Cobalt Spinel Catalyst for N2O Abatement in the Pilot Plant Operation

Article pubs.acs.org/IECR

Cobalt Spinel Catalyst for N2O Abatement in the Pilot Plant Operation−Long-Term Activity and Stability in Tail Gases Marek Inger,‡ Marcin Wilk,‡ Magdalena Saramok,‡ Gabriela Grzybek,*,† Anna Grodzka,† Paweł Stelmachowski,† Wacław Makowski,† Andrzej Kotarba,*,† and Zbigniew Sojka† †

Faculty of Chemistry, Jagiellonian University in Krakow, Ingardena 3, 30-060 Krakow, Poland Fertilizers Research Institute, Al. Tysiąclecia Państwa Polskiego 13A, 24-110 Puławy, Poland

‡

ABSTRACT: The catalytic activity of a doubly promoted K/Zn−Co3O4 spinel catalyst in the deN2O reaction was tested for 10 weeks in a nitric acid pilot plant. A charge of 15 kg of the catalyst was synthesized via precipitation method, dried, calcined, and formed into 5 × 5 mm tablets. The catalytic tests were carried out in the wide range of process parameters: 320 < T < 425 °C; 80 < VRG < 180 N m3·h−1, 3 < p < 9.5 bar(a). During the operation the chemical composition of tail gases from ammonia oxidation reactor outlet varied within the following range: 300 < CN2O < 1500 ppm(v); 30 < CNOx < 3000 ppm(v); 10000 < CO2 < 50000 ppm(v); 1000 < CH2O < 18000 ppm(v); N2 − rest. In these conditions, the N2O conversion was above 70% at 400 °C and was stable in time. Fresh and used catalysts were compared through detailed structural and morphological characterization (XRD, Raman, BET, SEM, XPS, H2-TPR), and their deN2O performance was verified in laboratory conditions. Long-term catalytic tests proved persistent stability of the catalyst with respect to its high activity, composition, structure, and morphology.

1. INTRODUCTION Among many potential methods of the abatement of N2O emissions from nitric acid plants, the most effective from the economic point of view, are undoubtedly the catalytic approaches. In principle, two technical routes of N2O removal can be distinguished: a direct high-temperature decomposition of N2O from the process gases in an ammonia burner (HTdeN2O) and a low-temperature N2O removal from tail gases, either by using auxiliary reducing agents (NH3 or hydrocarbons) or via direct decomposition (LT-deN2O).1 Currently, most of the implemented projects relate to high-temperature pathway, encompassing more than 80% of all implementations.2 However, since in some installations this technology cannot be employed due to, e.g., space limitation in the ammonia burner, low-temperature approach becomes an attractive solution. In the case of LT-deN2O process the reactor is located after the absorption column, preferably after the tail gases heat exchangers and before the expander, not affecting the process of nitric acid production directly. As the temperature of the tail gases stream varies usually in the range of 200−500 °C, its additional heating would result in extra costs.3 The temperature of the reaction can be decreased by the use of the reducing agents (hydrocarbons, ammonia, and even hydrogen).1,5−8 Although the low-temperature solution is more expensive for investments, as it requires extra spending to build a reactor and sometimes also a heat exchanger, proper design of the reactor geometry may allow for obtaining high N2O conversions. Moreover, application of the low-temperature method is preferred in high pressure and dual pressure plants. The high pressure (above 10 bar in the absorption column) provides longer residence time in the reaction zone. In the case of the HT-deN2O method, especially in high pressure plants, the high load of catalyst gauzes by ammonia-air mixture and the lack of available volume in the ammonia burner lead to a poor © 2014 American Chemical Society

reduction of the N2O emission. An average efficiency for the HT-deN2O method implemented in the high and the dual pressure plants is below 70%, but plants wherein the efficiency is below 50% also exist. At the same time, for the low pressure plants high-temperature reduction of N2O emission is usually greater than 85%.2 The specific low-temperature N2O decomposition implementation depends on the features of individual installation, especially temperature and NO x concentration in the tail gas stream. Low-temperature catalytic decomposition of nitrous oxide was a subject of numerous extensive studies involving simple and mixed oxides,9−11 perovskites,12 spinels,13−16 zeolites,17 hydrotalcites,18,19 mesoporous silica materials,20 and various supported catalysts.21−26 So far, one of the most promising systems for the low-temperature catalytic N2O removal is a modified cobalt spinel.14,27−31 The optimization procedure of the multicomponent K/Zn−Co3O4 spinel as deN2O catalyst is described in our previous works.14,27,32 Industrial application requires shaping of this catalytic material, and long-term testing was carried out in the real conditions. The aim of this study was to investigate the deN2O performance of the developed doubly promoted spinel catalyst in a pilot scale as well as to evaluate possible structural, morphological, and surface changes of the catalyst during the long-term operation. The last point was achieved by thorough physicochemical characterization for the fresh and used catalysts (after pilot plant prolonged reactivity test). Received: Revised: Accepted: Published: 10335

April 8, 2014 May 30, 2014 June 3, 2014 June 3, 2014 dx.doi.org/10.1021/ie5014579 | Ind. Eng. Chem. Res. 2014, 53, 10335−10342

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The pilot scale K/Zn−Co3O4 catalyst charge was obtained via the precipitation method using zinc and cobalt nitrate precursors and aqueous solution of K2CO3 (15 wt %) as a precipitation agent. The precipitate was aged for 2 h in the mother liquor under continuous stirring. It was next filtered under low pressure and rinsed repeatedly with demineralized water at 50 °C. The resultant precursor of the catalyst was dried at 100 °C for 24 h and calcined at 400− 600 °C for 4 h. The synthesis was repeated three times to obtain the total amount of the catalyst equal to about 15 kg. Finally, the K/Zn-doped cobalt spinel catalyst in the form of sieve fraction (diameter in the range of 1−1.6 mm) was formed in the 5 × 5 mm tablets on a rotary multicavity press (TR-5 Metalchem Gliwice). The resistance tests showed that the mechanical properties of the tablets of fresh and spent (after 800 and 1700 h) catalysts do not change significantly. The value of the axial strength varied from 45 to 44 and 33 N/tablet, respectively, for fresh and spent after 800 and 1700 h catalysts. 2.2. Pilot Plant Setup. The catalytic activity in deN2O reaction of the K/Zn-doped cobalt spinel was tested for more than two months in a pilot nitric acid plant shown in Figure 1.

exchangers, are directed to the deN2O reactor unit (Figure 2). The parameters of the tail gases (VRG, T, p) depend on the

Figure 2. Fragment of the pilot nitric acid plant with the view of the reactor for deN2O pilot scale studies. Insert−top view the investigated catalyst bed inside of the reactor.

operation parameters of the ammonia oxidation reaction carried out with the catalyst gauze in nitric acid plant. The reactivity of the catalyst expressed as a N2O-conversion was calculated on the basis of the N2O concentration in the tail gas stream at the inlet and outlet of the reactor. The N2O concentration was measured periodically by FT-IR spectrometer (GASMET) and by gas chromatography. A stainless steel tubular reactor with the deN2O catalyst was operated in the online flow mode. The reactor with 29 cm internal diameter was equipped with a mantle. It was loaded with the catalyst in the form of tablets (5 × 5 mm). The characteristic parameters of the catalyst bed are presented in Table 1. Catalytic tests were carried out in the wide range of process parameters: 320 < T < 425 °C; 80 < VRG < 180 N m3·h−1; 3 < p < 9.5 bar(a). The composition of the tail gas from the ammonia oxidation reactor outlet was in the following range: 300 < CN2O < 1500 ppm(v); 30 < CNOx < 3000 ppm(v); 10000 < CO2 < 50000 ppm(v); 1000 < CH2O < 18000 ppm(v); N2 − rest. During the tests, the gas temperature was measured directly below the catalyst bed, whereas the pressure was recorded at the reactor inlet and outlet to evaluate the pressure drop on the catalyst bed. Due to the constructional limitations of the pilot plant the flow rate of tail gases cannot be directly measured. Therefore, it is calculated on the basis of the balance for all known inlet and outlet streams (air, ammonia, process water, nitric acid). 2.3. Catalysts Characterization. Detailed physicochemical characterization of both the fresh and used spinel catalysts was carried out in order to investigate the influence of the real conditions in the pilot plant setup on the catalyst state. Moreover, the laboratory scale activity tests in deN2O reaction were also performed on the fresh and used catalyst to compare their catalytic performance. The elementary composition of the catalysts was determined with the use of Energy-Dispersive X-ray Fluorescence (XRF) spectrometer (Thermo Scientific, ARL QUANT’X). X-rays in the range of 4−50 kV (1 kV step). The X-ray photoelectron spectra (XPS) were measured with a Prevac photoelectron spectrometer equipped with a hemispherical VG SCIENTA R3000 analyzer. The spectra were recorded using a monochromatized aluminum AlKα source (E = 1486.6 eV) and an electron flood gun (FS40A-PS) to compensate the

Figure 1. Scheme of the pilot nitric acid plant (C1 - absorption column, C2 - bleaching column, E1 - heat exchanger, E2 - heat exchanger, E3 - heat exchanger, E4 - heat exchanger, E5 - ammonia evaporator, E6 - ammonia superheater, M1 -mixer, R1 - ammonia oxidation reactor, R2 - N2O decomposition reactor).

This installation consists of the following sections: preparation section of a feed gas, ammonia oxidation unit, heat exchange and NOx absorption section and nitric acid bleaching unit. In the gas preparation section streams of air and ammonia gases are mixed in an appropriate proportion and with the predefined operation parameters (flow rate, temperature, and pressure). In the next step, the prepared ammonia-air mixture stream is heated and then oxidized in the ammonia oxidation reactor with the catalyst gauze. In the reactor, oxidation of NH3 to the main product (NOx) and to the undesired byproducts (N2O, N2) takes place. Next, the oxidation products are leaving the reactor as the so-called process gases and are cooled in the heat exchanger. The evolved heat is used to warm up the other process streams, such as tail gases. Nitric acid is produced as a result of subsequent absorption of the nitrous gases in the absorption column. The tail gas (TG) composition depends on the operating conditions of the absorption column (pressure, load, temperature, and the amount of added air). The tail gases, warmed up in the heat 10336

dx.doi.org/10.1021/ie5014579 | Ind. Eng. Chem. Res. 2014, 53, 10335−10342


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Table 1. Characteristic Parameters of the Catalyst Bed Δm dr/m

hbed/m

Vcat/dm3

ρbed/kg·m−3

εbed

mcat0/kg

mcat1/kg

kg

%

0.29

0.155

10.24

1099

0.35

11.25

10.92

0.33

3

residual charge on the surface. The elemental composition of the sample surface was calculated on the basis of the areas of photoelectron peaks characteristic for Co 2p, Zn 2p, K 2p, and O 1s, using the sensitivity factors implemented in the CasaXPS software.33 To study the catalysts morphology a FEI Quanta 3D FEG SEM/FIB microscope was used. The samples were observed without using any prior coating. XRD patterns were recorded by a Rigaku MiniFlex powder diffractometer with Cu Kα radiation at 10 mA and 10 kV, for 2θ between 10° and 80° with a step of 0.02° and a counting time of 1 s per step. The microRaman spectra recorded at room temperature in ambient conditions were taken using a Renishaw InVia spectrometer equipped with a Leica DMLM confocal microscope and a CCD detector with an excitation wavelength of 785 nm. The Raman scattered light was collected in the spectral range 100−800 cm−1 with a resolution of 1 cm−1. At least five scans were accumulated to ensure a sufficient signal-to-noise ratio. The temperature-programmed catalytic reaction tests were performed in a quartz flow reactor in the range of 20−600 °C (10 °C/min), using 300 mg of the catalyst (sieve fraction of 0.2−0.3 mm), and the flow rate of the feed (50000 ppm(v) N2O in Ar) of 30 mL·min−1. The reaction progress was monitored with a quadrupole mass spectrometer (RGA200, SRS, lines for m/z = 44, 32, 30, 28, and 18, corresponding to N2O, O2, NO, N2, and H2O). Typically, prior to the activity measurement a catalyst sample was heated in flow of He up to 400 °C in order to remove the adsorbed substances such as water, oxygen, or carbon dioxide. Temperature-programmed reduction (TPR) characterization of the catalysts reducibility was performed using a catalytic microreactor system equipped with the QMS detector (Catlab, Hiden Analytical). Prior to the TPR measurements a catalyst sample (ca. 25 mg) was heated in flow of He up to 400 °C in order to remove the adsorbed water. The TPR profiles were recorded while heating the sample in flow of 25000 ppm(v) H2/He mixture (30 mL·min−1) up to 600 °C.

Figure 3. N2O conversion in the pilot scale reactivity test as a function of the wide range of temperature and gas flow.

Figure 4. Changes of the catalyst deN2O relative activity together with key operational parameters: bed temperature; NOx concentration; H2O concentration; time on stream during stability test of the double promoted (K, Zn) Co3O4 spinel catalyst.

3. RESULTS AND DISCUSSION 3.1. Catalytic Pilot Test. The N2O conversion in the pilot scale for different temperatures and gas flow rates is presented in Figure 3. Obviously, these factors have significant impact on the catalyst performance. Higher tail gas temperatures and lower flow rates result in higher N2O conversions. For the temperatures above 400 °C and the flow rates below 60 m3/h, the conversion reaches up to 70%. The observed changes in the deN2O reaction parameters arise from different operation conditions of the associated ammonia oxidation process. To show the long-term deN2O activity of the catalyst, the results obtained at the total pressure higher than 5 bar(a) and temperatures from 360 to 400 °C were selected. Relative activity of the catalyst, determined with respect to the catalyst activity of the shortest lifetime measurement (measurement number 1), for the corresponding time on stream as well as NOx and H2O concentrations are presented in Figure 4, along with the temperature variation in the reactor. Due to the undulations in the process conditions in the pilot

installation the data used for comparison (Figure 4) was selected from the measurements corresponding to nearly the same reaction conditions (temperature, pressure, flow rate). Notably, there is no sustained trend of the catalyst activity deterioration during the whole test. The investigated catalyst preserves its high initial activity for more than 2 months in the real tail gas conditions (360 < T < 400 °C; 80 < VRG < 180 N m3·h−1; 300 < CN2O < 1500 ppm(v); 30 < CNOx < 3000 ppm(v); 10000 < CO2 < 50000 ppm(v); 1000 < CH2O < 18000 ppm(v); N2 − rest) at the conversion level 50−60%. The observed variations in the activity may be attributed mainly to the changes in the temperature, NOx and H2O content in the tail gas stream, as was documented and discussed in our previous work.31 The increase of the NOx concentration within the typical range of variation from 500 to 2700 ppm resulted in 10337



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decrease of N2O conversion by about 10%. This process, however, is reversible and upon cutting of the NOx stream the initial N2O conversion is gradually recovered, as was shown previously.31 From the N2O conversion data, the apparent reaction rate constant (kapp) was calculated using the expressions X N2O = 1 − e−k

app

τ

(1)

where τ = 1/GHSV

(2)

The applied model assumes an ideal plug flow reactor behavior, first order rate law, isobaric and isothermal regime, constant molar flow and that the influence of oxygen pressure on the reaction rate is constant. This assumptions are valid since the isobaric and isothermal regime as well as constant molar flow were preserved. It has been shown, that while in some cases the oxygen partial pressure should be included in the kinetic equation, above a critical O2 partial pressure value the reaction rate no longer depends on it.34 Namely, the N2O decomposition with oxygen content in the gas feed CO2 > 20000 ppm(v) can be described by a first order rate law with respect to the pN2O. As mentioned above, in our test, the oxygen concentration ranged from 10000 < CO2 < 50000 ppm(v); however, for most of the collected data points its value was above 20000 ppm(v). To elucidate the impact of reaction variables: T, pNOx, pH2O, and pO2 on the N2O conversion we employed an Arrhenius dependence of the reaction constant on the temperature (eq 3). The variation of the pre-exponential factor was introduced due to adsorption of NOx, H2O at the catalyst surface which leads to blocking of available active sites. Oxygen is adsorbed at the cobalt spinel surface, especially in the presence of potassium promoter,28,35 so we took this fact into account in the model. The least-squares regression was used to fit the available data to eq 3. The sets of T, kobs, pNOx, pH2O, and pO2 for 36 measurements during the test were used kapp = A(1 − θNOx − θH2O − θO2)e−E / RT

Figure 5. Comparison of experimental (apparent) and predicted from eq 3 reaction rate for N2O decomposition.

macroscopic point of view, the catalyst bed during the 1735 h test did not alter: it did not change its volume, and the catalyst weight loss was only 3%. 3.2. Characterization of Fresh and Used Catalysts. XRD patterns of the fresh and used catalysts, shown in Figure 6, proved the spinel structure of both investigated samples. The

(3)

−ΔG/RT

where θ = (Kp)/(1+Kp) (coverage); K = e (adsorption equilibrium constant); ΔG = ΔH − TΔS. The adsorption was assumed to be described by Langmuir type isotherm with only one mode of adsorption for each molecule type (one value of adsorption enthalpy and entropy). The fitted parameters were as follows: A − pre-exponential factor; E − apparent activation energy; ΔH − enthalpies of adsorption for NOx, H2O, and O2; ΔS − entropies of adsorption for NOx, H2O, and O2. The reaction rate constant calculated with the model (kmodel) plotted as a function of experimental kapp value is shown in Figure 5. Figure 5 shows a good correlation between the experimental and modeled reaction rates. The value of the determined apparent activation energy equals 30 kJ/mol, which is similar to the activation energy determined in laboratory conditions for potassium doped cobalt spinel (28 kJ/mol).14 In the same time, the value of A equals 1.0 × 103 s−1. The surface coverage of H2O and O2 determined from the fitted thermodynamic data is negligible in both cases, being in the order of 1 × 10−3 for water and 1 × 10−6 for oxygen. The surface coverage (θNOx) obtained with this parameters changes from 0.1 for 425 °C to 0.99 for 345 °C. In the latter case the observed N2O conversion was very small, about 4−5%. After the long-term pilot test, the catalyst pellets were examined and compared with the fresh one. From the

Figure 6. XRD patterns of fresh and used spinel catalysts.

X-ray diffraction lines characteristic of the cobalt spinel structure were indexed within the Fd3m space group (69378ICSD). No differences in the diffractograms of the fresh and used catalyst samples confirmed no structural changes due to working in the real conditions. The intact catalyst structure was also confirmed by the micro-Raman spectra. Five Raman peaks present in Figure 7 correspond to the Eg, 3F2g, and A1g modes of crystalline Co3O4, ν1 = 190 (F2g), ν2 = 482 (Eg), ν3 = 521 (F2g), ν4 = 619 (F2g), and ν5 = 688 cm−1 (A1g).36 The micro-Raman spectra registered for several different points of the catalysts samples were similar, confirming the uniformity of the catalyst. The H2-TPR characterization was performed for fresh, used, and for Co3O4 as a reference (Figure 8). The H2 consumption plot for reference Co3O4 sample shows typical features with two main peaks, characteristic for the nanometric materials.37 Low-temperature, smaller and sharper peak appears around 310 °C, while the high-temperature peak, broader and much more intense ranges from 330 to 450 °C. Assuming that the H2 consumption is described by eqs 4 and 5, 10338



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temperatures ∼50 °C higher than Co2+ → Co0, as it has been reported for ZnCo2O4 spinel.40 The experimental results on the investigated catalysts summarized in Table 2 confirm the stability of the redox state of the catalyst. Table 2. Relative H2 Consumption for Low- and HighTemperature Steps of Spinel Catalysts Reduction low-temperature peak high-temperature peak

Co3O4

fresh

used

23% 77%

21% 79%

27% 73%

The spinel catalysts were also studied by means of XPS before and after long-term catalytic testing. All the expected elements, i.e. Co, Zn, K, O, and C (adventitious), are present on the surface of the fresh and used catalysts. The surface composition obtained from the XPS analysis is summarized in Table 3. The Zn/Co weight ratio, based on the decomposition

Figure 7. Raman spectra of fresh and used spinel catalysts.

Table 3. Comparison of the Surface (XPS) and Bulk (XRF) Content of the Main Components of Fresh and Used Catalysts XPS (wt %)

the relative contribution for each step should be 25% and 75%, respectively. (4)

3CoO + 3H 2 → 3Co + 3H 2O

(5)

Co

Zn

K

Co

Zn

K

fresh used

59.6 64.7

7.5 9.5

4.6 0.9

65.4 66.0

7.7 7.2

0.8 0.5

of the XPS spectra, matches well the data obtained from XRF. For the fresh catalyst it equals 0.13 and 0.12 from XPS and XRF, respectively. For the spent catalyst these values equal to 0.15 and 0.11 indicate some surface enrichment in Zn cations. Noteworthy, the potassium content obtained from the XPS analysis decreases substantially for the used catalyst and the K/ (Co+Zn) weight ratio changes from 0.05 to 0.01. On the other hand, less significant decrease of potassium content was observed from XRF studies, where the weight ratio changes from 0.010 to 0.007, which is within the error of the method. It means that the decreased amount of potassium viewed by the XPS can be a consequence of the surface agglomeration of this promoter. In general, an appreciable potassium desorption from TM oxide materials is not observed below 400 °C, as shown in our previous studies.41,42 It is probable that during the reaction potassium species transform into KNO3, which melts at 330 °C, and therefore the enhancement of the potassium leaching would take place. However, in the case of high potassium loading, especially in the form of nitrates, the loss of Kpromoter cannot be excluded.43 Comparison of the SEM images for the fresh and used catalysts indicates no significant changes in the morphology of the investigated samples (Figure 9). For both catalysts 50−150 nm aggregates are apparent. As it was discussed elsewhere,31 the aggregates consist of well-developed nanocrystallites with the average diameter of about 10 nm. However, it should be mentioned that the SEM image of the used catalyst, presented in the inset of Figure 9, recorded with higher magnification shows that during the deN2O test the particles start to sinter. Being confined to sintering of impenetrable micropores, it did not cause significant changes in the catalyst activity. Summarizing, the morphological and spectroscopic characterization results revealed that the laboratory optimized composition and morphology were successfully reproduced at

Figure 8. H2-TPR profiles of fresh and used spinel catalysts and cobalt spinel as a reference material.

Co3O4 + H 2 → 3CoO + H 2O

XRF (wt %)

catalyst

Zinc and potassium doped samples show different temperature-programmed reduction behavior, yet qualitatively similar to one another (Figure 8). While the low-temperature peak is shifted slightly toward higher temperature (330 °C for the fresh catalyst), the high-temperature peak differs to greater extent. First, the peak becomes much broader and extends from 360 °C (350 °C for the fresh catalyst) to 540 °C, reaching temperatures higher of about 100 °C in comparison to the reference spinel. Second, an additional peak appears around 375 °C (365 °C for the fresh catalyst). The difference in the shape of the high-temperature peaks (comparing with Co3O4) may arise from two effects: a morphological one and related with the K, Zn doping. The morphological effect may arise from the presence of spinel nanocrystals with different shapes or sizes, which exhibit distinct reduction profiles.37,38 Another reason for broader peak and appearance of additional feature may be due to the presence of alkali atoms at the catalyst surface. Indeed, it has been shown that low amounts of alkali, such as Li, Na, and K give rise to extra features in the Co3O4 reduction profile.39 Also the presence of Zn2+ in the spinel matrix may contribute to the broadening of the hightemperature band, since Zn2+ → Zn0 reduction appears at 10339



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progressive increase in the catalyst performance (see the inset in Figure 10). The observed increase in the catalyst activity is due to the fact that at elevated temperatures, desorption of residual NOx, CO2, and H2O adspecies from the active centers take place, as revealed by the QMS-TPD investigations (Figure 11).

Figure 9. SEM images of the fresh and used catalysts.

large scale (15 kg) synthesis and shaping of the prototype industrial catalyst. Moreover, the prolonged activity tests in the pilot plant conditions did not alter the catalyst performance. 3.3. Laboratory deN2O Activity Studies of the Used Catalyst. In order to check the influence of the long-term pilot testing on the catalyst performance, the temperatureprogrammed catalytic reaction studies of N2O decomposition over used and fresh (reference) catalysts were carried out. The N2O conversion measured at the laboratory scale for the fresh and used catalysts are presented in Figure 10. Whereas the Figure 11. Temperature-programmed desorption profiles for the fresh and used catalysts.

Whereas for the fresh catalyst CO2 is the main contaminant with two distinct desorption peaks at 100 °C and 200−300 °C, in the case of the used one the NO desorption dominates and the low-temperature CO2 peak is absent. At temperatures above 300 °C desorption of NO is accompanied by parallel release of O2 indicating that it can be associated with decomposition of surface nitrates and nitrites, whereas the broad desorption peaks at 200−300 °C are related to the liberation of the molecularly adsorbed NO and decomposition of surface carbonates. Those findings have a straightforward practical implication that the used catalyst can be regenerated either by an increase of the temperature or by flashing the catalyst bed in NOx free gas stream. 3.4. Economic Analysis of the Low-Temperature Catalyst Implementation. Until 2012, according to accepted legal regulations, the nitric acid producers could implement projects focused on reduction of N2O emission by Join Implementation or Clean Development Mechanism. The producers who had reduced N2O emission obtained the rights to Emission Reduction Unit (ERU), in amount corresponding to the level of greenhouse gas reduction, expressed as CO2 equivalent. The value of ERU depends on the market, and its course has ranged from 1 to 30 € during last several years.44 New legislation concerning greenhouse gas emission level came in EU January 2013.45 Nitric acid producers have obtained

Figure 10. Comparison of the conversion curves for the fresh and used catalysts together with the constant temperature conversion of N2O over the used catalyst showing the surface regeneration process (insert).

appreciable conversion for both catalysts was observed above ∼200 °C, the fresh catalyst exhibited distinctly higher activity the difference in T50% between the fresh (T50% = 254 °C) and used catalysts (T50% = 326 °C) reached 72 °C. Calcination of the used catalyst at 600 °C for 4 h in inert gas atmosphere resulted in complete recovery of the catalyst activity within the experimental error (green curve in Figure 10). It is worth mentioning, however, that a treatment of the catalyst already at 350 °C in the NOx free flow of 50000 ppm(v) N2O/He led to a 10340



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Figure 12. Long time loss/income prediction due to implementation of low-temperature catalytic N2O decomposition technology in a typical nitric acid plant.

of the nitrate and carbonates species resulted in partial, reversible poisoning of the catalyst. Full regeneration of the catalytic activity can be achieved simply by heating the sample under NOx free gas flow.

limits of free N2O emissions at the reduction of average level of 90%. Taking into account the current legal requirements relating to reduction of N2O emission a financial outcome of the implementation of N2O abatement technology with the developed spinel catalyst for the hypothetical nitric acid plant with a capacity of 1000 t HNO3/d was estimated. The total implementation costs include the prices of the spinel catalyst and plant modification (a new reactor and heat exchanger). In the presented calculation a 95% of reduction of N2O emissions was adopted. It fulfills the current legal requirements and provides extra ERU of N2O emission. It can be obtained through the use of a suitable amount of the catalyst. Lower N2O conversion would exclude this solution from the practical application. The result of the net financial effect of estimation, calculated for the average ERU values of 8 and 15 € is shown in Figure 12. These values should represent a reasonable approximation of the future ERU prices in the light of more and more restrictive EU environmental policies. The N2O abatement technology implementation, in the light of existing legislation, results mainly in the avoidance of additional financial burden resulting from the need to buy additional ERU. Nevertheless, the additional profits originated from exceeded reduction of N2O emissions can potentially take place, dependent on the course of ERU and the work time of the catalyst. Therefore, implementation of this technology besides environmental aspect can be considered as economically profitable.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the Polish National Centre for Research and Development funding awarded by the decision number PBS2/A5/38/2013. The research was partially carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08).

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REFERENCES

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4. CONCLUSION The optimized K/Zn-doped cobalt spinel catalyst for N2O decomposition was synthesized and shaped into tablets at the kilogram scale. The low-temperature, long-term (10 weeks) deN2O catalytic tests were performed in the pilot nitric acid installation. The resultant catalyst proved its structural and morphological stability, confirmed by several experimental techniques. The reactivity results revealed stable performance of the catalyst, with N2O conversion of about 70%. The observed activity in pilot plant conditions was influenced mainly by the NOx and H2O content in the tail gas. Adsorption 10341



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Cobalt Spinel Catalyst for N2O Abatement in the Pilot Plant Operation

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