Distillation: processes – separatory – With chemical reaction – Including step of adding catalyst or reacting material
Reexamination Certificate
2002-07-12
2004-03-30
Solola, Taofiq A (Department: 1626)
Distillation: processes, separatory
With chemical reaction
Including step of adding catalyst or reacting material
C203S046000, C549S541000
Reexamination Certificate
active
06712942
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a process for working up a mixture which contains at least one alkene and oxygen, the oxygen being removed from the mixture by a suitable method before the alkene is separated off by distillation. The present invention also relates to an integrated process in which propylene oxide is prepared from a hydroperoxide, preferably hydrogen peroxide, and propene, the resulting mixture containing propene and oxygen and being worked up by the novel process.
BACKGROUND INFORMATION
In processes in which mixtures of an alkene and oxygen are obtained, it is desirable in numerous procedures to separate off the alkene from this mixture and, if required, to recycle it to a process stage for economic reasons relating to the process. One problem encountered in these processes is the formation of ignitable mixtures, which of course have to be avoided in all circumstances for safety reasons.
A process in which this problem occurs in particular is the preparation of propylene oxide, an important intermediate in the chemical industry, starting from propene and hydrogen peroxide. In the course of working up the product, in this process unconverted propene is preferably separated off from the crude discharge of the epoxidation and is recycled to the process as starting material. In one possible embodiment, this crude discharge is subjected to a distillation, after which the crude product is separated into a low boiler fraction, which contains propene and compounds having a boiling point lower than that of propene, and a high boiler fraction, which contains propene oxide and compounds having a boiling point higher than that of propene. Inter alia, oxygen collects in this low boiler fraction, in a concentration which makes the low boiler fraction an ignitable mixture which constitutes a serious safety risk. The risk is all the more important because, as described above, one process requirement is that unconverted propene be recycled and hence the low boiler fraction in turn be worked up, for example by distillation.
In order to solve this problem, EP-B 0 719 768 proposes adding an inert substance having a boiling point which is lower than that of propene, preferably methane, during the separation of propene from the low boiler mixture by distillation, in the upper part of the separation apparatus, in an amount such that the oxygen is diluted to a concentration at which the mixture is no longer ignitable. In this procedure, the dilution component accordingly has to be added to the separation apparatus, it also being necessary to add a solvent by means of which the propene is washed out of the low boiler fraction.
BRIEF SUMMARY
It is the object of the present invention to provide a process which makes it possible to work up mixtures which comprise alkene and oxygen in a simple and safe manner.
We have found that this object is achieved by a process for working up a mixture (M1), comprising an alkene and oxygen, in which
(i) oxygen is removed from the mixture (M1) by a nondistillative method to give a mixture (M2) and
(ii) the alkene is separated off from the mixture (M2) by distillation.
In a preferred embodiment, the mixture (M1) worked up by the novel process is a gaseous mixture.
Of course, the mixture (M1) may also contain two or more alkenes differing from one another. In this case, it is possible, inter alia, for one or more of the alkenes to be converted into one or more other compounds during the removal of the oxygen, provided that it is ensured that the desired alkene is present in the mixture (M2).
DETAILED DESCRIPTION OF THE INVENTION
In general, there are no restrictions with regard to the nondistillative methods by means of which oxygen is removed from the mixture (M1), provided that it is ensured that, during this removal, the alkene which is contained in the mixture (M1) and which is to be contained in the mixture (M2) is not converted to a significant extent into an undesirable product.
In the novel process, oxygen is preferably removed by combustion. Another preferred embodiment is one in which the mixture (M1) is subjected to reaction conditions under which the oxygen contained in the mixture reacts with a suitable chemical compound.
Accordingly, the present invention also relates to a process, as described above, wherein (i) oxygen is removed from the mixture (M1)
by combustion of the oxygen or
by reaction of the oxygen contained in the mixture (M1) with at least one suitable chemical compound or
by a combination of these methods.
Regarding the combination of the methods described above, it is in principle possible to use the two methods simultaneously in the case of suitable reactants, so that the oxygen is combusted as well as being consumed by reaction with a suitable compound. It is also possible for the two different procedures to be carried out in succession in any desired order, it being possible for each method also to be carried out two or more times.
Regarding the combustion, all possible procedures are possible in principle, provided that it is ensured that the alkene which is present in the mixture (M1) does not react to give undesirable products. Inter alia, the combustion of oxygen can be effected without using a catalyst. This noncatalytic combustion can be carried out using all reactors suitable for this purpose. For example, temperatures of more than 300° C. and pressures in the region of atmospheric pressure are preferably employed, the reactors used being, for example, tubular reactors with, for example, an inert bed.
In a preferred embodiment of the novel process, the combustion of the oxygen which is contained in the mixture (M1) is carried out using at least one suitable catalyst. All suitable catalysts or catalyst mixtures may be used for this purpose. Inter alia, noble metals, for example Pt, Rh or Pd, which may be applied to suitable supports, for example metal oxides, are preferred. For example, Pd catalysts supported on Al
2
O
3
are used. Copper chromite catalysts may also be mentioned. Examples here include the commercially available catalysts R0-25/50 S6, R0-20/47 K2-4 or R3-20 S6 from BASF AG.
The temperatures which are used in the catalytic combustion of the oxygen which is contained in the mixture (M1) can be adapted to the requirements of the respective process, in particular the composition of the mixture (M1) and the type of catalyst used. In general, the temperatures are from 200 to 650° C., preferably from 280 to 580° C.
Regarding the reactor or the reactors in which the catalytic combustion takes place, all suitable reactors may be used. Examples of preferred reactors include tubular reactors, such as tube-bundle reactors or shaft reactors
The residence time of the mixture (M1) in the reactors described above, under the reaction conditions described above, can be adapted to the requirements of the respective process. The amount of oxygen which is removed from the mixture (M1) can be influenced by, inter alia, the specific choice of the residence time. For example, it is possible that the mixture (M2) which is obtained after combustion of oxygen which is present in the mixture (M1) should or can still contain a specific residual amount of oxygen, which is not critical for any further working up of the mixture (M2). For economic reasons relating to the process, it is therefore possible in principle for the mixture (M2) to have a certain residual oxygen concentration.
As stated above, it is possible for the oxygen to be removed from the mixture (M1) in two or more stages by combustion. Thus, it is possible, inter alia, for oxygen to be removed from the mixture (M1) by combustion in a first stage using a first catalyst and for oxygen to be removed from the resulting mixture using a second catalyst, it being possible for the reaction conditions in the first and the second stage to be identical to or different from one another, for example with respect to temperature or residence time of the respective reaction medium in the reactor. Furthermore, the different stages can be carried out in one reactor
Bassler Peter
Hefner Werner
Rehfinger Alwin
Rieber Norbert
Rudolf Peter
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Solola Taofiq A
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