GB2059947A - A continuous process for the production of 2,6-dichloro-3-nitropyridine - Google Patents

Abstract

A continuous process for the production of 2,6-dichloro-3-nitropyridine by nitrating 2,6-dichloro-pyridine with a mixture of concentrated sulphuric acid and fuming nitric acid at elevated temperature comprises passing a mixture of 2,6-dichloropyridine, concentrated sulphuric acid and fuming nitric acid (density 1.5) through one or more heat exchangers using one or more degassing units so that from 1800 to 2200 ml per hour of the reaction mixture pass through one square centimetre of reactor tube cross-section, the temperature of the bath liquid in the heat exchanger(s) being in the range from 120 to 145 DEG C, and the mixture containing from 1002 to 1087 g of concentrated sulphuric acid containing from 232.8 to 252.2 g of SO3 and from 88 to 264 ml of fuming nitric acid for each 148 g (1 mole) of 2,6-dichloropyridine.

Description

SPECIFICATION A continuous process for the production of 2,6-dichloro-3-nitropyridine This invention relates to a continuous process for the production of 2,6-dichloro-3-nitropyridine.

German Patent No. 1,670,558 describes the nitration of 2,6-dichloropyridine with an excess of fuming nitric acid (density 1.5), for example at 105 to 107"C, to form 2,6-dichloro-3-nitropyridine.

With large batches, however, the reaction temperature can easily rise to higher temperatures and it has been found that, at these higher temperatures, the reaction is irregular and, in some cases, can run out of control.

The present invention provides a process for the production of 2,6-dichloro-3-nitropyridine by nitrating 2,6-dichloropyridine with a mixture of concentrated sulphuric acid and fuming nitric acid at elevated temperature, which comprises passing a mixture of 2,6-dichloropyridine, concentrated sulphuric acid and fuming nitric acid (density 1.5) through one or more heat exchangers using one or more degassing units so that from 1800 to 2200 ml per hour of the reaction mixture pass through one square centimetre of reactor tube cross-section, the temperature of the bath liquid in the heat exchanger(s) being in the range from 120 to 1450C, and the mixture containing from 1002 to 1087 g of concentrated sulphuric acid containing from 232.8 to 252.2 g of S03 and from 88 to 264 ml of fuming nitric acid for each 148g (1 mole) of 2,6-dichloropyridine.

The process according to the invention surprisingly enables good yields of homogenous 2,6-dichloro3-nitropyridine to be obtained at elevated temperatures without any complications arising.

The process according to the invention may be largely automated so that it may be carried out with minimal labour costs.

Nitration of the 2,6-dichloro-3-nitropyridine is carried out with a mixture of concentrated sulphuric acid (density 1.84), which contains from 21 to 25% of SO3, and fuming nitric acid (density 1.5).

From 88 to 264 ml (2 to 6 moles) of fuming nitric acid, more particularly from 176 to 230 ml, and from 350 to 380 ml (6.57 to 7.13 moles) of the S03containing sulphuric acid are used per mole of 2,6-dichloropyridine. The S03-containing sulphuric acid used per mole of 2,6-dichloropyridine may be obtained for example by mixing from 350 to 380 ml of concentrated sulphuric acid (density 1.84) and from 180 to 195 ml of fuming sulphuric acid (concentration 65%, density 1.99). It may of course also be obtained using a fuming sulphuric acid of lower concentration. If, for example, a fuming sulphuric acid containing 30% of SO3 is to be used for producing the S03-containing sulphuric acid, from 132 to 246 ml of concentrated sulphuric acid (density 1.84) may be mixed with from 390 to 422.5 ml of fuming sulphuric acid containing 30% of SO3.

Where two or more heat exchangers are used for the reaction, it is best for example to add some of the fuming nitric acid before the second heat exchanger if it is used in a quantity of more than 2 moles per mole of 2,6-dichloropyridine. The quantity of fuming nitric acid (density 1.5) added before the second heat exchanger is for example the quantity which exceeds the quantity of 2 moles of fuming nitric acid where from 3 to 6 moles, for example from 1 to 3 moles, of HNO3 are used (always based on the use of 1 mole of 2,6-dichloropyridine). If for example a total of 5.2 moles of nitric acid in the form of fuming nitric acid (density 1.5) is used, up to 2.6 moles of this nitric acid may be added before the second heat exchanger.The rate of this addition before the second heat exchanger depends on the particular throughflow rate of the reaction mixture and on the amount of nitric acid added in the second fraction.

For example, if the throughput of reaction mixture (which contains 2.6 moles of KNO3 per mole of 2,6-dichloro-3-nitropyridine) amounts to between 1.8 and 2.2 litres per hour, the remaining 2.6 moles of fuming nitric acid (= 110 cc) will be added for example at a rate of from 50 to 55 ml per hour. If less HNO3 is used in this second phase, the dropwise addition rate will be reduced accordingly.

Preparation of the nitration solution, i.e. mixing of the nitric acid and the S03-containing sulphuric acid and subsequent introduction of the 2,6dichloropyridine, is carried out with thorough stirring and cooling. The reaction mixture thus obtained preferably has a temperature of about room temperature.

The nitration mixture thus obtained is then transferred to a storage vessel from which it is pumped through the heat exchanger(s) by means of a metering pump. The metering pumps may be formed for example by piston pumps with Tefloncoated valves. The heat exchangers used are, in particular, tubular exchangers, for example coil heaters, double-tube heat exchangers or even spiral heat exchangers.

The heat exchangers and, above all, the tubes through which the reaction mixture flows are preferably made of glass. However, other materials, such as porcelain, enamel, lead, fine steels, iron, may also be used. Materials such as these may also be used for the other vessels and parts of the manufacturing plant. So far as the heat exchanger(s) is/are concerned, the ratio between the amount of bath liquid and the amount of reaction liquid accommodated in the exchanger should be at least such that the bath liquid undergoes virtually no heating as a result of the nitration reaction. The heat exchangers may be identical or even different. Where only one heat exchanger is used, it should be provided with a standard degassing unit in that region where the main reaction takes place. If for example two heat exchangers are used, it is best to provide for degassing between both exchangers.The same also applies where several heat exchangers are used. Up to four heat exchangers may be used. In cases where two or more heat exchangers are used, a further metering pump has to be installed behind each degassing unit for further conveying the nitration mixture if the heat exchanger(s) is/are not situated at a lower level behind the respective degassing units.

In the latter case, the reaction mixture is further conveyed under the force of gravity. In most cases, the installation of a second metering pump behind the degassing unit will also be necessary where only one heat exchanger is used.

In general, the throughput amounts to between 1.8 and 2.2 litres of nitration mixture per hour, i.e. for a cross-section of the reaction tube in the heat exchanger of 1 cm2, from 1.8 to 2.2 litres per hour of nitration mixture (reaction mixture) pass through this cross-ection. Where two heat exchangers are used, the residence time of the mixture in the heat exchangers amounts for example to about 20 minutes. Coil heaters are particularly suitable. For producing from 200 to 250 g/hour of 2,6-dichloro-3nitropyridine, it is possible for example to use 2 coil heaters each with a filling volume of 0.25 or 0.35 litre in the tubes. The filling volume around the tubes of a coil heater such as this is for example 3.3 litres. A coil heater of the type in question has an exchange surface of for example 0.15 m2 for a tube diameter of 70 mm.Silicone oil is preferably used as the bath liquid for the coil heater. The tube diameter may of course also be larger, for example from 70 to 150 mm for the above-mentioned conversion. Accordingly, the heat-exchange surface may also be larger, for example from 0.15 m2 to 1.5 m2 (for the above-mentioned conversion).

The overall length of the coil heater (for the above-mentioned conversion) is for example 405 mm and the number of turns 8, the length of the coil, as measured from the middle of the lower inlet to the middle of the upper outlet, amounting for example to 180 mm. Forthe production of larger quantities, the two heat exchangers and particularly the heat exchange surface have to be enlarged accordingly, for example by increasing the reaction tube diameter and/or the number of turns in the tube. Naturally the same applies where only one heat exchanger is used.

The temperature in the heat exchangers is in the range from 120 to 145"C and preferably in the range from 125 to 141"C. In this connection, it is important for the temperature of the reaction mixture to be continuously monitored during the reaction, best before it enters and as it leaves the heat exchanger(s).

The reaction mixture should be transported through the heat exchangers preferably uniformly and at a constant rate so that, as far as possible, no admixture takes place between parts of the reaction mixture in which the reaction has advanced to different extents. For this reason, it is also important for the reaction mixture to be suitably and sufficiently degassed during the reaction. For example, in the production of 250g/h of 2,6-dichloro-3-nitropyridine, it is favourable to use two heat exchangers each with a volume of 0.25 litre, degassing taking place between both heat exchangers. If, instead, a single heat exchangerwith a volume of 0.5 litre is used, a degassing unit should be provided in the middle of this heat exchanger, i.e. in the middle of the corresponding reaction tube.

The bath liquid used for the heat exchangers is for example silicone oil (for example of the AR 200 type manufactured by Wacker-Chemie, Federal Republic of Germany). However, it is also possible to use other standard bath liquids, for example Diphyl.

Where more than two, for example three or four, heat exchangers are used, the bath temperature therein does not have to be as high as in the first heat exchanger. In their case, a bath temperature of for example from 100 to 125"C, more particularly from 115 to 120"C, is sufficient.

The process according to the invention is further illustrated in the following with reference to Figure 1 of the accompanying drawings taking advantageous embodiments into account, for example where it is carried out with two heat exchangers each having a volume of 0.25 litre (filling volume in the tubes).

However, the process may also be carried out in larger installations and using other fittings known to the expert.

The bath liquid of two heat exchangers W, and W2 is kept by a thermostat 4 at a temperature of 140"C, for example in the heat exchanger W1, and at a temperature of four example 138"C in the heat exchanger W2. The bath liquid is circulated by a pump 5. The temperature of the bath liquid in the heat exchanger W1 is measured by temperature sensors T1 and T2.It amounts to 141 0C for example at the measuring point T1 and to 1380C at the measuring point2. The temperature of the bath liquid in the heat exchanger W2 is measured for example by temperature sensors T5 and To. it amounts to 1 380C for example at the measuring point T5 and to 1 360C at the measuring point Te. The rate at which the bath liquid is moved through the heat exchangers amounts for example to between 400 to 900 litres per hour.

The previously prepared nitration mixture containing the 2,6-dichloropyridine is transferred in batches or even continuously to a storage vessel 1 from which it is conveyed under pressure through tubes 6 and 9 of the heat exchangers Wa and W2 by means of a metering pump 2. Before entering the heat exchanger W1, the nitration mixture is at room temperature.

The temperature of the reaction mixture on leaving the heat exchanger Wa is measured by a temperature sensor T3. For example, the reaction mixture has a temperature of from 125 to 145"C at the measuring point of the temperature sensor T3. The reaction mixture then passes through a conventional degassing unit 7 (in the most simple case a widened tube section with an opening through which the gases formed can escape). Thereafter the reaction mixture passes through the tube 9 of the heat exchanger W2. Since this heat exchanger W2 is at a lower level than the heat exchanger W1 and the degassing unit, the reaction mixture is transported from now on under the force of gravity. An additional metering pump is unnecessary in this case. The temperature of the reaction mixture before it enters the heat exchanger W2 is measured by a temperature sensor T4, its temperature on leaving the heat exchanger W2 being measured by a temperature sensor T7. For example, on entering the heat exchanger W2, i.e. at the measuring point of the temperature sensor T4, the reaction mixture has a temperature of from 80 to 1 10"C and on leaving the heat exchangerW2, i.e. at the measuring point of the temperature sensor T7, a temperature of from 120 to 1 300C. On leaving the heat exchanger W2, the reaction mixture passesthrnugha condenser 10, by which the mixture is brought to room temperature, and then enters a collecting vessel 11 where it is stirred with ice, resulting in precipitation of the 2,6-dichloronitropyridine.Stirring with ice does not have to take place immediately after cooling to room temperature, but instead may even be carried out for example after 24 hours or even after a few days. The reaction product obtained after stirring with ice is then filtered off under suction or centrifuged off, washed and dried. The apparatus as a whole, but above all the tubes 6 and 9 of the two heat exchangers, may be rapidly or immediately emptied through the outlet valves or discharge units 3 and 8.

If the nitric acid is not added all at once at the beginning, but instead partly at the beginning and partly at a later stage, it may be introduced through a feeder 13. The bath liquid may be emptied through outlet valves or drainage units 14 and 12.

The application of the process according to the invention using four heat exchangers is diagrammatically illustrated for example in Figure 2 of the accompanying drawings.

In Figure 2, the meaning of the references 1 to 14 and Wa, W2 and T, to T7 are the same as in Figure 1.

W3 and W4 with the corresponding coils 16 and 18 are another two heat exchangers arranged one behind the other. The exchanger W3 is followed by a degassing unit 17 and the exchanger W4 by a degassing unit 19. The reaction tube 16 of the heat exchanger W3 may be emptied through an outlet 20 and the reaction tube 18 of the heat exchanger W4 through an outlet 21. Outlets 22 and 23 are additional elements for emptying the bath liquid of W3 and W4.

The temperature of the reaction mixture before it enters W3 is measured by a temperature sensor T8 and, before it enters W4, by a temperature sensor T12. The temperature at the measuring point ofT8 may amount for example to between 125"C and 135"C and, at the measuring point of T12, to between 125"C and 135"C for example.The temperature of the reaction mixture on leaving the exchangerW3 is measured by a temperature sensorT11 and, on leaving the exchanger W4, by a temperature sensor Ta5. The temperature of the reaction mixture at the measuring point of T may amount for example to between 130"C and 140"C and, at the measuring point of T15r to between 130and 140"C for example.The temperature in the heat exchanger W3 is measured by the temperature sensors Tg and T10 and, in the heat exchanger W4, by temperature sensors T13 and T14. The temperature in the heat exchangers W3 and W4 (i.e. the bath temperature may for example be the same as the bath temperature of the heat exchanger W2, i.e. it may be between 135 and 140 C and more particularly between 136 and 138"C. However, it is also possible to keep the temperature in the bath liquid of W3 and W4 at a lower level, for example between 100 and 125"C and more particularly between 115 and 120 C. In this case, in contrast to the illustration in Figures 2, each of the heat exchangers W3 and W4 has to be provided with its own thermostat which provides these two heat exchangers with a separate bath liquid circuit. In this case, the thermostat 4 in Figure 2 only heats the heat exchangers W1 and Wand there is no connection with the bath liquid of W3 and W4.

For example, the temperature of the bath liquid may be between 115 and 120"C at the measuring point of Tg, between 115 and 120"C at the measuring point of T10, between 115 and 1 200C at the measuring point T13 and between 115 and 120 C at the measuring point T,4.

It is best to incorporate an electronic control system which, when the temperature at the measuring point of T3 (exit of the reaction mixture from the first heat exchanger W2) exceeds 145"C, switches off the metering pump 2 and at the same time opens the outlet valves for the individual heat exchangers W1 toW4. The measuring point T7 and, optionally, the measuring point T11 and T,5 may also be similary connected to the control system so that in their case, too, the reaction tubes 9 (of W2), 16 (of W3) and 18 (of W4) are immediately emptied when the temperature of 145"C is exceeded.

However, in the event of excessive generation of heat, the reaction may also be controlled by increasing the throughflow rate. For example, when the temperature at T3 exceeds 142 C, the throughflow rate may be increased in such a way that, instead of 2200ml/hour of reaction mixture, 2500 ml/hour are pumped through. In this case, the cold reaction mixture following up cools the content of the heat exchangers to a sufficient extent.

This process may also be automated, preferably by distinctly increasing the throughflow rate to between 2500 and 3000 ml/hour when a certain temperature, for example 142"C, is exceeded until the temperature at T3 falls back below the critical value.

The invention is further illustrated by the following example.

Example A mixture of 1635 ml of concentrated sulphuric acid, 825 ml of fuming sulphuric acid (D=1.99), 386 ml of fuming nitric acid (D=1.50) and 655 g (4.44 mole) of 2,6-dichloropyridine, which is at room' temperature (18 to 22"C), is pumped for 120 minutes through two coil heaters of glass each with a filling volume of 0.25 ml arranged one behind the other.

The heat exchangers are kept at 137 to 142"C by means of a bath liquid (silicone oil) pumped con tinuouslythrough theirjackets. Another 386 ml of fuming nitric acid (D=1.5) are added dropwise over a period of 120 minutes before the second heat exchanger. On leaving the second heat exchanger, the mixture is cooled to room temperature by means of a normal water cooler and, after passing through this cooler, is stirred with 6.5 kg of ice. The 2,6-dichloro-3-nitropyridine precipitated is filtered off under suction or centrifuged off, washed thoroughly with water and dried.

Yields: 550 g (64.5%), content of 2,6-dichloro-3nitropyridine 85.3%, content of 2,6-dichloropyridine 14.7%.

Claims (8)

1. Acontinuous processforthe production of 2,6-dichloro-3-nitropyridine by nitrating 2,6dichloropyridine with a mixture of concentrated sulphuric acid and fuming nitric acid at elevated temperature, which comprises passing a mixture of 2,6-dichloropyridine, concentrated sulphuric acid and fuming nitric acid (density 1.5) through one or more heat exchangers using one or more degassing units so that from 1800 to 2200 ml per hour of the reaction mixture pass through one square centimetre of reactor tube cross-section, the temperature of the bath liquid in the heat exchanger(s) being in the range from 120 to 145 C, and the mixture containing from 1002 to 1087 g of concentrated sulphuric acid containing from 232.8 to 252.2 g of SO3 and from 88 to 264 ml of fuming nitric acid for each 148 g (1 mole) of 2,6-dichloropyridine.

2. A process as claimed in Claim 1, wherein two heat exchangers with a degassing unit in between are used.

3. A process as claimed in Claim 1, wherein three or four heat exchangers are used, degassing units being provided between the respective heat exchangers.

4. A process as claimed in Claim 3, wherein the bath temperature in the third and/or fourth heat exchanger is in the range from 100 to 125"C.

5. A process as claimed in any of Claims 1 to 4, wherein part of the nitric acid is introduced before the second heat exchanger.

6. A processforthe production of 2,6-dichloro-3nitriopyridine substantially as described with particular reference to the accompanying drawings.

7. A processforthe production of 2,6-dichloro-3nitropyridine substantially as described with particular reference to the Example.

8. 2,6-Dichloro-3-nitropyridine when produced by a process as claimed in any of Claims 1 to 7.