Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-11-02
2002-03-05
Davis, Brian J. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
Reexamination Certificate
active
06353142
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the highly selective waste-acid-free preparation of 3,4-dichloronitrobenzene utilizing the heat of reaction.
BACKGROUND OF THE INVENTION
3,4-Dichloronitrobenzene is an important intermediate for the production of crop protection agents and dyes.
3,4-Dichloronitrobenzene is prepared industrially by isothermal nitration of 1,2-dichlorobenzene at temperatures between 40 and 70° C. (Ullmann's Encyclopedia of Industrial Chemistry, 5
th
Edition, Vol. A17, p. 430, 1991). In this procedure, large quantities of contaminated waste acid are obtained which have to be disposed of or worked up in a cost-intensive operation. A disadvantage of this process is that considerable heat of reaction has to be removed safely, which places high demands on the safety engineering to be implemented.
In order to avoid the production of waste acid, processes must be sought which comprise integrated sulphuric acid concentration operation with utilization of the heat of reaction. This necessitates a circulating acid in which byproducts may not accumulate.
The above-mentioned disadvantages can be circumvented by carrying out the nitration process adiabatically. Adiabatic mononitration has already been described for various aromatic compounds.
In adiabatic nitration processes, the heat of reaction produced does not constantly have to be removed reliably by expensive cooling but, as intended, remains in the reaction system. Thus, the above-mentioned energetic and safety disadvantages in the isothermal nitration of 1,2-dichlorobenzene do not occur.
The heat of reaction can be utilized for heating the circulating sulphuric acid and for facilitating the reconcentration of the acid.
A further advantage of carrying out the reaction adiabatically is that the reaction rate is high: the reactions have ended after considerably less than 10 minutes. Moreover, it is possible to use cost-effective weak acid (HNO
3
about 60-65% strength).
EP 668 263 A1 describes a process for the adiabatic production of mononitrotoluenes. A mixture of toluene, nitric acid, sulphuric acid and water is reacted under adiabatic reaction conditions, the reaction components being mixed intensively and the temperature during mixing being between 20 and 110° C. The adiabatic mononitration of 1,2-dichlorobenzene is likewise already known.
An adiabatic nitration process for the preparation of mononitrohalogenobenzenes is claimed in U.S. Pat. No. 4,453,027. The conditions described therein, however, are unsuitable for an efficient industrial preparation of 3,4-dichloronitrobenzene. The nitrating acid used contains 11.2% by weight of HNO
3
, 68.5% by weight of H
2
SO
4
and 20.3% by weight of H
2
O. In the case of adiabatic implementation, the high HNO
3
content leads to a temperature increase during the reaction of about 100° C., so that, at the end of the reaction, the temperature is considerably higher than 100° C., even if the reaction components are mixed, for example, at a temperature of as low as 45° C. As a consequence, in the production of 3,4-dichloronitrobenzene from 1,2-dichlorobenzene, higher amounts of the undesirable byproduct 2,3-dichloronitrobenzene are formed.
EP 675 104 A1 discloses a process for the adiabatic mononitration of mononitrohalogenobenzenes, and the nitration of 1,2-dichlorobenzene is mentioned explicitly. The adiabatic nitration is carried out by mixing the reaction components intensively, the temperature during mixing being between 60 and 160° C. and the temperature at the end of the reaction not exceeding 180° C. By this process, 1,2-dichlorobenzene can be converted into 3,4-dichloronitrobenzene. However, more than 16% by weight (based on the total amount of dichloronitrobenzenes) of the undesirable byproduct 2,3-dichloronitrobenzene are formed, which, in many cases, has to be disposed of at high cost or to be sold without covering the costs.
Accordingly, there has been a need for a process which combines the safety and economical advantages with respect to feed materials and the energy utilizable of an adiabatic process with a high selectivity for the formation of 3,4-dichloro-nitrobenzene.
We have now found reaction conditions at which the conflicting demands of a low formation of byproducts (2,3-dichloronitrobenzene and dinitrodichlorobenzenes) in combination with the high reaction rate required for carrying out the reaction adiabatically can be realized.
SUMMARY OF THE INVENTION
The invention relates to a process for preparing 3,4-dichloro-nitrobenzene by reacting 1,2-dichlorobenzene with an HNO
3
/H
2
SO
4
/H
2
O mixture with formation, essentially, of the dichloronitrobenzene and water of reaction. The process comprises the steps of
(a) feeding the reactants 1,2-dichlorobenzene, HNO
3
, H
2
SO
4
and H
2
O, in any sequence, into a reactor equipped with mixing elements, where
(a1) the quantity of HNO
3
is from 1 to 5% by weight, the quantity of H
2
SO
4
is from 70 to 92% by weight and the quantity of H
2
O is the remainder to 100% by weight, and 100% by weight represents the sum of HNO
3
+H
2
SO
4
+H
2
O,
(a2) the H
2
O is employed as such, as dilution H
2
O of the HNO
3
, as dilution H
2
O of the H
2
SO
4
or in a plurality of the forms mentioned, and
(a3) the molar ratio of 1,2-dichlorobenzene to HNO
3
is from 0.9 to 1.5;
(b) rapidly and intensively mixing the total quantity of the reactants, for which a mixing energy of from 1 to 50 watts per litre of the overall mixture, preferably from 3 to 30 W/l, is employed,
(c) carrying out the reaction under adiabatic conditions, the reactants being fed in at temperatures such that the mixing takes place in the range from 0 to 60° C. and the temperature at the end of the reaction does not exceed 100° C.,
(d) separating the reaction mixture, after carrying out the reaction, into an organic and an inorganic phase, and
(e) working up the substantially HNO
3
-free inorganic phase by distillation with removal of water.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
DESCRIPTION OF THE INVENTION
The process according to the invention can be carried out continuously or batchwise, preferably continuously.
The continuous procedure can be carried out, for example, as follows. The total quantity of the reactants is rapidly mixed with the aid of mixing elements and is fed as a mixture into a reactor. The mixing time for the continuous procedure is generally less than 3 seconds, for example, from 1 msec to 2.99 sec, preferably from 1 msec to 2 sec. The reactor is insulated, if desired, and substantially prevents back-mixing and is operated adiabatically. For the substantial prevention of back-mixing, the reactor is subdivided or is composed of a plurality of chambers or units; at the transitions between the reactor parts, the reaction mixture is redispersed. After full reaction, the mixture flows out and is separated in a separation vessel; the separation occurs rapidly. The organic phase is worked up in the usual manner, for example, by washing and distillation, or is fed immediately to a second nitration. In general, especially when there is an excess of 1,2-dichlorobenzene, the inorganic phase separated off is virtually free of nitric acid. Should this not be the case, especially when there is an excess of nitric acid, residual nitric acid can be consumed in a downstream reactor with addition of further 1,2-dichlorobenzene in the manner of a reactive extraction. The inorganic acid phase, substantially freed of nitric acid, is preferably fed to a flash evaporation with utilization of the heat of reaction absorbed and under reduced pressure. In this operation, water is removed from the acid and the acid is preferably and simultaneously brought to the initial concentration for step a). This recycling of the worked-up inorganic phase (H
2
SO
4
, H
2
O) to the process results in a circulation procedure for the H
2
SO
4
; it may be expedient to expel a small proportion of thi
König Bernd-Michael
Linn Thomas
Raatz Hans-Joachim
Weber Hans-Martin
Zirngiebl Eberhard
Bayer Aktiengesellschaft
Davis Brian J.
Eyl Diderico van
Gil Joseph C.
Henderson Richard E. L.
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