US3799993A

US3799993A - Process for the manufacture of trinitrotoluene

Info

Publication number: US3799993A
Application number: US20028940*7A
Authority: US
Inventors: Donald G Mc; M Hill; W Tolberg
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1972-08-25
Filing date: 1972-08-25
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26

Abstract

Process for preparing TNT comprises nitrating toluene with an anhydrous nitrating agent NO2HSO4 obtained by mixing HNO3 and oleum. The process eliminates pollution problems associated with conventional manufacturing processes and produces TNT in high yields and rapid reaction rates.

Description

United States Patent [191 Hill et al.

[ Mar. 26, 1974 [54] PROCESS FOR THE MANUFACTURE OF 2,475,095 7/1949 Hoek 260/645 TRINITROTOLUENE 2,934,571 4/1960 Bonetti 260/645 3,708,546 1/1973 Coon et al. 260/645 [75] Inventors: Marion E. Hill, Palo Alto; Wesley E.

w Atherton; Gerald FOREIGN PATENTS OR APPLICATIONS McDonald, Menlo Park, all of Calif. 752,608 7/1956 Great Britain 260/645 1,054,571 1 1967 G B 260 645 [73] Assignee: The United States of America as rem mam gr wzg g 2: of the Primary Examiner-Leland A. Sebastian g Attorney, Agent, or Firm-Edward J. Kelly; Herbert [22] Filed: Aug. 25, 1972 Berl; A. Victor Erkkila [21] Appl. No.1 283,694

[57] ABSTRACT [52 0.5. CI. 260/645, 260/688 Process for Preparing TNT comprises nitrating toluene [51] Int. Cl. C07c 79/10 with an anhydrous nitraiing agent NOZHSO4 obtained 58] Field of Search 260/645 y mixing Q and oleum- The Process eliminates pollution problems associated with conventional man- 56] R f re Ci ufacturing processes and produces TNT in high yields UNITED STATES PATENTS and rapid reaction rates.

2,297,733 10/1942 Wyler et a1. 260/645 14 Claims, 1 Drawing Figure TOLUENE HNO 80 l DNT H 2 50 l 80% TNT DINITRATOR r TRINITRATOR To r WORKUP NITRATING ACID ONT RECYCLE ACIDS SOLVENT I EXTRACTION SURPLUS H2 so ro ACID RECOVERY PMENTEDHARZS m4 3799.993

TOLUENE HNO 80 l DNI+ H2 30 80% l TNT DINITRATOR TRINITRATOR WORKUP A NITRATING ACID MT v RECYCLE ACIDS SOLVENT EXTRACTION SURPLUS H 504 TO ACID RECOVERY PROCESS FOR THE MANUFACTURE OF TRINITROTOLUENE The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon.

BACKGROUND OF THE INVENTION The invention relates to a process for producing TNT (2,4,6-trinitrotoluene). In particular the invention relates to a process for nitrating toluene to produce TNT.

TNT is conventionally manufactured by a multistep process, wherein toluene is first nitrated with a mixture of nitric acid and sulfuric acid to produce MNT (mononitrotoluene), the MNT is then nitrated to DNT (dinitrotoluene) and the DNT is finally nitrated to TNT, the acid strength and temperature being increased for each nitro group introduced. The TNT obtained contains small amounts of other TNT isomers in addition to the desired 2,4,6-isomer as well as DNT isomers, which must be removed in order to produce a TNT of sufficient purity for use as a military explosive. Such purification is usually accomplished by a selliting process, which involves treating the crude TNT with aqueous sodium bisulfite solution, which selectively extacts the undesired TNT isomers from the desired 2,4,6-isomer. The waste liquors resulting from such selliting operations, called red water," cannot be economically purified without creating a serious pollution problem. In addition, large amounts of nitrogen oxide fumes are evolved from the nitration reactors and create a serious atmospheric pollution problem, which is costly to alleviate.

SUMMARY OF THE INVENTION An object of the present invention is to provide a process for nitrating toluene to TNT in high yields and rapid reaction rates.

Another object is to provide a novel process for nitrating DNT to TNT in high yields.

A further object is to provide a process for nitrating toluene to TNT, which produces TNT of military specification grade, eliminates the polluting nitrogen oxide fumes emanated from the conventional nitration reactions and eliminates the sellite process with its attendant red water disposal problem.

A still further object is to provide a process for nitrating toluene which minimizes the formation of metal isomers, maximizes the ratio of 2,4- to 2,6-DNT isomers produced and maximizes the yield of desired 2,4,6- TNT isomer.

Another object is to provide a continuous, closed loop process for nitrating toluene to TNT.

These and other objects and advantages are achieved by the process of this invention, which is based on the use of an extremely active anhydrous nitrating mixture prepared from HNO and oleum. In accordance with the process ofthis invention, toluene is nitrated to DNT by reacting it at temperatures below about 40 C with an anhydrous mixture of I-INO and oleum, and the DNT thus produced is nitrated to TNT, without isolation from the dinitration reaction mixture, by treatment at a temperature of at least about 70 C with an anhydrous mixture of l-INO and oleum. When the trinitration reaction is complete, the reaction mixture is cooled to precipitate the TNT, which is separated, as

BRIEF DESCRIPTION OF THE DRAWING The drawing shows a schematic flow diagram of a continuous closed-loop process for manufacturing TNT from toluene according to this invention.

oEscRiRrIoN 0? THE PREFERRED EMBODIMENTS We consider that the high reactivity of the anhydrous nitrating mixture of HNO and oleum is due in part to the formation of nitronium bisulfate (NO HSO from HNO3 and S031 The stoichiometric conversion of HNO to this nitronium salt provides a high concentration of nitronium ion, which is recognized as the reactive species in most aromatic nitration reactions. The favorable solvation effect provided by anhydrous H 80 also contributes to the high reactivity of the system.

US. Pat. No. 2934571 discloses a process for producing DNT substantially instantaneously and in nearly quantitative yield by contacting toluene with a mixture of l-INO and oleum at a temperature between 50 C and 60 C. However, the process of this patent is limited to the production of DNT and other aromatic compounds containing not more than 2 nitro groups.

We have now found that TNT can be obtained in over 9Q% yield and rapid reaction rate by nitrating DNT with a mixture of HNO and oleum at a temperature of about C or higher. We have also unexpectedly found that the nitration of toluene in the aforesaid manner produces a DNT containing less than 2% of meta isomers, which is only half as much as in the current TNT process of manufacture; and that by nitrating the DNT further with a nitrating mixture of l-INO and oleum, a TNT product can be directly obtained having a purity sufficient for military use.

In carrying out the nitration of toluene to DNT with an anhydrous mixture of HNO; and oleum according to the present invention, two moles of l-INO are theoretically required to introduce two nitro groups into each mole of toluene; however, it is preferable to employ a small molar excess of I-INO e.g. about 5%, over the theoretical amount required so as to maximize the yield of DNT produced. Considerably higher molar ratios of HNOg to toluene, e.g. 3 to 8 moles of l-INO per mole of toluene, can be employed to accomplish the dinitration reaction at the low temperatures employed in the present process, but provide no technical advantage. Further, it is preferable to employ a small, e.g. about a 10% molar excess of I-INO relative to free S0 since an excess of SO; over HNO is undesirable due to the competing sulfonation reaction with toluene. The ratio of free 30;, to H 80, in the oleum employed is relatively less important and is conveniently about I part by weight per 4 parts H The nitration of toluene to DNT can be carried out at minus 40-50 C or lower; but it is preferably carried out at a temperature not exceeding about +l C and especially at between about l0 C and 20 C. At these preferred temperatures the dinitration is rapid and the production of undesired meta isomers is minimized. Reaction temperatures above about 40 C. are undesirable, since they promote the formation of undesired meta isomers.

After the dinitration reaction is complete, the reaction mixture containing the DNT and spent acid liquor can be charged with fresh HNO; and S0 as required, and the resulting mixture is heated to a temperature of about 70 C or higher, preferably between 70 and 100 C., and especially about 90 C., to effect the nitration of DNT to TNT. The amount of HNO is sufficient to provide at least one mole of ljNO per mole of DNT, as theoretically required. The amount of S0 is preferably about one mole, e.g. from 0.9 to 1.1 moles of S0 per mole of HNO The ratio of SO to H 50 employed in the trinitration reaction is not critical, since sulfonation is not a competing reaction in the nitration of DNT; it is conveniently about 0.43 to l by weight, which corresponds to oleum containing approximately 30% free S0 To ensure a rapid and complete nitration of DNT to TNT, an excess of HNO and S0 is employed, preferably from about 3 to about 5 moles each of l-lNO and 50;, per mole of DNT. The use of excess reagents in the trinitration step does not impair the economy of the total process for TNT manufacture, since the excess HNO and S0 can be employed in the dinitration step of a closed-loop process. Preferably, in a closed-loop process the amounts of HNO and S0 added for the trinitration step are such that the acid liquor, recovered by filtration of the trinitration reaction mixture to remove TNT, has approximately the composition of the HNO,,-oleum nitration mixture required for the dinitration step.

In the process of the invention additional amounts of sulfuric acid are SO by reaction of free S03 with the water formed in the nitration reactions. The production of such additional amounts of sulfuric acid results in an increase in both the volume and ratio of H50, to $0 which is undesirable in closedloop operation. To prevent such build-up of sulfuric acid in a closed-loop process, a sufficient portion of the reaction mixture obtained in the dinitration step is withdrawn and the DNT fontent is separated therefrom, e.g. by extraction with a suitable inert organic solvent, e.g. methylene dichloride, which is then separated from the DNT by distillation. The DNT thus recovered is returned to said reaction mixture for the trinitration step and the acid liquor remaining after removal of its content of DNT is disposed or utilized in suitable manner.

We have found that the reaction mass obtained in the trinitration step possesses a high capacity for retaining the unsymmetrical TNT isomers and other by-products and DNT in solution, with the result that TNT of excellent purity can be obtained directly from the cooled reaction mass by filtration followed by washing with water to remove adherent acid. The TNT crystals recovered from the reaction mass can be further purified by washing with sulfuric acid or nitric acid of a concentration at least about 50%, or mixtures of such acids.

The following example specifically illustrates the process of this invention.

EXAMPLE Part A. Preparation of Dinitrotoluene (DNT) A nitrating mixture of 25.1 g (398 mmoles) of absolute nitric acid and 143.9 g of 20% oleum consisting of 29.6 g (370 mmoles) free 80;, and I 14.3 g H 80, was placed in a two-piece glass reactor provided with a thermometer and a glass agitator shaft with a Teflon blade. The bottom part of the reactor was a 100 ml flask, which was joined to the top by a ground glass flange and was fitted with an internal glass coil for heating the contents with steam or cooling with water; and the top part was fitted with a thermometer port and three other ground glass joints. l 7.3 g (188 mmoles) of toluene (refined grade) were added dropwise during 100 minutes to the agitated nitrating mixture which was maintained at about -8 to -l0 C., after which the reaction mixture was agitated for '15 minutes at -8 to lO C. and then allowed to warm to room temperature.

A sample (18.2g) of the solution thus obtained was removed, poured on ice and the DNT was separated. A yield of 99.4% (based on toluene) of DNT having the following composition (by glc analysis) was obtained:

2.6- DNT 15.6% 2,3- and 2,5- DNT 0.6% 2,4- DNT 82.6% 3,4- DNT 1.2%

Thetotal meta isomers in the DNT thus obtained was 1.8%. Part B. Preparation of TNT The solution remaining in the reactor after removal of the 18.2g sample in part A was fortified by adding 60.1 g. mmoles) of (a stabilized SO marketed under the name Sulfan by Allied Chemical Corp.) and 46.1 g. (73l mmoles) of absolute HNO in that order with ice bath cooling from dropping funnels equipped with Teflon stopcocks. The total amounts of S0 and HNO in the resulting solution were 75] mmoles each, allowing for the HNO remaining after the dinitration step and also the amount removed in the 18.2 g. aliquot; and the amount of DNT remaining in the reactor was 171 mmoles. Trinitration was effected by heating the resulting solution at 70 C. for 30 minutes, then at 80 C. for 15 minutes and finally at C. for 15 minutes. This was accomplished by placing an oil bath preheated to 70 C. around the flask and introducing steam through the reactor coil to heat the solution to 70 C within 1 minute, and maintaining the temperature within i 2 C. by intermittent flow of a small amount of cooling water. The temperature was raised to 80 C. in 40 seconds and finally from 80 to 90 C. in 45 seconds. The hot reaction mixture was poured into an Erlenmeyer flask and'cooled to 0 C. and the TNT crystals thus obtained were separated by filtration on a coarse, sintered glass funnel. The mother liquor was chilled to l0 C. and filtered to obtain a second crop of TNT crystals on a separate filter. The crystal crops were combined, washed with water until acid free and dried. The dried crystals had a setting point of 802 C. and weighed 36 grams, corresponding to a yield of 94% of theory based on the toluene starting material. (A product of somewhat higher setting point was obtained by washing the filter cake with 70% H 80 prior to washing with water as described below).

About 5% of the DNT was oxidized and the remaining 95% was converted to TNT. About 1% of the TNT was subsequently oxidized, resulting in a net yield of about 94%. The products of the DNT oxidation were CO CO, N and H 0 while the TNT was converted, at least in part, to several by-products, mainly 2,4,6- trinitrobenzoic acid.

RECYCLE CLOSED-LOOP OPERATION The acid filtrate, obtained by removing the TNT crystals from the reactor mass in Part B above had very nearly the desired composition for the dinitration reaction, as described in part A above, and its composition was adjusted, as required, by addition of a small amount of l-lNO orO to provide the aforesaid rago of 398 mmoles HNOQW mmoles 8?} per 188 mmoles toluene as employed in part A above. The cycle was then repeated by adding 17.3 g. (188 mmoles) of toluene and conducting the dinitration and trinitration reactions as described in part A and part B above, respectively. For fortification of the reaction mass resulting from the dinitration reaction, the amount of DNT formed, the amount of HNO remaining and the amount of H SO produced were calculated on the assumption of a l 00 yield of ljNT according to the equation:

and adding the necessary amounts of S0 and HNO to adjust the acids for trinitration as described in part 8.

Nine cycles were carried out in the foregoing manner. The composition of the acid phases used for the dinitration and trinitration reactions in three of the cycles is shown in the following table:

ACID PHASE COMPOSITION Dinitration Trinitration The dinitration reaction in the first cycle was carried out with fresh acids so that neither nitrosyl sulfuric acid (NOHSOJ nor dissolved organics were present. NOHSO, was formed during each of the trinitration reactions due to the competing oxidation reaction. However, the buildup of NOHSO, was compensated by removing a portion of the filtrate after each trinitration reaction for analysis.

The NOHSO, concentration was lower in the trinitration step than in the dinitration step due to dilution by the S0 and HNO added to fortify the acid phase. The dissolved organics present in the acid phase at the start of the dinitration reaction were the impurities and TNT that had not crystallized out following the previous trinitration reaction; these materials plus DNT constitutecl the organics dissolved in the acid phase at the start of the trinitration reaction. The columns headed by give the relative weight percentage composition of the acids exclusive of the dissolved organics.

The average yield of TNT baed on toluene was 93% of theory for the nine cycles. This figure included the TNT that crystallized out of the acid phase, together with the impure TNT that remained in solution, (2,4,6- TNT was the major component of the organic materials in the acid phase after crystallization of the TNT; the total amount of dissolved organics was regarded simply as impure TNT). The method of operation did not allow determination of a value for the yield of crystallized TNT due to loss of a portion of the dissolved TNT in the by-product acid. The quality of the TNT obtained in the nine cycles of operation was consistently good, indicating that the acid phase possessed a high capacity for retaining the unsymmetrical TNT isomers and other by-products.

PURIFYING TNT The wet filter cake of TNT crystals obtained by separating the crystals from the nitration mixture in the last cycle, was slurried in about an equal weight of H (corresponding to about 1.5 g. of 70% H SO per gram of TNT solids) at room temperature. The slurry was filtered and the filter cake was washed acidfree with water and dried. The purified TNT crystals thus obtained had a setting point of 803 C., which is satisfactory for type I military explosive-grade TNT. The foregoing demonstrates that the novel process is capable of producing TNT of acceptable military explosive grade without the use of the conventional sellite purification process.

The present process can be readily adapted for continuous manufacture of TNT. The drawing shows a schematic flow diagram of a continuous closed-loop process for production of TNT from toluene according to the process of the present invention. Metered streams of toluene and fresh or recycled nitrating acid, in the ratio of 2.1 moles of HNO 2 moles ofSO and about 3-4 moles of H SO per mole of toluene, are fed continuously to the dinitrator which is operated at about l0 C. Fresh nitrating acid is used for startup; thereafter recycled nitrating acid from the trinitrator is employed (see below). The dinitrator can be a stainless steel tank provided with an agitator, cooling coils, bottom feed ports and top outlet for effluent reaction mixture. The average residence time of the reactants in the dinitrator is about 30 minutes, which as noted above is substantially longer than the time required to complete the nitration of toluene to DNT under these conditions. The effluent from the nitrator contaiing the DNT in H SO and little if any l-lNO or NO HSO is divided such that, for example, 80% thereof flows directly to the trinitrator and 20% flows to a solvent extraction unit, such as a Scheibel extractor, wherein the DNT is continuously extracted from the acid liquor with a suitable solvent, e.g. methylene chloride. The surplus acid from the extractor is pumped to storage for recovery or sale and the DNT extract flows to a still (not shown), where the solvent is separated by distillation, and the DNT thus obtained is fed to the trinitrator. Additional l-lNO and S0 are continuously fed into the trinitrator along with the eftluent from the dinitrator so as to provideappr oximately 3 moles of NO2HSO4 (3 moles each of l-lNO and S0 per mole of DNT. The trinitrator, which can be a vessel similar to the dinitrator but provided with heating coils, is operated at about C.

with an average residence time of reactants of about 1 hour, which is sufficient to complete the conversion of DNT to TNT under these conditions. Instead ofa single trinitration vessel, the reaction mixture can be fed, for example, through a series of three trinitrators, wherein the first is maintained at 70 C., the second at 80 C. and the third at 90 C. and the residence time in each is suitably adjusted, as illustrated in the foregoing example. The reaction mixture from the trinitrator flows to a workup installation, where it is cooled to about lO C. to crystallize the TNT, and the TNT crystals are filtered off and purified, if necessary, by suitable means, e.g. washing with 70% H 80 then washed with water and dried. The mother liquors separated from the crystallized TNT are of essentially the required concentration to convert the toluene feed to DNT and are recycled to the dinitrator, together with additional HNO and S if required.

It is thus evident that the process of this invention provides an important advance in the manufacture of TNT, including the following specific advantages:

1. It provides rapid nitration reaction rates and high yields of TNT.

2. It produces TNT of military specification grade directly by crystallization from the trinitration reaction mixture followed by washing with water. If necessary, the TNT can be further purified by washing with conc. H 80 3. It obviates the sellite process currently used to purify crude TNT and eliminates the red water disposal problem associated therewith, which is a major pollution problem that plagues the current method for TNT manufacture.

4. It does not produce N0 fumes as by-products because both NO HSO and NOT- S0 are thermally stable in the anhydrous HNO oleum system used in both the diand trinitration reactions, thereby eliminating the atmospheric pollution due to N0 fumes evolved in the current process for TNT.

We wish it to be understood that we do not desire to be limited to the exact method and detail of construction described for obvious modification wll occur to persons skilled in the art.

What is claimed is:

l. A process for producing TNT which comprises contacting toluene with an anhydrous nitrating mixture of HNO and oleum at a temperature below 40 C to produce DNT, heating the DNT in situ at a temperature of at least about 70 C with an anhydrous nitrating mixture of HNO and oleum to produce TNT, and recovering the TNT from the reaction mixture, wherein the amount of HNO is at least 2 moles per mole of toluene and the amount of free S0 in the oleum does not exceed 1 mole per mole of HNO in the nitration of toluene to DNT, and the amount of HNO is between 1 and 5 moles per mole of DNT in the nitration of DNT to TNT.

2. The process according to claim ll, wherein the toluene is nitrated at a temperature between about 20 C. and C. and the DNT is nitrated at a temperature between about 70 C. and 100 C.

3. The process according to claim 1, wherein the amount of HNO is between about 3 and 5 moles per mole of DNT and the amount of free SO;, in the oleum is about 1 mole per mole of HNO in the nitration of DNT.

4. The process according to claim 3, wherein the amount of HNO; is between about 2.l and 2.2 moles per mole of toluene.

S. The process according to claim 1, wherein the reaction mixture from which the TNT is recovered is recycled for use in the nitration of toluene.

6. The process according to claim 5, wherein a portion of the reaction mixture obtained by nitrating toluene to DNT, is contacted with an inert, immiscible liquid organic solvent for DNT, the resulting solvent solution of DNT is separated from the acid liquor, and the DNT is recovered from the solvent solution and submitted along with the remainder of the reaction mixture to the nitration of DNT to TNT.

7. The process according to claim 6, wherein the organic solvent is methylene chloride.

8. The process according to claim 1, wherein the TNT separated from the reaction mixture is washed with an acid of the group consisting of 50-100% H 50, and 50-100% HNO and mixtures thereof.

9. A continuous process for producing TNT, which comprises feeding into a first reaction vessel toluene and an anhydrous nitrating mixture of HNO and oleum wherein the amount of l-INO is at least 2 moles per mole of toluene and the amount of free 50;, in the oleum does not exceed 1 mole per mole of HNO maintaining the reactants in said vessel at a temperature below 40 C. to product DNT, passing the reaction mixture from said first reaction vessel into a second reaction vessel together with sufficient anhydrous nitrating mixture of HNO and oleum to provide a ratio of between 1 and 5 moles of HNO per mole of DNT, maintaining said mixture in the second vessel at a temperature of at least about C. to product TNT, separating the TNT from the reaction mixture, and recycling the recovered reaction mixture to the first reaction vessel.

10. The process according to claim 9, wherein a portion of the reaction mixture from the first reaction vessel is contacted with an inert, immiscible liquid organic solvent for the DNT, the resulting solvent solution of DNT is separated from said reaction mixture, and the DNT is recovered from said solvent solution and introduced along with the remainder of the reaction mixture to the second reaction vessel.

11. The process according to claim 10, wherein the organic solvent is methylene chloride.

12. The process according to claim 10, wherein the amount of HNO is between about 2.1 and 2.2 moles per mole of toluene and the amount of free 50;, in the oleum is less than 1 mole per mole of HNO in the first reaction vessel, and the amount of HNO is between about 3 and 5 moles per mole of DNT and the amount of free in the oleum is about 1 mole per mole of HNO in the second reaction vessel.

13. The process according to claim 12, wherein the oleum in the first reaction vessel contains about 20% by weight of free $0 and the oleum in the second reaction vessel contains about 30% by weight of free $0 14. The process according to claim 12, wherein the first reaction vessel is operated at a temperature between about 20 C. and +10 C. and the second reaction vessel is operated at a temperature between about 70 C. and C.

Claims

2. The process according to claim 1, wherein the toluene is nitrated at a temperature between about -20* C. and +10* C. and the DNT is nitrated at a temperature between about 70* C. and 100* C.
3. The process according to claim 1, wherein the amount of HNO3 is between about 3 and 5 moles per mole of DNT and the amount of free SO3 in the oleum is about 1 mole per mole of HNO3 in the nitration of DNT.
4. The process according to claim 3, wherein the amount of HNO3 is between about 2.1 and 2.2 moles per mole of toluene.
5. The process according to claim 1, wherein the reaction mixture from which the TNT is recovered is recycled for use in the nitration of toluene.
6. The process according to claim 5, wherein a portion of the reaction mixture obtained by nitrating toluene to DNT, is contacted with an inert, immiscible liquid organic solvent for DNT, the resulting solvent solution of DNT is separated from the acid liquor, and the DNT is recovered from the solvent solution and submitted along with the remainder of the reaction mixture to the nitration of DNT to TNT.
7. The process according to claim 6, wherein the organic solvent is methylene chloride.
8. The process according to claim 1, wherein the TNT separated from the reaction mixture is washed with an acid of the group consisting of 50-100% H2SO4 and 50-100% HNO3 and mixtures thereof.
9. A continuous process for producing TNT, which comprises feeding into a first reaction vessel toluene and an anhydrous nitrating mixture of HNO3 and oLeum wherein the amount of HNO3 is at least 2 moles per mole of toluene and the amount of free SO3 in the oleum does not exceed 1 mole per mole of HNO3, maintaining the reactants in said vessel at a temperature below 40* C. to product DNT, passing the reaction mixture from said first reaction vessel into a second reaction vessel together with sufficient anhydrous nitrating mixture of HNO3 and oleum to provide a ratio of between 1 and 5 moles of HNO3 per mole of DNT, maintaining said mixture in the second vessel at a temperature of at least about 70* C. to product TNT, separating the TNT from the reaction mixture, and recycling the recovered reaction mixture to the first reaction vessel.
10. The process according to claim 9, wherein a portion of the reaction mixture from the first reaction vessel is contacted with an inert, immiscible liquid organic solvent for the DNT, the resulting solvent solution of DNT is separated from said reaction mixture, and the DNT is recovered from said solvent solution and introduced along with the remainder of the reaction mixture to the second reaction vessel.
11. The process according to claim 10, wherein the organic solvent is methylene chloride.
12. The process according to claim 10, wherein the amount of HNO3 is between about 2.1 and 2.2 moles per mole of toluene and the amount of free SO3 in the oleum is less than 1 mole per mole of HNO3 in the first reaction vessel, and the amount of HNO3 is between about 3 and 5 moles per mole of DNT and the amount of free SO3 in the oleum is about 1 mole per mole of HNO3 in the second reaction vessel.
13. The process according to claim 12, wherein the oleum in the first reaction vessel contains about 20% by weight of free SO3 and the oleum in the second reaction vessel contains about 30% by weight of free SO3.
14. The process according to claim 12, wherein the first reaction vessel is operated at a temperature between about -20* C. and +10* C. and the second reaction vessel is operated at a temperature between about 70* C. and 100* C.