Energy & Fuels 1992,6, 315-317
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Effect of Ammonia on the Hydrogenation of Naphthalene or Butylbenzene during the Hydrodenitrogenation of Quinoline Chung M. Lee and Charles N. Satterfield* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received October 11, 1991. Revised Manuscript Received February 10, 1992
The hydrogenation of naphthalene in the presence of various concentrations of ammonia was studied during the hydrodenitrogenation of quinoline on a sulfided NiMo/A1203catalyst at 360 "C, 6.9 MPa total H2 pressure, in a vapor-phase tubular reactor. The selectivity to tetralin instead of to decalins was increased by increased ammonia concentration. This observation may have relevance to coal liquefaction. Brief studies on the effect of ammonia on the hydrogenation of butylbenzene are reported.
Introduction There is considerable information in the literature concerning the inhibiting effects of nitrogen compounds on the rate of hydrotreatment of various heterocyclic and aromatic compounds. However, there seems to be little information on how nitrogen compounds may affect the selectivity of these reactions. In a brief study on a presulfided NiMo/A1203 catalyst at 7.0 MPa and 360 OC of the HDS of dibenzothiophene, we observed that the addition of NH3 decreased the formation of partially hydrogenated products.' The present study focused on the effect of NH3 on the selectivity of naphthalene hydrogenation in a mixture with quinoline. In coal liquefaction, it is desirable to maximize partially hydrogenated aromatics such as tetralin which are good hydrogen donors rather than completely saturated hydrocarbons such as decalins, which are poor hydrogen donors. The reactions of quinoline are well-known and nitrogen removal is particularly of concern in hydrotreatment. Naphthalene is readily available and its hydrogenation products are only tetralin and cis/ tram-decalin, simplifying analysis. From a practical point of view, NH,can be readily added and removed from a reacting system if it is found that it produces a desired effect. The hydrogenation of butylbenzene during the HDN of quinoline was also studied in brief to compare the effect of ammonia on hydrogenation of a single-ring aromatic hydrocarbon. Experimental Section The reactor consisted of a vertical 0.52-cm4.d. tube immersed in a fluidized sand bath to maintain isothermal conditions. Liquid was fed through a preheater coil to the top of the catalyst bed by a high-pressure liquid chromatography pump (Milton Roy Constametric 111). Similarly, hydrogen was fed through a preheater coil by a mass flow controller (Brooks). The liquid and hydrogen feeds were mixed at the reactor inlet, passed downward through the catalyst bed, and flashed through a back-pressure regulator (Grove). This equipment has been used in our laboratory in previous studies of HDN, HDO, and HDS and is described in more detail elsewhere.2 Reactants. Seven different feed compositions were studied (Table I). For the f i t five, naphthalene (99+% pure, Aldrich) was dissolved at a concentration of 0.245 mol/L together with 0.0245 mol/L of quinoline (99% pure, American Tokyo Kasei) in a c16 paraffm solvent consisting of about 93% CleHa and 7% other paraffins, mostly C14Hmand C18H3&Ammonia concentrations were produced in situ by the hydrotreating of varying
* Author to whom correspondence should be addressed. 0887-0624/92/2506-0315$03.00/0
Table I. Feed Compositions (wt %) feed number component 1 2 3 4 5 6 7 naphthalene 4.2 4.1 4.1 4.1 4.1 butylbenzene 4.2 4.2 4.7 1.8 propylamine 0.94 1.9 4.1 quinoline 0.49 0.47 0.47 0.47 0.45 0.43 0.43 dodecanethiol 3.1 3.3 3.1 3.1 3.2 3.2 3.2 CIBparaffin 92.2 91.2 90.4 88.2 87.6 92.2 90.4 solvent
amounts of n-propylamine (98% pure, Aldrich) added to the feed. H a was also produced similarly from l-dodecanethiol(98% pure, Aldrich) which was added to the feed to comprise 0.12 mol/L. Under reaction conditions, n-propylamine and 1-dodecanethiol react rapidly to generate NH3 and H2S. The partial pressure of the H2S thus formed was about 12 kPa for all experiments and this kept the catalyst in the fully sulfided form. For feeds 6 and 7, naphthalene was replaced with butylbenzene at the same molar concentration. All runs were made at 360 "C and 6.9 MPa total pressure. Hydrogen (Airco grade 4.5 and Med-Tech high purity, >99.95% purity, were used interchangeably) was fed at a rate of 1520 mL (STP)/mL of liquid, equivalent to 9OOO scf H2/barrel (scf = standard cubic feet at 60 OF). Note that our definition of space-time is based on the moles of aromatic hydrocarbon fed, not total moles of liquid. A liquid hourly space velocity (LHSV) of 10.0 h-l corresponds to a space time of 296 (h/g of catalyst)/(mol of naphthalene) for feeds 1-5 and 263 (h/g of catalyst)/(mol of butylbenzene) for feeds 6 and 7. The reactants and products were all in the gas phase in the reactor and plug flow can be assumed. Catalyst. The catalyst was a commercial NiMo/A120s (American Cyanamid HDS-3A). It was crushed and sieved to a size range of 150-212 pm. After drying in air at 110 OC overnight, 1.6 g of catalyst was diluted with about 15 g of a similar-sized inert Sic and packed into the reactor tube. After the reactor was purged under helium flow for 12 h at 100 OC, the catalyst was presulfided with a 10% mol H a in H2gas mixture. The gas flow rate was 40 mL/min at a total pressure of 0.24 MPa. Sulfiding temperature was maintained at 175 "C for 6 h, and the temperature was then raised to 315 O C at a rate of 1 OC/min. The gas flow was switched to helium after sulfiding had continued for 1 h at 315 O C . For the first 200 h after sulfiding,the HDN of 4 w t % quinoline in the cl6 paraffin solvent was conducted at 360 "C and 6.9 MPa total pressure for routine deactivation of the catalyst until there was a steady level of activity. Standard catalyst activity was measured at a space time of 320 (h/g of catalyst)/mol of quinoline under the same conditions. During the conditions of this study, (1)LaVopa, V.; Satterfield, C. N. Chem. Eng. Commun. 1988, 70,171. ( 2 ) Satterfield, C. N.; Yang,S . H.Id. Eng. Chem.,Process. Des. Deu. 1984, 23, 11.
0 1992 American Chemical Society
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316 Energy & Fuels, Vol. 6, No. 3, 1992
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Figure 1. Space-time plot for the hydrogenation of naphthalene during the HDN of quinoline. Feed 1, no added ammonia. l
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Figure 3. Effect of ammonia on the HDN of quinoline. Space Time 320 (hi-gcotlmal Naphlhalsnrl A 640 (hi-gcat/mol Naphthalene)
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Figure 2. Space-time plot for the hydrogenation of naphthalene during the HDN of quinoline. Feed 4,48P a of added ammonia.
this batch of catalyst had a standard activity for HDN of quinoline to hydrocarbon products of 30%. Analysis. Liquid samplea of the reactor effluent were collected downstream from a low-pressure phase separator, and analyzed on a Perkin-Elmer Sigma 1B gas chromatograph equipped with dual flame ionization detectors. Concentrations of the products were related to the starting liquid feed. Carbon mass balances on total products compared to naphthalene or butylbenzene disappearance ranged from 95 to 105%. Separation of reaction species was accomplished by using either Supelco SPB-35 (30 m, 0.25 mm i.d., 0.25 pm) or Supelcowax 10 (30 m, 0.25 mm i.d., 0.25 pm) columns. Compounds were identified by comparing their retention times to those of pure standards.
Results The principal products from the HDN of quinoline are the two tetrahydroquinolines, decahydroquinoline, opropylaniline, and various hydrocarbons, plus NH3. More details are given by Satterfield and Smith.3 Tetralin and cisltrans-decalin are the main products from the hydrogenation of naphthalene. Each of feeds 1through 5 was studied a t six different space-times, resulting in data on the relative concentration of naphthalene, tetralin, and decalins in the product as a function of spacetime for reaction in the absence of added ammonia and for four increasing concentrations of NH3 from the added propylamine. Figures 1 and 2 show illustrative results for no added NH3 and for 48 kPa of NH3 present. Complete HDN of the quantity of quinoline present produces a NH3 partial pressure of only 3 kPa, so the effects of NH3 may be attributed to that deliberately added via propylamine. The solid lines in Figures 1 and 2 represent kinetic model predictions for naphthalene hydrogenation as discussed in the following. Figure 3 shows the percent HDN of quinoline for the same data (3) Satterfleld, C. N.;Smith, C. M. Znd. Eng. Chem. Process. Des. Dev. 1986,25,942.
Added Ammonia P a r t i a l Pressure, kPa
Figure 4. Selectivity to tetralin improved by the addition of ammonia at space times with essentially complete HDN of quinoline.
(HDN is defined here as the degree of complete removal of nitrogen from the sum of quinoline and N-containing intermediates, to form hydrocarbons). These figures show that ammonia inhibits both naphthalene hydrogenation and quinoline HDN. These are integral data, so the effect of NH3 on percent HDN of quinoline can only be seen in Figure 3 at the lower space-times where conversion ia less than complete. Figure 4 shows a plot of the ratio of decalins to tetralin as a function of NH3 concentration for two space-times at which HDN was complete. This shows the considerable effect of added NH3 on increasing the selectivity to tetralin. Kinetic Model. The detailed development of the kinetic model is available elsewhere (Lees). The hydrogenations of naphthalene and tetralin were each taken to be first order reactions, i.e., ki
N-T-D
kz
(1) where N is naphthalene, T is tetralin, and D is cis/ trans-decalin. For the HDN reactions, quinoline and the nitrogencontaining intermediates from the HDN of quinoline were grouped together as one component, B, and the reaction was taken to be first order. Ammonia was treated separately as A. ka
B-A
A Langmuir-Hinshelwood kinetic model was employed that used eqs 1and 2 and incorporated terms for the inhibition caused by N-containing intermediates as one class and ammonia. Since hydrogen was always present in great excess and all runs were made at the same pressure, no term for hydrogen was included and adsorption of decalins was assumed to be insignificant. The model assumes that hydrogenation and hydrogenolysis occur on the same site, that hydrogenated products are not appreciably adsorbed, and that reversible reactions
Energy & Fuels, Vol. 6, No. 3, 1992 317
Hydrodenitrogenation of Quinoline IO0
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Figure 6. Effect of ammonia on the HDN of quinoline in the presence of butylbenzene.
Figure 6. Spacetime plot for the hydrogenationof butylbenzene during the HDN of quinoline. Feeds 6 and 7.
are neglected. The so-called adsorption parameters were estimated from a correlation we published recently of adsorption inhibition terms for reacting species as a function of proton affinity (LaVopa and Satterfield4). That correlation gave values for quinoline, NH3, and naphthalene. For present purposes, the proton affinity of tetralin was assumed to be equal to that of benzene which in turn was estimated from data by Lau et aL5to be 188.7 kcal/mol. The resulting values of the adsorption parameters were, in descending order relative to tetralin: quinoline, 110; NH3, 9.2; naphthalene, 3.8; and tetralin, 1.0. The model was thus reduced to four adjustable parameters consisting of three rate constants and a proportionality constant. The calculated values of the rate constants were kl = 0.021, k2 = 0.00065,and k3 = 0.040, in units of mol of reactant/(h g of catalyst). The ratio of rate constants klto k2 in the hydrogenation of naphthalene is reported by Sapre and Gates' for reaction on a sulfided Co0-Mo03/yA1203at 325 OC and 7.5 MPa. For the conversioii of tetralin to cis- and trans-decalins, they reported that trans-decalin was formed considerably more rapidly than cis-decalin. The ratio of the rate constant for naphthalene to tetralin divided by that for tetralin to trawdecalin was 41 for their system with which our value of 33 is in good agreement. Our model was used to predict the concentrations of naphthalene and its hydrogenated products when ammonia was added. The model well fitted the selectivity of naphthalene to tetralin, as shown in Figure 4. It was less accurate a t predicting the concentration profile of tetralin, but it fitted the profiles of naphthalene and cisltrans-decalin well, as shown in Figures 1 and 2. Butylbenzene. The principal product from the hydrogenation of butylbenzene (BB) is butylcyclohexane (BCH). There was no significant reaction to butylcyclohexane under our conditions. Runs 6 and 7 were designed to in effect replace naphthalene with butylbenzene holding all other variables as constant as possible, maintaining the same solvent. Propylamine was added in run 7 to generate 24 kPa of NH3 but none was added in run 6. Figure 5 shows the percent HDN of quinoline versus space time for these two seta of runs. As with naphthalene, the presence of 24 kPa of NH3 slightly reduced the percent HDN of quinoline (runs 6 and 7). However, even after the HDN
of quinoline was completed, the 24 kPa of NH, moderately suppressed the hydrogenation of butylbenzene (Figure 6). Kinetic Model. In this case, a similar parallel reaction model as in the case of naphthalene was assumed. Butylbenzene is hydrogenated to butylcyclohexane as follows
(4) LaVopa, V.; Satterfield, C. N. J . Catal. 1988, 110, 375. (5) Leu, Y.K.;Saluja, P. P. S.;Kebarle, P.; Alder, R. W. J. Am. Chem. SOC.1978,100, 7328. (6) Lee, C . M. Sc.D. Thesis, MIT, 1992. (7) Sapre, A. V.; Gates, B. C. I d .Eng. Chem.,Process. Des. Deu. 1981, 20, 68.
BB -% BCH (3) where B is butylbenzene and BCH is butylcyclohexane, and quinoline reacts via eq 2. Similar assumptions as in the above kinetic model for naphthalene were made and butylbenzene was taken to behave similarly to tetralin. Using the same adsorption parameters as in the case of naphthalene, the effect of ammonia on the hydrogenation of butylbenzene during the HDN of quinoline was then made. Figure 6 shows space time plots of BB and BCH with model predictions. The model slightly overpredids the BCH profile. Discussion Ammonia inhibits the hydrogenation of tetralin to decalins more than the hydrogenation of naphthalene to tetralin. The effects can be interpreted in terms of competitive adsorption. Ammonia is weakly adsorbed, but as ita concentration is increased, its coverage of hydrogenation sites increases. Tetralin is much more weakly adsorbed than naphthalene or ammonia; thus, as ammonia concentration is increased, the added coverage of ammonia prevents the tetralin from adsorbing onto the surface preventing hydrogenation. Naphthalene, which is more strongly adsorbed than tetralin, better competes with ammonia for the hydrogenation site. The net effect is improved product selectivity to tetralin. Summary In a mixture of naphthalene and quinoline with a low concentration of added ammonia, there is an operating region in which essentially complete hydrodenitrogenation of quinoline can be achieved, but the naphthalene is converted to a greater extent to tetralin instead of decalin than is the case in the absence of added ammonia. This observation may be of importance in coal liquefaction in which tetralin is a good hydrogen donor but decalin is not. Acknowledgment. This study was supported by the Office of Fossil Energy, U.S.Department of Energy, under Grant No. DE-FG22-89PC89775. Registry No. BB, 104-51-8; NH3, 7664-41-7; naphthalene, 91-20-3; tetralin, 119-64-2;cis-decalin, 493-01-6; trans-decalin, 493-02-7; quinoline, 91-22-5.
