The present invention relates to catalysts for the oxychlorination of ethylene to dichloroethane (DCE) capable of providing high conversion rates without sacrificing selectivity by working in a fluid bed at high temperatures, and to the process in which the catalysts are used.
Dichloroethane is an important intermediate product for the production of vinyl chloride and therefore of PVC, one of the most widely used plastic materials.
Various technologies are used in the oxychlorination reaction. The reactors can be of the fixed- or fluid-bed type, and air and/or oxygen can be used as oxidizer.
The fluid-bed process is preferred over the fixed-bed process because it offers several advantages: lower investment costs for the reactors (because they are not made of steel), an almost isothermal thermal profile without hot spots (therefore with high selectivity and limited ageing phenomena).
Fluid-bed processes use catalysts based on copper salts, preferably CuCl2, mixed with various promoters, such as salts of alkaline metals, alkaline earth metals, and rare earths. The supports are based on alumina or various aluminum silicates (attapulgite, montmorillonite, silica gels, clays, et cetera); alumina having a particle size suitable for good fluidization is generally preferred.
Catalysts must provide the following performance:
ensure the highest possible yield of dichloroethane by virtue of satisfactory selectivity and high activity (high conversion rates of the hydrochloric acid);
be able to work with good fluidization conditions, avoiding sticking (sticking of the particles, due to a polymeric form of CuCl2 with a low melting point); sticking can be avoided by reducing the ratio between HCl and ethylene, but clearly this inevitably reduces the dichloroethane yield;
avoid losses of active elements and promoters, which in addition to penalizing the catalytic activity are a problem for the pollution of the process effluent-water;
provide high flexibility, in that production can be adapted to high market demand; in this case it is necessary to have catalysts capable of working at higher temperatures without sacrificing selectivity and without an increased loss of active element and promoter.
Currently, the most competitive fluid-bed process is the one that uses oxygen as oxidizer: in such conditions, the reaction is performed with partial conversion and therefore with recycling of the unconverted ethylene and of the carbon oxides that are by-products in the oxychlorination reaction. This technology has some important advantages: conversion of the hydrochloric acid is substantially complete; the efficiency of the ethylene is on average higher that that obtained in the process in air (because the ethylene is fully converted); the emission of incondensable gases into the atmosphere (venting) is reduced drastically, since it is not necessary to eliminate from the cycle, as in the case of the air process, the nitrogen supplied together with the air.
This aspect is particularly important for environmental impact, thanks to the low emission of noxious chlorinated compounds into the environment; the vented output can be sent into the atmosphere without further expensive treatments. Another advantage is the elimination, with respect to the air process, of the section for absorbing and stripping the dichloroethane contained in the gases that leave the system.
An important parameter that can affect the yield of the reaction is the molar ratio of HCl/C2H4 in the mixture of the reacting gases entering the reactor: this ratio is not stoichiometric (2), but is close to the stoichiometric value in the air process (1.9-1.96) and is between 1.7 and 1.9 in the oxygen process since the concentration of the ethylene also comprises the ethylene that is fed back to the reactor with the recirculation gas.
In the air process, with high HCl/C2H4 ratios, selectivity is generally high, but the limit is represented by the conversion of the hydrochloric acid and by defluidization.
In the oxygen process with lower HCl/C2H4 molar ratios, conversion of the hydrochloric acid is facilitated, but unfortunately reactions of combustion to carbon oxides are also facilitated, and this leads to a loss of selectivity and therefore to a higher specific consumption of ethylene.
In order to compensate for this aspect, the temperature of the fluid bed is usually kept low (210-225xc2x0 C.): in this manner, the final yield of the reaction is higher than 98% molar (moles of DCE produced with respect to moles of ethylene fed). The specific productivity of the system is low.
This fact is in contrast with the current trend of technology: the DCE producer tends to increase the specific productivity of the system without resorting to onerous investments for new reactors. To do so, the flow-rate of reagents in input to the reactor is increased, consequently reducing the conversion of the reagents (especially of hydrochloric acid), and this entails a reduction in the yield of the process but also entails severe corrosion problems arising from the unconverted hydrochloric acid. To overcome this problem, the temperature of the fluid bed is increased, but this causes an increase in the combustion reactions and in forming of unwanted chlorinated byproducts, which is not compensated by the decrease in residence time.
Therefore, in the field there is the strongly felt need to have an oxychlorination catalyst that is capable of providing high selectivities at high temperatures ( greater than 230xc2x0 C.) both in the oxygen process and in the air process.
Various patents published in patent literature disclose catalysts that have high selectivities at high temperatures.
For example, application EP-A-582165 discloses a catalyst based on copper salts that comprises various promoters (salts of Mg, K and rare earths). The synergistic action of three promoters allegedly allows to obtain good selectivities.
The maximum working temperature is 240xc2x0 C.; the selectivity of ethylene to pure dichloroethane is 94.98% molar; the selectivity to combustion products is 3.86% molar. Selectivity to triane (1,1,2-trichloroethane, the most important chlorinated byproduct) is 0.71%. Catalytic tests are conducted in the conditions of the air process; no information is given regarding the oxygen process. The support impregnation method is xe2x80x9cwettedxe2x80x9d (i.e., the method of dry impregnation by using a volume of solution that is equal to, or smaller than, the porosity of the substrate is not used).
U.S. Pat. No. 5,227,548 discloses a catalyst that comprises cupric chloride and chlorides of Mg and K, which have the synergistic effect of reducing the combustion of ethylene to CO and CO2. The method of preparation used in the examples is wet impregnation; a catalyst with an Mg/Cu ratio of 0.3 is used.
U.S. Pat. No. 5,527,734 discloses a catalyst that comprises cupric chloride and chlorides of Mg and Cs supported on gamma alumina, in which the atomic ration of Mg/Cu is at least 0.3 and can reach 2.6, but preferably does not exceed 1.5 and more preferably 1.
The combined use of Mg and Cs chlorides is necessary to avoid dirtying the surface of the tubes used to cool the fluid bed.
The Cu content of the catalyst is preferably 5-6% by weight. This content is high: it facilitates sticking and unwanted reactions (combustions and abundant forming of 1,1,2-trichloroethane; the catalyst is prepared with the dry impregnation method, but without using acid solutions (for hydrochloric acid or other acids).
U.S. Pat. No. 4,587,230 discloses a catalyst that comprises cupric chloride and Mg chloride in an Mg/Cu ratio of 0.2-1.1, in which the Cu atoms are arranged more inside the particle of the catalyst than at its surface (the X/Y ratio, where X=Al/Cu in the catalyst and Y=Al/Cu at the surface is at least 1.4).
Preparation is performed by dry impregnation, by using acid solutions of salts of Cu and Mg for hydrochloric acids or other acids in a quantity of 1 equivalent per g-atom of Cu or by treating a catalyst that contains Cu of the commercial type with an acid solution of Mg chloride.
The Mg/Cu ratio is preferably 0.5-0.8:1.
The catalyst has good selectivity to DCE up to temperatures of 230xc2x0 C.
It has now been found surprisingly that it is possible to obtain catalysts for the fluid-bed oxychlorination of ethylene to 1,2-dichloroethane (DCE) that are capable of providing a better performance (particularly selectivity at high temperatures) than hitherto known catalysts.
The catalysts according to the invention comprise a copper compound, preferably cupric chloride, in an amount expressed as Cu from 2 to 8% by is weight, and a magnesium compound, preferably the chloride, supported on alumina, and are characterized by:
an atomic ratio of Mg/Cu equal to, or greater than, 1.2, preferably between 1.3 and 2.5;
a distribution of the copper atoms more inside the particle of the catalyst than at the surface (layer of 20-30 xc3x85) and a higher distribution of the magnesium atoms at the surface (layer of 20-30 xc3x85) than inside the particle;
a specific surface of the catalyst of 30 to 130 m2/g, preferably 70 to 100 m2/g.
Moreover, it has been found that the use of gamma alumina containing less than 50 ppm of impurities derived from sodium compounds (expressed as Na), preferably less than 10 ppm, provides catalysts that are more stable (less crumbly), have high abrasion resistance, do not produce during reaction fines that would be lost through the cyclone separators and/or might deposit on the bed cooling tubes, thus hindering the heat exchange and accordingly the control of the reaction.