HK1020043B - An adiabatic process for producing dinitrotoluene - Google Patents

Description

Adiabatic process for preparing dinitrotoluene

The invention relates to an adiabatic process for the preparation of Dinitrotoluene (DNT) by nitration of toluene with nitric acid under adiabatic conditions.

It is known that toluene can be nitrated to dinitrotoluene by adiabatic processes. Toluene and nitrating acid (mixture of sulfuric and nitric acid, containing HNO)₃1-20 wt.%), while mixing well. The hot reaction products are phase separated, water is evaporated off and the concentrated spent acid is recycled to the nitration. As the spent acid is concentrated, the water produced during nitration is removed along with any other water that may be present. The heat of reaction is used to remove water.

One of the problems encountered with this process is that a portion of the DNT remains dissolved in the acid present in the separated aqueous phase. The DNTs remaining with the steam become solid under condensing conditions and cover the cold surfaces of the condensing heat exchanger. The curing point of a typical mixture of DNT isomers is about 55 ℃. Due to the formation of such precipitates, the heat transfer is significantly reduced, the steam can no longer be sufficiently condensed and the condenser has to be repeatedly shut down for cleaning.

In the conventional "isothermal" two-stage process for preparing DNT, the above-mentioned problems can be solved by injecting Mononitrotoluene (MNT), which is formed and separated in the first nitration stage, into the vapor space of an evaporator (DE-A-3,409,719). The MNT injected in this way lowers the melting point of the DNT and thus ensures that the vapour remains liquid under the conditions of water condensation. The organic phase separated from the vapour condensate is recycled to the reactor of the dinitration stage.

However, this solution cannot be used in the one-stage adiabatic dinitration of toluene (EP-A-597,361) process, since no separable MNT vapors are present in this process. MNT is produced as an intermediate in an adiabatic one-step process, but is immediately further nitrated to DNT.

EP-696,569 describes a one-stage adiabatic process for preparing DNT, which solves the problems described above. In the process disclosed, a small amount of mononitrotoluene is still present in the reaction mixture after the nitration is complete. The acid consumed in the reaction is condensed before the organic components are separated. The reaction product leaving the reactor, which still contains MNT, is passed directly to a concentration step, where the MNT is preferentially volatilized before the water and DNT fractions. The amount of MNT still remaining in the product mixture after the reaction has to be chosen such that no covering of the condensation heat exchanger by organic product occurs during the concentration step.

The process described in EP-696,569 has the disadvantage that MNT cannot be completely evaporated with water in the concentration step and that small amounts of MNT residue remain in the reaction product. This small amount of MNT leads to a reduction in the yield, since it cannot be separated from the DNT and recycled into the nitration stage. The separation is generally not affected until the DNT is hydrogenated to TDA (toluene diamine). Then, the aminotoluene produced from MNT was separated by distillation. This results in additional distillation losses for the hydrogenation process, as well as additional losses of hydrogen, catalyst and energy.

For these reasons, efforts are still made to keep the MNT content in the reaction product as low as possible and to operate with only the minimum amount of MNT necessary to keep the condenser clean. However, in practice, there is often a problem, if not impossible, in order to maintain constraints of the type described above.

Furthermore, the minimum amount of residual MNT necessary limits the purity of the DNT product that can be achieved.

This type of process control (i.e. condensation of the steam together with DNTs which are as free of MNTs as possible without any problems) places very high demands on the evaporation plant. In practice, it is almost impossible to construct and operate an evaporation apparatus that can meet these severe conditions. A certain amount of residual MNT in the nitrated product and the resulting yield loss are in most cases unavoidable. Furthermore, it is extremely difficult to ensure that the residual amount of MNT at the reactor outlet is low enough to allow the steam condensation step to be carried out efficiently and not affected by the formation of solids.

It is an object of the present invention to provide a continuous, adiabatic process for the preparation of dinitrotoluene.

It is a further object of the present invention to provide an adiabatic process for the preparation of dinitrotoluene which reliably prevents the formation of solid precipitates in the steam condensation step.

It is a further object of the present invention to provide an adiabatic process for preparing dinitrotoluene which can be carried out economically and is technically simple to operate.

These and other objects, which will be apparent to those skilled in the art, can be accomplished by the following process: toluene is reacted with nitric acid at an adiabatic temperature of from about 60 to about 200 ℃ and wherein the molar ratio of toluene to nitric acid is from about 1: 1.5 to about 1: 3. The reaction mixture thus obtained is then concentrated to a water content of not more than 30 wt.%. If the reaction is incomplete, Dinitrotoluene (DNT) present in the reaction mixture is at least partially removed by customary methods before or after concentration. A solvent is added to the stream containing DNT which is generated during the concentration of the reaction mixture. The resulting mixture of solvent and steam may then be recycled to the reaction vessel as a mixture. The solvent/vapor mixture may also be separated with recycled solvent and the DNTs may be combined with the product DNTs and also recycled to the reaction vessel.

The present invention relates to a process for the preparation of dinitrotoluene by the adiabatic nitration of toluene with nitric acid at a temperature of from about 60 to about 200 ℃ and a molar ratio of toluene to nitric acid of from about 1: 1.5 to about 1: 3.0. The resulting reaction mixture is concentrated to a water content of no more than about 30 wt.% (to contain H)₂O、HNO₃And H₂SO₄Based on the total weight of the concentrated mixture). Dinitrotoluene present in the reaction mixture is removed in whole or in part from the nitration reaction mixture by any of the well-known techniques, either before or after concentration of the mixture. Any DNT present in the vapor generated during the concentration of the reaction mixture is kept liquid by the addition of a solvent. The solvent added to the DNT-containing stream is separated together with the DNT from the water (aqueous phase) which is removed during the concentration of the reaction mixture. The solvent/DNT mixture is subsequently treated so that DNT is separated from the solvent and the solvent is recycled to the vapors generated during the concentration of the reaction mixture. The DNT separated from the solvent/DNT mixture may then be combined with the product DNT recovered from the reaction mixture or recycled to the nitration reaction vessel. The DNT/solvent mixture may also be recycled directly to the nitration reaction vessel. The solvent and any dinitrotoluene present therein are finally collected.

The nitration process of the present invention is preferably conducted at a temperature of from about 90 to about 180 deg.C, more preferably from about 95 to about 170 deg.C, and most preferably at about 100 deg.C and 160 deg.C.

The molar ratio of toluene to nitric acid in the nitration process of the present invention is preferably from about 1: 1.7 to about 1: 2.5, most preferably from about 1: 1.8 to about 1: 2.2.

Any of the well-known nitrating acids that are useful for nitrating aromatic compounds may be used as nitrating acids in the process of the present invention. The nitrating acid is typically a mixture of sulfuric and nitric acids, wherein the nitric acid typically contains from about 0.5 to about 15 wt.% (based on the total weight of the nitrating acid), preferably from about 1.5 to about 8 wt.% nitric acid.

In principle, any organic substance which dissolves DNT and prevents the formation of precipitates on the heat exchanger is a suitable solvent to be added to the steam which is generated during the concentration of the nitration reaction mixture. Particularly suitable solvents are those organic compounds having a boiling point at normal pressure of from about 80 to about 250 c, preferably from about 100 to about 200 c.

Such solvents include C which may be substituted₁-C₁₅Hydrocarbons, for example, substituted with nitro or halogen. Other suitable materials for the solvent include aromatic compounds and olefins which may be substituted with halogens, and white spirit. Specific examples of useful solvents include: toluene, xylene, chlorobenzene, dichlorobenzene, trichlorobenzene, isododecane, dodecane, and mononitrotoluene. Toluene and/or any mononitrotoluene produced in the present process is preferably used as the solvent. The solvent may of course be used alone or in admixture with other solvents.

The solvent was added to the steam or to the steam condensate in the following amounts: the weight ratio of the total solvent used to the DNT present in the steam or steam condensate is from about 50: 1 to about 1: 10, preferably from about 20: 1 to about 1: 5.

An important aspect of the successful performance of the process of the invention is that the solvent should be added separately to the steam and that the weight ratio of solvent to DNT in the steam falls within the above-mentioned range.

The reaction mixture resulting from the reaction of toluene and nitric acid is concentrated to a water content of not more than 30 wt.% (to contain H)₂O、HNO₃And H₂SO₄Based on the weight of the concentrated nitrating mixture). The reaction mixture is preferably concentrated to a water content of not more than 27 wt.%.

The DNT content in the vapour or vapour condensate obtained during concentration of the reaction mixture generally falls in the range from 2 to 50 wt.% (based on the total amount of vapour).

Concentration of the reaction mixture may be accomplished by any well-known technique such as standard distillation or flash distillation. The reaction mixture which is concentrated after the separation of the DNT consists essentially of an aqueous phase of concentrated sulfuric acid and may also comprise residual organic constituents such as dinitrotoluene, mononitrotoluene, nitrocresol, nitrobenzoic acid or nitrosulfuric acid. The concentrated mixture is recycled to the reaction vessel to which the toluene and nitrating acid are added.

The yield of DNT prepared by the process of the invention is > 96% after the customary purification steps of crude DNT. The purity of the obtained DNT is more than or equal to 98 percent.

The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.

Example 1: injection of solvent MNT for steam condensation

The reactor is a tubular reactor consisting of a lower part (about 1 metre long, 28 mm diameter) and an upper part (about 8 metres long, 80 mm diameter). The reactants are brought into contact with each other and thoroughly mixed by a nozzle. The dispersion produced is maintained in the direction of the longitudinal axis of the reactor by means of an internally mounted 30-mesh tray.

The following input streams were fed to the continuously operated adiabatic reactor with the following conversion:

toluene 55.1 kg/h (A)

68％HNO₃114.6 kg/hour (B)

80.5％H₂SO₄2002.1 kg/hr (C)

Toluene (a) and nitric acid (B) were weighed and added to the reactor at ambient temperature of about 20 ℃. Recycled sulfuric acid (C) was added to the reactor at about 110 ℃. The reaction mixture emerging at the bottom of the reactor had a temperature of 149 ℃ and consisted of 78 wt.% of circulating sulfuric acid, 89.3 kg/h of dinitrotoluene and 10.2 kg/h of mononitrotoluene. The reaction mixture was concentrated by flash evaporation at 30 mbar. The concentrated reaction mixture was then cooled to 110 ℃ by removing the heat of evaporation. The temperature in the flash evaporator was maintained at 110 c by providing additional heat. About 9.9 kg/hr of DNT and 5.7 kg/hr of MNT were carried away by the steam. An additional 39.9 kg/hr of MNT was added to the top of the steam condenser. Under preferred conditions, no DNT precipitation was seen on the walls of the steam condenser, which was operated at a condensation temperature of 25 ℃. Further nitration of the MNT to DNT can also be seen in the phase separator and in the flash distillation. After phase separation, this further nitration produced an isolated DNT containing 0.08% MNT.

Example 2: injecting solvent toluene for steam condensation

A cuspated tubular reactor is used herein as a reactor. The feed streams are a pure toluene stream and a stream consisting of a mixture of aqueous nitric acid and aqueous sulfuric acid (mixed acid). The temperature of the feed stream was maintained at 115 ℃ and continuously weighed into the reactor. The open ended tubular reactor is insulated to prevent temperature degradation during adiabatic operation. The composition of the mixed acid was determined gravimetrically to be 31.053g H₂SO₄，1.553g HNO₃And 9.358g H₂And O. The mixed acid was flowed at a volumetric flow rate of 4 liters/hour and HNO was added₃The stoichiometric ratio to toluene was adjusted to 2.14. To compensate for any heat loss, supplemental heat may be provided to the bottom of the reactor by an oil thermostat. The reaction mixture leaves the reactor at 160 ℃ and is subjected to rapid evaporation under vacuum of 70 mbar.

The steam was condensed in a condenser with cold water at a temperature of 12 c. 961g of toluene were uniformly injected directly into the vapor line upstream of the condenser, and the injection was completed in 7 hours. The vapor subcooled the condenser surfaces in a liquid stream without the precipitation of solids. The aqueous and organic phases of the resulting vapor condensate were separated and subjected to compositional analysis.

The liquid mixture of concentrated sulfuric acid and crude DNT leaving the flash evaporator was separated at 120 ℃ into an aqueous phase (41.236g) and an organic phase (660.2 g). The separated aqueous phase contained 1.39 wt.% of DNT and 78.5wt. -%)H of (A) to (B)₂SO₄. The residue consists essentially of water.

The following product distributions were determined in the organic vapor condensate and in the liquid organic discharge of the flash evaporator (after separation and washing).

(MNT-mononitrotoluene, DNT-dinitrotoluene; GC% area data, detector: FID, separation column: OV 1701):

	organic vapor condensate	Organic effluent of flash evaporator
			Toluene	7.10	0
2-MNT	1.14	0
			3-MNT	0.92	0
4-MNT	3.70	0
			2，6-DNT	32.85	27.65
2，5-DNT	1.27	2.04
			2，4-DNT	49.27	63.34
2，3-DNT	2.38	3.02
			3，4-DNT	1.36	3.94

Example 3: toluene as solvent and recycled solvent was injected into the reactor for steam condensation

The reactor is a tubular reactor consisting of a lower part (about 1 metre long, 28 mm diameter) and an upper part (8 metres long, 80 mm diameter). The reactants are brought into contact with each other and thoroughly mixed by a nozzle. The dispersion produced is maintained in the direction of the longitudinal axis of the reactor by means of an internally mounted 30-mesh tray.

The following input streams were fed to a continuously operated adiabatic reactor:

toluene 55.0 kg/hr

65％HNO₃119.0 kg/hour

78.8％H₂SO₄1647.7 kg/hr

Toluene and nitric acid were weighed and added to the reactor at ambient temperature of about 20 c in such a way that only 27.5 kg/hour of toluene was added directly to the upper stream of the reactor. Recycled sulfuric acid was added to the reactor at about 115 ℃. The temperature of the reaction mixture emerging at the bottom of the reactor was 155 ℃. The reaction mixture is introduced into a concentration apparatus and concentrated by evaporation at a pressure of 78 mbar. The concentrated reaction mixture was then cooled to 132 ℃ by removing the heat of evaporation. The temperature in the flash evaporator is maintained by indirectly providing additional heat. 12.8 kg/h of DNT and 1.8 kg/h of MNT are carried away by the steam. 27.5 kg/h of toluene were added to the top of the steam condenser. Under these conditions, no precipitation was seen on the condensing unit, which was operated at a temperature of 25 ℃. The condensed vapour is fed to a phase separation step in which the organic and inorganic phases are separated and the organic phase is recycled to the reactor. The bottom product from the concentration step was discharged under normal pressure through a barometric seal, cooled to 115 ℃ and phase separated. The 108 kg/h organic phase was separated. The organic phase consisted of 99.7% DNT and 0.03% MNT.

Example 4: separation of MNT and DNT isomers by distillation

A mixture of 362g of MnT isomer and 168g of DNT isomer was placed in a 1 l multi-necked flask and distilled by fractional distillation over a laboratory column (Vigreux column, 35 cm long) with stirring. The pressure at the top was set at 7.5 mbar. The temperature at the bottom was continuously raised from 95 ℃ to 145 ℃. At equilibrium, the temperature at the top increased from 85 ℃ to 98 ℃. The composition of the fractions collected by fractional distillation was determined by gas chromatography as follows:

the first 10g of distillate contained no DNT. The first DNT detection was carried out after 177.3g of distillate had been distilled off. The final fraction (after 362.5 g) contained 5.4 wt.% DNT. The final composition of the product remaining in the bottom (167.5g) was 0.6 wt.% MNT and 99.4 wt.% DNT.

Although the foregoing invention has been described in some detail for purposes of clarity 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 (11)

1. A process for producing dinitrotoluene, the process comprising:

a) reacting toluene and nitric acid at 60-200 deg.C under adiabatic condition, wherein the molar ratio of toluene to nitric acid is 1: 1.5-1: 3.0;

b) concentrating the reaction mixture of step a) to a water content of not more than 30% by weight;

c) the dinitrotoluene present in the reaction mixture resulting from step a) is removed completely or partly from the reaction mixture before or after the concentration step b);

d) adding a solvent to the vapor produced in step b) to maintain any dinitrotoluene in a liquid state;

e) recovering dinitrotoluene from the concentrated mixture formed in step b), and

f) the solvent and any dinitrotoluene present therein are collected.

Wherein the weight ratio of solvent added in step d) to dinitrotoluene present in the steam is from 50: 1 to 1: 10.

2. The process of claim 1 wherein the solvent and dinitrotoluene collected in step f) are separated.

3. The process of claim 2, wherein the separated solvent is recycled to the steam produced in step b).

4. The process of claim 3 wherein the separated dinitrotoluene is combined with the dinitrotoluene recovered in step e).

5. The process of claim 3 wherein the separated dinitrotoluene is recycled to the reaction mixture of step a).

6. The process of claim 1, further comprising step g): the solvent/dinitrotoluene mixture recovered in step f) is recycled to the reaction mixture of step a).

7. The process of claim 1, wherein step a) is carried out at 90-180 ℃.

8. The process of claim 1, wherein the molar ratio of toluene to nitric acid during step a) is from 1: 1.7 to 1: 2.5.

9. The process of claim 1 wherein the nitric acid used in step a) is in the form of a mixture of sulfuric and nitric acids, wherein the amount of nitric acid is from 0.5 to 15 weight percent based on the total weight of the sulfuric and nitric acid mixture.

10. The process of claim 1 wherein the solvent added in step d) is an organic compound having a boiling point at normal pressure of from 80 to 250 ℃.

11. The process of claim 1, wherein toluene and/or mononitrotoluene is used as solvent.