The present invention relates to a process for the continuous preparation of nitrobenzene by adiabatic nitration of benzene with a mixture of sulfuric acid and nitric acid in a reaction zone, in which the aqueous phase containing sulfuric acid in the reaction zone has a content of metal ions which form sparingly soluble metal sulfates of less than 900 mg/l, based on the volume of the aqueous phase containing sulfuric acid.
This invention relates to a continuous process for the preparation of nitrobenzene by adiabatic nitration of benzene by a mixture of sulfuric and nitric acid (mixed acid). Such a process was initially disclosed in U.S. Pat. No. 2,256,999, and present day embodiments are described in, for example, U.S. Pat. No. 4,091,042, U.S. Pat. No. 5,313,009 and U.S. Pat. No. 5,763,697.
The adiabatic nitration processes described have as a common feature the fact that the starting substances benzene and nitric acid are reacted in a large excess of sulfuric acid, which takes up the heat of reaction liberated and the water formed during the reaction. For carrying out the reaction, nitric acid and sulfuric acid are mixed to give so-called mixed acid, and benzene is metered into this. The benzene reacts with the nitric acid to give water and substantially nitrobenzene. The temperature of the reaction mixture and the concentrations of benzene, nitric acid and sulfuric acid are chosen such that after the reaction zone, a mixture of benzene, nitrobenzene, sulfuric acid and water is obtained, and this mixture is substantially free from nitric acid. The temperatures required for this are conventionally between 70 and 145° C. To establish the mixed acid, nitric acid of a concentration of 60 to 98 wt. % and sulfuric acid of a concentration of 60 to 96 wt. % are conventionally employed. Benzene is employed at least in the stoichiometric amount, based on the amount of nitric acid, but preferably in a 2 to 10% excess, compared with the amount of benzene required stoichiometrically. These processes and these parameters are preferably likewise realized in the process according to the invention.
The reaction zone in which benzene and nitric acid are reacted can comprise an arrangement of stirred tanks, a loop reactor or a tube reactor, as good thorough mixing is necessary for the reaction. A tube reactor in which several dispersing elements are arranged in distribution over the length of the tube reactor and ensure intensive thorough mixing of benzene, nitric acid and sulfuric acid and water is therefore preferably employed. Such a reactor and the form of dispersing elements which can be employed are described, for example, in U.S. Pat. No. 4,994,242 and in U.S. Patent Application 2003/0055300 A1. These processes and these parameters are, preferably, likewise realized in the process of the present invention.
The reaction mixture which is obtained after the reaction zone and which is substantially free from nitric acid, is fed to a phase separation apparatus in which two phases are formed. The first phase being called crude nitrobenzene and substantially comprising nitrobenzene, benzene and an amount of sulfuric acid and water dissolved in the nitrobenzene. The second phase, also called waste acid, substantially comprises water, sulfuric acid and nitrobenzene dissolved in the sulfuric acid.
The phase separation apparatus has the intended task of separating the phases of the crude nitrobenzene and the waste acid completely, so that only the physically dissolvable contents of the other particular phase cannot be separated off. Because of this physically dissolvable content, the crude nitrobenzene always contains some quantity of sulfuric acid and the waste acid always contains some quantity of crude nitrobenzene. This process and these parameters are preferably likewise realized in the process according to the invention.
In the adiabatic nitration, the crude nitrobenzene separated off in the phase separation apparatus is conventionally, and preferably, also subjected to a washing and a working up by distillation. This is described, for example, in EP 181 61 17 A1.
In adiabatic nitration, the waste acid separated off in the phase separation apparatus is conventionally, and preferably, also introduced into an apparatus for flash evaporation of the water. In this apparatus, by application of a reduced pressure and utilizing the high temperature of the waste acid which has been achieved by the adiabatic procedure, water is evaporated out of the waste acid, such that a concentrated sulfuric acid is obtained, the concentration of which substantially corresponds to the concentration before the reaction zone. According to the embodiments of the adiabatic nitration of benzene disclosed in the prior art, which are also preferably utilized in the process according to the invention, the sulfuric acid obtained by flash evaporation (i.e. the circulating acid) is collected in a buffer tank and recycled completely into the reaction zone. The heat of reaction is utilized most effectively by the complete recycling of the sulfuric acid. By recycling the sulfuric acid, a sulfuric acid circulation is formed, which substantially comprises the reaction zone, phase separation apparatus, evaporator, buffer tank and connecting lines.
It is known in the art that metal ions which form sparingly soluble metal sulfates together with sulfate in the sulfuric acid may be present in the sulfuric acid. These metals include the elements Al, Ca, Cr, Mg, Mn, Fe, Co, Ni, Cu, Sr, Cd and Ba, in particular Ca or calcium and Fe or iron. If the concentration of these metal ions which form sparingly soluble metal sulfates exceeds the solubility limit, metal sulfates precipitate in the sulfuric acid and form solids which are carried along in the circulation with the sulfuric acid, until they settle and accumulate on a surface or at a narrow point.
It is likewise known in the prior art that the solubility limit of the metal ions which form sparingly soluble metal sulfates depends greatly on the temperature of the solution, that is to say on the temperature of the sulfuric acid. Thus, metal ions dissolve less in cold sulfuric acid than in hot sulfuric acid. Consequently, metal sulfates are preferably obtained as a solid in cold sulfuric acid or at points where sulfuric acid is cooled, such as, for example, in heat exchangers. This production of solids in heat exchangers is to be regarded as problematic, since it can lead to a covering of the surface of the heat exchanger and therefore to a deterioration in the heat transfer coefficient. This production of solids also limits the possible amount of material flowing through the heat exchanger due to the reduction in the free cross-section of the lines in the heat exchanger. Table 1 shows the solubility limits (in mg/l) for calcium (Ca) and iron (Fe) for some selected temperatures (source: E K.-H. Wehde: Untersuchungen zum Löslichkeitsverhalten anorganischer Sulfate und zur Wärmeübertragung bei der Auflkonzentrierung verunreinigter Schwefelsäure, Doctorate Thesis, University of Essen, 1984, p. 65 & p. 70):
TABLE 1Solubility limits of calcium and iron ions in 70 wt. % strength sulfuricacidTemperatureCalcium, Ca2+ [mg/l]Iron, Fe2+ [mg/l] 20° C.105300 60° C.—770100° C.2301,670110° C.2602,360
The prior art takes into account the phenomenon described above of metal sulfates precipitating out in heat exchangers by periodic flushing of all those heat exchangers which carry circulating acid. This periodic flushing removes the metal sulfates which have crystallized out of the concentrated sulfuric acid. This is described in, for example, DE 340 97 17 C2.
Furthermore, it has now been observed that the problematic deposits of metal sulfates may occur not only in heat exchangers, but also at all points where the concentration of the metal ions which form sparingly soluble metal sulfates is high enough and the temperature is low enough to result in the formation of solid, and at the same time, at points where the flow rate of the sulfuric acid or the cross-section of the lines carrying sulfuric acid is low enough to bring about an accumulation of the metal sulfates which is troublesome for the process.
Deposits of metal sulfates can therefore be observed not only in heat exchangers, but also as deposits on the bottoms of tanks, at measurement points, such as level measurements, and on dispersing elements which, as intended, have small through-openings. Examples of such dispersing elements are described, for example, in U.S. Pat. No. 4,994,242 Deposits of metal sulfates can likewise also occur within the flash evaporator, in which, as intended, the sulfuric acid is cooled while water is evaporated and the concentration of the acid is increased. Deposits of metal salts can furthermore also form in the working up section following the reaction, such as, for example, in the waste water treatment due to entrained metal sulfates. To reduce the troublesome influence of these deposits, periodic cleaning of the installation components in question is considered necessary according to the prior art cited above. This cleaning, however, is associated with down times in the production and therefore with additional costs.
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