Synthesis of 15N-enriched urea (CO(15NH2)2) from 15NH3, CO, and S in a discontinuous process

KINETICS AND CATALYSIS; REACTION ENGINEERING; AND MATERIALS SCIENCE

Synthesis of ¹⁵N-enriched urea (CO(¹⁵NH₂)₂) from ¹⁵NH₃, CO, and S in a discontinuous process

C. R. Sant Ana Filho^*; A. L. R. M. Rossete; C. R. O. Tavares; C. V. Prestes; J. A. Bendassolli

Center for Nuclear Energy, University of São Paulo - CENA/USP, Stable Isotopes Laboratory, Phone: + (55) (021) (19) 34294770, Fax: + (55) (021) (19) 34294641, PO Box 96, Zip Code: 13400-970, Piracicaba - SP, Brazil. E-mail: [e-mail]

ABSTRACT

CO(¹⁵NH₂)₂ enriched with the stable isotope ¹⁵N was synthesized based on a reaction involving CO, ¹⁵NH₃, and S in the presence of CH₃OH. The method differs from the industrial method; a stainless steel reactor internally lined with polytetrafluoroethylene (PTFE) was used in a discontinuous process under low pressure and temperature. The yield of the synthesis was evaluated as a function of the parameters: the amount of reagents, reaction time, addition of H₂S, liquid solution and reaction temperature. The results showed that under optimum conditions (1.36, 4.01, and 4.48 g of ¹⁵NH₃, CO, and S, respectively, 40 ml CH₃OH, 40 mg H₂S, 100 ºC and 120 min of reaction) 1.82 g (yield 76.5%) of the compound was obtained per batch. The synthesized CO(¹⁵NH₂)₂ contained 46.2% N, 0.55% biuret, melting point of 132.55 ºC and did not exhibit isotopic fractionation. The production cost of CO(¹⁵NH₂)₂ with 90.0 at. % ¹⁵N was US$ 238.60 per gram.

Keywords: Reactor; Stable isotope; ¹⁵N; Enriched compounds.

INTRODUCTION

Urea (CO(NH₂)₂) is a solid product in the shape of pearls or grains; its main characteristic is N in the form of an amide (NH₂). The compound is commercially synthesized by a reaction of ammonia (NH₃) and carbon dioxide (CO₂) under conditions that are dependent on the technology employed in the industrial plant. In the majority of the processes, the synthesis reaction is performed in the liquid phase (solution) under a pressure of 13 to 25 MPa and at a temperature of 170 to 200 ºC. The process requires special equipment because of the working conditions and specific characteristics of the reaction (high corrosiveness and tendency to crystallize). In addition, urea synthesis is a multi-stage process that involves a large capital investment and consumes a large amount of energy (Karmazinov et al., 1971; Macdowell Junior, 1974; Kucheryavyi et al., 1976). A complete survey of the production technologies (operational systems and equipment) has been published by Chao (1967) and later by Uchino (1986) and Stamicarbon Staff (1986).

Currently, CO(NH₂)₂ production is a very important process that is practiced in various countries, and there are approximately 700 plants of different capacities. Worldwide production has reached 134.7 million tons per year, of which 49% is produced in China and India (Franco et al., 2007). CO(NH₂)₂ synthesis from NH₃ and CO₂, which are produced in the same unit, makes the product less onerous because the compound contains a high N content (46%) compared with other nitrogen fertilizers. This high nitrogen content provides an attractive price per ton.

The synthesis of CO(NH₂)₂ in a discontinuous system involving a chemical reaction between CO (carbon monoxide), NH₃ (ammonia), and S (sulfur) at a temperature of 100 ºC in the presence of CH₃OH (methanol) is an alternative method for urea production compared to the conventional process. This procedure was initially performed on a laboratory (or pilot) scale in an A-4 type reactor (spec. 94-40376) with a volume of 1.8 L (Franz and Applegath, 1961). The influence of temperature and amount of reagents on the yield in this method was evaluated. According to the results, a decrease in the reaction temperature from 100 to 70 ºC promoted a small increase in the reaction time of S in the formation of COS (carbonyl sulfide); however, it produced a significant increase in the production time of CO(NH₂)₂. Amounts of 10.9 g of NH₃, 9.8 g of S,8.7 g of CO, 0.6 g of H₂S, and 35 ml of CH₃OH were used, and approximately 13.5 g of CO(NH₂)₂ was produced, which represents a 73% yield.

An alternative method has been used to synthesize CO(NH₂)₂ under mild temperature (100 ºC) and pressure (1.4 to 2.07 MPa) conditions in a continuous industrial process (Monsanto Chemical Company, 1961). The reaction between NH₃, CO, S, CH₃OH, and H₂S (hydrogen sulfide) was performed in a stainless steel reactor in a countercurrent system with recycling. The product (CO(NH₂)₂), which was obtained in the crystalline form, displayed a chemical purity of 99.6%, and H₂S was produced as a by-product. The H₂S was converted to S (sulfur) in a recovery-and-reuse plant. The feed solution composition that displayed the most satisfactory results consisted of 20.72% CO, 20.45% NH₃, 3.1% H₂S, 15.22% S, and 40.51% CH₃OH. Under optimal operating conditions, the process yield in terms of NH₃ and S consumption was approximately 99% and for CO between 60-75%.

Studies on the preparation of CO(¹⁵NH₂)₂ enriched with ¹⁵N have been performed using an alternative method, and the synthesis reaction between ¹⁵NH₃, CO, and S under low pressures and temperatures has been investigated. The process was performed in a stainless steel reactor with a volume of 2 L that was equipped with a heated jacket, a thermometer, and a glass container that was used as the reaction vessel. A magnetic agitation system was used to avoid the loss of reagent gases (¹⁵NH₃ and CO). The reaction temperature was 100 ºC at a pressure of 1.22 to 1.91 MPa with a reaction time of 3 h. The effects of an increased reaction time and the addition of H₂O at the end of the process were investigated. The yield of the base process (1 mol of ¹⁵NH₃, 1 mol of CO, 0.75 mol of S, a temperature of 100 ºC, and a reaction time of 3 h) was 83%; the final product contained 0.1% biuret and 0.45-0.5% S, which would likely not present any problems in using the product (Stinson, 1975).

In Brazil, the synthesis of ¹⁵N-labeled urea has been evaluated with a low enrichment (5 at. % ¹⁵N) from the reaction between ¹⁵NH₃, CO, and S. In this study, a stainless steel reactor was used with a heating jacket, and a stainless steel manometer was used to determine the pressure of the system and regulate the amount of CO introduced into the reactor. Only a reaction temperature of 90 ºC was evaluated for 90 min, and the obtained CO(¹⁵NH₂)₂ exhibited a small degree of contamination of heavy metals from the walls of the reaction vessel (Bendassolli et al., 1989).

The thermodynamic aspects of CO(NH₂)₂ syntheses were evaluated using CO, S, and NH₃ as reagents. From this analysis, theoretical results were obtained for the conversion (%) of NH₃ to CO(NH₂)₂ as a function of the reaction temperature and for the equilibrium constant of the system at various temperatures (Bendassolli and Victoria, 1995). Although thermodynamic analysis indicated that the conversion of NH₃ to CO(NH₂)₂ was inversely proportional to the temperature, this result does not explain the kinetics of the reaction. Thus, at room temperature (25 ºC), an extremely long time is required to reach the reaction equilibrium (92.5% conversion).

Nitrogen and its isotopes are of great importance in the study of biological systems and participate in the majority of biogeochemical reactions (Fritz, 1989). In various fields of science, mainly in agricultural sciences (the fertility and chemical composition of the soil and plant nutrition) and in animal nutrition (IAEA, 1974), the isotopic technique employing ¹⁵Nlabeled urea has proven to be an important tool. A few studies published by Brazilian researchers are noteworthy (Gava et al., 2006; Faroni et al., 2007; Oliveira et al., 2007; Lange et al., 2009; Martha Junior et al., 2009). It should also be emphasized that there is an international tendency toward the use of stable isotopes in field studies whenever possible, which avoids the use of radiolabeled compounds in experiments of this nature (Zhao et al., 2001; Trivelin et al., 2002; Bendassolli et al., 1997).

A need exists in many research studies for the use of compounds that are highly enriched with the isotopes of interest because of isotopic dilution in the studied systems (Knowles and Blackburn, 1993). Until recently, these compounds were not produced in Brazil or South America because of methodological complexities. Research involving these compounds requires that they be imported from the United States, Europe, or Asia (Sant Ana Filho et al., 2008).

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 synthesis (in batches) of ¹⁵N-enriched CO(¹⁵NH₂)₂ (90 at. % ¹⁵N), and to evaluate the parameters (amount of reagents, temperature, reaction time, addition of H₂S, and liquid solution) involved in the production process.

EXPERIMENTS

Gases (N₂, NH₃, and CO), Reagents and Isotopes (¹⁵N)

Carbon monoxide (CO), ammonia (NH₃), and nitrogen (N2), all with a chemical purity of 99.0%, were used. The tests to evaluate the parameters involved in the process were performed using NH₃ with a natural isotopic abundance (0.366 at. % ¹⁵N) and an analytical grade reagent (chemical purity >99.5%).

Anhydrous ¹⁵NH₃ enriched with 90 at. % ¹⁵N, which was produced in the Laboratory of Stable Isotopes of CENA/USP (Bendassolli et al., 2002), was used as the isotopic source for the synthesis of CO(¹⁵NH₂)₂.

Synthesis of CO(¹⁵NH₂)₂

Production of Hydrogen Sulfide (H₂S)

Figure 1 shows the complete system (vacuum line, reaction tube, and glass reservoir) used in the process for obtaining H₂S. The reagents (0.2g of FeS and 2.5 ml of H₂SO₄ (50% v/v)) were added in different sides of reaction tube (TR). TR was isolated with a tap (T₁₃).

The glass reservoir (R₁) for the H₂S storage and tube T_R were connected by a vacuum line, as shown in Figure 1. The vacuum was activated using a mechanical pump (BM) by opening, in sequence, taps T₂, T₃, T₄, T₆, T₁₃, T₇, and T_12A. After reaching approximately 10^-3 MPa of pressure, T6 and T13 were closed. T_R was removed and the reagents (FeS and H₂SO₄) were placed in contact with each other (reaction 1).

In sequence, T_R was returned to the vacuum system. The taps T₆ and T₁₃ were opened and the H₂S gas was transferred to R1. After this process, the taps T_12A, T₇, T₆, T₁₃, T₄, T₃, and T₂ were closed, and the BM was turned off. R₁ and T_R were removed from the vacuum system and the residual solution, contained H₂SO₄, FeS, and FeSO₄ (byproduct of the reaction in T_R), was stored in an appropriate container, which was sent to a chemical waste management program.

The Process of Reactor Charging

Initially, sulfur (S) was added to the stainless steel reactor (internally lined with polytetrafluoroethylene PTFE). The S amount (g) added was a function of the limiting reagent (¹⁵NH₃ or CO) and based on the reaction stoichiometry (2). The reactor was closed and connected to the vacuum system (Figure 1), where vacuum was produced with a BM opening, in this order, T₂, T₃, T₄, T₈, T₉, and valve V1 until 10^-3 MPa of pressure was attained. After that, valve V1 was closed, the reactor was placed in a cooling bath (ethanol and CO₂(s)) at a temperature of -74 ºC. This procedure was performed to facilitate the transfer of ¹⁵NH₃, CO, and H₂S (gases) and CH₃OH (liquid solution) to the interior of the reactor.

The reservoirs containing CH₃OH (R₂), H₂S (R₁), and ¹⁵NH₃ (R₃) were connected to the synthesis reactor, through V₂, V₃ e V₄ , using stainless steel tubes (Ø ¼) and sphere-shaped valves (Figure 1). The maximum operating pressure of the system was approximately 1.2 MPa (limited by a safety valve). This pressure was limited by the reagent (gases) amount involved in the synthesis reaction, which ensured the safety of the process.

H₂S gas was added to the reactor by opening tap T_12B. During the process, R₁ was heated with a heat gun to facilitate gas admission. After the procedure, T_12B and V₃ were closed. The CH₃OH was added by opening V₃ (to the left) and T₁₁, which were subsequently closed. The volume (ml) of CH₃OH used in the synthesis was calculated according to the S amount (g). In the process, the use of CH₃CH₂OH (ethanol) was evaluated as a liquid solution instead of CH₃OH, because it is a nontoxic solvent and requires less care in handling. In sequence, valve V4 was opened and ¹⁵NH₃ (R₃) under heat was transferred to the reactor. Finally, V₄ and V₂ were closed, R₃ was disconnected and the ¹⁵NH₃ mass (g) was determined by gravimetry.

The CO contained in the stainless steel reservoir (R₄) was transferred into the reactor by connecting valve V₆. To facilitate the transfer of the gas (CO), the reactor temperature was maintained at approximately -74 ºC, which decreased the reagent vapor pressure (¹⁵NH₃, H₂S and CH₃OH) contained in the reactor. In sequence, valves V₆ e V₂ were opened and the CO (R₄) was transferred to the reactor. Finally, V₆ and V₂ were closed, R4 was disconnected and the CO mass (g) was determined by gravimetry.

Synthesis Reaction of CO(¹⁵NH₂)₂

The chemical reactions that occur in the synthesis process of CO(¹⁵NH₂)₂, which are based on the reaction of CO, ¹⁵NH₃, and S, include some reactions of interest. A series of possible reactions (intermediate and final) in the production of the compound is given below (Franz and Applegath, 1961).

In this stage, the reactor was removed from the cooling bath that contained ethanol and CO₂ (s). After reaching room temperature (25 ºC), it was placed in a heating system (metallic jacket) under mechanical agitation, as shown in Figure 2. The heating system consisted of heating plates and a temperature controller (thermostat), including a k-type thermocouple placed inside the reactor.

Because the PTFE coating makes thermal exchange difficult, more time was allowed for the interior temperature to reach the desired value. As previously discussed, for the safety of the process, it was not possible to evaluate reactions at temperatures above the urea melting point (132 ºC) because of the pressure (limited by the safety valve) exerted by the reagent gases in the reactor and different substances formed. The product principal of the reaction is biuret (NH₂CONHCONH₂).

At the end of the reaction time, the heating system was turned off, and the reactor was cooled to room temperature (25 ºC).

Recycling of the Exhaust Gases and Recovery of CH₃OH

The gases inside the reactor, possibly containing ¹⁵NOx, CO, ¹⁵NH₃ and H₂S, were removed through valve V₂ with a flow of N₂ (admitted through V₁) and underwent an oxidation process (reaction 8) using H₂O2 (30%) and NaOH (2 mol L^-1) solution. The function of the mixture was to retain H₂S (byproduct of the reaction), which is an extremely toxic and flammable gas that should not be released into the atmosphere.

These exhaust gases may also contain possible traces of ¹⁵NH₃ not converted in the syntheses process, which is neutralized in a 1 mol L^-1 H₂SO₄ solution (reaction 9). The (¹⁵NH₄)₂SO₄ obtained was reused in the synthesis process.

At the end of the process, CH₃OH was separated from the mixture containing CO(¹⁵NH₂)₂ and S. A glass system, connected to valve V₂ was used for the solvent separation process as shown in Figure 3. A thermos bottle containing liquid nitrogen (-196 ºC) was introduced around the trap to retain CH₃OH vapor (formed during the process). The entire operation (recycling) was performed inside a fume hood.

In this separation process, the glass system and reactor pressure was reduced with a mechanical pump (BM) by opening T14 and V2. At the same time, the reactor was heated to 70 ºC. At this temperature, the CH₃OH vapor was transferred through the glass system and retained in a cryogenic trap. After approximately 30 min, V₂ and T₁₄ were closed, and the BM was turned off. Finally, the solution (CH₃OH and traces of H₂S) obtained in the trap was transferred to an appropriate container to be reused in the CO(¹⁵NH₂)₂ synthesis

Purification of CO(¹⁵NH₂)₂

At this stage, the reactor was opened, and the mixture of CO(¹⁵NH2) and S contained inside was removed. The internal wall of the reactor was washed with deionized H₂O, which solubilized the CO(¹⁵NH₂)₂ and maintained the S in the solid phase because of its low solubility. The S was separated from the CO(¹⁵NH₂)₂ using a vacuum filter (mechanical) with a glass microfiber (1.6 µm) filter paper and membrane filter of cellulose esters (0.45 µm). The final solution, which contained CO(¹⁵NH₂)₂ solubilized in H₂O, was heated to 50 ºC, and the final product was obtained in solid form.

The synthesized CO(¹⁵NH₂)₂ was submitted to a chemical purification process to remove possible impurities, mainly S, acquired during synthesis. For purification, the CO(¹⁵NH₂)₂ was placed in a beaker, and 50 ml of CH₃COCH₃ was added under agitation. This solution was transferred to the vacuum filtration system using a microfiber filter (1.6 µm) to separate the solvent from the solid compound (CO(¹⁵NH₂)₂). Finally, the CO(¹⁵NH₂)₂ was dried (heated to 50 ºC) and the mass (g) synthesized determined by gravimetry.

Physico-Chemical and Isotopic Analysis of CO(¹⁵NH₂)₂

The samples of synthesized CO(¹⁵NH₂)₂ were subjected to physico-chemical analysis, such as: determination of the N concentration (%) in the system with an automatic analyzer (CF-IRMS: automated nitrogen and carbon analysis - gas, solid, and liquid sample, ANCA-GSL) (Barrie and Prosser, 1996); melting point (ºC) (OECD, 1995) and biuret concentration (%) (Ferreira et al., 2007).

A CO(NH₂)₂ standard of analytical grade with a chemical purity of 99.5% (46.4% N) and a natural isotopic abundance (0.366 at. % ¹⁵N) was used in the aforementioned analytical processes to compare the quality of the synthesized material. Before analysis, the synthesized CO(¹⁵NH₂)₂ samples and standard (P.A) were dried at 65 ºC.

To determine the isotope concentration (at. % ¹⁵N) in the synthesized CO(¹⁵NH₂)₂, ¹⁵N₂ (g) (Buchanan and Corcoran, 1957; Frazer and Crawford, 1963) was obtained an analyzed in a ATLAS MAT model CH4 mass spectrometer (Trivelin et al., 1973).

Experimental Design

The yield of the CO(NH₂)₂ synthesis was determined as a function of the following parameters: amount (0 to 350% excess) of NH₃, CO, and S in the synthesis reaction; reaction time; addition of H₂S; liquid solution and reaction temperature. These tests were performed in triplicate (n = 3) using anhydrous NH₃ with a natural isotopic abundance (0.366 at. % ¹⁵N).

First, the yield of the process was studied separately as a function of the limiting reagents (NH₃ and CO/S) in the synthesis reaction; a completely randomized experimental design with three replicates in a factorial layout was performed. In the procedure with CO (1.12 g) and S (1.28 g) as the limiting reagents, the yield of CO(NH₂)₂ synthesis was evaluated as a function of the amount (0, 25, 50, 100, 150, 200, 300, and 350% excess) of NH₃ in the synthesis reaction. For NH₃ (limiting reagent with 1.36 g), the yield of the process was evaluated as a function of the amount (0, 25, 50, 100, 150, 200 and 300% excess) of CO and S in the reaction. The amount of 0% NH₃, CO, and S represents the stoichiometric ratio of the reagents in the overall reaction (2).

The optimum conditions in terms of the ratio between the reagents (NH₃, CO, and S) were used to evaluate the yield of the process as a function of the following parameters: reaction time (60, 90, 120, and 150 min), addition of H₂S (W/H₂S and (N/H₂S) liquid solution (CH₃OH or CH3CH₂OH), and temperature (80, 90, 100, and 110 ºC) of the reaction.

The data obtained were evaluated using analysis of variance and Tukey's multiple comparison of means test with a 5% level of significance. For the analysis of variance, the appropriate model was used for completely randomized experiments with an appropriate factorial arrangement for each of the experiments involving the parameters of the CO(NH₂)₂ synthesis process. All statistical analyses were performed using the SAS system (SAS Institute, 2008).

Finally, tests were performed in triplicate to synthesize enriched CO(¹⁵NH₂)₂ (90 at. % ¹⁵N) using the optimal conditions (NH₃ limiting reagent) of the evaluated parameters. This procedure was subjected to economic evaluation to confirm its commercial viability.

RESULTS AND DISCUSSION

Evaluation of the Parameters

It was possible to arrive at the final product of interest using the proposed synthesis system involving the reaction between NH₃, CO, and S with CH₃OH (liquid solution). In the procedure, the reaction temperature was reached after approximately 30 min, at which point the temperature was held constant using a controlled heating system (thermostat/ thermocouple). The internal pressure of the reactor was varied from 0.26 to 1.06 MPa depending on the experimental conditions of the parameters involved in the synthesis. The complete CO(NH₂)₂ synthesis process required 8 h of work per batch.

Statistical analyses of the data related to the amount of reagents (NH₃, CO, and S) showed the effects of these factors and their effects on the CO(NH₂)₂ synthesis process. Table 1 shows the average results (three replicates) of CO(NH₂)₂ synthesis as a function of the amount (0, 25, 50, 100, 150, 200, 300, and 350% excess) of NH₃ added. These tests were performed under the conditions: 1.12 g CO, 1.28 g S, 40 ml CH₃OH, 100 ºC and 150 minutes of reaction, the maximum theoretical production (yield of 100%) was 2.4 g of CO(NH₂)₂. According to the data, there was a significant difference (F = 117.66) by Tukey's test at the 5% probability level. This difference was primarily attributed to the presence of NH₃ in two intermediate reactions (4 and 7) of the synthesis mechanism. In addition, the increase in the concentration of NH₃, which was varied from 25% to 350% excess, shifted the equilibrium of the reactions in the direction of the consumption of the reagent and toward the formation of the products (mainly H2NCOSNH₄) following Le Chatelier's principle (Russel, 1994). These alterations in the equilibrium of the reactions resulted in a significant increase of 105.32% (1.00 g of CO(NH₂)₂) in the yield of the synthesis process. Thus, under optimal conditions (350% excess), it was possible to obtain an average of 1.82 g of CO(NH₂)₂ (75.27% conversion), which is highly significant in relation to the theoretical maximum conversion value (75.7%) that can be obtained with the proposed system (Bendassolli and Victoria, 1995).

The conversion result, based on the synthesis data, was used to calculate the NH₃ and CO mass balance (%). It was possible to recover 59.0% of NH₃ (16.8% used in the synthesis reaction and 42.0% recovered in the form of (NH₄)₂SO₄)) with a 41% loss, and to recover 75.4% of CO (incorporated in the CO(NH₂)₂ molecule) with a 25% loss. These losses may be related to the formation of undesirable reaction products (CO₂, NOx) during the synthesis process.

The average results (three replicates) for the synthesis of CO(NH₂)₂ as a function of the amount (0, 25, 50, 100, 150, 200 and 300% excess) of CO and S are presented in Table 1. These tests were performed under the conditions: 1.36 g NH₃, 40 ml CH₃OH, 100 ºC and 150 minutes of reaction, the maximum theoretical production (yield of 100%) was 2.4 g of CO(NH₂)₂. According to the data, there was a significant difference (F = 94.06) at the 5% probability level (Tukey's test). This fact is mainly attributed to the increase in the concentrations of CO and S, which were varied between an excess of 25 to 300%. These increases shifted the equilibrium of reaction (3) toward the formation of COS (carbonyl sulfide). The reaction for the formation of COS is very rapid in comparison to reaction (4) for obtaining H2NCOSNH₄ (Ferm, 1957). However, in contrast to what was previously mentioned for NH₃, the response to an excess that ranged from 25% to 300% of CO and S was less intense because of the presence of the reagents (CO and S) in only one of the intermediate reactions that formed part of the synthesis mechanism. Thus, a significant increase of 95.7% (0.92 g of CO(NH₂)₂) was achieved for the yield. Under optimum conditions (300% excess of the CO and S), an average of 1.64 g of CO(NH₂)₂ (68.75% yield) was obtained.

The conversion result, based on the synthesis data, was used to calculate the mass balance (%) of NH₃ and CO. There was a recovery of 71.07% of NH₃ (65.67% incorporated into CO(NH₂)₂ and 5.4% recovered in the form of (NH₄)₂SO₄) with a 28.92% loss. Furthermore, 19.64% of CO (incorporated into CO(NH₂)₂) was recovered with a 80.35% loss. The primary reason for the losses in the system may be related to the formation of undesirable molecules (NOx, CO₂) during CO(NH₂)₂ synthesis. These results are important with respect to the goal of establishing the optimum conditions (% NH₃, CO, and S) for the synthesis of ¹⁵N-enriched CO(¹⁵NH₂)₂, which has a high cost (US$ 523.00/g of ¹⁵N, FOB price).

Another important point, which is shown in Table 1, is that the results obtained between 150% and 300% excess of CO/S did not differ statistically (P > 0.05) by Tukey's test. This result was observed because the mass (g) of NH₃, which was the limiting reagent, was essentially consumed in reaction (4) in the formation of H2NCOSNH₄. Results published in the literature indicate that the reaction to obtain H2NCOSNH₄ is the slowest of the synthesis of CO(NH₂)₂ (Franz and Applegath, 1961).

In addition, even with the statistically significant differences in the results, the use of a 350% excess of CO/S was not possible because of safety issues and limitations of the safety valve installed on the reactor (1.2 MPa).

The yield (%) of the CO(NH₂)₂ synthesis as a function of time (60, 90, 120, and 150 min), addition of H₂S (W/H₂S and N/H₂S), liquid solution (CH₃OH and CH3CH₂OH), and temperature (80, 90, 100, and 110 ºC) of the reaction were all evaluated. These tests were performed in triplicate under the conditions: 1.36, 4.01, and 4.48 g of NH₃, CO, and S, respectively.

The results of CO(NH₂)₂ synthesis as a function of time (60, 90, 120, and 150 min) are presented in Table 2. In this table, the values contained in the columns W/H₂S and N/H₂S correspond to the CO(NH₂)₂ samples obtained with and without the addition of H₂S, respectively. The tests were performed in triplicate at a temperature of 100 ºC. According to the data, a statistically significant difference (F = 36.20) at the 5% probability level was observed for the evaluated times. These variations led to increases of 27.13% and 28.48% with and without H₂S, respectively, and the increases were due to the difference in the reaction rates that characterize the synthesis mechanism. Reactions (3) and (4), which represent the formation of COS (rapid) and H2NCOSNH₄ (slow), respectively, stand out. The results obtained for a reaction time of 60 min at a temperature of 100 ºC differed from the results reported in the literature with a yield of 73% (Franz and Applegath, 1961). An average yield of 80.63% was obtained after a reaction time of 150 min, which is similar to the theoretical value of 81.1%. This result demonstrates that the reaction is close to equilibrium (Bendassolli and Victoria, 1995).

As is evident from the data in Table 2, the average results obtained for 120 and 150 min under the H₂S containing condition (W/H₂S) did not display a statistically significant difference (P > 0.05), although statistically significant differences were observed for other reaction times. Therefore, the reaction is close to equilibrium after 120 min, which allowed for a reduction of 30 min of the total time (approximately 8 h) per batch. Thus, with a reaction time of 120 min and the W/H₂S condition, approximately 1.90 g of CO(NH₂)₂ (79.75% of yield) was obtained.

This result, based on the synthesis data, was used to calculate the mass balance (%) of the main reagents (NH₃ and CO). A recovery of 52.26% of NH₃ (27.75% incorporated into CO(NH₂)₂ and 24.51% in the form of (NH₄)₂SO₄) was possible with a 47.74% loss, and an 82.59% recovery of CO (incorporated in CO(NH₂)₂) with a 17.41% loss.

The influence of temperature (80, 90, 100, and 110 ºC) was evaluated with regard to the synthesis yield. The tests were performed in triplicate with 40 mg of H₂S and 150 min of reaction time. According to the data in Table 3, a significant difference (F = 14.00) at the 5% probability level was observed in the yield for the temperatures evaluated. The results show a variation of 8.9% (equal to 0.17 g of CO(NH₂)₂) in the synthesis yield, which indicates that the increase in temperature (30 ºC) decreased the time for the reaction to reach equilibrium. Additionally, the average results obtained for the reaction temperatures of 100 and 110 ºC did not exhibit statistically significant differences (P > 0.05); however, these values were significantly different from each other. This fact shows that the reaction at 100 ºC is very close to equilibrium, which allows a reduction in temperature of the synthesis in relation the conventional methods (temperature of 170 to 200 ºC). The mild temperature avoids problems with the reactor (maintenance or material replacements) and decreases the energy costs of the synthesis. Thus, under a temperature of 100 ºC, an average yield of 80.63% (equal to 1.92 g of CO(NH₂)₂) was obtained.

Finally, the synthesis yield was evaluated in function of the liquid solution (CH₃OH or CH3CH₂OH) of the reaction. The tests were performed in triplicate with 40 mg of H₂S, 150 min of reaction time and temperature of 100ºC. A greater efficiency was observed for CH₃OH (methanol) in the synthesis process due to some of its characteristics, such as its simpler alcohol structure, greater polarity, and ability to solvate CO(NH₂)₂.

Thus, the use of CH₃OH resulted in an average increase of 55.6% in yield, with the possibility of obtaining 1.92 g (yield of 80.63%) of CO(NH₂)₂ per batch.

Physico-Chemical Analysis

The concentration (%) of N in the synthesized CO(NH₂)₂ was determined using a mass spectrometer with automated analyzer (ANCA-GSL). According to the results, in triplicate, the compounds exhibited an average N concentration of 46.09% and good analytical precision (CV = 1.1%). Furthermore, the samples did not display a significant (F = 2.9) difference relative to the P.A. (analytical grade) material.

The average value of the melting point obtained for the synthesized CO(NH₂)₂ samples was 132.6 ºC, which was mainly due to the efficiency of the vacuum filtration and chemical purification stages that were designed to remove impurities from the synthesis reaction (mainly S). The data obtained, in triplicate, did not display differences between the melting point values of the synthesized samples and the reference sample (P.A.). The reference sample exhibited a melting point of 133.7 ºC, which is within the specifications (132-134 ºC) provided by the manufacturer.

The synthesized CO(NH₂)₂ samples contained an average biuret of 0.55%. In addition, the data show a difference between the synthesized samples relative to the P.A. (analytical grade) sample. This fact was attributed to the rigid quality control of analytical reagents, which must often comply with international specifications (Afonso and Aguiar, 2004). The average concentration of 0.06% biuret in the P.A. sample agrees with the value certified by the supplier, which verifies the efficiency of the analytical method. In Brazil, commercial CO(NH₂)₂ contains 46.4% of N, 0.55% biuret, 0.008% free amine, 0.003% ash, and 0.003% iron and lead (Santos, 2006). A concentration of biuret in CO(NH₂)₂ greater than 1% is harmful to the majority of plants and may burn their foliage. In the soil, concentrations of biuret greater than 1.5% compromise the germination of seeds (Mithyantha et al., 1977; Brasil, 2007). Within this context, the biuret contained in the synthesized samples of CO(NH₂)₂ should not interfere with their use in agronomic and animal nutrition studies.

Synthesis of Enriched CO(¹⁵NH₂)₂ and its Economic Aspects

To prepare CO(¹⁵NH₂)₂ enriched with the ¹⁵N isotope, enriched ¹⁵NH₃ (limiting reagent) was used. In the proposed system, involving the reaction between ¹⁵NH₃, CO and S (1.36, 4.01 and 4.48 g, respectively), 40 mL of CH₃OH and 40 mg of H₂S, it was possible to obtain 1.82 g (76.5% of yield) of CO(¹⁵NH₂)₂. The entire process, including all of the stages described, required 9 h of work per batch.

In the process there was a recovery of 84.8% of ¹⁵NH₃ [79.0% incorporated in CO(¹⁵NH₂)₂ and 5.8% recovered in the form of (¹⁵NH₄)₂SO₄)] with a 15.2% loss (in the form of nitrogen oxides), and recovery of 20.8% of CO (incorporated in CO(NH₂)₂) with a loss 79.3%. The loss (15.2%) of ¹⁵N-NOx, with enrichment of 90 at. % ¹⁵N, has a cost of US$ 15.00 per batch (US$ 72.00/g of (¹⁵NH₄)₂SO₄).

The concentration (%) of N in the samples of CO(¹⁵NH₂)₂ was determined as described in section 2.3. According to the results, in triplicate, the compounds enriched with ¹⁵N exhibited an average N concentration of 46.11% (chemical purity of 98.94%). The isotopic values of the samples of CO(¹⁵NH₂)₂ were, on average, 90.4 ± 0.2 at. % ¹⁵N, which shows an absence of isotopic fractioning in the process.

An evaluation of the production costs (fixed and ¹⁵N-variable) was performed for the synthesis of enriched CO(¹⁵NH₂)₂. According to the results (Table 4), the production cost of CO(¹⁵NH₂)₂ with an enrichment of 90.0 at. % ¹⁵N was US$ 238.60 per gram, which is less (11%) than the international price (FOB) of US$ 268.00 based on an exchange rate per dollar of R$ 1.80. However, the value of the imported product does not include the fees and taxes applied to international trade, which increase the cost of the compound.

Furthermore, in relation to Table 4, (¹⁵NH₄)₂SO₄ with an enrichment of 90 at. % ¹⁵N, which represents 78% of the production cost of CO(¹⁵NH₂)₂, was obtained from the Stable Isotopes Laboratory of CENA/USP at a cost approximately 50% less than the international price (FOB).

CONCLUSIONS

The method employed for the synthesis of CO(¹⁵NH₂)₂ in a discontinuous process using a stainless steel reactor was adequate. Under the optimum synthesis conditions (1.36, 4.01, and 4.48 g of ¹⁵NH₃, CO, and S, respectively, 40 ml of CH₃OH, 40 mg of H₂S, 100 ºC and 120 min of reaction), 1.82 g (yield of 76,5%) of the compound was obtained per batch. The synthesized CO(¹⁵NH₂)₂ displayed 46.1% of N, melting point of 132.6 ºC and 0.55% of biuret. The procedure did not show isotopic fractioning.

The production costs (US$ 238.60 per gram) of CO(¹⁵NH₂)₂ were competitive with values on the international market (FOB). However, the lack of fees and taxes applied to the imported product should also be considered.

ACKNOWLEDGMENTS

We would like to thank the Stable Isotopes Laboratory for technical support in the execution of this study, the Foundation for Research Support of the State of São Paulo (FAPESP) for financing the research project and for the research grants, and Mr. João Geraldo Brancalion (CENA/USP) for the technical support in the illustrations of the synthesis process.

(Submitted: January 24, 2012 ; Revised: April 30, 2012 ; Accepted: May 17, 2012)

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*

To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  03 Jan 2013
• Date of issue
  Dec 2012

History

• Received
  24 Jan 2012
• Accepted
  17 May 2012
• Reviewed
  30 Apr 2012