DE1545462C3 - Process for the production of gases with a methane content of at least 50 percent by volume - Google Patents

Description

The invention relates to a process for the production of gases with a methane content of at least 50 percent by volume by splitting hydrocarbons with an upper boiling limit of up to 350 ° C and an average carbon number of at least 10 to at most 20 with steam and hydrogen under increased pressure in the presence of a Fission catalyst with 20 to 60 percent by weight nickel as the active component and a carrier made of alumina or magnesium aluminum silicate. Such gases are also referred to below as rich gases; they have an upper calorific value of at least 5000 kcal / Nm ³ .

In BE-PS 6 34 920 a process is described, according to which hydrocarbons of the gasoline range with steam at temperatures of 450 to 700 ⁰ C on catalysts, the nickel or cobalt and a metal of the platinum group on an oxidic carrier material, aluminum oxide and oxides or silicates the alkali or alkaline earth metal group can be split at normal or elevated pressure to mehtane-containing gases of town gas quality or at temperatures up to 1000 ⁰ C to synthesis gases which consist of CO and H ₂ and are poor in methane. In particular, to generate town gas, hydrogen can be added to the feed mixture.

From the DT-AS 11 80481 there is a method for generating Methane-rich gases from hydrocarbons with 4 to 10 carbon atoms per molecule are known in which the hydrocarbons mixed with water vapor at normal or increased pressure to 350 heated to 500 ° C and then passed over a nickel-containing catalyst, its temperature adjusts itself to a temperature of 400 to 550 ° C due to the positive heat of reaction. Preferably should pressures between 10 and 25 ata and a ratio of 2 to 5 parts by weight of water vapor to 1 part by weight of hydrocarbons nao.

This process is based on a reaction sequence in which the hydrocarbons are primarily mixed with water vapor converted to carbon monoxide and hydrogen and partly also to methane and carbon dioxide will. The carbon monoxide can still react with existing water vapor after the water gas reaction Form hydrogen and carbon dioxide, but can also with hydrogen to form methane and Carbon dioxide or water vapor react.

ao The hydrocarbons to be split must consist of low-boiling hydrocarbons up to about C ₅ . If a mixture of steam and hydrocarbons, which are predominantly in the range of C ₇ to C ₁₀ , reacted, then this mixture must be preheated to maintain the temperature in the catalyst bed to such an extent that, regardless of the addition of steam, thermal cracking reactions begin, which reduce the reactivity of the Reduce the starting mixture and the activity of the catalyst, polymerizing substances form and finally soot formation occurs.

According to a variant of this known method hydrocarbons with a final boiling point of 165 ° C with steam in the presence of hydrogen over a nickel on a support of alumina-containing catalyst under superatmospheric pressure at temperatures of 450-900 ⁰ C to methane-containing gases split (BE-PS 6 35 755). From FR-PS 13 94 202 a process for rich gas generation is known in which hydrocarbons with an average carbon number of 4 to 15 are reacted with steam and recycled product gas over nickel catalysts. The temperature of the gas mixture to be converted is not below 350 ° C and the reaction takes place at 400 to 600 ° C.

As the preheating temperature of the starting materials rises, the endothermic in the catalyst bed Reactions in which carbon monoxide and hydrogen are formed are favored, leading to a drop in temperature leads in the catalyst bed. However, this favors the relative carbon monoxide excess Boudouard reaction, that of nickel-rich catalysts, is anyway stronger than the water-gas reactions and causes the formation of free carbon (soot) and carbon dioxide.

The higher the average number of carbon atoms in the hydrocarbon mixture used, the more likely it is that the side reactions which are difficult to control and lead to the formation of polymers and soot and which greatly shorten the life of the catalyst occur. Experimental studies have shown that start at a pre-heating and the evaporation of liquid hydrocarbons to _C10 undesirable cracking reactions if the heat-transferring tube walls of the heat exchanger temperatures above 500 ° C accept. The higher the boiling range of the used

Hydrocarbon mixture lie, the lower the temperature limit, above which thermal ~ see cleavage reactions set in.

It is the object of the invention to carry out a method of the type mentioned at the beginning without interference and economically over a long period of time. This object is achieved in that 0.1 to 0.8 Nm ^{3 of} hydrogen per kilogram of hydrocarbons are added to the feed mixture of hydrocarbons and water vapor and that the hydrogen-containing feed mixture is preheated to a maximum of 350 ° C and reacted at a maximum of 450 ° C on the cleavage catalyst. The catalysts used can also contain 0.01 to 0.1% platinum or palladium as an activating component. Since the catalysts are sensitive to sulfur, it is advisable to use a known desulfurization compound, e.g. B. zinc oxide or iron oxide to conduct.

To maintain the preheating temperature, in particular to avoid exceeding the upper limit temperature at which thermal cracks set in, it is advantageous to preheat the already hydrogen-containing feed mixture exchanges heat with that flowing out of the reactor to bring about still hot product gas. In this way it is achieved that the heating medium is not significantly more than 100 ° C hotter than the feed mixture to be heated.

Hydrogen-containing gases which contain at least 20 percent by volume of H ₂ can be used to introduce the hydrogen, for example coke oven gas or refinery off-gas. If necessary, a small partial flow of the crude product gas can be split in a known manner to a high hydrogen content in a tube furnace.

The inventive method also higher boiling hydrocarbons, for example Heating oils or diesel oils, which were previously considered unsuitable for splitting into high-calorific supply Supply gases with the exclusion of oxygen were considered to be accessible to conversion to methane-rich gases made at surprisingly low temperatures. That is a consequence of the relatively low Addition of hydrogen. This measure also allows the water vapor content to be reduced considerably in the feed mixture. This will reduce the dew point in the Product gas significantly reduced. Accordingly, when the gas is cooled, the aqueous condensate falls only at lower temperatures, which is a reduction for a given apparatus material which means the risk of corrosion.

The method can be carried out, for example, with the following arrangement: A mixture of hydrocarbon vapor, water vapor and hydrogen at temperatures of 330 to 350.degree. C. is fed to a cracking reactor in the form of a shaft furnace filled with a catalyst. The product gas from the cleavage reactor is passed through a number of heat exchangers and gas treatment stages for further use. First, the cracking reactor is followed by a steam superheater, in which water vapor is brought to its temperature by a steam generator for the reaction in the cracking reactor. After the steam superheater, the product gas gives off heat in an evaporator, where a mixture of hydrogen and preheated hydrocarbons is evaporated and then mixed with the superheated steam before it enters the cracking reactor. After the evaporator, the product gas is passed through a feed water heater, which is connected upstream of the steam generator and is supplied with degassed feed water preheated to about 100 ° C. The product gas is further from any heat for preheating the hydrocarbons to about 16O ⁰ C, before it is used to preheat the feedwater. The product gas is introduced into the subsequent carbon dioxide _{scrubber at around 160 ° C and has a CO 2} content of 15 to 20 percent by volume. If the gas with this CO ₂ content is recycled, washing can be omitted. Otherwise, the washing is followed by gas cooling and drying.

try

III

IV

Feed hydrocarbon:

Boiling range ( ⁰ C)

Average carbon number per molecule

Reaction conditions:

Pressure (ata)

Preheating temperature ( ⁰ C)

Product gas ^{temperature (0} C)

Water vapor-hydrocarbon ratio (kg / kg) .......

Hydrogen addition per kg of hydrocarbons
(NnWkg)

Product gas composition:

CO ₂ (volume percent)

CO (volume percent)

H ₂ (percent by volume)

CH ₄ (volume percentage)

H ₂ O (Nm »/ Nm3)

Runtime to breakthrough (h)

40 to 10

21 350 420

2.5 0

23.8

0.2 10.0 66.0

1.28 40 to 255
10

21
350
432

2.5
0.10

22.4

0.3
11.6
65.7

1.28

730

190 to 335
16

21
350
425

2.5
0

24.2

0.2

10.1

65.5

1.28

190 to 335 16

21
350
445

2.5
0.20

21.4

0.3
13.5
64.8

1.26

170

Comparative example

The advantages of the method become clear from the above comparative tests and their results. Experiments I and III relate to the cleavage without the addition of hydrogen, experiments II and IV were each carried out with an admixture of hydrogen to the mixture to be converted. In all tests, the catalyst consisted of 40 percent by weight nickel on a magnesium-aluminum-silicate support with the molar composition MgO-Al ₂ O ₃ -SiO ₂ . The throughput was set so that 4000 Nm ^{3 of} rich gas per hour were generated per cubic meter of catalyst.

The breakthrough was registered when higher hydrocarbons with at least 5 carbon atoms per molecule were found in the product gas. In experiment III, the breakthrough was determined at the beginning of the experiment.

Claims (2)

Patent claims: 1. Process for the production of gases with a methane content of at least 50 percent by volume by splitting hydrocarbons with an upper boiling limit of up to 350 ° C and an average carbon number of at least 10 to a maximum of 20 with steam and hydrogen under increased pressure in the presence of a cracking catalyst 20 to 60 percent by weight nickel as the active component and a carrier made of alumina or magnesium aluminum silicate, characterized in that 0.1 to 0.8 Nm ^{3 of} hydrogen per kilogram of hydrocarbon are added to the feed mixture of hydrocarbons and water vapor and that the hydrogen-containing feed mixture is increased to a maximum 350 ° C is preheated and reacted at a maximum of 450 ° C on the cleavage catalyst. 2. The method according to claim 1, characterized in that that the heating of the hydrogen-containing feed mixture to the preheating temperature takes place by heat exchange with the product gas.