SE448684B - PROCEDURE FOR PREPARING AN EXOTERM, NICKEL CATALYZED METANIZATION REACTION - Google Patents

Description

10 hrs) 448 684 components, either with gases inactive below reaction conditions or by recycling product gas. Dilution with inert gases gives costs for the gases and for their separation from the final product, and recirculation of product gas causes energy loss in recirculation compressors.

In other cases, therefore, the exothermic pro- processes in cooled reactors, whereby the dilution and use of recirculation compressors.

As a cooling medium, e.g. air, salt bath, mixtures of biphenyl and biphenyl oxide, e.g. "Dowthern", and in the the above hydrocarbon formation reactions are often boiling water. In cooled reactors it is possible to achieve one low initial temperature for the product gases and thus a favorable reaction balance in cases where the desired It can, however not in cooled reactors with catalyst layers is avoided the product is promoted by low temperature. that shortly after the "ignition" of the reaction, i.e. a small piece into the catalyst layer a hot zone, a so-called "Hot spot", occurs, where the temperature is often immediately adjacent to a temperature thermodynamically determined of the adiabatic temperature increase. Only hereafter the reaction gas is cooled, i.e. later in the catalyst the layer. indicated in terms of catalyst stability, if any The same problem thus arises as above selectivity and in the case of hydrocarbon reactions carbon formation.

The problem of the "hot spot" temperature has in litt- raturen i.a. dealt with by van Welsenaere and Froment (Chemical Engineering Science, vol. 25, pp. 1503-1516, 1970), and it appears that "hot spot" can not be avoided in tubular reactors with constant wall temperature and that "threat the spot "temperature is very sensitive to small variations in the process variables such as inlet temperature, reactant concentrations and wall temperature. It therefore exists risk that the temperature may rise to an uncontrollable way, so that the so-called "runaway" takes place. Welsenaere and Froment indicates how the reactions can be controlled during '448 634 given conditions, e.g. when carried out in a fixed sator layer disposed in a tube surrounded by a coolant; a critical factor here is the pipe diameter. Welsenaere och Froment states in the said dissertation results from oxidation of p-xylene to phthalic anhydride at atmospheric pressure and with large excess of air.

The calculations are performed for an irreversible reaction of the first order, but can without significant changes also transferred to reversible reactions, which are not for meta- of the first order. One finds here e.g. the reaction reactions (1) Co + 3H2 (2) 250 + 252; == èC fl4 + CO2 + 59 kcal / mol och (3) C92 + 4H2; :: àCH4 + 2H2O + 39 kcal / mpl ; == åcH4 + H20 + 49 kcal / mol, that the diameter of the catalyst tubes can not exceed a few millimeters, if full security is to be provided against "runaway" under all circumstances. Such a small pipe diameter can not be used in industrial operation. The main difference between the Welsenaere and Froment examples and the methanation the reactions seem to be the high pressure and the large learn the concentration at the latter and the consequent large heat production per unit volume of catalyst.

Similar conditions apply to other hydrocarbons formative reactions, e.g. The Fischer-Tropsch image synthesis use of petrol and / or olefins: (4) nco + 2nH2; == å (cH2) n + H20 + ca. 40 kcal / g atom C or the so-called Mobil synthesis for the formation of hydrocarbons from alcohols, e.g. (5) nCH OH + (CH2) n + nH2O + ca. 12 kcal / g atom C 3 It is thus not possible without special measures to avoid the temperature rising to or to a value close to that determined by the adiabatic temperature increase for such reactions. Welsenaere and Froment suggest that 448 684 4 dilute the catalyst charge in the catalyst layer, e.g. with catalytically inert filler bodies, whereby the produced the amount of heat per volume of reactor is reduced. It proves however, that a relatively strong dilution of the catalyst tower is necessary, which presupposes an increase of themselves the catalyst volume to "ignite" the reaction. The method has thus the inconvenience that it requires an enlargement of the reactor and thus an increase in capital costs; what is often more important is that the dilution has to reduce the resistance of the catalyst charge or adsorption capacity for catalyst poisons, which is carried in the reaction stream. Such a dilution of the catalyst is therefore no satisfactory solution for temperature control the problems of cooled exothermic catalytic processes.

To study the conditions in more detail, experiments were performed in an air-cooled methanation reactor with a catalyst with the manufacturing designation MCR-2X. It is a microporous temperature stable, mechanically strong catalyst with nickel crystal smiles of the same order of magnitude as the pore diameter, on a carrier of Y-alumina (see Karsten Pedersen, Allan Skov and 1 J.R. Rostrup-Nielsen, ACS Symposium, Houston, March 1980).

The catalyst is used in the form of cylinders with height and diameter 4.3 mm. In some of the experiments, the catalyst was diluted. towers with catalytically inactive cylinders with the same geometric design. The pressure in the reactor was kept at 26 kg / cm * and man used a feed gas with the composition 70% H2, 9% CO, 10% CO 2 and 11% CH 4. determined adiabatic temperature increases of between 380 and These experimental conditions give thermodynamic 400 ° C regardless of the degree of dilution of the catalyst.

It turned out that without dilution of the catalyst it was not possible to limit the temperature increase, which lunda were the indicated 380-400oC. .In case of dilution conditions at 1: 3 and 1: 5 (ie 3 and 5 times as large volume, respectively inert cylinders as catalyst bodies) were actually measured temperature increases, which were up to 50 ° C less than expected adiabatic temperature increases of 380-400 ° C. In these search, the temperature of the pipe wall in the “hot spot” range was about 600 ° C 448 684 or more, which in industrial operation constitutes an unrealistic high pipe temperature; in an industrially cooled reactor more the maximum pipeline temperature not to exceed 400oC.

The achieved limitation of the temperature increase can be said be significant, even if it is in itself without major industrial importance, and even if catalyst dilution in this way for the reasons stated above is small suitable.

A closer analysis of the reduced temperature increase showed, however, that the reduced temperature increase due to the fact that the reaction rate had not risen so strongly as one might expect according to the reaction kinetics. surprisingly, it was found that this reduced reaction time speed depends on the calculated reaction rate at high temperature exceeds the speed of the reactants diffusion through the gas film surrounding the catalyst particles.

Since the gas phase diffusion is almost independent of the temperature pure (activation energy 1-2 kcal / mol) supported the reaction rate. against a barrier due to the lack of presence of reactive at the actual reaction sites, catalyst bodies internal, and therefore the reaction is prevented from rushing away. The object of the invention is to utilize this disruptive observation to indicate how to generally be able to prevent runaway in catalyzed exothermic gas phase reactions. down in cooled reactors. This is achieved according to the invention by using a catalyst whose individual parties articles ultimately have a zone with a reduced reaction rate for the reaction. lytic activity.

The outermost part of each individual catalyst particle thus according to the invention may consist of one for the reaction inactive sun; or it may be a reduced zone activity for the highly exothermic reaction in question. nen. In the case where at the same time a strongly exothermic main reaction and one or more side reactions (which may be minor exothermic, or t.o.m. thermally neutral or endothermic), takes place, the outer layer may or may not be catalytically active for the side reaction, but not for the main reaction. As example 448 684 e it may be mentioned that the reactions (1), (2) and (4) shown above, which are catalyzed by nickel, are accompanied by the water gas reaction. nen, also called the shift reaction: (6) CO + H 2 O ;; = to CO 2 + H 2 + 10 kcal / mol, which is catalyzed by i.a. copper.

Either the outer layer is catalytically inert or catalytic lytically active for a side reaction, but inactive for the main reaction, the above-mentioned film diffusion is extended to to also include the diffusion through the inert outer layer of the catalyst. It will therefore be possible to determine it maximum reaction rate in the reactor, since this determined by the thickness of the inert layer. In practice becomes the thickness of the layer inert to the main reaction a thickness that is to a few orders of magnitude greater than the thickness of the surrounding gas film; the thickness of this gas film can be set to approx. 1 h below normal industrial conditions; according to the invention can the indicated outer zone of the catalyst particles is therefore suitable preferably have a thickness of 0.01 - 2 mm. Thicknesses of specified layers of over approx. 0.5 or 1 mm comes in practice only in question when using relatively large catalysts particles.

In general, the explanation of the invention according to the above standing definition it, that a given catalyst body with homogeneous activity and homogeneous pore system exposed to a given mixture of reactants is subjected to different reaction restrictions as the reaction temperature changes (rises).

At low temperatures the reaction rate is limited by the catalyst material. The reaction is here in the so-called intrinsic velocity range, where there is almost none concentration gradient of the reactants by catalysis the pore system of the cod layer. As the temperature rises, catalyst activity increases, which results in reactant na get an increased gradient through the catalyst bodies. At a time point a temperature range is reached, in which the diffusion of 1 448 684 the reactants through the catalyst bodies become the limiting one the parameter for the reaction rate, which means that not all of the catalyst material is used in the reaction; the efficiency factor becomes <1.

If the temperature is increased further, the parameter will which determines the reaction rate to be mass transported through the gas film surrounding the catalyst bodies. This speed is under normal conditions too great to the cooling surface of the reactor must be able to limit the "hot spot" temperature the turn to a level considerably below that of the adiabatic temperature increase determined, which in many cases means that thermal sintering can not be avoided or for some methanation reactions that carbon formation does not can be averted.

According to the invention, however, a restrictive is incorporated surface in the individual catalyst bodies thereby, that one increases the gas film or in one way or another reduces the permeability of the reactants. This limits the reaction the rate of increase and so also the temperature increase.

One might expect the incorporation of such an inert layer, "delay layer", would lead to difficulties in starting or "igniting" the exo- thermal reactions. However, this does not happen because the reaction rate in the ignition zone in all circumstances is low, so that the gas diffusion through the inert layer does not become limiting (determining) the reaction rate called. This layer first becomes a limiting factor, ie. in fact, it only begins to ache when the reaction rate the heat becomes so great that you want it to slow down. Since the inert layer constitutes only a small part of the catalyst pill, the resistance of the catalyst to the marriage. _. _ _ In that context, it may be mentioned that the they can form the outer layer of very small catalyst pellets over 50% of their volume; but in normal cases the proportion becomes considerably smaller, thus for cylindrical catalyst bodies _.-._ _ .i 448 684 8 with height and diameter 4.2 mm typically 1-10%, in particular 2-10% and often 2-5% of the volume.

The reactor can be any type of cooled reactor for exothermic reactions, e.g. a tubular reactor or a reactor tor with a larger room of any shape, and through which passes cooling pipes, into which a cooling medium flows. Cataly the sator layer usually consists of a so-called solid layer (fixed bed), but the invention can also be used in combination tion with fluidized bed catalysts.

As mentioned, the outer layer must have reduced catalytic activity. and although it is often appropriate according to the invention to use a catalyst whose individual particles are ultimately has one for the desired reaction (or main reaction) inactive zone, it may in some cases be appropriate to use a catalyst where this outer zone has only one for reaction (or the main reaction) decreased activity.

The outer zone can be created in many different ways, often in analogy to that known from the pharmaceutical industry technology, where tablets are made with several layers of different composition or different concentration of active material.

Thus, catalyst bodies can first be prepared from normal type, consisting of a porous support with the catalytic technically active material. These can be prepared by known technique, e.g. by coexistence or by first produces porous support bodies, which are then impregnated with the catalytically active material. The thus produced the catalyst is then dipped in a gel or sol of inert support. materials, of the same kind as the catalyst support itself or another kind. If the outer layer is not to be completely inert, but have limited catalytic activity, one can then impregnate lower it, but ensure that the concentration of catalytic active material becomes smaller than in the inner layer. You can com- combine alternating impregnations, washes and chemicals. treatments and thereby achieve the desired structure and combination nation of structures. On a finished catalyst, one can, for example by electrolysis or precipitation from the vapor phase, impose (k, 448 684 an inert layer or even a catalytically active one layer, which partially clogs the pore mouths, so that the lysator activity in the outer layer and the diffusion rate to the interior of the catalyst is reduced. A special structure can to achieve by tableting a mixture of finely divided catalyst particles with an inert support material. Hereby the diffusion effect is combined with a dilution of the catalyst tower, so that the degree of dilution can be significantly reduced.

Practical implementations of these methods in combinations hence is close to the fi experts.§ As mentioned, the stated principle can not be used not only in the case of a single exotic reaction, but also in the case where a catalytically exotic main reaction and an electric several catalytic side reactions take place simultaneously in same reactor. In such cases, according to the invention, a catalyst active for the main reaction is used, the separate particles ultimately have a zone of material that is at least partially inactive in relation to the main reaction, but catalytically activates one or more side reactions.

A practical embodiment of such a catalyst, or generally of catalysts designed in accordance with The principle of the invention is a catalyst consisting of particles of a porous support material which is catalytically inactive for the desired reaction or the main reaction (but possibly catalytically active for a side reaction) and as in the pore system contains one for the desired reaction or main reaction catalytically active material in such a way that the pores in the outer zone of the carrier particles are free from it the reactions or the main reaction catalytically active material rialet. Pore content of catalytically active material can e.g. have been achieved by co-precipitation, by electro- light, by precipitation from the vapor phase or by impregnation from liquid phase. The design can also be as specified in requirement 6.

The catalyst can also be designed as specified in claim 7. 448 684 10 Catalysts designed according to the described principles can be used with special advantage for hydrocarbon reactions of the type indicated above, in particular the methanation reaction down. The catalyst can in this case consist of a known support material. rial, e.g. Y-alumina, magnesium aluminum spinel, silicic anhydride, zirconia, titanium dioxide or combined nations of two or more of these materials, together with catalytically active nickel, with the extreme occurring a zone without nickel or with reduced nickel concentration.

In this way, the desired scale effect is achieved immediately.

However, the principle can be applied to even more elegant sara (2) and Reactions (1) - (4) As mentioned above, the reactions (1) are followed. (4) of the water gas reaction (6). and also the water gas reaction is catalyzed by nickel, while the water gas reaction is also catalyzed by e.g. copper.

It is an advantage to have methanation reactions accompanied of the water gas or shift reaction in such a way that the partial pressure of carbon monoxide is reduced before this in too high degree comes into contact with the catalyst nickel, whereby CO to some extent, is a catalyst poison for nickel and can be reduced by reducing the CO partial pressure.

It should be noted that it is nickel metal and copper metal that catalyzes the reactions, but that the catalyst the metal is applied in the form of a compound, often a nitrate or a hydroxide, which is later oxidized, e.g. by calc- and eventually reduced to the free metal, often at the beginning of the desired reaction with the aid of hydrogen, present among the reactants; in the following these conditions and the description are disregarded will only deal with the free metals.

It is known that alloys of copper and nickel over a certain copper content has very little activity for methanation (see M. Araki and V. Ponec, J. Catalysis 44, 439 (1976)), which is illustrated in more detail in the example below 1. The Ni / Cu catalyst, on the other hand, has constant activity for conversion of carbon monoxide to carbon dioxide by tion (6). This can now be used in combination with 1 * 442 es4 the present invention in such a way that a nickel catalyst which ultimately has a nickel-copper shell. the alloy. In this way it is achieved, on the one hand, that the power heat-generating methanation reaction, but also reduces the partial pressure of carbon monoxide through the the nickel-containing catalyst core is spared from the so-called B deactivation (see the above dissertation by Karsten Pedersen, Allan Skov and J. R. Rost- rup-Nielsen in the ACS symposium). During the ß deactivation, adsorbed carbon monoxide is slowly added to carbon deposits with low reactivity, which deactivates the catalyst, but this is avoided when according to the invention procedures such as specified in claim 8.

The outer copper-containing catalyst film can be formed differently. For example. can one first in a known way, e.g. by impregnation or precipitation, producing particles of a nickel catalyst and then dipping the particles in water or another liquid and then in an impregnating liquid ka containing a copper compound, e.g. copper nitrate or copper hydroxide.

Another method is to precipitate copper hydroxide of copper nitrate in the outer parts of the catalyst system. It is an advantage in this process if the carrier the material is basic, e.g. contains free magnesium oxide.

This can be done e.g. achieve by using a carrier of magnesium aluminum spinel, which is fired at such a temperature (approx. 1100 ° C) that unreacted magnesium oxide has some reactivity. You can also first fill the por- the system with suspended magnesium oxide or another base such as calcium oxide or solutions of an alkali metal hydroxide.

Copper hydroxide precipitates in the outermost part of the pore system according to the reaction (7) Cu (NO 3) 2 + MgO + H 2 O + Cu (OH) 2 + Mg (NO 3) 2.

The described method can be used in nickel catalyst where the nickel has been evenly distributed in other ways, such as 448 684 12 mentioned e.g. precipitation, impregnation with nickel nitrate without the presence of MgO or other alkaline compounds in the pore system. If the catalyst support contains free silica or other alkaline compounds, nickel may be before or after copper by impregnation with nickel hexamine formate, which is itself alkališd; but does not entail precipitation of nickel by contact with basic substances. On this way it is possible to regulate the relationship between nickel and copper in the outer zone of the catalyst and at the same time achieve an even distribution of the nickel content.

The reaction rate during methanation and thus the reaction temperature can be controlled according to similar principles at use of a vanadium- or molybdenum-based catalyst sulfur-containing atmosphere, as described in the Danish patent application no. 5396/79 of 18 December 1979.

It can be particularly valuable to make use of pen according to the present invention in combination therewith in the Danish patent application 5395/79 the procedure for preparation of a gas mixture with a high content of C2- hydrocarbons by reacting a feed gas mixture containing hydrogen and carbon oxides using a catalyst containing molybdenum and / or vanadium and iron and / or nickel in the presence of gaseous sulfur compounds. It is this is a Fischer-Tropsch synthesis, and it is meaning to keep the temperature relatively low, because higher temperature shifts the equilibrium in the direction of formation of methane. Therefore, in accordance with the present invention, provide the catalyst bodies with a shell of inert carrier material or a shell containing copper for of the simultaneous water gas reaction.

The process according to the invention will in the following illustrated in more detail with a few examples.

Example 1 g. A series of catalysts were prepared by sodium silicate and varying amounts of copper and nickel nitrate with sodium bicarbonate. : v fä 448 684_ The precipitate product was formed into particles which washed out. taken from sodium compounds, dried at 120 ° C, calcined was heated at 500 ° C and reduced in hydrogen at 500 ° C. Metani- The filtration activity was measured at 1 atm and 250 ° C by a gas consisting of 1% CO in H2 in an amount of 100 Nl / h was passed over the catalyst in the form of irregular bodies with a size 0.3 - 0.5 mm (measured by screen mesh width).

The following results were obtained: Catalyst Weight% Atomic ratio Activity no. Ni Cu Ni / Cu + Ni 10-3 mel / 9 / h 1 68.5 0 1.0 93.75 2 54.2 14.6 0.8 22.32 3 40.2 29.0 0.6 8.93 4 26.5 43.0 0.4 3.57 13.1 56.7 '0.2 0.89 6 0 70.1 0 0 It can be seen that even a small amount of copper reduces drasticization activity drastically.

§. 3 catalysts were prepared by a support of Y-alumina with an inner surface area of approx. 40 m 2 / g impregnated in solutions of the respective copper nitrate, nickel nitrate and a mixture of nickel nitrate and copper nitrate. The impregnated The supported vehicles were calcined at SSOOC and reduced in hydrogen. gas at 750%. a pressure of 1 atm with 53.5 Nl / h gas consisting of 61.8 volume% H2, 18.2 volume% H 2 O and 20.0 volume% CO, above 0.2 This was measured The catalysts were tested in a reactor with g catalyst as 0.3 - 0.5 mm particles. the following rates of carbon monoxide conversion ( tion, shift reaction) and methanation respectively: -1 448 684 W Reaction rate at 375 ° C 3 "moi / g / h shift methanation cu 1.05 o N1, cu 73 5.2 Ni * i - 43 Ni * H 1431 1608 +) measured at 311oC, since the reactor temperature does not could be controlled at 375 ° C, ++) measured on the above catalyst MCR-2X.

It is seen that the Ni, Cu catalyst has good activity for the hydrogen gas reaction, while none of the Cu-containing catalysts the tores have significant activity for methanation.

Example 2 A carrier consisting of magnesium oxide with a smaller content of alumina (ratio Mg / Al 7: 1) impregnated in the form of cylinders with a height and diameter of 4.5 mm with a saturated aqueous solution of copper nitrate. The the impregnated support was calcined at 550 ° C. By refraction of the particles, it was clear that copper was accumulating. line as a thin layer on the outside of the wearer. Microscopic examination showed that the layer had a thickness of approx. 50 u.

The copper impregnated support was impregnated with a saturated aqueous solution of nickel hexamine formate prepared by dissolving 23 g of nickel formate in 50 ml of concentrated rat ammonia water. The impregnated catalyst calcined at 300 ° C. By breaking the particles were observed even dyeing and thus distribution of nickel. In analysis 0.6% by weight of Cu and 2.7% by weight of Ni were obtained. ' A corresponding catalyst is prepared by impregnation. ring first in nickel hexamine formate and then copper nitrate.

The copper shell appears as a dark coloration of the outer parts of the particles. zone. This catalyst is referred to as Catalyst A, and which J 448 684 reference, a copper-free methanation catalyst was prepared, designated as Catalyst B, by impregnating the support in nickel hexamine formate.

Example 3 A methanation catalyst containing approx. 25% by weight nickel on a stabilized alumina support impregnated in the form of 4.2 x 4.2 mm cylinders in a gel of alumina prepared by slurrying 11 g of aluminum oxide in 180 ml of water and gelled with 5.6 ml of concentrated uric acid. The impregnated catalyst was calcined at 5 ° C.

A similar petition was first made with it the difference being 10 ml of the alumina gel before that mixed with a solution of copper nitrate. The impregnated and the calcined catalyst had an outer shell containing copper, by microscopic examination determined to a thickness of approx. 0.5 mm. Smaller amounts of copper nitrate had penetrated into the interior of the catalyst, but the relationship between copper content in the outer zone and the inner zone could be determined to approx. 3: 1. This catalyst is denoted by C.

Example 4 This experiment shows how a coating on a catalyst applied with a gel may limit the reaction rate. at high temperature.

A methanation catalyst with nickel in the form of 4.2 x 4.2 mm cylinders were coated with an alumina gel and was then dried. Thereafter, the catalyst was reduced in pure hydrogen at 800 ° C for two hours. This catalyst is designated with D.

A catalyst without such a coating was activated (reduced cerades) in the same way. It is denoted by E and is identical with the above catalyst MCR-2X. 448 684 Example 5 This example shows testing of those in Examples 2-4 Catalysts A-E spoke.

The catalyst pellet used for testing is clamped between two thermocouples in an internal tubular reactor diameter 6 mm. It was then heated in pure hydrogen gas ca. 300 ° C to remove any traces of nickel oxide.

The hydrogen supply was then stopped and the system was added a synthesis gas consisting of 9% CO in H2 at a rate of ca. 100 Nl / h throughout the experiment. At approx. 300 ° C was pro- product gas stream analyzed in a gas chromatograph and the product the amount of methane produced was determined. The temperature was then increased stepwise and the methanation rate was determined each time.

The table below shows that catalysts with active surface layer applied with the gel has limited reaction rate. high temperature compared to the homogeneous reference the catalyst.

Table 3 Catalyst Test temperature Methanation ° c activity millimole / g / h A 300 1.4 Al2O3-MgO support 36 0 v 4 impregnated with 417 6 copper nitrate and 518 12 nickelhexamine formate 603 11 678 12 B 346 18 AIZO3 MgO Carrier 389 60 impregnated with 482 180 nickelhexamine formate 577 250 715 240 rv » 17 448 684 Table 3 (continued) Catalyst Test temperature Methanation oc activity millimole / g / h C 330 55 N10-Al2O3 Catalyst 425 95 coated with Al2O3 570 130 741 95 D 243 3.5 NiO-Al2O3 Catalyst 295 30 covered with a shell 336 50 containing copper oxide 440 65 and Al2O3 597 60 E 274 80 NiO-Al2O3 Catalyst 323 180 (the above known 379 250 catalyst MCR-2X 438 270 without restrictions 508 315 569 310 The reason why catalysts B and E have activity in the temperature range in question is gas the thickness of the film during the experiment, as the gas velocities significantly lower than in industrial operations. At normal industrial operation with the higher gas velocities, gas the film becomes thinner and thus slows down the diffusion less.

The mass transfer rate of the gas film is estimated at about 7 kmol / h / m2, where under industrial conditions it is around 100 kmol / h / m2. with and without shells will be larger than what appears of table 3.

This causes the difference in high temperature activity

Claims (8)

18 Patent claims Process for carrying out an exothermic, nickel-catalyzed methanation reaction in a cooled reactor containing a bed of nickel-containing porous catalyst particles, characterized in that each of the individual particles of the catalyst has an outer zone with reduced catalytic activity. 2. A process according to claim 1, characterized in that the outer zone is catalytically inactive in the methanation reaction. 3. A method according to claim 1 or 2, characterized in that the outer zone has a thickness of 0.01-2 mm. 4. A method according to any one of the preceding claims, characterized in that the outer zone constitutes 1-10 volume of the particles. Process according to any one of the preceding claims, characterized in that each of the catalyst particles consists of a porous support, which is catalytically inactive in the methanation reaction, the pores in the support containing nickel, but the pores in the outermost zone are essentially free of nickel. Process according to one of Claims 1 to 4, characterized in that the catalyst particles consist of a porous support which is catalytically inactive to the methanation reaction, the pores in the support containing nickel and the pores in the outer zone being partially blocked by catalytic inactive material. Method according to any one of the preceding claims, characterized in that said particles contain nickel present in statistically evenly distributed islands inside the 448 684-H9 particles, the outer zone adjacent to the particles being free of nickel or containing less nickel. Process according to any one of the preceding claims, characterized in that the methanation reaction is followed by a water gas reaction, in which the bed consists of porous catalyst particles, each containing nickel in the pores of the support and having an outer zone containing copper.