FR2482473A1 - PROCESS FOR CARRYING OUT EXOTHERMIC CATALYTIC REACTIONS IN THE GAS PHASE - Google Patents

Abstract

THE SUBJECT OF THE PRESENT INVENTION IS A METHOD FOR CARRYING OUT AN EXOTHERMAL CATALYTIC PROCESS IN A GAS PHASE USING A COOLED REACTOR CONTAINING A FIXED OR FLUIDIZED BED OR A PARTICULAR POROUS CATALYST. IN SUCH A PROCESS, A REDUCED REACTIONAL SPEED IN THE HIGH TEMPERATURE ZONE IS OBTAINED AND THUS BETTER SAFETY TOWARDS THE REACTION PACKAGING BY USING A CATALYST WHOSE INDIVIDUAL EXTERIOR PARTICLES HAVE A REDUCED ACTIVITY ZONE, OR NO ACTIVITY AT ALL IN RELATION TO THE DESIRED REACTION. THE REDUCED REACTION SPEED AVOIDS AN EXCESSIVE REACTION TEMPERATURE AND THUS AN AGGLOMERATION OF THE ACTIVE CATALYTIC MATERIAL AND THE PORE SYSTEM; IN SOME CASES, IT ALSO PREVENTS BORING SIDE REACTIONS AND UNINTENDED DISPLACEMENT OF REACTIONAL BALANCE. IN CASES WHERE A MAIN REACTION IS CARRIED OUT SIMULTANEOUSLY WITH A SIDE REACTION, THE EXTERNAL PARTS OF THE CATALYST PARTICLES MAY CONTAIN CATALYTIC MATERIAL THAT IS INACTIVE WITH RESPECT TO THE MAIN REACTION AND ACTIVE TOWARDS THE SECONDARY REACTION. THE REACTION PRINCIPLE IS SUITABLE, AMONG OTHERS, FOR HYDROCARBON REACTIONS SUCH AS METHANATION OR FISCHER-TROPSCH SYNTHESIS.

Description

METHOD FOR PERFORMING EXOTHERMIC CATALYTIC REACTIONS IN THE PHASE

GAS.

  The present invention relates to a method for performing gas-phase exothermic catalytic reactions using a cooled reactor containing a fixed or fluidized bed of a particulate active porous catalyst

  with regard to the desired reaction.

  Many gas phase catalytic processes give rise to high heat formation and, in many cases, cause a considerable rise in temperature. Examples of such processes are the catalytic conversion of alcohols to hydrocarbons by so-called Mobil synthesis, carbon oxides by methanation to methane, or by Fischer-Tropsch synthesis in gasoline and / or olefins (process accompanied by the so-called gas-to-water reaction); one can, moreover, quote the synthesis of

  ammonia, the preparation of formaldehyde from methanol, natural gas

  rel or other hydrocarbons, and the preparation of sulfuric acid vapors

  and sulfur trioxide from sulfur dioxide.

  High temperatures can have a number of

  vantages. In many cases, they can cause damage or destruction of the catalyst, for example by sintering the catalyst active material or the catalyst pore system. Unwanted side reactions can often occur, such as free carbon decomposition in the preparation of hydrocarbons from carbon oxides

  or alcohols, carbon that can block and destroy the catalyst.

  In many cases, higher temperatures can displace equi-

  free of the reaction and its selectivity in an undesired direction; thus, in the Fischer-Tropsch synthesis, a high temperature favors the formation of methane at the expense of the desired products, for example ethane,

  ethylene, or other olefins or gasoline.

  The exothermic catalytic processes are often carried out in adiabetic reactors and in such cases it is often attempted to limit the increase in temperature by diluting the reactants, either with gases that are inert under reaction conditions, or with

gas from the recycling product.

  Dilution with inert gases involves the price of gases and their separation from the final product, and the recycling of the gas produced involves a loss of energy.

in recycling compressors.

  For this reason, exothermic processes have been carried out in other cases.

  in cooled reactors, which avoids dilution and the use of recirculation compressors. Examples of suitable cooling medium are air, salt baths, synthetic heat transfer media such as "Dowterm" O, and

  processes for forming hydrocarbons, often boiling water.

  In cooled reactors, it is possible to obtain a low output temperature of the gases produced and thus a favorable reaction equilibrium

  in cases where the desired product is favored by a low temperature.

  However, in reactors cooled with a catalytic bed, it is not possible to avoid that shortly after ignition of the reaction, that is to say at a short distance from the entry into the catalytic bed, a hot zone, called the "hot spot", in which the temperature will often be close to the temperature thermodynamically determined by the adiabetic increase in temperature, and only after the reaction gas has cooled, is it That is, the same problems as mentioned above will occur with regard to the stability of the catalyst, the possible selectivity and the formation of the catalyst.

  carbon in the case of hydrocarbon reactions.

  The problems concerning the overheating temperature have been discussed in the literature, among others by van Welsenaere and Froment (Chemical Engineering Science, Vol 25, pp. 1503/1516, 1970), and it appears that

  hot spots are inevitable in tubular reactors with

  constant wall temperatures, and that the superheat temperature is very sensitive to small variations in process variables, such as inlet temperature, concentration of reactants, and wall temperature. It follows that there is a risk that the temperature

  may increase in an uncontrolled manner to lead to a "runaway".

  Welsenaere and Froment describe how reactions can be controlled under given conditions, for example when carried out in a fixed bed placed in tubes surrounded by a cooling medium; in this case, the diameter of the tubes is a critical factor. In this article, Welsenaere and Froment give results for the oxidation of para-xylene in anydride

  phthalic at atmospheric pressure and with a large excess of air.

  Calculations have been made for an irreversible reaction of the first order, but they can be transposed without notable change to reversible reactions which are not of the first order. For example, methanation reactions can be found for example: (1) CO + 3H 2 CH 4 + H 2 O + 49 i (caL / moLeO (2) 2CO 2 H 2 CH 4 d CO 2 + 59 Kcal / mol, and (3) CO 2 4H 2 CH 3 + 2H20 + 30 (cal / mot,

  that the diameter of the catalyst tubes should not exceed a few milli-

  meters if one wants to have total security against runaway under any

  what condition. Such a small tube diameter is not usable in an industrial operation. The greatest difference between the example of WeLsenaere and Fromernt and the methanation reactions appears to be the high pressure and high molar concentration in the latter, and the high heat production per unit volume of catalyst which in turn

  results. Similar conditions apply to other

  hydrocarbons, eg Fischer-Tropsch synthesis for the formation of

  of gasoline and / or olefins: (4) riCO + 2nH 2 - (CH 2) n + H 2 O + about 40 Kcal / gram atom Ce

  or the so-called Mobil synthesis for the formation of hydrocarbons from

  cools, for example: (5) nCH30H (CH2) n + nH20 + about 12 Kcal / gram atom C.

  Thus, it is not possible without special measures to prevent an increase

  temperature up to the temperature or a value close to the temperature determined by the adiabatic increase in temperature for

  such reactions. Welsenaere and Froment suggest a dilution of the

  The catalytic inert filler is used in the catalyst bed, whereby the amount of heat produced per unit volume of catalyst decreases. However, it has been found that a relatively high degree of catalyst dilution is required, which necessitates an increase in the volume of the catalyst itself to "ignite" the reaction. Thus, this method has the disadvantage of requiring an increase in the size of the reactor and, in this way, an increase in capital expenditures; more important is often the fact that the dilution has the disadvantage of decreasing the resistance of the catalytic filling or its absorption capacity for the catalyst poisons brought by the reaction stream. Therefore, such dilution of the catalyst is not a satisfactory solution to the problems of temperature control in the

  cooled exothermic catalytic processes.

  To further investigate the problems, some experiments were conducted in an air-cooled methanation reactor with a catalyst having the manufacturing designation MCR-2X. It is a microporous, temperature-stable, mechanically resistant catalyst with nickel crystallites of the same order of magnitude as the pore diameter, on a y-alumina support (see Karsten Pedersen, ALLan Skov and JR Rostrup-Nielsen). , ACS Symposium, Houston, March 1980). The catalyst in the form of a cylinder (having a height and a diameter of 4.3 mm) was used In some of these experiments, the catalyst was diluted by cylinders having no

  of catalytic activity of the same geometric form. We kept the pressure

  in the reactor at 26 kg / cm 2 and a feed gas was

  the composition was: 70% H2 - 9% CO - 10% CO 2 and 11% CH 4.

  These experimental conditions lead to thermally determined adiabatic temperature increases between 380 and 400 ° C., whatever

  the degree of dilution of the catalyst.

  It was found that it was not possible without dilution of the catalyst of

  limit the increase in temperature that became the 380 to 4000C cited.

  With dilution ratios of 1/3 and 1/5 (ie a volume of cylin-

  inert 3 and 5 times respectively, that of the catalytic bodies), actual temperature increases were measured which were up to 500C below the expected adiabatic temperature increases of 380 to 400 ° C. In these experiments, the temperature of the tube wall in

  the hot spot area was about 6000C or more, which means that

  an unrealistically high tube wall temperature in an industrial operation; in a cooled industrial reactor, the highest pipe wall temperature would not exceed 4000C. The limitation

  the temperature increase obtained can be considered as important.

  aunt, although by itself it is not of great industrial importance

  trielle practice, and although the dilution in this way is not very

  advantageous for the reasons given above.

  However, a detailed analysis of the reduced temperature increase showed that this reduced temperature increase was due to the fact that

  the speed of the reaction had not increased as much as

  depending upon the kinetics of the reaction, and it has surprisingly been found that this reduced reaction rate is due to the fact that the reaction rate calculated at high temperature exceeds the diffusion rate of the reactants through the gas film. surrounding the catalyst particles. Since gas phase diffusion is almost independent of temperature (activation energy 1-2 Kcal / mole), the reaction rate encounters a barrier due to the absence of substances to react atusites reactions inside the body of the

  catalyst, and that's why the reaction can not get carried away.

  It is an object of this invention to use this surprising observation to indicate how it is generally possible to prevent a runaway in gas phase exothermic catalytic reactions in cooled reactors containing a bed of a particulate porous catalyst which is active in regarding the desired reaction. According to the invention, this goal is achieved when each of the individual particles from outside the catalyst has a reduced catalytic activity zone with respect to the reaction.

desired.

  Thus, the outer part of each individual catalyst particle

  may consist of a completely inert layer, or a layer

  having reduced activity for the highly exothermic reaction in question.

  In the case of the simultaneous presence of a strongly exothermic main reaction and one or more secondary reactions (which may be less exothermic, or even thermally neutral, or endothermic), the outer layer may, if desired, be catalytically active

  for the secondary reaction but not for the main reaction.

  As an example, it can be mentioned that reactions (1), (2) and (4) above,

  which are catalyzed by nickel, are accompanied by the reaction of

  addition of the gas to the water, also called displacement reaction: (6) CO + H 2 CO 2 + H 2 + 10 Kcal / mot,

  which is catalyzed, inter alia, by copper.

  Whether the outer layer is inert or catalytically active

  With respect to a side reaction, but if it is inactive with respect to the main reaction, the diffusion through the aforementioned film also extends to include diffusion through the inert outer layer of the catalyst. Therefore, it will be possible to determine the maximum reaction rate in the reactor since it is determined by the thickness of the inert layer. The thickness of

  the inert layer towards the main reaction will be practically one to several

  if orders of magnitude greater than the thickness of the gaseous film surrounding the catalyst. On the basis of calculations, the thickness of this gaseous film can

  to be estimated at about lp under normal industrial conditions.

  According to the invention, the outer layer of the catalyst particles can

  therefore advantageously have a thickness of 0.01 to 2 mm.

  Thicknesses will be practically only about 0.5 or 1 mm thick for this layer when using catalyst particles

relatively large.

  A general explanation of the invention as defined is that a given catalyst body having a homogeneous activity and a homogeneous pore system, when exposed to a mixture of given reaction substances, is subject to various reaction restrictions when the reaction temperature changes (increases). At low temperature, the reaction rate is limited by the material

  catalyst. Here, the reaction is in the so-called intrinsic viscosity range.

  that in which there is almost no concentration gradient of the reagents through the pore system of the catalytic body. When the temperature

  increases, the activity of the catalyst increases and this gives an increased gradient.

  reagents through the catalytic body. At a certain point, a temperature range is reached where the diffusion of the reagents through the catalytic bodies becomes the parameter limiting the reaction rate; this means in fact that we do not use all the catalytic material in

  the reaction; the efficiency factor becomes <1.

  As the temperature increases further, the parameter determining the reaction rate m will be the mass transport through the gas film surrounding the catalytic bodies. Usually, this speed is too great for the reactor cooling surface to limit the temperature of the

  hotspots at a level well below that determined by increasing the

  adiabetic temperature, which means in many cases that thermal agglomeration can not be avoided or, in the case of certain methanation reactions, the formation of carbon can not be avoided. However, according to the invention, a restriction surface is incorporated in the individual catalytic bodies by increasing the gas film or decreasing in any way the penetration power of the reagents, whereby the reaction rate, and consequently also the 'increase of

temperature, are limited.

  It could have been expected that the incorporation of such an inert "retardation layer" would cause difficulties in starting or "igniting" the exothermic reactions. This however does not occur because the reaction rate is slow in the ignition zone in

  any condition, so that the diffusion of gas through

  towards the inert layer will not be restrictive (determining) for the

  reaction rate. This layer will become a limiting factor, i.e. practically it will not start to become effective, until the reaction rate becomes such that it is desirable to slow it down. Since the inert layer is only a small

  part of the catalyst pellet, the resistance of the catalyst to the emulsion

poisoning is maintained.

  In this connection, it may be mentioned that this outer layer can constitute for very small catalyst pellets more than 50% of their volume; but normally, its share will be much less, for example for cylindrical catalytic bodies of height and diameter of 4.2 mm, usually 1 to 10%, especially 2 to 10%, and frequently 2 to 5% of the volume. . The reactor can be of any type of reactor cooled for exothermic reactions, for example a tubular reactor or a reactor having a larger volume of the desired shape and containing tubes.

  in which a cooling medium flows.

  The catalytic bed will usually be a so-called fixed bed, but it can be used

  also the invention in relation to fluidized bed catalysts.

  As mentioned, the outer layer must have a reduced catalytic activity and, although it is often appropriate according to the invention to use a

  catalyst whose individual particles of the outer part possess

  If an area is inactive with respect to the desired reaction (or the main reaction), it may in some cases be advantageous to use

  a catalyst in which said outer zone has a certain amount of

  but reduced in reaction (or main reaction).

  The outer zone can be provided in various ways, frequently by analogy with techniques known in the pharmaceutical industry where pellets are prepared with several layers of various compositions or

  various concentrations of an active substance.

  Thus, it is possible to first prepare catalytic bodies of the

  usual kind, consisting of a porous support containing the catalytic material

  lytically active in pore systems. These can be prepared by known techniques, for example by coprecipitation or by first preparing porous carrier bodies which are then impregnated with the catalytically active material. The catalyst thus prepared is then immersed in a gel or solution of the inert carrier material of the same kind

  or of a kind other than that of the catalytic support itself.

  If the outer layer does not have to be completely catalytically inert but must have a reduced catalytic activity, then it can be impregnated by ensuring that the concentration of catalytic material

  active becomes weaker than that of the inner layer.

  We can combine impregnations, washes and treatments by alternating chemical bodies, thanks to which we can obtain the structure - and the

desired combination of structures.

  It is possible to apply to a finished catalyst an inert layer or even a catalytically active layer partially obstructing the orifices of the pores, for example by electrolysis or by deposition of the vapor phase, whereby the activity of the catalyst in the outer layer is reduced and the diffusion rate inward of the catalyst. A special structure can be obtained by pelletizing a mixture of disintegrated particles of the catalyst with an inert support material. The diffusion effect is thus combined with a dilution of the catalyst, whereby the

degree of dilution.

  Practical embodiments of these methods and their co! Imasams

  will be obvious to the technician.

  As indicated above, it is possible to use the described principle only

  in the case of a single exothermic reaction, but also in these

  o an exothermic main reaction and one or more seismic reagents

  The same reaction is carried out in the same reactor which contains a bed of particulate catalyst which is active towards the main reaction.

  each of the individual particles of said catalyst according to the inetaim pams-

  has an area of matter that is at least partial-

  inactive towards the main reaction, but catalytically

  to one or more side reactions.

  A form of practical reaction of such a catalyst or, alternatively, catalysts in accordance with the principle of the invention is a catalyst whose particles consist of a porous carrier material which is catalytically soluble with respect to the desired reaction. or the reacinu: pr-nr pa (but optionally catalytically active in that coermnMM a secondary ractimm), and which contains in a part of the pore system ume master that is catalytically active with respect to the reaction souit M reaction main, so that the pores of the zone-exerial

  support particles are devoid of the material which is catalytally

  the desired reaction or reactio. prfrcipale.

  The content in the pores of the catalytically active material may, for example, be provided by coprecipitation, by electrolysis, by deposition to

  from the gaseous phase, or by impregnation from the liquid phase.

  In cases where the catalyst is to be catalytically active to a reaction, the catalyst particles may according to the invention consist of a porous, inactive material which contains in a part of the porous system a material which is catalytically active with respect to desired reaction, the pores of the outer zone of the support being partially

  blocked by a catalytically inactive material.

  In another embodiment of the process, it can be used both in cases where a single reaction is catalyzed and in cases where the main reaction as well as one or more side reactions

  catalyzed by different components of the catalyst, the catalyst used

  built into the structure of "islands" embedded in the main structure of the support

  porous. In this case, the catalyst particles according to the invention

  in a porous carrier material which may itself be catalytically active or may comprise a component which is catalytically active towards one or more desired side reactions, the particles having a structure such that the "streams" of the support having a high the pores of the material which is active with respect to the desired reaction or the main reaction are statistically uniformly distributed within the catalyst, but more rarely present or absent in the

outer zone of the catalyst.

  Catalysts prepared according to the principles described in a particularly advantageous manner for hydrocarbon reactions can be used.

  such as those indicated above, including methanation reactions.

  The catalyst may be a known support material, for example γ-alumina, magnesium-aluminum spinel, silica, zirconia, titanium oxide or a combination of two or more of these materials, with catalytic nickel. active, the outermost zone being without

  nickel or with a reduced concentration of nickel. In this way, we ob-

  directly holds the desired shell effect.

  However, the principle can be used even more elegantly;

  as mentioned above, the reactions (1), (2) and (4) are accompanied -

  by the reaction of the gas with water (6). The reactions (1) - (4) and also the reaction of the gas with water are catalyzed by nickel, whereas the reaction

  from gas to water is also catalyzed, inter alia, by copper.

  It is advantageous that the methanation reactions are accompanied by the reaction of gas with water or displacement so that the partial pressure of the carbon monoxide is reduced before it comes too much in contact with the nickel catalyst. , since CO, to some extent, is a catalytic poison for nickel that can be reduced

  poisoning by decreasing the partial pressure of CO.

  It should be noted that it is the metallic nickel and the metallic copper which catalyze the reactions, whereas the metal catalyst is applied in the form of a compound, usually a nitrate or a hydroxide, which is then oxygenated, for example by calcination, and finally reduced to free metal, frequently during the start of the desired reaction using the hydrogen present among the reactants; in the following, we will ignore these

  circumstances, and the description will deal only with free metals.

  It is known that copper and nickel alloys have a very low catalytic activity above a certain copper content [cf. M. Araki and V. Ponec, J. CataLysis 44, p. 439 (1976) 1; this point is also illustrated in Exempt 1 below. However, the Ni / Cu catalyst still has activity for the conversion of carbon monoxide to carbon dioxide by the reaction of the gas with water (6). These can be used in accordance with

  the present invention to prepare a nickel catalyst the outer portion of which

  has a nickel-copper alloy shell. In this way, a slowing of the very exothermic methanation reaction is obtained first, but also a reduction of the partial pressure of the oxide.

  of carbon by the reaction of the gas with water, thanks to which the kernel

  lytically containing nickel avoids the so-called a-deactivation reaction (see The publications cited above by Karsten Pedersen, AlLan Skov and

  J.R. Rostrup-NieLsen in the ACS Symposium).

  By 6-deactivation, the adsorbed carbon monoxide is slowly converted to low reactivity carbon deposits, which deactivates the catalyst, but this is avoided by using porous nickel-containing catalyst particles in accordance with the invention. each particle contains

copper in its outer layer.

  The copper-containing external catalytic film can be formed by various means. For example, particles of a nickel catalyst can be first prepared in a known manner, for example by impregnation or coprecipitation, and then immerse the particles in water or in another liquid and then in a liquid. impregnation containing a compound of

  copper, for example nitrate or copper hydroxide.

  Another method is to precipitate copper hydroxide

  shot of copper nitrate in the outer parts of the catalyst pore system. It is this method that is advantageous if the catalyst is

  if it contains, for example, free magnesium oxide.

  This can be achieved for example by using a magnesium spinel carrier.

  sium-aluminum that is burned at such a temperature (about 1100C) that

  the unreacted magnesium oxide has some reactivity. Alternatively, one can first fill the pore system with a

  suspended magnesium oxide or other base, such as calcium oxide or solutions of an alkali metal hydroxide. The copper hydroxide will be precipitated in the outer 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 method described can be used for nickel catalysts in which the nickel has been evenly distributed by other means, as mentioned, for example, for coprecipitation, nickel nitrate impregnation in the absence of MgO or other alkaline compounds in the pore system. If the catalyst support contains free magnesium oxide or other alkaline compounds, the nickel can be applied before or after the copper by impregnation with the nickel-hexamine formate, which in itself is alkali but does not cause nickel precipitation by contact with alkaline compounds. In this way, it is possible to control the nickel / copper ratio in the outer zone of the catalyst

  and, at the same time, to obtain a uniform distribution of the nickel content.

  The reaction rate of the methanation and thus the reaction temperature can be controlled according to similar principles using

  a catalyst based on vanadium or molybdenum in an atmosphere containing

  sulfur, as described in French Application No. 80-26,525.

  It may be especially advantageous to use the principle of pre-

  This invention relates to the process described in French Patent Application No. 80-26,524 for preparing a gaseous mixture having a content

  high in C2 hydrocarbons, by reacting a mixture of feed gas with

  containing hydrogen and carbon oxides with a catalyst containing molybdenum and / or vanadium and iron and / or nickel in the presence of gaseous sulfur compounds. This reaction is a synthesis of

  Fischer-Tropsch, and it is very important to maintain the temperature

  because higher temperatures will shift the equilibrium towards methane production. Therefore, in accordance with

  the present invention, the catalytic bodies may be provided with a

  inactive support material keel, or shell containing copper

  to synthesize the reaction of gas with simultaneous water.

  The process according to the invention will be illustrated below by the description

  some examples given in a non-limiting way.

EXAMPLE 1

  A - Series of catalysts were prepared by coprecipitation of sodium silicate and varying amounts of copper and nickel nitrates by

sodium bicarbonate.

  The precipitated product is formed into particles which are washed to remove the sodium compounds, dried at 120 ° C., calcined at 5000 ° C. and reduced in hydrogen at 500 ° C. The methanation activity was measured at 1 atmosphere and 2500C by passing through a gas consisting of 1% CO in H 2 in an amount of 100 Nt / h on the catalyst in the form of irregular bodies having the size (determined by sieving). 0.3-0.5 mm. The following results were obtained:

TABLE I

  It appears that even a small amount of copper decreases very strongly

the activity of methanation.

  B-Three catalysts were prepared by impregnation of a yalumine support having an internal surface of about 40 m 2 / g in solutions of copper nitrate, nickel nitrate and a mixture of nickel nitrate and nickel nitrate respectively. copper nitrate. The impregnated supports were calcined at 550 ° C. and reduced in hydrogen at 720 ° C. The catalysts were tested in a reactor at a pressure of 1 atmosphere at 53.5 NZ / h of gas constituted by 61, 8% by volume of H2, 18.2% by volume of H20 and 20.0% by volume of CO,

  on 0.2 g of catalyst in the form of particles of 0.3-0.5 mm.

  In this way, the following rates were measured for the conversion of carbon monoxide (gas reaction to water, displacement reaction) and methanation:

TABLE II

Reaction rate at 3750C,

-3 mol / g / h -

  Displacement Methanation Cu ............... 1.05 0 Ni., Cu ........... 73 5.2 Ni +) ........... - 43 Ni ++ i) ........... 1431 1608 +) - ++) cf. next page Catalyst% by weight Report Atomic Activity No. Ni Cu Ni / Cu + Ni 10-3 mol / g / 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

Notes from Table II -

  +) measured at 311 ° C as the reaction temperature; could not be controlled at 3750C

  +> measured on the catalyst mentioned above MCR-2X.

  It appears that the Ni, Cu catalyst has a good activity for the reaction

  gas, while none of the Cu-containing reactors has a

  significant significance for methanation.

EXAMPLE 2

  A support constituted by magnesia with a lower alumina content (Mg / Al ratio 7/1) is impregnated in the form of cylinders whose height

  and the diameter were 4.5 mm by saturated saturation of copper nitrate.

  The impregnated substrates are calcined at 550 ° C. When the particles were broken, it was clear that the copper had accumulated in the form of

  black thin shell on the outside of the stand. Microscopic examinations

  that the thickness of the layer is about 50p.

  The copper impregnated support is impregnated with a saturated solution of nickel hexamine formate prepared by dissolving 23 g of nickel formate in 50 ml of concentrated aqueous ammonia. The catalyst impregnated at 3000C is calcined. The broken particles have a regular coloration and therefore,

  a regular distribution of nickel.

  By analysis, 0.6% by weight of Cu and 2.7% by weight of Ni were found.

  A corresponding catalyst was prepared by impregnating it first in nickel hexamine formate and then in copper. The copper shell

  appears in the form of a dark coloration of the outer zone of the

  cles. This catalyst was designated as catalyst A and, as a control, a copper-free methanation catalyst, called a catalyst, was prepared.

  B, impregnating the support in the nickel-hexamine formate.

EXAMPLE 3

  A methanation catalyst containing about 25% by weight of nickel is impregnated on a stabilized alumina support in the form of 4.2 × 4.2 mm cylinders in an alumina gel prepared by suspending 11 g of alumina in ml of aluminum. water, and is coagulated with 5.6 ml of concentrated nitric acid. We calcine

the catalyst impregnated at 55 ° C.

  A corresponding preparation was carried out with the difference that 10 ml of alumina gel was premixed with a nitrate solution of

  copper. The impregnated and calcined catalyst has an outer shell

  holding copper, estimated by microscopic examination as having a thickness

about 0.5 mm.

24 8247 3

  Small amounts of copper nitrate had penetrated inside the catalyst, but it could be estimated that the ratio between the copper content of the outer zone and that of the interior was about 3/1. This catalyst is designated as C.

EXAMPLE 4

  This example shows how a coating on a catalyst applied to

  By means of a gel, the reaction rate can be reduced to a high temperature.

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

  A catalyst was activated (reduced) without such a coating in the same way.

* It is designated E and it is identical to the catalyst MCR-2X mentioned above.

EXAMPLE 5

  This example illustrates the testing of the catalysts A-E mentioned in

Examples 2 to 4

  A tablet of the catalyst to be tested between two thermoelements is fixed in a tubular reactor having an internal diameter of 6 mm. Then, it is heated in pure hydrogen to about 300% C to remove any traces of nickel oxide. Then, the hydrogen inlet is cut off and the system is then fed with a synthetic gas consisting of 9% CO in hydrogen,

  at a speed of about 100 NQ / h throughout the experiment.

  At about 3000 ° C. the gaseous stream produced is analyzed by means of a gas chromatograph and the amount of methane produced is determined. Then, we increase the temperature by step and we determine the

  methanation rate in each step.

  Table III, below, shows that the catalysts having an inert outer layer applied by the gel have a reduced reaction rate.

  at high temperature compared to that of the homogeneous standard catalyst.

TABLE III

  I Activity Temperature Atvt Methanation test catalyst millimoles / g / h

A 300 1.4

  AL203.MgO support impregnated 360 4 with copper nitrate 417 6 and nickel formate 518 12 hexamine 603 11

678 12

B 346 18

  A1203.MgO support impregnated 389 with nickel formate 482 180 hexamine. 577,250

715,240

C 330 55

  NiO.A1203 coated catalyst 425 95 by Al 2 O 3. 570 130

741 95

D 243 3.5

  NiO.Al.sub.2 O.sub.33 coated catalyst 295 with a shell containing 33650 copper oxide and 440.degree.

A1203. 597 60

E 274 80

  NiO.A1203 catalyst. 323,180 (known MCR-2X catalyst 379,250 higher without restrictions). 508,315

569,310

  The reason that catalysts B and E have maximum activities

  males in the temperature range considered is the thickness of the gas film during the test, in which the gas velocities were considerably

  weaker than in an industrial operation_; in an industrial operation

  normal triplet, with higher gas velocities, the gas film

  would be thinner and, hence, slow down diffusion to a lesser degree.

  It has been estimated that the mass transfer value for the gas film is about 7 kmol / h / m2, whereas this value would be about 100 Kmol / h / m2

  in industrial conditions. This implies that the difference in

  high temperature with and without shell will be larger than indicated

in Table III, above.

  Of course, the present invention is not limited to the examples

  and modes of implementation mentioned above; it is likely to

  Many variants accessible to those skilled in the art, depending on the applications

  contemplated and without departing from the spirit of the invention.

2 L 8247 3

Claims (10)

REVENDI CATIONS   A process for carrying out a gas-phase exothermic catalytic reaction in a cooled reactor containing a bed of a particulate porous catalyst which is active with respect to the desired reaction, characterized in that each of the individual particles from outside the catalyst has a zone of reduced activity with respect to the desired reaction.   2. A process according to claim 1, characterized in that each of the individual particles from outside the catalyst has a zone   which is inactive with respect to the desired reaction.   3. A process according to claim 1 or 2, characterized in that this outer zone of the particles of the catalyst has a thickness of 0.01 at 2 mm.   4. A process according to any one of the preceding claims in   which a main catalytic exothermic reaction and one or more secondary catalytic reactions are carried out substantially   simultaneously in the same reactor which contains a bed of a particular catalyst   which is active towards the reactions, characterized in that each of the individual particles of the outside of said catalyst has a   an area of matter that is at least partially inactive to the reaction   and catalytically active against one or more side reactions.   5. Process according to any one of the preceding claims,   characterized in that the catalyst particles consist of a porous carrier material which is catalytically inactive with respect to the desired reaction or reaction and contains in a part of the pore system a material which is catalytically active with respect to the desired reaction or the main reaction or reaction, the pores in the outer zone of the support being free of the material which is catalytically   active towards the desired reaction or main reaction.   6. A process according to any one of claims lM 3, characterized   in that the particles of the catalyst consist of a porous, inactive carrier material which contains in a part of its pore system a material which is catalytically active with respect to the desired reaction, the pores of the outer zone of the support being partially   blocked by a catalytically inactive material.   7. A process according to any one of claims 1 to 4, characterized   in that the catalyst particles consist of a porous carrier material which is catalytically inactive with respect to the desired reaction or the main reaction, but which is optionally catalytically active or comprises a component which is catalytically active with respect to one of the secondary reactions. or more, the particles having a structure such that "islands" of the support having a high content in the pores of the material which is active with respect to the desired reaction or the main reaction are distributed statistically uniformly within the catalyst, but present more rarely or absent in the outer zone of the catalyst.   8. A process according to any one of claims 4 to 7 for   kill methanation reactions accompanied by the reaction of gas to water   in a reactor cooled with a particulate porous particulate catalyst   nickel, characterized in that nickel-containing porous catalyst particles are used, each of which contains copper in its outer layer. 9. A process for preparing the catalyst defined in claim 8, characterized in that a porous particulate support containing nickel or nickel oxide applied by impregnation or precipitation of the vapor phase or by coprecipitation of the carrier material and catalytic material, said carrier material comprising an alkaline material, with a solution of a copper salt for precipitating copper hydroxide in an outer zone, after which it is dried, calcined and reduced the catalyst.   10. A process according to claim 9, characterized in that the material support includes active MgO.