Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 1000-1004
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Catalytic Hydrodenitrogenation of Quinoline. Effect of Water and Hydrogen Sulfide Charles N. Satterfleld, C. Morrls Smlth, and Margaret Ingalls Department of Chemical Englneering, Massachusetts Institute of Technoiogy, Cambridge, Massachusetts 02 139
Studies at 375 "C and 6.9 MPa showed that the presence of water, either added as such or generated in situ from an oxygenated compound, increases the rate of hydrodenitrogenation on a NiMo/AI,O, catalyst. The effect occurs in either the presence of or absence of H,S. The degree of reversibility of this activity when water is removed varies with conditions. Any remaining enhancement is removed by a standard heating and resulfiding procedure that reduces the catalyst activity to the level it had before addition of water.
In a recent paper (Satterfield and Yang, 1983), we reported that the addition of water in a concentration of as low as 0.01 wt % to a solution of quinoline in a paraffin significantly increased the rate of hydrodenitrogenation (HDN) on a presulfided NiMo/Al2O3catalyst. The same enhancement was noted by addition of m-ethylphenol in amounts ranging from 0.19 to 4.7 wt %. m-Ethylphenol is rapidly hydrodeoxygenated to hydrocarbons and water, and presumably it is the product water that provides the enhancement. The effects appeared to be reversible, in that the sequential addition and removal of the m-ethylphenol or water over several cycles during the HDN reaction caused the catalyst to revert to the activity representing the original environment, except for a slow deactivation with time. These effects were observed in the presence of H2S. Similar effects occurred in the absence of H2S although larger amounts of m-ethylphenol were then required to produce the same degree of enhancement. Little information has been published on the effect of H 2 0 on hydrodenitrogenation, and the purpose of the present study was to explore in more detail the effects of the presence of H20, with or without the presence of H2S, in increasing the HDN rate of quinoline. The effects are subtle, and therefore considerable detail is given about how the experiments were done and the results that were observed. Experimental Section
The experiments were performed with the trickle bed reactor described previously (Satterfield and Yang, 1984), using quinoline dissolved in a pure paraffin liquid consisting mainly of n-hexadecane. The quinoline concentration was 3.87 X lo-* mol/g (5 wt %), equivalent to a solution containing 0.54 w t % N. All reaction data reported here were obtained at 375 "C and 6.9 MPa. Hydrogen gas was passed into the reactor concurrently with the liquid at a gas-to-liquid ratio of 1600 cm3of H2 (STP) per cm3 of feed solution. (This is equivalent to 9000 SCF/barrel.) Under these operating conditions, most of the paraffin was probably in the vapor phase. Water was either added as such or was generated in situ by adding decanol or m-ethylphenol to the liquid feed solution. Decanol is very rapidly converted to water in the reactor. Similarly H2S was generated in situ by adding CS2 to the feed solution'. Percent HDN is defined in terms of disappearance of nitrogen in the form of quinoline and other organonitrogen compounds formed as intermediates. The catalyst was a commercial NiMo/A1203 from the 0196-4305/85/1124-1000$01.50/0
same batch of American Cyanamid HDS-3A which we have used and reported on previously. It was presulfided with a 1:9 mixture of H2S and Hz at 0.24 MPa and 40 cm3/min, using a temperature program of 12 h at 175 "C, increasing of catalyst)/mol temperature to 315 "C, holding for 1h, and then decreasing the temperature to 150 "C. Under reaction conditions, the catalyst will decrease in activity over the first 100-200 h to a quasi-steady state. This will occur in either the presence or absence of H2Sin the vapor phase. However, at any given time, removal of H2S causes conversion to drop to a lower level over the course of a few hours. Readdition of H2S restores activity. The quasi-steady-state conversion is reached more rapidly in the absence of H2S than in its presence. Hence, each catalyst sample was subjected to an initial deactivation procedure in which the quinoline HDN reaction was carried out at a space time of 270 h (g of catalyst)/mol of Q, 375 "C, and 6.9 MPa of H2 in the absence of any H2S. Once the initial deactivation was complete, H,S was added back to the feed stream. Thereupon the conversion increases very rapidly, and a further enhancement occurs slowly over a period of many hours. For example, after an initial 173 h on stream in the absence of H2S for one sample of catalyst, the conversion was 42%. H2Swas then added, and a sample for analysis taken 5 h later showed 68% conversion. During the subsequent 75 h the conversion slowly increased to about 74 %, as shown on Figure 1. After reaching quasi-steady-state activity, we apply a standard resulfiding procedure from time to time in which the reactor is first flushed with a 50/50 mixture of liquid carrier and xylene for ' / 2 h at 350 "C and under 6.9 MPa of hydrogen. Helium is then passed through the catalyst for 30 min at 350 "C, following which the catalyst is resulfided with a 1:9 H2S/H2mixture at 40 cm3/min and 0.24 MPa at 350 "C for 1 h and allowed to cool (Satterfield and Yang, 1984). Experimental Errors. The experimental apparatus was capable of maintaining the temperature within f l " C of the set point. Changes of the ratio of gas to liquid flow rates can change the partial pressure of H2S in the reactor. This ratio is difficult to control under the low flow conditions studied here; accuracy is estimated to be about f10% of the mean for a given run. Run-to-run variation was somewhat higher and could produce an error of up to Et2% in quinoline HDN conversion. The analytical procedure reproduced HDN to about f l % absolute for repeat analyses of the same sample. No estimate of precision from replicate runs for quinoline HDN was undertaken; 0 1985 American Chemical Society
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maintaining the catalyst environment with respect to H2S and H20 exposure is the overriding factor in reproducing quinoline HDN results.
Results and Discussion This study consisted of four parts, each performed on a separate sample of the same NiMo/A1203catalyst. The purpose of the first part, with sample 9, was to investigate the interdependence of water, generated from decanol, and hydrogen sulfide on the HDN of quinoline. Studies with catalyst sample 10 were designed to compare the effect of water added as such with that generated in situ from decanol, both in the presence of H2S. This also showed the effect of water concentration with hydrogen sulfide present. Studies with catalyst sample 11 in the presence of H2S were to determine the effect of catalyst age on the percent HDN of quinoline at three different water partial pressures (0.7,7.1,and 71 kPa) and in the absence of water, water being generated from decanol. With sample 14, water was generated in situ from m-ethylphenol. This demonstrated that water produced in situ from decanol or m-ethylphenol affected the quinoline HDN reaction in a similar fashion. In the experiments that follow, it should be noted that if a large concentration of water is added to the feed stream, upon removal, the apparent steady-state conversion may not revert to the prior level during the time of the run. That is, a slow desorption of water from the catalyst may persist for a long time. However, the resulfiding process, which first involves drying under helium at 350 "C,effectively removes the water, and catalyst activity reverts to its level before water addition. Sample 9 The catalyst was first allowed to deactivate to steadystate conditions over a period of about 325 h at 375 O C for about the first 175 h and 390 "Cfor the next 150 h, in the absence of CS2. The catalyst was then resulfided, and the effects of adding and removing CS2and decanol to the feed stream were systematically studied, with results summarized in Figure 2. For all studies reported here with sample 9, the decanol mol/g solution and the CS2 concentration was 9.1 X concentration was 8.9 X mol/g solution. These correspond to partial pressures of 12.2 kPa of H2S and 6.4 kPa of H 2 0 under reaction conditions, assuming complete conversion to these products. A space time of 360 h (g>of catalyst)/mol of Q was maintained at all times to provide a reasonable range of conversions. This is equivalent to
a LHSV based on total feed of 3.5 h-l. In the absence of added CS2or decanol, conversion was about 42%, which we term the base-line conversion. At time a, decanol was introduced in the feed solution (no CS2 present). The conversion increased to 58% and then stayed at about 54%, remaining there for 15 h. At time b, decanol was removed from the feed solution and the conversion rapidly dropped back to about 42%. The initial spike above the steady-state conversion is probably due to a concentration effect; when switching feed solutions, our procedure included running at a space time of 46 h (g of catalyst)/mol of Q for a period of 10 min to flush the old liquid from the reactor. At time c, the feed solution was now changed to include the standard amount of CS2 The conversion increased to a value of about 78% over a 12-h period. (This is about 95% of its steady-state value.) At time d, decanol was also added to the feed solution (still containing CS2),and an additional enhancement of conversion to about 94% was observed. At time e, decanol was removed (solution still containing CS2),but now the enhancement was only partially reversed. The conversion dropped to approximately 90% and then declined very slowly over 60 h. At time f, CS2 was also removed from the feed solution so only quinoline was present. The conversion dropped back, at first rapidly and then more slowly, to the base-line conversion of 42% over 70 h. At time g, CS2was added to the feed again and conversion increased to 85% (h). (Note that this is essentially the same conversion obtained in the presence of CS2after water as such had been added and then discontinued.) Discussion. In the absence of CS2,addition of decanol caused an enhancement in percent HDN that rapidly reversed to the original activity when decanol was removed (a, b, and c). With CS2 added to the quinoline, percent HDN increases substantially, as we have observed repeatedly previously. The further addition of decanol to the reactor causes an additional increase in the percent HDN. When decanol was removed, however, in the presence of H2S, most of the enhancement continued and some degree of enhancement due to water remained for at least 60 h (e and f). After removal of decanol and subsequent removal of CS2 from the feed, the catalyst activity dropped to the level it had exhibited 200 h earlier before either CS2 or decanol had been introduced (g, a, and c ) . The addition of CS2a second time increased the activity to a level moderately higher than that observed upon first addition of CS2 and before addition of decanol (h vs. d).
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Figure 6. Response to addition and removal of decanol. Catalyst sample 11, first series. H2S present.
periments involving a wide range of operating conditions was run between times a and b; hydrogen sulfide was present during all this time. The base-line activity after these experiments at 350 h was 30% HDN. The remainder of Figure 5 is to summarize the base-line activity of this catalyst during and after studies described below. After resulfiding, we began the first of three series of runs to determine the response of the catalyst to the addition and removal of decanol in the presence of CS2 and how this response might change as the catalyst aged.
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Figure 7. Response to addition and removal of decanol. Catalyst sample 11, second series. H2S present.
Figure 8. Response to addition and removal of decanol. Catalyst sample 11, third series. H2S present.
First Series. Measurements for the first series, presented in Figure 6, were taken between 410 and 520 h on stream (between b and c on Figure 4). Percent HDN is now greater than in Figure 4 because of the presence of H2S. A base-line conversion for quinoline at 260 h (g of catalyst)/mol of Q and 375 OC was established at about 57% (points a, c, and e). At time a, 0.144 w t % decanol was added to the feed solution; this produces 0.7 kPa of water in situ and increased HDN to 63%. At time b, the decanol was removed, and conversion dropped back to base-line level, time c. Decanol (1.44 wt %), producing 7.1 kPa of water in situ, was then added to the feed solution. Conversion was enhanced from 57% to 70% but dropped rapidly back to 57%, time e, when the decanol was removed. Addition of a 14.4% decanol solution (71 kPa of water equivalent) increased conversion to 82% (f). However, now when the decanol was removed, the base-line conversion was not restored and some enhanced activity remained (g). Resulfiding reduced the catalyst activity essentially to the base-line level, Le., 61% at time h. This is presumably caused by the removal of residual water from the catalyst during the resulfiding procedure, which includes heating at 350 "C in helium. CS2 was now removed from the feed stream, and the catalyst was run under deactivation conditions at 375 "C for 99 h. The catalyst had now been on stream 604 h (time c, Figure 4); in the next 80 h, decanol was introduced to produce water at 71 kPa in the presence of hydrogen sulfide. The observed enhancement of HDN conversion was low compared to other results at this water concentration (data not shown), but when the decanol was removed, some enhancement still remained. After 714 h at reaction conditions, this catalyst sample was resulfided. Second Series. Resulfiding the catalyst lowered conversion to essentially the base-line conversion of 60%, Figure 7, time a. Adding decanol to form 0.7 kPa of water enhanced conversion to 66% (time b), and when decanol was added to constitute 7.1 kPa of water, the conversion was increased to about 73% (time d). In both of these cases, removal of the decanol caused complete reversibility (times c and e, respectively). Third Series. The catalyst was now deactivated again for 124 h without CS2p;esent in the feed stream (Figure 4, d). It was then resulfided and had now been on stream for 880 h. The base-line conversion was initially established at 54% (Figure 8, time a) and gradually increased to about 60% at times corresponding to c, e, and g. This is most likely due to the effect of hydrogen sulfide slowly
restoring the activity of the catalyst that had been severely aged in the absence of CS2. The same pattern of increasing conversion is observed with addition and removal of decanol of increasing concentrations. Upon addition of decanol equivalent to 71 kPa of water, conversion was enhanced from 60% at time e to about 75% at time f. At this time, however, the catalyst had been on stream for 1020 h, and the response to decanol addition was slow. When the decanol was removed from the feed, the enhancement in conversion was lost except for the apparent rise in base-line conversion. Discussion. The enhancement caused by the presence of H20 at 0.7 or 7.1 kPa was rapidly reversible when the water was removed. That caused by 71 kPa H20remained in part after the water was removed, after the catalyst had been on stream 480 h (second series) but not after 1050 h (third series). When a long-term promoting effect of water is observed, the standard resulfiding procedure, which involves drying the catalyst under helium at 350 OC, negates this effect and returns the catalyst to its original activity. With a more aged catalyst or one that has been highly stressed, a slower or lower response to water may be observed. Thus, with catalyst sample 9, following the studies described in Figure 1, a series of additional runs were made after which there was little response to decanol in the presence of CS2. Similarly, in a brief earlier study (Satterfield and Carter, 1981) utilizing a different vapor-phase apparatus, we observed little net effect on the HDN of quinoline in the presence of 13.3 kPa of H2Swhen water as such was added to constitute 13 kPa of partial pressure. The catalyst there had been on stream for 700 h during which it had been subjected to a variety of operating conditions.
Sample 14 After a series of studies in which the catalyst had been 320 h on stream, a base-line conversion of 71% was established in the presence of CS2(point a, Figure 9) for this comparatively fresh catalyst. The conversion was increased to 76% upon the addition of 0.19 wt % m-ethylphenol to the feed solution (b) equivalent to 1.1kPa of water in situ. The enhancement was lost when the m-ethylphenol was removed; conversion dropped back to about 70% (c). When a 0.95 wt % m-ethylphenol solution, equivalent to 5.4 kPa of water, was introduced to the reactor, conversion increased to 81% (d); the base-line conversion of 70% was restored when the m-ethylphenol was removed from the feed. m-Ethylphenol reacts rapidly to form water, and the
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enhancement effect is similar to that observed with decanol.
Enhancement Effect of Water-General In Figure 10 are plotted all the data from catalyst samples 10, 11, and 14. There is a slow drop in activity with total time on stream, during a portion of which deactivation was deliberately hastened by operation in the absence of HzS. In all cases, percent HDN increases with H 2 0 concentration, the fist small additions of water having the most marked effects. We can define an enhancement factor as the HDN rate with water present divided by the rate in the absence of water. If quinoline HDN is taken to be a first-order reaction (Figure ll),then this factor becomes EF = In (1 - HDN,,,,,)/ln
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The data analyzed in this way are shown in Figure 11. The HDN rate actually follows a Langmuir-Hinshelwood expression. Therefore, zero-order kinetics should match the data at high concentrations of the nitrogen intermediates while first-order kinetics should match the data at lower concentrations. A fist-order rate expression seems to provide a slightly better correlation, which is reasonable since all the data were obtained at conversions from 55% to 90% where the nitrogen intermediates are present at lower levels. However, the correlation breaks down at the highest water level.
Figure 11. Enhancement factor assuming first-order kinetics. H,S present.
It is evident that the presence of water can enhance the activity of a NiMo/Al,O, catalyst, but the effect is less pronounced than the enhancing effect of H,S. The enhancing effect of H2Sand HzOtogether is greater than that of either alone. The degree of reversibility varies with conditions. Further, the extent to which HzO increases activity is probably also bound up in the previous history of the catalyst. Possible mechanisms whereby water and HzS increase the HDN rate of quinoline are discussed elsewhere (Satterfield and Yang, 1983). Briefly, both species upon adsorption presumably increase the number or strength of acid sites on the catalyst surface and thereby enhance hydrogenolysis activity. The overall HDN network is a complex of hydrogenation and hydrogenolysis reactions, the relative importance of which can vary with circumstances. For example, during initial catalyst deactivation, hydrogenolysis activity decreases more than hydrogenation activity (Satterfield and Cocchetto, 1981). Also, changes in the relative rates of individual reaction steps may occur with little effect on the overall HDN. Thus, in an earlier study, we observed that above a level of about 1.5 wt % CS2in the feed, further increase in CS2concentration had little effect on percent HDN, although it markedly decreased the formation of decahydroquinoline and increased the formation of o-propylaniline. Similarly, the reason why the correlation in Figure 11 does not hold at very high concentrations may be because it is predicated upon the existence of a single rate-determining step in the reaction network. Acknowledgment This work was financially supported by the Office of Fossil Energy, U.S. Department of Energy, under Contract DE-AC22-80PC30075 and Grant DE-FG22-83PC60798. Registry No. H,O, 7732-18-5; H2S,7783-06-4; Ni, 7440-02-0; Mo, 7439-98-7; quinoline, 91-22-5; m-Ethylphenol, 620-17-7.
Literature Cited SatterfleM, C. N.; Carter, D. L. I n d . Eng. Chem. Process Des. D e v . 1981,
20,538. Satterfield, C. N.; Coccheno. J. F. Ind. Eng. Chem. Process Des. Dev.
1981, 20.53. Satterfield, C. N.; Yang, S.
H. Ind.
Eng. Chem. Process Des. Dev. 1904,
23,1 1 . SatterfieM, C . N.; Yang, S. H. J. Catal. 1983,81, 335. Yang, S. H.; SatterfieM, C. N. I n d . Eng. Chem. Process Des. Dev. 1984.
23,20.
Received for review May 14, 1984 Revised manuscript received November 16, 1984 Accepted December 12, 1984
