GB2094032A - Process for carrying out chemical reactions in which nitrous gases occur - Google Patents

Abstract

Problems with monitoring chemical reactions in which nitrous gases occur are overcome by measuring the concentration of NOx in a flushing stream which has been passed over the reaction mixture and controlling the reaction in such a manner that a predetermined NOx concentration in the flushing stream is not exceeded. This technique can be used to control diazotization reactions.

Description

SPECIFICATION Process for carrying out chemical reactions in which nitrous gases occur This invention relates to a process for carrying out a chemical reaction in which nitrous gases occur.

Diazotizations are carried out industrially in most cases by a batchwise procedure in a stirred kettle equipped with a fume-extraction unit. During the entire duration of the reaction, the fume-extraction unit draws a vigorous stream of the exit air through tubes or shafts into the exit air washer, whilst fresh air flows in over the reaction mixture through the kettle opening.

The reagent required for the diazotization, in most cases an aqueous, highly concentrated solution of sodium nitrite, is conventionally metered in under manual control. The required nitrite influx rate and the end point of the diazotization are deftermined by taking samples from the reaction mixture at regular, very short intervals of time, and testing their content of nitrous acid. This test is carried out in general by introducing a spot of a sample of the reaction mixture on to so-called nitrite paper, i.e. reagent paper prepared with potassium iodide and starch, or on to filter paper strips freshly moistened with sulfone reagent, and an excess of nitrous acid can be detected by an color change which occurs.If no color change occurs or, depending on the type of amine to be diazotized, only a slight color change occurs, the nitrite must be metered in in greater quantity or more rapidly. The end point of the diazotization is recognized by the fact that in spite of interruption of the supply of diazotization reagent several successive spot tests indicate excess nitrous acid in the reaction mixture by their color change.

This process has serious disadvantages. The spot samples are imprecise. They indicate only whether nitrous acid is present in the mixture, or not; they give only very imprecise information concerning the concentration. Furthermore, the reaction mixtures in most cases are more or less intensely colored. The coloration changes continuously from the beginning of the diazotization to the end point, since the starting materials and end products which frequently are colored are consumed or formed, respectively, in the course of the diazotization. In addition, the reaction mixture is frequently present in the form of a slurry instead of as a solution. For these reasons, the results of the spot tests are often difficult to interpret.

Some diazotizations require such high concentrations of nitrous acid that the nitrite paper or sulfone reagent gives a positive indication from the beginning to the end of the reaction; in such diazotizations,there is absolutely no possibility of monitoring the nitrous acid, and instead a calculated excess of diazotization reagent must be added, resulting in high loads of nitrous gases in the exit air washer at the end of the diazotization. Vigorous evolution of nirous gases also occurs when through lack of attention too much nitrite is added after the end point of the diazotization has been reached. If the exit air washer is not designed in correspondingly larger dimensions for such cases, poliution of the environment results. In addition, imprecise addition of nitrite adversely affects the quality and the yield of the diazo salts formed.

Industrial diazotizations can therefore be carried out reliably only by well-trained personnel who continuously monitor the reaction. Nevertheless, the diazotizations according to the prior art procedure described, with manual addition of nitrite, does not proceed at the optimum rate, so that unnecessarily long residence times in the diazotization kettle must be taken into account.

There have been no lack of attempts to improve the diazotization process. For example, the addition of the nitrite has been regulated with reference to Redox potential measurements. The measuring devices required for this purpose have the disadvantage that electrodes have to be dipped into the reaction mixtures.

However, these electrodes function unreliably in practice in most cases, since they rapidly become contaminated with particles which crystallize out or separate out in resinous form, and thereby become useless. The electrodes can suffer mechanical destruction owing to viscous or slurry-like reaction mixtures which have to be correspondingly vigorously stirred in large-scaie industrial diazotizations.

Attempts have also been made to regulate diazotizations automatically with the aid of fthe so-called "Redoxomats" from COLORA. This apparatus, which automatically determines the end potential of a diazotization, which potential changes as a rule from experiment to experiment, by adding reagent until no futher decrease in potential caused by the reaction occurs, has the serious disadvantage that it operates properly only in the case of amines which have a very high solubility in the reaction mass. In the majority of cases, industrial diazotizations involve masses in which the course of the chemical reaction is substantially determined by preceding physical processes and the transport of material. In such masses, it is extremely difficult to obtain a usable measured value which is sutable for regulating the reaction.

A device for determining "electroactive volatile materials", for example the oxides of nitrogen, is protected by British Patent 1,558,066. According to the patent, an attempt is made to monitor reactions, diazotizations inter alia, by passing an inert gas stream through the reaction mixture and then over electrodes over which a continuous stream of a liquid electrolyte, for example 0.5% strength aqueous potassium iodide solution, is simultaneously passed. The electrodes are connected to a voltage source, via an electrical measuring device. The measuring device is intended to indicate a jump in potential when, toward the end of a reaction, "electroactive volatile material" flows over the electrodes washed by the electrolyte. However, the protected measuring device does not by any means specifically indicate only nitrogen oxides.Diazotizations are known to involve, as a rule, the diazotization of amines in the presence of acids, in particular hydrochloric acid. Hydrochloric acid yields HCí gas to a gas stream which is passed through, and the HCl gas is also an electroactive volatile material. The frequent industrial practice of adding hydrochloric acid during the diazotization therefore affects the indication, as do amines contained in the carrier gas, which are also electroactive.The British patent mentions a complicated procedure for passing through and recovering the inert gas stream with the aid of the device described, which is scarcely practicable in-diazotizations on an industrial scale since the height of the charge in a reaction kettle during the diazotization changes substantially owing to the additions of nitrite solution and, if required, hydrochloric acid and/or ice; therefore, the depth of immersion of the devices described have continually to be adapted. In addition, the operation of devices such as that described, in reaction kettles of industrial size is hardly practicable and difficult to monitor. Continuous pumping of a stream of the reaction mixture through the devices described would be very expensive, and the measured values would be obtained after considerably delay.The only example of a diazotization contained in the British patent only describes the diazotization of laboratory amounts of aniline, which is known to be particularly easy to diazotize.

We have found that the diazotization of aromatic amines can be carried out, while avoiding the disadvantages described above, when the nitrous acid concentrations are measured not directly in the reaction mixture itself, but indirectly in the flushing stream passed over the reaction mixture beforehand, and the measured values obtained are used for automatically regulating the nitrite metering.

Basic principle: according to Henry's Law, the vapor pressure of a dissolved substance is proportional to its concentration in the case of adequate dilution: P =k.x P = partial pressure of the dissolved substance x = concentration of the substance in the solution k = constant Thus, within certain limits, there exists a proportionality between the concentration of the nitrous gases dissolved in the reaction mixture and that of the gases above the mixture. Furthermore, it is known that nitrite ions in aqueous acidic solution decompose into nitrous gases (a mixture of NO and NO2, also written as NO,). Accordingly, it is also possible to form a relationship between the concentration of the free nitrous acid in aqueous acidic solution and the concentration of nitrous gases in the gas space above the solution.

According to the process of the invention, the diazotization is carried out by taking gas samples from the flushing stream, which has been passed over the reaction mixture beforehand, and passing them into an NOX analysis apparatus. This apparatus operates according to the chemiluminescence method in which the gas samples are mixed with ozone produced in the apparatus. NO reacts with ozone to form NO2, some of the electrons of which are in an excited state and emit light between 590 and 2,600 mm on transition to the ground state.The intensity of this light is measured in a measuring cell within the apparatus and represents a measure of the NO concentration. NO2 present in the gas samples is split beforehand into NO and oxygen, and is also measured according to the principle described above, so that the NOX analysis apparatus gives the total NOX content in the gas space. From the continually obtained concentration data, it is possible, using the relationships described above, to estimate the concentration of nitrous acid in the diazotization mixture, and thus to use the measured values directly for automatically controlling the diazotization.

Since nitrous acid is very unstable and can occur in the diazotization mixture only in very low concentration, the absolute accuracy of this indirect measurement of the concentration of nitrous acid is not critical in most cases; it is only important to decide whether nitrous acid is present in a concentration above or below a critical concentration to be determined experimentally in each case, ie. whether it is necessary to add nitrite, or not.

However, it is also possible, using the apparatus described at the correct setting and an accurately functioning metering device, to maintain a very precisely constant concentration of nitrous acid in the reaction mixture, and thereby to achieve a significantly better diazotization than hitherto of amines which are sensitive or difficult to diazotize, in particular those which give undesired by-products when a temporary excess of deficiency occurs during the reaction.

It is surprising that such indirect measurement of the NO, concentration in the gas space above the reaction mixture, ie. not in the reaction mixture itself, can be effected sufficiently rapidly and so exactly that a reaction can be reliably controlled by this method, even on an industrial scale.

By working out reproducible procedures, in particular by establishing the lower and upper limits of the velocity of the exit air stream and determining the concentration of nitrous acid required for the particular amine to be diazotized, it is possible, however, to carry out an optimally controlled diazotization automatically, both on the laboratory scale and industrially on a large scale.

According to the process of the invention, all reactions can be controlled in which NOX can occur in the gas space above the reaction mixture, for example nitrations and nitrosations, but in particular diazotizations.

The invention is illustrated by Examples which follow, without being restricted to them. Parts denote parts by weight.

Example 1 A stirred flask is provided with a stirrer, a thermometer, an air inlet tube, an air outlet tube and a vessel with an electromagnetic valve. The outlet of the receiver passes under the surface of the reaction mixture in the stirred flask. A constant air stream is passed over the reaction mixture, via the air inlet tube and outlet tube. The air outlettube contains a branch from which an NOX analyzer (for example Beckman model 952 illegible text) continuously takes a part stream of the exit air and measures its NOX concentration. The electromagnetic valve is connected to the NOX analysis apparatus via a regulator in such a manner that the diazotization reagent can be metered according to the NOX concentrations measured.

300 parts of water, 70 parts of 33% strength hydrochloric acid and 27.4 parts of anthranilic acid are introduced into the stirred flask. A constant air stream is passed over the reaction mixture. The air stream is adjusted so that the amount of air present in the gas space above the reaction mixture is circulated approximately 2.5 times per minute. The vessel is charged with a 10% strength by volume aqueous sodium nitrite solution. The combined measuring/metering arrangement is regulated so that the addition of sodium nitrite solution is interrupted when the exit air contains more than 5 ppm of NOx. The lower the NOX concentration falls below 5 ppm, the more rapidly the sodium nitrite solution is metered in through the magnetic valve.The temperature is, kept below 5"C by external cooling from the beginning to the end of the diazotization. The diazotization is complete after 30 minutes; a clear diazo solution is present, and the supply of nitrite is virtually completely stopped. The amount of aqueous sodium nitrite solution consumed contains 14 parts of sodium nitrite, which is approx. 1.5% more than the theoretically expected amount.

Example 2 31.5 parts of 1-amino-2-methoxy-5-chlorobenzene are diazotized as described in Example 1. The automatic diazotization apparatus is again adjusted to 5 ppm. The amount of aqueous sodium nitrite solution consumed contains 14.2 parts of sodium nitrite.The diazotiazation time is 30 minutes. A clear diazo solution is formed.

The compounds below are diazotized as described in Example 1; the table gives the starting substances, the amount (in parts by weight) employed, the setting of the diazotization apparatus in ppm, and the diazotization time tin minutes: Example Starting substance Amount ppm t No.

3 1-amino-2,5-diethoxy-4- benzoylaminobenzene 60.1 5 30 4 1-amino-2-methyl-4- chlorobenzene 28.3 5 60 5 1-amino-2-methoxy-4 nitrobenzene 33.6 5 180 6 4'-methoxy-4-amino diphenylamine 42.8 5 60 7 1-amino-2-nitro-4- methoxybenzene 33.6 5 60 8 1-amino-4-nitrobenzene 27.8 10 15 9 1-amino-4-diethylamino benzene 32.8 9 60

Claims (9)

1. A process for carrying out a chemical reaction in which nitrous gases occur, which process comprises measuring the concentration of NOX in a flushing stream which has been passed over the reaction mixture and controlling the reaction in such a manner that a predetermined NOX concentration in the flushing stream is not exceeded.

2. A process according to claim 1 wherein measurement of the concentration of NOX is effected continuously or at regular intervals.

3. A process according to claim 1 or 2, wherein the chemical reaction is a diazotization and the reagent required for diazotization is metered in manually or automatically at such a rate that a predetermined NOX concentration is not exceeded, the predetermined NOX concentration being chosen such that the reagent flows in at a rate adapted to the course of the reaction.

4. A process according to any one of the preceding claims wherein the chemical reaction is the diazotization of an aromatic amine in an aqueous acidic solution by the addition of a solid alkali metal nitrite or an aqueous solution of an alkali metal nitrite.

5. A process according to any one of the preceding claims, wherein the NOX concentration is measured using an NOX analysis apparatus which automatically continually takes samples from the flushing stream which has been passed over the reaction mixture and analyzes the NOx content of these samples by chemiluminescence.

6. A process according to any one of the preceding claims wherein the chemical reaction is carried out under a gaseous stream and the NOX concentration in the gaseous flushing stream which has been passed over the reaction mixture is measured.

7. A process according to claim 6 wherein the gaseous stream is a stream of air or an inert gas.

8. A process for carrying out a chemical reaction in which nitrous gases occur substantially as hereinbefore described in any one of Examples 1 to

9.