GB2132111A

GB2132111A - Reaction tube for an exothermic heterogeneously catalysed gas reaction

Info

Publication number: GB2132111A
Application number: GB08333783A
Authority: GB
Inventors: Heinrich Josef Rudol Schiebahn; Manfred Vorwerk; Reinhard Menzer; Bernd Hohlein
Original assignee: Kernforschungsanlage Juelich GmbH; Rheinische Braunkohlenwerke AG
Current assignee: Forschungszentrum Juelich GmbH; Rheinbraun AG
Priority date: 1982-12-23
Filing date: 1983-12-19
Publication date: 1984-07-04
Also published as: GB8333783D0; JPS59133293A; GB2132111B; CA1214318A; DE3247821A1; FR2538265A1

Abstract

A reaction tube (1) for carrying out reaction between components of a reaction gas is filled with solid catalyst particles (3). The tube is subdivided into two successive sections, and in the upstream section (7) the ratio of the hydraulic tube diameter to the "equivalent diameter" of the catalyst particles (a dimension equal to the diameter of a sphere of corresponding volume) is smaller than the corresponding ratio for the downstream section (10) of the reaction tube. Additionally the upstream section (7) cooled, and is given a length such that the gas in it after reaching a maximum permissible reaction temperature can be cooled sufficiently prior to entering the downstream section (10) that the temperature in the downstream section remains below the maximum permissible temperature. The hydraulic tube diameter may be varied by mixing filler bodies with the catalyst. The reaction tube is especially suited to a methanation reaction performed on a synthesis gas containing carbon monoxide, carbon dioxide and hydrogen. <IMAGE>

Description

SPECIFICATION Reaction tube for an exothermic, heterogeneously catalysed gas reaction The invention relates to a reaction tube for an exothermic reaction between gaseous components of a reaction gas. In particular, although not exclusively, the reaction tube may be used for methanising a synthesis gas containing carbon monoxide, carbon dioxide and hydrogen. The reaction tube contains a solid catalyst which forms a bed through which the reaction gas reacting on the solid catalyst can flow and which is cooled from the outside. The reaction tube has an inlet connection for the supply of reaction gas and is connected at its outlet to a gas duct through which the gaseous product is discharged. The catalyst can be a loose fill or heap of catalyst particles, such as obtained by tipping into the tube.

In the case of exothermic gas reactions in which solid catalysts are employed, it must be ensured on the one hand that the heat of reaction does not inadmissibly overheat the solid catalyst. The rate at which the heat of reaction is liberated depends, not only upon the state of the reaction gas, but also upon the quality and the volume of the catalyst which actively participates in the course of the reaction. The heating-up of the catalyst is counteracted by external cooling of the reaction tube. On the other hand, the reaction product which is extracted as a gaseous product, should be in a predetermined thermodynamic equilibrium when extracted from the reaction tube. The length of the reaction tube and the pressure loss in it must be kept within acceptable structural and economic limits.The steps which must be taken in order to fulfil these conditions are, at least in part, diametrically opposed.

For methanising gases rich in carbon oxides, it is known to carry out the reaction using reaction tubes which are filled with solid catalyst and which have a constant tube diameter. Methanisation is applied to gases substantially comprising hydrogen H2, carbon monoxide CO, carbon dioxide CO2, and methane CH4.

The methanisation produces, for example, fuel gases which are used as a substitute for gases obtainable from natural sources, or gases which serve as energy vehicles in a long-distance energy circuit, such as that described, for example, in DT-PS 1 298 233 or DT-AS 1 601 001. In methanisation, the CH4 content of the gas is increased by exothermic reactions CO + 3H2 = CH4 + H2O At0298 K = -206 kJ/mole CO2+ 4H2 = CH4 + 2H20 AH0 298K = -165 kJ/mole proceeding on solid catalysts. The gas formed can be fed directly into the natural-gas network after separating off the water produced in the reaction, or it is again reformed with steam in the long-distance energy circuit. The heat generated in the reaction tube is carried away to a coolant (see, for example, DE-OS 25 29 316, DE-OS 29 49 588).

With reaction tubes of constant diameter, it has not hitherto been possible to satisfy at the same time all the aforesaid conditions for the performance of the process even when using qualitatively different catalyst materials. When using more than one catalyst material in the reaction tube so that the different catalyst materials are successively traversed by the reacting gas, considerable disadvantages arise in the performance of the process. In particular there are disadvantages arising from the varying ageing of the individual layers of catalyst and the resultant changes in the maximum temperature in the catalyst bed.

The object of the invention is to conduct the reaction with the use of a solid catalyst of predetermined quality, in such a way that, with limited pressure loss, overheating of the solid catalyst can be avoided on the one hand, while on the other hand there is made available at a predetermined temperature a gas mixture which is in thermodynamic equilibrium.

For this purpose the present invention provides a reaction tube for an exothermic reaction between gaseous components of a reaction gas, notably for methanising a synthesis gas containing carbon monoxide, carbon dioxide and hydrogen, the reaction tube containing a solid catalyst which forms a bed through which the reaction gas reacting on the solid catalyst can flow and which is cooled from the outside, the said reaction tube having an inlet union for the supply of reaction gas and being connected at its outlet to a gas duct through which a product gas is discharged, characterised in that the reaction tube is divided at least into two successive tube sections, of which the first tube section, upstream as seen in the direction of flow of the reaction gas has a hydraulic tube diameter which is smaller in relation to the "equivalent diameter" of solid catalyst particles present in this tube section than is the hydraulic diameter of the succeeding second tube section in relation to the equivalent diameter of solid catalyst particles present in the second, downstream tube section.

There is here meant by "hydraulic tube diameter" a diameter which is equal to the diameter of a cylindrical pipeline and which is given by the relation: dh = 4 f/U where: f = free cross-sectional area of flow of the pipeline U = circumference of the free cross-sectional area of flow.

By "equivalent diameter", there is to be understood a diameter of a sphere equal to the volume of a particle of catalyst. The equivalent diameter dg is calculated, starting from the sphere volume

in accordance with the relation:

Vp = particle volume.

This ratio of hydraulic tube diameter to equivalent diameter serves to take account of the fact that on the one hand, the dissipation of the heat of reaction is enhanced with decreasing hydraulic diameter, while on the other hand the ratio of geometrical surface to volume of the particles of catalyst influences the yield of the reaction. The calculation of the hydraulic tube diameter in the determination of this ratio is based solely upon the geometrical data of the tube section when it is empty, i.e. not filled with catalyst particles. The dimensions of the catalyst particles are disregarded for calculating the tube diameter.

According to a further feature of the invention, the first, upstream, tube section is designed with such a length that the reaction gas in this tube section can be cooled, after reaching a maximum permissible reaction temperature and until it passes into the second, downstream tube section, to such a temperature that the temperature of the reaction gas in the second tube section remains below the maximum permissible temperature. This proceeds, with advantage, from recognition that the danger of overheating of the solid catalyst exists essentially in the region of the inlet of the reaction tube. Therefore, particular care is taken to ensure that there is a considerable dissipation of heat in this region. In the succeeding, second tube section, the desired thermodynamic equilibrium is established at a predetermined temperature.This subdivision of the reaction tube into two tube sections which are individually adapted to the course of the reaction results in relatively short reaction tubes and low pressure loss.

In a further development of the reation tube, the upstream tube section contains, in addition to the particles of solid catalyst, space-filling bodies which reduce the hydraulic diameter of the tube and are catalytically inert in the exothermic reaction. In fixing the ratio of the hydraulic tube diameter to the equivalent diameter of the particles, it is necessary to take account of the displacement bodies only in the determination of the hydraulic tube diameter. The equivalent diameter of the particles remains unaffected by the dimensions of the displacement bodies.

Preferably, the upstream tube section has a smaller tube diameter than the succeeding downstream tube section. It would also be possible to use reaction tubes which widen conically as seen in the direction of flow of the reaction gas. For increasing the cooling action, the upstream tube section could also be subdivided into a number of individual tubes in which case the hydraulic tube diameter will be defined by the sum of the hydraulic diameters of these individual tubes.

In the following, the invention is more particularly explained by a description of exemplary embodiments.

The drawing diagrammatically illustrates a reaction tube comprising two tube sections having different diameters.

There is illustrated in the drawing a reaction tube 1 to which an exothermically reacting reaction gas can be fed by way of its inlet 2. The reaction tube 1 is filled with a solid catalyst bed 3 which promotes the reaction of the components of the reaction gas, and it is cooled over its entire length 4 from the outside. In the illustrated embodiment, the reaction tube 1 is surrounded by a pressure vessel, not illustrated in the drawing, which is filled with water under pressure. In taking up the heat evolved in the reaction, the water boils. The gaseous product withdrawn from the outlet 5 by way of a gas duct 6.

The reaction gas first flows through an upstream tube section 7 having a length 8. This first tube section has a smaller internal diameter 9 than the succeeding second tube section 10 downstream in the direction of flow of the reaction gas) of the reaction tube 1. In the transition portion 11, the reaction tube 1 widens from the diameter 9 to a diameter 12. In the illustrated embodiment, both tube sections 7 and 10 are of cylindrical form. In this case, the hydraulic tube diameters of these tube sections correspond to the internal tube diameters 9 and 12.

Solid catalyst particles 13, which all consisting of the same catalyst material, are provided in the two tube sections 7 and 10 of the reaction tube. The quantity of heat developed per catalyst mass during the reaction when a particular catalyst material is employed is known for each individual stage of the reaction. For a given throughput of a reaction gas having a particular composition and for a given solid catalyst volume, the ratio of hydraulic tube diameter to equivalent diameter in the upstream tube section 7 is selected so that, with a constant water temperature in the pressure vessel surrounding the reaction tube 1, the catalyst temperature does not exceed a maximum permissible temperature appropriate for the material of the solid catalyst. On entering the first tube section 7, the reaction gas rapidly reacts with a considerable evolution of heat. After the maximum temperature has been reached, the rate of transformation of the components decreases again and the velocity of the reaction is considerably reduced.

The ratio of hydraulic tube diameter to equivalent diameter acquires a higher value in the downstream tube section 10 than in the upstream tube section 7. The ratio is made such that as short a length of tube as possible is sufficient to set up the desired thermodynamic equilibrium at a preset temperature for the product gas. The length of the upstream tube section 7 is made such that the reaction gas is cooled to such a temperature before passing into the downstream tube section 10 that the reaction gas does not exceed the maximum permissible temperature even in the downstream-tube section, from which a smaller quantity of heat per unit time can be transferred to the water boiling in the pressure vessel.

Instead of being constructed as a single tube as in the illustrated embodiment, the upstream tube section 7 of the reaction tube 1 may be formed of a number of individual tubes which lead into the downstream tube section 10 at the transition portion 11 of the reaction tube. The individual tubes are filled with solid catalyst particles as in the illustrated embodiment and are washed by the boiling water in the pressure vessel for the purpose of cooling. For determining the ratio of hydraulic tube diameter to equivalent diameter in this case, the sum of the hydraulic tube diameters of all the individual tubes must be taken as the hydraulic tube diameter of the upstream tube section. It is also possible to employ, for.reducing the hydraulic tube diameter, space-filling bodies which have catalytically inert properties for the reaction.

For methanising a synthesis gas having a gas composition of 10% CH4, 9% CO, 10% CO2, 67% H2 and 4% N2 in a reaction tube 1 comprising two tube sections 7 and 10, a water pressure of 100 bar was established in the pressure vessel. At this pressure, the boiling point of water is 31 C. The reaction tube 1 was filled with solid catalyst particles consisting of a ceramic catalyst material suitable for high-temperature methanisation, which had a nickel base (Haldor Topsoe AIS, catalyst material MCR-2x). The catalyst particles, which were of cylindrical shape, had a mean diameter and a mean height of 4.3 mm each.This gives a value of about 4.9 mm for the equivalent diameter dg. The maximum permissible operating temperature for this catalyst material is 700"C. If the reaction gas entering the upstream tube section 7 at a temperature of 300"C and having the composition indicated in the foregoing reacted without cooling, a temperature of about 780"C would be attained as adiabatic limit temperature in the reaction tube 1. For the desired product gas quality, the escaping product gas must be set to a thermodynamic equilibrium temperature between 311 and a maximum of 370"C.

Under these boundary conditions, with a throughput of 1.8 kM/reaction gas per hour and with known data for the reaction kinetics of the catalyst MCR-2x, the reaction tube 1 having a total length of 8m was designed with a length of 3m and a diameter of 25 mm in its first, upstream tube section 7, and with a diameter of 50 mm in its second, downstream tube section 10.

For catalyst particles of the same catalyst material as in example above, but with a mean diameter and a mean height of 8 mm each for a throughput of 6.7 kM/reaction gas per hour, the reaction tube 1 had the following dimensions: upstream tube section 7, length 3 m, diameter 50 mm; downstream tube section 10, length 5 m, diameter 70 mm. The length of the whole reaction tube 1 was thus 8 m.

With the same catalyst material and under the same boundary conditions in the cooling of the reaction tube 1 as in the preceding examples, a reaction tube was designed for a throughput of 9 kM/reaction gas per hour, for two different particle sizes of the solid catalyst. In the upstream tube section 7 of the reaction tube 1, cylindrical solid catalyst particles having a mean diameter and a mean height of 8 mm each were employed, and the downstream tube section 10 was filled with solid catalyst particles having a mean diameter and a mean height of 4.3 mm each. In this case, the upstream tube section 7 had a length of 3 m and a diameter of 50 mm, and the downstream tube section 10 had a diameter of 70 mm. The total length of the reaction tube was less than 7 m. With this form of the reaction tube 1, it was possible to obtain, despite the considerably higher throughput of the escaping product gas, a composition corresponding to the thermodynamic equilibrium of the synthesis gas in the temperature range between 311 and 370"C.

Claims

1. Reaction tube for an exothermic reaction between gaseous components of a reaction gas, which tube contains solid catalyst forming a bed through which the reaction gas reacting on the solid catalyst can flow, and which is cooled from the outside, the reaction tube having an inlet connection for the supply of reaction gas, and an outlet connection to a gas duct for discharge of product gas, characterised in that the reaction tube is subdivided into two successive tube sections, of which the upstream section has a ratio of hydraulic tube diameter of the "equivalent diameter" of the solid catalyst particles in this tube section which is smaller than the ratio of the hydraulic diameter of the downstream section while the said upstream section has a length such that the reaction gas in the upstream tube section can be cooled after reaching a maximum permissible reaction temperature, and between these and entering the downstream section, sufficiently that the temperature of the reaction gas in the downstream tube section remains below the maximum permissible temperature.

2. A reaction tube according to claim 1 wherein in addition to solid catalyst particles the upstream tube section contains space-filling bodies which are catalystically inert toward the exothermic reaction, but reduce the hydraulic tube diameter.

3. A reaction tube according to claim 1 or claim 2 wherein the upstream tube section comprises a plurality of individual tubes.

4. A reaction tube according to claim 1 or claim 2 wherein the upstream tube section comprises a plurality of individual tubes, arranged so that all deliver into a common downstream tube, with the ratio of the total hydraulic tube diameter of the tubes of the upstream section to the solid catalyst particles therein being less than the ratio of the hydraulic tube diameter of the downstream tube section to the catalyst particles in the downstream tube section.

5. A reaction tube according to any one of the preceding claims containing a solid catalyst for methanising a synthesis gas containing carbon monoxide, carbon dioxide and hydrogen.

6. A reaction tube substantially as described herein with reference to the accompanying drawing.

7. A methanation plant incorporating a reaction tube to any one of the preceding claims.