Note: Descriptions are shown in the official language in which they were submitted.
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SINGLE STAGE FIXED BED OXYCHLORINATION OF ETHYLENE
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
15- 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 US patent 3,892,816 and US
patent 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
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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 HC1 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 HC1.
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 HC1, in
order to produce a high partial pressure of ethylene.
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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
HC1. -
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
l0 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
15 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
HC1 can be expressed as a percentage calculated according to
the formula:
(Q1 + Q2 - Q6)
200
Q3
where 100 is the stoichiometirc requirement, and wherein
Q1 - 1/2 Q3 -Q5 -Q6
Q2 = Q4
Q1 -1/2 Q3 + Q5
and
100Q7 - Q8% 02 in
3 5 Q4 =
%02 in - %02 rec
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where %02in - oxygen at inlet of reactor
02 rec ~ oxygen in recycle stream.
The symbols are defined as follows:
Q1 - mol/h fresh ethylene
Q2 - mol/h recycled ethylene
Q3 - mol/h HC1
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 millimetres. Diameters of less than 20 millimetres
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 millimetres result in excessively high hotspot
temperatures inside the catalytic bed.
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The preferred length of the reactor is between 3.5 and 8
metres. A length of less than 3.5 metres results in too
short a residence time and therefore either low reactant
conversion or low specific throughput; a length of more than
5 8 metres 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 USP 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,
cones, hollow cylinders, cylindrical pellets, multilobate
pellets and other shapes.
In addition to cupric chloride, the catalyst may also
comprise promoters such as potassium, magnesium, sodium,
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lithium, calcium, cerium and cesium chlorides for improving
the selectivity to EDC. The activity prolfile of the
catalyst in the catalytic bed would be arranged in such a
way as to have the HC1 conversion above 98% at a point 70 to
80~ of the distance along the catalytic bed. The last 20 to
30~ of the catalytic bed will perform as a finisher, so that
the whole reaction is assured of a high conversion even if
the first part of the catalytic bed loses activity over
time.
Preferably, the reactants are preheated to between 100 and
200~C. The reaction pressure can range up to 20 barg, the
preferred range being between 4 and 7 barg.
The invention will now be described, for the purposes of
exemplification only, in the following examples with
reference to the appended figure, which is a systematic
diagram of a single reactor oxychlorination apparatus.
Example 1
The reactor is a unit composed of 1 inch external diameter
(e. d) nickel tube 14 BWG 25 feet long; inside on the axis
there is a thermowell of 6 mm e.d. containing 8
thermocouples with which it is possible to record the
thermal profile of the reactor. The reactor is surrounded
with an external jacket in which steam at 210~C and 18 barg
is used to control the temperature of the reaction. The
reactor pressure is controlled with a pneumatic valve on an
effluent line.
The reagents were preheated in 18 barg steam heated
exchangers; subsequently, ethylene, HC1 and nitrogen were
mixed together and oxygen was added in a mixer where the
velocity of the gases is higher than the speed of ethylene
flame propagation. The catalyst used was a normal
industrial catalyst for oxygen three stage fixed bed process
consisting of hollow cylinders containing copper and
potassium chloride arranged such that the amount of the
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copper varies from 24 to 60gr/litre from inlet to outlet of
the reactor. The catalytic bed was 2.6 litres. In this
reactor, a mixture of 223.1 moles/h of ethylene, 66.8
'~ moles/h of HC1, 17.5 moles/h of oxygen and 18 moles/h of
nitrogen was introduced. The oxygen excess was 4.8%.
Nitrogen was used to stimulate the inert gases of the
recycle gas and its amount was a function of the recycle
ratio. In this case the recycle composition was 90%
ethylene and 10~ inert gases. The pressure at inlet was 5.1
barg and at outlet 3.5 barg. The temperature of cooling
jacket was held at 210~C. The outlet stream, consisting of
a mixture of ethylene, oxygen, HC1, nitrogen, EDC, water,
COx and byproducts, was analyzed and the results were:
-Oxygen conversion to crude EDC 94.7%
-HC1 conversion to crude EDC 99.40
-EDC production 32.8 mol/h.
-Selectivity of ethylene to COx 0.6% mol
-Selectivity of ethylene to EC1 0.24% mol
-Selectivity of ethylene to EDC 98,66% mol
-Selectivity of ethylene to chloral 0,15% mol
-Selectivity of ethylene to impurities 0,35% mol
-Hotspot 233~C
Example 2
This example was carried out with the same reactor and
catalytic scheme as in example 1. It was fed a mixture of
ethylene 169 moles/h, HC1 71.5 moles/h, oxygen 19.5 moles/h
and nitrogen 65 moles/h. The oxygen excess was 9%. Recycle
composition was ethylene 70%, inerts 30% v/v. The outlet
pressure was 7.2 barg. The cooling jacket temperature was
2IO~C.
The results were:
-Oxygen conversion to crude EDC 90.8%
-HC1 conversion to crude EDC 99.50
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-EDC production 35.6 moles/h
-Selectivity of ethylene to COx 0,34% mol
-Selectivity of ethylene to EC1 0,17% mol
-Selectivity of ethylene to chloral 0,20% mol ''
-Selectivity of ethylene to EDC 98,92% mol
-Selectivity of ethylene to impurities 0.37% mol
-Hotspot 2'17 ~C
Example 3
This example was carried out with the same reactor and
catalytic scheme of examples 1 and 2. It was a fed a
mixture of ethylene 267.55 moles/h, HC1 93.45 moles/h,
oxygen 25 moles/h, nitrogen 50moles/h. Oxygen excess was 7%.
Recycle composition: ethylene 80%, inerts 20% v/v. Outlet
pressure was 3.2 barg and the inlet pressure 6.6 barg. The
cooling jacket temperature was 210~C. The results were:
-Oxygen conversion EDC 90.9%
to crude
-HC1 conversion to crude EDC 99%
-EDC production 46.2moles/h
-Selectivity of ethylene to COx 0.13% mol
-Selectivity of ethylene to EC1 0,25% mol
-Selectivity of ethylene to chloral 0,10% mol
-Selectivity of ethylene to EDC 99,12% mol
-Selectivity of ethylene to impurities 0,40% mol
-Hotspot 233~C
Example 4
A reactor consisting of one 1.25 inch e.d. nickel tube 14
BWG 12 feet long was set up, the same as that used in a
normal three stage industrial reactor, inside of which there
was a thermowell of 6 mm e.d. containing 4 sliding
thermocouples. The temperature, pressure control and
reagent feeding systems were as in Example 1. The catalytic
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bed was 1.8 litres. The catalyst used was the same type as
in other examples. The copper content ranged from 24 to
46gr/litre.
It was fed a mixture of 272 moles/h of ethylene, HC1 97.5
~r
moles/h, oxygen 26.8 moles/h and nitrogen 24.6 moles/h. The
oxygen excess was 10 % . Recycle composition was 90 % ethylene,
10% inerts. The outlet pressure was 5.5 barg and the
pressure drop 0.55 barg. The results were:
-Oxygen conversion 89%
to crude.EDC
-HC1 conversion to crude EDC 98%
-EDC production 47.6moles%h
-Selectivity of ethylene COx 0.13% mol
-Selectivity of ethylene to EC1 0.20% mol
-Selectivity of ethylene to EDC 99.10% mol
-Selectivity of ethylene to chloral 0.15% mol
-Selectivity of ethylene to impurities o.42% mol
-Hotspot 258~C
Example 5
The same reactor and catalytic scheme as in Example 4 was
fed a mixture of 214.2 moles/h ethylene, 84.7 moles/h of
HC1, 24.1 moles/h of oxygen and 21.6 moles/h of nitrogen.
Oxygen excess was 13.4%. Recycle composition was 90%
ethylene, 10% inerts. The outlet pressure was 5.5 barg and
the pressure drop was 0.5 barg. The results were:
-Oxygen conversion 87.1%
-HC1 conversion 99.1%
-EDC production 42 moles/h
-Selectivity of ethylene to COx 0.30% mol
-Selectivity of ethylene to EC1 0.20% mol
-Selectivity of ethylene to EDC 98.50% mol
-Selectivity of ethylene to chloral 0.20% mol
-Selectivity of ethylene to impurities 0.80% mol
-Hotspot 270~C
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Example 6
Using the same reactor as in Examples 4 and 5 but with the
5 same loading pattern as in the first stage of a three stage
process consisting of a catalyst containing 26 gr/litre of
copper and l2gr/litre of potassium in the first 3/5 of the
reactor and 40 gr/litre of copper and l8gr/litre of
potassium in the remainder part, a mixture of ethylene 231
10 moles/h, oxygen 18 moles/h, HC1 64.8 moles/h and nitrogen 42
moles/h was reacted. The outlet pressure was 5.5 berg and
the pressure drop was 0.45 berg.
-Oxygen conversion EDC 88.5%
to crude
-HCl conversion to crude EDC 97.80
-EDC production 31.70 moles/h
-Selectivity of ethylene to COx 0.10% mol
-Selectivity of ethylene to ECl 0.20a mol
-Selectivity of ethylene to EDC 99.300 mol
-Selectivity of ethylene to chloral 0.10% mol
-Selectivity of ethylene impur 0.30% mol
-Hotspot 255~C
