DE2401617C2 - Process for producing a methane-containing gas by low-temperature steam reforming - Google Patents

Description

The invention relates to a process for producing a methane-containing gas in a low-temperature steam reforming process in which a hydrocarbon starting material containing a hydrocarbon having at least two carbon atoms per molecule and steam preheated to a temperature of 250°C to 600°C are introduced into (at least) one steam reforming reactor packed with a supported, hydrogen-activated nickel- and magnesium-containing catalyst with a solid solution of nickel oxide and magnesium oxide produced by calcination, and the hydrocarbon material is reformed under a reaction pressure of at least 101325 Pa and at a catalyst bed temperature (in the reactor) of 300°C to 600°C.

Steam reforming catalysts with a solid solution of nickel oxide and magnesium oxide and fused cement components are known from FR-PS 20 94 210. The solid solution of nickel oxide and magnesium oxide is produced by calcination at a temperature of 800° to 1300°C. At such high calcination temperatures, solid solutions are formed whose nickel component is difficult to activate by reduction. In contrast, the catalysts used according to the invention, whose solid solution of nickel oxide and magnesium oxide was produced at temperatures below 800°C, can be activated by hydrogen at much lower temperatures (than the known catalysts).

The use of α- aluminum oxide as a catalyst support is already known from DE-OS 20 08 936, but the known catalysts are used in the context of high-temperature steam reforming of gaseous hydrocarbons to form hydrogen-containing gases. The suitability of α -aluminum oxide in this regard cannot be taken as a basis for the conclusion that it is suitable as a support for nickel-magnesium oxide catalysts with a solid solution of NiO-MgO in the context of a low-temperature, high-pressure steam reforming process for producing methane-containing gases.

The invention was therefore based on the object of creating a process for producing a methane-containing gas, in particular a methane-rich gas, by, for example, adiabatic steam reforming of hydrocarbon starting materials in the presence of a novel nickel catalyst, during which the catalytic properties of the nickel catalyst used are retained over a long period of time.

This object is achieved according to the invention in the process for producing a methane-containing gas of the type mentioned at the beginning by carrying out the reforming in the presence of the hydrogen-activated catalyst containing nickel and magnesium with an atomic ratio Ni:Mg of 0.05 to 7.0 and a content of a solid solution of nickel oxide and magnesium oxide produced by calcination at a temperature of 350°C to 800°C, which is applied to an α- Al₂O₃, SiC , α-quartz and/or ZrO₂ carrier.

An advantageous embodiment of this process consists in introducing the hydrocarbon feedstock and the steam into (at least) one steam reforming reactor in a ratio H₂O/C (mol/atom) of 0.9 to 5.0 and operating at a reaction pressure of 981 kPa to 9810 kPa (absolute) and at a catalyst bed temperature of 400° to 570°C.

Steam reforming catalysts with a solid solution of nickel oxide and magnesium oxide and fused cement components are known from FR-PS 20 94 210. The solid solution of nickel oxide and magnesium oxide is produced by calcination at a temperature of 800° to 1300°C. At such high calcination temperatures, solid solutions are formed whose nickel component is difficult to activate by reduction. In contrast, the catalysts used according to the invention, whose solid solution of nickel oxide and magnesium oxide was produced at temperatures below 800°C, can be activated by hydrogen at much lower temperatures (than the known catalysts).

As hydrocarbon feedstock with at least two carbon atoms per molecule, refinery exhaust gases, light natural gas (LPG), light gasoline, heavy gasoline, kerosene and the like. These hydrocarbon feedstocks are introduced into one or more reactors together with steam in a mixing ratio of steam to hydrocarbons (H₂O/C, ie number of moles of steam relative to one carbon atom in the hydrocarbon feedstock) of suitably 0.9 to 5.0.

The catalysts usable in the process according to the invention can be represented as follows: Ni-solid solution of nickel oxide and magnesium oxide (hereinafter referred to as NiO-MgO) support at a time when the catalyst in question has been activated by reduction (NiO and/or MgO can also be present in a form other than in the form of a solid solution) and wherein the MgO is an active magnesium oxide capable of accelerating the low-temperature steam reforming in question.

Examples of support materials usable for the catalysts (b) and (d) are inorganic refractory supports which do not undergo chemical reaction with the other catalyst components, namely the nickel component and the magnesium oxide component, during the reforming reaction, for example, α -Al₂O₃, SiC, α -quartz, ZrO₂ and the like. In this connection, it should be noted that materials known as supports of conventional catalysts for low-temperature steam reforming reactions, such as γ- Al₂O₃, silica gel, kaolin and the like, are unsuitable as supports for the catalysts of the invention.

The use of α- aluminum oxide as a catalyst support is already known from DE-OS 20 08 936, but the known catalysts are used in the context of high-temperature steam reforming of gaseous hydrocarbons to form hydrogen-containing gases. The suitability of α- aluminum oxide in this regard cannot be used to conclude that it is suitable as a support for nickel-magnesium oxide catalysts with a solid solution of NiO-MgO in the context of a low-temperature, high-pressure steam reforming process for producing methane-containing gases.

The term "solid solution" here and in the following means the same as it usually means in inorganic chemistry. Such a solid solution can be analyzed by X-ray diffraction and the like. For example, in the case of X-ray diffraction analysis with CuK α, the peak of the NiO-MgO solid solution - although varying according to the Ni:Mg ratio - always lies between the peak of MgO and the peak of NiO.

Now, it is generally known that in catalyst compositions containing a nickel component and a magnesium oxide component for use in high-temperature steam reforming reactions, a solid solution of nickel oxide and magnesium oxide is readily formed as a result of partial oxidation upon calcination at temperatures in the range of 700° to 1200°C, particularly in the range of 1000° to 1200°C. However, since a catalyst obtained in this way in the form of a solid solution has poor activity and its activity deteriorates greatly over time, it has hitherto been considered necessary, with a view to further using such catalysts in a variety of reactions, to convert the nickel oxide contained therein in the form of a solid solution into metallic nickel in advance by subjecting the catalyst in question to sufficient reduction treatment with gaseous hydrogen and the like.

In contrast, in the case of the nickel-containing catalysts used in accordance with the invention, it is imperative that the solid solution in question is still present at a time after activation (of the catalyst) by reduction treatment. Such nickel-containing catalysts containing a solid solution can be used in a highly effective manner in the context of a low-temperature steam reforming process according to the invention. In particular, at high pressures in the range of 4905 to 9810 kPa (absolute), they can fully develop their valuable properties, i.e. their high activity and high stability. The atomic ratio of nickel to magnesium in the catalysts usable in accordance with the invention is 0.05 to 7.0. In contrast to the above-mentioned proposal to use nickel- and magnesium-containing catalysts with an atomic ratio Ni:Mg of 0.5 to 5.0, a wider range for the atomic ratio Ni:Mg can be maintained in the catalysts used in accordance with the invention, since all or most of the magnesium oxide contained therein has been converted into a solid solution.

If the Ni:Mg atomic ratio is less than 0.05, sufficient catalytic activity at low temperature cannot be expected. If the Ni:Mg atomic ratio exceeds 7.0, carbonaceous material will be deposited on the catalyst surface as the reforming reaction progresses, thereby impairing its catalytic activity.

• 1. Da das Magnesiumoxid, welches das einen Reaktionsteilnehmer bildende H&sub2;O-Molekül aktivieren soll und gemeinsam mit dem den aktiven Bestandteil bildenden metallischen Nickel vorhanden ist, bereits in Form einer festen Lösung vorliegt, findet während der Dampfreformierungsreaktion, insbesondere der unter hohem Druck durchgeführten Dampfreformierungsreaktion, kein merkliches Kristallitwachstum statt. Dies ist auf die Stabilität der festen Lösung zurückzuführen.
• 2. Ein eine feste Lösung enthaltender Katalysator entfaltet gegenüber H&sub2;O eine stärkere Aktivität als ein keine feste Lösung enthaltender Katalysator. Bei Verwendung eines Katalysators ohne feste Lösung erhöht sich die Reaktionsgeschwindigkeit selbst bei Erhöhung des Partialdrucks von H&sub2;O nicht wesentlich. Bei Verwendung eines Katalysators mit einer festen Lösung führt eine Erhöhung des Partialdrucks von H&sub2;O zu einer drastischen Erhöhung der Reaktionsgeschwindigkeit, so daß ein eine feste Lösung enthaltender Katalysator im Rahmen einer unter hohem Druck arbeitenden Dampfreformierungsreaktion sehr nützlich ist.

The reasons why the presence of a solid solution in the catalysts used in low-temperature steam reforming processes according to the invention is advantageous, in contrast to previous ideas, are not yet fully understood. However, the two factors described below certainly contribute to this:

• 1. Since the magnesium oxide, which is intended to activate the H₂O molecule constituting a reactant and is present together with the metallic nickel constituting the active ingredient, is already in the form of a solid solution, no appreciable crystallite growth takes place during the steam reforming reaction, especially the steam reforming reaction carried out under high pressure. This is due to the stability of the solid solution.
• 2. A catalyst containing a solid solution exhibits a stronger activity toward H₂O than a catalyst not containing a solid solution. When a catalyst without a solid solution is used, the reaction rate does not increase significantly even if the partial pressure of H₂O is increased. When a catalyst containing a solid solution is used, increasing the partial pressure of H₂O dramatically increases the reaction rate, so a catalyst containing a solid solution is very useful in a high-pressure steam reforming reaction.

In principle, any conventional process is suitable for producing the catalysts which can be used according to the invention, provided that the catalysts obtained meet the stated requirements.

Processes for the preparation of catalysts which can be used according to the invention are, for example:

1. Process for the preparation of a catalyst by coprecipitation

By mixing an aqueous solution of a nickel salt, an aqueous solution of a magnesium salt and an aqueous solution of an alkali metal compound, taking into account an atomic ratio Ni:Mg of 0.05 to 7.0 and a carrier content of less than 70% by weight, a precipitate of nickel and magnesium hydroxide or basic nickel and magnesium carbonates is produced. After aging the precipitate during prolonged stirring of the suspension, the precipitate is filtered off and washed. It is then dried and formed into tablets of a size of 0.5 x 20 mm with the aid of an admixed molding aid. These are then calcined at a suitable temperature to obtain a nickel-containing catalyst with a solid solution of nickel oxide and magnesium oxide.

To apply the catalyst to a support, it is added to the solution mixture in an amount of less than 70 wt.%, based on the total amount of catalyst, and the whole is then worked up in the manner described.

2. Process for the preparation of a catalyst by precipitation work

An active MgO powder and corresponding solutions of nickel and alkali metal compounds as in the process described under 1. are mixed, taking into account an atomic ratio Ni:Mg in the finished catalyst of 0.05 to 7.0, whereby a precipitate of nickel hydroxide or basic nickel carbonate is deposited on the MgO surface. The entire precipitate is then filtered off and worked up in the manner described under 1. The precipitate thus obtained is dried, shaped and calcined, whereby a nickel-containing catalyst with a solid solution of nickel oxide and magnesium oxide is obtained.

To apply the catalyst to a support, the support is combined with the precipitate after filtration and washing.

3. Preparation of a catalyst by a wet mixing process

The same solutions of nickel and alkali metal compounds as in the process described under 1 are mixed, whereby a nickel hydroxide or basic nickel carbonate precipitates. The precipitate obtained is then worked up in the manner described under 1. It is then mixed with active MgO powder in such an amount that the Ni:Mg atomic ratio in the finished catalyst is between 0.05 and 7.0.

To apply the catalyst to a carrier, it is combined with the mixture of precipitate and MgO powder and the whole is then processed in the manner described.

4. Manufacturing a catalyst by contract work

When preparing a catalyst mass according to one of the processes described under 1., 2. and 3., it is either immersed in an aqueous solution of a nickel salt or mixed with the precipitate of the hydroxides or basic carbonates of nickel and magnesium obtained in the process described under 1. The mixture obtained is then dried again, shaped and calcined, whereby a catalyst containing nickel and magnesium oxide is obtained with a nickel component which can be easily converted into metallic nickel by reduction and a content of a solid solution.

Calcination conditions

At high calcination temperatures and long calcination times, a solid solution is easily obtained. However, with excessive calcination, the finished catalyst becomes almost impossible to reduce, which is undesirable for reasons of catalytic activity.

When producing a catalyst according to the process described under 4., no problems of this kind arise. When producing a catalyst according to one of the processes described under 1., 2. and 3., the mixing state between a nickel salt and a magnesium salt becomes worse during the implementation of the process in question, so that higher temperatures or longer calcination times are required for correct calcination. Furthermore, catalysts produced according to the process described under 1. require calcination temperatures of over 700°C, depending on the starting salts and the precipitation conditions.

The calcination temperatures to be observed in the preparation of catalysts by the processes described under 1. to 4. should expediently be in the range from 350° to 800°C, preferably from 600° to 800°C.

The low-temperature steam reforming process according to the invention is started when a nickel/magnesium oxide catalyst prepared according to one of the processes described under 1. to 4. is introduced into (at least) a steam reforming reactor and activated by passing hydrogen as a reducing gas stream at a temperature of 300° to 600°C, preferably 400° to 500°C.

When the catalyst is fully activated, the hydrogen supply is shut off, whereupon a mixture of the respective hydrocarbon starting material and steam in a mixing ratio (H₂O/C - mole/atom) of preferably 0.9 to 5.0 is passed through the catalyst bed after preheating to a temperature of 250° to 600°C and is steam reformed there at a temperature of 300° to 600°C, preferably 400° to 570°C, to a methane-containing gas.

The methane-containing gas formed in this process usually contains, based on dry weight, more than 40% methane plus accompanying gaseous substances such as hydrogen, carbon monoxide and carbon dioxide.

If necessary, a gas containing even more methane can be produced by feeding the gas stream emerging from the reactor(s) to at least one methanation reactor for methanation of the carbon oxides contained in the gas stream. The methane-containing gas obtained is primarily suitable as a heating gas.

The following examples are intended to illustrate the process according to the invention in more detail.

For better comparison, the tests in the following examples 1 to 4 were carried out under the same conditions (explained in more detail below). Of course, other conditions can also be used, provided the desired result is achieved.

Firstly, all of the catalysts prepared according to the instructions in the following examples were activated by reduction for 10 hours in a stream of hydrogen gas at 500°C. After 1 g of each of the activated catalysts had been introduced into a reactor, a mixture of naphtha with an IBP of 33°C, an FBP of 122°C, a specific gravity d 4;¹⁵ of 0.675, a sulfur content of 0.2 ppm and an average of 6.04 carbon atoms and 13.4 hydrogen atoms per molecule at a rate of 35 g/st together with steam preheated to 500°C in a H₂O:C (mol/atom) ratio of 2.0 and steam reformed at a reaction temperature of 500°C and a reaction pressure of 6867 kPa (absolute) ("H") and 981 kPa (absolute) ("N"), respectively. The initial activity of each catalyst was determined.

Further, 25 cm3 of all activated catalysts were charged into a pressure reaction tube and the same naphtha feedstock (as used in determining the initial activity of the catalysts) was fed thereto at a rate of 30 g/hr. Steam reforming was carried out for 500 hours under the same conditions as those used in determining the initial activity of the catalysts. A portion of the catalysts was then removed from the respective reaction tubes and used for property determination. The rest of all catalysts were again charged into the reaction tubes and used for another 500 hours of steam reforming under the same conditions. Then all catalysts were removed from the reaction tubes. A portion of all catalysts was used as a sample for property determination. 1 g of all catalysts was again charged into reactors to determine the catalytic activity after 1000 hours of use under the same conditions as those used in determining the initial activity.

The determination of the "properties" was used to determine the crystallite growth by X-ray diffraction. The measurement of the initial activity and the determination of the activity after 1000 hours of use were used to determine the degree of conversion of the naphtha starting material into a gas containing methane, hydrogen and carbon oxides.

The quality of the individual catalysts can be evaluated based on the results of these tests.

To facilitate comparison, the experiments to determine the catalytic activity were carried out by controlling the conversion during steam reforming at a temperature of 500°C.

In the case where the steam reforming was carried out under conditions such that 100% conversion of the starting hydrocarbon material (naphtha) was achieved, i.e. in the case where the amount of catalyst relative to the feedstock was increased beyond a given amount, the gas formed in the dry state at the reactor outlet had approximately the following composition:

During steam reforming under a reaction pressure of 6867 kPa (absolute) ("H")
CH₄ about 65 vol.%; H₂ about 12.5 vol.%
CO about 0.5 vol.%; CO₂ about 22 vol.%

During steam reforming under a reaction pressure of 981 kPa (absolute) ("N")
CH₄ about 56 vol.%; H₂ about 21 vol.%
CO about 1 vol.%; CO₂ about 22 vol.%

example 1

When the various catalysts prepared according to the process described under 1. were used in the described experiments under the conditions summarized in Table II below under "A", the results summarized in Table II below under "B" were obtained. SiC was used as the carrier. Table I &udf53;vu10&udf54;&udf53;vz30&udf54; Table I (continued) &udf53;vu10&udf54;&udf53;vz30&udf54;&udf53;vu10&udf54;

From paragraph "B" of Table I it is clear that even in the case of a catalyst with a support portion, a solid solution could be obtained. Such a solid The catalysts containing the solution were very stable. In the case where the proportion of the support in the catalyst was 80 wt.%, not only was the initial activity insufficient, but also the stability was very poor.

Example 2

When the various catalysts prepared according to the process described under 2. under the conditions summarized in Table II below under "A" were used in the experiments described, the results summarized in Table II below under "B" were obtained. α- Al₂O₃ was used as the carrier component. Table II &udf53;vu10&udf54;&udf53;vz37&udf54;&udf53;vu10&udf54;

As can be seen from paragraph "A" of Table II, a solid solution could be prepared even if the preparation conditions were slightly modified, provided that the preparation conditions such as the calcination temperature and the like were appropriately selected. The catalysts containing a solid solution obtained thereby had a very stable activity.

Example 3

In Examples 1 and 2, SiC and α- Al₂O₃ were used as the carrier component. In the present example, experiments were carried out using other carrier components. The catalysts using these carrier components were prepared under the conditions given in paragraph "A" of Table III. The property "MgO efflux ratio" given in paragraph "B" of Table III represents the numerical value obtained by determining the weight ratio of MgO effluxed into the water from each of an unused catalyst and a catalyst used for 1000 hours after suspension in water into which gaseous carbon dioxide was continuously introduced at a pressure of 60 mm H2O and multiplying the weight ratio by 100. Table III &udf53;vu10&udf54;&udf53;vz33&udf54; Table III (continued) &udf53;vu10&udf54;&udf53;vz33&udf54;&udf53;vu10&udf54;

The carrier materials used were those listed in the following compilation. °=c:130&udf54;&udf53;vu10&udf54;&udf53;vz12&udf54;&udf53;vu10&udf54;

As is clear from paragraph "B" of Table III, catalysts b-16, b-17, b-18 and b-19 containing an inert carrier usable in the present invention exhibited a minor crystallite growth, a high MgO efflux ratio and a very stable catalytic activity in the process of the present invention. On the other hand, catalysts b-20 and b-21 did not have such favorable properties despite containing a solid solution.

Example 4

Energetic studies were conducted to determine the preferred range of the atomic ratio of the nickel component to the magnesium oxide component (Ni/Mg) in the nickel and magnesium oxide containing catalysts useful in the present invention. The results given in Table IV below were obtained.

The catalysts b-22 and b-23 were prepared by first preparing a solid solution with an atomic ratio Ni:Mg of 0.15 according to the method described under 1., then suspending the resulting solid solution in an aqueous magnesium nitrate solution in such a way that the weight ratio of solid solution to MgO (originating from the magnesium nitrate) of 3:7 was achieved, then a precipitate was precipitated by adding Na₂CO₃ to the resulting suspension and then the precipitate after drying, it was immersed in an aqueous nickel sulfate solution to impregnate it with 1 or 3 wt.% nickel oxide. Catalysts with the corresponding ratio of the components were obtained.

The catalysts described can also be prepared directly by the process described under 1. However, in the catalysts prepared in this way, the atomic ratio Ni:Mg is so low that the peak of the solid solution content in the catalysts cannot be easily determined using the X-ray diffraction equipment currently in use. In the present case, however, the catalysts were prepared in the manner described above.

Catalysts b-24 and b-25 of Table IV were prepared according to the procedure described in 1. The other preparation conditions are given in paragraph "A" of Table IV. Table IV &udf53;vu10&udf54;&udf53;vz32&udf54;&udf53;vu10&udf54;

From Table IV, it is apparent that catalyst b-22, whose Ni:Mg atomic ratio was less than 0.05, had both poor initial activity and poor stability and was unsuitable as a steam reforming catalyst despite the presumed presence of a solid solution. Catalyst b-25, whose Ni:Mg atomic ratio was above 7.0, was approximately satisfactory in its initial activity but tended toward very poor stability. Catalyst b-23, whose Ni:Mg atomic ratio was adjusted to above about 0.05, did not show particularly good initial activity but had superior stability. Its practical performance was satisfactory. Catalyst b-24, whose Ni:Mg atomic ratio was adjusted to below about 7.0, showed excellent initial activity and satisfactory stability.

For comparison with the measurement results presented in the above examples, corresponding tests were carried out with catalysts known from FR-PS 20 94 210. The catalysts used had the same Ni and Mg content as the catalysts b-15 and b-15a; however, they also contained alumina cement and were calcined at temperatures above 800°C. Table V &udf53;vu10&udf54;&udf53;vz21&udf54;&udf53;vu10&udf54;

As the comparison with the catalysts b-15 and b-15a shows, these show considerably higher conversion values than the known catalysts with the same content of NiO+MgO and the same Ni/Mg atomic ratio.

Claims (3)

1. A process for producing a methane-containing gas (as part of a (low-temperature steam) reforming), in which a hydrocarbon starting material containing a hydrocarbon with at least two carbon atoms per molecule and steam preheated to a temperature of 250°C to 600°C are introduced into (at least) one steam reforming reactor packed with a hydrogen-activated nickel- and magnesium-containing catalyst applied to a carrier with a solid solution of nickel oxide and magnesium oxide produced by calcining, and the hydrocarbon material is reformed under a reaction pressure of at least 101325 Pa and at a catalyst bed temperature (in the reactor) of 300°C to 600°C, characterized in that the reforming is carried out in the presence of the hydrogen-activated nickel- and magnesium-containing catalyst with an atomic ratio Ni:Mg of 0.05 to 7.0 and a content of a catalyst obtained by calcining at a temperature of 350°C to 800°C produced solid solution of nickel oxide and magnesium oxide, which is coated on an α- Al₂O₃, SiC, α -quartz and/or ZrO₂ support. 2. Process according to claim 1, characterized in that the support accounts for less than 70% by weight based on the total weight of the catalyst. 3. Process according to claim 1, characterized in that the hydrocarbon starting material and the steam are introduced into (at least) one steam reforming reactor in a ratio H₂O/C (mol/atom) of 0.9 to 5.0 and that the reaction is carried out at a reaction pressure of 981 kPa to 9810 kPa (absolute) and at a catalyst bed temperature of 400° to 570°C.