Process intensification in nitric acid plants by catalytic oxidation of

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Process Intensification in Nitric Acid Plants by Catalytic Oxidation of Nitric Oxide Carlos A. Grande,*,† Kari Anne Andreassen,† Jasmina H. Cavka,† David Waller,‡ Odd-Arne Lorentsen,‡ Halvor Øien,‡ Hans-Jörg Zander,§ Stephen Poulston,∥ Sonia García,∥ and Deena Modeshia∥ †

SINTEF AS, P.O. Box 124 Blindern, Oslo N0314, Norway Yara International ASA, Yara Technology Centre, P.O. Box 1130, Porsgrunn 3905, Norway § LINDE AG, Engineering Division, Dr.-Carl von Linde Straβe 6-14, Pullach 82049, Germany ∥ Johnson Matthey Technology Centre, Blount’s Court, Sonning Common RG4 9NH, United Kingdom

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‡

ABSTRACT: We have evaluated the kinetics of the catalytic oxidation of NO to NO2 using a Pt/alumina catalyst, under conditions relevant to industrial nitric acid production: NO and steam contents up to 5% and 20%, respectively, with temperatures from 250 to 350 °C, and pressures up to 4.7 bar. The objective is to replace the current homogeneous oxidation process, which requires cooling of the process gas and a long residence time, with a more intensive heterogeneous oxidation process, allowing the heat of reaction (114 kJ/mol) to be recovered. This may give a 10% improvement in overall heat recovery and, additionally, lead to reduced capital expenditure (CAPEX) and footprint of new build plants. With world production of nitric acid of 60 million tonnes per annum, the transformation from the homogeneous oxidation of NO to a heterogeneous oxidation can lead to significant environmental benefits and cost reduction.

1. INTRODUCTION Reduction of anthropogenic greenhouse gas emissions is one of the key challenges that must be addressed by mankind. Among the many technologies that should be implemented, improving the energy efficiency of existing industrial processes is one of the necessary steps to reduce the energy intensity of the manufacture of commodities. Nitric acid is a key industrial chemical for the production of fertilizers. In 2013, annual production capacity was ca. 78 million tonnes (Mt) and actual manufacturing ca. 58.5 Mt.1 Production growth of 2.3% is anticipated, considering an annual production of 65.5 Mt in 2018. The current production route of nitric acid is known as “the Ostwald process” and has been used for nearly a century.2 A simplified scheme of a modern plant is shown in Figure 1.3 In this process, ammonia is combusted (oxidized) in air, to nitric oxide (NO). This highly exothermic reaction is carried out over a highly selective platinum−rhodium catalyst. The temperature ranges between 800 and 930 °C. The pressure, on the other hand, varies from ambient to 15 bar, depending on the technology. Typically, 9.5−11.5% of ammonia in air is used as the feedstock. The exact concentration depends on the plant conditions. NO then reacts with oxygen to form nitrogen dioxide (NO2). The oxidation of NO to NO2 in nitric acid plants is currently carried out by a combination of two methods: gas cooling shifts the equilibrium toward NO2 formation; and giving the sufficient residence time to allow the homogeneous oxidation reaction to complete this process. © XXXX American Chemical Society

NO2 is subsequently absorbed in water to form nitric acid and nitric oxide. The tail gas of the absorption column is treated in a DeNOx unit before being discharged. All of the reactions in the Ostwald process are exothermic, and currently only a small fraction of the energy spent in making ammonia is recovered in its transformation to nitric acid. A summary of the reactions taking place in the process together with the heat of reaction is 4NH3(g) + 5O2 (g) → 4NO(g) + 6H 2O(g) (ΔH = −905.2 kJ/mol) 2NO(g) + O2 (g) ↔ 2NO2 (g)

(1)

(ΔH = −114 kJ/mol) (2)

3NO2 (g) + H 2O(l) → 2HNO3(aq) + NO(g) (ΔH = −117 kJ/mol)

(3)

The oxidation of NO in the state-of-the-art plants is a homogeneous process that occurs without the use of a heterogeneous catalyst. Because there is a molar reduction, the reaction rate of NO2 conversion (reaction 2) increases at higher pressures. The homogeneous reaction rate is third order Received: Revised: Accepted: Published: A

April 5, 2018 July 18, 2018 July 20, 2018 July 20, 2018 DOI: 10.1021/acs.iecr.8b01483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Simplified scheme of the Ostwald process for nitric acid production.

relative to pressure. Moreover, this is an unusual reaction as the observed rate is higher at lower temperatures.2−4 The temperature reduction that is required to shift the equilibrium toward NO 2 is achieved through a network of heat exchangers.5 There, a fraction of energy (heat) is recovered. Furthermore, heat loss through the pipe walls, where heat energy is not recovered, also occurs. In a large, high throughput plant, the tube between the ammonia burner superheater and the absorption column may have a volume of several hundred cubic meters. This column contains a relatively high pressure of highly corrosive gas. For these reasons, the conventional method of NO oxidation constitutes a significant capital expenditure (CAPEX) for a new plant. In addition, this pipework occupies a substantial part of the plant footprint. In existing plants with significant energy integration measures, at temperatures between 300−260 °C the degree of NO to NO2 oxidation is around 20%. However, according to equilibrium calculations, it could be up to around 90%. This means that there is a large unexplored potential of conversion of gas that is not exploited with existing technology; the heat transfer rate is much faster than the conversion rate. The bulk NO conversion is made at lower temperatures where the energy recovery is less effective and with lower efficiency producing low-pressure steam. An increase of rate of NO oxidation at temperatures around 300 °C could result in an additional recovery of the heat of reaction at temperatures where high-pressure steam can be produced. It is estimated that the overall plant heat recovery can be increased by around 10% if significant conversion of NO to NO2 can take place at temperatures around 300 °C. The recovery of the energy of oxidation of NO at intermediate temperatures (∼300 °C) is the main motivation of this work: using a catalyst, a faster conversion of NO to NO2

can be achieved. The conditions under which the catalytic oxidation of NO to NO2 would occur are challenging: the mixture is highly corrosive, and that imposes a large problem even for analysis of the results. The gas composition contains ca. 10% NOx (NO + NO2) and 15% water vapor. Kinetics of NO oxidation to NO2 at such high concentrations have not been reported in the literature in any catalyst. Because of the well-known properties of NO conversion to NO2 in platinum at lower concentrations and with low Pt concentration,6−12 we have selected this metal as an active site for the catalyst. A catalyst based on uniformly dispersed platinum deposited on alumina has been used to study the reaction rate. The lack of literature on NO oxidation close to the work operating conditions leads to a range of variables being evaluated: molar fractions of NO, NO2, and water, pressure, and temperature (yNO, yO2, yH2O, T, P). For this reason, the kinetic expression was determined with a total of 825 experimental points obtained at different operating conditions.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. The catalyst was prepared by impregnation of a Pt precursor, aiming at 2 wt % Pt, on γ-Al2O3 SCFa 140 from Sasol and subsequent reduction. Granule dimension of catalyst was determined to be ca. 40 μm using a Mastersizer-3000 laser particle size analyzer. The particle size of the platinum was derived from CO metal area measurements. The sample was reduced under H2/N2 at 300 °C for 10 min. The catalyst was then cooled under N2 to 35 °C. Subsequently, pulses of CO were introduced. A Pt:CO stoichiometry of 1:1 was assumed, and the Pt atomic area was assumed to be 0.0800 nm2. The Pt particle size calculated from the CO adsorption measurements was 8−10 nm.13 B

DOI: 10.1021/acs.iecr.8b01483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


A closer TEM image shown in Figure 3 indicates the presence of well-defined Pt nanoparticles on the catalyst. A

The samples were examined in a JEM 2800 scanning transmission electron microscope (STEM) using the following instrumental conditions: voltage (kV) 200; C2 aperture (μm) 70 (Z-contrast) imaging in scanning mode using an off-axis annular detector. The SE signal was acquired simultaneously with the other STEM images providing topological information on the sample. The STEM images showed the distribution of the metal in the support; in Figure 2a, the bright areas indicate the location of Pt on the surface of the alumina, while in Figure 2b, the particles inside the support are shown. These images illustrate a good dispersion and metal distribution of the metallic particles.

Figure 3. Transmission electron microscopy showing the Pt nanoparticles in the Al2O3 catalyst used in the kinetic measurements.

Figure 4. Particle size distribution of Pt nanoparticles obtained from TEM images in the prepared Al2O3 catalyst.

particle size analysis by TEM, shown in Figure 4, indicates that most particles were found between 1.20 and 4.40 nm. The discrepancy between these values and those obtained by CO metal area suggests the presence of some large metal particles that were not observed in the TEM areas studied. 2.2. Experimental Setup for NO Oxidation. The reaction rate data were collected in a differential bed reactor using 52 mg of catalyst. The unit and a detailed view of the reactor are shown in Figure 5. The reactor has a feed section where four mass flow controllers (Bronkhörst, The Netherlands) introduced air, nitrogen, and NO (20% diluted in nitrogen) into the system. All gases were acquired from Yara (Norway). A CEM (controlled evaporation and mixing) unit (Bronkhörst, The Netherlands) was used to introduce water

Figure 2. Scanning transmission electron microscopy of the prepared catalyst showing the Pt on the (a) surface and (b) subsurface of Al2O3. C


ÅÄÅ ÅÅ ÅÅ R dxi Å 1 i = ·ÅÅÅÅ w − dz WHSV ÅÅÅ ∑k Mk k ÅÅ ÅÇ


xi ,outlet =

into the system. All lines after the CEM were heated with electrical resistance elements to avoid condensation, which would result in flow and pressure pulsation as well as variations in molar fractions supplied to the reactor. Because the reaction also proceeds in the gas phase, it is very important to limit the contact time of NO and O2 before the catalyst. For the same reason, the length of the exit stream of the reactor has to be reduced as much as possible between the reactor and the analyzer (to limit unconverted NO and O2 interaction). Our initial reactor design presented a high dead volume, and some NO conversion was observed in the absence of a catalyst (homogeneous catalysis). In the final design, no measurable conversion of NO to NO2 was detected without the presence of a catalyst. After the reactor, pressure is controlled with a backpressure regulator (Equilibar, U.S.), followed by an online IR analyzer. Calibration of the analyzer system for NO, NO2, N2O, and water was carried out. Finally, all gases passed through a knockout drum filled with hydrogen peroxide to avoid high emissions of corrosive and dangerous NOx gases to the exhaust system. Because of the hazardous nature of the gases used in this process, the unit is equipped with gas alarms and was thoroughly tested for possible leaks. Using this system, the reaction rate was evaluated for NO concentrations from 0.25% to 5%, O2 concentrations from 1% to 8%, and water content from 0% to 20%. The flow rate in all of the experiments was set at 1.698 SLPM (standard liters per minute). Experiments were carried out at three different pressures, 1, 2.7, and 4.7 bar, and at four temperatures covering the range between 250 and 350 °C.

dxi dz dz

(∑ ) wk k Mk

(5)

(6)

where the coefficients are a0 = −7.955316 × 100, a1 = 6.074796 × 103, a2 = 1.890112 × 104, and a3 = −6.267597 × 106.

4. RESULTS AND DISCUSSION The typical results of experiments are shown in Figure 6. It should be noted that the aim of the work is to determine kinetics so complete or even very high conversion of NO to NO2 is not desirable. An additional advantage of using a differential bed is that the generation of heat of reaction is controlled, and for this reason the system was analyzed as isothermal. Temperature measurements close to the catalyst layer confirm this assumption. Equilibration of NO2 concentration was remarkably quick, and each of these points was determined after 5 min of stable operation. As shown in Figure 6, the typical run consisted of setting the temperature, the amount of water, and NO. Subsequently, the pressure of the system was modified. The

3. ASSUMPTIONS The data were analyzed assuming that the reactor worked like a plug flow reactor. The change of the mole fraction xi in the reactor tube along the dimensionless coordinate z was calculated according to the following equations:

∑ vij·rj j

1

ÑÉÑ ÑÑ ÑÑ ÑÑ R · ∑ ÑÑ kÑ 2 ÑÑ k ÑÑ ÑÑÖ

where rj are the reaction rates, with stoichiometric coefficients vij and species formation rates Ri. The dimensionless length coordinate is represented by z. Mole fractions are denoted by xi, mass fractions by wi, and molecular weights by Mi. The weight hourly space velocity (WHSV) is the gas mass flow divided by the catalyst mass. Equations 4−6 are the standard 1D plug flow reactor equations expressed in convenient variables for the purpose of parameter fitting. The reaction rate expression is based on catalyst mass, that is, moles of conversion per catalyst mass and time. This equation describes the change of mole fractions due to chemical conversion and/or due to dilution caused by the volume change of a reaction. The difference between calculated and the measured mole fractions was used to define a weighted error square sum, which was the objective function to a numerical minimization. The minimum that was found was then defined as the optimal parameter set. Simulations are stopped when the difference between successive optimization steps is smaller than 10−10. A statistical test was used to determine the significance of the identified parameters. The proposed algebraic structure of the kinetic equations obtained as result was modified until the fit quality was under the predefined acceptable difference and all parameters were significant. For kinetic parameter fits, a 20% error margin is often expected. This work was supported by an already existing program code termed “Siamod”.14 There was only one modeled reaction in the system (NO oxidation). N2O was excluded from the reaction system, because its inlet concentration was not available in all experiments and it was only present in few ppm, probably impurities in the feed gas. NO oxidation is limited by equilibrium. The equilibrium constant was calculated from thermodynamic data, standard enthalpy and entropy of formation, and the cp(T) profiles by15,16 a a a log10 K [bar] = a0 + 1 + 22 + 33 (7) T T T

Figure 5. Experimental unit to measure NO oxidation kinetics where heated lines are highlighted in red. MFC, mass flow controller; CEM, controlled evaporation and mixing; T, K thermocouple; BPR, backpressure regulator. Zoom of the differential reactor with dimensions.

Ri =

∫0

Wi Mi

Article

(4) D


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Figure 6. Reaction kinetics of NO oxidation to NO2 at (a) 250 °C and (b) 350 °C at 4.7 bar and with 5% NO for different amounts of oxygen and water.

ij 94.67 kJ yz j mol mol z zz × expjjjj− r1 = 1.01 × 10 z 3 jj R ·T zzz kg· s· bar k {

amount of oxygen then was modified. Experiments at different temperatures with constant amounts of water were initially made, and water content was the last variable to be evaluated. NO was varied from 1% to 5%, while the range of oxygen used was between 1% and 8%. Four levels of water, 0%, 2%, 10%, and 20%, were investigated. All experiments were carried out at 1.0, 2.7, and 4.7 bar and at 250, 275, 300, and 350 °C. The total number of test conditions amounted to 825. All experiments we made with the same catalyst. Every day that new runs were performed, the same reference test was performed. No deactivation has been observed for over 20 days of gas−catalyst contact. The estimated error in the measurements was ±10% and in most of the experiments within ±5%. Because no deactivation was found, our kinetic rate expression does not have deactivation terms caused, for example, by passivation of Pt by oxygen as reported by other researchers.17 The following reaction rate (including equilibrium and adsorption term) was found to fit well the experimental data:

17

×

2 ·pO − pNO 2

2 pNO

2

K

(1 + 3839 bar −1·pNO2 + 0.43 bar −1·pH O )2 2

(8)

A parity plot of measured versus simulated mole fractions is shown in Figure 7. The quality of the fitted reaction rate achieves approximately 20% accuracy; due to the experimental error of the experimental data, it was considered that such accuracy is acceptable for first catalyst evaluation. The number of the adapted parameters is small, and all of them are statistically significant. The pre-exponential factor is very large. However, this is multiplied by low numbers (Arrhenius exponent, the high adsorption factor, and the high reaction order) at small partial pressures and close to equilibrium. It is also important to note that the term related to water E


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obtained indicated that the conversion was rapid with an activation energy of 94 kJ/mol. This work is the first step in demonstrating that a catalytic conversion of NO to NO2 at higher concentrations is possible and efficient. A compromise between catalyst performance and pressure drop needs to be achieved for the catalytic process to accomplish the desired extra energy recovery.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +47 93207532. Fax: +47 22067350. E-mail: carlos. [email protected]. ORCID

Carlos A. Grande: 0000-0002-9558-5413 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 680414. The project belongs to the SPIRE programme, and information can be found in www.printcr3dit.eu. We want to acknowledge Martha Bricero de Gutiérrez from Johnson Matthey for the TEM pictures.

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Figure 7. Parity plot of the different species (a) NO, (b) NO2, and (c) H2O in all of the simulations. Dotted lines are deviations of 5%, 10%, and 20%.

REFERENCES

(1) Gubler, R.; He, X.; Suresh, B.; Yamaguchi, Y. Nitric Acid; Chemical Economics Handbook; IHS Chemical: London, 2014. (2) Sorgent, H. A.; Sachsel, G. F. Nitric Acid Manufacture. Theory and Practice. Ind. Eng. Chem. 1960, 52, 101−104. (3) Perez-Ramirez, J.; Kapteijn, F.; Schöffel, K.; Moulijn, J. A. Formation and Control of N2O in Nitric Acid Production. Where do we stand today? Appl. Catal., B 2003, 44, 117−151. (4) Connor, H. The Manufacture of Nitric Acid. Platinum Met. Rev. 1967, 11, 2−9. (5) Ivanis, G. R.; Lazarevic, M.; Radovic, I. R.; Kijevcanin, M. L. Energy Integration of Nitric Acid Production using Pinch Methodology. Hem. Ind. 2015, 69, 261−268. (6) Olsson, L.; Fridell, E. The influence of Pt oxide formation and Pt dispersion on the reactions NO2 ↔ NO + 1/2 O2 over Pt/Al2O3 and Pt/BaO/Al2O3. J. Catal. 2002, 210, 340−353. (7) Mulla, S. S.; Chen, N.; Cumaranatunge, L.; Blau, G. E.; Zemlyanov, D. Y.; Delgass, W. N.; Epling, W. S.; Ribeiro, F. H. Reaction of NO and O2 to NO2 on Pt: kinetics and catalyst deactivation. J. Catal. 2006, 241, 389−399. (8) Schmitz, P. J.; Kudla, R. J.; Drews, A. R.; Chen, A. E.; Lowe-Ma, C. K.; McCabe, R. W.; Schneider, W. F.; Goralski, C. T., Jr. NO oxidation over supported Pt: impact of precursor, support, loading, and processing conditions evaluated via high throughput experimentation. Appl. Catal., B 2006, 67, 246−256. (9) Weiss, B. M.; Iglesia, E. NO oxidation catalysis on Pt clusters: elementary steps, structural requirements, and synergistic effects of NO2 adsorption sites. J. Phys. Chem. C 2009, 113, 13331−13340. (10) Li, L.; Qu, L.; Cheng, J.; Li, J.; Hao, Z. Oxidation of nitric oxide to nitrogen dioxide over Ru Catalysts. Appl. Catal., B 2009, 88, 224− 231. (11) Bray, J. M.; Schneider, W. F. First-principles analysis of structure sensitivity in NO oxidation on Pt. ACS Catal. 2015, 5, 1087−1099. (12) Hong, Z.; Wang, Z.; Li, X. Catalytic oxidation of nitric oxide (NO) over different catalysts: an overview. Catal. Sci. Technol. 2017, 7, 3340−3352.

adsorption has only a small contribution under the conditions tested. The results obtained in this work indicated that there is potential to develop a catalytic step in the nitric oxide oxidation to recover extra energy in the production of nitric acid. The impact of this process can also have a significant effect on the final footprint of the nitric acid plant due to large reduction of piping now existing to provide time for conversion of NO. This work is only the first step in the process of catalyst evaluation, given that the velocities used in an industrial setting are significantly larger than those in this study raising diffusional limitation issues. Furthermore, although gains in production of heat can be achieved, pressure drop increase could become an important bottleneck for the catalytic process. For this reason, a good reactor design is required including a very efficient catalyst distribution.

5. CONCLUSIONS This work brings up a novel topic of research in production of nitric acid, by transforming the homogeneous noncatalytic nitric oxide oxidation into a faster catalytic step. This conversion may bring advantages both in reduction of footprint and in increased energy recovery. With this transformation, the energy recovery of production of nitric acid is expected to increase up to 10%. Considering that nitric acid is a major commodity for fertilizers, a 10% increase in energy is highly beneficial for the environment. A platinum catalyst on alumina was investigated for this purpose. The reaction was tested for NO concentrations as high as 5% with water content up to 20%, resulting in an extremely corrosive gas exiting the reactor. The reaction rate F


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Process intensification in nitric acid plants by catalytic oxidation of

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