DE2436297C2 - Process for producing methane - Google Patents

Description

The invention relates to a method for producing methane of a quality that is suitable for exchange with Natural gas is suitable from raw gases containing hydrogen and carbon oxides from the gasification of coal, Tars or heavy residual oils under increased pressure, which are cooled and removed from catalyst poisons, especially of sulfur compounds, purified and at least two-stage methanation Nickel catalysts with steam at pressures from 5 to 100 bar and temperatures in the range from 200 to 500 ° C are subjected, with the product gas of the previous methanation in the last Methanation stage on an indirectly cooled, fixed catalyst without recirculation of Product gas is implemented.

It is known that coals, tars and heavy residual oils can be converted into a raw gas by gasification with steam and oxygen under increased pressure and at higher temperatures, the content of carbon oxides of which is greater than the hydrogen content of the gas. The raw gases generated by the pressure gasification of coal at around 20 to 80 bar with steam and oxygen generally contain a lot of CO ₂ (28 to 32% by volume) and CO (15 to 20% by volume) with a relatively low H ₂ Content (35 to 44% by volume). The gasification of tars and residual oils at temperatures of about 1100 to 1500 ° C. results in a raw gas containing 3 to 6% by volume of CO ₂ , 46 to 50% by volume of CO and 40 to 48% by volume of H ₂ contains. Heavy residual oils are hydrocarbons that boil above 250 ° C.

The purified raw gases, in particular freed from sulfur compounds, can pass through known, single or multi-stage methanation

drive with the help of nickel catalysts to gases - calculated COj-free - with at least 90 vol- ⁰ zo methane, which are exchangeable or miscible with natural gas. The hydrogen content of these gases should not exceed 2% by volume; hydrogen contents of less than 1% by volume are also required in many cases. The gas usually still contains inert components in the form of nitrogen and argon as well as residual carbon dioxide.

During methanation, the oxides of carbon are catalytically converted to methane and hydrogen Water vapor converted by the following reactions, which are exothermic:

CO + 3 H ₂ = CH ₄ + H ₂ O
CO ₂ + 4 H ₂ = CH ₄ + 2 H ₂ O

Due to thermodynamic relationships, it is known that the reaction temperatures are as low as possible are necessary in order to achieve high methane contents or low residual contents of carbon oxide and hydrogen achieve. In many cases it is also necessary to have one before passing through the last catalyst layer To remove the majority of the water vapor formed in the previous reaction steps to the to encourage further methane formation at the same temperature.

In a known method, the scrubbed raw gas is first in a so-called cycle methanation stage implemented at higher outlet temperatures, the one above about 280 ° C active temperature range of the nickel catalyst is used. In this methanation stage, the Dilution of the feed gas a part of the product gas formed after cooling in a waste heat boiler returned to the entrance of this stage. This cycle methanation stage can be carried out in several stages by dividing the raw gas into several substreams and through reactors connected in series is directed. The outlet gas of the first reactor provides the diluent gas for the second reactor and the product gas of the last reactor in the series is partly used as a diluent gas for the first reactor returned.

That formed in the cycle methanation stage Due to the establishment of equilibrium, product gas still contains at a comparatively high temperature a considerable proportion of hydrogen, which is only available at lower temperatures with other carbon oxides can be converted to methane. It is therefore also a known procedure to use the product gas To cool cycle methanation and to subject a first Nachmethanisierung without prior removal of water vapor is operated as a so-called wet post-methanation stage. There the desired methane content in the product gas is not yet achieved due to the wet methanation, it is also customary to cool the gas obtained, a large part of the water vapor contained therein condense out, reheat this gas and again pass it over nickel catalysts. These so-called dry post-methanation results in the desired gas that can be exchanged with natural gas only needs to be washed out excess CO2.

Occasionally, by bypassing the wet post-methanation stage, the cycle stage is also used withdrawn product gas immediately cooled and after

Condensation of water vapor sent to the dry post-methanization. The methane content of this However, product gas is less, and so is the recovery of released heat of reaction is worsened, so that the thermal efficiency of the entire system compared to the first described Procedure decreases. In addition, due to the low proportion of water vapor and the comparatively high outlet temperature of the gas from the post-methanation, the risk of soot formation the Boudouard reaction versus the process with successive wet and dry post-methanation significantly increased.

In Ullmann's Encyclopedia of Industrial Chemistry, 3rd Edition (1957), Volume 9, Page 745, a two-stage Methanation process with fixed nickel catalysts without recirculation of product gas explained, in which the heat of reaction is dissipated by pressurized water. The one-stage Methanation without product gas circuit according to DE-OS 19 22 181 also has water cooling on. From GB-PS 1152 008, the single-stage methanation on an indirectly gas-cooled, im The catalyst maintained in a fluidized state is described. In US-PS 31 28 163 it is known to be rejected, to divide a methanation catalyst into layers or zones with cooled gas in between initiate, whereby this gas can participate in the implementation. In Eucken-Jakob »The Chemical Engineer« (1940) pages 83-88, tubular reactors for exothermic catalytic reactions are described in where the fresh gas first indirectly cools the catalyst before it passes through the countercurrent to the cooling gas K.ftalysatorbereich is directed. These reactors are used in particular for the catalytic oxidation of SO2 to SO3 provided.

The invention is based on the object of providing heat of reaction in a method of the type mentioned at the outset Not only to dissipate, but also to use and thereby the cooling in the last methanation stage to be carried out with gas. At the same time, an intensive course of the reaction should take place in this methanation stage can be achieved at high catalyst activity and the previous condensation of water vapor in the no gas to be converted is required. According to the invention, this is achieved by the procedural measures that are in Characteristics of the claim are mentioned. Since also the last stage in the conditions of the wet Methanation works, the residual gas can contain slightly more hydrogen than when it is used dry methanation is the case, but the hydrogen content is still below the required maximum limit remains.

Since in the method according to the invention, the reheating of the gas before the dry There is no methanation after the condensation of water vapor, so expensive heat exchange devices are used saved. Due to the lower pressure drop in the process according to the invention the discharge pressure of the end gas increases, creating energy in the compression that is usually required for the Tax is saved in the distribution network. It is also used to set the inert gas content of the end gas the leaching of excess carbon dioxide is required, the increased gas pressure promotes the CO2 absorption especially when using physically acting detergents. If the invention Methanation operated at such a high pressure that the end gas is released directly into the gas network can be, then at the same time the pressure in the preceding gasification for generating the raw gas be chosen lower. In this case, the energy saving benefits the gasification plant where the Gasification agents oxygen and water vapor are only made available at lower pressure to need.

It is also advantageous that the process according to the invention manages with a smaller volume of catalyst than would be required when using a wet and a dry post-methanation stage. A catalyst which contains 40 to 65% by weight of nickel on a support material which is resistant to steam is recommended for methanation. If the methanation carried out in two stages, Umseizungstemperaturen are from 250 to 500 ⁰ C in the first and from 250 to 400 ⁰ C in the second stage expedient.

An example circuit for the method according to the invention is explained with the aid of the drawing shows

Fig. 1 shows the two-stage methanation of the purified Raw gas and

F i g. 2 the temperature profile in the last methanation stage in the method according to FIG. 1.

In the procedure of FIG. 1, the raw gas purified of catalyst poisons, which contains predominantly carbon monoxide, carbon dioxide and hydrogen, is mixed in line 4a with the recycled product gas in line 7. The raw gas coming from the gas scrubbing is not shown with a temperature below 200 ° C and usually at most 100 ⁰ C. Has the Rectisol process used for gas washing, so the exit temperature of the gas is about 20 ° C.

The mixture is fed to the methanation reactor in line 7a 6 abandoned. The reactor 6 can, as shown schematically in the drawing, as a shaft reactor be formed with a hatched catalyst filling 6a. The methanation catalyst contains nickel as an active component, preferably in a proportion of 40 to 65% by weight, in one water vapor resistant carrier material. As a carrier material comes z. B. aluminum oxide, aluminum silicate, magnesium silicate, Magnesium spinel or mixtures of this compound in question. Notwithstanding FIG. 1 can do that Catalyst material 6a also subdivided into several layers or onto several one behind the other Reactors distributed.

Part of the product gas from the reactor 6 is cooled in the waste heat boiler 8; uncooled product gas is branched off in line 9 and part of the cooled product gas in line 7 with the aid of the compressor 10 returned in the cycle. Behind the waste heat boiler 8, a product gas partial flow is also withdrawn in line ί 1 and mixed with the gas in line 9, to set the desired temperature of the gas in line 12 in the last methanation stage (Reactor 30) is given.

The heat withdrawn from the product gas in the waste heat boiler 8 is used to generate water vapor, which in the line 13 is fed to a steam drum 14, which receives feed water supplied in the line 15, which has been preheated in the heat exchanger 31 and comes from the line 32. The water in the drum 14 is led in line 16 to waste heat boiler 8. Excess steam is the steam drum 14 taken through line 17. This steam is standing

ζ. B. as a gasification agent for pressurized gasification Generation of the raw gas available.

It is not absolutely necessary, but advantageous in practice, to pass an adjustable partial flow of the product gas in line 12 uncooled in line 33 past the heat exchanger 31 and to mix this partial flow with the cooled product gas partial flow in order to set the inlet temperature of the gas for the reactor 30 . The precooled product gas in the line 34 enters the reactor 30 and branches there to channels 35 which are formed between the catalyst containers 36. The catalyst containers 36 are open at the upper end, so that there the gas to be converted can penetrate unhindered into the catalyst material 5, as is indicated by the bent flow path A1. The pressure gradient required for this in the comparison of the gases in lines 34 and 39 is present in the system.

The catalyst material 5 in the containers 36 sits on grids 37, under each of which there is a gas collecting space 38 remains free. The gas collection spaces 38 of the various containers are what is not in the drawing is shown, interconnected so that the product gas formed in the various containers can be withdrawn together in the line 39.

The product gas in line 39 is cooled in the cooling stage 20, which is also only shown schematically, where air cooling and water cooling usually complement each other. This is done via line 21 Cooling condensate drawn off. The product gas in line 22 becomes a if necessary Post-treatment, not shown, to remove excess carbon dioxide and for drying fed

The catalyst containers 36 each contain shields 40 in their upper region, which the heat transfer largely hinder. As a result, the catalyst material in this area is replaced by the in the Channels 35 upwardly flowing gas is hardly cooled, and the temperature of the gas to be converted rises with increasing depth of penetration into the catalyst material. However, the cooling is for the Catalyst area between the grids 38 and the lower limit of the shields 40 effective, so that there the temperatures towards the grids 38 decrease again. The uncooled catalyst section is about 0.2 to 0.4 times the height of the catalyst bed.

In the method according to FIG. 1, the product gas of the cycle methanation enters the line 34 with the easily adjustable temperature C, compare FIG. 2, and heats up during upward flow through the channels 35 until the temperature H at the bottom of the shields achieved in the reactor 30 40 The flow path along this shielding does not bring further increase in temperature, which is taken into account in the curve section to the point K with the At a temperature K of about 300 ^° C., the product gas then enters the catalyst container 36 from above and flows there through the catalyst material to the grates 38, while the methanation reactions are taking place. The converted gas mixture, the temperature initially rises to the point L, as 40 does not heat of the exothermic formation of methane is discharged on the basis of the shields Thereafter, the temperature falls through the indirect cooling re gradually to the point M back from which the temperature of the line 39 The method of FIG. 1 is very flexible because the inlet temperature of the gas to be converted in the line 34 can be adjusted in a simple manner, which z. B. can be done with the help of the bypass line 33. The cooling effect of the gas in the channels 35 can also be adjusted with the temperature at which the gas enters the reactor 30. This inlet temperature thus also has an influence on the final temperature of the converted gas (point M) da M of the

ίο Inlet temperature of the product gas introduced depends. Due to the flexible cooling, the respective activity of the catalyst can be dependent on it Age should be optimally taken into account.

In the following numerical example, three different purified raw gases are considered during the conversion into gas that can be exchanged with natural gas according to the method according to the invention. Gases I and II originate from compressed coal gasification with oxygen and water vapor, with the volume ratio H2: (3 CO + 4 CO ₂ ) = 0.6 in gas 1, referred to as the stoichiometric number, and 1 in gas 11. Gas III comes from the thermal gasification of heavy residual oil (bunker C oil) at temperatures between 1200 and 1400 ^° C. with oxygen and water vapor.

example

The gases I, II and III go through the process circuit of Fi g. 1. There are 1000 kmol each in line 4a These gases are available at a temperature of 30 ° C and a pressure of 25 bar, their composition is that of Table 1

Table 1

Gas I. Gas II Gas 111 (Mole percent) CO ₂ 8.00 3.00 8.00 40 CO 16.00 17.20 27.30 H ₂ 59.10 62.40 69.20 CH ₄ 16.50 17.00 0.30 Inert 0.40 0.40 0.20

4300/4650/6000 kmol of moist gas in line 7 with a temperature of 345/340/330 ° C are the Gas mixed in with line 4a

Through conduit 7a to flow to the reactor 5300/5650/700 kmol moist gas with 300 ⁰ C,

dry gas) 03354 03585 0.5129

. The catalyst in the reactor 6 comprises 58 wt ° / o nickel on an alumina carrier, the bed height is 2.65 m, the product gas of the reactor 6 each has a temperature of 460 ⁰ C and the indicated in Table 3:

the Z, usi immense setting g lot laoen e ι an. I. Tabel 2 I.
,1
P. Gas I. Gas Il Gas III I CO ₂ (Mole percent) 12.08 3.41 20.46 I. CO (Mole percent) 4.31 4.24 5.19 I. H ₂ (Mole percent) 20.75 23.96 21.67 I. CH ₄ (Mole percent) 62.09 67.58 52.17 I. Inert (Mole percent) 0.77 0.81 0.51 I. H ₂ O (mol / mol 1

Table 3

Gas I.

Gas 11

Gas I.

CO ₂ (mole percent) 13.44

CO (mole percent) 0.41

H ₂ (mole percent) 7.92

CH ₄ (mole percent) 77.33

Inerts (mole percent) 0.90

H ₂ O (mol / mol

dry gas) 0.4475

3.54 23.89

0.14 0.47

11.78 8.58

83.60 66.47

0.94 0.59

0.4721 0.6542

In the waste heat boiler 8, the major part of the cooled gas from the reactor 6 on 335/330/320 ⁰ C. The gas withdrawn in line 12 in an amount (moist) of 643.0 / 627.4 / 557.3 kmo! and a temperature of 350 ⁰ C is cooled in the feedwater heater 31 is further to 195/175/195 ° C, wherein the influent in the line 32 at 105 ⁰ C feedwater to 175/175 heated / 150 ° C and in the conduit 15 Steam drum 14 is supplied. 14,500 / 15,600 / 19,200 kg of water are evaporated by the heat quantities given off in the waste heat boiler 8 and feedwater preheater 31 and are available in line 17 as saturated steam at a pressure of 70 bar.

Since the bypass line 33 is not used in the example, the temperatures of the gases in the line 34 are 195/175/195 ° C. (point C in FIG. 2). In the channels 35 of the reactor 30, the gas is ^{heated up to a temperature of 300 °} C. in each case (point H in FIG. 2) at the beginning of the shields 40. In the heat-insulated flow path of the cooling gas does not heat can be exchanged practically, so that the gas passes and with the above-mentioned temperature of 300 ⁰ C in the catalyst 5 (point K of Fig.2). The bed height of the catalyst 5 and thus the flow path to be taken into account is again 4 m in all cases; the same catalyst as in the reactor 6 is used. In the uncooled flow path in the catalyst, which is 1 m long, the temperature rises to the maximum value (point L in Figure 2) to 345/350/345 of ⁰ C. The gas to be converted still contains a remaining hydrogen content of 2.1 / 4.5 / 2.6 mol percent

correspond to those that are used in a conventional process with a single-stage wet post-methanation could be achieved.

In the cooled part of the catalyst 5, the temperature of the gases decreases steadily as they continue to flow through to the grates 37 until the end temperature (point M in FIG. 2) is ^{finally reached at 250 °} C. for the converted gases I, II, III. The gas with this final temperature is available in line 39. In the final cooling 20, water is condensed, and the product gas leaving the methanation through line 22, each with a pressure of 20.5 bar and a temperature of 30 ^° C., has the composition of Table 4:

^1J table 4th (Mole percent) Gas I. Gas Il Gas III (Mole percent) 12.87 1.22 24.18 CO ₂ (Mole percent) 0.01 0.01 0.01 ²⁰ CO (Mole percent) 0.83 1.75 0.90 H ₂ (Mole percent) 85.32 95.98 74.27 CH ₄ Water vapor (mol / mol 0.97 1.05 0.64 Inert ²⁵ dry gas) 0.0002 0.0002 0.0002

For gases I and III is usually the

Washing out of excess carbon dioxide required in the final wash, not shown, whereby End gases of the following qualities can be achieved, which are specified in Table 5:

Table 5

Gas I.
(Mole percent)

Gas 111

CO ₂
CO

CH ₄
Inert

0.50
0.01
0.95
97.43
1.11

0.50
0.01
1.17
97.47
0.85

The gas II can be used without CO ₂ scrubbing.

For this purpose 2 sheets of drawings

Claims (1)

Claim: Process for the production of methane of a quality that is suitable for exchange with natural gas from raw gases containing hydrogen and carbon oxides from the gasification of coal, tars or heavy residual oils under increased pressure, which are cooled and cleaned of catalyst poisons, especially sulfur compounds, and a at least two-stage methanation on nickel catalysts with steam at pressures of 5 to 100 bar and temperatures in the range of 200 to 500 ° C are subjected, the product gas of the previous methanation stage in the last methanation stage on an indirectly cooled, fixed catalyst without recirculation of product gas is, characterized in that the product gas of the previous methanation stage with (based on dry gas) at least 60 vol .-% methane and unchanged water vapor content of 0.25 to 0.7 kg / Nm ^{3 of} gas is passed into the last methanation stage, which Gas z initially used as cooling gas and then passes through the catalyst in countercurrent and that the inlet area of the catalyst is reduced over a distance of 0.2 to 0.4 times the total flow rate of the catalyst, a maximum temperature of 345 to 350 ° C. being reached in the catalyst will.