This invention relates generally to the production of dinitrotoluene. More particularly, the invention relates to an improved distillation process for removing dinitrotoluene impurity from process effluent water.

Dinitrotoluene (DNT) is a versatile and valuable chemical intermediate which is widely used in the production of toluene diisocyanate. However, if it iβ allowed to flow uncontrolled in the environment, for example if present as an impurity in a stream or pond, it may pose a potential pollution problem and a hazard to aquatic life. DNT is produced by a process involving the nitration of toluene first to mononitrotoluene and then to DNT. Historically, the nitration reaction has been carried out commercially using a mixed acid system, namely, nitric acid and sulfuric acid. However, as a result of recent developments in this field, an alternative, improved process is now available for the commercial production of DNT which, among other things, obviates the need for using sulfuric acid in the nitration of toluene. See for example U.S. Patents No. 5,001,272, issued r

March 24, 1991 to Mason and No. 5,009,078, issued March 24, 1992 to Quakenbush.

The nitration reaction, whether using a mixed acid system or nitric acid by itself, results in the generation of a substantial amount of water, i.e., water produced as a by-product of the nitration reaction, water contained in the nitric acid feed stock and water used throughout the production process for various utility functions. All this water, which is often referred to as the "process

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effluent water" or simply "process water" is collected and ultimately discharged as a process effluent stream. However, before being discharged, the process water must be treated to remove objectionable impurities and thereby meet environmentally acceptable standards or permitted discharge requirements. For a variety of practical reasons, distillation is often considered to be the most efficient and cost-effective purification operation.

However, it has been found that DNT process effluent water contains dissolved DNT in concentrations which, although relatively low, could pose a pollution or environmental problem. Accordingly, this invention is directed to a relatively simple and economically attractive improvement for solving this problem. And in a broader sense, the invention provides an improved process for substantially reducing the DNT concentration in any aqueous medium or stream containing DNT as an impurity.

In accordance with the invention, a process has been found for substantially reducing the concentration of DNT }.n water containing such DNT as an impurity, which comprises distilling off the water in the presence of nitric acid. Further according to the invention, an improved distillation process is provided whereby process effluent water generated in the production of DNT can be separated in suitable condition for biological treatment and proper disposal.

In one aspect, the present invention relates to a process for reducing the concentration of

dinitrotoluene in water containing said dinitrotoluene as an impurity, which process comprises distilling the water in the presence of nitric acid. In another aspect, the present invention relates to a process for reducing the concentration of dinitrotoluene in water in a composition containing said dinitrotoluene and said water which comprises distilling at least a portion of said water in said composition in the presence of nitric acid to provide vaporized water containing a reduced concentration of said dinitrotoluene.

In yet another aspect, the present invention relates to a process for reducing the concentration of dinitrotoluene from a composition containing said dinitrotoluene and said water which comprises:

(a) contacting said composition with nitric acid to provide a nitric acid-containing composition, and (b) distilling at least a portion of said water in said nitric acid-containing composition to provide vaporized water containing a reduced concentration of said dinitrotoluene, and (c) separating said vaporized water from said nitric acid-containing composition.

The invention is based on two discoveries relating to the separation and removal of process water which is generated during the production of DNT. The first discovery is that when contained in water in low concentrations, DNT has a volatility which iβ close to, and in the case of the 2,6- isomer, higher than, the volatility of water.

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Consequently, when such water is removed as a vapor phase, i.e., by distillation, a substantial proportion of the DNT will ordinarily evaporate or distill off with it; so that when the water subsequently iβ recondensed for ultimate disposal, it may well contain an environmentally objectionable level of DNT.

The second discovery is that the volatility of DNT, when contained in water in low concentrations, can be reduced by the addition of nitric acid. In accordance with the invention, this discovery not only provides the means for separating and removing process water which is substantially free of DNT, but also enables the achievement of this objective in a simple and cost efficient way. Thus no extra steps are required, the need for elaborate purification equipment is eliminated and the nitric acid employed in carrying out the distillation is the very acid used in the nitration reaction. As such it is easily and conveniently recycled to the nitration reactor either directly or after undergoing concentration, if necessary.

The process of the invention can be advantageously used tp treat any aqueous medium or stream containing dinitrotoluene in a concentration which cannot be readily removed by phase separation. It is of course well-known that dinitrotoluene has limited solubility in water. That iβ to say, it ordinarily will dissolve in water but only up to a limited concentration level. This concentration level limit, technically known or referred to as the saturation point, varies depending on the DNT isomer mix and also depending on temperature and other

phyβical or chemical conditions. For example, while under normal conditions the saturation point may range up to about 1,200 parts per million by weight (ppm) , if a substantial amount of nitric acid iβ preβent, it would raise the βaturation point dramatically. If more than the βaturation level iβ present in the water, it will phase separate, and aβ such it will be easy to remove. Removing the dissolved DNT, however, iβ another matter; and removed (or substantially reduced) it must be, before the water containing it can be responsibly disposed of without creating a hazard to the environment.

In view of the foregoing diβcuββion, it will become readily apparent that the process of the invention can be used to reduce the level of DNT preβent in any concentration, and any iβomer mix, in water. Accordingly, the term "impurity", aβ used herein with reference to the presence of DNT in the process water, iβ intended to mean and include any concentration which will reβult in βome DNT being dissolved in the water. Regarding the iβomer mix, it iβ to be noted that the 2,4- iβomer differe from the 2,6- iβomer in that itβ volatility (when preβent in relatively low concentrations in water) remains above, although quite cloβe to, the volatility of water. Nevertheless, using the improvement disclosed herein will facilitate the removal or reduction of the 2,4- isomer as well the 2,6- and other isomers inasmuch aβ the preβence of the nitric acid will enhance the effectiveness of the distillation. In general, the process of the invention iβ particularly suited for water

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containing diββolved therein either the 2,6- iβomer alone or a mixture thereof with other DNT isomers. And to the extent, as noted earlier, that any DNT present over and above the βaturation point can ordinarily be removed by simple phase separation, the procesβ of the invention iβ especially adapted for distilling water containing DNT aβ an impurity in a concentration up to about the βaturation point. As noted above, a critical parameter in practicing the distillation proceββ of the invention iβ to effect thiβ operation in the preβence of nitric acid. Some further scientific evidence has been developed to βuggeβt that an alternative source of nitrate ions, other than nitric acid, may also be effective. For example, it has been found, in accordance with the invention, that the addition of an alkaline earth metal salt, specifically magnesium nitrate, in lieu of the nitric acid, haβ eββentially a similar effect in lowering the volatility of DNT isomerβ. Based on this further finding, it iβ contemplated that various other sources of nitrate ions, including selected other nitrate salts, may be employed to achieve the same reβult or phenomenon aβ iβ achieved with the use of nitric acid. Nevertheless, the detailed deβcription provided herein will focus on nitric acid, inasmuch aβ itβ use is preferred for a variety of practical reasons. The nitric acid, in any suitable concentration iβ added to the water before or during the distillation operation. It haβ been found, moreover, that there iβ an inverse relationship between the concentration of nitric acid preβent and the concentration of DNT in the overhead water.

That is to say, within certain limits, the greater the concentration of nitric acid in the water being distilled, the lower the concentration of DNT in that water after distillation. Thus in accordance with the invention, although any concentration of acid may be used, it iβ preferable to employ such amounts of nitric acid as to provide an acid concentration in the water of at least about 10 percent by weight and more preferably from about 15 to about the azeotropic concentration of nitric acid in water, i.e., normally about 70 percent by weight. The most preferred acid concentration range is from about 25 to about 60 percent by weight.

In practicing the invention, the necessary amount of acid which is calculated to provide the desired concentration iβ simply added to the water either before commencement of the distillation operation or shortly thereafter. A further alternative, which would be particularly suited for continuous operation in a packed distillation column, is to inject the acid at a point above the water level βo that, aβ it enters the column, it will come in contact with the rising vapors. In accordance with yet another embodiment, the desired acid concentration is achieved by both including some acid in the water entering the column and injecting more acid into the column at a point above the water level as mentioned above.

The distillation can be carried out using any suitable or conventional apparatus or equipment ranging from a simple flaβh still to a fractional distillation column, which can be single-stage or multi-stage, with trays, packing or other mass

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transfer devices. Multi-stage distillation is preferred, particularly when the objective is to achieve a reduction of DNT concentration to less than about 100 ppm. Thus in accordance with the moβt preferred embodiments of the invention, DNT concentrations of below about 60 ppm by weight, and even below about 50 ppm (e.g., between 25 and 45 ppm) are achieved using multi-stage distillation. The diβtillation iβ performed at any βuitable temperature. Obviously, the actual boiling temperature of water will depend on the concentration of the nitric acid therein, and it is also governed by, and iβ a function of, the diβtillation pressure. As for the latter, it may range from aβ low aβ about 25 mm. of mercury to about 40 pβia, with a preferred range of from about 150 mm. of mercury to about 25 pβia.

The proceββ of the invention may be carried out batch-wiβe or aβ a continuouβ operation, the latter being preferred. In batch operation, the diβtillation can proceed, if deβired, until azeotropic conditions are attained, i.e., until sufficient water haβ been evaporated βo that the concentration of the acid in the remaining water reaches or approaches the azeotropic concentration. At this point, the distillation iβ stopped and the purified overhead water iβ taken out for further processing or biotreatment, as required, before it iβ properly diβcharged. In continuouβ operation, the water to be distilled is continuously fed to the diβtillation column at a rate which approximately equals the combined rate of bottoms and overheads coming out of the column. The water entering the

column may already contain the deβired concentration of nitric acid in it, or, alternatively, the acid may be injected into the column aβ a separate stream. Aβ noted earlier, a further alternative which, in accordance with one embodiment iβ preferred, iβ to provide for the preβence of nitric acid in two ways by (a) adding some acid to the water entering the diβtillation column (for example βuch aβ to provide an acid concentration from about 5 to about 25 percent by weight) , and (b) injecting further amounts of acid into the column, preferably at a point where it will come in contact with the rising vapor. The diβtillation column bottoms (whether resulting from a batch or continuous operation) , containing higher concentrations of both nitric acid and DNT, can be recycled as such to the toluene nitration reactor or, depending on the desired acid concentration in the recycle stream nd/or in the reactor, the bottoms may first be concentrated before being recycled to the reactor. The following examples are provided merely to illuβtrate the invention. All partβ and percentageβ are by weight unless otherwise specified. r

EXAMPLE 1

In this example, a βimple flash still iβ uβed to demonstrate the effect of nitric acid when water containing 2,6-DNT aβ an impurity iβ boiled. A water stream containing 2,6-DNT in a concentration of 0.0099 mole percent (based on the combined moles of water and DNT) is fed continuouβly to a βimple, flash still, with nitric acid being continuously

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injected into the βtream at a rate sufficient to provide an acid concentration in the stream of approximately 20 percent. After a 50% molar proportion of the stream has boiled overhead, the DNT concentration is checked in both the bottoms liquid stream and in the overhead stream using gas chromatography. The overhead βtream contained 0.0083 mole percent of 2,6-DNT; whereaβ, the bottomβ βtream contained 0.0102 mole percent. For compariβon purpoβeβ, the βame run waβ repeated, but this time no nitric acid waβ used. As a result, the overhead stream contained 0.0119 mole percent of 2,6-DNT, while the bottoms contained 0.0079 mole percent. EXAMPLE 2

In this example, a distillation column is used consiβting of a lower section housing a boiling still and an upper section containing a series of distillation trays for enhanced vapor-liquid contact. In a continuous operation, an aqueous solution containing 5,000 ppm of DNT and 14% nitric acid is fed to the column at the rate of 4.5 grams per minute. The DNT £β a typical iβomer blend, aβ uβed commercially in making toluene diisocyanate. It consists approximately of 76 percent of the 2,4- iβomer, 19 percent of the 2,6- iβomer and about 5 percent of other minor isomers. Approximately 3.5 grams per minute are taken out aβ bottomβ and one gram per minute aβ an overhead βtream. Using gas chromatography, the overhead βtream iβ found to containβ 470 ppm of DNT.

EXAMPLE 3

In this example, the identical procedure of Example 2 is followed, except that in addition to the 14% acid concentration in the water, more acid is added. Specifically, a 70% solution of nitric acid iβ injected at the rate of 3.5 grams per minute, this acid solution entering the column at the top of the trayed section, thereby coming in contact with the rising vapors. The overhead stream is found to contain only 40 ppm of DNT.

While the invention haβ been described above with referenceβ to βpecific embodiments thereof, it is apparent that many changes, modifications and variations can be made without departing from the inventive concept diβcloβed herein. Accordingly, it iβ intended to embrace all such changes, modifications and variations that fall within the spirit and broad scope of the appended claims.