Nitration reaction vessels

Explosion in a Nitration Reaction Vessel (February 1994). Loss Prevention Bulletin, 115, 1-5. 

The reaction vessel (nitrator) is constructed of cast iron, mild carbon steel, stainless steel, or glass-lined steel depending on the reaction environment. It is designed to maintain the required operating temperature with heat-removal capabiUty to cope with this strongly exothermic and potentially ha2ardous reaction. Secondary problems are the containment of nitric oxide fumes and disposal or reuse of the dilute spent acid. Examples of important intermediates resulting from nitration are summarized in Table 3. 

In a typical process 12001b (545 kg) of the mixed acids are run into the reaction vessel and 301b (13.5 kg) of the dried cotton linters are added. The mixture is agitated by a pair of contra-rotating stirrers and nitration is allowed to proceed at about 35-40°C for 20 minutes. It is interesting to note that the cellulosic material retains its fibrous form throughout the nitration process. 

On the completion of nitration the batch is dropped from the reaction vessel into a centrifuge and the acid mixture spun off and recovered. The nitrated linters, which still contain appreciable quantities of acid, are then plunged into a drowning tank, where the nitric acid is diluted with a large volume of water. The resultant ester is then pumped, as a slurry, into storage vats which may hold the products of several nitrations. 

Other methods of nitration that Laali investigated were with isoamyl nitrate in combination with a Bronsted or Lewis acid in several ionic liquids, with [EMIM][OTf] giving the best yields (69 %, 1.0 1.0 o p ratio). In the ionic liquid [HNEt( Pr)2] [CE3CO2] (m.p. = 92-93 °C), toluene was nitrated with a mixture of [NH4][N03] and trifluoroacetic acid (TEAH) (Scheme 5.1-37). This gave ammonium trifluoroacetate [NH4][TEA] as a by-product, which could be removed from the reaction vessel by distillation (sublimation). 

A simple jacketed pan or kettle is very commonly used in the processing industries as a reaction vessel. In many cases, such as in nitration or sulphonation reactions, heat has to be removed or added to the mixture in order either to control the rate of reaction or to bring it to completion. The addition or removal of heat is conveniently arranged by passing steam or water through a jacket fitted to the outside of the vessel or through a helical coil fitted inside the vessel. In either case some form of agitator is used to obtain even distribution in the vessel. This may be of the anchor type for very thick pastes or a propeller or turbine if the contents are not too viscous. 

Commercially, lead azide is usually manufactured by precipitation in the presence of dextrine, which considerably modifies the crystalline nature of the product. The procedure adopted is to add a solution of dextrine to the reaction vessel, often with a proportion of the lead nitrate or lead acetate required in the reaction. The bulk solutions of lead nitrate and of sodium azide are, for safety reasons, usually in vessels on the opposite sides of a blast barrier. They are run into the reaction vessel at a controlled rate, the whole process being conducted remotely under conditions of safety for the operator. When precipitation is complete, the stirring is stopped and the precipitate allowed to settle the mother liquor is then decanted. The precipitate is washed several times with water until pure. The product contains about 95% lead azide and consists of rounded granules composed of small lead azide crystals it is as safe as most initiating explosives and can readily be handled with due care. 

During air-pressurised discharge of a hot 53% aqueous solution of the nitrate salt from a reaction vessel via a filter press, a violent explosion occurred. The nitrate salt begins to decompose below 100°C, and at the likely internal temperature of 142°C, decomposition would be expected to be very rapid, involving much gas/vapour generation according to the equation below. 

The fluoro compound is resistant to nitration and an operating temperature of 90°C is necessary to ensure formation of the 5-nitro derivative. Under these conditions, the atmosphere (containing the fluoro compound, its nitro derivative and nitric acid vapours) in the nitration vessel is explosive and above the flash point. An unknown ignition source led to an explosion and rupture of the 3 cu. m vessel, and a maximum explosion pressure of 50 bar was confirmed experimentally. Such explosive atmospheres are not found in low temperature nitration reactions. 

The reaction is complete when the clear supernatant solution in the reaction vessel no longer contains chlorine as detected by the addition of dilute nitric acid and silver nitrate solution to ca. 1-ml. of the clear solution withdrawn from the reaction flask. After the reaction is complete, the ice bath is removed, and stirring is continued for 2 hours at room temperature. After standing overnight, the dimethylammonium chloride is filtered and washed with n-hexane, with careful exclusion of the moisture of the atmosphere. The combined filtrates are distilled at atmospheric pressure to remove the solvent. The tris(dimethylamino)arsine distills at 36°/2 mm. (55 to 57°/10 mm.). The yield is 402 g. (78%). 

Use of medium-scale heat flow calorimeter for separate measurement of reaction heat removed via reaction vessel walls and via reflux condenser system, under fully realistic processing conditions, with data processing of the results is reported [2], More details are given elsewhere [3], A new computer controlled reaction calorimeter is described which has been developed for the laboratory study of all process aspects on 0.5-2 1 scale. It provides precise data on reaction kinetics, thermochemistry, and heat transfer. Its features are exemplified by a study of the (exothermic) nitration of benzaldehyde [4], A more recent review of reaction safety calorimetry gives some comment on possibly deceptive results. [5],... 

Some work [5] has been performed on the photochemical reaction between sulfur dioxide and hydrocarbons, both paraffins and olefins. In all cases, mists were found, and these mists settled out in the reaction vessels as oils with the characteristics of sulfuric acids. Because of the small amounts of materials formed, great problems arise in elucidating particular steps. When NO and 02 are added to this system, the situation is most complex. Bulfalini [3] sums up the status in this way The aerosol formed from mixtures of the lower hydrocarbons with NO and S02 is predominantly sulfuric acid, whereas the higher olefin hydrocarbons appear to produce carbonaceous aerosols also, possibly organic acids, sulfonic or sulfuric acids, nitrate-esters, etc. ... 

The order and timing of the addition of reagents in the KA-process is varied but in a typical procedure three reagents, namely, acetic anhydride, a solution of ammonium nitrate in nitric acid, and solid hexamine dinitrate, are added slowly, in small portions and in parallel, into the reaction vessel which is preheated to 60-80 °C. On completion the reaction mixture is often cooled to 50-60 °C and the RDX filtered and sometimes washed with acetic acid. This process produces a product which melts over a 2 °C range but the RDX still contains up to 10 % HMX as a by-product. Dilution of the reaction mixture with water before removing the RDX produces a very impure product containing numerous unstable linear nitramine-nitrates. Based on the assumption that one mole of hexamine dinitrate produces two mole of RDX the KA-process commonly yields 75-80 % of RDX. 

Mercuric-5-nitrotetrazole [Structure (2.13)] was prepared according to the methods reported by Gilligan et al. [14] and Redman and Spear [15]. Thus, 5-aminotetrazole was treated with sodium nitrite and copper sulfate to obtain Cu(NT)2HNT-4H20 (where NT nitrotetrazole). The copper salt was subsequently converted to the ethylene diamine complex MNT was then obtained by treating the complex with mercuric nitrate in HN03 medium. The precursors and final product were air dried. The synthesis of these compounds is carried out in a fume hood behind a protective polycarbonate shield in a stainless steel reaction vessel. 

Nitration reactions are carried out in closed vessels that are provided with an agitating mechanism and means for controlling the reaction temperature. The nitration vessels are usually constructed of cast iron and steel, but often acid-resistant alloys, particularly chrome-nickel steel alloys, are used. 

Considerable heat evolution accompanies the nitration reaction, oxidation increases it, and the heat of dilution of the sulfuric acid increases it still further. Increased temperature favors dinitration arid oxidation, so the reaction must be cooled to keep it under control. Good heat transfer can be assured by the use of jackets, coils, and good agitation in the nitration vessel. Nitration vessels are usually made of stainless steel, although cast iron stands up well against mixed acid. 

---