Improved flow synthesis

Abstract

A method for the flow synthesis manufacture of RDX, comprises the steps of preparing input flow reagent A, comprising hexamine pre- dissolved in water or glacial acetic acid, preparing input flow reag

Description

The following invention relates to methods of producing explosives from the direct nitration of hexamine by flow synthesis. Particularly to a method of producing RDX. Before the present invention is described in further detail, it is to be understood that the invention is not limited to the particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. According to a first aspect of the invention there is provided a method for the flow synthesis manufacture of RDX, comprising the steps of i. preparing input flow reagent A, comprising hexamine pre-dissolved in water or glacial acetic acid, ii. preparing input flow reagent B comprising greater than 95% concentration nitric acid, iii. causing the input flow reagents A and B to enter a flow reactor at a flow rate, so as to cause a total nitric acid concentration of greater than 90%, in said flow reactor, iv. cooling the reaction chamber to less than 30°C, v. causing the output mixed flow to be quenched, to cause precipitation of RDX The addition of dry hexamine to nitric acid may cause unwanted exothermic reactions. Therefore it is desirable for flow synthesis to avoid this method step and highly preferably the step i) of introducing pre-dissolved hexamine i.e. aqueous hexamine or hexamine in glacial acetic acid directly into the reactor mitigates this problem, as the only time concentrated nitric acid comes into contact with the hexamine is in the reactor cell. The pre-dissolved hexamine is stable, and can be either prepared as input reagent A, immediately before introduction into the reaction cell or it may be procured and / or prepared and stored as a stable solution, ready for later use. The hexamine when pre-dissolved may be present in any amount up to a saturated solution. For hexamine dissolved in water, preferably the hexamine may be in the range of from 0.2g to 0.8g of hexamine per gram of water. The hexamine when dissolved in water, is aqueous hexamine. The input flow reagent A is alternatively hexamine pre-dissolved in acetic acid, which may be in the form of glacial acetic acid. When the input flow reagent A, is glacial acetic acid, the hexamine may be dissolved in the range of from 0.2g to 0.62g of hexamine per gram of glacial acetic acid. Without being bound by theory, the use of glacial acetic acid, has been shown to decrease the concentration of the nitric acid required to nitrate the hexamine when in the reactor cell, thereby improving the efficiency of the process. The may comprise ammonium nitrate solution, that is aqueous hexamine pre-dissolved in water and further comprising ammonium nitrate, wherein the ammonium nitrate may be present greater than 30%wt% preferably at least 60%wt% of aqueous solution with hexamine dissolved in water. The ammonium nitrate may be dissolved in the range of from 0.2g / g to 1.3g / g of hexamine per gram of water. The use of ammonium nitrate increases the solubility of hexamine in water, thereby allowing more reagent into the reaction cell. In a further arrangement, input flow reagent A hexamine in glacial acetic acid may comprise ammonium nitrate. The flow reagent A is pre-dissolved hexamine, this may reduce the total concentration of the nitric acid in the flow reactor, therefore preferably in step iii) the ratio of reagent A to reagent B may be greater than 1:3, to ensure a high excess of nitric acid in the reactor, preferably in step iii) the ratio of reagent A to reagent B is in the range of 1:3 to 1:20, more preferably 1:5 to 1:15. The ratio may be greater than 1:20. The excess of nitric acid may be any value such as for example 1:20 to 1:40 or higher. A further consideration, with an excess of nitric acid in the system is that it may restrict the volume of other starting materials in the reactor limiting the throughput of pre-dissolved hexamine, and hence reducing the yield. Preferably, in step iii), the total nitric acid concentration is in the range of 93- 99%, in said flow reactor, this is caused by ensuring the correct ratio of A:B i.e. an excess of concentrated nitric acid via input reagent B is present in the reactor. The input flow reagent B preferably comprises 95-99% concentration nitric acid, more preferably at least 99% concentration nitric acid. In one arrangement in step ii), the nitric acid in input flow reagent B further comprises a further nitrating agent, such as for example further acids, such as, for example oleum, metal nitrites, such as, for example NaNO?, or ammonium nitrate. The use of flow synthesis provides a facile means of preparing RDX at both laboratory R&D scale of ~100g, and to provide the ability to add further flow reactors to readily scale up production, without the associated dangers of forming +100Kgs of RDX explosive in a single reactor vessel. Further, it also avoids the use of hundreds of litres of highly concentrated acid in a large reactor vessel in a batch process. The use of flow synthesis allows for the continuous removal and safe stowage of final explosive product RDX material from the flow reactor or flow reactors, to avoid the build-up of large quantities of explosive material. This may allow explosive processing buildings to process a greater mass of explosive and / or associated safety distances to be reduced, as the explosive material may be distributed to safe areas, away from the flow reactor, as it is synthesised. The avoidance of nitric acid in input flow reagent A, allows the hexamine starting material to be dissolved, without the nitration reaction starting. This prevents product from precipitating out before it is caused to enter into the flow reactor, and may prevent blockage of the flow reactor and associated mixing chambers. The total nitric acid concentration when input flow reagent B contains only nitric acid as the sole nitration agent, the concentration must be sufficient for nitration to occur, such as for example greater than 92% concentration. The flow rate of input flow reagent A may be selected from any suitable flow rate with input flow reagent B, to provide a total nitric acid concertation capable of causing nitration of hexamine, such as for example in the range of greater than 92%. The actual flow rate of input flow reagent A may be pL through to millilitres to litres, depending on the capacity of the flow cell. The flow rate of input flow reagent B may be selected from any suitable flow rate with input flow reagent A to provide a total nitric acid concertation capable of causing nitration of hexamine, such as for example in the range of greater than 92% concentration. The actual flow rate of input flow reagent A may be pL through to millilitres to litres, depending on the capacity of the flow cell. The use of higher concentrations of acid in input flow reagent B, allows the volume / flow rate of Input flow reagent B to be reduced, i.e. a lower ratio, which may lead to reducing the quantity of nitric acid being used. This may be caused by using other strong acids, such as for example oleum. The temperature in the flow reactor needs to be controlled to prevent a highly exothermic reaction from occurring, preferably the temperature is caused to be less than 30°C, preferably between -10°C to 30°C, more preferably between 10°C to 25°C. The temperature is monitored by water circulators. The flow reactor may be cooled by any suitable means such as for example water circulator or electric coolers. The reaction in step v, the output mixed flow is quenched, to stop the reaction and to cause precipitation of the RDX product. The output flow may be transferred in to a large volume of quench medium or mixed in a mixing chamber. Preferably the output mixed flow, which comprises the RDX dissolved in the nitric acid, is mixed with the quench medium via an SOR mixer at the end of the flow reactor. The quench medium may have a pH 7 or less, and may be selected from an aqueous acidic solution or water. The quenching agent may be cooled to induce crystallisation, preferably less than 20°C, preferably in the region of 10°C or less. The RDX precipitate is filtered and collected and then washed in an aqueous solution, preferably the quenching solution may have a pH 7 or less, preferably water. Experimental reagents 99 % HNO3 was purchased from Honeywell in a 500 mL quantity. Cat. 84392-500ML, Lot. No. I345S. 70 % HNO3 was purchased from Fisher scientific in a 2.5 L quantity. Code: N / 2300 / PB17. Lot: 1716505. Hexamine was purchased from Sigma-Aldrich in a 250 g quantity. Cat. 797979-250G, Lot. No. MKCJ7669. Flow reactor used: Protrix Reactor General Experimental Method Pump A: hexamine in water, (or acetic acid, or ammonium nitrate) Pump B: 99 % HNO3 The general reaction is shown above, where the input flow reagent A comprises hexamine dissolved in a solvent (water, water / ammonium nitrate(AN) and glacial acetic acid), and input flow reagent B comprises the nitrating agent 5 99% concentrated nitric acid.. The input flow reagent A and input flow reagent B are caused to react in the flow reactor to furnish the product RDX. Experimental results Exp. Input A solvent Temp °C hexamine concentration / gg-i Flow rate (hexamine) Flow rate(B) (acid) Final cone, / g g-i Reaction time / s Yield % 14 Water 14 0.429 1.000 8.000 0.0378 59 42.4 16 Acetic acid 14 0.387 1.330 8.000 0.0470 57 56.7 17 Acetic acid 20 0.387 1.330 8.000 0.0470 57 51.5 18 Acetic acid 20 0.387 0.667 4.000 0.0471 114 52.5 19 Acetic acid 20 0.387 2.333 6.998 0.0899 59 22.4 20 Acetic acid 20 0.387 1.167 3.490 0.0902 114 26.8 21 Acetic acid 20 0.387 1.866 7.464 0.0690 57 41.9 22 Acetic acid 20 0.387 0.933 3.732 0.0690 109 23 AN / water 14 0.342 1.330 8.000 0.0453 59 25.1 24 AN / water 14 0.342 0.667 4.000 0.0454 114 32.6 25 AN / water 20 0.342 1.330 8.000 0.0453 56 24.3 26 AN / water 20 0.342 0.667 4.000 0.0454 112 31.7 28 Water 5 0.444 0.500 4.000 0.0391 134 47.7 29 Water 5 0.444 0.900 3.600 0.0753 119 7.4 30 Water 5 0.444 0.711 3.790 0.0576 116 16.5 31 Water 10 0.444 0.500 4.000 0.0391 121 42.4 32 Water 14 0.444 0.500 4.000 0.0391 118 35.3 33 Water 1 0.444 0.500 4.000 0.0391 170 51.0 34 Water 1 0.444 1.000 8.000 0.0391 83 38.5 35 Water 1 0.444 0.362 4.138 0.0277 178 57.8 36 Water 1 0.444 0.432 4.068 0.0335 178 57.4 37 Water 1 0.444 0.566 3.934 0.0448 155 46.0 38 Water 1 0.444 0.629 3.871 0.0503 152 30.2 39 Water 1 0.444 0.250 2.000 0.0391 355 57.8 41 Water 1 0.444 1.000 8.000 0.0391 50 25.6 43 Water 1 0.444 0.298 2.390 0.0391 170 54.2 44 Water 1 0.444 0.587 3.910 0.0466 155 34.2 45 Water 1 0.444 0.608 3.890 0.0485 151 32.2 46 Water 1 0.444 0.362 4.140 0.0277 178 55.7 47 Water 14 0.429 1.000 8.000 0.0378 74 42.4 48 Acetic acid 14 0.387 1.330 8.000 0.0470 71 56.7 49 Acetic acid 20 0.387 1.330 8.000 0.0470 71 51.5 50 Acetic acid 20 0.387 0.667 4.000 0.0471 143 52.5 51 Acetic acid 20 0.387 2.333 6.998 0.0899 74 22.4 52 Acetic acid 20 0.387 1.167 3.490 0.0902 143 26.8 53 Acetic acid 20 0.387 1.866 7.464 0.0690 71 41.9 54 Acetic acid 20 0.387 0.933 3.732 0.0690 137 - 55 AN / water 14 0.342 1.330 8.000 0.0418 73 25.1 56 AN / water 14 0.342 0.667 4.000 0.0419 142 32.6 57 AN / water 20 0.342 1.330 8.000 0.0418 70 24.3 58 AN / water 20 0.342 0.667 4.000 0.0419 140 31.7 Table 1 Table 1 shows the different carrier solvents for the input reagent A, selected from water, acetic acid, or ammonium nitrate solution. 5 The temperature in Table 1 is the temperature within the reactor cell. There are trends which may indicate that the use of lower temperatures increases the throughput and yield. Without being bound by theory this may be due to the limitation on competing hydrolysis reactions. The flow rates in table 1 are indicated as (ml / minute), the results indicate 10 that a suitable concentration of nitric acid in the reactor can be achieved, that is sufficient to cause nitration of the pre-dissolved hexamine (aqueous hexamine, aqueous hexamine and ammonium nitrate, and hexamine in glacial acetic acid) to furnish RDX. Previous attempts have relied upon using hexamine predissolved in nitric acid, as one of the flow reagents. As mentioned earlier, the 15 dissolving of dry hexamine powder in nitric acid, causes large exothermic reactions, so the process defined herein allows the hexamine to be introduced into the reactor in a more safe pre-dissolved form. The maximum percentage yield, appears to be when the maximum concentration is around 0.035 g / g, or 0.85 mL hexamine solution per 8 mL acid. 5 To maximise RDX output, the optimum concentration is around 0.045 g / g (1.15 mL) hexamine solution per 8 mL acid. The optimum output is achieved by finding a compromise between percentage yield and residence time, i.e. time in the reactor. RDX synthesis using a flow reactor poses more challenging design issues 10 than simply pumping solutions from well-known and quantified batch chemistry. This is mainly due to the fact that the starting material hexamine is solid, and RDX can potentially precipitate out of solution during the reaction. Precipitation of the RDX during the transition through the flow reactor can happen as the acid concentration drops and water content increases, thereby leading to potential 15 blockages in the flow reactor, this could lead to catastrophic events, and so the nitric acid concentration in the flow chemistry.

Claims

1. A method for the flow synthesis manufacture of RDX, comprising the steps ofi. preparing input flow reagent A, comprising hexamine pre-dissolved in water or glacial acetic acid,ii. preparing input flow reagent B comprising greater than 95% concentration nitric acid,iii. causing the input flow reagents A and B to enter a flow reactor at a flow rate, so as to cause a total nitric acid concentration of greater than 90%, in said flow reactor,iv. cooling the reaction chamber to less than 30°C,v. causing the output mixed flow to be quenched, to cause precipitation of RDX.

2. The method according to claim 1, wherein in the input flow reagent A comprises hexamine pre-dissolved in water, and is in the range of from 0.2g to 0.85g of hexamine per gram of water.

3. The method according to claim 1, wherein the input flow reagent A comprises hexamine pre-dissolved in glacial acetic acid, and is in the range of from 0.2g to 0.62g of hexamine per gram of glacial acetic acid.

4. The method according to any one of the preceding claims, wherein input flow reagent B comprises at least 99% concentration nitric acid.

5. The method according to any one of the preceding claims, wherein in step iii) the ratio of reagent A to reagent B is greater than 1:3.

6. The method according to claim 5 wherein in step iii) the ratio of reagent A to reagent B is in the range of 1:3 to 1:15.

7. The method according to any one of the preceding claims, wherein the input flow reagent A, the hexamine pre-dissolved in water, further comprises ammonium nitrate.

8. The method according to any one of the preceding claims, wherein step i) the hexamine is pre-dissolved to achieve a saturated solution.

9. The method according to any one of the preceding claims, wherein in step iii), the total nitric acid concentration is in the range of 93- 99%, in said flow reactor.

10. The method according to any one of the preceding claims, wherein the temperature in the reactor is in the range of from -10°C to 30°C.

11. The method according to any one of the preceding claims, wherein step ii, the nitric acid in Input flow reagent B further comprises a further acid, oleum, ammonium nitrate or NaNO2.

12. The method according to any one of the preceding claims, wherein, step v), the quench is caused by mixing the output mixed flow and a quenching agent.

13. The method according to claim 12, wherein the quenching agent is an aqueous solution, such as to cause precipitation of RDX.

14. The method according to any one claims 12 to 13, wherein the quenching agent is cooled below 10°C.