MXPA00000287A - Catalytic composition for controlling exothermic reactions on a fixed bed - Google Patents

Abstract

Catalysts for exothermic reactions conducted in a fixed bed, comprising an inert diluent constituted by metal granules in which the metal has a thermal conductivity of more than 0.5W/cm/K, particularly catalysts for the oxychlorination of ethylene to 1,2-dichloroethane.

Description

CATALYSTS FOR EXOTHERMAL REACTIONS IN A FIXED BED DESCRIPTION OF THE INVENTION The present invention relates to compositions comprising a catalyst for exothermic reactions carried out in a fixed bed, as well as a metal diluent used to reduce or eliminate the formation of hot spots in the fixed bed. Particularly, it relates to compositions in which the catalyst is a catalyst used for the oxychlorination of ethe to 1,2-dichloroethane. In the exothermic reactions, the elimination of the heat by means of the cooling fluid is a decisive factor for the control of the reaction and, therefore, for the possibility of achieving high conversions and selectivities. While this problem is hardly important in fluid bed operations due to the high overall coefficient of exchange that can be achieved in these conditions, fixed-bed technology the problem of heat removal is extremely important, since the The concentration of the reactants is high at the entrance of the bed and, therefore REF .: 32481, the reaction rate and heat generation are the highest. Consequently, the temperature inside the catalytic bed usually increases rapidly, creating high temperature zones (hot zones) which give rise to considerable problems in terms of the rapid aging of the catalyst and cause the resulting loss of selectivity due to the increase of secondary reactions. Considering that the quantity of heat exchanged, for a given cooling surface and for a given general exchange coefficient, is governed by the difference between the temperature inside the bed and the temperature of the cooling fluid, and that under Normally the heat exchange rate is regulated by said temperature difference, the temperature in the hot zone will tend to increase until the temperature difference eliminates all the heat generated by the reaction. However, in the final part of the bed, the speed of the reaction (and therefore, the generation of heat) is very low and, consequently, the hot zones are not produced. In order to reduce the temperature of the hot zones acting on the catalyst, two solutions can be adopted: use a catalyst that is not very active in the region of the catalytic bed located at the entrance of the reagents; - diluting the catalyst in said region using inert solid diluents. The diluents used to date comprise materials such as graphite, silicon carbide, macroporous carbon, low surface area alumina, silica and glass beads. Said diluents, due to their low coefficient of thermal conductivity, are not suitable for the efficient transfer of heat from the region of the hot zone to the wall of the heat exchanger. In addition, also due to their low thermal conductivity, the diluents are not able to transfer heat properly from the areas where peaks form in the concentration of the catalysts due to the irregular mixing of the catalyst with the diluent being created, consequently the hot Now, it has been unexpectedly discovered that if metals which are inert toward reactants and reaction products, and which are of high thermal conductivity, are used as diluents, not only the performance and selectivity of the catalyst is improved, and therefore the productivity of the installation, but also it is possible to reduce or prevent the loss and / or aging of the catalyst in cases in which said problems usually occur. Particularly, in the case of the oxychlorination of ethe to 1,2-dichloroethane, the diluents of high thermal conductivity allow the reaction to be carried out in a single step instead of in multiple stages, as is normally the case. The diluents that can be used in the compositions according to the invention are metals with a thermal conductivity greater than 0.5 W / cm / K (the value considered in the temperature range ranging from 400K to 1573K, equivalent to between 127 ° and 1000 ° C). Copper has a thermal conductivity (W / cm / K) of 3.93 to 400K and 3.39 to 1573K; the values for aluminum being from 2.4 to 400K and from 2.18 to 800K; the values for nickel being 0.8 and 0.76 to 400K and 1200K, respectively; Zinc has a conductivity greater than 1 in the temperature range considered. The following data are examples of the coefficients related to the materials not included among the usable materials: 0.13 W / cm / K to 673K for alumina; 0.04 and 0.01 for graphite at 400K and 1200K; 0.19 and 0.25 for stainless steel at 573K and 973K.

The metals that can be used in the compositions according to the invention are chosen so that they are substantially inert with respect to the reactants and the products of the reaction in which they are used. The preferred metal is copper, due to its high thermal conductivity and high density that allow a high heat capacity per unit volume of metal and, therefore, absorption and then the rapid transfer of considerable amounts of heat. Aluminum and nickel can also be conveniently used, particularly under reaction conditions that require high chemical inertness. Preferably, metal diluents having a shape and geometrical dimensions similar to those of the granular catalyst with which they are mixed are used. It is also possible to use different shapes and dimensions. The preferred forms are those that provide a large surface area for each unit volume of diluent associated with significant percentages of cavities, which facilitates the exchange of heat and reduces pressure losses.

Examples of such forms are cylindrical configurations with a through bore having a large diameter and annular configurations. Examples of cylindrical shapes are the configurations that have multiple lobes with through holes in the different lobes and other configurations that have a large geometric area. Forms of this type are described (described for catalysts and vehicles) in USP 5,330,958, the disclosure of which is incorporated herein by reference. The dimensions of the cylindrical shapes are generally between 3 and 10 mm in height and 5 to 10 mm in diameter. The percentage of diluent is a function of the exothermic nature of the reaction and its kinetics. Percentages of 10 to 80% by volume of the mixture can be conveniently used. The catalytic compositions containing the metal diluent are used to form the bed in the area at the entrance of the reagents. It is also possible to use different layers of the bed in which the concentration of the catalyst increases towards the lower part of the bed. A typical example of an exothermic reaction carried out in a fixed bed in which the compositions according to the invention can be conveniently used is the oxychlorination of ethylene to 1,2-dichloroethane. The following reactions are cited as examples: the oxidation of n-butane to maleic anhydride; the oxidation of o-xylene or naphthalene to phthalic anhydride; synthetic natural gas from methane; vinyl acetate from ethylene and acetic acid: ethylene oxide from ethylene. As mentioned above, in the case of the oxychlorination reaction, it has been found that, in addition to the advantage of achieving higher yields and selectivities, the use of diluted catalysts according to the invention makes it possible to carry out the reaction in a single step instead of in multiple stages, as normally occurs in the prior art processes. The catalysts diluted according to the invention are used under the reaction conditions usually applied; however, it is possible to optimize said conditions in order to use the highest possible result of the catalysts in the best possible way, both as regards their performance and their selectivity. The catalysts that can be diluted with the metal diluents comprise all the catalysts that can be used in exothermic reactions carried out in a fixed bed. In the case of catalysts for the oxychlorination of ethylene to 1,2-dichloroethane, the preferred and representative catalysts that can be used are based on cupric chloride or cupric hydroxychloride, comprising promoters selected from the alkali metal chlorides and / or alkaline earth metal chlorides, optionally rare earth. Said catalysts are supported on inert porous supports, particularly alumina having a surface area of between 50 and 300 [mu] g / g. Catalysts of this type are widely described in the literature and particularly in EP-A-176432, the disclosure of which is incorporated herein by reference. In the catalysts described in EP-A-176432, the concentration of cupric chloride is lower on the surface than inside the catalyst pellet.

The following examples are provided to illustrate, but in no case limit, the scope of the invention.

Examples A) PREPARATION OF THE CATALYST 300 g of alumina were heated at 450 ° C, in the form of cylindrical granules with three lobes provided with three equidistant through-holes drilled parallel to the axis of the cylinder. They were then impregnated with an aqueous solution containing 9.33 g of CsCl and heated at 500 ° C for 1 hour. An aqueous solution containing 58.33 g of CuCl_.2H20 and 12.45 g of KCl was prepared independently (in order to obtain a Cu content of 4% and a K content of 2%, expressed as weight percentage in the final catalyst). In order to facilitate the dissolution of the chlorides, 8 g of HCl in a 35% aqueous solution was added. This solution was used to impregnate the support granules previously treated with CsCl. The resulting catalyst was dried in an oven at 120 ° C overnight and ready to be used.

B) DESCRIPTION OF THE REACTOR In order to check the performance of the catalysts, diluted with different materials, a tubular reactor having an internal diameter of 26 mm and a height of 130 cm was used. The construction material of the reactor was Ni 200. The reactor was provided with a thermostatic control jacket inside which oil circulated and which had conduits to supply the reagents. The reagents were dosed (HCl, CX4, 02 and N; ) and were controlled by means of mass flow meters. The reaction products were cooled in the reactor outlet: the liquid products (EDC, unconverted HCl, chlorinated by-products and water of reaction) were collected in a flask, while the non-condensable ones were sent (0, X, CO and C0) at the exit of gases after having been measured and analyzed by means of chromatography. The liquid products were composed of two phases, an aqueous phase and another organic phase; the two phases were separated in a separatory funnel, weighed and analyzed: the hydrochloric acid titration was carried out for the aqueous phase, while the organic phase was analyzed by means of chromatography in order to determine the purity of the EDC. The reagents were normally supplied at a temperature of 210 ° C; the reaction was brought to the selected temperature and, when stable and constant conditions were reached, the collection of the liquid products and the gas control were carried out for a period of 1 to 2 hours.

COMPARATIVE EXAMPLE 1 The catalyst according to the above description was prepared and charged into the reactor. It was mixed with graphite in the following manner: a layer of the undiluted catalyst having a thickness of 50 cm was loaded in the lower part of the reactor (the part near the product outlet duct); an amount of 185.2 g (equivalent to 270 cc) was charged; the catalyst (45.5 g, equivalent to 64 cc) mixed with graphite (82.2 g, equivalent to 96 cc) was charged in the upper part of the reactor, up to a height of 30 cm; the resulting mixture contained the 40% of the catalyst by volume. Accordingly, the total height of the catalytic bed was 80 cm. A support coating for a thermocouple was disposed coaxially in the reactor. 9 thermocouples of said coating were introduced at a distance of 10 centimeters each to detect the temperature of the reactor. The thermal profile of the reactor was obtained by means of the different thermocouples; said profile is represented in the graph of Figure 1. Samples were collected in order to determine the yield: the test conditions and the associated results are related in Table 1.

EXAMPLE 1 The same procedure was used as in comparison example 1. The amounts of the catalyst are the same, the only difference being the type of diluent used, which is in the form of copper rings of 7 mm in height, 6 mm in outer diameter and 5.6 mm in internal diameter. The amount of diluent by weight is 225.7 g (96 cc). The results of the tests are given in Table 1. The graph of Figure 1 also represents, for comparison purposes, the thermal profile obtained in example 1. The influence on the efficiency of the use of copper as a diluent is evident; due to a less hot lane zone, a significant increase in activity (expressed by the conversion of hydrochloric acid) and selectivity (due to reduced formation of carbon oxides and chlorinated by-products) is achieved. The results of the tests are given in Table 1. The graph of Figure 1 also represents, for comparison purposes, the thermal profile obtained in Example 1. The influence on the performance of the use of copper as a diluent is evident; due to a hotter hot zone, a significant increase in activity (expressed by the conversion of hydrochloric acid) and selectivity (due to the reduced formation of carbon oxides and chlorinated byproducts) is achieved.

Table 1 CONSTANT CONDITIONS Bed height cm 80 Total volume (tot) Nl / h 720 Atms Pressure 3 Oil temperature ° C 210 Linear speed cm / s 18.5 Contact time sec. 4.3 Note: EDC: 1,2-dichloroethane EC: ethyl chloride It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers.

Claims (13)

1. Having described the invention as above, the content of the following claims is claimed as property: 1. Catalyst compositions comprising a catalyst in the form of granules having a defined geometrical configuration, suitable for exothermic reactions carried out in a fixed bed, and a metallic diluent having a geometrical shape and dimensions corresponding to those of the catalyst or which are different, characterized in that the metal of the diluent has values of thermal conductivity greater than 0.4 W / cm / K in the range of 400K to 1573K .
2. 2. The compositions according to claim 1, characterized in that the metal of the diluent is copper.
3. 3. The compositions according to claim 1, characterized in that the metal is selected from aluminum, zinc and nickel.
4. 4. The compositions according to the preceding claims 1 to 3, characterized in that the metallic diluent is in the form of cylindrical granules having at least one through-hole or annular granules.
5. 5. The compositions according to claim 4, characterized in that the cylindrical granule has a configuration of multiple lobes provided with through holes in said lobes.
6. 6. The compositions according to the preceding claims 1 to 5, characterized in that the diluent is used in an amount of 10 to 80% by volume of the volume of the composition.
7. 7. The compositions according to the preceding claims 1 to 6, characterized in that a catalyst is used for the oxychlorination of ethylene to 1,2-dichloroethane.
8. 8. The compositions according to claim 7, characterized in that the catalyst comprises a cupric compound supported on an inert porous support medium.
9. 9. A composition according to claim 8, characterized in that the catalyst comprises a cupric compound selected from copper chloride and cupric hydroxychloride supported on alumina having a surface area between 50 and 300 m2 / g.
10. 10. The compositions according to claim 9, characterized in that the catalyst comprises a promoter selected from alkali metal chlorides and alkaline-earth metal chlorides optionally mixed with rare earth metal chlorides.
11. 11. The process for carrying out exothermic reactions in a fixed bed, characterized in that the catalytic compositions according to the preceding claims 1 to 10 are used.
12. 12. The process for the preparation of 1,2-dichloroethane by means of the oxychlorination of ethylene in a fixed bed, characterized in that the catalytic compositions according to the preceding claims 8 to 10 are used.
13. 13. The method according to claim 12, characterized in that the reaction is carried out in a single step.