EP0000491B1 - Procédé de traitement des sous-produits obtenus par la nitration de composés aromatiques - Google Patents

Procédé de traitement des sous-produits obtenus par la nitration de composés aromatiques Download PDF

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Publication number
EP0000491B1
EP0000491B1 EP78100350A EP78100350A EP0000491B1 EP 0000491 B1 EP0000491 B1 EP 0000491B1 EP 78100350 A EP78100350 A EP 78100350A EP 78100350 A EP78100350 A EP 78100350A EP 0000491 B1 EP0000491 B1 EP 0000491B1
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European Patent Office
Prior art keywords
nitric acid
nitration
column
aromatic
water
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Expired
Application number
EP78100350A
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German (de)
English (en)
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EP0000491A1 (fr
Inventor
Karl-Werner Dr. Thiem
Lutz Dr. Neumann
Bernd Thelen
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Bayer AG
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Bayer AG
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C201/00—Preparation of esters of nitric or nitrous acid or of compounds containing nitro or nitroso groups bound to a carbon skeleton
    • C07C201/06—Preparation of nitro compounds
    • C07C201/16—Separation; Purification; Stabilisation; Use of additives

Definitions

  • the present invention relates to a process for working up aromatic aldehydes and nitrous gases which occur as undesired by-products in the nitration of aromatics with aliphatic side chains in excess nitric acid.
  • the carboxylic acids can e.g. from the reaction mixture after leaving the nitration reactor and subsequent phase separation. be removed from the organic phase of the reaction mixture by washing with an aqueous alkaline solution and then neutral washing with water. Due to its high boiling point, the aldehyde remaining in the organic phase accompanies the nitroaromatics to a greater or lesser extent in the further workup of the organic phase by rectification in a column in which the unreacted aromatic and adhering water are separated off.
  • oxidation reactions to aromatic aldehydes and aromatic carboxylic acids which take place as a side reaction to the nitration, are simultaneously associated with the reduction of part of the nitric acid to nitrogen oxides with a lower oxidation number than 5 (nitrous gases). This leads to an undesirable additional consumption of nitric acid.
  • nitrous gases escaping as exhaust gas are also loaded with the unreacted aromatic, the nitroaromatic and nitric acid in accordance with their partial vapor pressure, as a result of which a further reduction in yield occurs.
  • a process for the separation and processing of undesired by-products consisting of aromatic aldehydes and nitrous gases which occur during the nitration of optionally substituted aromatics containing aliphatic side chains in excess nitric acid by partial oxidation of the side chains was found, which is characterized in that simultaneously distilling the optionally substituted aromatic aldehyde as an azeotrope with water above the top of the column during the rectification of the nitroaromatic by maintaining an aqueous phase within the rectification column in addition to the organic phase and removing it after oxidation to give the corresponding carboxylic acid and the nitrous gases contained in the offgas in an adsorption column an aqueous phase obtained in the nitriding process, which may optionally contain nitric acid, is converted back into nitric acid in countercurrent in the presence of oxygen.
  • the applicability of the process according to the invention follows from the nitration of reactive aromatics which carry an aliphatic side chain in excess nitric acid.
  • aromatics are toluene, xylene, ethylbenzene, isopropylbenzene, 1-methyl-naphthalene or 2-methyl-naphthalene.
  • the nitroaromatics obtainable from the compounds mentioned and substituted by alkyl radicals pass through a separation stage for the nitric acid used in excess and a nitric acid extraction. This is followed by a washing step in which the acidic constituents, for example benzoic acid or substituted benzoic acid, formed in side reactions are separated off with the aid of aqueous alkaline solution. After a subsequent neutral wash, the organic phase with the desired nitroaromatics enters the rectification column.
  • the desired bottom product can be obtained as the bottom product To obtain nitro aromatics with an aldehyde concentration of less than 0.005% by weight.
  • the formation of the required aqueous phase occurs from about 1% by weight of water, based on the total product feed to the rectification column.
  • the amount of additional water fed in is not critical upwards, it is only determined by economic considerations, for example by the steam consumption increasing with higher amounts of water, limited. To a small extent, the optimal amount of water also depends on the operating temperature or the operating pressure of the column due to the temperature-dependent solubility of water in the organic phase. An amount of water of 1 to 5% by weight, based on the total product feed, is preferred.
  • the inorganic phase is fed into the neutral wash immediately in front of the rectification column, while the organic phase with the aromatic aldehyde separated according to the invention, the unreacted starting product and the nitroaromatic is returned to the inlet stream to the nitration reactor .
  • the aromatic aldehyde separated according to the invention is then oxidized to the associated aromatic carboxylic acid, which in turn can then be discharged in the alkaline wash and in the neutral wash.
  • nitrotoluene feed to the rectification oven contains, for example, 0.5% by weight of benzaldehyde and 4.5% by weight of unreacted toluene
  • 7 (or 11 or 14) 'must be theoretical without the addition of water according to the invention
  • Stages in the stripping section of the rectification column, a reflux ratio of 23 (or 18 or 15), based on the added organic phase, is set in order to obtain an almost aldehyde-free bottom effluent with a benzaldehyde content of less than 0.005% by weight.
  • the amount of steam required for this is 1190 kg (or 950 kg. Or 790. kg), based on 1000 kg of product feed.
  • nitrotoluene feed to the rectification column contains 1% benzaldehyde and 4% unconverted toluene
  • a reflux ratio of 23 (or 18 or 18) or 9 (or 14 or 18) theoretical stages in the stripping section must be achieved without the inventive addition of water 15) can be set in order to achieve the aldehyde level of less than 0.005% by weight in the bottom drain with the same steam consumption as above.
  • reflux ratios of 10 to 2.5 are possible if, for example, 1 to 5% of water, based on the amount of the product stream feed, is fed in. Under these conditions, the amount of steam required to heat the sump is only about 50% of the amount of steam required without the measures according to the invention.
  • part of the nitric acid becomes nitrogen oxides with a lower oxidation number than 5 reduced.
  • nitrogen oxides with an oxidation number lower than 5 are, for example, nitrogen dioxide, nitrous oxide, nitrogen monoxide and dinitrogen trioxide. Overall, they are referred to as nitrous gases.
  • This exhaust gas is also loaded with unreacted starting product, with the nitro atoms produced by the nitration and with nitric acid in accordance with their partial vapor pressure.
  • nitric acid treated and recycled to nitric acid.
  • the compounds further contained in the exhaust gas, namely the unreacted starting material, and the nitroaromatic produced in the nitriding process are condensed after the exhaust gas treatment and returned to the circuit.
  • a special embodiment of this exhaust gas treatment with the recovery of nitric acid, unreacted starting product and nitroaromatic consists in carrying out the exhaust gas treatment in a column in which the exhaust gas supplied from below in countercurrent countercurrently contains the aqueous phase, which may already contain nitric acid, via one or more Inlet points is counteracted.
  • the column can be charged with an aqueous phase which already contains more nitric acid and is obtained, for example, when the organic phase is extracted with water or dilute nitric acid.
  • the upper part of the column can then be charged with water or with dilute nitric acid in order to increase the efficiency of the conversion of the nitrogen oxides into nitric acid.
  • the conversion of the nitrogen oxides to nitric acid requires the presence of oxygen and is advantageously carried out in the presence of atmospheric oxygen. Atmospheric oxygen is expediently fed into the column below the exhaust gas feed. In this way, a drain stream largely free of dissolved nitrous gases can be obtained, which can be fed to a nitric acid concentration or another suitable use.
  • the column should be divided by residence time chambers. Otherwise, the column can be equipped, for example, with packing or with exchangeable trays. However, the exhaust gas treatment can also be carried out, for example, in a boiler cascade with appropriate gassing devices.
  • the absorption of the nitrous gases and their conversion into nitric acid is exothermic, so that cooling of the reaction mixture may be necessary. This can be done, for example, by pre-cooling the input stream in aqueous phase or by passing the reaction mixture through separate cooling devices. Temperatures between 10 and 50 ° C have proven to be the most favorable working conditions.
  • the reaction can be carried out at normal pressure or at slightly elevated pressure up to about 3 bar. Working under pressure allows smaller apparatus dimensions with the same performance.
  • the organic phase then goes through an alkaline wash and a neutral wash, in which e.g. Benzoic acid and nitrobenzoic acid in the form of their alkali salts are discharged with the waste water.
  • a neutral wash e.g. Benzoic acid and nitrobenzoic acid in the form of their alkali salts are discharged with the waste water.
  • the washed organic phase is then fed into a rectification column which at the same time has a water feed below the feed of the organic product.
  • the bottom product of the rectification column is the desired nitrotoluene with a benzaldehyde content of less than 0.005% by weight.
  • the top product of the rectification column contains, in addition to a small amount of nitrotoluene, the water, the unreacted toluene and the benzaldehyde to be removed.
  • This top product is separated into an inorganic and an organic phase; the inorganic phase is returned to the washing stage and the organic phase can be returned directly to the nitriding reactor.
  • nitrous gases are mainly generated as exhaust air from the nitriding reactor. They are fed into the lower part of an absorption column. Below the exhaust air feed is a supply point for atmospheric air, which contains the oxygen required to convert the nitrogen oxides into nitric acid.
  • a dilute nitric acid solution is fed into the middle of the absorption column and is obtained from the organic phase of the nitration mixture during the extraction of the nitric acid.
  • Water is fed in at the top of the column. Instead of feeding in water, it is also possible to use a very dilute nitric acid, which is obtained, for example, as a top product in the concentration of sub-azeotropic nitric acid.
  • the running bottom product of the absorption column is worked up together with the excess nitric acid obtained on the nitric acid separation for reuse in the nitration reactor.
  • the inventive method allows Separation of aromatic, optionally substituted aldehydes from optionally substituted aromatic nitro compounds with lower specific steam consumption compared to rectification without the addition of water according to the invention.
  • the process according to the invention allows the entire organic portion of the top product of the rectification column to be returned to the nitration stage and thereby avoids a complicated distillative separation of the aromatic aldehyde to be discharged from the other reusable aromatic compounds, such as the unreacted aromatics and a small amount occurring in the top product of the rectification column Part of nitro aromatics. Losses in yield of organic substances are avoided in this way.
  • the method according to the invention enables the conversion of nitrogen oxides to nitric acid and thereby avoids losses in the yield of nitric acid.
  • a nitriding reactor e.g. a cascade of 3 ring tube reactors
  • 39.079 kg per hour of a 65.8% by weight nitric acid with 4.033 kg per hour of toluene, which contains 0.6% by weight nitrotoluene and 0.8% by weight benzaldehyde are added continuously 70 ° C and a residence time of 6 minutes.
  • the reaction mixture is then cooled and continuously separated into an inorganic and an organic phase at 30 ° C. in a centrifuge.
  • the organic phase (content of nitric acid: 18.2% by weight, water: 4.9% by weight, benzoic acid: 0.3% by weight, benzaldehyde: 0.5% by weight) Nitrobenzoic acid: 0.2% by weight, of toluene: 2.8% by weight) is in a four-stage cascade consisting of a ring tube reactor, centrifuge and three washers and separators each, in countercurrent at 45 ° C at 0.746 kg per hour Washed water.
  • the aqueous phase obtained after this countercurrent extraction is used to treat the waste gases.
  • the alkaline wash of the organic phase consists of a treatment with 3% by weight aqueous sodium hydroxide solution (2.671 kg per hour).
  • the subsequent neutral wash takes place in a cascade of stirred washers and subsequent separators at approx. 45 ° C. In these two washing stages, the acids contained in the organic phase are completely removed as sodium salts with the waste water streams.
  • the washed organic phase (amount: 5.803 kg per hour; 0.6% by weight of benzaldehyde; 3.5% by weight of toluene) is added to water in a tray column (12 stripping trays; 18 reinforcing trays) with the addition of 0.145 kg per hour Rectified 160 ° C bottom temperature and a reduced pressure of about 100 torr head pressure and 130 torr bottom pressure at a reflux ratio of 4: 1 based on the organic phase. This requires 2.175 kg of steam per hour to heat up the sump.
  • the bottom effluent is free of benzaldehyde (less than 0.005% by weight).
  • the organic part of the distillate is returned to the nitration (amount: 0.259 kg per hour; approx. 77.5% toluene; 12.5% benzaldehyde; 10% nitrotoluene).
  • the yield of nitrotoluene, based on the toluene, is 98.3%.
  • the aqueous phase obtained after countercurrent extraction (2.429 kg per hour with 53% by weight nitric acid) is fed into a 20-stage plate column for the treatment of the nitrous gases in the exhaust gas.
  • the top floor is charged with 0.242 kg per hour of dilute nitric acid (2% by weight nitric acid).
  • the waste gas stream (1.010 kg per hour) is fed to the 5th tray of the column and an air stream (0.638 kg per hour) to the 1st tray of the column.
  • the liquid feed streams are pre-cooled to 10 ° C, the absorption is operated at 15 ° C and a dwell time of 6 minutes.
  • the exhaust gas stream of the exhaust gas scrubbing free of organic compounds (0.799 kg per hour) is fed to an absorption with 25% aqueous sodium hydroxide solution in order to remove residual residues of the nitrous gases.
  • 1,000 kg per hour of the washed organic phase obtained according to Example 1 are continuously fed to a column with 15 theoretical plates in the stripping section and 10 steps in the rectifying section to separate toluene and benzaldehyde, with 0.05 kg per hour on the 5th tray below the feed Water can be added.
  • the rectification is operated with a reflux ratio of 2.5: 1 based on the organic phase under the conditions given in Example 1, 0.425 kg of steam per hour being required to heat the bottom.
  • the bottom effluent from the column contains less than 0.005% by weight of benzaldehyde.
  • An equally good separation of the benzaldehyde is obtained if water is added at 0.05 kg (or 0.025 kg) per hour and a reflux ratio of 6: 1 (or 10: 1) is set.
  • the amount of steam required in this case is 0.600 kg (or 0.675 kg) per hour.
  • the rest of the process is carried out as described in Example 1.
  • the nitriding process is carried out continuously, as described in Example 1, but at a nitriding temperature of 80 ° C.
  • the organic phase obtained after washing (amount: 1,000 kg per hour; 1% by weight of benzaldehyde) is continuously added to a column 12 theoretical stages in the stripping section and 10 steps in the reinforcement section, with 0.02 kg per hour of water being added on the 6th floor below the inlet.
  • the rectification is carried out at a reflux ratio of 10: 1 based on the organic phase under the conditions given in Example 1, with 0.675 kg of steam per hour being required to heat the bottom.
  • benzaldehyde In the bottom effluent, benzaldehyde can no longer be detected by gas chromatography (less than 0.005% by weight).

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  • Organic Chemistry (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

Claims (4)

1. Procédé de séparation et de traitement de sous-produits inopportuns constitués d'aldéhydes aromatiques et de gaz nitreux se formant lors de la nitration d'hydrocarbures aromatiques éventuellement substitués et contenant des chaînes latérales aliphatiques dans de l'acide nitrique en excès par oxydation partielle de ces chaînes latérales, caractérisé en ce que, lors de la rectification de l'hydrocarbure aromatique nitré, on distille simultanément l'aldéhyde aromatique éventuellement substitué sous forme d'un azéotrope avec l'eau par le sommet de la colonne de rectification en maintenant, à l'intérieur de cette dernière, une phase aqueuse en plus de la phase organique, puis on l'exclut après oxydation en un acide carboxylique correspondant et l'on retransforme, en acide nitrique, les gaz nitreux contenus dans le gaz résiduaire dans une colonne d'adsorption avec une phase aqueuse se formant dans le procédé de nitration et pouvant éventuellement contenir de l'acide nitrique et ce, en contre-courant et en présence d'oxygène.
2. Procédé suivant la revendication 1, caractérisé en ce que, comme oxygène, on utilise l'oxygène atmosphérique.
3. Procédé suivant la revendication 2, caractérisé en ce qu'on effectue l'oxydation de l'aldéhyde aromatique en un acide carboxylique aromatique correspondant à l'interieur du réacteur de nitration.
4. Procédé suivant la revendication 3, caractérisé en ce qu'on introduit l'oxygène atmosphérique en dessous du point d'admission du gaz résiduaire dans la colonne d'adsorption.
EP78100350A 1977-07-19 1978-07-11 Procédé de traitement des sous-produits obtenus par la nitration de composés aromatiques Expired EP0000491B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE2732558 1977-07-19
DE19772732558 DE2732558A1 (de) 1977-07-19 1977-07-19 Verfahren zur aufarbeitung von bei der nitrierung von aromaten erhaltenen nebenprodukten

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EP0000491B1 true EP0000491B1 (fr) 1980-10-01

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DE700642C (de) * 1935-07-24 1940-12-28 Lignose Sprengstoffwerke G M B Verfahren zur Reinigung von technischem Mononitrobenzol, Mononitrotoluol, aehnlichen aromatischen Mononitrokohlenwasserstoffen oder deren Substitutionsprodukten

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IT1106266B (it) 1985-11-11
EP0000491A1 (fr) 1979-02-07
IT7850326A0 (it) 1978-07-17
DE2732558A1 (de) 1979-02-01

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