JP2914972B2 - Carbon film and method for producing the same - Google Patents

Description

The present invention relates to the production of carbon membranes that can be used for gas separation. The membrane has a very sharp cutoff of the upper pore size, which can be on the order of 10% of the effective pore size, preferably 5%, for a given membrane. Accordingly, such membranes are suitable for the separation of various gas species that differ in size from one another up to about 10% of the molecular size. The method is one of the pretreatments of the membrane material, pyrolysis at a predetermined rate, followed by activation or treatment to change the pore size. The use of membranes for separation methods has been known for decades. Such separation methods were based on the selective permeation of different components through the membrane. In many methods, separation has been by permeation selection due to differences in the size of the various component species in the mixture. The first method was used for the separation of colloidal species and for the separation of other suspended small size particles from liquids. Among the first type of membranes, mention may be made of membranes having rather large and heterogeneous pore diameters (> 10 °). This was satisfactory for separating mixtures having very large size differences of the species to be separated. Later developed was a reverse osmosis membrane used for sweetening brackish water. In parallel, ion exchange membranes were used for separation methods based on electrodialysis. Among important developments are hollow fiber membranes, which produce very large overall membrane surface areas in small modules. This also offers the possibility of easy sealing of the ends of such modules. Subsequent developments relate to the invention of an asymmetric membrane with a thick, porous support that provided a very thin, dense and highly selective membrane, where thin membranes were used for practical separation methods, while coarse membranes were used. The support provides the required mechanical strength. Conventional methods for the separation of a gas (s) from a gas mixture are based on differences in the chemical or physical properties of various components. Chemical methods include cyclic methods that use certain chemicals to remove components. For example, removal of hydrogen sulfide from natural gas. This can be done in an iron sponge process using a reactor containing a bed of finely pulverized in a single run, leaving iron sulfide waste. Another method is the cyclic amine method, where organic ammonium salts are obtained during natural gas purification, which must be pyrolyzed to recycle the amine. Among the most common physical separation methods are those based on fractional distillation. The most common is the separation of nitrogen and oxygen from air. On a large scale (about 1000 t / day), the multi-stage cryogenic method is economical from an energy point of view. Other methods are more economical on a small scale. Adsorption methods based on the different adsorption properties of the various components of the gas mixture are also used. Such a method is advantageously performed as a multistage circulation method in a column arrangement. After use, the adsorbent must be regenerated for recycling, which can be done at low pressure or by heating. Various separation methods are based on the use of special zeolites or carbon molecular sieves. The present invention relates to novel carbon membranes for use in separating gaseous species from each other. The invention further relates to a method for separating gaseous species from each other whenever performed by such a method. The carbon membrane is made from a suitable precursor material that undergoes pyrolysis and has no pore-holes or cracks after pyrolysis. The permeability is due to the formation of an open pore system of molecular size, which pores have a sharp cutoff at a predetermined desired upper limit size. In this regard, molecules that are of a small size can pass through the carbon membrane, while gas molecules having a size larger than the pore size cannot pass through the membrane. Thus, the selectivity of the membrane is determined by providing essentially uniform pores whose size is between the size of the molecular gas species that must pass through and the size of the molecular gas species to be excluded. The term "hole" should not be interpreted as defining a fixed hole of a given size, but rather a hole system having a bottle neck along the path of the permeable gas, which determines permeability and selectivity. I do. A high degree of selectivity is obtained when the effective size of such a restriction is essentially uniform. The term "pore size" is used as the principle of action to indicate the cut-off value of the size of the molecule of any gas that passes through or is blocked from passing through such a membrane. The material of the membrane is carbon, which is a solid black material that does not melt up to about 3500 ° C. and is insoluble in organic solvents, it contains at least 70% by weight of carbon (element) and has a high content of fused aromatic rings. It appears to be a high molecular weight polymer. The term "film" refers to a thin layer or sheet of material. It can also be tube-shaped, the diameter and wall thickness of which define a dimension. The membrane of the present invention is about 2-500μ, preferably about 1-100μ,
More preferably, it is about 5 to about 30 microns thick. The tube-like membrane has a diameter of about 5 to 1000 microns and a wall thickness of 1 to about 30.
Has 0μ. Starting materials include a wide variety of non-melting materials containing carbon, such as regenerated cellulose, caprophane (Cu
prophan) Cellulose, cellulose, thermoset polymers and acrylics can also be used. The pyrolysis temperature varies over a wide range from 250 ° C to 1500 ° C, preferably from about 400 ° C to about 900 ° C. A predetermined heating rate is used, for example, from about 1 ° C / min to about 10 ° C / min. At high heating rates, pinholes can form and even microscopic cracks can form. This reduces the selectivity of the membrane and, in extreme cases, can render the membrane useless for its intended purpose. Pyrolysis causes a slow decomposition of the material used as the starting material, leaving the morphology and dimensions of the material essentially unchanged during pyrolysis. Since the gas species to be separated has an effective size on the order of a few Angstroms, it has a predetermined value, generally 2Å-
It is essential to produce a membrane with a sharp upper cut-off of the pore size in the range of 10 °, especially 2.5 ° to 6 °. In order to produce a carbon film having a desired pore size, the method of the present invention uses a sheet or tube-like film of an organic precursor in which the film shape is set, and maintains the shape and size given in the pyrolysis step. The precursor material must not undergo melting or softening during pyrolysis. The pyrolysis (carbonization) rate is chosen to be low enough to prevent crack or pinhole formation. The atmosphere in which the pyrolysis is performed is controlled to prevent unwanted burning of the membrane material that creates pinholes or holes larger than the allowable size. The pyrolysis is advantageously carried out under an inert atmosphere. Suitable media for such pyrolysis are inert gases such as argon, carbon dioxide and the like. The heating rate can vary over a wide range depending on the type of starting membrane material. It is generally at a rate of 1 ° C / min to about 10 ° C / min. Slower or faster heating rates may be used. The membrane material can be advantageously subjected to a pretreatment with certain chemicals to provide enhanced uniformity of the pore system formed during pyrolysis. Among these are ammonium salts such as ammonium chloride, chlorine, bromine, fluorine, flame retardant materials, hydrogen chloride,
Can include hydrogen bromide, oxygen and air. Permeability was measured by conventional means and the results were measured using a Barre
r) Shown in units. When the membranes have a certain size after pyrolysis, certain continuous treatments such as heating at elevated temperatures can increase the effective size of the pores and maintain their uniformity. Such expansion can be effected, for example, by subjecting the membrane to a stream of air at 400 ° C. for about 15 minutes, which can result in a pore size increase of about 3 ° to 6 °. The permeability of the formed membrane is measured for a given pair of gases, and the ratio of permeability provides the selectivity of the membrane for such gases. Following the thermal decomposition, the film formed at high temperature in oxygen or an oxygen-containing atmosphere is oxidized. Next, the formed film is exposed to a hydrogen atmosphere at a high temperature for a predetermined time. In certain experiments, the membrane material was sprayed with ammonium chloride, pyrolyzed in a carbon dioxide atmosphere at ambient pressure, oxidized in oxygen or oxygen atmosphere at high temperature, exposed to hydrogen at ambient temperature and high temperature, and exposed to hydrogen under hydrogen. Performed by cooling and exposing to air. Pyrolysis already starts at about 200 ° C. and under certain circumstances the pyrolysis step of the present invention can already be carried out at 150 ° C. or lower. After pyrolysis, it is advantageous to expose the membrane to air for an extended period of time, generally at ambient temperature and at a temperature of about 200 ° C. Advantageously, this is generally followed by a further step, in which the membrane is exposed to reduced pressure, generally at an elevated temperature in the range from 400 to 800 ° C. Instead of such a vacuum treatment, the membrane can be purged with an inert gas at a high temperature. After thermal decomposition, the film is heated for about 10 minutes in an oxidizing atmosphere containing oxygen.
When heating at 200-800 ° C. for about 16 hours, this results in an increase in pore size. The films formed have highly uniform pores and their upper limits do not differ from each other by more than about 5-10%. Membranes with an upper limit on pore size allow for separation of gas species having similar dimensions, of course, with a uniform upper limit on the pore size being between the sizes of the gas molecules to be separated. Heating at a temperature of 600 ° C. to 2000 ° C. in hydrogen for about 15 minutes to 12 hours reduces the pore size by sintering. The initial heating step-thermal decomposition-is performed in an inert atmosphere, for example, an inert gas, carbon dioxide or the like. Part of the thermal decomposition may be performed in an atmosphere of air or oxygen, and the thermal decomposition may be performed in an inert atmosphere or under reduced pressure. Further, the membrane may be subjected to a degassing step at a temperature in the range of about 200 ° C to about 1000 ° C under reduced pressure. Also, the film is kept for about 1-2 hours at a pressure of 1-1000 Torr for about 500-
It can be passivated by treatment with hydrogen at a temperature in the range of about 1000 ° C. The membrane can also be exposed to vapors of organic materials at a temperature of 450-1200 ° C. for about 15 minutes to about 4 hours, causing carbon deposition in the pores and modifying the permeability and selectivity of the membrane. Reference Example 1 A cellulose membrane was pyrolyzed in the presence of pure argon in an oven temperature programmed from ambient temperature to 800 ° C for 1 minute at a constant heating rate of 1 ° C. It was then exposed to air for at least 16 hours and then degassed at 800 ° C. for 0.5 hours.
A carbon molecular sieve membrane having a hydrogen permeability of 300 barrels and a methane permeability too low to be measured by existing equipment was obtained. That is, its permeability to methane was less than 0.25 barrels. This means that the selectivity of this membrane for hydrogen-methane mixture physics is greater than 300 / 0.25 = 1200. Reference Example 2 The experiment was performed as in Reference Example 1. However, the maximum pyrolysis temperature and degassing were performed at 600 ° C. The permeability of the formed carbon membrane to oxygen and nitrogen was 98 and 8.2 barrels, respectively. Thus, its selectivity for separating an oxygen / nitrogen mixture was 98 / 8.2 = 12. Reference Example 3 The experiment was performed as in Reference Example 1, except that the maximum pyrolysis and degassing temperature was 400 ° C. The permeability of the formed carbon membrane for oxygen and nitrogen is 56 and 56, respectively.
5.1 barrels. Therefore, the selectivity for separating the oxygen / nitrogen mixture was 56 / 5.1 = 11. Reference Example 4 A cellulose membrane was subjected to the following treatment. 1. Sprayed with ammonium chloride powder; 2. Pyrolyzed in the presence of carbon dioxide at ambient pressure in a temperature programmed open from ambient temperature to 620 ° C. for 1 minute at a constant heating rate of 1 ° C .; Exposure to hydrogen at ambient pressure at 620 ° C. for a time and cooling to room temperature in the presence of hydrogen. The permeability of the formed membrane to oxygen and nitrogen was 12.6 and 0.85 barrels, respectively. Thus, its selectivity for separating an oxygen / nitrogen mixture was 14.8. Reference Example 5 The film of Reference Example 4 was subjected to the following additional treatment. 4. Exposed to air at room temperature for at least 16 hours; 5. Exposed again to hydrogen at 620 ° C. for 30 minutes. The permeability of the formed carbon membrane to oxygen and nitrogen was 59 and 4.8 barrels, respectively. Therefore, its selectivity for this gas combination was 11.6. Example 1 A cellulose membrane was subjected to the following treatment. 1. Sprayed with ammonium chloride powder; 2. Pyrolyzed in the presence of carbon dioxide at ambient pressure in a temperature programmed open from ambient temperature to 620 ° C for 1 minute at a heating rate of 1 ° C; 3. 200 for 1 hour 4. Oxidation in oxygen at 0 ° C .; 4. Exposure to hydrogen at ambient pressure at 620 ° C. for 1 hour and cooling in the presence of hydrogen to room temperature; 5. Exposure to air at room temperature for at least 16 hours. The permeability of the formed membrane to oxygen and nitrogen was 49 and 4.9 barrels, respectively. Therefore, its selectivity for this gas combination was 10. Example 2 The membrane of Example 1 was further treated as follows: 6. Degassed at 200 ° C. for 1 hour. O ₂ and N _2, respectively permeability 115 and 16 are formed.
It was 7. The corresponding selectivity was 6.8. Example 3 A cellulose membrane was subjected to the following treatment. 1. Sprayed ammonium chloride powder; 2. Pyrolyzed from ambient temperature to 620 ° C. for 1 minute in a temperature programmed oven at a constant heating rate of 1 ° C. at ambient pressure in the presence of carbon dioxide; 3.3 Oxidation in oxygen at 180 ° C. for time; 4. Exposure to hydrogen at 620 ° C. for 1 hour at ambient pressure and cooling to room temperature in the presence of hydrogen; 5. Exposure to air at room temperature for at least 16 hours. Each formed O ₂ and N ₂ permeability 61 and 5.5
Barrel, with a corresponding selectivity of 11. Example 4 The membrane of Example 3 was further treated as follows: 6. Degassed at 600 ° C. The O ₂ and N ₂ permeability formed were 117 and 21 respectively.
It was a barrel. The corresponding selectivity was 5.6. Example 5 A cellulose membrane was subjected to the following treatment. 1. Sprayed with ammonium chloride powder; 2. Pyrolyzed in the presence of carbon dioxide at ambient pressure in an oven programmed from ambient temperature to 620 ° C. for 1 minute at a constant heating rate of 1 ° C .; 4. Oxidation in oxygen at 180 ° C. for time; 4. Expose to hydrogen at ambient pressure at 620 ° C. for 15 minutes, cool to room temperature in the presence of hydrogen, then pump off hydrogen; 5. Repeat steps 3 and 4 for 2 (The first time of step 3 was extended to 16 hours instead of 1 hour); 6. The membrane was exposed to air for at least 16 hours. The O ₂ and N ₂ permeability formed were 68 and 7 barrels, respectively. The corresponding selectivity was 9.6. Example 6 The membrane of Example 5 was further treated as follows: 7. Degassed at 200 ° C. for 1 hour. O ₂ and N ₂ permeability formed are 300 and 50 respectively
It was a barrel. The corresponding selectivity was 6. Example 7 A cellulose membrane was subjected to the following treatment. 1. Sprayed ammonium chloride powder; 2. Pyrolyzed in room temperature to 620 ° C. for 1 minute in a temperature programmed oven at a constant heating rate of 1 ° C. at ambient pressure in the presence of carbon dioxide; 3. 16 hours 4. Oxidation in oxygen at 180 ° C .; 4. Expose to hydrogen at 620 ° C. at ambient pressure for 10 minutes, cool to room temperature in the presence of hydrogen, then pump off hydrogen; 5. Repeat steps 3 and 4 for 2 6. Membranes were exposed to ambient air for at least 16 hours. O ₂ and N _2, respectively permeability 106 and 13 are formed.
7 barrels. The corresponding selectivity was 7.7. Example 8 The membrane of Example 7 was further treated as follows: 7. Degassed at 200 ° C. for 1 hour. O ₂ and N _2, respectively permeability 361 and 51 are formed.
7 barrels. The corresponding selectivity was 7. Reference Example 6 A cellulose membrane was subjected to the following processing steps. 1. Sprayed ammonium chloride powder; 2. Pyrolyzed from ambient temperature to 620 ° C in a temperature programmed oven at a constant heating rate of 1 ° C per minute at ambient pressure in the presence of carbon dioxide; At 0 ° C., the CO ₂ gas phase was replaced with pure argon, the temperature was raised to 900 ° C. at the same constant rate of 1 ° C./min, and the membrane was then cooled to room temperature. It was not permeable to oxygen. This membrane allowed small molecular species, such as hydrogen, to pass through to the other side of the membrane. Reference Example 7 The film of Reference Example 6 was further subjected to the following treatment. 4. Oxidized with oxygen at 300 ° C for 7 hours. The permeability of O ₂ and N ₂ formed is 850 and 1 respectively
95 barrels. The corresponding selectivity was 4.4. Reference Example 8 The film of Reference Example 7 was further subjected to the following treatment. 5. Oxidized with oxygen at 300 ° C. for 7 hours. The O ₂ and N ₂ permeability formed were 1913 and 78, respectively.
It was 6. The corresponding selectivity was 2.4. Example 9 A cellulose membrane was subjected to the following treatment. 1. Sprayed ammonium chloride powder; 2. Pyrolyzed in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C. per minute from ambient temperature to 620 ° C .; 3. 16 hours 4. Oxidation in oxygen at 180 ° C .; 4. Expose to hydrogen at 620 ° C. at ambient pressure for 15 minutes, cool to room temperature in the presence of hydrogen, then pump off hydrogen; 5. Repeat steps 3 and 4 for 2 Repeated times. The last hydrogenation step was at 800 ° C. for 1 hour. The O ₂ and N ₂ permeability formed are 700 and 212, respectively.
It was a barrel. The corresponding selectivity was 3.3. This example was made to serve as a reference for the next example in which chemical vapor deposition was used. Example 10 The film of Example 9 was treated for another 20 minutes at 720 ° C. by chemical vapor deposition of methane. The O ₂ and N ₂ permeability formed were 147 and 19.1 barrels, respectively. The equivalent selectivity is
It was 7.7. Comparing this result with that of Example 7, the chemical vapor deposition treatment resulted in a decrease in oxygen permeability and an increase in selectivity for nitrogen. Reference Example 9 A cellulose membrane was subjected to the following treatment. 1. Sprayed ammonium chloride powder; 2. Pyrolyzed in the presence of carbon dioxide at ambient pressure in a temperature programmed oven from ambient temperature to 700 ° C for 1 minute at a constant heating rate of 1 ° C; Degas for 1 hour. The H ₂ and Ar permeability formed were 520 and 0.5, respectively.
It was a barrel. The corresponding selectivity was 1040. Reference Example 10 A cellulose membrane was subjected to the following treatment. 1. Sprayed ammonium chloride powder; 2. Pyrolyzed in the presence of carbon dioxide at ambient pressure in a temperature programmed oven from ambient temperature to 650 ° C. for 1 minute at a constant heating rate of 1 ° C .; Activated at 1 ° C. in a 1: 1 mixture of hydrogen and carbon dioxide. The O ₂ and Ar permeability formed are 122 and 13 respectively.
It was a barrel. The corresponding selectivity was 9.4. The H ₂ and Ar permeability formed were 2340 and 23 barrels, respectively. The corresponding selectivity was 180. H formed
e and Ar permeability were 724 and 13 barrels, respectively. The corresponding selectivity was 55.7. Reference Example 11 The film of Reference Example 10 was further subjected to the following treatment. 4. Further hydrogenation with hydrogen at 600 ° C. for 30 minutes. O ₂ and Ar permeability formed were 285 and 40 respectively
It was a barrel. The corresponding selectivity was 7.1. Reference Example 12 A cellulose membrane was subjected to the following treatment. 1. sprayed with ammonium chloride powder; 2. pyrolyzed from ambient temperature to 750 ° C. in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C. for 1 minute; Activated at 500 ° C. in a 1: 1 mixture of hydrogen and carbon dioxide. O ₂ and Ar permeability formed were 23 and 3.3, respectively.
It was a barrel. The corresponding selectivity was 7.0. SF ₆ did not transmit in all of the above-described examples.

Continuation of front page    (72) Inventor David Rosen               Arad, Karanit, strike in Israel               REIT 7 (72) Inventor Shiuromo Sago               Beiasiba, Mollier, Su               Treat 3 (72) Inventor Jakob Kolesyu               Beer-Shiba, Omagh, Tsue, Israel               La, Street 10                (56) References JP-A-64-26723 (JP, A)                 JP-A-63-4812 (JP, A)                 JP-A-60-179102 (JP, A)                 JP-A-60-202703 (JP, A)                 JP-A-61-222916 (JP, A)                 JP-A-61-222912 (JP, A)

Claims (1)

(57) [Claims] A method for producing a molecular sieve carbon membrane having a pore size of greater than 2 ° and less than 10 ° from a membrane precursor, said membrane being an effective predefined, adapted to preferably allow the passage of a desired constant gaseous species. A method for producing a molecular sieve carbon membrane having a controlled pore size, while having substantially low permeability to other gases to be separated from the gas, to maintain the precursor dimensions and increase the carbon yield Pretreating the membrane precursor with a chemical adapted to perform thermal decomposition of the precursor at a predetermined heating rate in an inert atmosphere up to a certain temperature; formed at a high temperature in oxygen or an oxygen-containing atmosphere. Oxidizing the formed film; exposing the formed film to a hydrogen atmosphere at a high temperature for a predetermined time. 2. Claim 1 further comprising subjecting the membrane to a reduced pressure treatment.
The production method described in the section. 3. When the membrane precursor has a thickness of 2 to 500 μm and is in the form of a tube, a wall thickness of 1 to 300 μm and 5 to 100 μm
3. The method according to claim 1, wherein the non-melting carbon-containing material has a diameter of 0 .mu .. 4. The method according to any one of claims 1 to 3, wherein the heating is performed at a rate of 1 ° C / min to 10 ° C / min. 5. 5. The method according to claim 1, wherein the pore size of the already pyrolyzed film is increased by heating at a temperature of 200 to 800 [deg.] C. for 10 minutes to 16 hours in an atmosphere of an oxidizing gas containing oxygen. The production method described in Crab. 6. Pore size in hydrogen at 600 ° C to 2000 ° C for 15 minutes to 12 hours
The method according to any one of claims 1 to 4, wherein the temperature is reduced by sintering at a temperature in the range of: 7. The method according to any one of claims 1 to 4, wherein the membrane is subjected to a degassing step at a temperature in the range of 200 to 1000C under reduced pressure. 8. The membrane is kept for 1-2 hours at a pressure of 1-1000 Torr for 500-100
The method according to any one of claims 1 to 4, wherein the passivation is carried out by treating with hydrogen at a temperature in the range of 0 ° C. 9. Subjecting the membrane to vapor exposure of organic matter at a temperature of 450-1200 ° C. for 15 minutes to 4 hours, causing carbon deposition in the pores,
The method according to any one of claims 1 to 4, wherein the permeability and selectivity of the membrane are modified.