Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
1997-04-18
2001-01-30
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
C570S225000
Reexamination Certificate
active
06180841
ABSTRACT:
The present invention relates to the oxychlorination of ethylene in a fixed bed reactor system which consists of a single reactor, to produce chlorinated hydrocarbons, particularly 1,2-dichloroethane (EDC).
It is well known that hydrocarbons such as ethylene may be chlorinated by reacting them with hydrogen chloride and gases containing elemental oxygen, particularly air or oxygen enriched air, in the presence of a catalyst at elevated temperatures and pressures in order to produce chlorinated hydrocarbons such as EDC. The reaction may be carried out with two different reactor technologies. The first is fluid bed reactor technology wherein a gaseous mixture of reactants is contacted with a fluidizable catalyst powder. The second is fixed bed reactor technology, in which the gaseous reactants flow over a fixed catalyst inside the reactor.
Fluid bed reactors have a number of drawbacks, such as potential stickiness of the catalyst powder, unsteady operation, poor selectivity owing to the gas and catalyst solids back mixing in the reactor, loss of heat transfer owing to fouling of the cooler bundle and limits in reagent velocity imposed by the need to avoid catalyst loss by elutriation from the reactor.
Fixed bed reactor technology has been developed in order to overcome these problems (see U.S. Pat. Nos. 3,892,816 and 4,123,467).
Although the fixed bed reactor overcomes many of the problems incurred with the fluid bed reactor system, a number of new problems have been encountered. A major problem is the difficulty, in the fixed bed reactor, of transferring the heat developed by the exothermic oxychlorination reaction away from the reactor to prevent overheating. For this reason, all the necessary reagents may not be fed in the correct stoichiomentric ratio to the reactor. Moreover, because it can be unsafe to have an oxygen concentration of above 8% in the mixture feeding the reactor, for flammability reasons, the reaction is carried out in two or more subsequent stages (usually three) such that the ethylene is introduced into the first reactor while the HCl and oxygen feeds are split between the reactors. Unreacted ethylene plus some inert gases are recycled back to the first reactor.
In a further attempt to reduce the incidence of hot spots and the like, it is known to alter the activity profile of the catalyst within a fixed bed reactor such that the activity increases in the direction of flow. For example, see European patent application 0146925. However, in the prior art, even when a profiled catalyst is used it has been deemed necessary to use a multi-reactor system.
We have now developed a new process for the catalytic oxychlorination of ethylene which makes use of a single fixed-bed reactor. Nevertheless, hot-spots are avoided and good selectivity to ethylene is achieved, as well as over 99% utilisation of HCl.
According to a first aspect of the invention, we provide a method for the oxychlorination of ethylene to produce 1,2-dichloroethane (EDC), comprising reacting ethylene, a chlorine source and an oxygen source in a fixed-bed oxychlorination reactor in the presence of a catalyst, characterised in that a single reactor is used and ethylene is present in a large molar excess with respect to chlorine.
Preferably, the chlorine source is HCl.
Preferably, the ethylene is introduced in a 200-700% molar equivalent excess with respect to stoichiometric HCl, in order to produce a high partial pressure of ethylene.
The oxygen source may be pure oxygen, or an oxygen-enriched gas. Oxygen is preferably supplied in a molar excess of up to 15%, more preferably between 2 and 8%, with respect to HCl.
The large excess of ethylene present functions to increase the selectivity of the reaction, as well as acting as a heat sink, exploiting its high specific heat capacity. Unreacted ethylene is preferably recovered and recycled back to the reactor, or to other processes requiring ethylene such as in direct chlorination reactions.
The composition of the recycle gas reaches an equilibrium depending mainly on combustion rate, the amount of inert gases in the raw materials and the purge rate. Depending on these factors, ethylene concentration can vary between 10 and 90%. As a consequence, the actual ethylene excess used will depend on its concentration in the recycled vent gas and on the recycle flow rate.
In general, the ethylene excess with respect to its stoichiometric requirement as determined by the amount of HCl can be expressed as a percentage calculated according to the formula:
200
(
Q1
+
Q2
-
Q6
)
Q3
where 100 is the stoichiometirc requirement, and wherein
Q2
=
Q4
Q1
-
1
/
2
Q3
-
Q5
-
Q6
Q1
-
1
/
2
Q3
+
Q5
and
Q4
=
100
Q7
-
Q8
%
O
2
in
%
O
2
in
-
%
O
2
rec
where
% O
2
in—oxygen at inlet of reactor
% O
2
rec=oxygen in recycle stream.
The symbols are defined as follows:
Q1=mol/h fresh ethylene
Q2=mol/h recycled ethylene
Q3=mol/h HCl
Q4=mol/h total recycle
Q5=mol/h burned ethylene
Q6=mol/h fed inert gases
Q7=mol/h fed oxygen
Q8=mol/h total fresh reagents
Control of the recycled gas flow rate may be used to adjust the oxygen concentration at the inlet of the reactor and thereby the hotspot temperature. In the conditions of temperature and pressure existing in the inlet of the reactor, the lower flammability limit of the mixture occurs when the oxygen concentration is around 8%. For safety and operational reasons, the concentration is advantageously between 5 and 6% v/v, as use of a higher concentration can result in an elevated hotspot temperature in the catalytic bed.
Typically, the hotspot temperature would be about 230-280° C., depending on a number of factors, including reactor diameter.
The reactor employed in the method of the invention is a tubular reactor. Advantageously, it consists of a plurality of tubes stacked together within a single coolant jacket. The internal diameter of each tube is preferably between 20 and 40 millimeters. Diameters of less than 20 millimeters are disadvantageous as an excessive number of tubes is required in an industrial reactor in order to obtain a satisfactory throughput of materials, while diameters larger than 40 millimeters result in excessively high hotspot temperatures inside the catalytic bed.
The preferred length of the reactor is between 3.5 and 8 meters. A length of less than 3.5 meters results in too short a residence time and therefore either low reactant conversion or low specific throughput; a length of more than 8 meters is not necessary in order to achieve both high HCL and oxygen conversion and large specific throughput.
Catalyst layers within the reactor can be arranged in a number of ways. For example, the reactor may simply be filled with catalyst in the normal manner, not employing a profiled catalyst distribution. Alternatively, a simple loading pattern maybe employed whereby the catalyst is loaded in two layers, a first of low activity catalyst or diluted catalyst (See U.S. Pat. No. 4,123,467) in order to avoid hotspots and a second of a more active or more concentrated catalyst, in order to increase the rate of reaction. A further, more complex loading pattern consists of a succession of several layers of catalyst with increasing activity (or concentration) from the first to the last layer. The choice of suitable catalyst loading pattern will depend on the maximum temperature of the hotspot, as well as the inside diameter and length of the tubular reactor and on the projected throughput.
Invariably, it is advantageous to fill the last part of the reactor with a high activity catalyst as used in the third reactor of a three stage oxychlorination process.
Catalysts for use in the invention are known in the art and are supported catalysts in which cupric chloride is the major active component and alumina, silica gel, alumino-silicate and the like form the supports. The support material may be present in the form of spheres, cubes, con
Fatutto Pierluigi
Marsella Andrea
Vio Dario
EVC Technology AG
Siegel Alan
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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