4406
Ind. Eng. Chem. Res. 1996, 35, 4406-4411
Effect of Temperature and O2 Concentration on N-Containing Emissions during Oxidative Regeneration of Hydroprocessing Catalysts Edward Furimsky,*,† Anneli Siukola, and Alain Turenne Energy Research Laboratories, Canada Centre for Mineral and Energy Technology, Natural Resources Canada, 555 Booth Street, Ottawa, Ontario, Canada K1A 0G1
The effect of temperature and O2 concentration on the formation of NO, N2O, HCN, and NH3 was studied during oxidative regeneration of the spent CoMo/Al2O3 and NiMo/Al2O3 hydroprocessing catalysts. The experiments were performed isothermally in successive steps lasting 6 h each at 350, 450, and 500 °C. Helium and 2 and 4% O2 were used as the media. For both catalysts, the amount of N-containing emissions accounted for about one-third of the total nitrogen in the coke. Most of the N2O, HCN, and NH3 formation occurred in the same temperature range as that of CO and CO2, whereas the NO formation persisted until the very end of every burning step. The amount of coke on the catalyst influenced burning patterns. For the spent NiMo/Al2O3 catalyst, the chemically controlled burn was much more evident than that for the CoMo/Al2O3 catalyst. The deposits of metals such as vanadium and nickel present in the former have contributed to the difference. The availability of O2 was a much more important factor during burn of the CoMo/Al2O3 catalyst than during that of the NiMo/Al2O3 catalyst. Introduction The oxidative regeneration of hydroprocessing catalysts has been conducted on a commercial scale for several decades. The essential step of the regeneration process involves the removal of coke from the spent catalysts under controlled conditions to avoid temperature excursions. These and other aspects of regeneration of the spent hydroprocessing catalysts have been extensively reviewed (Furimsky and Massoth, 1993). It appears that the mechanisms for the removal of carbon, hydrogen, and sulfur from the spent catalysts are better understood than that for removal of nitrogen. The complexity of nitrogen removal has been indicated by recent studies published by Zeuthen et al. (1991a,b). It appears that N-containing compounds such as HCN, NH3, NO, and N2O are among the possible regeneration products (Furimsky et al., 1995)). However, their yields under typical regeneration conditions have never been quantified. These compounds originate from N-containing structures such as pyrrolic and pyridinic rings and amino groups. The presence of ammonia in the spent hydroprocessing catalysts cannot be ruled out. Ammonia is the main product of the hydrodenitrogenation reactions (HDN). Its strong interaction with Al2O3 was confirmed by Amenomiya (1977). Once adsorbed on Al2O3 at room temperature, part of the ammonia remained adsorbed even after evacuation at 500 °C. Thorough quantification of the N-containing compounds formed during regeneration has not yet been conducted although their emissions are regulated by the environmental authorities (Furimsky, 1996). There are some indications that part of the nitrogen may still remain in the catalyst unless the final temperature is well above that usually employed during regeneration (Furimsky and Massoth, 1993; Furimsky et al., 1995). The fate of this nitrogen is perhaps the least understood. It may still be associated with active species on the catalyst and as such affect recovery of †
Present address: IMAF Group, 184 Marlborough Avenue, Ottawa, Ontario K1N 8G4, Canada.
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the catalyst activity. Most of the results published in the literature were obtained in the temperatureprogrammed mode, with the final temperatures frequently exceeding those employed during commercial regeneration. It has been shown that, at these temperatures, part of the nitrogen can be removed as N2; however, the catalyst activity may be adversely affected due to sintering (Furimsky and Massoth, 1993). In the present study, most of the experiments were performed isothermally at 350, 450, and 500 °C. The duration of the experiments was prolonged significantly compared with the previous studies performed in the temperatureprogrammed mode (Furimsky and Massoth, 1993; Furimsky et al., 1995). The aim was to maximize the nitrogen removal from the catalyst. The two catalysts used in the study included the microporous CoMo/Al2O3 deposited only by coke and macroporous NiMo/Al2O3 deposited by both the coke and metals. The different surface structures of the catalysts and the amount of deposited coke offered the opportunity to identify the effect of these parameters on the burning patterns. The main objective was to quantify the N-containing emissions formed in 2 and 4% O2 in the temperature range typical for commercial regeneration (Furimsky and Massoth, 1993). Little attention was paid to the possible effect of the chemical composition of the catalysts on the emission evolution. Experimental Section Catalysts. The spent catalysts, i.e., the chestnut bur like macroporous NiMo/Al2O3 catalyst (catalyst A) was used for hydroprocessing an atmospheric residue, whereas the CoMo/Al2O3 catalyst (catalyst B) was used for hydroprocessing a conventional distillate. The former was extracted by toluene and subsequently treated under vacuum to remove the solvent. Prior to their experimental use, the catalysts were treated in helium at 350 °C. The elemental analysis of such treated catalysts is given in Table 1. In addition to coke, catalyst A contained small deposites of vanadium, nickel, and iron. © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4407 Table 1. Properties of Catalystsa catalyst A carbon hydrogen nitrogen H/C N/C vanadium nickel BET, N2 surf. area, m2/g
15.5 0.57 0.35 0.44 0.019 3.3 2.7 71
catalyst B 7.5 0.34 0.12 0.54 0.014 187
a
Analysis performed after catalysts were treated at 350 °C in helium for 20 min.
Procedures. A description of the experimental procedure has been published elsewhere (Furimsky et al., 1995). In the present case, 1 and 3 g of catalysts A and B, respectively, were used for the experiments. The quartz tube reactor (10 mm i.d.) was externally heated using a Lindberg furnace. The catalyst was supported on a quartz wool plug. The oxidation medium (0.3 L/min) entered at the bottom and exited at the top. The gas entered a Balston filter to remove moisture, tar, and dust before entering the analysis system. All the experiments were performed isothermally at 350, 450, and 500 °C. Helium, 2% O2 + balance N2, and 4% O2 + balance N2 were used as the media for pyrolysis and/ or oxidation. Analysis. The analysis of HCN, NH3, CO, and CO2 was performed using the on-line Bruel & Kjaer FTIR analyzer type 1301. CO and CO2 determination was also performed using the MTI 200 gas chromatograph equipped with a thermal conductivity detector. The HCN and NH3 calibration was performed by the manufacturer of the analyzer. The chemiluminescent NO/ NO2 analyzer Model 10 AR was used for analysis of NO. The analyzer was always in the NOx (NO + NO2) mode. The reason for reporting NOx as NO is explained later. The N2O was determined using an FTIR on-line analyzer. The yields of compounds and their rates of formation were calculated from their concentrations and a total flow rate normalized to STP conditions. Results The experimental conditions were chosen to ensure a limited availability of O2 as to avoid significant temperature excursions. The choice was based on the previous study from this laboratory (Furimsky, 1988). Nevertheless, when helium was replaced by the gas containing 4% O2 at 340 °C, the temperature increased to about 375 °C and then approached 350 °C within about 5 min. During the subsequent burn at 450 °C, the same change of the gases at 450 °C increased the temperature by 5 °C only. For these reasons, the reported initial rates are the average rates estimated during the first 20 min of the oxidation. Although HCN, NH3, N2O, and NO are reported only, parallel experiments were performed and analysis was carried out in the NO mode and NOx (NO + NO2) mode to verify the formation of NO2. For this purpose, the lines connecting the reactor and analyzer were externally heated to about 450 °C. In the NOx mode, the concentration slightly increased during the experiment in 4% O2 after about 50 min on stream at 350 and 450 °C. The slight increase was attributed to the presence of NO2. The total amount of the NO2, estimated from the increase accounted for less than 0.2 wt % of the converted nitrogen. During repeated runs, but without the external heating of the lines, the concentration in
Figure 1. Cumulative formation of CO2, CO, HCN, and NH3 at 350 °C in 2% O2 (catalyst A).
the NO mode significantly decreased during the same period of the oxidation. This confirms the secondary oxidation of NO to NO2 between the reactor exit and the analyzer. A similar observation was made by Yamashita et al. (1993). Such oxidation is known to increase with decreasing temperature; i.e., the decrease in the temperature resulted in an increase of the equilibrium constant for the oxidation of NO to NO2 by about 6 orders of magnitude (Barin and Knacke, 1973). The delayed appearance of small amounts of NO2 suggests its initial formation and subsequent reduction when still enough coke was present. It is believed that some inorganic species in the catalyst could reduce NO2 as well. Based on these observations, NO2 was considered to be a minor product. Thus, although the analyzer was always in the NOx mode, the results are reported as the total NO. In their fresh form, structures of the catalysts differed significantly. The amount of coke (C + H + N) and surface area are parameters which indicate different properties of the spent catalysts as well. With respect to burning, the surface area may be more important because it can favorably influence access of O2 to the coke. The presence of metals such as vanadium and nickel in catalyst A should also be noted. The higher H/C ratio of the coke on catalyst B indicates lower aromaticity compared with that of catalyst A. This may positively influence the burning of coke on catalyst B as well. Catalyst A. The formation of N-containing compounds was verified at 350, 450, and 500 °C during 1 h blank experiments performed in steps in helium. At 350 °C, about 0.2 and 1.9 wt % of the coke’s nitrogen were converted to HCN and NH3, respectively. Formation of these compounds ceased completely after a few minutes and has not reappeared for the duration of the 6 h experiment. Only NH3 was formed at 450 °C, i.e., about 0.6 wt % of coke’s nitrogen. No N2O or NO was formed during pyrolysis. Thus, less than 3 wt % of the coke’s nitrogen was removed during the two-step pyrolysis. None of the investigated N-containing compounds was detected in helium at 500 °C. The gas analysis was also carried out during the heating period in helium from room temperature to 350 °C to ensure that all measured emissions formed are accounted for. Only traces of HCN and NH3 have been observed while approaching 350 °C. Cumulative formations of HCN, NH3, CO, and CO2 in 2% O2 are shown Figure 1. These results confirm that the evolution of HCN and NH3 coincided with that
4408 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 2. Distribution of N-Containing Compounds during Burnoff of Catalyst Aa
Table 3. Rates of NO Formation for Catalyst A O2 conc, vol %
O2 conc 2% N totalb HCN NH3 NO N2O C totalb N/C of res. coke
4%
350 °C
450 °C
350 °C
450 °C
12.8 5.7 5.1 2.0 0 54.4 0.0036
25.2 4.4 5.9 12.8, 2.10 41.6 0.0366
19.2 5.8 3.5 1.9 8.0 80.9 0.0086
17.6 1.1 1.9 12.0, 2.6tr 14.0 0.0285
a Results expressed as percent of nitrogen in coke converted to the corresponding compound. b Note: N total and C total in wt % of N and C in catalyst, respectively. c The amount formed at 500 °C and higher temperatures including.
Figure 2. Cumulative formation of NO during burn of catalyst A (1 g of catalyst; 2% O2).
of CO and CO2. Thus, most of the HCN and NH3 arose from the destruction of the organic matrix caused by the burning of the coke rather than from the coke pyrolysis; i.e., the total yield of HCN and NH3 at 350 and 450 °C in 2% O2 (Table 2) was significantly greater than that from the pyrolysis. As the results in Figure 2 show, at 350 °C, the initial formation of NO was very slow. Thus, it can be estimated from Figures 1 and 2 that, after a 30 min burn at 350 °C in 2% O2, the cumulative yield of HCN and NH3 was at least 20 times greater than that of NO. However, while the formation of HCN, NH3, CO, and CO2 at 350 °C completely ceased after about 30 min, the formation of NO persisted until the end of the experiment at a gradually increasing rate. It is well established that most of the hydrogen in coke will be removed as H2O before a sizable amount of carbon is removed. Results in Table 2 show that during the burnoff stage performed at 350 °C in 2% O2, 12.8% and more than 50% of the coke’s nitrogen and carbon were removed, respectively. This should result in a significant increase of the N/C ratio of the residual coke. During the second burning stage, performed at 450 °C, the initial rate of NO formation markedly increased and then reached a steady state which persisted until the end of the run. The total yields of NO, HCN, and NH3 accounted only for about one-third of the total nitrogen in coke removed during the three burnoff stages performed subsequently at 350, 450, and 500 °C in 2% O2. At the same time, almost all the carbon had already been removed at 450 °C. NO was the only product detected during the additional burnoff step performed at 500 °C. After about 150 min of burn at 500 °C, the temperature was raised by 50 °C in steps to 550, 600, and 650 °C for three consecutive burns lasting 50 min each. Every temperature increase resulted in a slight increase of the rate of NO formation. Nevertheless, only
a
350 °C
450 °C
500 °C
2 2a 4
Initial Rate (mol/g‚min × 106) 0.001 1.8 0.003 0.8 0.001 2.0
0.2 0.2 0.2
2 2a 4
Steady-State Rate (mol/g‚min × 108) 1.3 1.5 1.3
1.0 0.3 1.0
2 g of catalyst.
about 40% of the total nitrogen in coke was removed as NO, HCN, and NH3 during all these burnoff stages. Three burnoff stages, each 6 h in duration, were performed at 350, 450, and 500 °C using 2% O2 and twice the amount of the catalyst. In this case, the total amount of NO generated was about twice that formed during the experiment using 1 g of catalyst A. At 350 °C, doubling the O2 concentration in the burning gas from 2 to 4% increased the total yield of the N-containing compounds by about 50%; however, increasing the temperature to 450 °C decreased the total yield. Results in Table 2 suggest that the total amount of nitrogen removed at 350, 450, and 500 °C was similar for both 2 and 4% O2 oxidation mixtures. However, the O2 concentration influenced the product distribution. In 4% O2, N2O appeared at 450 °C, whereas no N2O was formed in 2% O2. Also, at 450 °C, yields of HCN and NH3 were significantly lower in 4% O2 when compared with yields in 2% O2. It is quite evident that a portion of NO arises from the nitrogen in the coke which is the most difficult to remove. The initial and steady-state rates of its formation are shown in Table 3. The initial rates at 350 °C represent the average rates recorded during the entire 6 h of the experiments, whereas at 450 and 500 °C they are the average rates determined during the first 25 min. The increase of temperature from 350 to 450 °C resulted in an abrupt increase of the initial rate. Then, all NO produced at 350 °C resulted from a chemically controlled burn, i.e., mostly from the oxidation of the skin of the coke layer. The diffusion phenomena are partly responsible for the decrease in the initial rate during the burnoff performed at 500 °C. It can be estimated from Figure 2 that, in 2% O2, more than 50% of the NO was formed during the chemically controlled burn, presumably from the oxidation of the bulk coke. Catalyst B. The low content of nitrogen in the coke required the use of 3 g of catalyst B to ensure accuracy of the analysis. During the first heating step in helium at 350 °C, about 1.6 and 0.6 wt % of the coke’s nitrogen were converted to NH3 and HCN, respectively. Trace amounts of HCN and NH3 were detected during the subsequent steps at 450 °C and even at 500 °C. Trace amounts of NO and N2O were formed during pyrolysis at 350 °C, whereas none of these compounds were formed during pyrolysis at 450 °C. Similar to catalyst A, the evolution of HCN and NH3 during the isothermal oxidation of catalyst B in 2 and 4% O2 coincided with the formation of CO2 and CO (Figure 3). All concentration-time profiles obtained at 350 °C showed that HCN, NH3, CO, and CO2 reached their maximum value and then approached zero at about the same time. The results in Table 4 show that more of the coke’s nitrogen was removed at 350 °C than at 450 °C. Also, at 350 °C, HCN and NH3 were the main contributors; however, at 450 °C their contribution was
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4409
Figure 4. Formation of N2O during burn of catalyst B (3 g of catalyst; 4% O2; 450 °C).
Figure 3. Formation of CO2 and CO during burn of catalyst B (3 g of catalyst; 4% O2; 350 °C). Figure 5. Cumulative formation of NO during burn of catalyst B (3 g of catalyst; 2% O2).
Table 4. Distribution of N-Containing Compounds during Burnoff of Catalyst B
Table 5. Rates of NO Formation for Catalyst B
O2 conc 2%
O2 conc, vol %
4%
350 °C
450 °C
350 °C
450 °C
N HCN NH3 NO
20.7 10.8 8.3 1.5
13.0 0.6 0.2 5.2
23.9 11.3 5.1 1.3
N2O C totala
0 63.9
7.0 36.0
6.2 98.3
8.8 0.9 0.4 5.1 0.8 1.6 0.8
totala
a Note: N total and C total in wt % of N and C in catalyst, respectively.
significantly decreased. For 2% O2, the temperature increase from 350 to 450 °C resulted in a significant increase in the NO. Also, N2O has appeared as a new product. For 4% O2, most of the N2O was formed at 350 °C. However, a spike of N2O appeared while heating the catalyst in helium to 450 °C after the oxidation step at 350 °C as well as on introduction of 4% O2 at 450 °C (Figure 4). During the same heating in helium, a spike of NO has appeared as well. The results in Table 4 show that, at 350 °C, the sums of HCN + NH3 were significantly greater than the amount of the coke’s nitrogen converted to HCN + NH3 during pyrolysis (2.2 wt %). This confirms that the O2-aided destruction of the coke matrix is the main contributor to the formation of HCN and NH3. These trends were the same for both the 2 and 4% O2 oxidation mixtures. Cumulative yields of NO (Figure 5) confirm its formation during the entire experiment. Thus, most of the NO was produced after the formation of CO, CO2, HCN, NH3, and N2O had ceased completely. Increasing O2 concentration from 2 to 4% increased the NO formation. For NO formation, the initial rates estimated during
a
350 °C
450 °C
500 °C
2 4
Initial Rate (mol/g‚min × 108) 0.48a 1.94 0.82a 2.70
3.01 4.63
2 4
Steady-State Rate (mol/g‚min × 108) 0.24 0.83 0.28 0.75
0.21 0.33
Average rate during the first 20 min of burn.
the first 25 min and steady-state rates estimated during the last 200 min are shown in Table 5. The former increased with increasing temperature from 350 to 500 °C, whereas the steady-state rates reached a maximum at 450 °C. The increase of the O2 concentration from 2 to 4% increased the initial rate as well; however, it had little effect on the steady-state rate. Nitrogen Balance. For both catalysts, removal of carbon, determined from the total yield of CO + CO2, was significantly greater than that of nitrogen, determined from the total yields of NO, N2O, HCN, and NH3. Thus, while almost complete carbon removal had already been achieved, almost 60% of the original nitrogen in coke was still unaccounted for (Tables 2 and 4). This would significantly increase the N/C ratio of the residual coke if most of the unaccounted nitrogen was still in the catalyst. It was, however, shown previously (Furimsky et al., 1995) that more than one-third of nitrogen in coke is converted to N2. It is, also, well established that tarlike species are being formed during the oxidation of coke. Then, other portions of the coke’s nitrogen end up in the tar. Moreover, the steady evolution of NO until the end of the burn at 500 °C and even at higher temperatures suggests that a portion of the original nitrogen still remained in the catalyst. In
4410 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
the present work, no attention was paid to these portions of nitrogen. Discussion In 2% O2, NO + N2O accounted for about 45% of the measured N-containing compounds for both catalysts A and B. However, in 4% O2 this sum was 67% and 46% for catalyst A and B, respectively, suggesting higher oxidation of the former. All results suggest that one portion of the NO originated from a rather different type of nitrogen in the coke than N2O, HCN, and NH3. Thus, while the formation of N2O, HCN, and NH3 is complete at about the same time as that of the CO and CO2, formation of this portion of NO persisted until the end of each burnoff stage. This was evident for both catalysts at 350 and 450 °C. NO was formed during the entire burn at 500 °C, although almost all of the coke’s carbon had already been removed at 450 °C. This indicates a strong association of the remaining nitrogen with the catalyst surface. For catalyst A, N2O appeared only in 4% O2 (Table 2). The recent information suggests that, during the oxidation of carbonaceous solids, N2O arises from the reduction of NO while diffusing from the pores (Goel et al., 1996; Miettinen and Abul-Mihl, 1996). As shown by Rodriguez-Mirasol et al. (1994), complete reduction of NO yielding N2 as the final product can also be achieved if a suitable porosity is available. It is well established that the burn of the bulk coke is accompanied by the development of new pores which in the case of catalyst A occurred in 4% O2. Such porosity may have been present on the catalyst B because of a much thinner layer of coke as suggested by the appearance of N2O in 2% O2 (Table 4). For both catalysts, increasing temperature from 350 to 450 °C in 4% O2 decreased the formation of N2O, HCN, and NH3 and increased that of NO. This indicates decreasing reduction of NO because of decreasing porosity in the final stage of the burn. Thus, while burn is progressing, porosity is reaching a maximum and then decreases as the result of the pores growing into each other (Nguyen and Watkinson, 1990). Figures 2 and 5 indicate a significant difference in burnoff patterns between catalysts A and B. This is confirmed by a marked difference between the initial rates of NO formation shown in Tables 3 and 5 for catalysts A and B, respectively. For the former, rapid NO formation on the introduction of O2 at 450 °C indicates chemically controlled burn which accounted for about 50% of the total NO formed. Almost all of CO, CO2, HCN, NH3, and N2O were removed during the same initial burning period. It is believed that, for catalyst A, chemically controlled burn was predominant during the entire burn at 350 °C and during early stages at 450 °C. The later stages of burning at 450 °C as well as the entire burn at 500 °C could have been affected by diffusion. Thus, for catalyst A, the increase of temperature from 450 to 500 °C decreased the initial rate. The decreased amount of nitrogen in the catalyst and its greater resistance to oxidation could also be partly responsible for the decrease of the initial rate at 500 °C. For catalyst B, initial rates of the NO formation (Table 5) were significantly lower than that of catalyst A; i.e., they approached the steady-state rates for catalyst A. A delay in NO formation at 450 °C in 2% O2 (Figure 5) is attributed to its reduction to N2O. The difference between the burning patterns of catalysts A and B can be traced to the difference in surface
area. First of all, the surface area of the fresh macroporous catalyst A was about 140 m2/g compared with 210 m2/g for the fresh microporous catalyst B. Yet, the amount of coke on the former was about twice that of catalyst B. It can be estimated that the amount of coke per unit of surface area of catalyst A is about 3 times that of catalyst B. This will result in a much thicker coke layer on the former. In the case of catalyst A, much more coke (mostly bulk coke) has to be removed from the external surface and macropores before diffusion becomes evident. Thus, vanadium and nickel deposits on catalyst A hindered diffusion of the feedstock molecules into small pores, ensuring that most of the coke was deposited in macropores or on the external surface of the particles. Then, most of the HCN, NH3, N2O, and NO removed during chemically controlled burn originated from nitrogen in the bulk coke rather than from the nitrogen which is in intimate contact with the catalyst surface. For catalyst B, only spikes of HCN, NH3, and N2O were observed initially at 450 °C in 4% O2, e.g., such as shown in Figure 4 for N2O. Besides diffusion phenomena, the O2 availability may also be partly responsible for the large difference in the NO formation rates between catalysts A and B. Thus, O2 could have been consumed during the oxidation of inorganic sulfur which, in the case of catalyst A, may have been initially prevented by a thick coke layer (Furimsky and Massoth, 1993; Bartholdy et al., 1995). This is supported by the increased rates of the NO formation which resulted from the increase of the O2 concentration from 2 to 4% (Table 5). At the same time, for catalyst A, the same change in the O2 concentration had little effect on the initial rates of NO formation (Table 3), suggesting that the availability of the Ncontaining groups in the bulk coke for the NO formation was the rate-controlling factor unless the deposited metals (vanadium and nickel) hindered the access of O2. Formation of the N-containing compounds, such as HCN, NH3, NO, and N2O, from the model coke prepared from the pyrrolic and pyridinic compounds, under conditions which are typical of catalyst regeneration, was confirmed (Furimsky et al., 1995). The pyrrolic and pyridinic rings are predominant N-containing structures in the coke. Amino groups may also be present in the coke because their formation is favorable under hydroprocessing conditions. It has been shown that during pyrolysis of the catalyst deposited with pyrrolic coke much more NH3 was formed than that from the catalyst deposited by pyridinic coke. A trace of HCN was formed only during pyrolysis of the pyrrole coke, whereas in the presence of O2, both NH3 and HCN became important products for both pyrrole- and pyridine-deposited catalysts. Also, the pyrrole-deposited catalyst yielded relatively large amounts of N2O compared with the trace amount of N2O formed during the oxidation of the pyridine-deposited catalyst. Yields of NO were similar for both catalysts. Ha¨ma¨la¨inen et al. (1994) studied the oxidation of three N-containing groups of compounds, i.e., pyrrolic, pyridinic, and amino. The formation of HCN, NH3, N2O, and NO was observed during oxidation of both pyrrolic and pyridinic groups of compounds, however, yielding no well-defined pattern. As expected, NH3 was the main product during oxidation of the amino group compounds, but N2O, NO, and trace amounts of HCN were also formed. This suggests a complex mechanism for the evolution of N-containing compounds during oxidative regeneration. It is therefore difficult either to identify or to exclude these
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4411
N-containing structures as a sole source of these compounds. The situation becomes even more complex because of a possible interconversion of pyrrolic structures into pyridinic structures (and vice versa) as recently reported by Stanczyk et al. (1995). These authors further observed that, when present in the coke, these structures can be ultimately transformed into a more stable form of nitrogen, i.e., quaternary nitrogen. Such interconversion may have also occurred during the present experiments. During the late stages of burnoff, NO formation persisted even after almost all carbon had already been removed. This portion of NO may have been formed from a more stable form of nitrogen, such as the quaternary form. A high stability of this nitrogen may have been partly responsible for the slow evolution of NO in the steady-state burning region. The presence of metal nitrides in catalysts cannot be ruled out completely. As was shown by Ramanathan and Oyama (1995) and Kapoor and Oyama (1995), molybdenum, tungsten, and vanadium nitrides can be formed from the corresponding oxides by treating with ammonia. In this case, the residual nitrogen may have no adverse effect on the catalyst activity. Thus, as has been shown by these authors, the metal nitrides were quite active catalysts for HDN reactions. It was shown by DjegaMariadassou et al. (1995) that, during mild oxidation, the metal nitrides can be converted to metal oxynitrides. Ammonia itself could have been responsible for the NO formation as well. Ammonia, as the main HDN product, is known to be strongly associated with Lewis acids of the catalyst (Amenomiya, 1977). Then, at least two different associations of nitrogen with the catalyst surface may have been present, i.e., one with active metals and the other with the support uncovered with the metals. It is believed that, during later stages, the NO has originated from a gas-solid reaction in which active metals function as the active agents transferring oxygen from the gas phase to the surface nitrogen. In this scheme, oxynitrides could be intermediate species in converting surface nitrogen to NO. It is believed that it may be more difficult to oxidize the surface nitrogen associated with the support rather than that associated with active metals. All N-containing oxides have originated from the gassolid reactions. Thus, at temperatures employed in the present study, the gas phase oxidation of HCN and NH3 could not occur (Wojtowicz et al., 1993; De Soete, 1993). It appears that NO is the major primary product. However, NO2 cannot be ruled out as the primary product. It was shown by Yamashita et al. (1993) that NO2 is a strong oxidizing agent and as such can be reduced to NO by coke or some metals on the catalyst surface. This suggests that a portion of NO may have resulted from NO2. It is unlikely that N2O can be formed as the primary product. The reduction of NO in the secondary gas-solid reaction is believed to be the only source of N2O.
Conclusions During oxidative regeneration of hydroprocessing catalysts, about one-third of the nitrogen in coke is converted to emissions such as NO, N2O, HCN, and NH3. Most of these compounds were formed during the early stages of the isothermal oxidation. NO was the major N-containing compound formed during the late stages of regeneration. However, traces of NO2 may have been present as well. The nitrogen unaccounted for could have been removed in the tar and/or as N2. It was evident that a small portion of the nitrogen still remained in the catalyst, in spite of the prolonged regeneration. The form of this nitrogen and its effect on the recovery of the catalyst activity deserve some additional attention. Literature Cited Amenomiya, Y. J. Catal. 1977, 46, 326. Barin, I.; Knacke, O. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1973. Bartholdy, J.; Zeuthen, P.; Massoth, F. E. Appl. Catal. 1995, 29, 53. De Soete, G. Rev. Inst. Fr. Pet. 1993, 4, 113. Djega-Mariadassou, G.; Boudart, M.; Bugli, G.; Sayag, C. Catal. Lett. 1995, 31, 411. Furimsky, E. Appl. Catal. 1988, 44, 189. Furimsky, E. Catal. Today 1996, 30, 4. Furimsky, E.; Massoth, F. E. Catal. Today 1993, 17, 4. Furimsky, E.; Nielsen, M.; Jurasek, P. Energy Fuels 1995, 9, 439. Goel, S.; Zhang, B.; Sarofim, A. F. Combust. Flame 1996, 104, 213. Ha¨ma¨la¨inen, J. P.; Aho, M. J.; Tummavuori, J. L. Fuel 1994, 12, 1894. Kapoor, R.; Oyama, S. T. Catal. Lett. 1995, 31, 179. Miettinen, H.; Abul-Milh, M. Energy Fuels 1996, 10, 421. Nguyen, Q. T.; Watkinson, A. P. Can. J. Chem. Eng. 1990, 68, 814. Ramanathan, S.; Oyama, S. T. J. Phys. Chem. 1995, 99, 16365. Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. Combust. Flame 1994, 99, 499. Stanczyk, K.; Dziembaj, R.; Piwowarska, Z.; Witkowski, S. Carbon 1995, 33, 1383. Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Proc. Technol. 1993, 34, 1. Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85. Zeuthen, P.; Blom, P.; Muegge, B.; Massoth, F. E. Appl. Catal. 1991a, 68, 117. Zeuthen, P.; Blom, P.; Massoth, F. E. Appl. Catal. 1991b, 78, 265.
Received for review January 2, 1996 Revised manuscript received September 2, 1996 Accepted September 10, 1996X IE960003D
X Abstract published in Advance ACS Abstracts, October 15, 1996.
