Acid temperature control mixing heat

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. 

This reaction can be carried out both in concentrated aqueous solution and, preferably continuously, in the molten state. In the latter case, sodium dichromate is mixed with sulfuric acid in a twin screw and the mixture fed into an externally heated rotary tube furnace. The water first evaporates, then the. sodium hydrogen sulfate melts (at I70°C) and finally the chromium(VI) oxide is formed (I98°C). Temperature control is critical, since chromium(Vl) oxide decomposes at temperatures slightly above this temperature. The reacted mixture then separates in a settling tank. Chromium(Vl) oxide is taken off from below and liquid sodium hydrogen sulfate is taken off from above, both being converted into solid material by cooling rollers. 

Derivation (1) By heating phenol with dilute sulfuric acid, cooling the product, and then nitrating, keeping the temperature approximately 50C (2) by nitration with mixed acid with careful temperature control. 

The commercial dinitrophenol mixture is produced by heating phenol with dilute sulfuric acid, cooling the product, and then nitrating while keeping the temperature below 50 °C, or by nitrating with a mixed acid under careful temperature control (Sax and Lewis 1987). 2,3-, 2,5-, and 3,4-DNP are prepared by nitration of m-nitrophenol. 3,5-DNP is prepared by the replacement of one nitro group by methoxyl in 1,3,5-trinitrobenzene and demethylation of the dinitroanisole by anhydrous aluminum chloride. 2,6-DNP is prepared by sulfonation and nitration of o-nitrophenol (Harvey 1959). 2,6-DNP is also produced as a byproduct in the synthesis of 2,4-DNP by way of 2,4-dinitrochlorobenzene. 

Calcium carbonate is usually added during the batch fermentation to control the pH, and the highest concentration of lactic acid (12-15 %) is limited by the precipitation of calcium lactate from the broth. Temperature control is maintained with heat transfer coils using circulating water, and the cells are retained in a mixed suspension by mechanical agitators or... 

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

Acetylene mixed with acetic acid is stable at 70°C, but an accidental temperature rise in a cylinder containing a solution of acetyiene in acetic acid resuited in a temperature rise reaching 185°C on a specific point of the cylinder walls. There was an extreme heating of the cylinder, which could not be controlled by severe external cooling, and caused the cylinder to detonate. 

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