Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
2000-05-03
2001-07-03
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S790000, C210S805000, C210S806000, C210S903000, C210S909000
Reexamination Certificate
active
06254789
ABSTRACT:
The process of the invention relates to treating aqueous solutions from washes, more particularly from acid washes from nitroaromatic compounds.
Processes for preparing nitroaromatic compounds, more particularly dinitro-aromatic compounds, have been employed industrially for many years, Such compounds are intermediates in the preparation of aromatic diamines, these latter being used to synthesise the corresponding isocyanates. The isocyanates are then used to synthesise polyurethanes, which have many applications.
Briefly, dinitration reactions are usually carried out in two steps, the first consisting of preparing mononitro compounds, and the second consisting of preparing the dinitro compounds. In addition to the aromatic compound to be reacted, a nitrating acid is used, generally a nitric acid/sulphuric acid mixture, the sulphuric acid being the catalyst for the reaction.
At the end of the nitration step, the nitro compound is separated from the residual acid. That operation is usually carried out by direct decantation of or by centrifuging the reaction mixture.
The nitro compounds obtained after separating the nitrating acid (also known as crude nitro compounds) cannot be used as they are unless they are intended to be nitrated again. They still contain a fraction of the nitrating acid in the dissolved state, as well as organic impurities.
It should be noted that nitration reactions carried out using nitric acid alone as the nitrating acid cause the same difficulties with purification of the nitro compounds obtained.
The aromatic nitro compounds which are separated from the residual acid also contain a fraction of nitric acid representing several percent by weight of the nitro compounds, which must be recovered. They also contain organic by-products which have to be eliminated.
Such problems, connected with purifying the aromatic nitro compounds, and more particularly to the separation and recovery of the dissolved acids, have formed the basis of a number of studies.
The recovery of residual acids has non negligible economic consequences, given the tonnages of nitro compounds. Further, their recovery also has an impact on the environment as it can limit aqueous discharges. In addition, acid recovery can reduce the cost of treating residual water which cannot be discharged as it is, as it is polluted with salts such as sulphates and in particular nitrates.
Methods for separating and recovering dissolved nitric and sulphuric acids are all directed towards improving the efficiency of the various nitro compound washes.
In a first possibility, described in European patent EP-A-0 279 312, crude dintrotoluene is washed with a verv small amount of water. In this way, the wash water, charged with nitric and sulphuric acids, is sufficiently concentrated to enable it to be returned directly to the nitration process. However, such a method has the disadvantage of requiring the use of a particular apparatus to separate the aqueous and organic phases, in that instance a coalescer. Because of the very small amount of water used for washing, decantation is difficult. Further, the extraction yields are at most 72%.
A further possibility, constituting the subject matter of European patent application EP-A-0 736 514, consists of washing the crude dinitrotoluene with the water charged with the acids used for the nitration reaction in a plurality of counter-current steps. The water which is recovered can be recycled to the nitration process, without prior concentration, or preferably with prior concentration of the acid water. That process, however, does not satisfy all the necessary safety requirements. The acid water concentration step, which is necessary in the majority of cases, is associated with a number of risks, as the aqueous phase to be distilled contains both nitrating acids and dissolved dinitrotoluene. As a result, dinitrotoluene nitration conditions are satisfied, resulting in trinitrotoluene, the particular properties of which are well known. Further, the dinitrotoluenes present in the aqueous acidic phase from washing are not recovered in the case where that water is concentrated. Such compounds are entrained with the water during distillation and are lost. Further still, the nitro compounds can cause blockages or soiling in the concentration column as thev are condensed at the column head and thus solidify. They can also be found in the water intended for discharge, thus causing pollution or oxvercosts as they have to be eliminated.
Thus the subject matter of the present invention is the recovery of the acid or acids contained in crude nitro compounds by carrying out a simple and efficient process with maximum safet during implementation.
REFERENCES:
patent: 2773911 (1956-12-01), Dubois
patent: 5554299 (1996-09-01), Joulak et al.
patent: 5696305 (1997-12-01), Klingler et al.
patent: 5820764 (1998-10-01), Joulak et al.
patent: 0 047 331 (1982-03-01), None
Berrod Gerard
Dovergne Georges
Le Bris Louis
Marion Philippe
Perrona Philippe
Hoey Betsey Morrison
Rhodia Chimie
Seugnet Jean-Louis
Simmons David A.
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