HK1113567A - A process for the production of dinitrotoluene - Google Patents

Description

Process for producing dinitrotoluene

Technical Field

The invention relates to a process for the production of dinitrotoluene by nitration of toluene with nitrating acid (mixture of nitric acid and sulfuric acid), wherein toluene is converted in a first stage into Mononitrotoluene (MNT) and the mononitrotoluene is then converted in a second stage into Dinitrotoluene (DNT).

Background

Dinitrotoluene is an intermediate in the production of Toluene Diisocyanate (TDI), an important starting material for the industrial scale production of polyurethanes.

The nitration of toluene with nitrating acid (a mixture of nitric and sulfuric acids) to produce dinitrotoluene is known and has been described many times (see, e.g., Ullmann's Enzyklopadie der technischen Chemie, 4 th edition, volume 17, p.391, 1979, Verlag Chemie Weinheim/New York). For example, as described in h.hermann, j.gebauer, p.konieczny, "Industrial Nitration of cellulose to dinitrocellulose" in ACS-Symposium, series, 234-:

a) separating the reaction mixture obtained by dinitration (MNT nitration into DNT) by phase separation, using nitric acid to enhance the used acid, mixing with toluene, and entering a mononitration process (toluene nitration into MNT);

b) the reaction mixture obtained from mononitration is separated in a separation stage after completion of the reaction into an organic phase containing mononitrotoluene and an aqueous phase containing mainly sulfuric acid ("spent acid");

c) the organic phase containing mononitrotoluene obtained in b) is subjected to a dinitration process, where the mononitrotoluene is reacted with nitric acid in the presence of sulfuric acid to form dinitrotoluene.

To achieve commercial specifications, the crude DNT thus obtained is generally treated in downstream stages, mainly by washing, so that the dissolved sulfuric and nitric acids and the secondary components formed in the reaction stage, such as cresols and their degradation products, can be largely removed. Typical commercial DNT products contain greater than 98.5 wt.% DNT, less than 0.1 wt.% mononitrotoluene, less than 0.1 wt.% trinitrotoluene, and less than 0.1 wt.% other minor components, based on the weight of the DNT product mixture, with DNT yields greater than 98% and toluene conversions greater than 99.9%. The ratio of the total weight of the 2, 4-and 2, 6-DNT isomers to the total weight of the 2, 3-, 3, 4-, 2, 5-and 3, 5-DNT isomers is also very important. According to commercial specifications, the total content of 2, 4-and 2, 6-DNT isomers in the crude DNT is greater than 95% by weight, based on the weight of the crude DNT. The content of 2, 4-DNT is preferably from 79.0% by weight to 81.0% by weight, based on the total weight of 2, 4-DNT and 2, 6-DNT. Accordingly, the content of 2, 6-DNT is from 19.0% by weight to 21.0% by weight, based on the total weight of 2, 4-DNT and 2, 6-DNT.

In addition to the crude DNT, acid losses are obtained in the separation of the reaction mixture obtained in the mononitration, which acid losses leave the system as a minor stream. The sulfuric acid content of the spent acid is generally from 70 to 74% by weight, and generally contains more than 0.1% by weight, preferably more than 0.2 to 1.5% by weight, of unreacted nitric acid, nitrosulfonic acid obtained during the oxidation which takes place in the secondary reactions, more than 0.2% by weight of MNT (not separated off in the phase separation), and generally water concentrations in the range from more than 26% to less than 30% by weight (including water introduced with fresh sulfuric acid added to the process, water contained in nitric acid and water formed during the nitration of toluene and mononitrotoluene), these contents being based on the weight of spent acid.

In the two-stage isothermal nitration of toluene to dinitrotoluene with nitric acid in the presence of sulfuric acid, the use of high-concentration nitric acid (DE 102004005913A) and of azeotropic and sub-azeotropic (sub-azeotropic) nitric acid in the concentration range from more than 57 to 69% by weight (EP 0903336A 2) is described. Concentrations of more than 69 wt.% require an increased cost-intensive technical input, mainly due to overcoming azeotropes.

In industrial DNT plants, the concentration of sulfuric acid used as catalyst and water scavenger is influenced primarily by the concentration of nitric acid added to the process and the reaction conditions in the mononitration stage.

It is known that, under suitable conditions, nitric acid is capable of not only nitrating but also oxidizing organic compounds in the two-stage isothermal nitration of toluene to dinitrotoluene with nitric acid in the presence of sulfuric acid. Possible compounds from the oxidation of toluene, MNT or DNT are cresols, phenols and nitration products thereof. The low concentration of sulfuric acid enhances the tendency of nitric acid to oxidize, and thus the content of organic by-products in the reaction mixture increases as the concentration of sulfuric acid decreases. On the other hand, as the sulfuric acid concentration decreases, the rate of nitration reaction required for mononitration decreases. Both of these phenomena require industrial DNT plants to set a lower limit on the concentration of sulfuric acid required for economic operation of the process. This lower limit is generally about 70% by weight (EP 0903336A 2).

This lower limit determines the sulfuric acid concentration of the spent acid obtained in the phase separation of the dinitration stage and recycled to the mononitration stage. The sulfuric acid concentration of the spent acid obtained in the second nitration stage is generally greater than 80% by weight, which on the one hand limits the sulfuric acid output by mononitration and on the other hand ensures a high reaction rate of the nitration reaction in the dinitration stage. Since the concentration of sulfuric acid has a very large influence on the reaction rate required in the dinitration stage, decreasing the concentration leads to a decrease in the reaction rate.

In order to meet the above concentration requirements and specifications for industrial DNT, H is generally used in industrial DNT plants using isothermal two-stage processes, based on the weight of sulfuric acid₂SO₄Sulfuric acid having a concentration of 93 to 98 wt% or more. When HNO is used, based on the weight of nitric acid₃In the case of a concentration of 60 to 65% by weight of the sub-azeotropic nitric acid, H is generally used based on the weight of sulfuric acid₂SO₄Sulfuric acid at a concentration of over 95 wt.%. In contrast, EP 0903336A 2 describes the use of 86 to 91% by weight, preferably 87 to 89% by weight, of sulfuric acid. The acid used may be freshly produced or else concentrated in a concentration unit from the spent acid obtained in the phase separation of the reaction mixture obtained in the mononitration.

In addition to this standard process for the 2-stage continuous isothermal nitration of toluene with nitric acid in the presence of sulfuric acid, it has also been proposed to carry out the nitration of toluene to dinitrotoluene with nitrating acid continuously in three stages (EP 903336A), or adiabatically in one or two stages according to the processes described in EP 597361A and EP 696570A, the total heat of reaction from the nitration of toluene to DNT or only from the DNT stage being used to separate off the water produced by the nitration and the water introduced by the nitric acid into the spent acid, as described in EP 696571. Furthermore, US 5948944 a and US 2362743A propose carrying out the nitration of toluene to DNT only in the presence of nitric acid as the reaction medium, which makes it possible to avoid the use of sulphuric acid.

In all processes for the production of DNT by nitration of toluene with nitric acid, as described, for example, in EP 155586 a and US 5275701 a, the process is operated economically on the premise that the spent acid formed during the process can be reprocessed in such a way that it can be reintroduced into the reaction as reaction medium.

However, very important considerations for the selective nitration process are the safety of the process itself, the stability of the process operation, the selectivity and completeness of the conversion of toluene to dinitrotoluene and the specific use of nitric acid, which is necessary for the conversion of toluene to dinitrotoluene.

In view of the above criteria, the production of dinitrotoluene on an industrial scale from toluene using nitric acid is carried out mainly according to the so-called nitrated or mixed acid process, in which toluene is reacted continuously with nitric acid in the presence of sulfuric acid in a two-stage isothermally operated reaction stage to form dinitrotoluene. In the process of the nitrating acid,

a) the reaction mixture obtained by dinitration (nitration of MNT to DNT) is separated by phase separation, the acid thus obtained is re-enhanced with nitric acid and then mixed with toluene, entering the mononitration process (nitration of toluene to MNT);

b) the reaction mixture obtained from mononitration is separated in a separation stage after completion of the reaction into an organic phase containing mononitrotoluene and an aqueous phase containing mainly sulfuric acid ("spent acid");

c) the organic phase containing mononitrotoluene obtained in b) is subjected to a dinitration process, where the mononitrotoluene is reacted with nitric acid in the presence of sulfuric acid to form dinitrotoluene.

The selectivity of the toluene reaction is mainly influenced by the concentration of sulfuric acid in the two reaction stages of the process. H is, as stated above, based on the weight of sulfuric acid₂SO₄Sulfuric acid concentrations of less than 70% by weight lead to an increased tendency of nitric acid to oxidize during the mononitration, with the result that toluene, MNT or DNT is oxidized to give cresols, phenols and their nitration and degradation products. On the other hand, a sulfuric acid concentration that is too high in the dinitration stage leads to an increased formation of trinitrotoluene due to the constant presence of nitric acid.

The required completeness of reaction in the reaction stage can also be influenced by the given residence time, the chosen sulfuric acid concentration and the reaction temperature. The completeness of the reaction also depends on the nitric acid concentration in the sulfuric acid phase. The completeness of the reaction also depends on the interface produced in the reaction stage, since the reaction mixture tends to decompose into an organic phase containing only traces of acid and a sulfuric acid phase in both nitration stages.

Depending on the components to be nitrated, the highly specific use of nitric acid can promote their reaction, but on the other hand leads to a significant loading of nitric acid in the spent acid obtained in the phase separation of the mononitro stage or in the DNT removed in the phase separation of the dinitration stage. It also results in large amounts of unreacted nitric acid which must be removed from the two streams, reprocessed in a later stage, and then returned to the reaction stage.

The economic efficiency of isothermal two-stage reactions of toluene using nitric acid in the presence of sulfuric acid has been attempted since the production of dinitrotoluene is carried out on a very large industrial scale, so that even very small economic improvements in the process of this important industry are of great economic interest.

EP 903336A describes the use of preferably 87 to 89% by weight of sulfuric acid, which is obtained by reprocessing the spent acid obtained in the mononitration and which can be obtained at very low cost, as is sulfuric acid with a content of more than 89% by weight. The use of lower sulfuric acid concentrations is considered based on the fact that the process is carried out in a triple nitrification stage rather than a double nitrification stage. The organic phase containing dinitrotoluene obtained in the phase separation of the second stage is passed to a third reaction stage, known as the polishing zone, in which it is reacted with a mixed acid containing an aliquot (aliquot) of nitric acid and all fresh sulfuric acid which is passed to the process. The basis of this process is that the sulfuric acid entering the polishing zone is diluted only by an aliquot of the water contained in the nitric acid feed and the water formed in the residual nitration carried out in the polishing zone. Thus, the acid concentration obtained in the finishing zone is even higher than that obtained in the standard process. In addition to the additional investment and operating costs of the third stage, a disadvantage of this three-stage process is that the polishing zone has only one reactor. Thus, in order to achieve complete conversion of the MNT, sufficient nitric acid must be added so that the nitric acid content of the spent acid obtained from the third stage thereafter is about 0.4% by weight. Such nitric acid contents, together with increased sulfuric acid concentrations, on the one hand entail an increased risk of TNT formation (US 3157706 a) and, on the other hand, according to practical experience, result in DNTs being discharged which have a nitric acid content of significantly more than 0.4% by weight and therefore a large amount of unreacted nitric acid.

Minimizing the loss of nitric acid through DNT emissions is a goal of EP 279312 a 2. EP 279312 a2 describes a process for separating sulfuric acid and nitric acid from dinitrotoluene containing sulfuric acid and nitric acid obtained in the dinitration of toluene with mixed acids. In the process disclosed, the dinitrotoluene obtained after the separation of the major part of the sulfuric acid and nitric acid, which still contains up to 6% by weight of sulfuric acid and up to 5% by weight of nitric acid, is mixed with up to 10% by weight of water, based on the amount of dinitrotoluene, and the aqueous phase containing sulfuric acid and nitric acid is removed, these two acids being subsequently separated off. The DNT wash is performed in one or more stages of the process.

EP 736514 a1 describes improved acid recovery compared to EP 279312 a 2. EP 736514 a1 discloses a process for removing and recovering nitric acid, sulfuric acid and nitrogen oxides from crude dinitrotoluene formed in the nitration of toluene or mononitrotoluene after removal of nitrating acid. In the process disclosed, the crude dinitrotoluene is extracted in countercurrent with dilute aqueous solutions of nitric acid, sulfuric acid and nitrous acid in a plurality of stages, in particular two or four stages, in each case in a volume ratio of dinitrotoluene to aqueous solution of from 1: 3 to 10: 1, preferably from 1: 1 to 4: 1. The aqueous extract is recycled to the nitration either directly or after concentration.

It is advantageous to carry out the extraction in all extraction stages at temperatures above the melting point of dinitrotoluene. The density of the aqueous solution should be different from that of the dinitrotoluene (preferably the former is lower than the latter) in all stages. It is advantageous to circulate the dilute aqueous solution in the extraction stages. The desired volume ratio of dinitrotoluene to dilute aqueous solutions of nitric acid, sulfuric acid and nitrous acid can be adjusted by adding fresh water to the extraction circuit of the dilute solution obtained in the last extraction stage. In particular, a concentrate is added which is formed when the aqueous extract is concentrated. The aqueous solution withdrawn from the first extraction stage is a nitric/sulfuric acid mixture containing a total of 25-40% by weight of acid. The mixture is concentrated either alone or, preferably, together with nitric acid from the treatment of the final mononitrotoluic acid to a total acid content of 65% by weight (based on HNO)₃Calculation).

The nitric acid content of the spent acid obtained in the phase separation of the mononitration stage can also be studied in a number of ways from the viewpoint of minimizing unused nitric acid.

US 2947791 a describes an improved continuous process for the nitration of toluene in which equimolar amounts of nitric acid (nitrating acid component of nitric and sulfuric acids containing 50-60% by weight of sulfuric acid, 20-40% by weight of nitric acid and 10-20% by weight of water) and toluene are reacted at a temperature of 50-100 ℃ in a well-stirred system consisting of two reactors in series, wherein 0.4-0.7 mole of toluene per mole of nitric acid is fed to the first reactor and the remainder of toluene is subsequently added to the reaction mixture leaving the first reactor and reacted in the second reactor.

According to US 2947791 a, the toluene fed in portions results in a toluene conversion of more than 95% in the mononitration stage and a lower content of nitrogen oxides and nitric acid in the aqueous phase. In the subsequent dinitration, an excess of 5-10 mol% of nitric acid is always used.

US 2475095 a describes that residual amounts of toluene in the mononitration reaction mixture make phase separation after the reaction mixture more difficult. Thus, US 2475095 a describes that an excess of nitric acid compared to toluene has been added to the mononitration process. In this case, the nitric acid content of the spent acid obtained from the mononitration stage is said to be 1% by weight.

US 4496782 a is also based on an excess of nitric acid in the mononitration process, so that the spent acid obtained from the mononitration stage contains a significant amount of nitric acid. To utilize the nitric acid content, US 4496782 a describes adding additional nitric acid to the spent acid obtained from the mononitration process to a nitric acid content of greater than 2 wt.%, and then reacting the nitric acid with an aliquot of mononitrotoluene in a stirred reactor under adiabatic conditions at a temperature of greater than 110 ℃, thus obtaining a spent acid having a nitric acid content of less than 0.25 wt.% in a subsequent phase separation. In addition to increasing technical input, this approach also presents security risks.

DE 102004005913 a1 emphasizes the technical input required for the two-stage isothermal reaction of toluene with nitric acid in the presence of sulfuric acid. According to the disclosure of DE 102004005913A 1, it is important to reduce the technical input compared with the prior art (according to DE 102004005913A 1, in the mononitration and dinitration stages, the technical input is a two-stage or four-stage stirred vessel cascade). DE 102004005913 a describes the use of one stirred vessel in the mononitration stage and a cascade of two stirred vessels in the dinitration stage.

DE 102004005913 a1 also describes a non-complete conversion of toluene with a significant amount of nitric acid (0.96% by weight in the examples of the process described in this document) in the spent acid of the mononitration stage and a complete conversion of the second nitration stage by a suitable excess of nitric acid.

DE 102004005913 a1 refutes the problem of recovering large amounts of unreacted nitric acid from acid-consuming systems from mono-or dinitration, as noted below: "to minimize the loss of nitric acid, i.e. not to nitric acid which is converted into the end product, the nitric acid from the crude DNT washing is recycled to the nitration process as a weak acid with a total acid content of 23.73% to 40%, together with nitric acid from the offgas washing and spent acid stripping (directly or after concentration), as described in EP 0736514". Thus, DE 102004005913 replaces the lower costs of the reaction stage with the added costs of DNT and spent acid treatment stages and also includes the risk of further reactions beyond control of the reaction in the mononitration stage and the increased risk of trinitrotoluene (TNT) formation in the dinitration stage.

Disclosure of Invention

It is therefore an object of the present invention to provide a process for the reaction of toluene with nitric acid in the presence of sulfuric acid to form dinitrotoluene which leads to high toluene conversion, high selectivity for DNT, efficient and selective utilization of the nitric acid fed directly into the reaction stage, using a simple equipment set-up.

This object is achieved by the two-stage nitration process for producing dinitrotoluene of the invention, in which: (1) in the mononitration step, the weight ratio of the aqueous phase to the organic phase is greater than 2: 1; (2) in each nitration reaction, the organic phase is dispersed in an acid-containing aqueous phase; (3) the amount of nitric acid used is less than 2.06 moles per mole of toluene.

Detailed Description

It is known that, in the desired isothermal nitration of toluene or mononitrotoluene with nitric acid in the presence of sulfuric acid, the nitration of aromatics takes place essentially in a two-phase aqueous phase (Ullmann's Enzyklopadie der technischen Chemie, 4 th edition, volume 17, page 391, 1979, Verlag Chemie, Weinheim-N.Y.). To react together, the toluene in mononitration and the mononitrotoluene in dinitration must diffuse through the interface between the two phases to react in the aqueous phase containing nitric acid in the presence of sulfuric acid.

It is also known that in this case, as the reaction conditions, such as the reaction temperature, the concentration of the components to be reacted in the organic phase and the concentration of nitric acid and sulfuric acid in the aqueous phase, vary, the effective rate of reaction depends to a large extent on the size of the constantly renewed interface, which can be increased, for example, by vigorous stirring. This has a favorable effect on the reaction rate.

The possible dependence of the available constantly updated interfaces on the type of mixing and composition of systems consisting of two immiscible liquids is also described in the literature (j.m. zaldivar et al, chemical engineering and Processing 34(1995)), 529. Thus, the corresponding dependence of the mononitration of toluene with nitric acid in the presence of sulfuric acid, etc. (see above) is also described in particular.

Surprisingly, it has now been found that the fundamental dependence described for the toluene reaction is observed to be more pronounced for the nitration of mononitrotoluene by nitric acid in the presence of sulfuric acid to dinitrotoluene.

Thus, in the two reaction stages of the isothermal nitration of toluene by nitric acid to dinitrotoluene in the presence of sulfuric acid (mononitration and dinitration), the size of the constantly renewed interface achievable depends not only on the energy of the stirring introduced into the reaction system, but also on the weight ratio of the phases present in the reaction system. But also on which phase is dispersed in which phase.

Thus, if the weight ratio of aqueous phase to organic phase in the mononitration stage is greater than 2: 1, preferably greater than 3: 1, most preferably greater than 3.5: 1, and the weight ratio of aqueous phase to organic phase in the dinitration stage is greater than 1.5: 1, preferably greater than 3: 1, most preferably greater than 3.5: 1, while the organic phase is dispersed in the aqueous phase (rather than a reverse phase dispersion or an "drop in drop" emulsion), a very large constantly renewed interface is obtained, so that essentially complete conversion of toluene to mononitrotoluene and of mononitrotoluene to dinitrotoluene is obtained despite less than 2.06 moles of nitric acid per mole of toluene being used in the overall process.

The invention relates to a method for producing dinitrotoluene by nitrating toluene with nitrating acid, wherein:

a) reacting toluene with nitrating acid to form mononitrotoluene to obtain reaction mixture containing mononitrotoluene,

b) separating the reaction mixture containing mononitrotoluene into an organic phase containing mononitrotoluene and an aqueous phase containing sulfuric acid,

c) the organic phase containing mononitrotoluene is reacted with nitrating acid to obtain a reaction mixture containing dinitrotoluene,

d) separating the reaction mixture containing dinitrotoluene into an organic phase containing dinitrotoluene and an aqueous phase containing sulfuric acid,

wherein

1) The weight ratio of aqueous phase to organic phase in the nitration in step a) is greater than 2: 1, preferably greater than 3: 1, most preferably greater than 3.5: 1, and the weight ratio of aqueous phase to organic phase in step c) is greater than 1.5: 1, preferably greater than 3: 1, most preferably greater than 3.5: 1,

2) in step a) and step c), the organic phase is in each case dispersed in an aqueous phase,

3) in general, less than 2.06 moles of nitric acid are used per mole of toluene.

It is important in the process of the present invention that the weight ratio of aqueous phase to organic phase in the mononitration stage (step a) is adjusted to be greater than 2: 1, preferably greater than 3: 1, most preferably greater than 3.5: 1 and in the dinitration stage (step c) the weight ratio of aqueous phase to organic phase is adjusted to be greater than 1.5: 1, preferably greater than 3: 1, most preferably greater than 3.5: 1. It is also important that in steps a) and c), in each case the organic phase is dispersed as a disperse phase in the aqueous phase (homogeneous phase), a total of less than 2.06 mol of nitric acid per mol of toluene being added to the process (steps a) to d)).

Of particular importance in the process of the invention is the form of the reaction mixture present in the reaction steps a) and c) being a dispersion in which in each case the organic phase is dispersed in the aqueous phase as a homogeneous phase. The input mixing or dispersing energy is preferably chosen such that the desired dispersion is produced with a very large interface, while avoiding the formation of stable "droplet-in-droplet" emulsions. The mixing or dispersing energy advantageously introduced into the reaction step is easily determined by simple experimentation. Even when following the phase ratios required for the present invention, the amount of energy required depends on the geometry of the reactor and stirrer chosen and the physical data of the reaction mixture. Once adjusted, however, it may be advantageous to monitor the state of the dispersion present in the reaction system by the conductivity (conductivity) of the uniformly mixed reaction mixture.

The nitration reaction in step a) and/or step c) is preferably carried out isothermally.

In a preferred embodiment of the invention, the reactions in the individual reaction steps a) and c) are carried out in reactors of a cascade, in which reactors mixing takes place, preferably in 2 to. ltoreq.4 reactors of the cascade. In a particularly preferred form, a loop reactor containing a circulation pump and a heat exchanger is used as the reactor.

In a further preferred embodiment of the invention, two loop reactors in series, comprising a circulation pump and a heat exchanger, are used for the mononitration stage. In the dinitration stage, two loop reactors in series are used, with a circulation pump and a heat exchanger, and in addition, one loop reactor with a circulation pump but no heat exchanger is used. The circulation pump should be sized so that the organic phase is always dispersed in a uniform aqueous phase. In this embodiment, the conductivity measurement is preferably carried out in the at least one reactor used for continuously monitoring the state of the circulating dispersion.

Phase separation steps b) and d) are carried out after reaction steps a) and c), respectively. Any equipment suitable for phase separation may be used. Both dynamic and static separators are suitable. In a preferred embodiment, static separators are used in both stages (steps b) and d)).

In the process according to the invention, toluene is added to the mononitration stage of step a). Toluene is preferably added to the first reactor, but it is also possible to add toluene in several portions to several reactors. The toluene entering the first reactor is preferably fed to a reactor which is mixed with nitrating acid through one or more nozzles. For this purpose, the aqueous phase containing sulfuric acid (spent acid from the dinitration stage) of step d) is preferably mixed with the previous nitric acid, thereby producing nitrated acid. However, it is also possible to use fresh sulfuric acid or a mixture of fresh sulfuric acid and spent acid from the dinitration stage.

Separate addition of toluene and nitrating acid is also possible. It is also possible to add nitric acid and sulfuric acid (e.g. spent acid from the dinitration stage) separately. The weight ratio of the phases in the reactor of the mononitration stage is preferably adjusted by the amount of sulfuric acid (e.g., spent acid from the dinitration stage) added to the reactor. However, it is also possible to recycle the aqueous phase containing sulfuric acid obtained in step b) from the mononitration stage, the recycled aqueous phase containing sulfuric acid preferably being fed to the first reactor of this stage. However, it may also be added to several reactors in this stage. It is also possible to feed the nitric acid in several portions to several reactors of the mononitration stage.

In a particularly preferred embodiment of the invention, 1.03 mol of nitric acid per mol of toluene are added to the mononitration stage (step a)). In general, less than 2.06 moles of nitric acid per mole of toluene are used in the two nitration stages of the present invention.

The reaction of toluene is carried out in the mononitration stage (step a)) at a temperature of from 30 to 70 ℃ and the reactor of the mononitration stage can be operated at the same reaction temperature. However, the reaction temperature may be different in each reactor to suit the progress of the reaction.

The dispersion present in the reactor of the mononitration stage (step a)) is preferably monitored continuously by conductivity measurements. The deviations which can be identified, for example, by a significant drop in the conductivity of the circulated reaction mixture, are therefore corrected by varying the mixing energy which is fed into the reactor via the mixing or dispersing device or, preferably, by varying the phase ratio in the reactor of the nitration stage.

In the process of the invention, very high toluene conversions can be achieved with very efficient use of the nitric acid added to the mononitration stage. The aqueous phase containing sulfuric acid (spent acid) obtained in the subsequent phase separation generally has a nitric acid content of less than 0.1% by weight, based on the weight of spent acid. Such low nitric acid levels make the process of the present invention very low in the requirement for specific nitric acids. In addition, such low nitric acid levels also reduce the cost of concentrating spent acid from the mononitration stage.

In the process of the present invention, the organic phase obtained from the phase separation of the mononitration stage (step b)) is preferably passed into the reactor of the dinitration stage (step c)) without any further treatment. The organic phase containing the MNT is preferably passed into the first reactor, but may also be divided into several portions into several reactors. The MNT entering the first reactor is preferably added to the reactor where it is mixed with nitrating acid using one or more nozzles. For this purpose, the aqueous phase containing sulphuric acid from step b) (spent acid from the mononitration stage) is optionally reprocessed, preferably mixed with previous nitric acid, to produce nitrated acid. However, it is also possible to use fresh sulfuric acid or a mixture of fresh sulfuric acid and spent acid from the mononitration stage. It is also possible to add the organic phase containing MNT and the nitrating acid from step b) separately. It is also possible to add nitric acid and sulfuric acid (e.g. reprocessed spent acid from the mononitration stage) separately. The weight ratio of the phases in the reactor of the dinitration stage (step c)) is preferably adjusted by the amount of sulfuric acid (e.g. the reprocessed spent acid from the mononitration stage) added to the reactor. However, it is also possible to recycle the aqueous phase containing sulfuric acid from the dinitration stage obtained in step d), the recycled aqueous phase containing sulfuric acid preferably being fed to the first reactor of this stage. However, it may also be added to several reactors in this stage. It is also possible to feed the nitric acid in several portions to several reactors of the dinitration stage.

In a particularly preferred embodiment of the invention, 1.03 mol of nitric acid per mole of mononitrotoluene are added to the dinitration stage (step c)). In general, less than 2.06 moles of nitric acid per mole of toluene are used in the two nitration stages of the present invention.

The reaction of mononitrotoluene is carried out in the dinitration stage (step c)) at a temperature of from 55 to 80 ℃ and the reactor of the dinitration stage can be operated at the same reaction temperature. However, the reaction temperature may be different in each reactor to suit the progress of the reaction.

The dispersion present in the reactor of the dinitration stage (step c)) is preferably monitored continuously by conductivity measurements. The deviations which can be identified, for example, by a significant drop in the conductivity of the circulated reaction mixture, are therefore corrected by varying the mixing energy which is fed into the reactor via the mixing or dispersing device or, preferably, by varying the phase ratio in the reactor of the nitration stage.

In the process of the present invention, very high mononitrotoluene conversions can be achieved with very efficient use of the nitric acid added to the dinitration stage. The aqueous phase containing sulfuric acid (spent acid) obtained in the subsequent phase separation generally has a nitric acid content of less than 0.2% by weight, based on the weight of spent acid. The organic phase containing DNT obtained in step d) has a nitric acid loading of less than 0.4 wt.% HNO, based on the weight of the organic phase₃。

Overall, the process of the present invention produces DNT in greater than 97.7% yield based on the nitric acid used. Such high DNT yields based on the nitric acid used have been achieved in the prior art only by expensive treatment of the spent acid obtained in the mononitration stage or of the aqueous phase obtained in downstream extractions and/or washings.

The process of the invention therefore differs from the processes of the prior art in the very high DNT yields (based on the nitric acid used) while using simple apparatus. The low requirement of the invention for nitric acid can be achieved without the costly and energy intensive additional nitric acid recovery step employed in prior art processes to achieve low consumption of the particular nitric acid.

The key to achieving low consumption of a particular nitric acid without reprocessing any of the spent acid and wash water is that the organic phase is dispersed in the aqueous phase in both reaction stages. This can be achieved or is easier to achieve by setting the weight ratio of the phases in reaction steps a) and c) to a certain value in advance. As a result, by appropriate choice of the reactor configuration, a significantly enlarged, rapidly renewed interface between the organic and aqueous phases can be achieved in comparison with a reverse dispersion with the same energy input. Thus, surprisingly high DNT yields (based on the nitric acid used) can be achieved with approximately the same sulfuric acid concentration and lower nitric acid concentration in the reaction stage compared to the prior art.

The proportion of the reaction in the dinitration stage (step c)) is of particular importance in the context of the invention. The transport limitation of the mononitrotoluene reaction is largely eliminated because of the large, rapidly renewed interface that can be achieved in the process of the invention. Thus, at approximately the same sulfuric acid concentration, a lower residual concentration of nitric acid in the reaction mixture leaving this stage is necessary for complete conversion of MNT. The lower content of nitric acid also makes it possible to significantly reduce the nitric acid loading of the dinitrotoluene-containing organic phase obtained in the subsequent phase separation (step d)), so that the unreacted nitric acid discharged from the reaction systems a) to d) is significantly reduced.

Examples

SUMMARY

The nitration is carried out in a continuously operable two-stage laboratory apparatus which in each case consists of the following components:

a feed container with a metering pump is provided,

two reactors in series (stirred vessel with heating/cooling jacket and high speed laboratory stirrer),

a separator installed at the outlet of the second reactor,

a downstream receiver for collecting the separated phases,

in each case, the collected aqueous phase can be recirculated to the first reactor of the cascade via additional metering pumps.

In carrying out the nitration reaction, sulfuric acid is first added to the apparatus (which has been rendered inert) at the concentration desired for continuous operation, and then the components are metered in the following weight ratios and mass while the mixing energy input to the reactor is adjusted by means of a stirrer in order to obtain the optimum value of the conductivity measured in the finely divided reaction system.

After reaching a stable equilibrium, the organic phase discharged from the separator was analyzed for conversion, and for the DNT phase, also for the composition of their main components and their nitric acid loading. The nitric acid content of the aqueous phase discharged from the separator was also measured.

In addition, the by-products of each phase were qualitatively analyzed. Small amounts of nitrocresols and nitrobenzoic acids were found in the different degrees of nitration.

Example 1(not in accordance with the invention)

720 g/h 96% sulfuric acid and 331 g 98.5% nitric acid are continuously mixed (forming nitrating acid), this mixture is continuously metered together with 685.7 g/h crude MNT from the MNT stage into the first reactor of the DNT stage, where it reacts at 70 ℃ and the conductivity in the reactor cascade is close to 90 mS. The fully reacted reaction mixture is separated in a subsequent separator into an organic phase and an aqueous phase.

The aqueous phase of the dinitration stage is continuously mixed with 331 g/h of 98.5% nitric acid and this mixture is continuously metered into the first reactor of the MNT stage together with 460.7 g/h of toluene and reacted in this reactor at 50 ℃ with a conductivity in the reactor cascade of about 80 mS. The reaction mixture which overflows into the separator is separated here into an organic phase and an aqueous phase, and the organic phase is then passed successively to the dinitration stage, the aqueous phase being transferred to the work-up stage.

The results obtained are shown in Table 2.

Example 2(according to the invention)

715 g/h 96% sulfuric acid and 328 g 98.5% nitric acid are continuously mixed (forming nitrating acid), and this mixture is continuously metered into the first reactor of the DNT stage, where it reacts at 70 ℃ together with 685.7 g/h crude MNT from the MNT stage and 2190 g/h aqueous phase from the separator of the DNT stage, the conductivity in the reactor cascade being close to 160 mS. The fully reacted reaction mixture is separated in a subsequent separator into an organic phase and an aqueous phase.

The aqueous phase of the dinitration stage is continuously mixed with 328 g/h of 98.5% nitric acid, and this mixture is continuously metered into the first reactor of the MNT stage together with 460.7 g/h of toluene and 2310 g/h of the aqueous phase from the separator of the MNT stage, reacted in this reactor at 50 ℃ and a conductivity in the reactor cascade of about 120 mS. The reaction mixture which overflows into the separator is separated here into an organic phase and an aqueous phase, and the organic phase is then passed successively to the dinitration stage, the aqueous phase being transferred to the work-up stage.

The results obtained are shown in Table 2.

Example 3(according to the invention)

275.4 g/h 92% sulfuric acid was continuously mixed with 474.9 g 68% nitric acid (forming nitrating acid), and this mixture was continuously metered into the first reactor of the DNT stage, where it was reacted at 70 ℃ together with 685.7 g/h crude MNT from the MNT stage, with a conductivity in the reactor cascade of approximately 160 mS. The fully reacted reaction mixture is separated in a subsequent separator into an organic phase and an aqueous phase.

The aqueous phase of the dinitration stage is continuously mixed with 474.9 g/h of 68% nitric acid, and this mixture is continuously metered into the first reactor of the MNT stage together with 460.7 g/h of toluene, reacted in this reactor at 50 ℃ and a conductivity in the reactor cascade of about 120 mS. The reaction mixture which overflows into the separator is separated here into an organic phase and an aqueous phase, and the organic phase is then passed successively to the dinitration stage, the aqueous phase being transferred to the work-up stage.

The results obtained are shown in Table 2.

From the column "Nitric acid in dinitration stage (Nitric acid di stage)" in Table 2, it can be seen that the amount of Nitric acid required in examples 2 and 3 according to the invention, in which the weight ratio of aqueous phase to organic phase in the nitration in step a) is greater than 2: 1 and the weight ratio of aqueous phase to organic phase in step c) is greater than 1.5: 1), is less than the value of example 1 not according to the invention. This is due to the fact that under the optimized operating conditions of the process of the invention, the nitric acid used is almost completely consumed in such a way that the HNO in the "spent acid in dinitration stage" of Table 2 is eliminated₃Residue (HNO)₃The amount of nitric acid discharged in the column "residual in specific acid di)" can be concluded as described above.

Such as columns "Nitric acid (Nitric acid mono stage) in the mononitration stage" and "HNO in the spent acid in the mononitration stage₃Residue (HNO)₃The data in residual in specific acid mono) "shows that similar statements also apply to the mononitration stage.

TABLE 1 acid concentration

Example No. 2	Concentration of nitric acid	Concentration of sulfuric acid
				[ weight% ]]	[ weight% ]]
			1	98.5	96
2	98.5	96
			3	68.0	92

TABLE 2

Example No. 2	Toluene	Nitric acid of mononitration stage	HNO3Residue of	Conversion of toluene		Temperature of the mononitration stage	Recycling spent acid (mineral acid monorycyling) from mononitration stage	Comparative examples Mass (inorganic)/Mass (organic)
									In the spent acid of the mononitration stage
				[ mols ] of]	[ mols ] of]	[％]							[ mol% ]]		[℃]	[ gram of]
1	5.00	5.175					0.09	＞99.9		50	0	1.34
			2	5.00	5.125	0.005							＞99.9		50	2310	4.69
3	5.00	5.125					0.005	＞99.9		50	0	4.72

Table 2 (continuation)

Example No. 2	Nitric acid in the dinitration stage	Sulfuric acid from the dinitration stage	HNO in spent acid in secondary spent stage3Residue of	MNT conversion	TNT content	Temperature of the dinitration stage	Recycling spent acid (acid) from the dinitration stage	Comparative examples Mass (inorganic)/Mass (organic)
										[ mols ] of]	[ gram of]	[％]	[ mol% ]]	[％]	[℃]	[ gram of]
1	5.175	720	0.43	＞99.9	＜0.05	70	0	0.89
									2	5.125	715	0.13	＞99.9	＜0.05	70	2190	3.30
3	5.125	2752	0.13	＞99.9	＜0.05	70	0	3.29

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims (12)

1. A process for producing dinitrotoluene by nitrating toluene with nitrating acid comprising:

a) reacting the organic phase containing toluene with the aqueous phase containing nitrating acid to form a reaction mixture containing mononitrotoluene,

b) separating the reaction mixture containing mononitrotoluene into an organic phase containing mononitrotoluene and an aqueous phase containing sulfuric acid,

c) reacting the organic phase containing mononitrotoluene with nitrating acid to form a reaction mixture containing dinitrotoluene,

d) separating the reaction mixture containing dinitrotoluene into an organic phase containing dinitrotoluene and an aqueous phase containing sulfuric acid,

wherein

1) In step a), the weight ratio of the aqueous phase to the organic phase is greater than 2: 1,

2) in step c), the weight ratio of aqueous phase to organic phase is greater than 1.5: 1,

3) in step a), the organic phase is dispersed in the aqueous phase,

4) in step c), the organic phase is dispersed in the aqueous phase,

5) less than 2.06 moles of nitric acid are used per mole of toluene.

2. The process according to claim 1, wherein a portion of the aqueous phase containing sulfuric acid obtained in step d) is recycled to step c) so that the weight ratio of the aqueous phase containing sulfuric acid to the organic phase containing dinitrotoluene in step d) is greater than 1.5.

3. The process according to claim 2, characterized in that at least a part of the aqueous phase containing sulfuric acid obtained in step d) is recycled to the first reactor in a cascade of at most 4 reactors.

4. The process according to claim 1, characterized in that the aqueous phase containing sulfuric acid obtained in step d) is wholly or partly recycled to step a).

5. The process according to claim 1, wherein the aqueous phase containing sulfuric acid obtained in step b) is passed completely or partially to step c).

6. The process according to claim 1, wherein the reaction in step a) and/or step c) is carried out isothermally.

7. The process according to claim 1, wherein in step b) and/or step d) the phase separation is carried out using a static separator.

8. The process according to claim 1, wherein step a) and/or step c) is carried out in a cascade of up to 4 reactors.

9. The process of claim 8, wherein the last reactor in step c) is a tubular reactor.

10. The process of claim 1, wherein step a) and/or step c) is carried out in a loop reactor.

11. The process of claim 1 wherein in step a) less than or equal to 1.03 moles of nitric acid are used per mole of toluene.

12. The process of claim 1 wherein in step c) less than or equal to 1.03 moles of nitric acid are used per mole of mononitrotoluene.