Exothermic catalytic chemical process

Details Exothermic catalytic chemical process Exothermic catalytic chemical process

1999-04-06

2001-01-16

Barts, Samuel (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Group ib metal containing catalyst utilized for the...

C518S700000, C518S705000, C518S712000, C518S728000

Reexamination Certificate

active

06174927

ABSTRACT:

This invention relates to an exothermic catalytic chemical process and in particular to the control of such processes, especially methanol or ammonia synthesis processes, wherein at least part of the heat evolved during the reaction is employed for pre-heating at least part of the reactants to the desired reaction inlet temperature.
Exothermic catalytic chemical processes are often effected by passing a pre-heated reactants stream, usually gaseous, through a fixed bed of a particulate catalyst for the desired reaction. Heat exchange means are often employed to transfer heat evolved during the reaction to at least part of the inlet reactants stream. Thus an adiabatic catalyst bed may be employed with a heat exchanger downstream of the bed to transfer heat from the effluent from the catalyst bed to the reactants stream to be fed to the catalyst bed. Alternatively, the heat exchanger means may be disposed within the catalyst bed so that heat is transferred from the reactants undergoing the reaction to the reactants to be fed to the bed. An example of a suitable reactor for such a process is described in U.S. Pat. No. 4,778,662 where the heat exchanger means comprises a plurality of tubes extending through at least the inlet portion of the catalyst bed. In this type of reaction, herein termed a tube-cooled reactor, the tubes, which are open at their upper ends, extend into, and communicate with a zone above the top of the catalyst bed. The reactants stream is fed to the lower end of the tubes and passes up the tubes into said zone and then passes down through the catalyst bed outside said tubes. As the reactants pass down through the bed, the reaction proceeds and heat evolved by the reaction is transferred through the walls of the tubes to heat the reactants passing up the tubes.
Exothermic reactions, such as ammonia or methanol synthesis reactions, are generally reversible: production of the desired product is favoured, from equilibrium considerations, by employing relatively low temperatures. However the rate of reaction depends on the catalyst activity and temperature: increasing the reaction temperature generally increases the rate of reaction. The catalysts employed for the reaction generally have an optimum usable temperature. If the temperature is too low, the reaction proceeds only relatively slowly, if at all, while if the temperature is too high, not only may the extent of the reaction be limited by virtue of equilibrium considerations, but also the life of the catalyst may be decreased through thermal sintering of the catalyst. Consequently it is generally desirable to operate at temperatures at which the reaction proceeds at an acceptable rate but to limit the maximum temperature to which the catalyst is subject for any prolonged period of time in order to obtain maximum catalyst life and the desired extent of reaction.
Examination of the temperature profile of the reactants as they pass down the bed in reactors of the aforementioned tube-cooled type reveals that the reactants temperature generally passes through a maximum part way down the catalyst bed. We have devised a method of operation of the process whereby the peak temperature is decreased. While the method of the present invention is of particular applicability to processes in which the aforementioned tube-coated type of reactor is employed, it is also applicable to processes using the other reactor designs wherein the heat evolved during the reaction is used to heat at least part of the reactants fed to the reactor, so that the temperature of the reactants entering the bed is at least in part determined by such heat exchange. Examples of such other designs include reactors with one or more adiabatic beds with a feed/effluent heat exchanger and also reactor designs having a feed/effluent heat exchanger together with a “quench” system wherein additional cool reactants, or one of the reactants, are introduced into a bed, or between bed, to effect cooling of the reactants undergoing the reaction.
In a process wherein the heat evolved by the reaction is used to heat at least part of the reactants fed to the catalyst, the temperature of that part of the reactants fed to the bed will depend on the amount of reaction that is taking place and the temperature of the reactants fed to the heat exchanger in which the heat evolved by the reaction is transferred to the reactants.
For simplicity, the invention will be described in relation to processes employing reactors of the aforesaid tube-cooled type, wherein all of the reactants fed to the bed undergo heating by the heat evolved by the reaction. Extension of the invention to processes using an adiabatic bed and a feed/effluent heat exchanger and/or to processes using a quench gas in addition to heating of the reactants by heat evolved by the reaction will be readily apparent to the skilled person.
We have found that for any given temperature T
0
at which the reactants are fed to the heat exchanger, there are two possible operating regimes.
In the first regime, hereinafter termed the stable regime, an upward deviation from the temperature T
1
to which the reactants are heated in the heat exchanger and at which the reactants enter the catalyst bed has little effect on the amount of reaction occurring in the bed since the regime is “equilibrium limited”, i.e. the reaction rate is limited by equilibrium considerations rather than by catalyst activity and temperature. As a result, the temperature of the catalyst bed changes very little: as a consequence the amount of heat transfer decreases, giving a decrease in the temperature T
1
. As a consequence the amount of heat transfer decreases, giving a decrease in the temperature T, counteracting the aforesaid upward deviation. Conversely with a downward deviation of T
1
. The regime is thus stable in the sense that for a given temperature T
0
at which the reactants are fed to the heat exchanger, the temperature to which the reactants are heated in the heat exchanger and hence at which they enter the bed tends to maintain itself at a constant value. This is the regime normally used for operation: control of the process, to vary the temperature T
1
at which the reactants enter the catalyst bed, can thus be achieved by controlling the temperature T
0
at which the reactants are fed to the heat exchanger.
In the other regime, herein termed the metastable regime, the reaction rate is not limited by equilibrium considerations, but is determined by the catalyst activity and temperature. As a consequence, an upward deviation of the temperature T
1
at which the reactants are fed to the bed effects a significant increase in the amount of reaction occurring in the bed: as a result the bed temperature increases significantly increasing the amount of heat transferred to the reactants undergoing heating. This in turn leads to a further increase in T
1
. The result is that the process tends to move away from this operating regime towards the aforesaid stable regime, unless checked by decreasing T
0
. In the converse situation at the metastable regime, a downward deviation of the temperature T
1
at which the reactants are fed to the bed, leads to less reaction in the bed as a result of decreased catalytic activity at the lower temperature. Less heat is thus transferred to the reactants undergoing reaction so the temperature T
1
of the reactants entering the bed drops further, leading to still further decrease in the reaction rate and hence the amount of reaction occurring. Unless this temperature decrease is checked, by increasing T
0
, the reaction will die. Thus in the stable regime, increasing T
0
has the effect of increasing the value of T
1
, while in the metastable regime, increasing T
0
has the effect of decreasing T
1
. For a given value of T
0
, the temperature T
1
for the stable regime is greater than the temperature T
1
of the metastable regime.
In the present invention we have found that there are significant advantages in operating in the metastable regime.
Accordingly the present invention provides an exothermic cataly

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