Nitrification refers toOrganic compoundThe reaction of introducing nitro into the molecule.Nitro can be linked with carbon atoms in organic compounds to formNitro compound；It can also be connected with oxygen atoms to form nitroester compounds;It can also be connected with nitrogen atoms to form nitramine, etc.[1]
Nitrification is a kind ofChemical unit processIt is the process of introducing nitro into organic compound molecules. Nitro is the univalent NO2 formed by nitric acid losing a hydroxyl group.Aromatic compoundThe reaction mechanism of nitrification is: OH base coating of nitric acidProtonation, and then one of the water is removed by the dehydrating agent to form an intermediate of NO2+, and finally aromaticElectrophilic substitution reactionAnd remove one molecule of hydrogen ion.In this kind ofNitrificationinAromatic ringOfelectron densityIt will determine nitrificationreaction rateWhen the electron density of aromatic ring is higher, the reaction rate is faster.Since the nitro group itself is an electron withdrawing group, after the first nitration, the secondary nitration reaction is often inhibited due to the decrease of the electron density of the aromatic ring.It must be carried out under more intense reaction conditions (such as high temperature) or stronger nitrating agent.
The nitrification process is carried out in the liquid phase, usually usingKettle reactor。Enamel kettle, steel kettle, cast iron kettle or stainless steel kettle can be used according to different nitrating agents and media.useMixed acidTo remove as soon as possible during nitrificationreaction heatIn order to maintain a suitable reaction temperature, in addition to jacket cooling, a cooling coil is also installed in the kettle.Most nitrification processes with small production use intermittent operation.Nitrification process with large output can be operated continuously, and kettle type continuous operation is adoptedNitrificationMultiple sets of continuous nitrification reactors in series are used to complete the nitrification reaction.The advantages of the annular continuous nitrification reactor are large heat transfer area, good mixing, large production capacity, and less by-products of nitro compounds and nitrophenols.
Nitrification method
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Nitrification methods mainly include the following:
（1）Dilute nitric acidNitrification is generally used forAromatic compoundThe nitration reaction is carried out in stainless steel or enamel equipment, and the excess nitric acid is about 10-65%.
（2）Concentrated nitric acidNitrification This kind of nitrification often uses too much nitric acid, and the excessive nitric acid must be utilized or recovered, so its practical application is limited.
(3) Homogeneous nitrification in concentrated sulfuric acid medium When the nitrated matter or nitrification product is solid at reaction temperature, the nitrated matter is often dissolved in a large amount of concentrated sulfuric acid, and then the mixture of sulfuric acid and nitric acid is added for nitrification.This method only requires a small amount of nitric acid in excess. Generally, the yield is high. However, the consumption of sulfuric acid is high.[3]
Heterogeneous nitrification
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Heterogeneous mixed acid nitrification When the nitrates or nitrification products are all liquid at the reaction temperature, the method of heterogeneous mixed acid nitrification is often used to makeOrganic phaseIt is dispersed into acid phase to completeNitrification。
The advantage of nitrification in organic solvents is that different solvents can often change theNitroThe proportion of isomeric products, avoid using a large amount of sulfuric acid as solvent, and use nitric acid close to the theoretical amount.Common organic solvents include acetic acidacetic anhydride、DichloroethaneEtc.
Common nitrating agents include nitric acid, mixture of nitric acid and sulfuric acid (i.eMixed acid）, mixture of nitric acid and acetic anhydride, etc.According to the nature of the nitrates and the different nitrating agents used, the main nitration methods are: dilute nitric acid nitration, concentrated nitric acid nitration, nitric acid nitration in concentrated sulfuric acid, nitric acid nitration in organic solvents and heterogeneous mixed acid nitration.Mixed acid nitration is mainly used for nitration of benzene, toluene and chlorobenzene.Mixed acid nitrification is the most important nitrification process because of the large demand for mixed acid nitrification products.
Process characteristics
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Nitrification requires maintaining appropriate reaction temperature to avoid formation of multi nitro compounds, oxidation, etcside reaction。Nitrification isExothermic reaction, and the reaction rate is fast, and poor control will cause explosion.In order to maintain a certain nitrification temperature, nitrification reactors are usually required to have good heat transfer devices.
The mixed acid nitrification method also has the following characteristics: ① The nitrified matter or nitrified product is liquid at reaction temperature and insoluble in waste sulfuric acid, so the waste acid can be recovered by layering method after nitrification; ②The amount of nitric acid is close to the theoretical amount or too little, and the waste sulfuric acid can be used to prepare mixed acid after being concentrated, that is, the consumption of sulfuric acid is very small; ③Mixed acid nitrification is a heterogeneous process. The nitrification reactor is required to be equipped with a good stirring device to fully contact the acid phase and the organic phase; ④The composition of mixed acid is an important factor affecting the nitrification capacity. The nitrification capacity of mixed acid is expressed by sulfuric acid dehydration value (DVS) or nitrification activity factor (FNA).DVS is the calculated mass ratio of sulfuric acid and water in waste sulfuric acid after the nitric acid in mixed acid is completely nitrated to generate water.FNA is the calculated mass of sulfuric acid in waste acid after the complete nitrification of nitric acid in mixed acid to generate waterPercent concentration。High DVS or FNA indicates strong nitrification capacity.For each specific nitrification process, its mixed acid composition, DVS or FNA should be determined through experiments.For example, when nitrating benzene to produce nitrobenzene, the mixed acid composition (%) is H2SO446~49.5, HNO344~47, and the rest is water,DVS2.33～2.58，FNA70～72。
Nitrification product
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nitrobenzene
BenzeneMixed acidAnd cyclewaste acidAfterRotameterContinuously feed the firstNitrificationAfter passing through the second and third nitrification reactors, the reaction material flows into the continuous separator.The separated nitrobenzene is washed with waterAlkali washing, water washing and distillation to obtain industrial nitrobenzene.Part of the separated waste acid is sent back to the first nitrification reactor as circulating waste acid to absorb part of the heat released by nitrification reaction and dilute the mixed acid to reduce the excessNitroThe formation of things.Most waste acid should be concentrated intoconcentrated sulfuric acidAnd then used to prepare mixed acid.
Nitroalkane
Alkane nitrationGas phase reactionThe preheated propane and liquid nitric acid are sent into the reactor at the same time, and the reaction is carried out in an adiabatic reactor at 370-450 ° C and 0.8-1.2MPa.The reaction heat is removed by vaporization of excess propane and acid.The nitration products are condensed, and the liquid phase products are chemically treated and refined to obtain four nitroalkanes. The gas phase products are sent to propane and nitrogen oxide recovery systems respectively.
Product use
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Nitroalkane is an excellent solvent, which can reducepolyvinyl chloride、polyamide, epoxy resin, etcSolubilityAnd can be used as solvent additive andFuel additive。They are also raw materials for organic synthesis, such as synthesis of hydroxylamine and trimethylolNitromethane, explosives, medicines, pesticides andsurface active agent Etc.Various aromaticsNitro compound, such as nitrobenzene, nitrotoluene and nitrochlorobenzeneDye intermediate(Benzene intermediate)。Some nitro compounds are simple explosives, such as2,4,6-Trinitrotoluene(i.e. TNT).Reduction of aromatic nitro compounds can produce various aromatic primary amines, such asaniline etc.
The color of organic compounds will increase after nitration. If multiple nitro compounds are introduced, their oxidation function will be very strong, so they will become explosive substances.asDynamiteTNT is formed by nitration of tolueneTrinitrotoluene；Nitrobenzene is the raw material for making dyes;Methane is obtained by nitrationNitromethane, is a rocket fuel;Celluloid, a transparent plastic formed by nitration of fibers, was first used to make film. Later, because it was extremely flammable, it often caused fire in cinemas and has been eliminated.
