MEMBRANE GAS SEPARATION UNDER MAGNETIC FIELD

Abstract

Apparatus, methods and uses for separating gas mixtures of at least two different gases under magnetic field and employing a membrane are described.

Claims

1. Apparatus for the separation of a gas mixture of at least two different gases, wherein the apparatus comprises a first chamber and a second chamber, wherein the first chamber has at least one inlet for feeding the gas mixture into the first chamber, wherein the first chamber has at least one outlet for retrieving at least part of one of the at least two gases, wherein the second chamber has at least one outlet for retrieving at least part of another one of the at least two gases, wherein the first chamber and the second chamber are in contact and separated by a membrane, wherein the membrane is surrounded by a magnet.

2. The apparatus of claim 1, wherein in a particular embodiment the membrane is made of hollow fibers.

3. The apparatus of claim 2, wherein the hollow fibers have a basically circular wall and a void inside the basically circular wall.

4. The apparatus of claim 3, wherein the basically circular wall has a wall thickness as determined by Scanning Electron Microscopy (SEM) in the range of 130 to 160 m.

5. The apparatus of claim 1, wherein the polyimide-based membrane is made of a commercial polyimide.

6. The apparatus of claim 1, wherein the membrane is a polymer-based membrane which has been carbonized.

7. The apparatus of claim 1, wherein the magnet is a permanent magnet.

8. The apparatus of claim 1, wherein the magnet is an electromagnet.

9. The apparatus of claim 1, wherein the magnet generates a magnetic field of 40 mT or more.

10. A method for separating a gas mixture of at least two gases which is carried out in an apparatus according to claim 1.

11. The method of claim 10, wherein the gas mixture of at least two gases is selected from the following mixtures of two gases: H.sub.2/CH.sub.4, He/CH.sub.4, H.sub.2/CO.sub.2, He/CO.sub.2, CO.sub.2/N.sub.2, CO.sub.2/CH.sub.4, and He/O.sub.2.

12. A method for separating a gas mixture of at least two gases, wherein the gas mixture is fed into a first chamber of a gas-separating apparatus via a first inlet, wherein at least part of one of the at least two gases is retrieved from the first chamber via at least one outlet of the first chamber, wherein the first chamber and a second chamber of the gas-separating apparatus are in contact and are separated by a membrane, wherein at least part of another one of the at least two gases is retrieved from the second chamber via at least one outlet of the second chamber, wherein the polyimide-based membrane is surrounded by a magnet, whereby an external magnetic field is applied to the membrane.

13. The method of claim 12, wherein the gas mixture of at least two gases is selected from the following mixtures of two gases: H.sub.2/CH.sub.4, He/CH.sub.4, H.sub.2/CO.sub.2, He/CO.sub.2, CO.sub.2/N.sub.2, CO.sub.2/CH.sub.4, and He/O.sub.2.

14. Use of a membrane which is surrounded by a magnet such that an external magnetic field can be applied to the membrane for separating a gas mixture of at least two gases.

15. The use of claim 14, wherein the gas mixture of at least two gases is selected from the following mixtures of two gases: H.sub.2/CH.sub.4, He/CH.sub.4, H.sub.2/CO.sub.2, He/CO.sub.2, CO.sub.2/N.sub.2, CO.sub.2/CH.sub.4, and He/O.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be described with reference to the accompanying drawings of which:

[0034] FIG. 1 is a cross sectional representation of the membrane cell used for the gas permeance experiments under application of external magnetic field.

[0035] FIG. 2 shows an apparatus used in the present invention for permeance measurements.

[0036] FIG. 3 is a cross sectional view of the hollow fiber membranes used in the present invention.

[0037] FIG. 4 shows a hollow fiber spinning setup for a dry/wet phase inversion process.

DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention inter alia provides an apparatus for the separation of a gas mixture of at least two different gases,

[0039] wherein the apparatus comprises a first chamber and a second chamber,

[0040] wherein the first chamber has at least one inlet for feeding the gas mixture into the first chamber,

[0041] wherein the first chamber has at least one outlet for retrieving at least part of one of the at least two gases,

[0042] wherein the second chamber has at least one outlet for retrieving at least part of another one of the at least two gases,

[0043] wherein the first chamber and the second chamber are in contact and separated by a membrane,

[0044] wherein the membrane is surrounded by a magnet. The magnet is especially such that an external magnetic field can be or is applied to the membrane.

[0045] This apparatus allows for a reliable and effective separation of gas mixtures, in particular of mixtures of at least two different gases. The used membrane improves the selectivity of the separation and at the same time overcomes disadvantages previously often associated with membranes in gas separation, especially insufficient membrane performance which is regularly caused by gas flow direction through membrane gaps and not pores, pore orientation, membrane thickness, uniform pore morphology and functionality. Furthermore, using the apparatus according to the present invention allows to reduce the energy required for the desired gas separation.

[0046] In the context of the present invention a membrane is made from a material which comprises an imide polymer, i.e. a polyimide. The material or material blend may comprise additional materials, especially further polymers, such as polyaniline. Moreover, the material may be further treated, especially carbonized, to yield carbon fibers. Furthermore, the material may be any type of membrane suitable for mixture separation, including but not limited to organic, inorganic, polymeric, ceramic, mixed matrix, composite, and hybrid membranes.

[0047] It has been found that particularly good separation results are achievable when the membrane is made of hollow fibers. In a particular embodiment within this context, an advantageous hollow fiber has a basically circular wall and a void inside the basically circular wall. Basically circular means that the wall will not necessarily have a circular cross-section which is circular in a mathematically strict sense, but one which is considered circular in the concerned technical field, as illustrated for example by FIG. 3. In order to achieve a good separation result, the basically circular wall has preferably a wall thickness as determined by Scanning Electron Microscopy (SEM) in the range of 130 to 160 m. Especially for the concerned gas mixtures to be separated, it is further beneficial that the polyimide-based membrane is made of a commercial polyimide, and/or that the polyimide-based membrane has been carbonized to yield carbon membrane.

[0048] In order to achieve a reliable, continuously stable and homogenous magnetic field, it is preferred that the magnet used in this invention is a permanent magnet. For an enhanced flexibility and the possibility to easily alter the magnetic field strength, it is advantageous to employ an electromagnet. In either case, the magnet is preferably a cylindrical magnet which allows a geometrically homogenous construction of the separation apparatus. It has furthermore proven advantageous that the magnet generates a magnetic field of 40 mT (400 Gauss) or more in order to achieve a desired ratio of separation of the gases contained in the treated gas mixture.

[0049] An advantageous separation method for separating a gas mixture of at least to gases into its components according to the invention is a method for separating a gas mixture of at least two gases which is carried out in an apparatus as outlined herein. A separation method as such as provided by the invention can be a method for separating a gas mixture of at least two gases, wherein the gas mixture is fed into a first chamber of a gas-separating apparatus via a first inlet, wherein at least part of one of the at least two gases is retrieved from the first chamber via at least one outlet of the first chamber, wherein the first chamber and a second chamber of the gas-separating apparatus are in contact and are separated by a polyimide-based membrane, wherein at least part of another one of the at least two gases is retrieved from the second chamber via at least one outlet of the second chamber, wherein the membrane is surrounded by a magnet, whereby an external magnetic field is applied to the membrane. Similarly useful is a use according to the invention which is a use of a polyimide-based membrane which is surrounded by a magnet such that an external magnetic field can be or is applied to the membrane for separating a gas mixture of at least two gases. All these methods and uses allow for a reliable and effective separation of gas mixtures, in particular of mixtures of at least two different gases. The aforementioned methods and uses improve the selectivity of the separation and at the same time overcome disadvantages previously often associated with membranes in gas separation, especially insufficient membrane performance which is regularly caused by gas flow direction through membrane gaps and not pores, pore orientation, membrane thickness, uniform pore morphology and functionality. Furthermore, the aforementioned methods and uses of the invention allow to reduce the energy required for the desired gas separation.

[0050] It has been established that particularly good separation results for the aforementioned methods and uses of the invention can be achieved when the separated gas mixture is selected from one of H.sub.2/CH.sub.4, He/CH.sub.4, Hz/CO.sub.2, CO.sub.2/N.sub.2, He/CO.sub.2, CO.sub.2/CH.sub.4, and He/O.sub.2.

[0051] In general, the invention teaches a method of enhancing the performance of gas separation membranes for gas mixtures (specifically separation of He, Hz, CO.sub.2, O.sub.2, N.sub.2 and CH.sub.4. gas mixtures) and an apparatus for carrying out the enhanced gas separation technique, which utilizes the addition of an external magnetic field surrounding a gas separation apparatus.

[0052] The invention targets the enhancement of membrane performance through application of external magnetic field.

[0053] The targeted enhancements are: changes in permeability and selectivity performance of the membranes due to externally applied magnetic field, enhancement of membrane performance to overcome present drawbacks of membrane performance, caused by gas flow direction through membrane gaps and not pores, pore orientation, membrane thickness, uniform pore morphology and functionality (which currently limit the scale up of membranes).

[0054] The present invention has the potential to overcome barriers in current membrane technology.

[0055] Traditional separation techniques are highly energy consuming and lack efficiency. For example liquid amine absorption used to separate CO.sub.2 or H.sub.2S from gas mixtures suffers from slow kinetics due to diffusion limitations in bulk amine solutions, slippage of the liquid to the atmosphere, foaming etc., while it entails a highly energy consuming regeneration step. Employment of solid adsorbents or membranes that would separate mixtures based on preferential adsorption and/or sieving of molecules with minute size and shape differences, could lead to energy-efficient, high-resolution molecular separation processes (R. W. Baker, Membrane Technology and Applications, 2nd Edition, John Wiley & Sons, Ltd, 2004, England). Two major mechanisms for membrane separation dominate, namely, molecular sieving, where the size of membrane pores restrict the passage of molecules with certain size and shape while allowing others having smaller dimensions, and sorption separation, where pore functionalities interact with certain molecules that exhibit higher affinity than others thus inducing selectivity (K. Varoon, X. Zhang, B. Elyassi, D. D. Brewer, M. Gettel, S. Kumar, J. A. Lee, S. Maheshwari, A. Mittal, C-Y. Sung, M. Cococcioni, L. F. Francis, A. V. McCormick, K. A. Mkhoyan, M. Tsapatsis, Synthesis of self-pillared zeolite nanosheets by repetitive branching, Science 334 (2011) 72-75). However, intensive efforts are being dedicated to overcome challenges associated with fabrication and operation of membranes, the function of which is based on the aforementioned mechanisms. Specifically, of particular importance is the fabrication of defect-free membranes so that flow is directed exclusively through the pores of the membrane and not through gaps that can sacrifice selectivity, a challenge that becomes even more dominant when scaling up is attempted towards fabrication of large membrane areas as required by industrial applications. Other issues such as pore orientation, membrane thickness, uniform pore morphology and functionality also limit the use of membranes at industrial scale.

[0056] The present inventors found solid evidence that magnetic field can enhance the performance of membranes for separation of gas mixtures. The concept can have great impact on the field of mixture separations at industrial scale. The inventors tested the concept by studying various gases passing through a membrane, and the inventors found that the permeance and selectivity performance of the membrane changed after applying external magnetic field depending on the type of gas molecules that are being separated. The inventors also found out that additional tuning of the membrane performance can be achieved after loading the membrane with magnetic particles. These particles can act alone or in conjunction to external magnetic field, the latter having the ability to alter the magnetic properties of the particles, which in turn alter the transport properties of the gas molecules of the mixture to be separated. This is attributed to the magnetic susceptibility of the molecules, and the interaction of the molecules and the particles with the applied magnetic field. These data are available and are described below.

[0057] The repulsive or attractive forces induced by magnetic field could act selectively on mixtures of gases, e.g. consisting of oxygen, nitrogen, or carbon dioxide that exhibit high magnetic susceptibility, thus providing an additional mode of tuning the performance of microporous membranes. The concept is tested herein using polymeric hollow fibers (HFs), which were produced from a commercial co-polyimide precursor via phase inversion, and carbon HFs prepared by carbonization of the polymeric ones. Separation performance of the above membranes was evaluated and compared to performance under the presence of an external magnet surrounding the membrane cell for a variety of gases under the effect of magnetic field. Permeation through the membranes of a series of gas molecules was carried out, namely He, Hz, CO.sub.2, O.sub.2, N.sub.2 and CH.sub.4. The results showed that the introduced magnetic field modified the gas separation properties of the membranes. The highest enhancement of permselectivity values were obtained for the carbon HFs under magnetic field in the cases of Hz/CH.sub.4 and He/CH.sub.4.

[0058] Moreover, operation of the same membranes under magnetic field resulted in a 7-fold increase in CO.sub.2/CH.sub.4 selectivity. The membranes that were used as a case study were prepared according to previous reports involving some of the inventors (E. K. Chatzidaki, E. P. Favvas, S. K. Papageorgiou, N. K. Kanellopoulos, N. V. Theophilou, New PolyimidePolyaniline Hollow Fibers: Synthesis, Characterization and behavior in Gas Separation, Europ. Polym. J. 43 (2007) 5010-5016, and E. P. Favvas, E. P. Kouvelos, G. E. Romanos, G. I. Pilatos, A. Ch. Mitropoulos, N. K. Kanellopoulos, Characterization of highly selective microporous carbon hollow fiber membranes prepared from a commercial co-polyimide precursor, J. of Porous Materials 15, 2008, 625-633). More specifically, the preparation route described below was followed

[0059] The precursor was an asymmetric hollow fiber prepared from commercial P84 co-polyimide. P84 (BTDA-TDI/MDI) was obtained from Lenzing and is a thermally stable co-polyimide of 3,34,4-benzophenone tetracarboxylic dianhydride and 80% methylphenylene-diamine+20% methylene diamine Its chemical structure is shown following (in Formula 1), while the calculated density is about 1.30 g/cm3. The glass transition temperature (Tg) of this polymer is 588 K as determined by differential scanning calorimetry (heating rate of 10 K/min) under dry nitrogen environment.

##STR00001##

[0060] The polymer was dried overnight at 393 K under vacuum prior to be used for the production of the hollow fibers. The solvent applied was N-methyl-pyrolidone (NMP). The P84 polymeric hollow fiber precursors were prepared by the dry/wet phase inversion process in a spinning set-up. The spinning dope, consisting only of P84 polymer and NMP as solvent, was mixed overnight at 323 K in a stainless-steel vessel of 3 liters in order to prepare a homogeneous solution. In a second step the solution was filtered through a 15 m metal filter to remove impurities existing in the raw polymers. Both vessels as well as the spinneret were thermostated at 323 K in order to facilitate the flow of the polymer solution. After filtering, the dopes were allowed to degas inside a second stainless steel vessel for 2 days. The bore liquid was a degassed mixture of NMP and deionized water. The polymer solution (28.5% w/w P84/NMP) and bore fluid (70% w/v NMP/H.sub.2O) were simultaneously pumped through a tube-in-orifice spinneret using gear pumps. The i.d. of the spinneret was 500 m and the o.d. 700 m. The extruded fibers passed first through a 6 centimeters air gap before entering to the coagulation bath, which was filled with tap water at room temperature. The nascent fibers were oriented by means of two guiding wheels and pulled by a third wheel into a collecting reservoir. In order to remove residual NMP, the produced fibers were washed with tap water overnight and then solvent exchanged in plastic containers with ethanol for 6 h.

[0061] Permeation measurements of various gases were performed using the variable pressure method in a high-pressure (70 bar) stainless steel permeation rig, the characteristics and operation of which are described in S. F. Nitodas, E. P. Favvas, G. E. Romanos, M. A. Papadopoulou, A. Ch. Mitropoulos, N. K. Kanellopoulos, Synthesis and Characterization of Hydrogen Selective Silica-based Membranes, J. Porous Mater. 15 (2008) 551-557. More specifically, the measurements were performed following the protocol below.

[0062] The separation of various gas mixtures gases was determined for the used membranes at a temperature of 373 K and an absolute pressure of 2 bar. The permeance measurements were conducted in a homemade apparatus (laboratory MESL, Institute of Nanoscience and Nanotechnology, NCSR Demokritos), which is depicted in FIG. 2. The device can operate under high vacuum conditions (10.sup.3 Torr) and its feed pressure can reach 70 bar. The permeance apparatus consists of the following parts:

1. the high pressure area;
2. the low pressure area; and
3. the membrane area.

[0063] A bottle of high volume (5 L) is placed in the high pressure area in order to maintain the experimental conditions at the set-point value, i.e. in the case of small pressure drop. In the low-pressure area, there is the capability of volume selection that ensures the measurement of the pressure increase, independently of the permeance value. The apparatus is immersed in oil bath, which is maintained at constant temperature (0.05 K) with the use of a PID thermostat (controller)-stirrer (Julabo E07). For the minimization of the temperature fluctuations, the area above the oil bath is thermally insulated and its temperature is regulated (0.1 K) by an air circulator and a thermostat PD (CENON TSL 100 with supply-thermal protection unit CENON DP800). Concurrently, the room temperature is kept stable (2 K) with the help of an air conditioner. The membranes were placed in the permeance apparatus and degassed at the maximum operating temperature (373 K).

[0064] The experiments were performed by continuously monitoring the pressure increase in the low-pressure side of the rig by means of a differential pressure transducer. These kinds of experiments, under the effect of external magnetic field, were performed by fixing a cylindrical permanent magnet around the permeation cell, as illustrated in FIG. 1, which is part of the overall apparatus used in the present invention for permeance measurements as shown in FIG. 2. More specifically, FIG. 1 is a cross sectional representation of the membrane cell used for the gas permeance experiments under application of external magnetic field, wherein 1 represents high pressure, 2 Represents low pressure, and 3 represents a hollow fiber membrane.

[0065] Permeability, K, (Barrer) and permeance, Pe (GPU), coefficients were calculated for membranes with accurate thickness of the separating layer and for membranes where this separating layer is not known with accuracy, respectively. The permeance measurements were conducted at room temperature for the carbon hollow fiber membranes (CHFMs). In addition, the effect of external magnetic field in both membranes was examined using a 400-Gauss cylindrical permanent magnet placed around the permeation cell containing the membrane. Decrease of permeance follows the increase of the kinetic diameter of the studied gas. This behavior indicates that the main separation mechanism is molecular sieving.

[0066] Table 1 shows the permeance results related to the carbon hollow fibers. For all cases, the studied membranes give molecular sieving characteristics with high selectivity coefficients. N.sub.2 and CH.sub.4, having the largest kinetic diameter among all gases tested, present very low permeance so as to be practically considered impermeable, confirming the molecular sieving mechanism. Experiments on the CHFMs revealed that application of external magnetic field affects the permeation behavior. Indeed, permeance was enhanced with application of the external magnet for all gases tested. The effect was stronger for O.sub.2, where permeance almost doubled after exposure to the magnetic field.

TABLE-US-00001 TABLE 1 Room temperature permeance values for carbon hollow fiber membranes (CHF) and CHF under magnetic field. Permeance (GPU) Kinetic CHFM Gas Diameter () CHFM under magnetic field He 2.6 1.41 1.83 H.sub.2 2.89 1.29 1.58 CO.sub.2 3.3 0.012 0.0174 O.sub.2 3.46 0.0072 0.0133 N.sub.2 3.64 0.0009 0.0001 CH.sub.4 3.8 0.0009 0.0002

[0067] Table 2 illustrates the ideal selectivities (permselectivities calculated as the ratio of the permeances) in relation to the Knudsen factors for six pairs of gases, H.sub.2/CH.sub.4, He/CH.sub.4, H.sub.2/CO.sub.2, He/CO.sub.2, CO.sub.2/CH.sub.4, and He/O.sub.2.

[0068] For the cases of CH.sub.4 and N.sub.2 in particular, the gases with the smaller permeance values (in the order of 10.sup.4 GPU), the corresponding ideal selectivities are very high, confirming the molecular sieving diffusion mechanism. The drastically higher He/CH.sub.4 and H.sub.2/CH.sub.4 separation factors compared to the He/CO.sub.2 and H.sub.2/CO.sub.2 ones indicate that the mean pore size of the carbon matrix should be close to the kinetic diameter of methane (3.8 or 0.38 nm). The significant deviation of the observed selectivity values compared to respective Knudsen selectivities further confirms the ultra-microporous structure of the studied membranes, in analogy to permselectivity properties reported for zeolite membranes.

[0069] In the case of CO.sub.2/CH.sub.4 gas pair, the presence of the magnetic field increases the selectivity coefficient. A 7-fold increase is observed in particular for the CHFM fibers. This is the combined result of the enhancing effect of the magnetic field on the CO.sub.2 permeance and the depressing action on the CH.sub.4 permeance, as indicated by the respective permeance values reported in Table 1. In contrary, for the He/CO.sub.2 and H.sub.2/CO.sub.2 pairs, the presence of the magnetic field results in reduction of the separation factors. The anisotropy in membrane performance induced by the effect of the external magnetic field action is better demonstrated by the He/O.sub.2 pair, where selectivity of the CHF membranes is suppressed by applying the magnetic field.

TABLE-US-00002 TABLE 2 Room temperature ideal selectivities of the studied membranes compared to the respective Knudsen selectivity values. T = 298 K H.sub.2/CH.sub.4 He/CH.sub.4 He/CO.sub.2 H.sub.2/CO.sub.2 CO.sub.2/CH.sub.4 He/O.sub.2 Knudsen 2.83 2.83 3.74 3.74 0.60 2.00 selectivity CHFM 1436.11 1569.89 120.76 110.47 13 195 CHFM 7897.50 9159.50 105.28 90.78 87 137 (under magnetic field)

[0070] The present invention is of a great interest since it has the potential to overcome challenges in gas separation of current membrane or adsorption techniques, towards practical application in development of energy efficient separation processes applicable at industrial scale. It can also address one of the major challenges existing in membrane gas separation: how to overcome limitations of the Robeson upper bound. Specifically, of particular importance is the fabrication of defect-free membranes so that flow is directed exclusively through the pores of the membrane and not through gaps that can sacrifice selectivity, a challenge that becomes even more dominant when scaling up is attempted towards fabrication of large membrane areas as required by industrial applications. Other issues such as pore orientation, membrane thickness, uniform pore morphology and functionality also limit the use of membranes at industrial scale. The present invention has the potential to enhance performance of membranes even if the aforementioned hurdles cannot be overcome.

[0071] All industries treating or utilizing gases/gas mixtures can benefit from the technology, mainly the gas processing industry. Examples of application include H.sub.2S removal, H.sub.2 purification, CO.sub.2 capture, HC separation, O.sub.2 purification, CO.sub.2 recycle for EOR, etc.

[0072] The foregoing disclosure and showings made in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.