Pilot-Plant-Scale Continuous Manufacturing of Solid Dinitrogen

Chapter 8

Pilot-Plant-Scale Continuous Manufacturing of Solid Dinitrogen Pentoxide

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on May 20, 2013 | http://pubs.acs.org Publication Date: April 24, 1996 | doi: 10.1021/bk-1996-0623.ch008

T. E. Devendorf and J . R. Stacy Energetic Materials Research and Technology Department, Indian Head Division, Naval Surface Warfare Center, Indian Head, MD 20640-5035

The Indian Head Division, Naval Surface Warfare Center (NSWC), has completed design, specification, installation, and start-up test/evaluation of a pilot plant scale system capable of producing solid N O at a rate of 360 grams per hour (ca. 130 pounds per 160 hour month). N O is produced by the gas phase oxidation of dinitrogen tetroxide (N O ) with ozone (O ) (i.e., N O + O -> N O + O ). Ozone is produced on site at an approximate rate of one pound per hour with a concentration of 8% in oxygen. The N O -ozone gas phase reaction takes place in a plug flow reactor. Four reactors in parallel are available to accommodate various production rates. The N O collects as a solid in four parallel trains of three traps in series, each trap suspended in a dry ice/acetone solution. The solid N O is dissolved in various solvents for use as a non-aqueous nitrating agent. 2

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The Indian Head Division, Naval Surface Warfare Center (NSWC), was funded to develop a process for pilot plant scale continuous manufacturing of solid dinitrogen pentoxide ( N 0 ) . The principle objectives of the program were to: (1) Transition the technology of N 0 gas phase synthesis from the White Oak Detachment, NSWC, to the Indian Head Division, (2) Develop and demonstrate the technology to manufacture N 0 in the gas phase with a 2.5 kilogram per day capacity, and (3) Manufacture sufficient N 0 to support the parallel development effort of novel nitration processes at the Indian Head Division. 2

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Synthesis of N 0 2

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N 0 is a white crystalline solid which sublimes without melting. N 0 is thermally unstable, and readily decomposes to oxygen and nitrogen dioxide. The decomposition of N 0 is thermally dependent with a half life of approximately one hour at 0°C and 2

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This chapter not subject to U.S. copyright Published 1996 American Chemical Society In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

8. DEVENDORF & STACY

Manufacturing of Solid Dinitrogen Pentoxide 69

one week at -20°C. At temperatures below -60°C, N 0 has been stored for up to one year (Paul, N . , Defense Research Agency, personal communication, 1993). N O is the anhydride of nitric acid, and readily converts to the acid in the presence of moisture. N 0 is produced by the gas phase oxidation of dinitrogen tetroxide ( N 0 ) with dry ozone (0 ) according to literature methods (1-3). 2

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(1)

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The anhydrous N 0 is trapped as a solid at low temperatures (ca. -78°C). Downloaded by UNIV OF CALIFORNIA SAN DIEGO on May 20, 2013 | http://pubs.acs.org Publication Date: April 24, 1996 | doi: 10.1021/bk-1996-0623.ch008

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Indian Head Process. The pilot plant scale facility produces ozone with a PCI Ozone and Control Systems model G-28S ozone generator. This generator produces ozone by corona discharge at an approximate rate of one pound per hour with a concentration of 8% ozone in anhydrous oxygen. The N 0 -ozone gas phase oxidation reaction takes place in a plug flow reactor. To accommodate various production rates, four reactors in parallel are available. Maximum capacity requires all reactors be operated simultaneously. The reactors are cooled externally by water. The N 0 collects as a solid with at an approximate rate of 360 grams per hour in four parallel trains of three glass traps in series. Each N 0 collection trap is suspended in a dry ice and acetone solution. The N 0 is dissolved in solvents such as methylene chloride for use as a non-aqueous nitrating agent. N O is valuable as a nitrating agent when the use of nitric acid would not be possible or effective. 2

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Bench Top Scale Process Development The initial N 0 bench top scale process development conducted at the Indian Head Division was based on the nitric acid dehydration reaction (4-5): 2

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+ P 0 2

3N 0

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+ 2 H P0

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(2)

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This method produced low yields of poor quality N 0 contaminated with N 0 and small quantities of H N 0 . In addition, the waste stream contained 1.5 grams of H P 0 per gram of N 0 produced. Both the N 0 's poor quality and the high ratio of waste to product made this method unacceptable for scale-up. A higher quality product was produced with reduced waste when the gas phase reaction (7-3) was pursued: 2

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Original Process. Figure 1 is a schematic of the original N O synthesis apparatus which was a large scale modification of an apparatus developed at China Lake (Fisher, J., N A W C China Lake, actual N O manufacturing apparatus provided). The ozone 2

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In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

70

NITRATION

generator used was a PCI Ozone and Control Systems model G L - 1 . This ozone generator produces ozone in a cell consisting of an inner stainless steel grounded electrode and a silver plated glass dielectric. The stainless steel electrode is cooled internally by water and the glass dielectric is cooled externally by an insulating oil. High voltage is applied to the glass dielectric. Oxygen passes between the electrodes through a high intensity corona discharge which converts a portion of the oxygen in the gas stream to ozone. The approximate rate of ozone production is one pound per day at a concentration of 5-8% in oxygen. The oxygen is provided as a compressed gas with a dew point of less than -60°C. When used with this N 0 synthesis apparatus, the oxygen feed rate to the ozone generator was 10 SCFH at 15 psig. Operation of this N 0 synthesis apparatus was semi-continuous. The N 0 feed reservoir was filled with liquid N 0 prior to each use. When introduced, the ozone stream was split in two with the flow rate of each stream controlled by valves (2) and (3) respectively (Figure 1). Valve (3) controlled the ozone sweep through the N 0 reservoir where the reaction was initiated. Partially reacted N 0 , along with any remaining ozone and oxygen, was swept into the primary reaction zone where the unreacted N 0 contacted a second ozone stream. The flow rate of the second ozone stream was controlled by valve (2). The reaction zone was cooled using a water condenser. The reaction rate was manually controlled by varying the ratio of ozone passing through valves (2) and (3). To determine what adjustments to valves (2) and (3) were needed, the color and temperature in the primary reaction zone were monitored. Visually observing a brown tint in the reaction zone indicated incomplete N 0 oxidation. The temperature in the reaction zone was maintained between 90 and 100°C


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The N 0 product (90% yield based on N 0 ) was collected in two glass traps suspended in a dry ice and acetone solution. The N 0 was introduced to the traps through a nozzle extending half way into the trap. An optimized production rate of 43.1 grams of N 0 per hour was achieved using this apparatus. The limitations of this synthesis technique became apparent during scale-up of the process. The production rate was limited by the geometry of the N 0 reservoir and the fixed length and diameter of the reaction zone, which did not provide sufficient retention time at higher flow rates. The reaction rate was difficult to control because there was no way to maintain a constant N 0 vaporization rate. N 0 vaporization depended on the ozone/oxygen sweep rate, N 0 reservoir temperature, and the head space and N 0 level in the reservoir. The large number of variables associated with the N 0 vaporization rate along with the two valves controlling ozone addition, made this process extremely difficult to control. Consequently, this apparatus was unsuitable for scale-up. 2

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Modified Process. To develop an N 0 process which could more easily be scaled up, several modifications were made to the original apparatus. The resulting configuration is shown in Figure 2. The N 0 reservoir was replaced with a gas phase reactor. Both the ozone and the N 0 were introduced through two separate nozzles extending half way into the reactor. The N 0 feed rate was controlled using valve (4) independent of the ozone addition rate which was controlled using valves (2) and (3). The larger reaction zone increased the residence time allowing the reaction to go to completion over a wide range of reactant flow rates. The N 0 reservoir was placed in a constant temperature bath to control the vaporization rate and feed pressure of the N 0 . 2

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DEVENDORF & STACY

Manufacturing of Solid Dinitrogen Pentoxide

Figure 1. Original N O Process. 2

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Figure 2. Modified N O Process. 2

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This apparatus was run under a variety of conditions. These included variations in ozone feed rate and split between the two reaction zones, variations in N 0 feed rate, and different N 0 bath temperatures. An optimized production rate of 70.1 grams per hour was obtained using this apparatus. This result was obtained by operating the ozone generator near maximum capacity and supplying as much N 0 as possible without causing incomplete conversion to N 0 . To achieve the optimized production rate, the model GL-1 ozone generator was operated with an oxygen feed rate of 20 SCFH at 8 psig. The entire ozone output was injected into the reactor through the nozzle with out any split to the second reaction zone. The N 0 bath was maintained at a constant temperature of 36°C and the N 0 reservoir feed pressure was 6 psig. In this manor the entire process was controlled by adjusting the N 0 feed valve and observing the color and temperature of the resulting reaction products as they left the reaction zone. The limitations of the apparatus were primarily a result of the reactor configuration. The reactor did not provide sufficient mixing of the reactants which limited the overall yield and increased the amount of ozone required to completely convert the N 0 to N 0 . In addition, un-reacted N 0 accumulated in the reactor dead spaces. 2

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Plug Flow Reactor Process. To achieve better mixing and flow patterns, and thereby improve the N 0 production rate, the original tank type reactor was replaced with a plug flow reactor. The new reactor was a modified Allihn condenser with a 500 mm jacket. Nine thermal wells were inserted into the condenser as illustrated in Figure 3, to more clearly track the extent of reaction. The reactor had two inlet paths. The first allowed the introduced gas to flow in a direct path with the only restrictions in the flow pattern caused by the bulbs in the Allihn condenser. The second inlet was through an 8 mm inside diameter concentric tube that tapered to a 4 mm inside diameter tip at the outlet. Turbulence and mixing were dramatically increased in this reactor as a result of the bulbs in the Allihn condenser and the increased gas velocity caused by the tapered tip of the second inlet Tests showed no significant variation in N 0 production rate when the ozone and N 0 were introduced through either inlet Therefore, to prevent unnecessary back pressure on the ozone generator, the N 0 was introduced through the tapered inlet. The operation of this apparatus was also semi-continuous with the N 0 supplied from a two liter stainless steel pressure vessel which was filled with liquid N 0 prior to each operation. The pressure vessel was maintained in a steam heated water bath with the temperature controlled at 37°C. The feed rate of N 0 was controlled by adjusting a stainless steel needle valve. This valve provided better flow control than the glass stopcock used with the previous apparatus. An optimized production rate of 86.0 grams per hour was obtained using this apparatus. This result was obtained by operating the ozone generator near its maximum capacity and feeding as much N 0 as possible while ensuring complete conversion to N 0 . To achieve the optimized production rate, the model GL-1 ozone generator was operated with an oxygen feed rate of 20 SCFH at 13 psig. As before, the N 0 was collected in two glass traps suspended in a dry ice and acetone solution. This apparatus was considered suitable for scale-up and the pilot plant scale process was developed based on this model. 2

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Tests indicated that when no external cooling water was running, the reactor temperature reached a maximum of 73°C at the first thermal well, and rapidly cooled to ambient temperature in the later thermal wells. The majority of tests were conducted with cooling water running to maintain the temperature between 50 and 60°C at the first thermal well of the reactor. Measurements of the cooling water flow rate and the inlet and outlet temperatures provided a rough estimate of the heat of reaction. A value 246 calories per gram of N 0 produced was determined in this manner which compares with a value of 294 calories per gram using handbook heats of formation. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on May 20, 2013 | http://pubs.acs.org Publication Date: April 24, 1996 | doi: 10.1021/bk-1996-0623.ch008

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Pilot Plant Scale Process Development The introduction of the plug flow reactor provided a method of manufacturing N 0 that was more appropriate for scale-up. However, it was not possible to determine the maximum capacity of the plug flow reactor or a scaling factor, because the model GL-1 ozone generator was operating at its maximum capacity during most of the bench top scale tests. For the pilot plant scale process four reactors identical to the one used for the bench top scale process were installed in parallel to provide the required capacity. In this manner various production rates less than the maximum capacity could also be accommodated. The reactors are cooled externally by water. Figure 4 illustrates the pilot plant scale process. 2

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Ozone Generation. One of the earliest tasks undertaken was to purchase an ozone generator capable of providing sufficient ozone to meet the 2.5 kilogram per day N O manufacturing requirement. Calculations showed the amount of ozone required to be approximately one pound per hour. The ozone generator selected was a PCI Ozone and Control Systems model G-28S which produces ozone by corona discharge as previously described at an approximate rate of 28 pounds per day with a concentration of 8% ozone in oxygen. The model G-28S ozone generator is operated with an oxygen feed rate of 120 SCFH at 15 psig. At the time it was purchased, this ozone generator was the largest capacity generator available that was not custom built. Several oxygen cylinders are connected to a manifold which supplies the ozone generator. This ensures continuous operation while empty oxygen cylinders are replaced by full cylinders. An ozone monitor was purchased for use with the model G-28S ozone generator to provide continuous display of the ozone concentration in oxygen as it is fed to the N O reactors. The sampling system consists of inlet valves for the sample and zero gas along with a solenoid valve which changes position to allow either the sample or zero gas into the sample chamber. The U V absorption of the gas in the sample chamber is measured and the ozone concentration is calculated using Beer's Law. The ratio of intensities is determined and the resulting ozone concentration displayed. Since the concentration determined by the photometer is based on the ratio of light intensities, the actual intensity of the light is not important 2

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N 0 Supply. A continuous N 0 feed system was achieved by applying moderate heat to the bulk storage cylinder using electric heat tape and a temperature controller with a set point between 30 and 35°C. This induces flow of liquid N 0 from the cylinder bottom outlet (dip tube) into an expansion chamber. The expansion chamber is an empty two liter pressure cylinder maintained in a steam heated water bath with the 2

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NITRATION


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Figure 3. Plug Flow Reactor N 0 Process. 2

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Figure 4. Pilot Plant Scale N O Process. 2

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temperature controlled between 35 and 40°C. The N 0 is vaporized in the expansion chamber generating approximately 10 psig pressure which is used to drive the N 0 into the four reactors. An advantage of this feed system was the elimination of the need to handle the dangerous liquid N 0 because the small feed cylinders no longer were required. Both the N 0 feed rate and the ozone feed rate to each of the four reactors is controlled by adjusting a stainless steel needle valve and by observing the accompanying flow meter. The typical ozone in oxygen feed rate to each reactor when all four are being used is 30 SCFH with an accompanying N 0 feed rate of 1.0 SCFH. An individual reactor can be shut down and the flow of ozone and N 0 diverted to the other three reactors if a problem arises. Individual reactors have been operated successfully with an ozone in oxygen feed rate of as much as 60 SCFH and an accompanying N 0 feed rate of 2.0 SCFH. 2

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Solid N 0 Trapping. The N O collects at an approximate rate of 360 grams per hour as a solid in glass traps suspended in a dry ice and acetone solution. Four trains in parallel of three glass traps each in series are used to collect the N 0 when operating at maximum capacity. The flow rate of N 0 to each of the four trains of traps is controlled by adjusting a stainless steel needle valve and observing the accompanying flow meter. This ensures a balanced quantity of N 0 collects in each of the traps. An individual train of traps can be shut down and the flow of N 0 diverted to the other three trains if a problem occurs or if full traps need to be replaced by empty ones. Operating by this technique the pilot plant scale manufacture of N 0 is essentially continuous and has in fact operated non-stop for 24 or more hours on several occasions. The solid N 0 manufactured is stored in a low temperature freezer at -70°C. When needed, the required amount of solid N 0 is dissolved in a solvent for use as a nitrating agent There are several advantages to producing solid N O . First, is the flexibility of being able to choose a particular diluent solvent because the N 0 is not manufactured in solution. Second, is the reduced volume needed for low temperature storage. Finally, N 0 dissolved in a solvent requires higher storage temperatures to prevent the N 0 from precipitating out of solution, or temperature cycling to re-dissolve the N 0 prior to use. Because N O is thermally unstable, storage at higher temperatures and/or temperature cycling will increase the rate of N 0 decomposition and shorten the shelf life of the solution. 2

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Analytical Determination of N 0 Solutions. To support N 0 manufacturing, an analytical method for determining the shelf life NjO^methylene chloride solutions was developed (6). This method uses Fourier Transform Infrared spectroscopy (FTIR) using 0.1 mm fixed path length liquid K B r cells to determine the concentration of N 0 , N 0 , and nitric acid. The absorbence maxima for N 0 is 560 c m and for N 0 is 428 cm* . Calibrations curves were established and the decomposition of N 0 with time was studied at 0°C. An estimate of the quality of N O in solution with methylene chloride was obtained using a Milton Roy Company "Spectronic 20" UV/visible light spectrophotometer. Solutions with a known weight of N 0 in methylene chloride 2

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N204 Concentration In Methylene Chloride (%)

Figure 5. N 0 Concentration vs. Absorbence. 2

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A calibration curve (Figure 5) was constructed using standard regression techniques. A 15 % solution of N O in methylene chloride was tested using this technique and the minimum purity of the product determined to be 93.9 % (92.9 mol %) N 0 and 6.1 % (7.1mol%)N 0 . 2

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Conclusion The Indian Head Division, Naval Surface Warfare Center (NSWC), currently is capable of manufacturing solid dinitrogen pentoxide ( N 0 ) at the pilot plant scale. The N 0 is produced by the gas phase oxidation of dinitrogen tetroxide ( N 0 ) with ozone (0 ). During the past two years the rate of N O rnanufacturing improved by an order of magnitude, from approximately 35 grams per hour to the current rate of 360 grams per hour. This represents a significant improvement in the cost of N O . In addition to developing a process to manufacture N O in the gas phase with a 2.5 kilogram per day capacity, the other objectives of the sponsor were met. These included transitioning the technology of N O gas phase synthesis from the White Oak Detachment to the Indian Head Division, and manufacturing approximately 60 pounds of N 0 to support the parallel development of a novel nitration processes. The Indian Head Division is one of two domestic sources of solid N 0 . The advantages of solid N 0 are greater flexibility in preparing various solutions in a particular solvent, reduced storage volume at low temperatures, and the ability to store the solid product for indefinite time periods at temperatures below -70°C.


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Acknowledgments The authors wish to express their appreciation to Dr. Richard Miller of the Office of Naval Research who funded this work. They would also like to thank Walter Carr, Ricky Cox, Tim Dunn, Jody Lang, and A l Stern for their assistance in developing and operating the facility. References 1. 2. 3. 4. 5. 6.

Harris, A. D.; Trebellas, J. C.; Jonassen, H. B. Inorganic Synthesis; McGrawHill: New York, NY, 1950; Vol 9.; pp 83-88. Guye, P., United States Patent 1,348,873, 1920. Gruenhut, N. S.; Goldfrank, M.; Caesar, G. V.; Cushing, M. L. Inorganic Synthesis; McGraw-Hill: New York, NY, 1950; Vol 3.; pp 78. Caesar, G. V.; Goldfrank, M. J. Am. Chem. 1946, vol 68, pp 372-375. Russ, F.; Pokorny, E. Monatshefte Fur Chemie. 1913, vol 34, pp 1051-1060. Smith, V. G. JANNAF Propellant Development and Characterization Subcommittee Meeting; CPIA: Columbia, MD, 1994; No. 609.

RECEIVED February 29, 1996


Pilot-Plant-Scale Continuous Manufacturing of Solid Dinitrogen

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