METHOD OF PREPARING A GAS CONTAINING A HIGH PORTION OF METHANE

Abstract: A Method to prepare a gas containing a high portion of methane by methanisation of a gas mixture comprising car­ bonmonoxide, carbondioxide and hydrogen and reaction with a gas mixture comprising methanol and/or hydrocar­ bons having up to 20 C-atoms and a portion of less than 20% by weight methane in a reactor, in which a hydrogen-rich gas is fed to a first catalyst bed which catalyses the formation of methane, the temperature at the entrance of the catalyst bed being adjusted to from 250 to 550°C, the gas mixture after having left the first catalyst bed is cooled down to the entrance temperature of the second catalyst bed and the gas mixture is fed to a second catalyst bed in which the conversion of hyd­ rocarbons with steam and the other components of the gas mixture is catalysed to form methane.

Description: Method to prepare a gas containing a high portion of methane The invention relates to a method to prepare a gas containing a high portion of methane by methanisation of a gas mixture comprising carbonmonoxide, carbondioxide and hydrogen and reaction with a gas mixture comprising methanol and/or hydrocarbons having up to 20 C-atoms and a portion of less than 20 % by weight methane in a reactor. From DE-AS 1 180 481 (= GB-PS 820 257) a process for the production of gases containing methane is kno n, wherein the vapour of paraffinic hydrocarbons containing an average of 4-10 carbon atoms and steam are passed through a bed of a nickel catalyst under atmospheric or superatmospheric pressure, the proportion of steam being within the range of 2 to 5 parts by weight for one part by weight of the hydrocarbons or when using hydrocarbons containing an average of 4-7 carbon atoms within the range of 1,5-5 parts by weight for one part by weight of the hydrocarbons. The hydrocarbon vapour and the steam are passed into the catalyst bed at a temperature above 350 C such that the bed is maintained by the reaction at a temperature within the range of 400 C to 550 C. By this process products are obtained having a composition after the removal of steam of about 61-65 % by volume of CH4, 21-23 % by volume of Cm2, 1 5' by volume of CO and 12-17 % by volume of H2. If a gas having a higher content of methane is desired the methane-rich gas produced in the abovementioned way may be subjected to the action of a nickel catalyst at a lower temperature, e.g. at 4000C or below in order to bring about the formation of methane by reaction between carbon dioxide, carbon monoxide and hydrogen present in the gas. For this process a number of nickel catalysts may be used, examples of which can be found in DE-AS 1 199 427 and 1 227 603. BE-PS 634 920 describes catalysts which are suitable for reforming hydrocarbons with steam and contain nickel or cobalt in the form of the metal or in the form of a compound which may be reduced to form the metal as well as a metal of the platinum group upon a refractory support such as alumina. These catalysts may also contain a small amount of an alkali metal or an earth alkali metal in order to prevent the deposition of carbon upon the catalyst. It is also known from said Belgian patent specification 634 920 that by carrying out the reforming of hydrocarbons having a boiling point of at most 3500C with steam at a temperature of 600-1000 C synthesis gas or a gas having a low methane content are obtained but that by carrying out said process at a temperature of 450-7000C a methane-rich gas is obtained. According to FR-PS 1 394 202 the life of the catalysts in the process as described in DE-AS 1 180 481 can considerably be increased by recycling hot reaction gases containing water vapour through the catalyst bed mixed with the hydrocarbon vapour and steam being supplied to said bed. According to the US-PS 3 420 642 the reaction between the hydrocarbons and the steam is carried out in two stages. To the first stage the whole of the steam is supplied but only part of the hydrocarbons. The product gas and undecomposed steam from the first stage are supplied to the second stage together with the remaining portion of the hydrocarbons. According to DE-AS 1 922 182 the reaction as described, is carried out in two reactors. The gas produced in the first reactor and containing methane, hydrogen, carbon monoxide, carbon dioxide and undecomposed steam is cooled by the addition of at least one of the reaction components and the resulting gaseous mixture is introduced into a second reactor in which the components of the mixture react in the presence of a catalyst in such a way that the methane content of the mixture is increased. According to DE-OS 2 314 804 the conversion of hydrocarbons in the vapour phase having a boiling range up to about 2500 C with steam is carried out under a pressure of 15-100 at in the presence of nickel catalysts in two reactors. To the first reactor a mixture of 40-50 96 by weight of the total amount of preheated hydrocarbons and 80-90 % by weight of the total amount of steam is supplied, the resulting product is cooled after having been discharged from the first reactor and then mixed with the remaining portion of preheated hydrocarbons and steam and further converted in a second reactor. According to NL-PS 296 207 a methane containing gas is prepared by contacting a heated mixture of a hydrocarbon material, a hydrogen containing gas and steam with a very active metal catalyst at a temperature of 450-9000C under a pressure between 3 and 50 at. The hydrogen containing gas may also contain carbon oxides which react with the hydrogen in the presence of the catalysts to form methane. The amount of hydrogen is 0.1-2.5 moles and the amount of steam is 2.5-0.3 moles per atom of carbon in the hydrocarbon material. According to DE-AS 1 545 462 hydrocarbons with an average number of carbon atoms of 10-20 are converted with steam and hydrogen to form methane-rich gases. To the mixture of hydrocarbons and steam 0.1-0.8 Nm3 of hydrogen is added fcr 1 kg of hydrocarbon, so that the hydrogen containing feed mixture needs only be preheated to a maximum temperature of 350 C and the reaction can be carried out at a maximum temperature of 4500 C. This is favourable because it is generally known that with an increasing preheating temperature of the hydrocarbons endothermal reactions are promoted in the catalyst bed whereby carbon monoxide and hydrogen are formed. This leads to a temperature reduction-in the catalyst bed. This reduction of the temperature and the excess of carbon monoxide favour the Boudouard reaction according to which carbon dioxide and carbon are formed from carbon monoxide. The carbon formed'in this way is, however, deposited upon the catalyst and upon the walls of the reaction vessels. It is also known that many of the reactions which take place when passing hydrocarbons and steam over a nickel catalyst are exothermal and thus again reduce the. formation of methane. The purpose of the invention is to provide an improved process in which the disadvantages of the processes of the prior art are at least partly avoided. Accordingly this invention provides a method to prepare a gas containing a high portion of methane by methanisation of a gas mixture comprising carbonmonoxide, carbondioxide and hydrogen and reaction with a gas mixture comprising methanol and/or hydrocarbons having up to 20 C-atoms and a portion of less than 20 % by weight methane in a reactor, characterised by the fact that a) as a gas mixture comprising carbonmonoxide, carbon dioxide and hydrogen a hydrogen-rich gas containing from 25 to 90 % by volume hydrogen b) is fed to a first catalyst bed which catalyses the formation of methane, the temperature at the entrance of the catalyst bed being adjusted to from 250 to 550 C, c) the gas mixture after having left the first catalyst bed is cooled down to the entrance temperature of the second catalyst bed from 250 to 5500 C by addition of a mixture of steam and hydrocarbons the mixture of which has a lower temperature than the entrance temperature of the second catalyst bed, d) and the gas mixture is fed to a second catalyst bed in which the conversion of hydrocarbons with steam and the other components of the gas mixture is catalysed to form methane. Preferably the reactor is operated under a pressure of from 0.5 to 10 MPa, more preferably from 1 to 6 MPa and most preferably from 2 to 5 MPa. According to the invention a hydrogen-rich, carbonmonoxide containing gas is forced to react in a reactor over a first catalyst bed to form methane in an exothermic reaction after which the product gas is cooled down by the addition of a hydrocarbon-steam mixture in the same reactor whereupon the resulting mixture is led over a second catalyst bed, to form a methane-rich gas. The gas mixture coming out of the first catalytic bed may still contain so much hydrogen that it protects the catalyst of the second bed, as with the known hydrogasification. Considering the nature of the catalyst to be used, the first catalyst stimulates the reaction of the carbonmonoxide and carbondioxide with hydrogen to form methane (and water). The second catalyst bed stimulates the conversion of the added hydrocarbons and steam to methane. As to the catalysts used in the procedure the catalyst in the first bed should be a supported Nickel or Cobalt methanisation catalyst according to the state of the art. This first-bed catalyst in particular should be thermostable and active already at relatively low temperatures; it can be prepared according to DE-OS 1 767 202 but is not limited to this. Aluminum oxide is preferred as a support for the catalyst. In the second bed, well-known CRG-type (catalytic rich gas) catalysts can be used but alumina-supported Nickel catalysts based on the above-mentioned preparation procedure (DE-OS 1 767 202) are preferred here too. Typical for the invention is that the reaction heat of the methanisation reaction, taking place in the first catalyst bed, is being used to bring the reacting mixture at the required entrance temperature of the second catalyst bed. With this the preheating of the reacting mixture outside the reactor can be limited. Due to this the temperature of the added steam/hydrocarbon mixture will be lower than the entrance temperature of the second catalyst bed. The temperatures at the entrance of both catalyst beds can be between 2500C and 5500C. The temperature at the entrance of the first bed will preferably be between 275 and 4000C; at the entrance of the second bed preferably between 300 and 450 C. The reaction temperatures can be controlled accurately by adding steam to the hydrogen rich gas which is added to the first catalyst bed. It is advisable to feed steam in such an amount that the temperature of the gas mixture when leaving the first catalyst bed is reduced to a temperature not exceeding 800 C, preferably not exceeding 7000C and even more preferably not exceeding 6000C. The hydrogen-rich gas generally contains: from 30 to 80 5' by volume hydrogen from 5 to 25 % by volume carbonmonoxide from 0 to 25 5' by volume methane less than 50 5' by volume carbondioxide and/or inertgas. Preferably the hydrogen-rich gas contains: from 40 to 65 96 by volume hydrogen from 12 to 22 % by volume carbonmonoxide from 0 to 10 96 by volume methane less than 10 5' by volume carbondioxide and/or inertgas. It is preferred to use lean gas as hydrogen-rich gas. With lean gas, as described in the prior art, is meant a mixture consisting of H2, CO, C02, CH4, HzO and possible inert components with a volume percentage of H2 bigger than the separate volume percentages of CO, CO2 and CH4, but not necessarily bigger than the volume percentage of H20. As hydrogen-rich gas which is supplied to the first catalyst bed there can be used a gas mixture obtained by subjecting a gas resulting from coal gasification or hydrocarbon reforming to a subsequent carbonmonoxide shift conversion step and optionally to a carbondioxide removal step. Preferably the hydrogen-rich gas contains from 2.5 to 5 parts by volume hydrogen per 1 part by volume carbonmonoxide, preferably from 2.9 to 3.2 parts by volume hydrogen per 1 part by volume carbonmonoxide. The total steam amount required in a certain situation depends upon the hydrogen-ricb gas composition, the catalyst-bed entrance temperatures, the operating pressure and the reactor throughput. Insufficient steam addition may lead to carbon deposition in the catalyst bed (s). Due to the number of parameters involved it is difficult to state comprehensively how the change in one of the parameters may affect the required mass input ratios. Obviously the mass input ratios are determined by the thermodynamical conservation laws from which they can be calculated for specific cases. For the procedure according to the invention therefore state the following general mass input ranges: steam in an amount of about .6 to 1.2 ton and hydrocarbons in an amount of about .3 to .8 ton referred to 1000 ms3 gas input to the first catalyst bed (msn is cubic metres at ISOC and .101 MPa). The mixture of steam and hydrocarbons which is fed to the gas mixture after having left the first catalyst bed has preferably a temperature of from 120 to 3O00C, more preferably of from 1800C to 25O0C. The hydrocarbons used in the method of the present invention is preferably the sorcalled light fraction of hydrocarbons consisting essentially of hydrocarbons having up to 12 C-atoms. The product gas coming out of the second catalyst bed can still have a relatively high temperature. of this product gas can be utilized to heat a mixture of steam and hydrocarbons to a temperature required for the reaction in a third catalyst bed. Although this principle extends to a number of catalyst beds, the thermal profit decreases rapidly, so that the capital costs will then become prohibitive. The invention is explained by way of the Figures and examples. Figure 1 schematically represents the method according to the invention, figure 2 represents the mentioned well-known "hydrogasification". In the Figures is 1 the reactor, 2 a pre-heater for the reaction mixture, added to 3, the first catalyst bed according to the invention, respectively 4 according to the well-known performance. The discharging of the product gases is performed via a discharge pipe 5. According to the invention a mixture of steam and hydrocarbons is supplied through a pre-heater 6 to the reactor 1 and mixed with the gas coming from the first catalyst bed 3, which mixture is lead into a second catalyst bed 7. Example 1: In the method A according to the invention only hydrogenrich gas is supplied through pre-heater 2. The steamhydrocarbon mixture is supplied through the pre-heater 6. In the known method B this happens through pre-heater 2 together with a hydrogen-rich gas. The composition and quantities of reaction mixture supplied per time-unit are equal in both cases, Just as the pressure and the temperature at the entrance of the first catalyst bed (respectively 3 and 4). The pressure is 2 MPa. The compared results are represented in tables 1 and 2. Table 1 Survey of results according to methods pointed out as A and B in Figure 1. In both cases the quantity of raw materials is identical: 1000 m3 hydrogen-rich gas 1,728 tons of steam of 1900C 1,012 tons of butane of 600C The temperature of the mixture of steam and butane is 1650C. EMI11.1 <tb> Type <SEP> Ti+) <SEP> Tm+) <SEP> Tu+) <SEP> Caloric <SEP> value <SEP> Q <SEP> in <tb> <SEP> dry <SEP> product <tb> <SEP> gas <tb> <SEP> A <SEP> 300 <SEP> 205 <SEP> 447 <SEP> 28,0 <SEP> MJ/ms3 <SEP> 369 <tb> <SEP> B <SEP> 300 <SEP> - <SEP> 480 <SEP> 26,9 <SEP> MJ/ms3 <SEP> 951 <tb> Composition of obtained dry product gas for A: 11,9% H2 0,3% CO 70;3 CH4 17,5% CO2 Composition of obtained dry product gas for B: 1,15% H2 0,5 CO 66,2% CH4 17,8% CO2 *) The hydrogen-rich gas is of the following composition: 50% H2, 10% CO, 5% CH4, 5 % CO2, 30% H2O +) Ti is the temperature at the entrance of the reactor in B and at the entrance of each catalyst bed in A. Tu is the temperature at the exit of the reactor. Tm is the temperature of the pre-heated steam/ hydrocarbonmixture at A. Temperatures are given in C. **) Q is the necessary preheat for the hydrocarbon steammixture. Table 2 Survey of results according to methods pointed out as A and B in Figure 1. In both cases the quantity of raw materials is identical; 1000 m3 hydrogen rich gas*) 1,066 tons of steam of 1900C 0,572 tons of butane of 600C The temperature of the mixture of steam and butane is 167 0C. EMI12.1 <tb> Type <SEP> Ti+) <SEP> Tm+) <SEP> Tu+) <SEP> Caloric <SEP> value <SEP> Q <SEP> in <tb> <SEP> dry <SEP> product <tb> <SEP> gas <tb> <SEP> A <SEP> 350 <SEP> 205 <SEP> 492 <SEP> 26,3 <SEP> MJ/m3 <SEP> 213 <tb> <SEP> B <SEP> 350 <SEP> - <SEP> 531 <SEP> 24,7 <SEP> MJ/m3 <SEP> 748 <tb> Composition of obtained dry product gas for A: 19,2% H2 0,6% CO 63,3% CH4 16,88 C02 Composition of obtained dry product gas for B: 24,3% H2 1,1% CO 57,4% CH4 17,2% CO2 *) The hydrogen-rich gas is of the following composition: 50% H2, 10% CO, 5% CH4, 5% CO2, 30% H2O +) Ti is the temperature at the entrance of the reactor in B and at the entrance of each catalyst bed in A. Tu is the temperature at the exit of the reactor. Tm is the temperature of the pre-heated steam/ hydrocarbon mixture at A. Temperatures are given in C **) Q is the necessary preheat for the hydrocarbon steammixture From the examples appears that at analogous conditions: 1. the performance A according to the invention delivers a product gas at a higher caloric value at a lower temperature; 2. the reaction mixture of the performance A according to the invention demands considerably less preheat energy. In the figure the pre-heaters ? and 6 are represented. Another configuration in relation to the pre-heaters belongs to the possibilities.

Claims: What we claim is Method to prepare a gas containing a high portion of methane by methanisation of a gas mixture comprising carbonmonoxide, carbon dioxide eldd hydrogen and reaction with a gas mixture comprising methanol and/or hydrocarbons having up to 20 C-atoms and a portion of less than 20 % by weight methane in a reactor,' characterised by the fact that a) as a gas mixture comprising carbonmonoxide, carbondioxide and hydrogen a hydrogen-rich gas containing from 25 to 90 % by volume hydrogen b) is fed to a first catalyst bed which catalyses the formation of methane, the temperature at the entrance of the catalyst bed being adjusted to from 250 to 5500 C, c) the gas mixture after having left the first catalyst bed is cooled down to the entrance temperature of the second catalyst bed from 250 to 550 C by addition of a mixture of steam and hydrocarbons the mixture of which has a lower temperature than the entrance temperature of the second catalyst bed, d) and the gas mixture is fed to a second catalyst bed in which the conversion of hydrocarbons with steam and the other components of the gas mixture is catalysed to form methane. 2. Method according to claim 1, characterised by the fact that the reactor is operated under a pressure from 0.5 to 10 MPa. 3. Method according to claim 2, characterised by the fact that the reactor is operated under a pressure fromt o¯¯6¯¯MPa. - 4. Method according to claim 3, characterised by the fact that the reactor is operated under a pressure from 2 to 5 MPa. 5. Method according to any of claims 1 to 4, cha racterised by the fact that the temperature at the entrance of the first catalyst bed is adjusted to from 275 to 4000C. 6. Method according to any of claims 1 to 5, cha racterised by the fact that the temperature at the entrance of the second catalyst bed is adjusted to from 300 to 4500C. 7. Method according to any of claims 1 to 6, cha racterised by the fact that the'hydrogen-rich gas contains from 30 to 80 % by volume hydrogen from 5 to 25 % by volume carbonmonoxide from 0 to 25 % by volume methane less than 50 % by volume carbondioxide and/or inertgas. 8. Method according to claim 7, characterised by the fact that the hydrogen-rich gas contains from 40 to 65 % by volume hydrogen from 12 to 22 % by volume carbonmonoxide from 0 to 10 % by volume methane less than 10 % by volume carbondioxide 2nd/or and/or inertgas. 9. Method according to claim 7, characterised by the fact that the hydrogen-rich gas is lean gas. 10. Method according to any of claims 1 to 9, cha .racterised by the fact that the hydrogen-rich'gas contains from 2.5 to 5 parts by volume hydrogen per 1 part by volume carbonmonoxide. 11. Method according to claim 10, characterised by the fact that the hydrogen-rich gas contains from 2.9 to 3.2 parts by volume hydrogen per 1 part by volume carbonmonoxide. 12. Method according to any of claims 1 to-lI, cha racterised by the fact that the temperature of the gas mixture when leaving, the first catalyst bed is reduced to a temperature not exceeding 8000C by feeding steam to the first catalyst bed. 13. Method according to claim 12,characterised by the fact that the temperature of the gas mixture when leaving the first catalyst bed is reduced to a temperature not exceeding 7090C. 14. Method according to claim 13, characterised by the fact that the temperature of the gas mixture when leaving the first catalyst bed is reduced to a temperature not exceeding 600 C. 15. Method according to any of claims 1 to 14, cha racterised by the fact that the mixture of steam and hydrocarbons which is fed to the gas mixture after having left the first catalyst bed has a temperature of from 120 to 3000C. 16. Method according to claim 15, characterised by the fact that the mixture of steam and hydro carbons which is fed to the gas mixture after having left the first catalyst bed has a temperature from 180 to 2500C. 17. Method according to any of claims 1 to 16, cha racterised by the fact that in step c) there are added steam in an amount of about 0.6-to 1.2 tons and hydrocarbons in an amount of about 0.3 to 0.8 tons per each 1000 m35 gas input to the first catalyst bed. 18. Method according to any of claims 1 to 17, cha- racterised by the fact that the hydrogen-rich gas which is supplied to the first catalyst bed is obtained by subjecting a gas, resulting from coal gasification or hydrocarbon reforming, to a subsequent carbonmonoxide shift conversion step and optionally to a carbondioxide removal step.

Inventors: BOTTELBERGHS, PIERRE HENRI, GEUS, JOHN WILHELM, TAKENS, WALTHER
Applicants: VEG-GASINSTITUUT N.V.
Application year: 1980
Main classification: C07C9
IPC codes: C07C9, C10G47, C07C1, C07C67, C07C4, C10K3