Energy & Fuels 1994,8, 1384-1393
1384
High-pressure Hydrogenation of Naphthalene Using a Reduced Iron Catalyst Xiaodong Zhan and James A. Guin* Chemical Engineering Department, Auburn University, Alabama 36849 Received June 27, 1994. Revised Manuscript Received August 26, 1994@
Based upon the interest in iron as a potential dispersed phase catalyst for synfuels production, the high-pressure hydrogenation of naphthalene in mineral oil and cyclohexane solvents was studied in trickle flow and vapor-phase reactors, respectively, with an unsupported iron oxide powder as catalyst. Several forms of iron catalyst were examined including the original iron oxide, hydrogen prereduced, and in-situ sulfided forms. A commercial Ni-Mo/AlzOs catalyst was used for comparison. Iron oxide was not active for hydrogenation; however, in situ reduction by hydrogen yielded a remarkably active hydrogenation catalyst. The secondary hydrogenation reaction to form decalins from tetralin occurred only with the hydrogen reduced iron. Kinetic behavior was determined in the vapor phase at 6.9 MPa and at temperatures over a range of 160-300 "C. Trickle bed experiments showed the iron oxide catalyst to be much more rapidly deactivated than the baseline NiMo/AlzOs. In vapor-phase operation using reduced iron as catalyst, rapid deactivation occurred with a 2.0 wt % naphthalene feed, whereas little deactivation was found with a 0.2%feed. Transport limitations found with NiMo/AlzO3 catalyst extrudates were consistent with estimated effectiveness factors, and were eliminated by crushing the extrudates to 40-50 mesh. Sulfur rapidly poisoned the hydrogenation function of the iron catalyst with the order of decreasing activity being Fe >> FeS, or FezOs. A proposed LangmuirHinshelwood rate expression, which reduced to a first-order dependence on both naphthalene and hydrogen, was found to satisfactorily fit the data. Although the reduced iron catalyst exhibited high hydrogenation activity under certain conditions, its propensity toward deactivation and poisoning would limit its application in direct coal liquefaction.
Introduction Recently there has been much interest in the use of iron based compounds as disposable catalysts for coal 1iquefaction.l The importance of iron as a catalyst lies in the fact that it can serve as an economical and environmentally acceptable disposable catalyst.2 The advantages of the dispersed phase catalyst in coal liquefaction over conventional catalysts have been extensively discussed in the l i t e r a t ~ r e . ~During -~ the course of our investigation of iron as a potential dispersed phase catalyst for coal liquefaction, it was found that the Hz-reduced form of iron yielded a remarkably active catalyst for the hydrogenation of aromatic ring compounds.2,6 Although hydrogenation is usually carried out using active metals such as Ni, Pt, and Pd, consideration of iron is of interest for the reasons given above. At the high-pressure conditions typical of coal liquefaction, several classes of reactions occur simultaneously, including hydrodesulfurization, hydrodenitro@Abstractpublished in Advance ACS Abstracts, October 1, 1994. (1)Symposium on Iron Based Catalysts for Coal Liquefaction, 205th National ACS Meeting, Denver, Co. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1993,38 (11,1-239. Also see: Energy Fuels 1994,8. (2)Guin, J.A,; Zhan, X.; Linhart, R. S. Energy Fuels 1994,8,105112.
(3)Hirschon, A. S.;Wilson, R. B. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1989,34(3),881-885. (4)Pradhan, V. R.;Tierney, J. W.; Wender, I. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990,5 (31,793-800. ( 5 ) Utz, B.R.; Cugini, A. V.; Frommell, E. A. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1989,34 (41,1423-1430. (6)Guin, J.A.; Zhan, X.; Linhart, R. S. Prepr. Pap.-Am. Chem. SOC., Diu.Fuel Chem. 1993,38 (11, 86-92.
0887-0624/94/2508-1384$04.50/0
genation, hydrodeoxygenation, and hydrogenation of aromatics. Among these, hydrogenation reactions are the lo west.^ The importance of hydrogenation stems from several factors including, for example, the fact that denitrogenation of aromatics does not occur until the N-containing aromatic ring has been saturated. Hydrogenation reactions are also important in coal liquefaction from the aspect of maintenance of donor solvent quality, e.g., production of hydroaromatic compounds. In addition, regulations currently being promulgated for transportation fuels specify reduced aromatics contents, thus requiring a greater degree of hydrogenation, especially with regard to highly aromatic coal derived liquids. A literature review revealed that kinetic studies using iron catalysts were sparse, especially in the high-pressure reaction regime representative of coal liquefaction. For these reasons we have performed a study of the hydrogenation activity of iron catalyst in the high-pressure region. In previous work, Yoon and Vannicea studied the kinetic behavior of iron catalysts for benzene hydrogenation at low pressures. They found that a simple power rate law could not adequately describe the kinetic behavior exhibited by their iron catalysts. The H2 dependence was observed to be near third order and the benzene dependence increased from inverse first order to near zero order as temperature increased. Their work also showed a maximum in the rate of hydrogenation as a function of temperature with (7) Sapre, A. V.; Gates, B. C. Ind. Eng. Chem. Process Des. Deu. 1981, 20,68-73. (8)Yoon, K.J.;Vannice, M. A. J. Catal. 1983,82,457-468.
0 1994 American Chemical Society
High-pressure Hydrogenation of Naphthalene
Energy & Fuels, Vol. 8, No. 6,1994 1386
Figure 1. Experimental apparatus.
the rate decreasing at higher temperature. Recently the activity of three iron carbonyl based catalyst precursors for the hydrogenation of 4-(naphthylmethy1)bibenzyl (NBBM),a model compound representative of coal, was r e p ~ r t e d .The ~ catalytic activity of iron in this reaction was attributed to the formation of a reduced iron phase capable of dissociating Hz into atoms which were then inserted into the naphthalene system of NBBM. The activity of iron in our work is also attributed to a reduced iron phase. In our study, naphthalene was chosen for the investigation of the hydrogenation activity of iron catalyst at high pressure because it is the simplest compound bearing resemblance to the polynuclear aromatics in coal and coal liquids. Naphthalene hydrogenation is representative to some degree of reactions believed to occur during primary coal liquefaction and associated secondary upgrading processes. Most research concerning naphthalene hydrogenation has been conducted using commercial hydrotreating catalysts such as NiMo/Al2O3 and Co-Mo/AlzOs and their sulfided forms. The results of these studies show that hydrogenation of naphthalene is sequential, with the rate of the primary reaction, i.e., from naphthalene to tetralin, being an order of magnitude faster than that of the ~J~ secondary reaction, i.e., from tetralin to d e ~ a l i n s . The reversibility of the primary hydrogenation of naphthalene and the high irreversibility of the secondary hydrogenation of tetralin have been well documented.7J1J2 Equilibrium relations for these reactions have been developed under vapor-phase conditions.l3 Researchers studying naphthalene hydrogenation have been principally concerned with catalyst performance, e.g., catalyst activity testing,14deactivation behavior,15 (9) Walter, T.D.;Casey, S. M.; Klein, M. T.; Foley, H. C. Energy Fuels 1994,8,470-473. (10)Broderick, D. H.; Sapre, A. V.; Gates, B. C.; Kwart, H.; Schuit, G. C. A. J. Catal. 1982,73,45-49. (11)Girgis, M. J.; Gates, B. C. Znd. Eng. Chem. Res. 1991,30,20212058. (12)Bacaud, R.;Besson, M.; Qega-Mariadassou, G. P r e p . Pap.-&. Chem. Soc., Diu. Fuel Chem. 1993,38 (l), 1-7. ( 1 3 ) h y e , C. G.; Weitkamp, A. W. J. Chem. Eng. Data 1969,14, 372-376. (14) Johnson, B. G.;Massoth, F. E.; Bartholdy, J. AlChE J. 1986, 32 (12),1980-1987.
and inhibition by sulfur.16 In the present work, the hydrogenation of naphthalene was investigated with an unsupported iron powder catalyst in several forms, including the original iron oxide, prereduced, and sulfided forms. A commercial Ni-Mo/AlsOs catalyst also was examined for the purpose of comparison. Catalyst activities were examined in both trickle-bed and vaporphase conditions.
Experimental Section Chemicals. The following chemicals were used as received: naphthalene (Fisher, purified), cyclohexane (Fisher, reagent), decane (Fisher, certified), mineral oil (Humco Laboratory Inc, specific gravity 0.8478at 25 "C, kinematic viscosity 16.1 mm2/s at 40 "C, and 86.8 Saybolt universal seconds at 100 OF, respectively), CS2 (Fisher, reagent), hydrogen (Airco, UHP), nitrogen (Airco, UHP). Catalysts. Ni-Mo/AlzOa (Shell 324)was a 1/16in. extrudate having a composition of 2.8% Ni and 13.6% Mo, particle density 1.38 g/cm3, pore volume 0.471 cm3/g, average pore diameter 100 A, and surface area 174 m2/g. It was used as both extrudate (1/16in.) and crushed particles (40-50 mesh, 80-100 mesh) without sulfidation. Iron oxide powder was 99.8% a-FezO3 from Strem Chemicals, Inc, with specific gravity 5.24,BET surface area 2.78 m2/g,and particle size 400 mesh. Three different forms of iron catalyst were employed: iron oxide as received, iron sulfide, and hydrogen prereduced iron. Iron sulfide was obtained from in-situ sulfidation by introducing 2 wt % CS2 into the feed. In some experiments, the iron catalyst was prereduced in situ in a hydrogen atmosphere at 6.9MPa, 400 "C, with a hydrogen flow rate of 100 sccm for 24 h and then cooled down in the same hydrogen atmosphere for 1h t o the reaction temperature before starting the feed. Experimental Apparatus and Procedure. All experiments were performed in the continuous flow tubular reactor system shown schematically in Figure 1. The system is capable of operation at pressures to 17 MPa, temperatures to 680 "C, liquid flows to 5 cm3/min, and gas flow rates to 2500 sccm. Liquid and gas feeds are mixed to produce a dispersive stream entering an externally heated reactor tube. The NiMo/Al203 extrudates were packed in the center of the reactor, a 316 stainless steel tube 450 mm long, 13 mm i.d, and were (15)Wang, W. P.; Guin, J . A. Fuel Process. Technol. 1991,28, 149166. (16) Rhee, Y.W.; Guin, J. A.; Curtis, C. W. Fuel Process. Technol. 1988,19,1-5.
1386 Energy & Fuels, Vol. 8, No. 6, 1994 preceded by 3 mm glass beads to preheat and to uniformly distribute the reactant mixture. The catalyst was followed by glass beads supported on glass wool to minimize end effects. For the crushed particles of Ni-Mo/AlzOs and unsupported iron particle catalysts, a 0.5 pm pore size stainless steel frit welded inside the reactor tube 150 mm from the bottom served as a catalyst support. Glass beads 0.1-0.12 mm diameter preceded the powder catalysts. Reaction products were separated in a liquidgas separator. Liquids were analyzed by gas chromatography using temperature programming t o improve peak resolution and decane as an internal standard. Peak identification was made from previous literature results confirmed by standard addition of authentic compounds. In all of the experiments in this work, the reactor was operated at 6.9 MPa with a pressure drop across the reactor bed of about 0.07 MPa. Nitrogen was used to purge and pressurize the reactor before starting the reactant feeds. Hydrogen was always present in large stoichiometric excess. In many cases, the same catalyst charge was used for several reaction conditions, although for each type of reaction, at least two separate runs with fresh catalyst charges were performed to assure the data were reproducible. Steady state was ascertained by invariance of the liquid product composition. The extent of catalyst deactivation was determined by returning to previous run conditions after conducting a series of runs at various conditions. Trickle-Bed Reactions. In the trickle-bed reactions, except for the runs for kinetic studies, the catalyst activity was investigated on Ni-Mo/AlzOs extrudate (10 g) and iron powder (22 g) at 6.9 MPa (1000 psi) and 200 to 425 "C. The flow rate of liquid feedstock, consisting of 2 wt % naphthalene in mineral oil, was held as 0.2 cm3/min and Hz flow rate at 100 sccm. The WHSV was 1 h-' for Ni-Mo/AlzOs and 0.5 h-l for iron powder, molar ratio of hydrogen to naphthalene was 170, and ratio of hydrogedliquid was 500 cm3 (STP)/cm3 for both catalysts. The flow rates of liquid and gas were such that the liquid was in the trickle flow regime with continuous gas phase.17 Vapor-Phase Reactions. Fixed-bed vapor-phase reactions were carried out at 6.9 MPa and 160,200,250, and 300 "C. A liquid reactant feed solution containing 0.2 wt % naphthalene in cyclohexane was vaporized prior to entering the reaction zone by a preheating section consisting of glass beads. NiMo/Al203 catalyst was used in three different particle sizes (1/16 in. extrudate, 40-50 mesh and 80-100 mesh) to ascertain the extent of internal diffusion limitations. Iron catalyst was prereduced in situ in hydrogen following the procedure described earlier. In a typical run, the ratio of gasfliquid feed was held at 2000 scc per cm3 of liquid feed so as to keep the naphthalene feed concentration constant. The molar ratio of hydrogen to naphthalene was about 7000 so that hydrogen was in large excess. WHSV values ranged from 7.3 to 37.1 h-I for Ni-Mo/AlzOs and 3.3 t o 16.6 h-l for reduced iron. Flow rates from 0.05 t o 0.5 cm3/min for liquid and from 100 to 1000 sccm for hydrogen were used t o determine reaction order for naphthalene. The hydrogen rate dependence was determined by reducing the hydrogen flow rate by 30 % with nitrogen providing the balance so that the total gas flow rate and naphthalene feed concentration remained constant. Thermodynamic calculations showed that the preceding range of flow rates assured complete vaporization of liquid feed at reaction conditions.ls
Results and Discussion Trickle-Bed Reactions. Hydrogenation of naphthalene (NAPH) yields tetralin (TET), trans- and cis~~~~~
(17)Satterfield, C. N. AIChE J. 1975,2 (2), 209-228. (18)Zhan, X. Ph.D Thesis, Auburn University, in progress.
Zhan and Guin 1.0 -3
i AL&&A
-A
m
0
25
50
75
Ni-Mo/A1203. FezO,, FezO3, ! Pre-reduced
300°C 300'C
250'C Fe, 250°C
100 125 150 175 200 225 250
TIME
(hr)
Figure 2. Extent of hydrogenation of Ni-Mo/AlzOs and prereduced iron catalysts at different temperatures in the trickle-bed reactor.
decalin (DEC) according the following ~toichiometry.~
m w
'
trans-decalin
\
naphthalene
tetralin cis-decalin
In order to represent the hydrogenation activity of the catalyst in a simple manner, an extent of hydrogenation AH was defined based on the above stoichiometry.
In eq 1, the Mi are the moles of compound i in the reaction products. According to eq 1,AHvaries from 0 to 1 as the products vary from totally aromatic (no hydrogenation, pure naphthalene) to completely saturated (100% hydrogenation, pure decalin). If there is no decalin in the products, the maximum value of AHis 0.4corresponding to 100% tetralin. Control experiments performed in the trickle-bed reactor with glass beads alone at a liquid feed flow rate of 0.2 cm3/minand HPflow rate of 100 sccm produced a maximum AH of 0.05 in a temperature range of 250400 "C. This conversion is negligible compared to that with catalyst indicating that the observed reactions were due t o catalytic effects. Material balances for naphthalene and its hydrogenation products (tetralin and trans- and cis-decalin) in most of the experiments were typically within 95% with the balance probably being volatile gaseous products, coke deposits on the catalyst, and unidentified products mixed with the cracking products of mineral oil at high temperature. Effect of Catalyst Prereduction on AH. Figure 2 shows the extent of hydrogenation AH defined by eq 1 vs time for Ni-Mo/AlzOs extrudate and iron at 250 and 300 "C. No significant deactivation was observed for Ni-Mo/AlzOs with the extent of hydrogenation basically the same at identical conditions after 340 h. During
High-pressure Hydrogenation of Naphthalene
the 340 h period, several other reaction conditions were employed including a temperature of 425 "C; however, all data points shown were at 300 "C. A s indicated, the extent of hydrogenation was unchanged when the temperature was returned to 300 "C. In contrast, for Fez03 at 300 "C, the catalyst activity increased gradually to a maximum in the first 48 h followed by significant deactivation. Based on results shown later, the increase in activity in the first 50 h is believed to reflect an activation of the iron catalyst by H2 reduction. This view is further substantiated by the results of experiments at 250 "C also shown in Figure 2, where the catalyst was examined with and without in-situ H2 prereduction and with more frequent product analysis. With Fez03 at 250 "C, samples were taken every 2 h during the first 24 h; however, there was essentially no conversion for the first 6 h. AH then increased rapidly during the next 18 h and reached a relatively stable level without further deactivation in the following 54 h. In contrast, the H2 prereduced catalyst showed immediate high activity at 250 "C without any apparent deactivation for 150 h. The level of activity (AH= 0.6) of the prereduced catalyst reflects a product distribution of 1.0% naphthalene, 66.3%tetralin, and 32.7%decalin. Comparing the two runs at 250 "C showing the behavior of the iron catalyst with and without prereduction, it can be concluded that the original Fez03 has very little hydrogenation activity while in-situ prereduction in hydrogen yields a highly active catalyst. On the basis of the two runs at 300 "C one can conclude that the ironbased catalysts are also more susceptible to deactivation than the Ni-Mo/AlaOs. Prior to the continuous flow reactions reported herein we performed some batch reactions for the hydrogenation of naphthalene and biphenyl in a tubing bomb microreactor using a dispersed iron oxyhydroxide catalyst.2,6 In this earlier work, we found that when the catalyst was used in its original oxide form at 350 "C, the extent of naphthalene hydrogenation increased sharply in a very narrow range of catalyst loading t o a constant level, correspondingto the complete conversion of naphthalene to tetralin. The secondary ring hydrogenation to decalins took place only when the iron catalyst was prereduced. No decalin was produced even when the loading of original oxide catalyst was almost 10 times that of the baseline amount. Conclusions from these earlier batch reactions were that the hydrogenation of the second aromatic ring required catalytic sites of greater activity, e.g., reduced iron sites, rather than merely the increase in number of original sites. It was speculated that when the original iron oxyhydroxide catalyst was used, a partial in-situ self-reduction occurred and the active sites were a reduced iron phase. In the continuous trickle-bed reactions in this work with iron oxide as the initial catalyst, it appears that a similar self-reduction process occurs. Significant secondary hydrogenation (production of decalins), as reflected by the value of AH greater than 0.4 (Figure 2, 300 "C), can be attributed to the formation of an increasing number of reduced iron phase active sites. At a naphthalene conversion of loo%, the ratio of decalins to tetralin was about 3.5 at 300 "C for the continuous reaction data in Figure 2 while it was essentially zero at 350 "C in the tubing-bomb batch reactor.2 The fact that no secondary hydrogenation
Energy & Fuels, Vol. 8, No. 6, 1994 1387 1 .o CXX2W C S p in feed Pinal 32 hrs
uC S p
in feed first 92 hrs
*a CSz * in feed first 48 hrs
4
z 0.6
f
L4 0
b
E b
0.2
X
w
0.0 5
Figure 3. Effects of Fez03 catalyst sulfidation on the extent of hydrogenation at 250 "C in the trickle-bed reactor. (a) CS2 introduced, (b) CS2 removed.
occurred in the batch reactor was probably caused by the accumulation of water produced during the reduction process. In the batch reactor, the water concentration in the gas increases as the reduction process proceeds. As reported by Pernicone,lgowing to the high activity of the iron surface toward oxygenated compounds, repeated oxidation-reduction reactions may occur in a water-containing hydrogen atmosphere with consequent recrystallization and growing of iron crystallites. Furthermore, it is known that at temperatures below 570 "C, the reduction of Fez03 proceeds in a stepwise manner, i.e., complete reduction to Fe304 and then to Fe.20 The possibility of Fe304 reduction to Fe is limited by the HzO/H2 ratio (for example, (H2O/H2),, = 0.09 at 400 "C).19 Therefore, in the batch reactor, little or incomplete reduction of Fe304 t o Fe would be expected. On the other hand, when the iron catalyst is prereduced in the continuous flow reactor, water is removed and complete reduction to Fe is eventually possible as long as the reduction time is sufficient. Jung20 observed that unsupported a-Fe203 was completely reduced to Fe in minutes by flowing hydrogen at 400 "C. The difference in extent of reduction probably accounts for the greater activity of the iron catalyst in the continuous flow reactor. Effect of Catalyst Sulfidation on AH. Results of three runs at 250 "C with a sulfur source (CS2) added t o the feed in different periods are shown in Figure 3. The iron catalyst was charged in its original oxide form. The sulfur source was CS2 which converts rapidly to H2S under reaction conditions, reportedly at least 8000 times faster than that of the aromatic hydrogenation reaction7. In the first run (also partly shown in Figure 21, liquid feed without CS2 was charged for the first 72 h at which time the liquid feed was spiked with 2% CS2. AH then fell continuously to zero after 32 h, indicating sulfur poisoning 'of the hydrogenation sites. The other two runs were performed by feeding the CS2 spiked feed from the beginning and then switching to a CS2-free feed. In these two runs, no conversion was observed (19) Pernicone, N.;Traina, F.Preparation of Catalysts IZ.Proc. 2nd Znt. Symp. 1978,321-351. (20)Jung, H.; Thomson,W. J. J. Catal. 1991,128,218-230.
Zhan and Guin
1388 Energy & Fuels, Vol. 8, No. 6, 1994
4.0 4 using CS2 spiked feed until introduction of CS2-free feeds at 48 and 92 h, respectively, following which both * * * * * 3.01 g Fez03 6.03 g Fen03 runs showed small, but stable AH as the residual HzS O O O O D 10.1 g Fe203 concentration in the reactor became depleted. This __ In( l / l - X ~ ) = 0 . 0 4 1 8 ( W / q L ) small activity indicates that the catalyst following the 3.0 removal of CSZfrom the feed has finite, albeit significantly lower, aromatic hydrogenation activity than H2 x"I prereduced iron. The difference in conversion in the presence and absence of CS2 in the feed may be caused by some structural change in the catalyst involving a loss of S in the pure H2 atmosphere. It is known that 4 , the particular phase of iron sulfide present in the reactor depends on the H2S/H2 partial pressure. Thus some loss of S with an associated catalyst structural change is likely upon the removal of the H2S precursor. The exact form of the sulfided iron phase was not determined; however, iron catalysts are known t o quickly form pyrrhotite, FeS, ( x =1.05-1.15), under w/qL ( m i n - d m l ) high-pressure coal liquefaction conditions in a H n z S environment, regardless of the initial form of the iron.' Figure 4. First-order kinetic plot using eq 2 at 200 "C on prereduced iron catalyst in the trickle-bed reactor. The significance of the slight maxima at 75 and 100 h may indicate that the catalyst composition with regard as observed at 200 "C, however, the deviation between to sulfur, e.g., x in FeS,, influences the extent of various runs was considered too great to allow deterhydrogenation. A second factor which cannot be commination of kinetic parameters.18 These differences pletely dismissed in the presence of CS2 is the possibility were believed to result from different flow paths, i.e., of H2S inhibition. It is known that HzS inhibits contacting efficiency, in the trickle-bed regime as a hydrogenation of aromatics with the exact degree of inhibition depending on the reaction c o n d i t i ~ n s . ~ ~ result ~ ~ ' ~of~slight ~ ~ ~differences ~ ~ ~ ~ in catalyst packing in the reactor tube. Such differences did not occur in the The exact degree to which each of these factors, i.e., single-phase (vapor-phase) fixed bed experiments decatalyst structural change and inhibition, is operative scribed in the following sections. Therefore, to examine cannot be ascertained from these runs; however, we can the kinetics using the iron-based catalyst in more detail, conclude that in the absence of H2S, the aromatic attention was focused on a fixed-bed vapor-phase operahydrogenation activity of the iron catalyst is such that tion. Fe >> FeS, or FezOs. Vapor-PhaseReactions. By changing the feed solvent Trickle-Bed Reaction Kinetics. A liquid-phase from mineral oil to cyclohexane and suitably adjusting hydrogenation reaction was carried out in the tricklethe flow rate, the reaction was caused t o proceed in the bed reactor using the prereduced iron at 200 "C and 6.9 vapor phase. In a typical vapor-phase reaction experiMPa. Liquid flow rates varied from 0.15 t o 2.5 cm3/ ment, various flow rates of liquid and gases were used min at several catalyst loadings (WHSV = 0.8-42.2 h-l) to determine the naphthalene order of the reaction rate with a Ha flow rate of 500 sccm per cm3 liquid. a t constant temperature. At the end of an experiment, Satterfield17has pointed out that, for many systems of the first reaction condition was repeated to determine interest where a high percent of conversion is not the extent of catalyst deactivation. In most cases, the required, the kinetics of a reaction in a trickle-bed naphthalene conversion decreased only about 10%after reactor can be satisfactorily represented as a first-order a 72 h reaction period. Using this data, a small process even though the true kinetics may be different. correction to the experimental data was made by asIn the present work, an ideal first-order plug flow model suming the catalyst activity decline (conversion dedescribing the reactor performance can be expressed as crease) was proportional to time on-stream. The secondary hydrogenation reaction, Le., from tetralin to In(-) 1 = KL-W decalins, also occurred to a certain extent in most cases. 1 qL This secondary reaction was assumed to have negligible influence on the kinetics of the primary reaction of naphthalene t o tetralin. Equilibrium between naphThe linear plot of ln(l/l - X N ) vs W/qL in Figure 4 thalene and tetralin was not approached under our indicates that the hydrogenation reaction follows apreaction conditions with the equilibrium conversion of parent first-order kinetics. The effects of backmixing naphthalene being greater than 99.97% according to and liquid holdup are not important since the reaction Frye's ~orre1ation.l~ can be described with an ideal plug flow The Catalyst Deactivation. Figure 5, plotted as naphapparent rate constant, kL, as obtained in Figure 4, was thalene conversion vs the total amount of naphthalene/ 0.0418 f 0.0015 cm3/(min.g). This value will be comcyclohexane feed passed over the catalyst bed, compares pared with that in the vapor-phase reaction in the the activities of catalysts with two different naphthalene following section. Trickle phase reactions were also concentrations in the feed. It can be seen that when carried out a t 250 "C with similar qualitative behavior the feed contained 2 w t % naphthalene, the reduced iron (21) Sapre, A. V.; Gates, B. C. Ind. Eng. Chem. Process Des. Dew. catalyst deactivated too rapidly for meaningful kinetic 1982,21, 86-94. studies. Furthermore, this deactivated catalyst could (22) Paraskos, J. A.; Frayer, J. A,; Shah, Y. T. Ind. Eng. Chem. not be regenerated by H2 treatment at 400 "C. HowProcess Des. Dew. 1975, 14, 315-322.
-
-x,
11
I
6 A b b A
Energy & Fuels, Vol. 8, No. 6, 1994 1389
High-pressure Hydrogenation of Naphthalene 1 .o
0.8
x"
.O
0
1 0.6
0 c-(
m X
w > z
0.4 1
I
0.4
3
..-.*
mesh, 80-100 m e s h , A A A ~ 40-50 A mesh, * a * . & 40-50 mesh, o o o o o extrudate, extrudate, 0
h
2 . 0 wtw NAPH, R e d u c e d Iron wt% N A P H , R e d u c e d Iron 2 . 0 wt% N A P H , Ni-Mo/A120,
0ooOOO.2
500
1000
1500
~
0
80-100
0
-
0.0 0
0
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c m 3 liquid per g c a t . Figure 5. Effect of the liquid reactant loading on the naphthalene conversion at 250 "C with reduced iron in vaporphase reactions.
ever, the catalyst deactivation was less than 5% when the naphthalene feed concentration was reduced to 0.2 wt % in cyclohexane. The exact mechanism of catalyst deactivation at the higher naphthalene concentration cannot be ascertained from these runs. Appleby et al.23 have shown that naphthalene can form coke on the catalyst surface; however, their catalyst was probably more acidic than the one used here and their study was performed in the absence of hydrogen. Thus their results cannot be directly applied to this work. It is possible that the rapid deactivation is due to irreversible adsorption of naphthalene on the catalyst surface; however, this possibility cannot be verified without additional study. When the vapor-phase reactions were conducted with Ni-Mo/AlsOs catalyst using 2 wt % naphthalene in cyclohexane at 300 "C, a catalyst deactivation of less than 10%of naphthalene conversion was observed, which is in contrast to that of reduced iron a t 250 "C (Figure 5). This behavior is analogous to the more rapid deactivation of the iron catalysts observed in the trickle-bed reactions shown earlier in Figure 2. Since the reduced iron showed fairly stable activity for the 0.2 wt % naphthalene feed, kinetic studies were carried out at this concentration at 160, 200, and 250 "C on both reduced iron and Ni-MoIAl203 catalysts, except for the runs at 300 "C with Ni-Mo/ A 1 2 0 3 where kinetic data also were obtained with the 2 wt % naphthalene feed. Mass Transfer. Several tests were performed t o check for the presence of internal and external masstransfer limitations. Plots of naphthalene conversion XN vs Wlq with Ni-MoIAlzOa catalyst having three particle sizes at 300 "C are shown in Figure 6. It appears that there exists an internal diffusion limitation with the 1/16 in. extrudate. External diffusion and wall effects may also be important with the extrudates since the Reynolds number was typically less than 2 and the diameter of the reactor tube was only 6 times that of average catalyst particle size. In addition, the conversion data for the 1/16 in. extrudate did not follow (23)Appleby, W. G.; Gibson, J. W.; Good, G. M. Ind. Eng. Chem. Process Des. Deu. 1962, 1 , 102-110.
0.0 0.00
0.02
W/q
0.04
0.06
0.063 g 0.079 g 0.135 g 0.060 g 0.259 g 0.320 g 0.08
0. 0
(min-g/cm3)
Figure 6. Effect of internal pore diffusion on vapor-phase naphthalene conversion at 300 "C on Ni-Mo/AlzOa.
apparent first-order kinetics as did the 40-50 mesh and 80-100 mesh particles. Conversions with particle sizes of 40-50 mesh and 80-100 mesh agree with each other, indicating that the internal diffusion limitation is eliminated when the particle size is 40-50 mesh. Therefore, this particle size was used for subsequent investigation. An estimated Thiele modulus of 1.33 for the Ni-MoIAlzO3 extrudate and 0.22 for the 40-50 mesh particle was calculated using the first-order apparent rate constant obtained together with the molecular diffusion coefficient of naphthalene in the mixture of hydrogen and cyclohexane, estimated using Chapman-Enskog equation and Blanc's law.24 A tortuosity factor of 2 was used to calculate the effective diffision coefficient of naphthalene in the catalyst pores. These values give effectiveness factors of 0.59 for NiMolAl203 extrudate and 0.97 for the 40-50 mesh particles which are consistent with the experimental observations. A test was performed for the presence of external gas film resistance by varying the gas velocity (flow rate) while using different catalyst loadings to keep Wlq constant as shown in Figure 6. The results showed external diffision limitations to be absent since identical conversions were obtained at constant Wlq with different catalyst loadings over a wide range. In fact, Petersen has demonstrated that with realistic values of the mass-transfer and diffusion parameters, external transport limitations will never exist unless internal diffusion limitations are also present.25 Kinetics. The empirical power-law rate model shown in eq 3 was used t o fit the vapor-phase experimental data using the integral method.
rN= k,,,CNUCHb
(3)
CH was taken as a constant since less than 1%of entering Hz reacted. For a first-order reaction of naphthalene in a plug flow reactor, the value Of k,,CHob can be determined from eq 4 resulting from substitution (24) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids; McGraw-Hill, Inc.: New York, 1977. (25) Petersen, E.E.Chem. Eng. Sci. 1966,20,587.
1390 Energy &Fuels, Vol. 8, No. 6, 1994
Zhan and Guin
Table 1. Vapor-Phase Experimental Conditions Using Ni-Mo/AllOs at 6.9 MPa
CNOx IO7,moVcm3 CHOx 103, mol/cm3 W/q x 102, min-g/cm3
160 "C
0.2 wt % NAPWCH 200 "C
250 "C
2.36 1.73 0.52-5.16
2.16 1.59 0.31-3.14
1.96 1.44 0.17-2.11 4.0
2 wt % NAPWCH 300 "C 17.9 1.31 0.18-2.48
17.9 0.918 0.19-3.97
,
J
CH0=1.436x10-3 m o l / c m 3 * t . i CH0=1.005x10-3 * mol/cm3
AGACGA
i ' -i h
i
4
\
2.0 l
1 .o
0.0
0.00
W/q
(min-g/cm3)
0.02
W/q
Figure 7. Hydrogen dependence of vapor-phase reactions at 300 "C on Ni-Mo/AlzO3. Table 2. Vapor-Phase Experimental Conditions Using Reduced Iron at 6.9 MPa
0.04
0.06
0.08
0.10
(min-g/cm3)
Figure 8. Hydrogen dependence of vapor-phase reactions at 250 "C on prereduced iron. 4.0
,
..... - 160°C
0.2 wt % NAPWCH -0
200°C 250°C
160°C 200°C 250°C 250°C CNOx lo7, mol/cm3 2.36 2.16 1.96 1.96 CHOx lo3, mol/cm3 1.73 1.59 1.44 1.01 W/q x lo2, min.g/cm3 2.93-27.7 1.65-18.5 0.45-8.25 5.31-42.1
of eq 3 into the plug flow material balance followed by integration. ln(=)
1
= kappCHO
b w
7
(4)
In a typical run, as q was varied, the inlet naphthalene and Ha concentrations CNOand CHOwere held constant by adjusting the feed rates. The H2 dependence (exponent b) of the reaction rate was determined by reducing the Hz concentration CHOin a separate run with N2 providing the make-up to keep the total gas flow rate and naphthalene concentration CNOunchanged. The detailed reaction conditions for the Ni-Mo/AlzOs and prereduced iron catalyst are shown in Tables 1 and 2, respectively. Figures 7-10 are plotted as In (1/1 - X N ) vs W/q according to eq 4. The linear plot with least-squares regression yields a standard error of the slope, kappCHOb in eq 4, within 4% in all cases as summarized in Tables 3 and 4. In order to determine the value of b in eq 3, the initial concentration of H2 was reduced t o 70% of the baseline condition for Ni-Mo/AlzOs at 300 "C and reduced iron at 250 "C. The results shown in Figures 7 and 8 indicate that hydrogen also follows first-order kinetics, i.e., the value of b is about 1, since as shown in Tables 3 and 4,the slopes of the plots are reduced correspondingly to about 70% of those under baseline conditions in both cases. Because the naphthalene concentration in the liquid feed was 2 wt % for the
6
W/q
(min-g/cm3)
Figure 9. First-order kinetic plots of vapor-phase reactions on Ni-Mo/AlzOs.
studies at 300 "C, one additional run was performed at 300 "C with 0.2 w t % naphthalene with the other conditions the same to check for the influence of inlet reactant concentration on the reaction rate. As shown in Figure 7, in agreement with eq 4, the conversion of naphthalene was the same regardless of the feed concentration thereby further substantiating the firstorder kinetics. As the naphthalene dependence follows first-order kinetics in both trickle-bed and vapor-phase conditions, a comparison of the rate constants in these two cases can be made on the reduced iron at 200 "C. By defining the rate constant k L in eq 2 for the liquid phase as k L = kapp,L CHO,L, where CHO,L is the saturated Ha concentration in the liquid, estimated t o be 2.68 x moYcm3
Energy & Fuels, Vol. 8, No. 6, 1994 1391
High-pressure Hydrogenation of Naphthalene
Table 3. Vapor-Phase Experimental Results Using Ni-Mo/AlaOs at 6.9 Mpaa 160 "C
(a = 1) kapp x 10-4 ( b = I),moV(min*g)/(moVcm3)2
KappCHob
0.2 wt % NAPWCH 200 "C
28.5 f 1.04 1.65
kapp,0
Eapp, kJ/mol a
m = kappCi&Hb.
kapp
= kapp,O
2 wt % NAPWCH 300 "C
250 "C
68.2 f 2.76 4.30 1.18 109 40.2
161 f 2.53 11.2
89.8 f 2.17 9.79
133 f 2.18 10.2
eXp(-Eap@T).
Table 4. Vapor-Phase Experimental Results Using Reduced Iron at 6.9 MPaa 0.2 wt % NAPWCH
(a = 1) kapp x 10-3 (b = I), moV(min+g)/(mol/cm3)2 kappCHOb
160 "C
200 "C
250 "C
250 "C
6.68 f 0.19 3.85
15.60 f 0.31 9.83 6.05 x lo8 43.2
43.42 f 1.16 30.3
5.82 f 0.18 29.6
/
surface, (2) naphthalene and hydrogen chemisorb competitively on the same type of active sites, (3) adsorbed hydrogen atoms are the dominant surface species, and (4)the addition of a H atom to naphthalene occurs sequentially, a reaction mechanism consisting of the following sequential elementary steps can be proposed.
+ 2" 5 2H* CloH8+ * 3 CloH,* CloH8*+ H* CloHg*+ * CloHg*+ H* !? CloHl,* + * H,
+ H*5 CloHll* + * CloHll* + H* k CloH12*+ * ClOH1,* EKTC H 12 + *
CloHlo* W/q
(min-g/cm3)
Figure 10. First-order kinetic plots of vapor-phasereactions on prereduced iron.
a t 200 "Cusing Shaw's correlation,26kapp,lis equivalent and thus comparable to kappin eq 4 for the vapor-phase reaction. The recalculated apparent rate constant for the liquid-phase reaction is kapp,L = 156 mol/(min*g)/(mol/ cm3I2,in contrast to that in the vapor phase of 9833 mol/ (min*g)/(m~l/cm~)~ (Table 4). The higher reaction rate constant in the vapor phase may be due to the fact that hydrogen and naphthalene adsorb on the catalyst surface more readily from a vapor phase than from a liquid. In addition, the surface structure of the catalyst and the surface coverage of the reactants may be different in vapor-phase and liquid-phase operations because of the solvent influence, e.g., mineral oil vs cyclohexane. Kinetic Mechanism. As stated above, it appears that a first-order power law dependence on both naphthalene and hydrogen can adequately describe the kinetic behavior of vapor-phase naphthalene hydrogenation at our experimental conditions. However, the actual mechanism is probably more complex than indicated by this dependence. A possible mechanism leading to a first-order dependence under our conditions can be postulated as follows. Assuming that (1)hydrogen molecule chemisorbs dissociatively on the catalyst (26) Shaw,J. M. Can. J. Chem. Eng. 1987, 65, 293-298.
10
(i) (ii) (iii) (iv) (V)
(vi)
(vii)
where * represents a surface active site. If the adsorbed naphthalene and adsorbed hydrogen are assumed to be in equilibrium with the gas-phase species and the ratecontrolling step to be the irreversible addition of the last H atom to a CloH11* surface species (eq vi), a Langmuir-Hinshelwood rate expression of the form in eq 5 can be derived. = ~WH~KNKEC,CH~/[(I
+
+ + + K&T)2]
K H ~ ~ c H ~ /KNCN ~
KH3'2KNKEc&H312
(5)
where KE is the product of K E ~K, E ~and , K E ~ .The denominator of eq 5 also can be expressed with the concentrations of adsorbed species and unoccupied surface active site.
den = 1
+ "('C* 1+ F )+ CCloH1l*
CCloH8*
-k CCloH12*
C*
(6)
Considering that in our vapor-phase reactions the concentration of H2 was about 7000 times larger than that of naphthalene and that the reactor was operated a t high pressure, a further assumption can be made that most of the surface active sites are occupied by adsorbed hydrogen atoms, i.e., CH*>> Ci, where i is the unoccupied
Zhan and Guin
1392 Energy & Fuels, Vol. 8, No. 6, 1994 12.0
.
-\
which are temperature dependent. The apparent activation energy for the combined product can be expressed by
Eapp=E+ A H E
+ A H N + A€€H
where E is positive and AHls are all negative. At low temperatures, E dominates the apparent activation energy Eappand the apparent rate constant appears to r" increase with temperature. At high temperatures, the dependence of adsorption heats hHi on temperature may become strong enough t o retard the apparent activation energy. In other words, Eappmay decrease significantly at high temperature due to the more m-m Ni-Mo/Alq03 negative value of adsorption heat (AHN AHH) with Reduced iron the decrease of surface coverage. The reason for the 7.0 L T T variation of the AHi with temperature is related to the 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 fact that on a nonuniform catalyst surface, various 1 0 ~ 1( K~- ' ) active sites with different energy levels exist. As the Figure 11. Arrhenius plots based on the apparent rate reaction temperature is increased, adsorption on sites constants. with a higher adsorption energy becomes dominant and the surface coverage decreases leading to a change in active site or the other adsorbed surface species except the adsorption enthalpy. A decline in apparent rate hydrogen atoms. Therefore, the denominator of eq 5, constants at high temperature has also been observed or eq 6, can be simplified to be CH*/C*= ( K H ~ T H ~ ' ~ ) ' for benzene8 and toluenez7 hydrogenation reactions, whereby eq 5 becomes although, AHH,were postulated to be constant in these reactions. (7) 10.0
A
E l
+
A&L&&A
thereby predicting a first-order dependence on both naphthalene and hydrogen in agreement with the preceding rate model in eq 3. Of course, a much greater range of experimental data would be required to verify the complete mechanism represented by eq 5. Temperature Effects. Arrhenius plots based on the observed apparent rate constants at temperatures of 160-250 "C are shown in Figure 11 and the kinetic parameters are given in Tables 3 and 4. For Ni-Mol A1203 catalyst, the apparent activation energy (40.2 kJ/ mol) is approximately the same as that of reduced iron (43.2 kJ/mol), and the frequency factor (1.18 x lo9), which is related to number of active sites of the catalyst is about twice that of reduced iron (6.06 x lo8). Note that the surface area of Ni-Mo/AlzOs is about 60 times that of original iron oxide catalyst. Assuming that the surface area of the prereduced iron catalyst is not too different from the original FezO3, then on a per unit surface area basis, the reduced iron is more active than the Ni-Mo/AlsOs. It can be seen from Figure 11 that on the Ni-Mol A1203 catalyst, the reaction rate constant at 300 "C is slightly lower than that at 250 "C. This effect is not due to (1)thermodynamic limitations, as the equilibrium conversion of naphthalene at 300 "C and 6.9 MPa is 99.7% according to Frye's equilibrium correlation,13 (2) catalyst deactivation, since as stated earlier, the recheck of catalyst activity at the end of an experimental run revealed no significant deactivation at this temperature, or (3) diffusion limitations, since as shown in Figure 6, the internal diffusion limitations at 300 "C are eliminated for the catalyst particle size used. A possible reason for the decline in apparent rate constant with temperature can be found by examination of eq 7 which reveals that the apparent rate constant is actually a product of the intrinsic reaction rate constant k, intermediate reaction equilibrium constant K E , and adsorption equilibrium constants KH and K N , all of
Conclusion
The in-situ prereduction of iron oxide with hydrogen gave a very active catalyst for naphthalene hydrogenation. When the original iron oxide was employed without any pretreatment, an in-situ reduction process appeared to occur before the hydrogenation reactions proceeded. Sulfur rapidly poisoned the hydrogenation function of the iron catalyst, indicating that iron sulfide phases had much lower aromatic hydrogenation activity than reduced iron. A Langmuir-Hinshelwood rate expression reduced under our experimental conditions to a first-order dependence on both naphthalene and hydrogen, consistent with the empirical power law used to fit the data. The apparent activation energy of NiMo/A1203 was approximately the same as that of prereduced iron, whereas the frequency factor was about twice that of prereduced iron. At low naphthalene concentration and in the absence of sulfur, the prereduced iron catalyst was more active than the Ni-Mo/ A1203 on a BET surface area basis. Acknowledgment. This work was supported by the U.S. Department of Energy Under Contract No. DEFC22-90PC90029as part of the research program of the Consortium for Fossil Fuel Liquefaction Science. Nomenclature AH = extent of hydrogenation as defined in eq 1 a, b = exponents of naphthalene and hydrogen in eq 3,
respectively CNO,CN = initial and local concentrations of naphthalene in the gas mixture under reaction conditions,respectively CHO,CH = initial and local concentrations of hydrogen in the gas mixture under reaction conditions,respectively E , Eapp = intrinsic and apparent activation energy, respectively (27) Lindfors, L.P.;Salmi, T.Znd. Eng. Chem. Res. 1993, 32, 3442.
High-pressure Hydrogenation of Naphthalene M E= standard enthalpy of intermediate reactions AHH, AHN= standard heat of adsorption of hydrogen and naphthalene, respectively KE = equilibrium constant in intermediate reactions KH,KN,KT = adsorption equilibrium constants of hydrogen, naphthalene, and tetralin, respectively kapp= apparent rate constant of vapor-phase reaction in eq 3 kapp,O= frequency factor of kapp k L = rate constant of trickle-bed reaction defined in eq 2
Energy & Fuels, Vol. 8, No. 6, 1994 1393 MNAPH, MTET,MDEC= moles of naphthalene, tetralin, and decalin in the reaction products, respectively q = total volumetric flow rate of gas mixture under reaction
conditions q L = volumetric flow rate of liquid feed in liquid state = reaction rate of naphthalene W = weight of catalyst ~ S = weight V hourly space velocity, g of liquid feed/(h.g of catalyst) X N = conversion of naphthalene ~
