Industrial Nitration of Toluene to Dinitrotoluene - ACS Symposium

Chapter 21

Industrial Nitration of Toluene to Dinitrotoluene

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Requirements of a Modern Facility for the Production of Dinitrotoluene H. Hermann, J. Gebauer, and P. Konieczny Josef Meissner GmbH and Company, Bayenthalgürtel 16-20, 50968 Cologne, Germany

Technical and chemical aspects of the industrial nitration of toluene to mono- and dinitrotoluene in mixed acid are discussed. In modern mixed acid nitration plants for DNT the spent acid from the MNT-stage is purified, reconcentrated, and recycled back into the nitration process. Thus the consumption of sulfuric acid per one tonne of DNT is reduced to almost zero. Moreover, also the sulfuric-, nitric-, nitrous acid and MNT/DNT from the washing of the crude DNT and from the purification and reconcentration of the MNT spent acid are recovered and recycled back into nitration. By doing so not only the nitrate load of the waste water from a DNT nitration plant is reduced by 95 % but also the consumption figures for nitric acid are considerably improved. More than 98 % of the nitric acid needed for nitration can thus be converted to DNT.

Dinitrotoluene (DNT), a starting material for the manufacture of toluene diiso cyanate (TDI) is one of today's basic chemical commodities. The production output of about 1 million metric tonnes per year and an estimated growth rate of 4 - 8 % of TDI per year (7) turn it into the most important product which results from the nitration of toluene. Thirty years ago, things were different. Main products from the nitration of toluene were the mononitrotoluenes (MNT), as intermediates for the manufacture of dyestuff, drugs, optical brighteners, etc. (2) and trinitrotoluene (TNT). Research work in the field of toluene nitration was consequently dealing with questions as to how to nitrate toluene to yield M N T or TNT, how to optimize the manufacturing process for these products, how to minimize the meta-MNT-content and how to shift the o/p-MNT ratio in favor of the p-MNT (5). This has changed dramatically since that time. TNT has been replaced by high-performance explosives. Compared to the amount of DNT manufactured, the quantity of various

0097-6156/96/0623-0234$15.00/0 © 1996 American Chemical Society In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Industrial Nitration ofToluene to Dinitrotoluene

industrially produced MNT-isomers and TNT is relatively small, about 80,000 tonnes of MNT-isomers were produced in 1985 (2). In terms of economy, two-step nitration of toluene to DNT in mixed acid is still the most favorable procedure for manufacturing DNT.


Chemical Aspects of Nitration of Toluene to DNT in Mixed Acid Industrial nitration of toluene to DNT in mixed acid is always performed in a heterogeneous two-phase system. Three chemical parameters are important: The rate of nitration, the isomer distribution, and the amount of by-products formed (HN0 , nitrocresols, products of oxidation and decomposition). 2

Rate of Nitration. The conversion of toluene to M N T and M N T to D N T only takes place in the aqueous phase of the aqueous heterogeneous system. Two totally different types of kinetics are observed when nitrating toluene to DNT in mixed acid: The nitration of toluene at a constant acid concentration in the constant flow stirred tank reactor (CFSTR) was thoroughly examined (4-12). Within the techni cally interesting range (70 - 72 % b.w. H S 0 and 30 mole % respectively) the rate of nitration of toluene to M N T is governed by the transfer rate of toluene from the organic phase to the aqueous phase, and thus by the interfacial area between the two phases (8,9,13-15). Hence, the technical parameters like agitating speed, design of the reactor, and type of emulsion have a deciding influence on the nitration of toluene. The nitration of M N T to DNT in the aqueous heterogeneous system with mixed acid has not been examined in such detail as has been the nitration of toluene to MNT. The nitration of the M N T to DNT is performed in mixed acid at acid concentrations in the range of about 78 - 82 % b.w. sulfuric acid. Its rate is no longer controlled by mass transfer but kinetically (14-18) and strongly depends on the acid strength (19-21). In industrial nitration the excess of nitric acid in the DNT spent acid is selected in such a way that, at a given residence time and space velocity, the amount of M N T still unconverted and the amount of TNT in DNT are within product specification (Table I). 2

4

Isomer Distribution. Besides the product purity according to specification the isomer ratio of 2.4-DNT / 2.6-DNT is of major importance for TDI manufacture. It should be 4 : 1 (80/20) and is principally adjusted during nitration of toluene to M N T by the o/p-MNT-ratio in the MNT-isomer-mixture. The isomer ratio on the MNT-stage depends on the nitration temperature and the acid concentration employed. In particular the amount of meta-isomer in MNTisomer-mixtures and thus the portion of the so-called "ortho isomers" in the DNT-isomer-mixture will be reduced when nitrated at low temperatures in the MNT-stage (13,22). The o/p-MNT ratio varies little with temperature and is only slightly shifted in favor of the p-MNT (75). In mixed acid nitration the o/p-MNT ratio will be shifted in favor of the p-MNT

In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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with increasing acidity in the aqueous phase (13,16,21), or in the presence of phosphoric acid in the mixed acid (23). A significant shifting of the isomer ratio is not possible during the nitration of the M N T isomers to DNT within the technically accessible range of parameters (temperature, composition of the acid, phase ratio, type of emulsion, and agitating speed) (75). Formation of By-Products. Besides being nitrated, the toluene will be partially oxidized to nitrocresols due to the oxidative power of the mixed acid (24, 25). During the nitration of toluene to M N T from 0.3 to 1.75 % (average 0,7 %) (Table IT) nitrocresols are formed, which are mainly dinitro-p- and dinitro-o-cresol (80 % 2.6-dinitro-p-cresol). Per each mole of nitrocresol, one mole of nitric acid is consumed and converted to H N 0 With increasing acidity in the mixed acid, for instance from 70 % H S 0 to 75% H S 0 in the spent acid, the amount of cresols formed decreases by about 25 % (26). At a higher acid concentration in the DNT step (80 - 82 % H S 0 b.w.) the nitrocresols formed in the M N T step (70 - 72 % H S 0 b.w.) are further oxidatively degraded (14,15,27-29). The amount of nitrocresols will be reduced from about 0.7 % in the crude MNT to 400-1300 ppm (average 850 ppm) in the crude DNT (Table II). The nitrocresols which remain in the crude DNT are, besides trinitro-ortho- and trinitro-para-cresols, mainly trinitro-meta-cresol (44). It is surprising that dinitrocresols are almost completely oxidized in the DNT nitration mixture which has a concentration of 82 % sulfuric acid, although nitrocresols are supposed to be stable under this condition (30). The high concentration of N 0 in the crude DNT (Table II) might be responsible for the rapid oxidative degradation of the nitrocresols (25,31). The oxidation does not only yield C 0 and H 0 , moreover small organic molecules like formic acid, acetic acid, hydrocyanic acid (29), oxalic acid, etc. are obtained. Assuming that about 11-12 mole of nitric acid are consumed during oxidation of nitrocresols on the DNT stage (as in case of the oxidative degradation of 3.5-DNT (28)), then almost all of the nitrous acid formed at the DNT stage originates from the oxidative degradation of the nitrocresols formed at the MNT-stage. 2

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2

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2

2

4

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2

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Industrial Manufacture of DNT Each industrial nitration process of toluene to DNT consists of three parts: • Nitration of toluene to DNT Purification of the crude DNT • Treatment of the effluents. Nitration. When toluene is nitrated to DNT in mixed acid, besides kinetic aspects, the following parameters determine the selection, design, and operation of a nitration plant for the manufacture of DNT: 1. The quality of the raw materials like sulfuric acid and nitric acid and their concentrations. The lower the respective concentrations, the more water will be fed into the mixed acid and the larger the amount of spent acid which has to be disposed.



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2. The materials employed for all parts which come into contact with the mixed and spent acids must be corrosion resistant to sulfuric acid. Sulfuric acid of about 70 % b.w. is a limit value for most types of stainless steel. 3. The actual heat of reaction to be removed and hence the heat exchange capacities of the nitrators. 4. General safety aspects and legal safety requirements. 5. The following requirements should be met: • The quantity of spent acid obtained per each tonne of DNT should be as low as possible • Nitration should be performed in a two-phase liquid state • Nitration of toluene to M N T should be performed with fortified spent acid from the nitration of M N T to DNT. • Toluene should be completely converted to the desired product, and the residual content of M N T and TNT in the product should not exceed the values specified for the final product (Table I), so that additional purification processes are not required for removal of said substances. Table I: DNT-Isomer-Mixtures (Technical Grade) e

Standard Supplier Supplier US-Patent Germ. Appl. 4 224 249 D E 3 705 091 I II Specific. 96.65 95.33 96.06 min. 95.8 96.01 a

Purity % Σ 2.4-,2.5-,2.6-DNT Isomer Ratio 80/20 ± 1 79.9/20.1 79.8/20.2 79.7/20.3 (2.4-/2.6-DNT) 4.11 4.04 Other Isomers (tot)' 4.08 3.96 (2.3- +3.4-DNT) % max. 4.2 3.97

80.60/19.40

3.35 b

0.55 0.78 0.56 (2.5- +3.5-DNT) % max. 1.0 0.63 traces 0.05 93 purification + reconcen destillate H 0 tration waste water acidic 7

7

waste gas

Figure 2. Modern 2-Stage Nitration of Toluene to DNT with Spent Acid Extraction


21. HERMANN ET AL.


per hour (62). Before discharging the waste gas into the open air, the concentration of nitrous oxides (mainly NO) (e.g. max. 0.5 g/m N 0 ) as well as the organic contaminants like toluene, MNT, and the steam volatile DNT have to be reduced to the legally permitted limit values, e.g. in a NO -treatment unit. 3

2

x


Safety Measures. A n integral part of a modern plant for DNT manufacture is the complete safety engineering. Extensive safety measures are required to prevent thermal explosions caused by uncontrolled nitrations and other kinds of non-routine operating conditions and the uncontrolled leakage of reactants into the open air. To ensure the required safety conditions the following prerequisites must be fulfilled: The reactants - toluene, nitric acid and sulfuric acid - have to be admixed in defined order and specified proportions • The heat of reaction generated during nitration, mixing and dilution of acid must be removed efficiently A homogeneous emulsion of the type o/w or w/o must be maintained permanently. • The concentrations of the product to be nitrated (toluene or MNT) in the nitration mixture should always be kept at such a low level that, in case of circulation failure, the nitration mixture does not reach decomposition temperature or spontaneous degassing and evaporation are avoided. • Amounts of critical substances (acids, DNT) inside the plant must be kept as small as possible. This is achieved by: • Exact control of mass and energy flow in the plant • Exact supervision of the raw material feed with redundant flow measurement • Redundant temperature control • Use of conditioned cooling water and redundant cooling water control Monitoring of proper emulsion conditions in the nitrators by true watts measurement of the circulation pump or stirrer and, in case loop reactors are concerned, by means of flow control • Redundant temperature and speed control of dynamic separators Blanketing with nitrogen to minimize hazard of ignition Contained plants to minimize critical escape of spent gases or product vapors (2.6-DNT is a carcinogenic substance) Application of a gas pendulum system, e.g. for dynamic separators (centrifuges) Spent gas scrubbing with an acid suitable for nitration to prevent toluene, MNT, DNT and nitrous fumes from escaping • Numerous actions to control level and overflow A l l safety measures described above are interlocked according to a hierarchical order of safety. Exceeding the set limit values always effects an immediate disconnection of the raw material dosage. A l l these measures help to avoid uncontrolled nitration and hence prevent personal injury and harm to the environment.


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Industrial Nitration of Toluene to Dinitrotoluene - ACS Symposium

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