NL8102436A - METHOD FOR PERFORMING EXOTHERMAL CATALYTIC REACTIONS IN THE GAS PHASE - Google Patents

Description

Br / B1 / ll / 23

Process for conducting exothermic gas phase catalytic reactions.

The invention relates to a process for carrying out gas phase exothermic catalytic reactions using a cooled reactor containing a solid or fluidized bed of a porous particulate catalyst which is active with respect to the desired reaction.

A large number of catalytic processes carried out in the gaseous phase cause a strong heat development and in many cases this causes a considerable rise in temperature. Examples thereof are the catalytic conversion of alcohols to hydrocarbons according to the so-called Mobil synthesis, of carbon oxides by methanation to methane or according to the Fischer-Tropsch synthesis to petrol and / or olefins (processes involving the so-called water gas reaction); furthermore the ammonia synthesis, the preparation of formaldehyde from methanol, natural gas or other hydrocarbons and the preparation of sulfuric acid vapor and sulfur trioxide from sulfur dioxide.

20 The high temperatures can have a number of drawbacks. In many cases, they can lead to damage or destruction of the catalyst, for example by sintering the active catalyst material or the catalyst pore system. Undesirable side reactions can often occur, for example, decomposition to free carbon in the preparation of hydrocarbons from carbon oxides or alcohols, which can lead to clogging and destruction of the catalyst. In many instances, the higher temperatures can shift the reaction equilibrium and selectivity in an undesired direction; for example, a high temperature in the Fischer-Tropsch synthesis favors the formation of methane at the expense of the desired products, for example, ethane, ethylene, 81 02 4 3 6 - - 2 - 2 - - - or other olefins or gasoline.

Exothermic catalytic processes are often carried out in adiabatic reactors and in such cases attempts are often made to limit the increase in temperature by diluting the reaction components either with gases inert under the reaction conditions or by recirculation of the product gas. A dilution with inert gases entails the cost of the gases as well as the separation of the final product, while the recirculation of product gas entails a loss of energy to be applied for the recirculation compressors.

In other cases, therefore, exothermic processes have been carried out in cooling reactors, so that a dilution and the use of recirculation compressors can be avoided.

✓

As cooling media, air, salt baths, synthetic heat transfer media such as "Dowterm" ®, among others, are often used with boiling water in the above hydrocarbon production processes. In cooled reactors it is possible to achieve a low discharge temperature of the gases formed as a product and, consequently, a favorable reaction equilibrium where the formation of the desired product is favored by a low temperature. In cooled reactors with a catalyst bed, however, it is not possible to avoid a hot zone, a so-called "hot place", shortly after the "ignition" of the reaction, ie a short distance from the catalyst bed feed. where the temperature will often be close to the temperature then determined dynamically by the adiabatic temperature rise. Only then is the reaction gas cooled, that is to say further down the catalyst bed. As a result, the same problems as mentioned above will arise with regard to the stability of the catalyst, possibly selectivity and carbon formation in the case of hydrocarbon reactions.

The problems with regard to the temperature of the hot site have been described in the literature, among others by van Welsenaere and Froment (Chemical Engineering 8102436 f i -3-

Science, Vol. 25, pp. 1503-1516, 1970) and it appears that the hot site is unavoidable in tubular reactors with a constant wall temperature and that the temperature of the hot site is very sensitive to slight variations in the process variables, such as the supply temperature, concentration of the reaction components and wall temperature. There is therefore a chance that the temperature may increase irregularly, leading to what is known as "getting out of hand".

Van Welsenaere and Proment indicate how the reactions can be controlled under certain conditions, for example if they are carried out in a fixed catalyst bed, which is contained in tubes surrounded by a cooling medium, in which case the tube diameter forms a critical factor. . In the said article, Van Welsenaere 15 and Froment give the results for the oxidation of p-xylene to phthalic anhydride under atmospheric pressure and with a large excess of air.

The calculations have been performed for an irreversible first order process, but can also be transferred without significant changes to non-first order reversible reactions. For example, for the methanation reactions there are: (1) CO + 3H2 - - * CH4 + H20 + 49 kcal / mol, (2) 2C0 + 2H2 CH4 + CO2 + 59 kcal / mol, and 25 (3) CO2 + 4H2 - ----- CH4 + 2H20 + 39 kcal / mol, that the diameter of the catalyst tubes can be no more than a few mm, if complete protection against hand-run-out can be achieved under all circumstances. Such a small tube diameter is unsuitable for industrial application. The main difference between the example of Van Welsenaere and Froment and the methanation reactions seems to lie in the high temperature and high molar concentrations in the latter reactions and the resulting high heat production per unit of catalyst feed. Similar conditions apply to other reactions for the formation of hydrocarbons, for example the Fischer-Tropsch synthesis for the preparation of gasoline and / or olefins: 81 02 436 f * * -4-

(4) nCO + 2nH „-λ (CH0) _ + nHo0 + about 50 kcal / g atom C

2 »\. ^ Γ1 δ or the so-called Mobil synthesis for the preparation of hydrocarbons from alcohols, for example (5) nCH2OH -> ^ CS2 ^ n + nI2 ° + about * 2 kcal / .g atom C.

Thus, without special measures, it is not possible to prevent an increase in temperature to the temperature or a value close to this temperature, which is determined by the adiabatic temperature rise for such reactions. Van Welsenaere and Froment propose a dilution of the catalyst filling of the. catalyst bed, for example, with catalytic inert filler bodies, thereby reducing the amount of heat produced per unit of the reactor volume.

However, it has been found that a relatively high degree of dilution of the catalyst is necessary, necessitating an increase in the catalyst volume as such to "ignite" the reaction. Thus, this method has the disadvantage that an increase in the reactor and consequently an increase in investment costs is necessary, and, more importantly, the fact that. the dilution has the drawback of reducing the resistance of the catalyst fill or its adsorption capacity to catalyst poisons entrained with the reaction stream. Such dilution of the catalyst 25 is therefore not a satisfactory solution to the problems of temperature control in cooled exothermic catalytic processes.

For a further investigation of the problems, some tests were carried out in an air-cooled methanation reactor with a catalyst known under the production designation MCR-2.X. This is a microporous, temperature stable, mechanically strong catalyst with nickel crystallites of the same order of magnitude as the pore diameter on a Y-alumina support (see Karsten 35 Pedersen, Allan Skov and JR Rostrup-Nielsen, ACS Symposium, Houston, March 1980). The catalyst was used in the form of cylinders with a height and diameter of 4.3 mm. In some of the runs, the catalyst was diluted with 81 O 2 4 3 6 * 1 * 5 catalytically inactive cylinders of the same geometrical 2 shape. The pressure in the reactor was kept at 26 kg / cm while using a gas feed consisting of 70% (2.9% CO, 10% CO2 and 11% CH 2. These test conditions will produce a thermodynamically determined adiabatic rise in temperature. of 380-400 ° C regardless of the degree of dilution of the catalyst.

It was found that without dilution of the catalyst it was not possible to limit the temperature rise, which therefore reached the aforementioned 380-400 ° C. At dilution ratios of 1: 3 and 1: 5 (i.e., a volume of inert cylinders of 3 and 5 times the volume of the catalyst bodies, respectively), actual temperature increases that were up to 50 ° C less than the 15 expected adiabatic temperature increases of 380-400 ° C.

In these tests, the temperature of the tube wall in the region of the hot site was about 600 ° C or higher, which would be an unrealistically high tube wall temperature in the industrial embodiment because in an industrially cooled reactor, the highest tube wall temperature is no more than 400 ° C would amount. The temperature rise limitation achieved can be considered to be substantial, although as such it is not of great industrial importance, and although the catalyst dilution is not very suitable in this way for the sake of the above considerations.

However, a detailed analysis of the reduced temperature rise showed that the reduced temperature rise was due to the fact that the rate of reaction had not increased to the extent expected from the reaction kinetics and was surprisingly found to reduce this rate of reaction. the reaction rate was due to the fact that the calculated high temperature reaction rate was greater than the diffusion rate of the reaction components through the gas film surrounding the catalyst particles. Since this gas phase diffusion is almost independent of the temperature (activation energy 1-2 kcal / mol), the reaction rate reached a limit, which is caused by the absence of the 81 02 4 3 6-6 reaction components. the actual reaction sites, the interior of the catalyst bodies, thereby preventing the reaction from getting out of hand.

The object of the invention is now to take advantage of this surprising observation and to indicate how one can generally avoid a run-out in the gas-phase catalyzed exothermic reaction in cooled reactors, which contain a bed of a porous particulate catalyst which is active is with respect to the desired reaction. According to the invention this is achieved when each of the individual particles of the catalyst in the outer part has a zone of reduced catalytic activity with respect to the desired reaction.

Thus, the outermost portion of each separate catalyst particle may consist of a completely inert layer or of a layer with reduced activity for the particular highly exothermic reaction in question. In the case of the co-occurrence of a strong exothermic main reaction together with one or more side reactions (which may be less exothermic or even thermally neutral and endothermic) - the outer layer may optionally be catalytically active for the side reaction but not catalytically active for the main reaction? as an advantage it can be mentioned that the above-mentioned reactions (1), (2) and (4), which are catalyzed by nickel, are accompanied by the water gas reaction, also called shift reaction: (6) CO + H 2 O --- * CO2 + H2 + 10 kcal / mol,.

which is catalyzed, among other things, by copper.

If the outer layer is catalytically inert or catalytically active with respect to a side reaction but inactive with respect to the main reaction, the above-mentioned diffusion through the film may be extended to also include diffusion through the inert outer layer of the catalyst. This will make it possible. the maximum speed in the reactor can be determined, because it is determined by the thickness of the reactor. the inert layer. The thickness of the inert layer inert for the main reaction will in practice be one to a few orders of magnitude greater than the thickness of the surrounding 81 0 2 4 3 6 ______

- X

Gas film, which thickness of the gas film is estimated to be about 1 µm under normal industrial conditions by calculation, so that the outer zone of the catalyst particles of the invention is therefore suitably 0.01-2 mm thick can own. In practice, thicknesses of this layer of greater than about 0.5 or 1 mm will only come into contact if relatively large catalyst particles are used.

A general explanation of the invention, as described, is that a given catalyst body with a homogeneous activity and a homogeneous pore system exposed to a given mixture of the reaction components is subject to various reaction limitations when the reaction temperature is changed (elevated) .

At low temperature, the reaction rate ¥ is limited by the catalyst material. The reaction proceeds within the so-called intrinsic velocity range, with virtually no concentration gradient of the reaction components occurring over the pore system of the catalyst body 20. As the temperature increases, the activity of the catalyst increases, leading to an increased gradient of the reaction components across the catalyst bodies. At some point, a temperature range is reached, where the diffusion of the reaction components through the catalyst bodies becomes the limiting parameters for the reaction rate, which effectively means that not all of the catalyst material is utilized in the reaction, so that the efficiency factor is 1 becomes.

As the temperature is further increased, the parameter determining the reaction rate will be the mass transport through the gas film surrounding the catalyst bodies. Normally this speed will be too high to limit the temperature of the hot site by cooling the surface of the reactor to a significantly lower value than the value determined by the adiabatic temperature rise, which in many cases means that thermal sintering cannot be avoided or, in the case of certain methanation reactions, that the carbon formation cannot be prevented.

According to the invention, however, a restrictive surface is now incorporated into the individual catalyst bodies by expanding the gas film or by somehow reducing the penetrating power of the reactants, thereby limiting the rate of reaction and consequently the temperature rise.

It would be expected that incorporation of such an inert layer or "retarding layer" would cause difficulties in starting or "igniting" the exothermic reactions. However, this is not the case because the reaction rate in the ignition zone is small under all conditions, so that the gas diffusion through the inert layer will not be restrictive (determining) of the reaction rate. This layer will only become a limiting factor, that is, it will actually only be effective when the reaction speed becomes so high that it is desired to slow it down. Since the inert layer makes up only a small portion of the catalyst bead, the catalyst's resistance to poisoning is maintained.

In this regard, it may be noted that the outer layer of very small catalyst grains may account for about 50% of its volume, but that the fraction thereof will usually be considerably smaller, for example cylindrical catalyst bodies having a height and diameter of 4. 2 mm usually 1-10%, especially 2-10% and usually 2-5% by volume.

The reactor may be any supercooled reactor for exothermic reactions, for example a tubular reactor or a reactor, which has a larger space of arbitrary shape and contains cold stores through which a cooling medium flows. The catalyst bed will usually be a so-called fixed bed, although the invention can also be applied to fluidized bed catalysts.

As noted, the outer layer must have a reduced catalytic activity and although it will often be effective according to the invention to use a catalyst, the individual particles of which are in the 81 024 3 6? In the outer part having a zone which is inactive with respect to the desired reaction (or main reaction), it may in some cases be useful to use a catalyst in which this outer zone still has a reduced activity for the reaction (or main reaction).

The outer zone can be provided in various ways, often in analogy to the technique known in the pharmaceutical industry for preparing multi-layer tablets of varying composition or varying concentrations of the active substance.

For example, it is possible to first prepare normal-type catalyst bodies consisting of a porous support containing catalytically active material in the pore system. These can be prepared according to a known technique, for example by coprecipitation or by first preparing porous support bodies, which are subsequently impregnated with catalytically active material. The catalyst prepared in this way is then dipped in a gel or sol of inert support material of the same or different type as the type used for the catalyst support as such. If the outer layer is not to be completely catalytically inert but must have a limited catalytic activity, an impregnation can then take place, ensuring that the concentration of the catalytically active material becomes smaller than the concentration in the inner layer. Alternate impregnations, washes, and chemical treatments can be combined to provide a desired structure and combination of structures. On a ready catalyst.

30, an inert layer or even a catalytically active layer can be applied, which partially clogs the openings of the pores, for example by electrolysis or deposition from the vapor phase, resulting in the catalyst activity of the outer layer and the rate of diffusion to the interior of the kata. lysator are reduced. A special structure can be obtained by pelletizing a mixture of disintegrated catalyst particles with an inert support material. This combines the diffusion effect with a dilution of the catalyst, whereby the degree of dilution can be considerably reduced.

Practically suitable embodiments of these methods and combinations thereof will be obvious to a person skilled in the art.

As noted, the described principle can be applied not only in the case of a single exothermic reaction, but also in cases where an exothermic main reaction and one or more side reactions take place simultaneously in the same reactor, which is a bed of a particulate catalyst which is active with respect to the main reaction. In that case, each of the individual particles of the catalyst of the invention may have on the outside a zone of a material which is at least partially inactive with respect to the main reaction, but is catalytically active with respect to one or more side reactions.

A practically suitable embodiment of such a catalyst or in general of catalysts according to the principles of the invention is a catalyst: the particles of which consist of a porous support material which is catalytically inactive with respect to the desired reaction or main reaction (but possibly catalytically active with respect to a side reaction) and in a part of the pore system contains a material which is catalytically active with respect to the desired reaction or main reaction in such a way that the pores of the outer zone of the carrier particles are free from the material, which is catalytically active with respect to the desired reaction or main reaction. The catalytically active material present in the pores can be provided, for example, by coprecipitation, by electrolysis, by deposition from the vapor phase or by impregnation from the liquid phase.

In cases where the catalyst is only catalytically active with respect to one reaction, the catalyst particles of the invention may consist of a porous, inactive material, which is in a portion of the 81 02 4 3 6 - * * -11- pore system. contains a material which is catalytically active with respect to the desired reaction, while the pores in the outer zone of the support are partially blocked by catalytically inactive material.

According to another embodiment of the method applicable in cases where only one reaction is catalyzed, but also in cases where a main reaction as well as one or more side reactions are catalyzed by different components of the catalyst, the catalyst used has the structure of "islets" embedded in the actual structure of the porous support. In that case, the catalyst particles according to the invention consist of a porous support material, which as such can be catalytically active or can contain a component 15 which is catalytically active with regard to one or more desired side reactions, the particles having such a structure that "Islets" of the carrier having a high content in the pores thereof of a material active with respect to the desired reaction or main reaction are distributed statistically uniformly within the interior of the catalyst, but are present to a lesser extent or are absent in the outer zone of the catalyst.

Catalysts prepared according to the described principles can be used with particular advantage for hydrocarbon reactions, such as those mentioned above, in particular methanation reactions. The catalyst may then consist of a known support material, for example Y-aluminum oxide, magnesium-aluminum spinel, silicon oxide, zirconium oxide, titanium oxide or combinations of two or more of these materials, together with catalytically active nickel, in the outer part of which a zone without nickel or with a reduced nickel concentration. In this way, the desired jacket effect is achieved directly.

However, the principle can also be applied more elegantly, for example in the case of the above-mentioned reactions (1), (2) and (4), which are associated with the water gas reaction (6). The reactions (1) - (4) and also the 8102436-12 water gas reaction are catalyzed by nickel, while the water gas reaction is also catalyzed by, inter alia, copper. It is advantageous that the methanation reactions are accompanied by the water gas or shift reactions in such a way that the partial carbon monoxide pressure is prevented before it becomes too high in contact with the catalytic nickel, because CO is to some extent a nickel catalyst poison. , so that the poisoning can be reduced by reducing the partial CQ pressure. It should be noted here that it is metallic nickel and metallic copper which catalyze the reactions, while the catalyst metal is applied in the form of a compound, usually a nitrate or hydroxide, which is subsequently oxidized, for example by calcination, and finally reduced by the free metal, often at the start of the desired reaction, using the hydrogen contained in the reaction components. These conditions will no longer be taken into account below and the description will relate only to the free metals.

It is known that alloys of copper and nickel above a certain copper content exhibit poor methanation activity (see M. Araki and V. Ponec, J. Catalysis 44, 439 (1976)), which is further explained in using Example 1 below. The Ni / Cu catalyst is still active in the conversion of carbon monoxide to carbon dioxide by the water gas reaction (61- In connection with the invention, this can be utilized in the preparation of a jacketed nickel catalyst nickel-copper alloy 30.

on the outside. On this wedge, first of all, a delay of the methanation reaction, which is accompanied by a strong heat development, is prepared, but also a decrease of the partial carbon monoxide pressure by the water gas reaction, whereby the nickel-containing catalyst core is cleared of the so-called deactivation (see the the above article by Harsten Pedersen, Allan Skov and JR Rostrup-Nielsen in the ACS symposium). Due to the / i-des activation, the adsorbed carbon monoxide slowly becomes 8102436 * 5. Converted to low reactivity carbon deposits which deactivate the catalyst, but this is avoided when using nickel-containing porous catalyst particles containing copper in the outer layer of each particle according to the invention.

The outer copper-containing catalyst film can be formed in various ways. For example, it is possible to first prepare particles of a nickel catalyst in a known manner, for example, by impregnation on coprecipitation, and then immersing the particles in water or another liquid and then in an impregnation liquid, which copper compound, for example copper nitrate or copper hydroxide.

In another method, copper hydroxide 15 from copper nitrate is precipitated in the outer portions of the catalyst pore system. In this method it is advantageous that the carrier material is basic, for example contains free magnesium oxide. This can be achieved, for example, by using a magnesium-20 aluminum spinel support which has been annealed at such a temperature (about 1100 ° C) that the unreacted magnesium oxide will still have some reactivity. suspended magnesium oxide or another base, such as calcium oxide or solutions of an alkali metal hydroxide, are then filled- Copper hydroxide will then precipitate in the outer part of the pore system according to the reaction (7) Cu (NO3) 2 + MgO + H20 -> Cu (OH) 2 + Mg (NO 3) 2 30

The described process can be used for nickel catalysts in which the nickel is otherwise evenly distributed, such as, for example, by co-precipitation or impregnation with nickel nitrate without the presence of MgO or other alkaline compounds in the pore system. If the catalyst support contains free magnesium oxide from other alkaline compounds, the nickel may be applied before or after the copper by impregnation with nickel hexamine formate, which is alkaline as such, but no precipitation of the nickel caused on contact with alkaline compounds. In this way it is possible to control the ratio of nickel to copper in the outer zone of the catalyst and at the same time achieve an even distribution of the nickel content.

The reaction rate in the methanation and thereby also the reaction temperature can be controlled by the same principles as when using a vanadium or molybdenum catalyst in a sulfur containing atmosphere, as described in British Patent Application No. 80,40165.

It may be particularly useful to use the principle of the invention with respect to the process described in British Patent Application No. 80.40166 / for the preparation of a gas mixture having a high C 1 carbon hydrogen content through an input gas mixture containing hydrogen and carbon oxides to be converted using a catalyst containing molybdenum and / or vanadium and iron and / or nickel in the presence of gaseous sulfur compounds. This reaction is a Fischer-Tropsch synthesis and it is particularly important to keep the temperature relatively low, because higher temperatures will shift the equilibrium towards the production of methane. According to the invention, therefore, catalyst bodies can be provided with a jacket of inactive support material or a jacket containing copper, which will give rise to the simultaneous water gas reaction.

The method according to the invention is now further elucidated on the basis of a few examples.

Example I

A. A series of catalysts were prepared by coprecipitation of sodium silicate and various amounts of copper and nickel nitrates with sodium hydrogen carbonate.

The precipitated product was formed into particles which were washed to remove sodium compounds, dried at 120 ° C, calcined at 500 ° C and reduced in hydrogen at 500 ° C at 500 ° C. The methanation activity was measured at 1 atmosphere and 250 ° C. By passing a gas consisting of 1% CO in hoeveelheid 100 NI / h over the irregular-shaped catalyst 5 having a size (determined by sieving) of 0.3-0.5 mm. The results were as follows:

TABLE A

Atomic Activity

Catalyst Wt% Ratio, NNi, _Cu Ni / Cu + Ni-10 mol / g / h 1 68.5 0 1.0 93.75; 2 54.2 14.6 0.8 22.32 i I 3 40.2 29.0 0.6 8.93 4 26.5 43.0 0.4 3.57: 5 13.1 56.7 0.2 0.89 I 6_0_700_0_0_

It turns out that even a small amount of copper drastically reduces the methanation activity.

B. Three catalysts were prepared by impregnating a support of aluminum alumina with an internal surface area of about 40 m / g in solutions of copper nitrate, nickel nitrate or resp. a mixture of nickel nitrate and copper nitrate. The impregnated supports were annealed at 550 ° C and reduced in hydrogen at 720 ° C. The catalysts were tested in a reactor by passing 53.5 Nl / h gas consisting of 61.8 vol.% H 2, 18.2 vol.% H 2 O and 20.0 vol.% CO under a pressure of 1 atmosphere. a 0.2 g catalyst in the form of 0.3-0.5 mm particles. In this way, the following rates of carbon monoxide conversion (water gas reaction, shift reaction) and methanation were measured: TABLE "B"

Reaction rate at 375 ° C, 10 mol / g / h

Shifting reaction Me th anis eration reaction _

Cu 1.05 0

Ni, Cu ... 73 5.2

Ni +) - 43

Ni ++) 1431 ..... 1608 8102436 -16- * * ► · -Τ Ι

Measured at 311 ° C as the reactor temperature is not at 375 ° C

could be set.

"H *)

Measured by the above-mentioned catalyst MCR-2X.

It has been found that the Ni, Cu catalysts have a good activity for the water gas reaction, while none of the Cu-containing catalysts have a significant activity for the methanation.

Example II

A support consisting of magnesium oxide with a minor amount of alumina (Mg / Al ratio 7: 1) in the form of 4.5 mm height and diameter cylinders was impregnated with a saturated aqueous copper nitrate solution. The impregnated support was calcined at 550 ° C. When the particles were broken, it was clearly visible that the copper had accumulated in the outer part of the support as a thin black jacket. Microscopic examination revealed that the thickness of the layer was approximately 50 μl. amount.

; The copper-impregnated support was impregnated with a saturated aqueous nickel hexamine formate solution prepared by dissolving 23 g of nickel formate in 50 ml of concentrated aqueous ammonia. The impregnated catalyst was calcined at 300 ° C. The fractured particles showed an even coloration and thus an even distribution of the nickel. Upon analysis, 0.6 wt% Cu and 2.7 wt% Ni were found.

A corresponding catalyst was prepared by first impregnating in nickel hexamine formate and then in copper nitrate. The copper jacket was visible as a dark discoloration of the outer zone of the particles. This catalyst is referred to as Catalyst A, while as a comparison, a copper-free methanation catalyst, designated Catalyst B, was prepared by impregnating the support in nickel hexamine formate.

Example III

A methanation catalyst containing about 25 wt% nickel on a stabilized aluminum oxide support was produced in the form of 4.2 mm × 81 02 4 36 3 cylinders. J * -17- 4.2 mm impregnated in an alumina gel prepared by suspending 11 g of alumina in 180 ml of water and gelling with 5.6 ml of concentrated nitric acid. The impregnated catalyst was calcined at 550 ° C.

A similar preparation was carried out, however, with the difference that 10 ml of the alumina gel was previously mixed with a solution of copper nitrate.

The impregnated and annealed catalyst exhibited an outer jacket containing copper, which was estimated to have a thickness of about 0.5 mm upon microscopic examination. Minor amounts of copper nitrate had entered the interior of the catalyst, but the ratio of the copper content in the outer zone to the copper content in the interior was estimated to be about 3: 1. This catalyst was designated as Catalyst C.

Example IV

This test serves to demonstrate how a coating on a catalyst applied by gel can reduce the reaction temperature at high temperature.

A nickel-containing methanation catalyst in the form of 4.2 mm x 4.2 mm cylinders was coated with an alumina gel and then dried. The catalyst was then reduced in pure hydrogen at 800 ° C for 2 hours. This catalyst was designated as Catalyst D.

A catalyst without such a coating was activated (reduced!) In the same manner. It is referred to as Catalyst E and is identical to the above-mentioned MCR-2X catalyst.

Example V

This example relates to the investigation of Catalysts A-E listed in Examples II-IV.

The catalyst granule to be tested was fixed between 2 thermo-elements in a tubular reactor with an internal diameter of 6 mm. Then, in pure hydrogen, it was heated to about 300 ° C to detect possible trace amounts. .

-18- how to remove nickel oxide quantities. The hydrogen feed was then turned off, and a synthesis gas consisting of 9% CO in Hj was supplied to the system throughout the run at a rate of about 100 Nl / h. At about 300 ° C, the gas stream obtained as product was analyzed with the aid of a gas chromatograph and the amount of methane formed was determined. Then, the temperature was gradually increased and the methanation reaction rate was determined at each step.

The table below shows that catalysts with an inert outer layer applied from the gel give rise to a reduced reaction rate at a high temperature compared to the homogeneous comparative catalyst.

15. TABLE C

Catalyst Test Temp. Methanization / ° C activity '' mmol / q / h__ Λ 300 1.4

Carrier of A ^ O ^. MgO 360 4 impregnated with copper 417 6 nitrate and nickel hexamine- 518 12 formate 603 11 678 12 B 346 18

Carrier of Al203.Mg0 389 60 impregnated with nickel 482. 180 hexamine formate 577 250. . 715.240 C 330 55

NiO. Al 203 - catalyst 425 95 coated with Al203 570 130 741 95 D 243 3.5

NiO.Al2 O3 catalyst 295 30 coated with a "jacket, 336 50 containing copper oxide and A12C> 3 440 65 597 60 Γ & ΖΤΠΓ - -19- CONTINUED TABLE C_ E 274 80

NiO.AJ ^ O ^ catalyst 323 180 (the aforementioned known 379 250 catalysts MCR-2X without 438 270 limitations) 508 315 569 310

The reason why catalysts B and E have maximum activities within the considered temperature range is due to the thickness of the gas film during the test, the gas velocities being considerably lower than in the industrial version; in the normal industrial version with higher gas velocities, the gas film would become thinner and diffusion would be slowed to a lesser extent. The mass transfer value for the gas film is estimated to be about 7 kgmol / h.m, while under industrial conditions this value would be about 100 kgmol / h / m. This means that the difference in high temperature activity with and without the jacket would be higher than indicated in Table C.

81 02 4 3 6

Claims (11)

Process for carrying out a gas phase catalytic exothermic reaction in a cooled reactor containing a bed of a porous particulate catalyst which is active with respect to the desired reaction, characterized in that each of the individual particles of the catalyst on the outside has a zone of reduced catalytic activity with respect to the desired reaction. 2. Process according to claim 1, characterized in that each of the individual particles of the catalyst has on the outside a zone which is inactive with respect to the desired reaction. 3. Process according to claim 1 or 2, characterized in that the outer zone of the catalyst particles has a thickness of 0.01-2 µm. 4. Process according to one or more of the preceding claims, in which a catalytic exothermic main reaction and one or more catalytic side reactions take place almost simultaneously in the same reactor, which contains a bed of a particulate catalyst which is active with respect to the reaction. characterized in that each of the particles of the catalyst on the outside has a zone of a material which is at least partially inactive with respect to the main reaction and is catalytically active with respect to one or more of the side reactions. 5. Process according to any one or more of the preceding claims, characterized in that the catalyst particles consist of a porous support material, which is catalytically inactive with respect to the desired reaction or main reaction and contains a material in a part of the pore system, which is catalytically active with respect to the desired reaction or main reaction, while the outer pores of the support are free from the material which is catalytically active with respect to the desired reaction or main reaction. Process according to one or more of Claims 1 to 3, characterized in that the catalyst particles consist of a porous, inactive carrier material, which contains in a part of the pore system a material which is catalytically active with respect to the desired reaction, while the pores in the outer zone of the support are partially blocked by a catalytically inactive material. 7. Process according to one or more of claims 1-4, characterized in that the catalyst particles consist of a porous support material, which is catalytically inactive with respect to the desired reaction or main reaction, but which is optionally catalytically active or contains a component which is catalytically active with respect to one or more of the desired side reactions, which particles have such a structure that "islets" of the carrier having a high pore content of a material active with respect to the desired reaction or main reaction are statistically uniformly distributed throughout the interior of the catalyst but are present or absent in the outer zone of the catalyst in a much smaller amount. A method according to any one of claims 1 to 4 for carrying out methanation reactions accompanied by the water gas reaction in a cooled reactor using a particulate catalyst containing nickel in the pores of a porous support, characterized in that nickel Containing porous catalyst particles are used which contain copper in the outer layer of each particle. 9. Process for preparing the catalyst, as defined in claim 8, characterized in that a particulate, porous support containing nickel or nickel oxide is applied by impregnation or precipitation from the vapor phase or by coprecipitation of support material and catalyst material, which support material is a. alkaline material, treated with a solution of a copper salt to precipitate copper hydroxide in an outer zone, after which the catalyst is dried, calcined and reduced. Method according to claim 9, characterized in that the support material contains active .MgO. 81 02 4 36 'f 1 {F ~ - F -22- Catalyst particle for catalyzing exothermic reactions characterized by a modified outer zone with a reduced or no catalytic activity at all for the reaction in question. 81 024 3 6