Flow synthesis

Abstract

A method for the flow synthesis manufacture of RDX, comprises the steps of i. preparing input flow reagent A comprising greater than 95% concentration nitric acid and NH4NO3; ii. preparing input flow

Description

The following invention relates to methods of producing explosives from the indirect 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 greater than 95% concentration nitric acid and NH4NO3; ii. preparing input flow reagent B comprising hexamine dissolved in acetic acid; iii. preparing input flow reagent C, comprising acetic anhydride; iv. causing the input flow reagents A, B and C to enter a flow reactor, v. maintaining the flow reactor to less than 90 °C, vi. causing the input flow reagents to react in the reactor. The use of flow synthesis, (ie use of a pumped micro reactor), provides a facile means of preparing RDX at both laboratory R&D scale of ~100 g / hr and a larger industrial scale at multiple Kg / hr. The ability to add further flow reactors in parallel allows the ability to scale-out, rather than traditional scale-up production, with the associated dangers of forming +100 Kgs of RDX explosive in a single reactor vessel. Further, it also reduces the volume of highly concentrated acid, at any given time, when compared to a large reactor vessel as found in 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 per unit of time 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. In step i, the input flow reagent A may preferably be 99% concentration nitric acid. The ammonium nitrate may be present in the nitric acid in the range of from 50%wt to 75.5%wt. In step ii, the flow reagent B, the hexamine may be dissolved in acetic acid in the range of from 5%wt to 37.8%wt, preferably, in the range of from 30 %wt to 37.8%wt, preferably a saturated solution. Preferably the acetic acid is glacial acetic acid (anhydrous acetic acid). In step iii, preparing input flow reagent C, the acetic anhydride may be anhydrous ie dried, input flow reagent C may optionally comprise glacial acetic acid. In step iv, the input flow reagents A, B and C are pumped and combined in the flow reactor, a micro flow reactor, such that there is a continuous flow of the materials, and that the reaction occurs within a chamber / cell in the flow reactor. Preferably the total nitric acid concentration in the flow reactor may be in the range of from 90- 99%. There is a balance of reaction time vs maximum theoretical yield, ie time to pass the reactants through the flow reactor, and waiting for the maximum yield. In one arrangement the input flow reagents may be caused to be passed through the reactor for less than 45 seconds, preferably less than 15 seconds. This allows for a fast throughput of reagents, to provide a high throughput, rather than the maximum theoretical yield. In one arrangement the input flow reagent B and input flow reagent C may be premixed together, before being reacted with flow reagent A. The first step of the synthesis reaction is the reaction between the hexamine and the acetic anhydride. In a further arrangement, input flow reagent B and input flow reagent C may be passed through a first flow reactor, the output of the reaction product of input flow reagent B and input flow reagent C may then be passed through a second flow reactor, such that said reaction product of input flow reagent B and input flow reagent C, being reacted with flow reagent A to form RDX. o In a preferred arrangement the molar ratio of nitric acid:hexamine in the reactor is at least 4:1, this is below the stoichiometry of the reaction, it will provide a yield, but will furnish a sub optimal yield, preferably, the ratio of nitric acid:hexamine may be greater than 6:1, more preferably in the range of from 9:1 to 30:1. In a preferred arrangement the molar ratio of ammonium nitrate:hexamine in the flow reactor in step iv, is in the range of from 1:1 to 10:1. The reaction once it has commenced is exothermic, therefore the temperature has to be controlled ie maintained, to ensure that the reaction does not cause too much heat to be generated. The temperature of the flow reactor may be maintained in the range of from 20 °C to 90 °C, and may involve both heating and cooling to retain the temperature within the working range. The flow reactor may preferably be formed from a ceramic, such as for example a nonoxide engineering ceramic, such as silicon carbide, which provides high thermal conductivity, good chemical compatibility and a high surface area, which allows precise thermal control of the reaction. The non-oxide ceramics have a thermal conductivity orders of magnitude larger than borosilicate glass. Preferably the flow reactor is a liquid cooled / heated reactor, rather than air cooled. The reactor preferably has an integrated heat exchanger, and the use of a liquid as the working fluid, provides increased thermal control of the reaction. Preferably, the temperature of the reactor is in the range of 50 °C to 90 °C, at temperatures in excess of 90 °C there may be formation of acetyl nitrate, a highly undesirable side product. The temperature may be monitored by any measurement means, the flow reactor may be heated and / or cooled by any suitable means such as for example water circulators or alternatively electric heater / coolers. The product may be isolated, such that at step vii, causing the output flow to be quenched, after the reagents have passed through the reactor. A quenching agent may be is an aqueous solution, such as to cause precipitation of RDX. It may be undesirable to rapidly cool the mixture, as this reaction furnishes unwanted side reaction products that are only sparingly soluble. 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 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. 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 Experimental o A solution of ammonium nitrate (AN) dissolved in nitric acid was prepared by the addition of ammonium nitrate (45 g) to nitric acid (99%, 138.75 g) whilst stirring, once the ammonium nitrate had fully dissolved the solution was then attached to Line A of the Protrix reactor. A second solution consisting of hexamine (hex) dissolved in acetic acid was prepared by the addition of hexamine (31.5 g) to glacial acetic acid (50 g) whilst stirring followed by sonication ensure full dissolution of the hexamine. This was then attached to Line B of the Protrix reactor. Acetic anhydride was attached to Line C of the Protrix reactor. Each line included a Vapourtech SF-10 pump and thermal control was provided by a Julabo heating circulator, which heated the reactor to 75 °C. The ammonium nitrate(AN) / HNO3 solution attached to line A was introduced to the reactor first at a rate of 10 mL / min, once nitric acid solution was observed exiting the product port, lines B and C were started at flow rates of 2.5 and 5.25 mL / min respectively. After equilibration, the product was collected as a clear yellow solution directly into an empty collection vessel for 2 minutes (Exp F003). The flow rate of Line A was reduced in subsequent experiments (Exp F004-008) with Lines B and C kept constant. Details of the reaction condition are shown in Table 1. A clear yellow solution was observed in the collection vessels for the experiments with flow rates of 10-8 mL / min. A precipitate was observed in the collection vessels for the experiments with flow rates of 7-5.3 mL / min. The solution B (hexamine and acetic acid (AcOH) and solution C (acetic anhydride AC2O) were fixed, to allow the study of the variable of solution A (ammonium nitrate(AN) and nitric acid (HNO3). Exp Line A (mL / min) Line B (mL / min) Line C (mL / min) T (°C) Collection Time (minutes) Residence time (s) F003 10 2.5 5.24 75 2 11.0 F004 9 2.5 5.24 75 2 11.7 F005 8 2.5 5.24 75 2 12.4 F006 7 2.5 5.24 75 2 13.3 F007 6 2.5 5.24 75 2 14.2 F008 5.3 2.5 5.24 75 2 15.0 Table 1. List of reaction conditions for experiments F003-008. All six of the reactions were diluted with water and then heated at -95 °C for 30 minutes to remove any linear nitramines that may have formed. Once 10 cooled to room temperature, the solids present in each reaction were filtered and washed with water. Molar ratios, yields and percentages of RDX and HMX have been calculated and are shown in Table 2. The relative percentages of product was determined using their relative integrals from the 1H NMR (500 MHz, CDCI3) spectrum of the 15 product mixture. Exp Line A Flow Rate (mL / min) AN:Hexamine HNOsiHexamine Yield % RDX % HMX F003 10 6.21:1 24.34:1 21.40% 98.97% 1.03% F004 9 5.59:1 21.91:1 22.70% 98.54% 1.46% F005 8 4.97:1 19.47:1 24.80% 97.45% 2.55% F006 7 4.35:1 17.04:1 24.50% 96.70% 3.30% F007 6 3.73:1 14.6:1 25.40% 95.69% 4.31% F008 5.3 3.29:1 12.9:1 28.60% 94.50% 5.50% Table 2. List of molar ratios, yields and percentages of RDX and HMX for experiments F003-008. There are three variables that change due to the flow rate here, residence time, acid concentration and ammonium nitrate concentration. Increasing the residence time may have a positive effect on yield, but this needs to be balanced with throughput. The HMX is a side product in this reaction, and is itself a useful product. This synthetic route allows for a rapid transit time of reagents through the reactor, with minimal impurities. Whilst the invention has been described above, it extends to any inventive combination of the features set out above, or in the following description, drawings or claims. Exemplary embodiments of the device in accordance with the invention will now be described with reference to the accompanying drawings in which:- Figures 1 show a flow reactor arrangement. Turning to figure 1 there is provided a schematic of a flow reactor 1 (a micro reactor) The reagents in Line A, Line B and Line C, are fed via associated pumps 6, 7, 8, to the Protrix flow reactor 2, in the arrangement shown, lines B and C briefly premix before being mixed with line A, however the reactor cell can be configured to allow all three Line A, B and C to enter at the same time. A thermometer 9 monitors the temperature in the flow reactor 2, and applies heating / cooling from the heating and cooling source, which may be water jackets or electrical heating and cooling elements. The reactants flow through the reactor plate 2, and emerge at the product exit 3, which feeds into a collection vessel 5, 5 for further processing,

Claims

1. A method for the flow synthesis manufacture of RDX, comprising the steps ofi. preparing input flow reagent A comprising greater than 95 % concentration nitric acid and NH4NO3ii. preparing input flow reagent B comprising hexamine dissolved in acetic acid;iii. preparing input flow reagent C, comprising acetic anhydride; optionally acetic acid;iv. causing the input flow reagents A, B and C to enter a flow reactor;v. maintaining the flow reactor to less than 90°C;vi. causing the input flow reagents to react in the reactor.

2. The method according to claim 1, wherein in step i the input flow reagent A is 99% concentration nitric acid.

3. The method according to any one of the preceding claims wherein the ammonium nitrate is present in the nitric acid in the range of from 50%wt to 75.5%wt.

4. The method according to any one of the preceding claims, wherein step ii the hexamine is dissolved in acetic acid in the range of from 5%wt to 37.8%wt5. The method according to claim 4, wherein the hexamine is dissolved in acetic acid in the range of from 30%wt to 37.8%wt.

6. The method according to any one of the preceding claims, wherein in step iv, the total nitric acid concentration is in the range of from 90-99%, in said flow reactor.

7. The method according to any one of the preceding claims, wherein input flow reagent B and input flow reagent C are premixed before being reacted with flow reagent A.

8. The method according to any one of the preceding claims, wherein the molar ratio of nitric acid:hexamine in the reactor is at least 4:1.

9. The method according to claim 8, wherein the molar ratio of nitric acid:hexamine is in the range of from 9:1 to 30:1.5 10. The method according to any one of the preceding claims, wherein themolar ratio of ammonium nitrate:hexamine in the reactor is in the range of from 1:1 to 10:1.

11. The method according to any one of the preceding claims causing the input flow reagents to react in the reactor for less than 45 seconds.10 12. The method according to any one of the preceding claims, wherein thetemperature of the reactor is in the range of 20 °C to 90 °C.

13. The method according to any one of the preceding claims, wherein the material of the flow reactor cell comprises a non-oxide engineering ceramic.14 The method according to claim 13 wherein the non-oxide engineering 15 ceramic is silicon carbide.

15. The method according any one of the preceding claims, wherein the flow reactor comprises an integrated heat exchanger, with a liquid as the working fluid.

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