Fume-off

The factors affecting the performance of a local exhaust system are well known. For fume control, an added factor is the effect of heat release or buoyancy. Important design parameters are process heat release and the size and geometry of air-supply openings and their location relative to major surfaces of the enclosure, lire kxation of the fume off-take is usually only of secondary importance. 

Charge-transfer complexes with pyrimidine and purine bases as well as with solvents like hexa-methylphosphoramide and dimethyl sulfoxide are reported in Ref 66. The action of aromatic amines (primary, secondary, or tertiary) resulted in fume-offs or unidentifiable tars, in all cases purple or red colors developed prior to more violent reactions (Ref 66)... 

Other preparations and isolations. If damp methylenedi(nitroformamide) is allowed to stand for several days, the odor of formic acid is noticed, and MEDINA can be isolated from the residue (Ref 11, p 14). The details of scale-up to 150 lb batches, including exp details and flow sheets, and further scale-up with the aim of prodn of 1000 lbs are given. The report describes a fume-off and fire which occurred during the S3rd run. The cause was attributed to a stuck valve which allowed nitric acid to build up in the reactor (Ref 13, p 57). In Ref 16, p 73 there are cost analysis data for pilot plant and large scale prodn, flow sheet for a proposed coml plant, and material balances. The action of acet anhydr on N,Nf-bis(hydroxy-methyl)MEDlNA regenerates MEDINA (Ref 6) the diNa salt of N. N trinitrotrimethylene-diamine, on warming with me ale, ppts the Na salt of MEDINA... 

Interaction of nitromethane and formaldehyde in presence of alkali gives not only 2-nitroethanol, but also di- and tri-condensation products. After removal of the 2-nitroethanol by vacuum distillation, the residue must be cooled before admitting air into the system to prevent a flash explosion or violent fume-off. 

Vacuum distillation must be effected at below 80°C to avoid fume-offs, particularly if air be admitted to the warm residue. The lachrymatory material also polymerises in contact with alkalies. 

A fume-off during distillation of this compound (bp., 63-65°C/2 mbar) illustrates the inherent instabilty of this type of compound. 

The reaction conditions necessary to obtain a good yield of the title compound (a difficult isomer), and to avoid hazards during the nitration of resorcinol, are critical and strict adherence to those specified is essential. The necessary 80% white fuming nitric acid must be completely free from oxides of nitrogen and nitrous acid, and procedures for this are detailed. Then the temperature dining addition of the diacetate must be kept between -10 and 0°C by regulating the rate of addition. The alternative use of 80% sulfuric acid as solvent for the 80% nitric acid (5 equiv.) is preferred as more reliable, but both methods have led to violent exothermic decomposition, accompanied by fume-off, after an induction period. In any event, the explosive 2,4,6-trinitroresorcinol ( styphnic acid ) is produced as a by-product. 

Aromatic amines, Sulfuric acid Nielsen, A. T. etal., J. Org. Chem., 1980, 45, 2341-2347 The acid, prepared from 90-98% hydrogen peroxide and oleum or 100% sulfuric acid, is one of the most powerful known oxidants and its use for oxidising aromatic amines to nitro compounds has been studied. Some mono- di- and tri-amines are destroyed exothermically with violent fume-off. Precautions for use are detailed. 

The pot temperature should not be allowed to rise above 70° (the submitters used a hot-water bath at 75°), as a fume-off which may proceed with explosive violence is likely to occur. A nitrogen bubbler may be used to eliminate bumping. The distillation should be carried out behind a safety shield. 

Commercial 70 % nitric acid can be used for the 6>-nitration of low molecular weight alcohols like ethanol and 2-propanol. The nitrate ester products are isolated from the cautious distillation of a mixture of the alcohol and excess 70 % nitric acid. The presence of urea in these reactions is very important for the destruction of nitrous acid and its omission can lead to very violent fume-off. However, this method is not recommended on safety grounds. Using temperatures above ambient for the O-nitration of alcohols, with either nitric acid or mixed acid, is dangerous and greatly increases the risk of explosion. 

An unusual feature of the KA-process is that the reaction is conducted at 60-80 °C. Solutions of nitric acid in acetic anhydride are known to be prone to dangerous fume off at temperatures above ambient. However, a saturated solution of ammonium nitrate in fuming nitric acid can be added to warmed acetic anhydride without such danger. In fact, these reactions are commonly conducted at 60-80 °C as a matter of safety by preventing a build-up of unreacted starting material. The hexamine used in these reactions is in the form of the dinitrate salt, which is formed as a crystalline salt on addition of a saturated aqueous solution of hexamine to concentrated nitric acid below 15 °C. The use of hexamine dinitrate in this process reduces the amount of nitric acid needed for the nitrolysis. 

In 1902 Rutherford and Soddy added ammonium hydroxide to a thorium solution, filtered off the thorium hydroxide precipitate, and found that, after they evaporated the thorium-free filtrate to dryness and fumed off the ammonium salts, the residue was much more active than the original thorium salt (18). This observation led them to the discovery of a new member of the thorium series, which they called thorium X. 

This stopper also contains a thermometer. The bulb of the tube and that of the thermometer are so arranged that they are on the same level. After the liquid is preheated to about 100°, the bulb with the sample is introduced and heating is continued in such a manner that the rise of temperature is uniform, say 5° to 20° per minute. The behavior of the sample is observed during the entire period of heating and the temperatures are noted at which the sample begins to fume off, when the flash appears and finally when the tube fills with black smoke... 

Fermentol (also Pratol Glycerine or Pro-toglycerine), A glycerine produced in Germany from sugar by a fermentation process. Trimethylene glycol in the product evolves more heat than glycerine on nitration, is more soluble in the mixed acid, and thus contributes to fume-offs Refs 1) Naoura, NG(1928), pp 31, 229 2) Marshall 3(1932), p 42... 

Fume-Off. The oxidative destruction of byproducts in nitration reactions Ref Coll, "Summary Report of Div 8 NDRC", Vol 1(1946), p 153... 

Nitroethanol prepared by the formaldehyde-nitromethane method should not be distilled without use of diphenyl ether as a heat-dispersing agent. The residue, consisting of di- and tricondensation products of formaldehyde with nitromethane, when hot and concentrated, and particularly when the vacuum is broken and air is let in on the hot distillation residue, is very likely to undergo a flash detonation, or at least a fume-off which may proceed with explosive violence. Use of diphenyl ether is a wise safety precaution in the distillation of 2-nitroethanol made by other methods as well. 

Carbon disulphide was used as an extraction solvent when analysing epoxy resins. On one occasion, adding to a hardener produced a vigorous fume-off leaving a residue looking like sulfur [1]. Amines and complexes thereof are used as hardeners, and the reaction with, especially, polyamines to give dithiocarbamates is surprisingly exothermic [2]. 

This and the isomeric nitroester (below), both produced in the nitroacetoxylation of isoprene, are of limited thermal stability and it is recommended that neither be heated above 100°C, either neat or in solution. Vacuum distillation of the nitro esters should be limited to 1 g portions, as decomposition fume-offs have been observed. 

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