Production of 15N-enriched nitric acid (H15NO3)

KINETICS AND CATALYSIS; REACTION ENGINEERING; AND MATERIALS SCIENCE

Production of ¹⁵N-enriched nitric acid (H¹⁵NO₃)

C. R. Sant Ana Filho^I,*; J. A. Bendassolli^I; A. L. R. M. Rossete^I; S. M. S. Piedade^II; C. V. Prestes^I

^ICentro de Energia Nuclear na Agricultura, CENA, Universidade de São Paulo, USP, Avenida Centenário 303, C.P. 96, CEP: 13.400-970, Piracicaba - SP, Brazil. E-mail: [e-mail], E-mail: [e-mail]

^IIDepartamento de Ciências Exatas, Escola Superior de Agricultura Luiz de Queiroz, Universidade de São Paulo, USP, Avenida Pádua Dias 11, C.P. 9, CEP: 13418-900, Piracicaba - SP, Brazil

ABSTRACT

Techniques that employ ¹⁵N have proved to be an important tool in many areas of the agronomic and biomedical sciences. Nevertheless, their use is limited by methodological difficulties and by the price of compounds in the international market. Nitric compounds (¹⁵NO₃^-) have attracted the interest of researchers. However, these compounds are not currently produced in Brazil. Thus, in the present work H¹⁵NO₃ was obtained from the oxidation of anhydrous ¹⁵NH₃. The method we used differs from the industrial process in that the absorption tower is replaced with a polytetrafluoroethylene-lined, stainless-steel hydration reactor. The process output was evaluated based on the following parameters: reaction temperature; ratio of reagents; pressure and flow of ¹⁵NH_3(g) through the catalyst (Pt/Rh). The results showed that, at the best conditions (500 ºC; 50 % excess O₂; 0.4 MPa; and 3.39 g.min^-1 of ¹⁵NH₃), a conversion percentage (N-¹⁵NH₃ to N-¹⁵NO₃^-) of 62.2 %, an overall nitrogen balance (N-¹⁵NH₃ + N-¹⁵NO₃^-) of 86.8 %, and purity higher than 99 % could be obtained.

Keywords: Nitric compounds; Catalyst; Reactor; Stable isotope.

INTRODUCTION

Nitrogen is considered to be a limiting factor for primary production, both on land and in the aquatic environment, since it is the main component of enzymes that control the biochemical reactions in which carbon is reduced and oxidized (Schelsinger, 1991). In most studies involving stable isotopes (tracer), isotopes of light elements are used, such as nitrogen and its isotopes, which have great importance in biological systems studies, and also take part in most geochemical reactions (Fritz and Fontes, 1989).

Under natural conditions, nitrogen has two stable isotopes, ¹⁴N and ¹⁵N, with isotopic abundances of 99.73 and 0.37% in atoms, respectively (Weast, 1998). The existence of the heavy nitrogen isotope (¹⁵N) allows the production of compounds enriched in this isotope, characterized by having isotopic abundances above those naturally found. These compounds, in turn, allow the isotopic technique to be used in many research studies, and allow many of the aspects involved in the nitrogen cycle to be elucidated, in both the agronomic (soil-plant-atmosphere system) (Trivelin et al., 2002) and biomedical fields (Dichi et al., 1998).

The isotopic technique that uses ¹⁵N as tracer basically consists of supplying the organism under study with a chemical compound in which the isotopic ratio (¹⁵N/¹⁴N) of the compound under consideration is different from that naturally found, and then evaluating the isotope's distribution in the system under study. In many research studies involving the ¹⁵N isotopic technique in the agronomic and biomedical fields, nitrogen compounds highly enriched in the isotope have to be used, due to the isotopic dilution of the systems under study (Knowles and Blackburn, 1993). Until recently, these compounds were not produced in Brazil and South America, due to difficulties of a methodological nature. Such compounds had to be imported from the United States, Europe, or Asia at high prices, which varied proportionally depending on enrichment and type of compound. H¹⁵NO₃ (50% m/v solution) enriched at 95 % in atoms of ¹⁵N has a FOB price on the order of U$90.00 (ninety dollars) per gram on the international market.

Consequently, the production of ¹⁵N-labeled nitric compounds is being requested by researchers to be used in agronomic investigations, particularly for factors that allow information to be obtained on the nitrogen cycle. Within this context, we could highlight the marked mobility of nitrogen, with emphasis on the occurrence of losses caused by volatilization (loss in gaseous forms), biological immobilization, erosion, and leaching.

The technology of isotopic separation and production of ¹⁵N-enriched compounds is not shared by other countries, due to economic or even strategic factors. In this respect, some studies have been developed in Brazil (Maximo et al., 2000; Bendassolli et al., 2002; Bendassolli et al., 1988; Bendassolli et al., 1989; Bendassolli, 1991; Tavares et al., 2006; Maximo et al., 2005; Oliveira, 2000; Tavares, 2005).

From the above, considering the control over the methodology of isotopic separation and the importance of making other ¹⁵N-enriched nitrogen compounds available for the country's researchers at lower-than-FOB prices, the objective of this study was the production (in batches) of ¹⁵N-enriched H¹⁵NO₃, and to evaluate the parameters (reaction temperature, ratio between reagents, system pressure and flow of ¹⁵NH_3(g) through the catalyst) involved in the process.

EXPERIMENTS

Gases (O₂, N₂, NH₃) and Isotope (¹⁵N)

Oxygen (O₂) 5.0 (99.999 %) and commercial nitrogen (N₂) (97 to 98 %) were used. The tests, aimed at evaluating the parameters involved in the process, were performed using NH₃ (1,8 g) with natural isotopic abundance (0.366 % in atoms of ¹⁵N), and purity in the order of 99 % (2.0). Anhydrous ¹⁵NH₃ enriched to 90 % in atoms of ¹⁵N (Bendassolli, et al., 2002), was used as an isotopic source for the production of H¹⁵NO₃.

H¹⁵NO₃ Production Process

The complete H¹⁵NO₃ production system is represented schematically in Figure 1, while Figure 2 shows in detail the gas loading system in the mixing reactor.

Figure 2

Using this value and the stoichiometric ratio (eq. 1), we determined the amount (mols) of O₂ to be transferred into the mixing reactor. This amount was admitted by V₁ and controlled by the reactor's pressure gauge, using the ideal gas equation. When needed, the inert gas N₂ was added (V₁) into the same reactor in order to maintain pressure in the system at the desired conditions.

Next, the mixture of gases (¹⁵NH₃, O₂, and N₂) in the mixing reactor was heated to 110 ºC on a hot plate (Figure 1) for 10 minutes. At the same time, the catalyst was heated in an infrared oven equipped with a temperature controller. After heating both parts of the system, V₂ was opened and the gases were conducted through a stainless steel pipe (diameter ¼ inch) until reaching contact with a fine screen (8g), disposed in several layers, consisting of platinum (90 %) and rhodium (10 %). In this step, the anhydrous ¹⁵NH₃ was catalytically oxidized and converted into ¹⁵NO, according to Ostwald's industrial method (Tageder and Mayer, 1980). The velocity of the catalytic reaction is very rapid, giving conversion in a short contact time (3.10^-4s) (Swaddle, 1997).

Next, the ¹⁵NO containing excess O₂, needed for the oxidation process, was cooled by a cooling coil (S_R) (Figure 1), using H₂O as cooling fluid. As the resulting gases cool down, the ¹⁵NO formed reacts with the O₂ present in excess, forming ¹⁵NO₂. This reaction is a slower oxidative process, but equilibrium is favored by a reduction in temperature (Shreve and Brink, 1977). In this process, the formation of ¹⁵N₂O₄ may occur.

Finally, the V₃ valve was opened, displacing the gases into the stainless steel reactor (2 L) at a constant flow, where most of the ¹⁵NO not yet oxidized to ¹⁵NO₂ was then converted by the excess O₂. Next, when the pressures at the M₁ and M₂ manometers became equal, two procedures were employed to evaluate the effectiveness of the production process. In the first procedure, which used a complete flow of gases, valve V₄ was opened and the gases were exhausted (30 minutes), passing through a recovery system. Finally, valves V₄, V₃, and V₂ were closed, in this order. In the second procedure, which used partial flow of gases, valves V₃ and V₂ were closed, in this order, and valve V₄ was opened, causing only the hydration reactor gases to be exhausted, passing through a recovery system. Finally, valve V₄ was closed, and the gases that were in the mixing reactor, containing mainly anhydrous ¹⁵NH₃, were recovered to be reused in the process.

In the reactor, ¹⁵NO₂ was absorbed in H₂O, forming H¹⁵NO₃ at a variable concentration, depending on the evaluated parameters. The process used here is different from the industrial method (Tageder and Mayer, 1980) with respect to the replacement of the absorption tower with a stainless steel reactor (internally lined with polytetrafluoroethylene) containing refrigerated H₂O (2 ºC), in order to hydrate the ¹⁵NO₂. This absorption reaction is favored because of the reduced volume in the production system and the use of high pressure values (0.80 to 1.2 MPa), according to Le Chatelier's principle (Shreve and Brink, 1977). In the reactor, the reactions of ¹⁵NO₂ absorption in H₂O and ¹⁵NO oxidation with residual O₂ were processed simultaneously (Cekinski, 1990).

After absorption, the exhaust gases, possibly containing gases such as N₂, residual O₂, ¹⁵NO, and ¹⁵NO₂, underwent an oxidation process using H₂O₂, at the exit of the hydration reactor. These exhaust gases may also contain possible traces of ¹⁵NH₃ not converted in the process, which is neutralized in a 2.5 mol L^-1 H₂SO₄ solution.

At the end of the process, the hydration reactor was opened and the solution (H¹⁵NO₃) contained inside it was transferred to a proper reservoir (amber flask); the N content (N-NH₄⁺ and N-NO₃^-) was then determined by distillation and titration (Trivelin et al., 1973). This procedure was also used to analyze the solutions obtained from the recovery system, in order to obtain the overall nitrogen balance (in relation to initial ¹⁵NH₃). The isotopic determination (% in atoms of ¹⁵N) to verify the isotopic abundance of the final product was performed in an ATLAS MAT model CH4 mass spectrometer (Mulvaney, 1993).

Experimental Design

The process output for H¹⁵NO₃ production was verified based on the following parameters: reaction temperature (350 to 900 ºC); ratio between reagents (Stoichiometric, 25, 50, and 100 % excess O₂) and flow of gases (total or partial).

The best conditions regarding reaction temperature, ratio between reagents, and flow of gases were used to evaluate process output in relation to the following parameters: flow of NH₃ through the catalyst (0.02; 0.04; 0.20; 0.62; and 3.39 g.min^-1) and system pressure (0.2; 0.34; 0.49; 0.64; and 0.78 MPa).

The tests were conducted with three replicates, according to a completely randomized experimental design, in factorial combination. The means were submitted to statistical analysis (Tukey 5 %).

Finally, tests were run with three replicates, aimed at the production of H¹⁵NO₃ enriched at 90 % in atoms of ¹⁵N, at the best conditions for the (physicochemical) parameters evaluated.

RESULTS AND DISCUSSION

The statistical analysis of data on temperature and reagent amounts showed an effect of these factors and their interactions on the H¹⁵NO₃ production process. Table 1 presents data on mean conversion output (N-NH₃ to N-NO₃^-) and overall nitrogen balance (in relation to initial NH₃), as a function of reaction temperature and excess amount of O₂, using total flow of gases in the mixing reactor. The best results were obtained at temperatures of 500, 600, and 900 ºC for 50 and 100 % excess O₂. Although no statistical differences were observed among the results, the increase in reaction temperature was disadvantageous, due to the fact that during the tests a reduction in catalytic activity was verified: the catalyst had a usable life of 150 batches (tests). This phenomenon, defined as deactivation, is a strong chemisorption of reagents and impurities on the active sites of the catalyst, causing changes on the metal surface. Therefore, the best conditions for the production system were obtained when a temperature of 500 ºC was used for 50 and 100 % excess O₂.

It was also verified that the mean overall nitrogen balance results (Table 1) at 500 ºC and O₂ excess values between 50 and 100 % showed a 2 % difference, which is equivalent to R$20.00 per batch, when enriched ¹⁵NH₃ is used (90 % in atoms of ¹⁵N), while the addition of 50 % excess O₂ has an additional cost of R$0.16. Therefore, the best conditions were obtained when 100 % excess O₂ was used at 500 ºC. Under these conditions, the conversion of 47.5 % was possible, and an overall nitrogen balance of 51.2 % was achieved. The difference to complete the nitrogen mass balance refers to actual losses, which might be related to the ineffectiveness of the ¹⁵NO₂ hydration step and the influence of residence time of the gases with the catalyst's active metal (Pt), causing undesirable reactions in the process (N₂ and N₂O formation).

The temperature variation (400 to 900 ºC) results indicated a low conversion output for temperatures of 400, 700, and 800 ºC, using total flow of gases in the mixing reactor. The low conversion obtained at 400 ºC can be attributed mainly to the ineffectiveness of the 1st oxidation (NH₃ to NO), as demonstrated by the overall nitrogen balance values (Table 1).

In order to maintain high pressure in the system, a partial flow of gases was used in the mixing reactor. The mean conversion and overall nitrogen balance results as a function of the amount of O₂ (0, 25, 50, and 100 % excess O₂) and reaction temperature (350, 500, 650, and 800 ºC), with three replicates, are presented in Table 2.

It can be seen that the best results (conversion and overall nitrogen balance) were obtained at temperatures of 500 and 650 ºC for 0 and 50 % excess O₂, without statistical differences. However, with regard to the means (Table 2), the best system operation condition was observed with the use of 50 % excess O₂ at 500 ºC, since the use of high temperatures in the catalysis oven may increase the catalyst's deactivation rate (usable lifetime). Another important fact that justifies the best conditions herein defined was the 5 % difference in the mean overall nitrogen balance results (Table 2) for excess O₂ values between 0 and 50 %. This difference is equivalent to R$51.00 per batch, using enriched ¹⁵NH₃ (90 % in atoms of ¹⁵N), while the addition of 50 % excess O₂ in the production reaction costs R$0.16. At the best conditions, a conversion of 55.2 % was possible, and an overall nitrogen balance of 82.9 % was achieved. The difference to complete the mass balance refers to actual losses in the system, which were lower (about three times) than those obtained under total flow of gases. The reasons for these losses were an ineffective 2nd oxidation (NO to NO₂) and the occurrence of undesirable reactions (production of N₂ and N₂O), considering that no NH₃ was quantified in the gases exhausted from the hydration reactor.

By comparing the results in Tables 1 and 2, decreases in conversion output and overall nitrogen balance can be observed when low temperatures (350 to 400 ºC) as well as high temperatures (700 and 800 ºC) were used, in both gas flow conditions at the mixing reactor. However, at intermediate temperatures (500, 600, and 650 ºC), superior results were observed, especially at 500 ºC, where the highest process output was obtained. This result was obtained using partial flow of gases in the mixing reactor, with the addition of 50 % excess O₂ in the reaction (eq. 1).

Using the best conditions (temperature of 500 ºC, 50 % excess O₂ and partial flow of gases) established in the process and a mean mass of 1.8g NH₃ (0.366 % in atoms of ¹⁵N), tests were carried out with three replicates to evaluate process output as a function of NH₃ flow through the catalyst and production system pressure. The gases were conducted with exhaust time of 30 minutes (hydration reactor).

Under these conditions, there was an influence on conversion output and on overall nitrogen balance as a function of NH₃ flow (0.02; 0.04; 0.20; 0.62; and 3.39 g.min^-1) through the catalyst. The results obtained (Table 3) demonstrated a significant difference (P<0.05) in conversion output for all times evaluated. However, it can also be observed that there were no statistical differences for overall nitrogen balance results at flow values of 0.62 and 3.39 g.min^-1, which were statistically different from other values. Consequently, the best conversion results were obtained using a NH₃ flow of 3.39 g.min^-1. Under this condition, a conversion of 62.2 % was possible, and an overall nitrogen balance of 86.8 % was achieved. Flow rates higher than 3.39 g.min^-1 were not studied due to operational limitations of the system (piping diameter).

An influence of pressure (0.20 to 0.78 MPa) on conversion output was also verified. The results obtained did not show significant differences (P<0.05) for pressure values from 0.20 to 0.64 MPa, for which mean conversion output was 55.2 ± 2.3 %, achieving an overall balance of 82.9 ± 1.7 %. However, there was a statistical difference at a pressure value of 0.78 Mpa; such reduction is mainly due to an operational difficulty of the system at high pressure values, near the reactor's safety valve limit of 0.98 MPa.

The entire production process (batch) involving all steps described, using 1.8g anhydrous ¹⁵NH₃, was performed in 60 min. Therefore, based on the best conversion result (62.2 %) and using previously enriched ¹⁵NH₃, the production line has a daily capacity to obtain 1,020 mL of 0.05 mol L^-1 H¹⁵NO₃ solution (approximately 3.2g) enriched at 90 % in atoms of ¹⁵N. The amounts produced (g), corresponding to two batches, can be considered adequate, since most research studies involving the ¹⁵N isotopic technique use compounds enriched at about 5 % in atoms of ¹⁵N. As a result, starting from 3.2g H¹⁵NO₃ enriched at 90 % in atoms of ¹⁵N and using the isotopic dilution technique, we were able to produce 62g H¹⁵NO₃ enriched at 5 % in atoms of ¹⁵N. The isotopic enrichment of H¹⁵NO₃ occurs because of the ¹⁵N isotopic concentration in the reagent (anhydrous ¹⁵NH₃), since no isotopic fractionation was verified in the conversion from N-¹⁵NH₃ into N-¹⁵NO₃.

CONCLUSION

The best conditions for H¹⁵NO₃ production were: catalysis oven temperature of 500 ºC; 50 % excess O₂; use of partial flow of gases in the mixing reactor; pressure of 0.4 Mpa and NH₃ flow through the catalyst of 3.39 g.min^-1. Under these conditions, a conversion of 62.2 % (N-NH₃ into N-NO₃^-) was possible and an overall nitrogen balance (N-NH₃ + N-NO₃^-) of 86.8 % was achieved and 510 mL of 0.05 mol L^-1 H¹⁵NO₃ solution (90 % in atoms of ¹⁵N) per batch were obtained, starting from 1.8g anhydrous ¹⁵NH₃.

ACKNOWLEDGMENT

To the Stable Isotopes Laboratory for technical support during the study, to Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) for funding the research project, to project designer João Geraldo Brancalion, and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for granting a scholarship.

(Received: August 20, 2007 ; Accepted: February 18, 2008)

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Publication Dates

• Publication in this collection
  24 Nov 2008
• Date of issue
  Dec 2008

History

• Accepted
  18 Feb 2008
• Received
  20 Aug 2007