Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2003-02-06
2004-09-21
Parsa, J. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S936000
Reexamination Certificate
active
06794549
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a continuous adiabatic process for preparing nitrochlorobenzene, where the waste acid produced during the practice of the process is reconcentrated and recycled into the nitration reaction. By adding phosphoric acid, it is possible to increase significantly the proportion of the product of value ortho-nitrochlorobenzene. The production of the co-product para-nitrochlorobenzene is suppressed in favour of the product of value.
2. Brief Description of the Prior Art
Nitrochlorobenzenes are important intermediates in the preparation of dyes, pharmaceuticals and pesticides. There is a particularly high demand for ortho-nitrochlorobenzene, which is obtained, in the customary nitration of chlorobenzene in about 35%. para-Nitrochlorobenzene, too, has a number of industrial uses. Since there are only very limited industrial applications for meta-nitrochlorobenzene, its production is generally undesired.
Industrially, nitrochlorobenzenes are prepared by nitration of chlorobenzene. For nitration, it is customary to use a mixture of sulphuric acid, nitric acid and water. Under these conditions, the proportion of the para-isomer is very high. The ratio of para- to ortho-nitrochlorobenzene is usually about 1.95. Additionally, a considerable proportion of waste sulphuric acid contaminated with organic compounds is produced, requiring complicated and expensive work-up. The reconcentration of the waste sulphuric acid succeeds in a particularly elegant manner when the nitration reaction is carried out under adiabatic conditions, since in this case, there is no heat exchange with the environment, and the energy released during the process can be used to preheat and/or reconcentrate the waste sulphuric acid. Moreover, under adiabatic conditions, it is possible to obtain a considerably improved ratio of para- to ortho-nitrochlorobenzene. If part of the sulphuric acid is replaced by phosphoric acid, it is possible to obtain a considerably improved ratio of para- to ortho-nitrochlorobenzene whilst maintaining the energetically favourable conditions for reconcentrating the waste acid.
The effect of phosphorous compounds on nitrations is described in the literature. Thus, DE 2 422 305 describes the addition of phosphoric acid in the isothermal nitration of chlorobenzene and the associated higher yield of ortho-nitrochlorobenzene. Here, in general, the use of phosphoric acid is accompanied by a reduced rate of reaction. To obtain economical space-time yields, it is necessary to employ highly concentrated phosphoric acids (>100% H
3
PO
4
). However, the reconcentration and recycling of these phosphoric acids is very complicated. Alternatively, the nitration may also be carried out at an elevated reaction temperature. However, this results inter alia in a higher proportion of unwanted m-nitrochlorobenzene which has to be removed at high cost.
U.S. Pat. No. 4,453,027 claims an adiabatic nitration process for the preparation of mononitrohalobenzenes. However, the nitrating acid used contains such large amounts of nitric acid that, although the end temperature described is from 100 to 110° C., the start temperature of the reaction, and consequently of the reactants, has to be from 0 to 10° C. Such a process is neither technically nor economically sensible, since cooling brine and expensive apparatus have to be used.
EP 675 104 A describes an adiabatic process for the nitration of halobenzenes, where the reactants are mixed with application of a certain mixing energy and mixing is carried out in a temperature range of from 60 to 160° C. Using the temperatures mentioned, it is possible to obtain the high reaction rates required for adiabatic operation. However, this process has the disadvantage that relatively high proportions of unwanted meta-nitrochlorobenzene are obtained, the removal of which, as already mentioned, is complicated and costly.
Accordingly, there was a need for a process for the continuous preparation of nitrochlorobenzene, which process permits easy work-up and recycling of the waste acid and a considerably improved yield of the target product ortho-nitrochlorobenzene, substantially avoiding the unwanted byproduct meta-nitrochlorobenzene.
SUMMARY OF THE INVENTION
Surprisingly, we have now found a process for the continuous preparation of nitrochlorobenzene by reacting chlorobenzene with sulphuric acid, phosphoric acid, nitric acid and water, which process is characterized in that
a) the feedstocks chlorobenzene, sulphuric acid, phosphoric acid and water are introduced simultaneously or successively in any order into a reactor equipped with mixing elements and are mixed such that the temperature of the reaction mixture during initial mixing is from 10 to 80° C.,
b) the content of phosphoric acid in the reaction mixture during mixing, based on the sum of sulphuric acid, phosphoric acid, nitric acid and water, is from 10 to 50% by weight,
c) the reaction proceeds under adiabatic conditions,
d) at the reactor outlet, the crude nitrochlorobenzene is separated from the waste acid and
e) the waste acid is reconcentrated and recycled as a recycled acid mixture into the nitration reaction and
f) if appropriate, up to 3000 ppm of a silicon compound may be added.
DETAILED DESCRIPTION OF THE INVENTION
The feedstocks used in the process according to the invention are chlorobenzene, sulphuric acid, phosphoric acid, nitric acid and water, where the chlorobenzene used may contain nitrochlorobenzene in an amount of, for example, from 0 to 20% by weight. A higher amount is possible, but uneconomical. Here, water can be used as such or else is introduced into the reaction as dilution water in the nitric acid and/or the sulphuric acid and/or the phosphoric acid.
In the process according to the invention, feedstock chlorobenzene, sulphuric acid, phosphoric acid, nitric acid and water can be introduced individually or as mixtures into a reactor equipped with mixing elements. The feedstocks can be introduced into the reactor simultaneously or successively in any order. Introduction in to the reactor can be carried out, for example, by adding chlorobenzene and nitric acid, and, if appropriate, water as separate streams simultaneously or successively to the recycled acid mixture, where the nitric acid may be diluted with water. It is also possible to pre-mix chlorobenzene with water and sulphuric acid and/or phosphoric acid and to introduce the resulting mixture as a separate stream into the reactor. Mixing of nitric acid, with sulphuric acid and/or phosphoric acid and/or water, takes place in the reactor. It is furthermore possible to introduce chlorobenzene and a nitrating acid prepared by mixing sulphuric acid, phosphoric acid, nitric acid and, if appropriate, water in separate streams into the reactor. In a preferred embodiment of the process according to the invention, nitric acid and the recycled acid mixture are mixed to give a nitrating acid, and nitrating acid and chlorobenzene are introduced into the reactor in separate streams. For the reaction to succeed, the reactants are introduced into the reactor, such that the reaction mixture obtained once all reactants have been mixed has the composition according to the invention and mixing takes place at the temperature according to the invention.
At the time of mixing, the content of phosphoric acid, based on pure H
3
PO
4
in the reaction mixture, based on the sum of sulphuric acid based on pure H
2
SO
4
, phosphoric acid based on pure H
3
PO
4
, nitric acid based on pure HNO
3
and water, may be from 10 to 50% by weight, preferably from 15 to 50% by weight and particularly preferably from 20 to 45% by weight.
At the time of mixing, the content of sulphuric acid, based on pure H
2
SO
4
in the reaction mixture, based on the sum of sulphuric acid based on pure H
2
SO
4
, phosphoric acid based on pure H
3
PO
4
, nitric acid based on pure HNO
3
and water, may be from 30 to 80% by weight, preferably from 40 to 75% by weight and particularly preferably from 40 to 65% by weig
Demuth Ralf
Gotta Matthias
Linn Thomas
Weber Hans-Martin
Zirngiebl Eberhard
Akorli Godfried R.
Bayer Aktiengesellschaft
Eyl Diderico van
Parsa J.
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