2humidified feed gas. Similar to porous supports a substantial increase of CO2/N2selectivity is observed due to the presence of CA enzymes. The additional layer of CA-PAA biopolymer decreases the CO2permeability compared to reference membranes, but still presenting high CO2permeabilities values, i.e. above 1000 Barrer.

The results obtained using CA enzyme-PAA biopolymer on dense supports at 1.2 bar, 25° C., and using 5% CO2in N2humidified feed gas are shown in Table 4 for PTMSP supports and in Table 5 for PDMS supports.

TABLE 4CO2permeabilityCO2/N2Membrane/modification(Barrer)selectivityPTMSP reference182506CA enzyme-PAA 1/PTMSP107060CA enzyme-PAA 2/PTMSP repeat191740

TABLE 5CO2permeabilityCO2/N2Membrane/modification(Barrer)selectivityPDMS reference178815CA enzyme-PAA 1/PDMS122334

Example 4 UV Grafting with AEMA) Followed by Coating of the Biopolymer (PAA-CA Enzyme) Solution According to Method 5

Membrane CO2permeability and CO2/N2selectivity at 1.2 bar, 25° C., 5% CO2in N2fully humidified is shown in Table 6:

TABLE 6CO2permeabilityCO2/N2Membrane/modification(Barrer)selectivityPTMSP reference182506PTMSP UV grafted with AEMA1365011CA enzyme-PAA 1/PTMSP9142grafted with AEMACA enzyme-PAA 2/PTMSP18324grafted with AEMA

The CA enzyme-PAA 2 membrane has 2.5 times less CA enzymes per milligram of polyacrylamide (PAA) compared to the CA enzyme-PAA1 membrane.

For the UV grafting procedure 10% in water aminoethyl methacrylate (AEMA) was used.

The results are expressed in permeability (Barrer) instead permeance (m3(STP)/(bar m2h) in order to compensate for the variation of the relatively thick dense support (25 to 50 microns). For conversion reasons 1000 Barrer represent a permeance of 2.7 m3(STP)/(bar m2h) for 1 μm thick membrane.

The results show a big decrease of CO2permeability, but show 4 and respectively 7 times CO2/N2selectivity increase compared to PTMSP due to the introduction of CA enzymes.

Example 5: UV Grafting with AEMA Followed by Coupling of CA Enzymes by Activation with Glutaraldehyde According to Method 6

Membrane CO2permeability and CO2/N2selectivity at 1.2 bar, 25° C., 5% CO2in N2fully humidified are shown in Table 7.

TABLE 7CO2permeabilityCO2/N2Membrane/modification(Barrer)selectivityPTMSP reference182506PTMPS UV grafted med AEMA1365011Glutaraldehyde + CA enzymes/21025PTMSP UV grafted with AEMA

For the UV grafting procedure 10% in water aminoethyl methacrylate (AEMA) was used.

The results are expressed on permeability (Barrer) instead of permeance (m3(STP)/(bar m2h) in order to compensate for the variation of the relatively thick dense support (25 to 50 microns). For conversion reasons 1000 Barrer represent a permeance of 2.7 m3(STP)/(bar m2h) for 1 μm thick membrane.

The results show a big decrease on CO2permeability, but show 4 times increase of CO2/N2selectivity compared to PTMSP due to the introduction of CA enzymes.

Test of Membrane Durability

One membrane, PAA-CA Enzyme 1/PTMSP prepared according to procedure in Method 3 was selected and the membrane exposed for over 350 hours to the following test conditions: 5% CO2, 85% N2, 10% O2, 300 ppm SO2; 1.2 bar pressure, 25° C., and humid gases. The composition is very typical for flue gases from power plants. Both CO2flux (permeance) and CO2/N2selectivity remained relatively constant in time showing the potential applicability for CO2capture from flue gases.

The results are presented inFIG.10.