DE3726730A1 - CARBON MEMBRANES AND METHOD FOR THEIR PRODUCTION - Google Patents

Abstract

The invention relates to carbon membranes for the separation of gaseous species from each other and to a process for the production of these. The membranes have uniform pore sizes of a predetermined size, with a very sharp upper size cutoff. The membranes are produced from carbon containing non-melting material in sheet form by strictly controlled pyrolysis at a certain rate of increase of temperature per unit time, in a inert atmosphere. The thus obtained membranes can be treated by heating in an oxidizing atmosphere at elevated temperature to modify pore size. The final product can be exposed to a further treatment by means of air, hydrogen or carbon dioxide, and if desired to vacuum treatment. The starting material an be pretreated with certain materials to increase dimensional stability and increase carbon yield.

Description

The invention relates to a method for producing Carbon membranes, which are used for the separation of gases can be used. The membranes have a very sharp closure of the upper pore size, which for a given membrane in the order of 10% and preferred 5% of the effective pore size is. So these are Membranes for the separation of a variety of Suitable gas species that differ in size differ by about 10% in molecular size.

The process involves pretreating the membrane material, pyrolysis at a predetermined rate and the subsequent activation or further treatment for changing the pore size.

The use of membranes for separation processes has been around since known for many decades. These separation processes are based on the selective penetration of the different parts of the molecule through thin membranes. Based in many systems the separation on the selectivity of the penetration, due to differences in size of the different species in the mixture. The first procedures were for the Separation of colloidal species and for the separation of other small sized suspended particles from liquids used. Among the first types of membranes can membranes with a fairly large and non-homogeneous Pore diameters (<10 Å), which are suitable for the separation of mixtures with very large size differences the species that should be separated  were. A later development concerns the reversal Osmosis membranes used for the desalination of salt water were used. In parallel, ion exchange membranes for separation processes based on electrodialysis used.

Of the most important developments can be hollow fiber membranes be mentioned, which have a very large total membrane surface result in a small module and also open up the possibility that the edges of each module can be easily sealed.

Another development relates to the invention more asymmetrically Membranes that have a coarse-pored thick support, on a very thin, dense and highly selective membrane attached, the thin membrane for the actual separation process is used while the coarse support the required mechanical strength results. The known methods for the separation of a certain gases or gases from gaseous mixtures are based on differences in chemical or physical Properties of the various components.

The chemical processes include cyclic processes, where those intended for the removal of the components Chemicals are used. Distance is one example of hydrogen sulfide from natural gas. This can by the sponge iron process, in which a Reactor containing a bed of finely divided iron is used in a single pass and in which Iron sulfide waste remains. Another process is the cyclic amine process, in which an organic Ammonium salt is obtained during natural gas cleaning, which must be thermally decomposed so that the amine is recycled  can be. Among the most common physical Separation processes are those based on fractional ones Distillations are based. The most common is separation of nitrogen and oxygen from air. In a big one The benchmark (around 1000 t / day) is the multi-stage cryogenic Process from the energy point of view uneconomical. On a smaller scale, other processes are more economical. Adsorption processes are also used which on the different adsorption capacity of the different components of a gas mixture are based. Such methods are preferred as multi-stage cyclic processes carried out in columns. To the adsorbent must be used for recycling be regenerated what at low pressure or can be done by heating. Different separation processes are based on the use of specific zeolites or of carbon molecular sieves.

The invention relates to new carbon membranes for the Use when separating gas species from each other. The invention further relates to a separation process from Gas species from each other when using such a procedure or using such a carbon membrane be performed.

The carbon membranes are made from suitable precursor materials, that can undergo pyrolysis, and they show no pore holes or cracks the pyrolysis process.

The permeability is based on the formation of an open pore System of molecular dimensions, the pores a sharp cut or a sharp termination or  a sharp cutoff of the predetermined desired show upper size. Due to this fact can molecules of smaller size through the carbon membranes go through, whereas any gaseous molecules with a size that is larger than the pore size, do not pass through the membrane can. The selectivity of the membranes is thus the creation of essentially uniform pores determined, the size between that of the gaseous Molecular species that should go through and the one that is held back should be.

The term "pore" is not intended to identify a particular hole Size, but define a pore system which Bottle necks along the path of the penetrating gas which determine the permeability and selectivity. If the effective size of such constrictions in the is essentially uniform, a high degree of Get selectivity. The term "pore size" is used as working theory and shows the final value of the molecular size of gases that either go through or who are held back by being held by such Go through membranes.

The material of the membrane is carbon, which is a solid black material that is not up to about 3500 ° C melts and which in organic solvents is insoluble. It contains at least 70% by weight of carbon (the element), and it appears to be a condensed polymer aromatic rings with high molecular weight to be. The term "membrane" denotes a thin one Layer or foil from the material. It can also be in tube shape if the diameter and wall thickness define the important dimensions.  

The membranes according to the invention have a thickness of about 1 to 100 microns, with the preferred range is about 5 µm to about 30 µm. Tubular membranes have a diameter of about 5 to 1000 microns a wall thickness of 1 µm to about 300 µm.

A variety of carbon can be used as the starting material containing non-melting materials, such as regenerated cellulose, cuprophan cellulose, Cellulose, thermosetting polymers and also Acrylver bonds.

The pyrolysis temperature varies within a wide range from 250 ° C to over 1500 ° C. The preferred range is about 400 ° C to about 900 ° C. There is a preferred rate of heating, such as from about 1 ° C / min to about 10 ° C.

At higher heating speeds, pinholes (small holes) and even microscopic cracks formed are reduced, which reduces the selectivity of the membrane will, and in extreme cases, the membranes for the intended purpose become useless.

Pyrolysis results in the gradual decomposition of the material, which was used as the starting material, wherein the morphology and dimensions of the material essentially remain unchanged during pyrolysis.

Have the gaseous species that can be separated an effective size on the order of a few Angstrom. It is essential that membranes with a sharp delimitation of the pore size with a predetermined Be formed, generally in the range of  2 Å to 10 Å and preferably in the range of 2.5 Å to 6 Å.

In the manufacturing method according to the invention of carbon membranes of desired pore size Films or plates or tubular membranes of organic Precursors, which specified the membrane geometry have used. The pyrolysis process remains get the given geometry.

The precursor materials should be during pyrolysis do not melt or soften. The speed the pyrolysis (carbonization) is chosen so that it is so low that the formation of cracks or pinholes is avoided.

The atmosphere in which the pyrolysis is carried out is controlled so that an undesired burning of the Membrane material is avoided, which pinholes or Pores larger than the allowed size would result.

The pyrolysis is preferably carried out in an inert atmosphere. Suitable media for such pyrolysis are Inert gases such as argon, carbon dioxide etc.

The heating rate or rate can be within large limits, also depending on the nature of the Starting membrane material can be varied. The speed is generally in the range of 1 ° C / min to about 10 ° C / min. You can go faster or slower Use heating rates.

The membrane material is preferred with certain chemicals subjected to pretreatment so that uniformity of the pore system, which occurs during pyrolysis  is formed, is improved.

Among them, ammonium salts such as ammonium chloride, Chlorine, bromine, fluorine, flame retardant, hydrogen chloride, Hydrogen bromide, oxygen and air can be mentioned.

The permeability is determined in a manner known per se; the results are reported as a Barrer unit ben.

If a membrane has a given pore size after pyrolysis possesses, it is possible to determine the effective size of the pores enlarge and maintain their uniformity, by doing various post-treatments such as heating at elevated temperature. Such an enlargement can be obtained, for example, by using the Air flow membrane at 400 ° C for about 15 minutes subjects what an increase in pore size from about 3 Å to about 6 Å.

The permeability of the resulting membrane is for a given gas pair measured, and the ratio of permeabilities gives the selectivity of the membrane for such Gases.

It is preferred to membrane after pyrolysis of an atmosphere from air, oxygen etc. at elevated temperature to suspend. It is also preferred to use the membrane then heat in vacuum for a period of time. Certain attempts were made after the membrane material was sprayed with ammonium chloride. Farther was pyrolysis under a carbon dioxide atmosphere performed at ambient pressure, treatment was given at elevated temperature and ambient pressure with  Hydrogen and cooling under hydrogen and one Air exposure.

The process can be carried out in the same way with an oxidation step at elevated temperature under oxygen after the Pyrolysis stage carried out before the subsequent stages will.

Pyrolysis begins at around 200 ° C, and at certain It is possible to use the conditions according to the invention Pyrolysis level already at 150 ° C and even at perform lower temperatures.

It is preferred to air the pyrolysis after pyrolysis a few hours, generally at one temperature between room temperature and about 200 ° C, suspend.

It is generally preferred to go to a further stage afterwards carry out: the membrane in vacuum at elevated temperature generally in the range of 400 to 800 ° C to treat. Instead of such a vacuum treatment, it is also possible to increase the membrane with an inert gas Rinse temperature. If the membrane becomes an oxidizing Exposed to atmosphere after the pyrolysis stage, this results in an increase in pore size. The emerging Membranes have particularly uniform pores, the upper one Border from it differs by no more than about 5 to 10% of each other. Membranes with an upper limit the pore size allow the separation of gas species of similar dimensions, with the uniform top Range of pore size naturally between the size of the Gas molecules that are to be separated are located. A Heating to over 700 ° C reduces the pore size, conditionally by sintering.  

The initial heating stage, the pyrolysis, is carried out in an inert atmosphere, like an inert gas, carbon dioxide or one similar gas. It is still possible part of the pyrolysis in an air or oxygen atmosphere perform and pyrolysis in an inert atmosphere or quit in a vacuum.

The following examples illustrate the invention. you are summarized in Table I.

Example 1

A cellulose membrane was made in the presence of pure argon in a temperature programmed oven at a constant Heating rate of 1 ° C / min from room temperature pyrolyzed to 800 ° C. Then it became air exposed for at least 16 hours and then until Evacuated at 800 ° C for 0.5 hours. You get a Koh lenstoff molecular sieve membrane, which has a hydrogen permeability of 300 barrers. The methane permeability could, because it was too low, with the existing device cannot be measured, d. H. the permeability compared to methane was less than 0.25 barrers. This shows that the selectivity of this membrane over water Substance-methane mixture in the separation over 300 / 0.25 = 1200 lies.

Example 2

An experiment was carried out in the same way as in Example 1 described, carried out, but the final pyrolysis temperature and the evacuation took place at 600 ° C. The permeability the resulting carbon membrane against oxygen and nitrogen was 98 and 8.2 barrers, respectively, making their  Selectivity for the separation of an oxygen / nitrogen Mixture 98 / 8.2 = 12.

Example 3

A test was carried out as described in Example 1, but the final pyrolysis and evacuation temperature was 400 ° C. The permeability of the resulting carbon membrane for oxygen and nitrogen was 56 or 5.1 barrers, so their selectivity for separation an oxygen / nitrogen mixture 56 / 5.1 = 11 be wore.

Example 4

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C / min from ambient temperature up to 620 ° C;
• 3. Exposure to hydrogen at ambient temperature at 620 ° C for 1 hour and cooling in the presence from hydrogen to room temperature.

The permeability of the resulting membrane to oxygen and nitrogen was 12.6 and 0.85 Barrer, respectively that their selectivity for the separation of an oxygen / Nitrogen mixture was 14.8.  

Example 5

The membrane of Example 4 was added to the following Treated:

• 4. Exposure to air at room temperature for at least 16 hours;
• 5. Exposure to hydrogen at 620 ° C for 30 mi grooves.

The permeability of the resulting carbon membrane for Oxygen and nitrogen are 59 and 4.8 barrers, respectively that their selectivity for this gas pair is 11.6.

Example 6

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide Ambient pressure in a temperature programmed Oven at a constant heating rate from 1 ° C / min from ambient temperature to 620 ° C;
• 3. Oxidize in oxygen at 200 ° C for 1 hour;
• 4. Exposure to hydrogen at ambient pressure 620 ° C for 1 hour and cooling in the presence from hydrogen to room temperature;
• 5. Exposure to air at room temperature for at least 16 hours.

The permeability of the resulting carbon membrane for Oxygen and nitrogen were 49 and 4.9 Barrer, respectively that their selectivity for this gas pair was 10.

Example 7

The membrane of Example 6 was further treated by:

• 6. Evacuation at 200 ° C for 1 hour. The emerging O₂ and N₂ permeabilities were 115 and 16.7 Barrer, and the corresponding selectivity was 6.8.

Example 8

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with an ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C / min from ambient temperature up to 620 ° C;
• 3. Oxidation in oxygen at 180 ° C for 3 hours;
• 4. Exposure to hydrogen at ambient pressure 620 ° C for 1 hour and cooling in the presence from hydrogen to room temperature;
• 5. Exposure to air at room temperature for at least 16 hours.

The resulting O₂ and N₂ permeabilities were 61 or 5.5 barrers, and the corresponding selectivity was 11.  

Example 9

The membrane of Example 8 was further treated by:

• 6. Evacuation at 600 ° C. The resulting O₂- and N₂- Permeabilities were 117 and 21 barrers, respectively, and corresponding selectivity was 5.6.

Example 10

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C / min from ambient temperature up to 620 ° C;
• 3. Oxidation in oxygen at 180 ° C for 1 hour;
• 4. Exposure to hydrogen at ambient pressure 620 ° C for 15 minutes and cooling in the presence from hydrogen to room temperature, then the Pumped off hydrogen;
• 5. Levels 3 and 4 were repeated twice (the Level 3 was held for the first time at 16 hours extended by 1 hour);
• 6. The membrane became air for at least 16 hours exposed.

The resulting O₂ and N₂ permeabilities were 68 or 7 barrers, and the corresponding selectivity was 9.6.  

Example 11

The membrane of Example 10 was further treated by:

• 7. Evacuation at 200 ° C for 1 hour.

The resulting O₂ and N₂ permeabilities were 300 or 50 Barrer, and the corresponding selectivity was 6.

Example 12

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with an ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide Ambient pressure in a temperature programmed Oven at a constant heating rate from 1 ° C / min from ambient temperature up to 620 ° C;
• 3. Oxidize in oxygen at 180 ° C for 16 hours;
• 4. Exposure to hydrogen at ambient pressure 620 ° C for 10 minutes and cooling in the presence from hydrogen to room temperature, then the Pumped out hydrogen;
• 5. Levels 3 and 4 were repeated twice;
• 6. The membrane was exposed to ambient air for at least Exposed for 16 hours.

The resulting O₂ and N₂ permeabilities were 106 or 13.7 Barrer, and the corresponding selectivity was 7.7.  

Example 13

The membrane of Example 12 was further treated by:

• 7. Evacuation at 200 ° C for 1 hour.

The resulting O₂ and N₂ permeabilities were 361 and 51.7 Barrer, respectively, and the corresponding selectivity was 7.

Example 14

A cellulose membrane was the following process steps subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C / min from ambient temperature up to 620 ° C;
• 3. at 620 ° C, the CO₂ gas phase was pure argon replaced, and the temperature was reduced to 900 ° C the same constant speed of 1 ° C / min elevated. The membrane was then brought to room temperature cooled, and she had none against oxygen Permeability.

Example 15

The membrane of Example 14 was further treated by:

• 4. Oxidation with oxygen at 300 ° C for 7 hours.

The resulting O₂ and N₂ permeabilities were 850 or 195 Barrer, and the corresponding selectivity was 4.4.

Example 16

The membrane of Example 15 was further treated by:

• 5. Further oxidation with oxygen at 300 ° C during 7 hours.

The resulting O₂ and N₂ permeabilities were 1913 or 786 Barrer, and the corresponding selectivity was 2.4.

Example 17

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C / min from ambient temperature up to 620 ° C;
• 3. Oxidation in oxygen at 180 ° C for 16 hours;
• 4. Exposure to hydrogen at ambient pressure 620 ° C for 15 minutes and cooling in the presence from hydrogen to room temperature, then the Pumped off hydrogen;
• 5. Levels 3 and 4 were repeated twice. The last hydrogenation stage was 800 ° C for 1 hour de.

The resulting O₂ and N₂ permeabilities were 700 or 212 Barrer, and the corresponding selectivity was 3.3. This example was used as a comparative example for the next example, where a chemical Vapor separation was used.

Example 18

The membrane of Example 17 was further modified by chemical Vapor separation of methane at 720 ° C for 20 minutes treated. The resulting O₂ and N₂ permeabilities were 147 and 19.1 barrers, respectively, and the corresponding selectivity was 7.7. If you compare this result with that of Example 12, it is evident that the CVD Treatment a reduction in oxygen permeability and an increase in selectivity over nitrogen works.

Example 19

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide Ambient pressure in a temperature programmed Oven at a constant heating rate from 1 ° C / min from ambient temperature to 700 ° C;
• 3. Degassing at 200 ° C for 1 hour.

The resulting H₂ and Ar permeabilities were 520 or 0.5 Barrer, and the corresponding selectivity was 1040.  

Example 20

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide Ambient pressure in a temperature programmed Oven at a constant heating rate from 1 ° C / min from ambient temperature to 650 ° C;
• 3. Activate in a mixture of 1: 1 hydrogen Carbon dioxide at 500 ° C for 2 hours.

The resulting O₂ and Ar permeabilities were 122 or 13 barrers, and the corresponding selectivity was 9.4. The resulting H₂ and Ar permeabilities were 2340 or 23 Barrer, and the corresponding selectivity was 180. The resulting He and Ar permeabilities were 724 and 13 barrers, respectively, and the corresponding selectivity was 55.7.

Example 21

The membrane of Example 20 was further treated by:

• 4. further hydrogenation with hydrogen at 600 ° C during 1/2 hour.

The resulting O₂ and Ar permeabilities were 285 or 40 Barrer, and the corresponding selectivity was 7.1.  

Example 22

A cellulose membrane was subjected to the following treatments subject:

• 1. spray with ammonium chloride powder;
• 2. Pyrolysis in the presence of carbon dioxide at ambient pressure in a temperature programmed oven at a constant heating rate of 1 ° C / min from ambient temperature to 750 ° C;
• 3. Activate in a mixture of 1: 1 hydrogen-carbon dioxide at 500 ° C for 2 hours.

The resulting O₂ and Ar permeabilities were 23 and 3.3 barrers, respectively, and the corresponding selectivity was 7.0.

Claims (10)

1. A process for the preparation of carbon membranes from a membrane precursor material which contains and does not melt carbon and has dimensions corresponding to the desired product, the membranes having an effective predetermined pore size selected to allow the passage of desired given gas species, while being essentially impermeable to other gas species to be separated from the first, characterized in that the membrane precursor is optionally pretreated with an agent which increases the carbon yield and maintains the geometry of the precursor, the pyrolysis of the precursor with a predetermined heating rate in an inert atmosphere up to a given temperature, optionally oxidizing the resulting membrane in oxygen or an oxygen-containing atmosphere at elevated temperature, optionally the resulting membrane of an air-hydrogen or carbon dioxide exposes the atmosphere for a predetermined time and, if necessary, the resulting membrane is subjected to a vacuum treatment. 2. The method according to claim 1, characterized in that that the membrane precursor is a non-melting carbonaceous material with a thickness is from 2 to 500 µm and that when it is in tubular form is present, a wall thickness of 1 to 300 microns and a diameter from 5 to 1000 µm. 3. The method according to claim 1 or 2, characterized in that heating at a rate from 1 ° C / min to 10 ° C / min. 4. The method according to any one of claims 1 to 3, characterized characterized in that the pyrolyzed membrane through air, oxygen, carbon dioxide, water or Steam is activated. 5. The method according to any one of claims 1 to 3, characterized characterized in that the pore size of the pyrolyzed Membrane by heating in the range of 200 to 1200 ° C from 10 minutes to about 16 hours in one atmosphere from nitrogen, argon, hydrogen or in a vacuum is increased. 6. The method according to any one of claims 1 to 3, characterized characterized by the pore size Sintering at a temperature in the range of 700 ° C to about 1500 ° C for about 15 minutes to 12 hours in one inert atmosphere, such as an inert gas, nitrogen or in hydrogen or in a vacuum.   7. The method according to any one of claims 1 to 3, characterized characterized in that the membrane at a Temperature in the range of about 200 ° C to about 1000 ° C in is subjected to a vacuum of a degassing stage. 8. The method according to any one of claims 1 to 3, characterized characterized in that the membrane by treatment with hydrogen at a temperature in the range from about 500 ° C to about 1000 ° C at a pressure of 1 torr passivated up to 1000 torr for about 1 to 2 hours becomes. 9. The method according to any one of claims 1 to 3, characterized characterized in that the membrane the steam an organic substance at a temperature in the range from 450 ° C to 1200 ° C for about 15 minutes to exposed for about 4 hours, which is a deposition of Carbon in the pores, a modification of the permeability and the selectivity of the membrane. 10. Carbon membranes for the separation of gas species from each other, characterized in that them by a method according to any one of claims 1 to 9 have been obtained.