Comparison of Hydrodenitrogenation of Model Basic and Nonbasic

A systematic study has been conducted in a trickle-bed reactor using a commercial NiMo/Al2O3 catalyst to understand the effects of different variables...
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Energy & Fuels 2003, 17, 164-171

Comparison of Hydrodenitrogenation of Model Basic and Nonbasic Nitrogen Species in a Trickle Bed Reactor Using Commercial NiMo/Al2O3 Catalyst Deena Ferdous, A. K. Dalai,* and J. Adjaye† Department of Chemical Engineering, Catalysis and Chemical Reactor Engineering Laboratories, University of Saskatchewan, Saskatoon, Canada Received June 5, 2002

A systematic study has been conducted in a trickle-bed reactor using a commercial NiMo/ Al2O3 catalyst to understand the effects of different variables such as H2S concentration in the feed by adding different amount of butanethiol, reaction pressure, temperature, liquid hourly space velocity (LHSV), and H2/feed ratio on the hydrodenitrogenation (HDN) of typical basic (acridine) and nonbasic (carbazole and 9-ethylcarbazole) nitrogen compounds present in heavy gas oil. The HDN conversion of basic compound was higher than that of nonbasic compounds at all butanethiol concentrations (0-4 wt %) in the feed. The HDN conversion of acridine was 9899 wt % at 355-400 °C, whereas, with an increase in temperature from 355 to 400 °C, the conversion of carbazole and 9-ethylcarbazole increased somewhat from 92 to 95 wt % and from 94 to 97 wt %, respectively. Pressure (1120-1420 psig) had no effect on the HDN conversion of basic and nonbasic nitrogen compounds. Also, an increase in LHSV did not have a significant effect on the conversion of acridine and 9-ethylcarbazole. However, the conversion of carbazole increased from 92 to 99 wt % with a decrease in LHSV from 2 to 0.5 h-1. The increase in H2/feed ratio from 200 to 800 mL/mL caused a significant increase in conversion of carbazole from 90 to 98 wt %. The present studies showed no steric hindrance effect of the alkyl group present in 9-ethylcarbazole.

Introduction Oil-sand-bitumen and the bitumen-derived gas oil contain high levels of nitrogen (∼4000 ppm) and sulfur compounds (∼4 wt %) as compared to those present in conventional crude oils. Thus, the products such as heavy gas oil obtained from the processing of bitumen also contain high levels of nitrogen. The presence of higher levels of nitrogen compounds also adversely affect the stability of fuel during storage and deactivates the catalysts used in the downstream secondary processing of these stocks such as fluid catalytic cracking and hydro cracking at a faster rate. Thus, the nitrogen compounds present in the oil-sand-derived heavy gas oil need to be removed before they are used for further processing. Nitrogen in these feedstocks is present predominantly as heterocyclic aromatic compounds.1-4 Two main types of aromatic nitrogen compounds have been identified in the Athabasca bitumen: neutral pyrrole benzologues and basic pyridine benzologues.5,6 Alkyl-substituted carbazoles are the major type of neutral nitrogen * Corresponding author. E-mail: [email protected]. Tel.: (306) 966-4771. Fax: (306) 966-4777. † Syncrude Canada Ltd., Edmonton Research Center, Canada. (1) Katzer, J. R.; Sivasubramanian, R. Catal. Rev. Sci. Eng. 1979, 20, 155. (2) Harvey, T. G.; Matheson, T. W.; Pratt, K. C.; Stanborough, M. S. Fuel 1985, 64, 925. (3) Holmes, S. A.; Thompson, L. F. Fuel 1983, 62, 709. (4) Mushrush, G. W.; Beal, E. J.; Hardy, D. R.; Hughes, J. M. Fuel Processing Technol. 1999, 61, 197-210.

compounds in synthetic crude oil derived from Athabasca bitumen.7 Nitrogen is present in the heavy gas oil in the form of basic and nonbasic heterocyclic compounds such as pyridine, acridine, carbazole, quinoline, etc.8 Jokuty and Gray7 demonstrated that alkyl-substituted carbazoles are the major type of neutral nitrogen compounds in synthetic crude oil derived from Athabasca bitumen. Nonheterocyclic organonitrogen compounds such as aliphatic amines and nitriles are also present, but in considerably smaller amounts, and can denitrogenate much more rapidly than the heterocyclic compounds.1 Jokuty and Gray9 developed a retention chromatographic method to concentrate basic nitrogen compounds from a commercial synthetic crude oil derived from Athabasca bitumen. Their findings concluded that the most abundant ring system were alkyl-substituted 5,6,7,8-tetrahydroquinolines and octahydrobenzoquinolines. The amount of nitrogen present in these tar-sandor shale-oil-derived gas oils is also higher than those originated from conventional crude oil. Hydrodenitrogenation (HDN) is the commonly used catalytic process by which nitrogen is removed as NH3 (5) Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 3, 25-33. (6) Frakman, Z.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1987, 3, 131. (7) Jokuty, P. L.; Gray, M. R. Energy Fuels 1991, 5, 791. (8) Yu, C. Y.; Hatchery, W. J.; Borsch, W. Ind. Eng. Chem. Res. 1989, 28, 13. (9) Jokuty, P. L.; Gray, M. R. Ind. Eng. Chem. Res. 1992, 31, 1445.

10.1021/ef020126c CCC: $25.00 © 2003 American Chemical Society Published on Web 12/06/2002

Hydrodenitrogenation of (Non)Basic Nitrogen Species

when heavy gas oil is refined to fuel or petrochemical feedstock. HDN is an important step in converting fuels from coal, oil shale or tar sands to synthetic fuels, which are used in further processing in refineries. If nitrogen is not removed from these feedstocks, products with undesirable characteristics form during refining. More importantly, the basic nature of the nitrogen containing species effectively poisons the acidic hydrocracking and reforming catalysts. Thus HDN is necessary in producing high-quality, low-cost fuel and feedstocks. Because of the difficulties in removing nitrogen from heavy gas oil, HDN studies are often conducted using model compounds.10-12 In the past, most of the HDN work has been done using one- and two-ring heteroaromatics such as quinoline, isoquinoline, pyridine, indole, etc. Stern13 determined the rates of conversion of pyrrole, indole and carbazole with a batch reactor operated at 350 °C and 68 atm with reactants in hexadecane solvent using presulfided catalyst. The relative first-order disappearance rate constants for pyrrole, indole and carbazole were 3.30, 1.16, and 0.12 min-1, respectively, with a Ni-Mo catalyst. The author reported that hydrogenation and not nitrogen removal was the ratedetermining step. The rate decreased with an increase in degree of substitution on the nitrogen-containing fivemembered ring. Though the experimental work by Stern13 determined the rate controlling step for HDN reaction, the experiments were conducted in a batch reactor and under experimental conditions, that are not used in industry. Zawadski et al.14 studied the kinetics and reaction network of HDN reaction network of acridine at 367 °C and at 137 atm. They found that overall nitrogen removal followed first-order kinetics. The acridine HDN rate increased markedly with hydrogen partial pressure; the HDN rate constant increased almost 6-fold as hydrogen partial pressure increased from 60 to 100 atm. They reported that the rate constant was only a factor of 1.3 greater at 170 than at 100 atm. Ho15 reported that the HDN of 3-ethylcarbazole was 79 wt % over a commercial sulfided NiMo/Al2O3 catalyst at 290 °C, 7.0 MPa, and 1.4 h-1 LHSV. The principal nitrogen-containing products were partially and fully hydrogenated tetrahydro-3-ethylcarbazole being the dominant product. From this work and product analysis, Ho concluded that the rate of hydrogenation of 3-ethylcarbazole, since this rate is relatively slower than than that of hydrogenolysis, is limiting, which agrees with the work of Stern.13 It is important to note that the pressure and temperature conditions used by this author were quite low. Moreau et al.16 studied the structure activity relationship of NiMo- and NiW-based alumina catalyst for the HDN of pyridine, quinoline, (10) Satterfield, C. N.; Yang, H. S. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 11. (11) Hadjiloizou, G. C.; Butt, J. B.; Dranoff, J. S. Ind. Eng. Chem. Res. 1992, 31, 2503. (12) Miki. Y.; Sugimoto, Y. Appl. Catal. 1999, 180, 133. (13) Stren, E. W. J. Catal. 1979, 57, 360. (14) Zawadski, R.; Shih, S. S.; Reiff, E.; Katzer, J. R.; Kwart, H. Tenth and Eleventh Quarterly Reports for the Period August 16, 1981, to February 15, 1982, Prepared for the Office of Fossil Energy, Department of Energy, Washington, DC, 1982. (15) Ho, T. C. Catal. Rev. Sci. Eng. 1988, 30, 117. (16) Moreau, C.; Aubert, C.; Durand, R.; Zmimita, N. Geneste, P. Catal. Today 1988, 4, 117.

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acridine, pyrrole, and carbazole using a batch method at 340 °C and 70 bar. They concluded the existence of two types of catalytic sites, one responsible for hydrogenation and the other for the hydrogenolysis. From their work, it was also found that the rates of hydrogenation of the heteroaromatic rings were mostly influenced by the aromaticity of the rings (π-electron delocalization) and the cleavage of CdN bonds were considerably influenced by the basicity of nitrogen atoms. Nagai et al.17 studied the HDN conversion of carbazole using nitrided molybdena alumina at 300 °C and a total pressure of 10.1 MPa. The molybdenum concentration in the catalyst was varied from 12.5 to 97.1 wt %. The HDN rate of nitrided 12.5% Mo/Al2O3 catalyst 30 min after the start of the run was 0.736 µmol h-1m-2 and decreased to 0.34 µmol h-1m-2 after 3 h, but remained constant thereafter until at least 9 h. They reported that the activity of nitride catalysts for the hydrogenation in carbazole HDN is not related to surface acidity but rather to the reduced molybdenum ions Mo2+ and Mo0 on the surface of the molybdenum nitride catalysts. To our knowledge, no extensive work has been done using multiple ring heteroaromatics such as acridine, carbazole, and 9-ethylcarbazole, etc., using industrial operating conditions and reactor configuration. Also, long time in situ sulfidation (24 h at 193 °C and 24 h at 343 °C) and catalyst stabilizations, which influence the long term catalyst activity and product selectivity that are usually practiced in industry, has not been performed before for the HDN study of these model compounds. Also, no significant studies using model nitrogen containing compounds have been undertaken to define the interactions between six membered (basic) and five-membered (nonbasic) nitrogen heterocycles under industrial HDN conditions. A systematic study of this subject should proceed by HDN studies of five and six-membered nitrogen heterocycles. Also, a considerable research has taken place to investigate the effect of hydrogen sulfide on HDN.18,19 In the past, most of the studies have been conducted with quinoline as model compound. So, the objectives of this present research is to study the HDN activity of different model compounds (acridine, carbazole and 9-ethylcarbazole) at different industrial operating conditions such as temperature (355-400 °C), pressure (1120-1420 psig), LHSV (0.5-2 h-1), and H2/feed (200-800 mL/mL) ratio using a commercial NiMo/Al2O3 catalyst. Experimental Section Experiments were performed using model nitrogen compounds such as acridine, carbazole and 9-ethylcarbazole. The solvent used for these experiments was dodecane. Initially, it was difficult to dissolve carbazole in dodecane. Therefore, 4 vol % of acetone was added into dodecane solution to dissolve carbazole. To compare the conversion of carbazole with those of acridine and 9-ethylcarbazole under identical process conditions, the feed solutions of these compounds were also made (17) Nagai, M.; Goto, Y.; Irisawa, A.; Omi, S. J. Catal. 2000, 191, 128. (18) Satterfield, C, N.; Gu¨ltekin, S. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 62. (19) Yang, M. H.; Grange, P.; Delmon, B. Appl. Catal. A 1997, 154, L7.

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Figure 1. Schematic diagram of the experimental setup. (1) Liquid feed tank, (2) weighing balance, (3) liquid feed pump, (4) mass flow controller for hydrogen and helium, (5) check valve, (6) reactor and furnace assembly, (7) water scrubber, (8) high-pressure gas liquid separator, (9) back pressure, (10) gas flow meter.

using the same amount of acetone. The experiments were performed at different temperatures (355-400 °C), pressures (1120-1420 psig), LHSVs (0.5 to 2 h-1), H2/feed ratios (200800 mL/mL), and at different sulfur concentration in feed (0-4 wt %). Sulfur concentration of the solution was varied using butanethiol (C4H10S). The catalyst used for these experiments was trilobe-shaped commercial Ni-Mo-based alumina with a diameter of ∼1.5 mm. The BET surface area and the pore volume of the catalyst were 160 m2/g and 0.5 mL/g, respectively. All the experiments were performed in a microscale trickle bed reactor using 5 mL of catalyst. The schematic diagram of the experimental set up is given in Figure 1. The system consisted of liquid and gas feeding sections, a high pressure reactor, a heater with temperature controller for precisely controlling the temperature of the catalyst bed, a scrubber for removing the ammonium sulfide from the reaction products, and a high-pressure gas-liquid separator. The length and internal diameter of the reactor were 240 and 14 mm, respectively. The catalyst bed inside the reactor was diluted with 9 mL of 90mesh silicon carbide. Before loading, the catalyst was dried for 2 h at 200 °C. For loading the catalyst, the reactor was packed from bottom to top in nine parts. The bottom 3.5 cm was first loaded with 3 mm size glass beads followed by 2 cm with 16 mesh, 2 cm with 46 mesh, and 0.8 cm with 80 mesh silicon carbide. The bed length of the catalyst was 9 cm. The catalyst bed was packed with 5 mL catalyst and 9 mL of 90 mesh silicon carbide. Small quantities of catalyst and silicon carbide were alternately loaded in the reactor with spatula and mildly vibrating the reactor intermittently. Then the top part of the catalyst bed was loaded with 80 mesh, 46 mesh, 16 mesh silicon carbide, and 3 mm glass beads. After loading the reactor with catalyst and diluents, it was put in the unit and 50 mL of distilled water was injected into the scrubber. The pressure of the reactor was raised to 1500 psig for pressure testing with helium. Then the reactor pressure was decreased to 1275 psig. Helium was then fed at the controlled rate of 50 mL/min, and the temperature of the reactor was then raised to 100 °C. After the reactor was raised to 100 °C, the sulfidation of the catalyst was started. The sulfidation of the catalyst was carried out using sulfidation solution containing 2.9 vol % of butanethiol in a straight run atmospheric gas oil. Initially, the flow rate of the sulfiding solution was kept high (at 2.5 mL/min) to wet the catalyst bed.

Ferdous et al. After passing sulfiding solution for 2 h at this rate, the flow rate was reduced and adjusted to maintain a LHSV of 1 h-1. Hydrogen flow was then started corresponding to an H2/ sulfiding solution ratio of 600 mL/mL. Helium flow was then stopped. The temperature of the reactor was then slowly increased from 100 to 193 °C and was maintained at this temperature for 24 h. After that the temperature of the reactor was again increased slowly to 343 °C and was then maintained at 343 °C for another 24 h. After sulfidation was over, the catalyst was then precoked by passing heavy gas oil at the rate of 5 mL/h. The temperature of the reactor was then increased slowly to 375 °C. The precoking of the catalyst was continued under this set of conditions for 7 days. After this initial period of precoking, the operating parameters were set at the desired level and experiments were performed using feed containing different model compounds. The sample was collected after 24 h interval. The products were stripped with nitrogen for removing the dissolved ammonia and hydrogen sulfide and then were analyzed for their total nitrogen contents. The nitrogen content of the liquid product was measured by combustion/chemiluminence technique (using Antek 9000 analyzer) following ASTM D4629 method. Also, on limited samples 13C NMR (Bruker Avance 500) and GCMS (VG-70-VSE) analyses were performed to identify different product species produced during HDN process.

Results and Discussion The experiments were conducted in order to study the conversion of basic (acridine) and nonbasic (carbazole and 9-ethylcarbazole) nitrogen model compounds at different operating conditions. The conversions of different model compounds during HDN reaction are calculated on the basis of eq 1. % conversion ) total nitrogen content in feed - total nitrogen content in product total nitrogen content in feed

(1) Some experiments were repeated, and the percentage error in conversion of total nitrogen compounds was found to be (0.25 wt %. The effects of different operating conditions on HDN of acridine, carbazole, and 9-ethylcarbazole are discussed in the following section. Effect of H2S Concentration. Hydrodesulfurization of sulfur compounds present in heavy gas oil takes place in the commercial HDN operation. The presence of different additives, such as sulfur and also the types of compounds present in the feed material, may affect the performance of catalyst. Also, the byproduct hydrogen sulfide formed during hydrodesulfurization generally affects the relative rate of HDN of basic and nonbasic nitrogen compounds. On the other hand, it is wellknown that HDN reaction occurs over sulfided catalyst and hydrogen sulfide may be required to maintain the chemical state of the catalyst.20 However, the studies by Yang and Satterfield21 using a model compound (quinoline) indicated that prehydrogenation reactions are slightly inhibited by hydrogen sulfide, but the hydrogenolysis reactions that are responsible for scission of carbon-nitrogen bonds were enhanced with increasing hydrogen sulfide concentration. On the other (20) Perot, G. Catal. Today 1991, 10, 447. (21) Yang, S. H.; Satterfield, N. C. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 20.

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Figure 2. Effect of C4H10S concentration on HDN conversion of basic and nonbasic nitrogen model compounds at temperature of 385 °C, pressure of 1275 psig, LHSV of 1 h-1, and a H2/feed ratio of 600 mL/mL.

hand, Gultekin et al.22 reported that the overall HDN activity of quinoline enhanced with increasing hydrogen sulfide concentration. To address these discrepancies in the literature, excess hydrogen sulfide inside the reactor (see eq 2) was generated by varying butanethiol (0-4 wt %) in the feed solution containing six (basic) or five membered (nonbasic) nitrogen heterocycles using butanethiol (C4H10S). The experiments were carried out at 385 °C, 1275 psig, LHSV of 1.0 h-1, and hydrogen/feed ratio of 600 mL/ mL.

C4H10S + H2 f H2S + C4H10

(2)

The effects of butanethiol concentration on the conversion of acridine, carbazole, and 9-ethylcarbazole during HDN are given in Figure 2, which indicates that without butanethiol in feed, the conversion of basic nitrogen compound such as acridine is much higher than that of nonbasic compounds. For example, the conversion of acridine is ∼99 wt % under industrial conditions and is much lower in the other two cases (9697 wt %). Their conversion over 96 wt % may be due to prior sulfidation of the catalyst as described in Experimental Section. In the case of acridine, the conversion increased from 99 to ∼100 wt % with butanethiol concentration in feed over 0.25 wt %, indicating that hydrogen sulfide has positive effects on its conversion. Saturation of the catalyst surface with hydrogen sulfide may be the reason for constant conversion of acridine. It is reported in the literature that a certain degree of sulfidation of the catalyst is necessary for high conversion of basic model nitrogen species, since sulfur sites could serve as a hydrogen donor for the hydrogenation reaction.23 In the present work constant conversion of acridine suggests that 0.25wt % of butanethiol concentration is sufficient to accomplish the total sulfidation of the catalyst surface. This result agrees with those reported by Gestel et al.24 that in the HDN of 2,6dimethyl aniline over CoMo/Al2O3 and NiMo/Al2O3, the (22) Gultekin, S.; Khaleeq, M. A.; Al-Saleh, M. A. Ind. Eng. Chem. Res. 1989, 28, 729. (23) Calafat, A.; Laine, J.; Lopez-Agudo, A. Catal. Lett. 1996, 40(34), 229. (24) Gestel van J.; Leglise, J.; Duchet. C. J. Appl. Catal. 1992, 92, 143.

catalyst can maintain high activities at high sulfur concentration in the feed. Hanlon25 also reported that the hydrogenation of pyridine (a basic nitrogen species) to piperidine over a commercial NiMo/Al2O3 catalyst was unaffected by the partial pressure of H2S. Figure 2 shows that with the increase in butanethiol concentration from 0.25 to 4 wt % the HDN conversion of carbazole increased from 95 to 99 wt %. Satterfield and Gu¨ltekin18 reported that hydrogen sulfide, if present during HDN reaction, enhances the hydrogenolysis reactions and inhibits hydrogenation reactions to some degree. The net effect is the enhancement of the overall HDN rate. Hanlon25 suggested that a surface acidic species, X-S (where X is an active site), play an active role in the hydrogenolysis reaction i.e., C-N bond breaking reaction. This species, proposed to be of the form NiMoS in the bimetallic catalysts, is formed from an equilibrium between H2S, H2 and the catalyst surface,25

H2S + X f X-S + H2

(3)

It was mentioned that the NiMoS pseudophase, the possible synergistic structure in HDN, is favored by increasing H2S concentration. Calafat et al.23 reported that the promotion effect of Ni continues to be operative only when H2S is present and it increases with increasing H2S/H2 ratio. So, probably because of these reasons, HDN conversion of carbazole increased with increasing butanethiol concentration in the feed. Initially, the conversion of 9-ethylcarbazole was high (97 wt %). This high conversion compared to that of carbazole could be due to reduction of steric hindrances, which may be due to the presence of the ethyl species at the 9th carbon position. Its conversion decreased from 98.5 to 94 wt % with increase in butanethiol concentration from 0.25 to 4 wt %. Bej et al.26 also reported a decrease in HDN conversion of nonbasic nitrogen compounds in heavy gas oil with increasing H2S concentration in the feed. They indicated that when the hydrogen sulfide is in excess of completing sulfidation of the catalyst, it leads to a decrease in overall HDN conversion. Data in Figure 2 show that an optimum butanethiol concentration (∼2 wt %) in the feed may be necessary to obtain a high total nitrogen conversion of basic and nonbasic nitrogen compounds. Therefore, unless otherwise stated, a butanethiol concentration of 2 wt % was maintained to study the effects of temperature, pressure, LHSV, and H2/Feed ratio on the conversion of nitrogen containing species in feeds, containing one or more model nitrogen compounds. Effect of Temperature. The effects of temperature (355-400 °C) keeping pressure, hydrogen/feed ratio, LHSV constant at 1275 psig, 600 mL/mL, and 1 h-1, respectively, on the HDN conversion of acridine, carbazole, and 9-ethylcarbazole are studied and given in Figure 3. It is observed from Figure 3 that the HDN conversion of acridine remained at >99 wt % at all temperatures. Nagai and Masunaga27 also reported 100 wt % conversion of acridine in a mixture of carbazole and xylene at 280 °C and at 1450 psig when LHSV was maintained at 10 h-1. (25) Hanlon, R. T. Energy Fuels 1987, 1, 424. (26) Bej, K. S.; Dalai, K. A.; Adjaye, J. Energy Fuels, 2001, 15, 377. (27) Nagai, M.; Masunaga, T. Fuel 1988, 67, 771.

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Figure 3. Effect of temperature on HDN activity of basic and nonbasic nitrogen model compounds (1600 ppm of each in feed containing 2 wt % C4H10S) at pressure of 1275 psig, LHSV of 1 h-1, and a H2/feed ratio of 600 mL/mL.

The HDN conversion in case of carbazole increased from 92 wt % to 95 wt % when temperature was increased from 355 to 400 °C (Figure 3). It is well-known that, during HDN reaction, nonbasic nitrogen compounds convert to basic nitrogen compounds via a hydrogenation step, and that is the most important step for the HDN reaction of nonbasic nitrogen compounds. Then it goes to on CdN bond cleavage. The CdN bond cleavage and overall HDN equilibria are favorable with increasing temperature.28 Probably because of that the HDN conversion of carbazole increased steadily with increasing temperature. Bej et al.26 also reported the steady rate of increasing HDN conversion of nonbasic nitrogen compound for the temperature range of 365415 °C. For 9-ethylcarbazole, the HDN conversion also increased with increasing temperature from 355 to 370 °C and then labeled off. For example, at 355 °C, the HDN conversion of 9-ethylcarbazole was 94 wt %, whereas at 370 °C the conversion was 97 wt %. From the results in Figure 3, it is observed as explained earlier that the elevated temperature has no effect on the HDN conversion of acridine but has some effect on HDN conversion of carbazole and 9-ethylcarbazole. At any given temperature, the conversion of 9-ethylcarbazole is higher than that of carbazole, which shows that 9-ethylcarbazole does not have steric hindrance effect on the HDN conversion. It is reported that steric hindrance around the nitrogen heteroatom is expected to be less important in five-membered rings than in six membered rings because of higher electron density. It is also reported that alkyl substituents are expected not to have a large effect on the reactivity of multiring heterocycles.15 From the final product analysis of non basic nitrogen compounds, it is observed that the basic nitrogen content in the product is approximately zero for all reaction temperatures which means that all basic nitrogen compounds formed due to the hydrogenation of non basic nitrogen compound have undergone CdN bond cleavage. Effect of Pressure. The effect of pressure on the HDN conversion of basic and nonbasic nitrogen model compounds were studied by increasing pressure from (28) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021.

Ferdous et al.

Figure 4. Effect of LHSV on HDN activity of basic and nonbasic nitrogen model compounds (1600 ppm of each in feed containing 2 wt % C4H10S) at temperature of 385 °C, pressure of 1275 psig, and a H2/feed ratio of 600 mL/mL. Table 1. Effect of Pressure on the HDN Conversion of Basic and Nonbasic Nitrogen Model Compounds at the Temperature, Pressure, LHSV, and H2/Feed Ratio of 385 °C, 1275 psig, 1 h-1, and 600 mL/mL HDN conversion,wt % pressure, psig

acridine

carbazole

9-ethylcarbazole

1120 1275 1420

99.8 99.1 99.8

93.7 95.7 95.3

98.9 98.1 98.1

1120 psig to 1420 psig while keeping temperature, LHSV, and the H2/feed ratio constant at 385 °C, 1 h-1, and 600 mL/mL, respectively. The HDN conversion of acridine, carbazole, and 9-ethylcarbazole did not change significantly with pressure (see Table 1). Ho et al.29 reported no change in HDN conversion of 2,4-dimethylpyridine on a sulfided NiMo/Al2O3 for a wide range of pressure change (4-13 MPa). This table also shows that the HDN conversion of 9-ethylcarbazole (98.1-98.9 wt %) is higher than that of carbazole. The higher conversion of 9-ethylcarbazole than that of carbazole is probably because of the presence of alkyl substituent, which has a higher affinity for Lewis acid sites on the catalyst surface. This selective affinity makes it easier for 9-ethylcarbazole to adsorb on the catalyst surface compared to that of carbazole.15 The conversion of acridine is higher than that of carbazole, especially at low pressure (1120 psig). Nagai and Masunaga27 have also reported that acridine denitrogenated faster than carbazole at low pressure. Our study shows that a low pressure of 1120 psig is suitable to convert over 93% of nitrogen compounds at a reaction temperature of 385 °C, LHSV of 1 h-1, and H2/feed ratio of 600 mL/mL. Effect of Liquid Hourly Space Velocity (LHSV). The effect of LHSV on the HDN activity of acridine, carbazole, and 9-ethylcarbazole has been studied by changing LHSV from 0.5 to 2 h-1 keeping temperature, pressure, and H2/feed ratio constant at 385 °C, 1275 psig, and 600 mL/mL, respectively. The results are plotted in Figure 4. From this figure, it is seen that the HDN conversion of acridine does not change significantly with LHSV. (29) Ho, T. C.; Montagna, A. A.; Steger, J. J. In Proceedings of the 8th International Congress on Catalysis, DECHEMA, 1984; II-257.

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Figure 5. Effect of H2/feed ratio on HDN activity of basic and non- basic nitrogen model compounds (1600 ppm of each in feed containing 2 wt % C4H10S) at temperature of 385 °C, pressure of 1275 psig, and LHSV of 1 h-1.

Figure 6. Effect of mixture of basic and non basic nitrogen model compounds (feed containing 2 wt % C4H10S) on HDN activity at temperature of 385 °C, pressure of 1275 psig, LHSV of 1 h-1, and a H2/feed ratio of 600 mL/mL.

The HDN conversion of carbazole decreased from 96 to 92 wt % with increasing LHSV from 0.5 to 2 h-1. On the other hand, the conversion for 9-ethylcarbazole, which remained constant at 97 wt %, is higher than that of carbazole. It is probably because of the steric effect of the alkyl group attached to the nitrogen atom that makes it easier to be adsorbed on the catalyst surface. Figure 4 shows that at a fixed space velocity the conversions of basic nitrogen compounds (acridine) are higher than those of the nonbasic nitrogen compounds (carbazole and 9-ethylcarbazole). It is important to note that, at higher LHSV, the conversion difference between basic and nonbasic compounds is higher whereas at lower LHSV, the difference is significantly lower. It is probably because, at lower LHSV, most of the nonbasic nitrogen compounds are converted to basic compounds and undergo CdN bond cleavage reaction. So, at lower LHSV, the HDN conversion difference of basic and nonbasic nitrogen compounds is low. This also indicates that nonbasic nitrogen compounds require higher reaction time for their conversion than basic nitrogen compounds. Effect of H2/Feed Ratio. The H2/feed volumetric ratio is an important parameter in the hydrotreatment process. Though some researchers have reported the influence of H2/feed volumetric ratio on the efficiency of hydrodesulfurization reaction, information about the effect of H2/feed ratio on the HDN conversion of basic and nonbasic nitrogen compounds are not available. In this study, the H2/feed ratio was varied from 200 to 800 mL/mL keeping temperature, pressure, and LHSV constant at 385 °C, 1275 psig, and 1 h-1. The results in Figure 5 show that HDN conversion of acridine remained constant at ∼98% with the increase in H2/feed ratio 200 to 800 mL/mL. This is because as the H2/feed ratio increased, the partial pressure of hydrogen increased. The partial pressure of hydrogen favors the prehydrogenation reaction and hence the conversion of nitrogen compounds. But when H2 is present in much excess, the reaction may behave like pseudo-first-order reaction with respect with hydrogen partial pressure.26 So, probably because of that, the HDN activity of acridine did not change with increase in H2/feed ratio. It is seen that the conversion of carbazole increased from 90 to 98 wt % with an increase in H2/feed ratio

from 200 to 800 mL/mL (see Figure 5). It is well-known that the HDN reaction for nonbasic nitrogen compounds requires two steps: (1) hydrogenation of the heterocyclic ring and (2) hydrogenolysis (cleavage of CdN bond).15,28 An increase in hydrogen partial pressure causes an increase in the hydrogenation reaction, which causes an increase in HDN conversion. The increase in H2/feed ratio did not have any significant effect on the HDN conversion of 9-ethylcarbazole. The conversion remained constant at ∼96 wt %. It is probably because of that at higher H2/feed ratio the reaction may behave like pseudo first-order reaction as discussed above in case of acridine. The above results (Figure 5) show that the increase in hydrogen partial pressure is beneficial for nonbasic nitrogen compounds especially in cases containing no alkyl substituent (carbazole), whereas lower H2/oil ratio is better for basic nitrogen compounds. For example, in this present work, ∼98 wt % conversion of basic nitrogen compound was obtained at the H2/feed ratio of 200 mL/ mL, whereas at this H2/feed ratio the conversion of non basic nitrogen compounds (carbazole and 9-ethylcarbazole) were 90 and 96 wt %, respectively. On the other hand, at 800 mL/mL, the basic (acridine) and nonbasic (carbazole and 9-ethylcarbazole) nitrogen compound conversion were 98, 98, and 96 wt %, respectively. Effect of the Presence of Basic Nitrogen Compound in the Mixture of Basic and Nonbasic Nitrogen Compound. The experiments were performed using feed containing different concentrations of acridine and carbazole. In case of feed containing acridine and carbazole, the total concentration of model compounds was maintained at 1600 ppm. The concentration of acridine was varied from 0 to 1600 ppm. The 0 ppm concentration of acridine in the feed indicates that the feed contains only carbazole at 1600 ppm and vice versa. The H2S concentration of the feed solution was maintained constant using 2 wt % butanethiol. Three feed solutions were prepared using different concentrations of acridine and carbazole: 400 ppm acridine and 1200 ppm carbazole, 800 ppm acridine and 800 ppm carbazole, and 1200 ppm acridine and 400 ppm carbazole. The HDN results are shown in Figure 6. It is evident from this Figure that HDN conversion increased with an increase in basic nitrogen concentration in the feed. This is expected since the reactivity of

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basic nitrogen compounds is more than nonbasic nitrogen compounds. As a result, the HDN conversion increased with increasing acridine concentration in the feed. The HDN conversion increased from 89 to 98 wt % when the acridine concentration increased from 400 to 1200 ppm in the feed. It is also observed from this figure that if the feed contains only nonbasic nitrogen compound, its conversion is higher than that the feed containing low concentrations of both basic and nonbasic nitrogen species. For example, the HDN conversion decreased from 94 to 89 wt % when the feed contained 400 ppm of acridine and 1200 ppm of carbazole. It is probably because of the competitive adsorption of acridine and carbazole on the catalyst surface. It is discussed earlier that acridine conversion during HDN reaction is higher than that of carbazole. If the feed contains both acridine and carbazole, because of the basic nature of acridine, it adsorbs faster on the catalyst surface compared to carbazole and therefore, occupies higher number of active sites on the catalyst surface, thereby decreasing the total nitrogen conversion. This hypothesis also supports higher nitrogen conversion (∼99 wt %) when feed contained only acridine (see Figure 6). Possible Reaction Path for the HDN Conversion of Acridine, Carbazole, and 9-Ethylcarbazole. To investigate the presence of different compounds in the final products, 13C NMR and GC-MS analyses were performed for these samples. These products were obtained from acridine, carbazole and 9-ethycarbazole at the reaction conditions of temperature, pressure, LHSV, and H2/Feed ratio of 385 °C, 1275p sig, 1 h-1, and 600 mL/mL. No aromatics were present in the product sample obtained from acridine, which was also confirmed from ∼100 wt % conversion of acridine obtained from total nitrogen analyzer (Figure 3). NMR spectra as well as GC-MS data showed the presence of different alkanes in this product sample. These analyses indicate that hydrogenation of this compounds is complete during HDN reaction. The possible reaction path for acridine HDN reaction is as follows:27

Ferdous et al.

compounds could not be identified. The possible reaction path for the HDN reaction of this compound can be written as follows:27

C13 NMR analysis was done for the liquid product obtained from 9-ethylcarbazole at temperature, pressure, LHSV, and H2/feed ratio of 385 °C, 1275 psig, 1 h-1, and 600 mL/mL. Its conversion was ∼97 wt % (Figure 3). NMR spectra showed that final product from HDN reaction of 9-ethylcarbazole did not contain any aromatics and it contained only alkanes. It is probably because of the complete hydrogenation of 9-ethylcarbazole. The conversion of 9-ethylcarbazole was higher than that of carbazole. The spectra confirms that the presence of ethyl group in carbazole causes an increase in HDN reactivity. The possible reaction path for HDN reaction of 9-ethy carbazole is as follows:

Conclusions

Conversion of carbazole during HDN process was ∼94 wt % at the temperature, pressure, LHSV and H2/feed ratio of 385 °C, 1275 psig, 1 h-1, and 600 mL/mL (Figure 3), which is significantly lower than that of acridine. C13 NMR of this product sample showed the presence of aromatics as well as alkanes. The individual aromatic

The basic nitrogen compound is more easily hydrogenated than nonbasic nitrogen compounds under the process conditions studied. The HDN conversion of acridine was unaffected by butanethiol concentration whereas that of carbazole increased with the increase in butanethiol concentration from 0.25 to 4 wt %. The conversion of acridine was 98-99 wt % in the temperature, pressure, and LHSV ranges of 355-400 °C, 1120-1420 psig, and 0.5-2 h-1. Pressure (1120-1420 psig) did not have any significant effect on the HDN conversion of basic and nonbasic nitrogen model compounds. A lower pressure of 1120 psig is required to obtain HDN conversion over 93 wt % at 385 °C, LHSV of 1 h-1, and H2/feed ratio of 600 mL/mL. With a decrease in LHSV from 2 to 0.5 h-1, the HDN conversion of carbazole was increased from 92 to 99 wt %. However,

Hydrodenitrogenation of (Non)Basic Nitrogen Species

LHSV did not have any effect on HDN conversion of 9-ethylcarbazole. The increase in H2/feed ratio from 200 to 800 mL/mL caused a significant increase in HDN conversion of carbazole from 90 to 98 wt %. The presence

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of the alkyl group in 9-ethylcarbazole did not have a steric hindrance effect on HDN conversion. EF020126C