Commercial aromatic nitration processes are generally operated with a mixed acid system comprising sulfuric and nitric acids. A substantial amount of nitric acid is typically lost in an economically and environmentally unsound manner in the spent acid of nitration. For example, conventional dinitrotoluene processes comprise reacting toluene in a first nitration stage to form mononitrotoluene and separating the organic product from the aqueous spent acid phase. The nitric acid in the spent acid phase is usually removed or recovered so that the remaining sulfuric acid can be reconcentrated and recycled. The crude mononitrotoluenes are then nitrated with fresh nitrating acid in a second nitration stage. The dinitrotoluene product is then separated from the aqueous spent acid phase which is recycled to a nitration stage.
Ideally, the nitric acid in the aqueous spent acid of the mononitration stage should be recovered with a minimum of processing and with no risk of discoloration (charring) of the spent acid. One such method involves the extraction of the spent acid with the aromatic hydrocarbon being nitrated in the process. This method removes the excess nitric acid from the spent acid in the form of the mononitro derivative of the aromatic hydrocarbon. Specifically, in a dinitrotoluene process toluene is used to extract the spent acid to recover the nitric acid as mononitrotoluene in a post-reactor. Impractically tight control over the stoichiometric reaction of the aromatic hydrocarbon is necessary to prevent formation of sulfuric acid soluble color bodies which render the final denitrified sulfuric acid black or dark red. "Charred" or black spent acid is unacceptable in the industry particularly if the denitrified spent acid is sent to a phosphate producer.
In another method, treatment of the spent acid of nitration with a variety of oxidizing or reducing agents which destroy the nitrous acid present in spent acid prior to extraction with the aromatic hydrocarbon will prevent the formation of discolored sulfuric acid. The cost of these reagents precludes their use in the industry.
U.S. Pat. No. 2,402,180 discloses a method for the manufacture of trinitrotoluene involving nitration in three steps: mono, di and trinitration stages. The spent acid from the trinitration stage is fortified in the nitrator itself to provide a mixed acid for the dinitration of added mononitrotoluene.
U.S. Pat. No. 2,849,497 discloses a continuous process for the preparation of nitrobenzene which involves contacting the spent acid that remains after the nitration with fresh benzene to use up the major portion of any unreacted nitric acid in the spent acid mixture. The improvement resides in the step of extracting dissolved nitric acid in the crude nitrobenzene product by contacting the crude nitrobenzene with the denitrated waste acid and thereafter denitrating the waste acid containing this nitric acid with fresh benzene.
U.S. Pat. No. 2,012,985 discloses the nitration of aromatic hydrocarbons such as benzene, toluene, xylene and the like using a spent acid resulting from the nitration of an alphatic alcohol containing at least two hydroxy groups, such as nitroglycerine spent acid.
U.S. Pat. No. 2,256,999 discloses a nitration process which is carried out with the utilization of the entire heat of mixing and of reaction, and the absorption of the heat in the contents of the nitrator. The acids and organic compounds are mixed in the nitrator and no attempt is made to cool the reaction. Accordingly, the temperature of the residual or spent acid remaining at the completion of the reaction is raised considerably. This hot residual acid is then concentrated by any desired method to substantially the strength of the initial sulfuric acid without the loss of the heat of reaction. This reconcentrated residual acid is then used in a subsequent nitration cycle.
U.S. Pat. No. 3,975,452 discloses a process for treating the final acid obtained in forming a nitrate of a polyhydric alcohol, the final acid comprising a mixture of sulfuric acid, nitric acid, water and the nitrate. The process comprises mixing the final acid with an aromatic nitro compound, allowing the mixture to stratify whereby the nitrate is dissolved into the aromatic nitro compound as one layer leaving a substantially nitrate-free waste acid layer comprising sulfuric acid, nitric acid and water, and separating the layers. The waste acid obtained in the extraction, which is free from the nitration product, is utilized for the nitration of aromatic compounds or aromatic nitro compounds, especially for the preparation of that aromatic nitro compound which serves as extracting agent in the process.
U.S. Pat. No. 4,021,498 discloses an adiabatic process for the mononitration of nitratable aromatic hydrocarbons which yield a mononitration product containing less than 500 ppm of dinitrated product. In order to insure complete denitration of the mixed acid in the adibatic reaction, the aromatic nitratable compound is used in slight excess over the nitric acid.
U.S. Pat. No. 4,257,986 discloses an improvement in a process for the manufacture of a nitroaromatic compound produced by the mixed acid nitration method. The improvement resides in the refining of the aqueous acid mixture which comprises (a) contacting the spent mixed acid with an oxidizing or a reducing agent under conditions effective for removing contaminent nitrous acid, (b) contacting the aqueous spent acid mixture in step (a) with feed aromatic compound to remove contaminent organics and residual nitric acid and then, if necessary, (c) contacting the remaining acid mixture with sufficient oxidizing agent under oxidizing conditions to remove residual organic components.