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I n d . E n g . C h e m . Res. 1990,29, 1840-1846
Scheffer, B.; Arnoldy, P.; Moulijn, J. A. Sulfidability and Hydrodesulfurization Activity of Mo Catalysts Supported on Alumina, Silica, and Carbon. J . Catal. 1988, 112, 516-527. Schmidt, J. L.; Castellion, J. R.; Schuit, G. A. Hydrodesulfurization Catalysts Supported on Carbon. US. Patent 3,997,473, 1976. Schuit, G. C. A.; Gates, B. C. Chemistry and Engineering of Catalytic Hydrodesulfurization. AIChE J. 1973, 19, 417-438. Scott, J. W.; Bridge, A. G. Origin and Refining of Petroleum. Adu. Chem. Ser. 1971, 103, 119. Shah, Y. T. Gas-Liquid-Solid Reactor Design; Wiley: New York, 1979. Stanislaus, A.; Absi-Halabi, M.; Al-Dolama, K.; Katrib, A.; Ismail, M. Effect of Thermal Treatment on the Sintering and Structure Change of Cobalt-Molybdenum/Alumina and Nickel-Molybdenum/Alumina Hydrotreating Catalysts. Appl. Catal. 1988, 41, 109-1 19. Stirton, R. I. U S . Patent 2,441,297, 1948. T a ” , P.; Harnsberger, H.; Bridge, A. G. Effect of Feed Metals on Catalyst Aging in Hydroprocessing Residum. Ind. Eng. Chem. Res. 1981, 20, 262-273. Takahashi, T.; Nishi, Y.; Otsuji, N.; Kai, T. Hydrogenation of Cyclohexene and Benzene over an Amorphous Pd-Zr Alloy. Can. J . Chem. Eng. 1987,65, 274-279. Thomas, R.; de Beer, V. H. J.; Moulijn, J. A. Characterization of r-Alumina-Supported Molybdenum Oxide. Bull. SOC.Chim. Belg. 1982a, 90,1349-1352. Thomas, R.; van Oers, E. M.; de Beer, V. H. J.; Moulijn, J. A. Characterization of r-Alumina-Supported Molybdenum Oxide and
Tungsten Oxide. Reducibility of the Oxidic State versus Hydrodesulfurization Activity of the Sulfided State. J. Catal. 1982, 76, 241-253. Thomas, R.; van Oers, E. M.; de Beer, V. H. J.; Moulijn, J. A. Characterization of Silica-Supported Molybdenum Oxide and Tungsten Oxide. Reducibility of the Oxidic State versus Hydrodesulfurization Activity of the Sulfided State. J. Catal. 1983,84, 275-287. Topsoe, H.; Clausen, B. S.; Topose, N. Y.; Pedersen, E. Recent Basic Research in Hydrodesulfurization Catalysts. Ind. Eng. Chem. Fundam. 1986,25, 25-36. Vogel, R. F.; Marcelin, G. Preparation of Stoichiometric Aluminum Phosphate. J . Catal. 1983a, 80, 492-493. Vogel, R. F.; Marcelin, G. The Preparation of Controlled Pore Alumina. U S . Patent 4,376,067, 198313. Voorhies, A., Jr. Carbon Formation in Catalytic Cracking. Ind. Eng. Chem. 1945, 37, 318-322. Wang, W. J. Characterization of Binary Metal Oxides. Master Thesis, National Central University, 1987. Weekman, V. W.; Nace, D. M. Kinetics of Catalytic Cracking Selectivity in Fixed, Moving, and Fluid Bed Reactors. AIChE J. 1970, 16, 397-404. Yao, H. C. Surface Interaction in the MoO3/r-AI2O3System. J. Catal. 1981, 70, 440-444.
Received for review January 25, 1989 Revised manuscript received November 8, 1989 Accepted April 10, 1990
Reaction Network and Kinetics of the Pyrolysis of Tetralin/Naphthalene Mixtures in Nitrogen and Hydrogen Atmospheres Fabio Murena and Francesco Gioia* D i p a r t i n e n t o d i Ingegneria Chimica, Uniuersitci degli S t u d i d i Napoli Federico I I , Piazzale Tecchio, I80125 Napoli, Italy
Fundamental studies on the kinetics of hydrogen donation, during the process of coal hydroliquefaction, require that the reactions undergone by the donor and their kinetics be defined as clearly as possible. In this work, the pyrolysis of tetralin is investigated on the assumption that this compound is a convenient model hydrogen donor. The reaction is run in a stirred autoclave a t a constant temperature of 430 “ C , in the presence of either nitrogen or hydrogen, and a t different naphthalene initial concentrations. Runs in the presence of the mineral matter of coal are also performed for testing catalytic effects on the reactions. The principal chemical compounds produced during the pyrolysis process are identified and linked in a network. The first-order kinetic constants are evaluated by regressing the experimental data. The theoretical concentration vs time curves based on the calculated kinetic constants fit the data satisfactorily.
Introduction Any investigation aimed at understanding the fundamentals of coal depolymerization in the presence of a hydrogen donor is faced with the problem of clearly defining the chemical transformations undergone by the donor. Chemical compounds present in the liquefaction products may be derived from either reactions involving the coal and the donor or from reactions undergone by the donor even when coal is not present. The latter reactions include those which, although specific to the donor, are catalyzed by the coal. Since the disappearance of the donor may not be exclusively due to its reaction with coal, it is useful when investigating the kinetics of coal hydroliquefaction to clearly define the reactions undergone by the donor during pyrolysis and the corresponding kinetics, so as to avoid the incomplete or erroneous interpretation of the data. For example, the calculation of donated hydrogen based exclusively on the amount of donor disappeared gives incorrect results if the pyrolysis reactions of the donor are not accounted for (see Gioia and Murena, 1988). 0888-5885/90/2629-1840$02.50/0
The present work is an investigation of the pyrolysis of tetralin. This specific model donor was chosen because the majority of liquefaction experiments which investigate the fundamental aspects of the kinetics of coal liquefaction use tetralin as the donor. The potential complications resulting from more complex donors are thus reduced, and the basic steps of the process can be identified with greater precision. The pyrolysis of tetralin has been investigated by several authors, among them Benjamin et al. (19791, Hooper et al. (1979),de Vlieger et al. (1984),McPherson et al. (19851, and Curran et al. (1967). Most investigators, however, deal only with the chemical aspects of the transformations that tetralin undergoes. Few authors (such as de Vlieger et al., 1984) make an attempt to quantitatively investigate the reaction network and, above all, the kinetics of the reactions involved. However, the network is not completely identified, and the kinetics of the reactions which form the network are not thoroughly explored. The results of the present work, nevertheless, are an advance over those of previous investigators for the folC 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1841 lowing reasons: (i) A large number of reaction products and intermediates are identified, and, of greater importance, their concentrations are measured as a function of time. Consequently, it has been possible to link these compounds quantitatively in a network and evaluate the kinetic constant of each single reaction. (ii) The effect of hydrogen pressure is investigated over a pressure range of 30-120 bar.
Experimental Procedure The reaction is run in a 300-mL autoclave (Autoclave engineers with magnetic stirring). The procedure is as follows: About 200 mL of tetralin (or a mixture of tetralin/naphthalene) is loaded in the autoclave, flushed with nitrogen, and then heated. When the required temperature is reached (after about 1 h), the pressure is adjusted to the required value with either nitrogen or hydrogen. The time of the pressure adjustment is assumed to be the zero time of the reaction run. The pressure and temperature are then continuously monitored and when necessary reset, particularly after each sampling of the liquid mixture. For the runs under hydrogen, a frequent reset of the pressure is necessary during the few initial minutes due to the high solubility of this gas in the tetralin (Simnick et al., 1977) and to the high mass-transfer coefficient realized in the autoclave (Kara et al., 1983). The loading of the coal mineral matter for runs 6 and 7 is made at the end of the heating period by a technique similar to that described by Gioia and Murena (1988). Liquid samples, averaging about 1.5 mL, are periodically collected. The samples are taken by opening a valve on a sampling tube present in the autoclave. The large pressure difference between the inside and outside of the autoclave is the driving force for collecting the samples. The sampling line is flushed before each sampling by extracting a volume of liquid roughly equal to the volume of the line ( ~ 1 . mL). 5 The sampling frequency is adapted to the rate of the chemical transformations taking place. The first sample is collected at the end of the heating period ( t = 0, as defined before) and provides the initial concentrations to be used in the kinetic analysis. The total reaction time for all runs is about 450 min. A t the end of the run, the reactor is left to cool to room temperature, and a sample is taken. This is useful for checking that samples collected during the run are genuinely representative and are not affected by the flashing. Chemical Analysis of Samples The samples are analyzed by a combination of GC-MS and GC-FID fitted with capillary columns. The routine analysis of samples is done by GC-FID to obtain the concentrations of the identified compounds as a function of time. More details on the analysis technique are reported by Murena (1989). The number of chemical species detected is quite large, with about 150 having concentrations larger than 10 ppm. It is neither possible nor of interest to identify all these compounds and follow their evolution with time. The most abundant compounds are identified by GCMS and GC-FID. The presence of other compounds of interest but, in lesser amounts which are not clearly identified by GC-MS, is checked by GC-FID. Other compounds, not identified in detail, fall into three groups according to their ranges of elution times with boundaries marked by the elution times of known compounds. The first, expressed as GI, includes mainly alkylindans and, to a lesser extent, alkylbenzenes. The second group, G P ,includes alkyltetralins and alkylnaphthalenes with elution times between those of naphthalene and fluorene. The
Table I. Maximum Concentrations and Relative Elution Times of Identified and Unidentified Chemical ComDounds max concn, compd g/g of soln (%) tE, % benzene 0.15 34.1 toluene 2.51 37.4 2.00 42.9 ethylbenzene 0.06 45.6 o-xylene 51.1 0.07 propylbenzene 0.58 58.8 sec-butylbenzene (SBB) 0.63 64.2 indan (I) 6.36 65.9 n-butylbenzene (BB) 0.32 69.1 trans-decalin 14.8 1-methylindan (MI) 21.38 19.7 cis-decalin 100 tetralin (T) 108.9 naphthalene (N) 8.73 groups ~~
G1 G2 G3
1.41 2.41 0.42
~~
~
C108.9 108.9-214 >214
third group, G,, includes four-ring aromatics. The compounds included in group G2are probably adduction products of tetralin and naphthalene with alkyl radicals while those in group G3are the result of the addition of two-ring molecules. The presence of these kinds of compounds is also reported by other authors (Benjamin et al., 1979; Mc Pherson et al., 1985). All the identified compounds and groups are reported in Table I with their elution times relative to that of tetralin, whose value tE = 12.2 min is assumed to be 100%. In the list, the maximum concentration detected for each compound (expressed as g/g of solution (5%)) is also reported to give a quick representation of the importance of the reactions leading to the compound. Tetralin and cis-decalin concentrations are not reported, as they are consumed during the process; i.e., their maximum concentrations are the initial ones. Naphthalene maximum concentration refers to the runs made with only tetralin as feed. The concentrations of G groups are the sum of the concentrations of the compounds belonging to the group. The identified compounds include the most relevant products of tetraline pyrolysis (Benjamin et al., 1979; Hooper et al., 1979; de Vlieger et al., 1984). It is interesting to note that the alkylbenzenes present in significant amounts are those having the alkyl group(s) in position 2 and/or 3. This is consistent both with the structural formula of tetralin and with the fact that the alkyl radicals are formed upon the rupture of the hydrogenated ring.
Experimental Results Figures 1-3 report for runs 1-5 (no mineral matter present) the experimental results expressed as mol/g of solution of all identified species and groups vs time. For the groups, average molecular weights are used. For the sake of conciseness, the results regarding benzene, toluene, ethylbenzene, and propylbenzene (which have very similar trends) are summed up together and reported as a group named, in short notation, CAB. The fitting curves are model predictions to be discussed later. Inspection of these figures shows a few characteristics of the concentration vs time data (referred as to c w t data) which are useful for setting the network. In particular: P -Butylbenzene (BB), 1-Methylindan (MI), and Naphthalene (N). The c w t data show a finite derivative at time zero. This implies that they are produced directly from the reactant, i.e., tetralin. The o-xylene data, not reported in the figures, show the same trend. Furthermore, n-butylbenzene and 1-methylindan show, at longer times,
1842 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 .-T-
-
c
- -__--___~----
n
4
S BB
0
-2
/
W-
N
2
A
0 0
2:
150 t,
MI
1
min
300
L 50
Figure 3. Concentration of indan (I) and sec-butylbenzene (SBB) versus time for runs 1 (v),2 (O),3 (0),4(A),and 5 (0). Operating conditions are in Table 11. Fitting curves based on network of Figure 4 and on kinetic constants of Table 111.
150
0
t , min
300
450
Figure 1. Concentration of tetralin (T), naphthalene (N), and 1methylindm (MI) versus time for runs 1 (V),2 ( O ) ,3 (D),4 (A),and Operating conditions are in Table 11. Fitting curves based on 5 (0). network of Figure 4 and on kinetic constants of Table 111.
-.
= 5 0
-0
'
L
i
I A B
A
v)
>c n 3 ; 2 E
t
0
i
150
1 , min
300
450
Figure 2. Concentration of n-butylbenzene (BB), identified alkylbenzenes (ZAB), and group G , versus time for runs 1 (v),2 ( O ) ,3 (a),4 (A),and 5 (0). Operating conditions are in Table 11. Fitting curves based on network of Figure 4 and on kinetic constants of Table 111.
a reduction (more significant for n-butylbenzene) of the slope of their c vs t data so that they may be assumed to be intermediate compounds in the network. Indan (I), sec-Butylbenzene (SBB), GI, and CAB (Benzene, Toluene, Ethylbenzene, Propylbenxene). Their c vs t data show zero derivative at time zero, which implies that they are not formed directly from tetralin and must be considered to be "late products". Indan and sec-butylbenzene are presumably formed from l-methylindan as suggested by their structural formula. Moreover, the GC-MS analysis has shown that the group GI is essentially formed of alkylindans which are also considered to be decomposition products of 1-methylindan. The other alkylbenzenes (CAB) are assumed to be decomposition products of n-butylbenzene. The shape of the c vs t data of these alkylbenzenes excludes the possibility that they are formed from one another. cis-Decalin and trans-Decalin. The data (not reported in the figures) indicate that the main reaction taking place is cD tD; namely, the cis-decalin originally contained as an impurity in the tetralin is converted to trans-decalin. Only at larger H, pressures is there a net production of decalin; this production, however, is small. Naphthalene. The trend of the naphthalene concentration profiles of runs 3-5 (see Figure 1)could be interpreted by considering the tetralin dehydrogenation as a reversible reaction. In fact, as the hydrogen pressure and the naphthalene initial concentrations are increased (cf. runs 3 and 4), a decrease of the rate of production of naphthalene is observed. This interpretation, however, would disagree with the fmdings of Benjamin et al. (1979), who have shown that naphthalene does not rehydrogenate in the absence of a catalyst. In our experiments, however, the stainless steel walls of the autoclave could provide some catalytic activity. In order to check beyond any doubt if the rehydrogenation reaction of tetralin at our experimental conditions took place or not, run 8 was carried out: naphthalene was reacted in the presence of hydrogen (at pH2= 120 bar) with hexadecane as solvent. The reaction mixture was sampled as for the other runs, and the samples were analyzed by GC-FID to detect any formation of tetralin. After 7.5 h, no significant naphthalene rehydrogenation to tetralin took place. In fact, the chromatograms showed tetralin only in trace amounts (less than 0.1%).
-
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1843 Neither 1-methylindan nor n-butylbenzene was detected in significant amounts. At 430 OC, the equilibrium constant of the reaction T N + 2H2 as obtained by Whitehurst et al. (1980) (reported in correct form by de Vlieger et al. (1984)) is 1.54 X lo3 bar2. At the hydrogen pressure of run 8 (pH,= 120 bar), the equilibrium ratio C,/C, = 10. On the contrary, at the end of run 8 that ratio was The results of this run can be considered as conclusive of the fact that in our experimental conditions and in the absence of any added catalyst the rehydrogenation of naphthalene does not occur. It is necessary now to find a different interpretation of the influence of hydrogen pressure and naphthalene initial concentration on the trend of concentration profiles of runs 3-5. A plausible interpretation of the observed influence of pH2on the formation of naphthalene can be based on the assumption that the reaction T+N+2Hz (a) takes place through the mechanism T DHN + H2 (b) DHN N + H2 (C) where DHN is 1,2-dihydronaphthalene (or 1,Cdihydronaphthalene). In this hypothesis, the overall rate of naphthalene production from tetralin is (1) r12 = kKbC1/PH2= k12c1
,- 5
-
+
where Kb is the equilibrium constant of reaction b, k is the first-order kinetic constant of reaction c, and k12 is the overall kinetic constant of reaction a. Accordingly, the kinetic constant k12 is inversely proportional to the hydrogen partial pressure; this is consistent with the observed kinetic behavior of naphthalene. Our chromatographic column is not able to separate DHN, but de Vlieger et al. (1984), using GC-MS analysis, established the presence of dihydronaphthalene in small amounts. Small amounts do not exclude, however, a role of DHN as an intermediate. In fact, according to the above mechanism, the concentration level of DHN which is regulated by the value of the equilibrium constant Kb could be very small. The observed reduction of the rate of naphthalene production at larger initial naphthalene concentration must be due to reactions which consume this compound and whose rates depend on the concentration of naphthalene. It has not been possible to identify these reactions. They are without doubt active, as the chemical analysis of the reaction products shows that compounds which may be formed from naphthalene are present, e.g., the alkylnaphthalenes and the binaphthalenes which belong to group G2 and G3. Another feature shown by the c vs t data of naphthalene, particularly in the presence of hydrogen, is more apparent at higher pressures and low initial naphthalene concentrations (runs 2 and 3). The data show an increasing rate of formation of naphthalene with time. To confirm this phenomenon, run 3 was replicated, and the same results were obtained. This behavior was also observed by Hooper et al. (1979), who interpret it as an autocatalytic effect in which one of the cracking reactions may take part in the further formation of naphthalene. Alternatively, they attribute the behavior to a catalytic effect of the walls of the reactor, which becomes active after an induction period. The increased rate of naphthalene formation with time is not apparent in the data of the runs with larger naphthalene initial concentrations (runs 4 and 5). At larger naphthalene concentrations, the reactions which consume naphthalene reach a rate high enough to hide the in-
k3
(2)
k,?
(1)
It51
> ' S L B B A TAB
(81
(4)
Figure 4. Reaction network for tetralin pyrolysis: dashed arrow, in the presence of a catalyst; chain-dot arrow, at large naphthalene concentrations.
creasing rate of formation observed at the lower naphthalene concentrations. The increased rate of formation cited is a minor effect (see Figure 1)and has been discussed for completeness.
Reaction Network The consideration of the previous paragraph concerning the relative position of chemical compounds in the reaction network is not sufficient, however, to precisely identify the reaction network. Although many possibilities are excluded, several alternatives remain; the procedure we have adopted for evaluating them is based on trial and error. The kind of kinetic equations is assumed, and their parameters are estimated by using a suitable statistical method. The set of differential equations describing the network is then integrated by using the estimated parameters, and the resulting theoretical ci vs t curves are compared with the experimental ci vs t data. The procedure is repeated for other possible networks until that which best fits the data is identified. The most natural assumption for the homogeneous reactions at hand is a simple first-order kinetics. Therefore, for any reaction leading from the compound i to product j it is assumed ' i j = kijCi (2) pH2is not considered here, as it is kept constant during the reaction run. Its effect is included in the kinetic constants hi., and will be discussed later. The statistical method utiiized to regress the data for estimating the kinetic constants k i j is that proposed by Himmelblau et al. (1967). The network resulting from the described procedure is shown in Figure 4. Many networks chemically possible have been excluded because the analysis of the data produces inconsistent kinetic constants. It is useful to point out that for the runs under a hydrogen atmosphere (runs 3-5) the statistical regression of data gives inconsistent results if the naphthalene rehydrogenation reaction is included in the network. The previously discussed finding that naphthalene does not rehydrogenate in the absence of a catalyst is thus confirmed. The first-order kinetic constants corresponding to the network of Figure 4 are reported in Table 111. The theoretical curves obtained by integrating the set of differential equations with the constants of Table I11 are drawn through the data in Figures 1-3. For the sake of simplicity, the reaction of formation of o-xylene from tetralin is not included in the network. The absence of this reaction does not affect the calculated kinetic constants due to the low concentration level of o-xylene. For the same reason, the reaction leading from cis- to trans-decalin is analyzed separately. The data show that this reaction takes place with first-order kinetics with a min-'. Only at larger kinetic constant k = (2.5 f 0.3) X hydrogen pressures (runs 3 and 4) does the sum of transand cis-decalins show a net increase, thus indicating that
1844 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 Table 11. Experimental Conditions for the Runs (T= 430 "C) feed," wt % run P,b bar gas tetralin naphthalene' ash 1 60 N2 >98 0.5 0 >98 0.5 0 2 150 N2 0.5 0 3d 150 H2 >98 80 20 0 4 150 H2 20 0 5 60 H2 80 6 150 H2 >98 0.5 1.8 7 150 H2 >98 0.5 1.1 8O 150 H2 0 10 0 "The feed concentrations refer to the loading of the reactor. At zero time of the reaction run they are slightly different due to the reaction taking place during the heating of the autoclave. *Total pressure P is reported. Subtracting the tetralin vapor pressure (-30 bar a t 430 "C), the partial pressure of the gas may be obtained. 'Naphthalene was present in the tetralin, as an impurity, a t a concentration of 0.5%. d T ~ identical o runs. eRun 8 was performed with hexadecane as solvent.
some trans-decalin is produced from tetralin. This production, however, is very limited; i.e., the estimated first-order kinetic constant is of the order of lo4 min-'. Groups G2 and G3 are not explicitily included in the network because the kinetics of their formation reactions are not identifiable. Inspection of Figures 1-3 shows that the fitting of the data is generally satisfactory, particularly in relation to the principal compounds. However, a closer look at the nbutylbenzene concentration data shows that in this case the fitting is not as accurate as that obtained for other compounds. The reason may be that not all identified alkylbenzenes (CAB) are produced from n-butylbenzene. In fact, if a simpler reaction network is considered (where the butylbenzene decomposition reaction is included but the products are left undefined), a kinetic k , smaller than ~15% of that in Table I11 is calculated, and the fitting of the butylbenzene data improves noticeably. Thus, the products actually coming from the butylbenzene are somewhat less than the sum of benzene, toluene, ethylbenzene, and propylbenzene as assumed in the network of Figure 4. Finally, it must be pointed out that at the lowest naphthalene concentrations and highest pressures (runs 2 and 3) the fitting of the naphthalene data points (based on a first-order hypothesis) is not as accurate as in the other cases due to the described phenomenon of the increased rate of naphthalene formation with time.
Kinetic Constants Inspection of Table I11 shows that the reactions involved in the network can be roughly divided into two categories: (i) reactions whose kinetic constant is insensitive to the variations of the operating conditions and to the kind of gas atmosphere in the range explored in this work; (ii)
-
-
- -
-
-
-
reactions whose rate is influenced by hydrogen pressure. Reactions T MI, MI I, and BB CAB belong to category i. This finding, as far as reaction T MI is concerned, is in agreement with the findings of Benjamin et al. (1979) and de Vlieger et al. (1984). The average values of the kinetic constants of these reactions in the range of operating conditions of this work are, respectively, k,, = (6.23 f 0.88) X lo4, k,, = (1.1f 0.2) X lo4, and k , = (29.5 f 6) X min-l. The value of K35 is consistent with that reported in the literature (de Vlieger et al., 1984). Reactions T BB, MI SBB, and N products belong to category ii. They all show, even though to different extents, rates which increase with increasing hydrogen pressure. These reactions, when taking place in the nitrogen atmosphere, are insensitive to the value of the total pressure. The reaction T N + 2H2, as discussed before, has a rate which decreases with hydrogen pressure. The results of Table I11 show that the kinetic constant It,, is reduced by a factor of 3 when the hydrogen partial pressure is increased from 30 to 120 bar. This behavior is interpreted in the light of the reaction mechanism leading to eq 1. Surprisingly, the results of Table I11 also show that this reaction has a rate depending, to a certain extent, on the total nitrogen pressure (cf. kIz of runs 1and 2). However, the naphthalene concentration has no effect on the rate of this reaction (cf. kIz of runs 3 and 4). Reaction N Products. This reaction can be accounted for in the network only for runs 4 and 5, which have the largest naphthalene concentrations. The firstorder kinetic constant reported in Table I11 for these two runs has been calculated only for completeness. In fact, having ascertained that the "products" can be alkylnaphthalenes and binaphthalenes (groups G2and G3),the reaction (actually a sum of reactions) cannot have firstorder kinetics. The rate of naphthalene disappearance due to this reaction must show a rate which depends, apart from the naphthalene concentration, on the unknown concentrations of alkylic fragments and radicals produced by pyrolysis.
-
-
Catalytic Effect of Coal Mineral Matter In order to analyze the effect of coal mineral matter on the tetralin pyrolysis reactions, runs 6, 7, I,, and I, are to be considered. Run 6 has been carried out by running the pyrolysis in the presence of LTA (low-temperature ash) obtained by oxidation at low temperature of the Illinois No. 6 coal utilized in our laboratory (Gioia and Murena, 1988). Run 7 is similar to run 6, but a residue of liquefaction runs is used. This residue is obtained by washing with tetrahydrofuran and filtering the solid phase remaining in the reactor at the end of several liquefaction runs (reported by Gioia and Murena, 1988).
Table 111. Results of Regression Procedure: Pseudo-First-Order Kinetic Constants kij X 10' ( m i d ) of Reactions of the Network of Figure 4 run" run typec gas p , d bar k37 k4a 3.2 33.2 1 PYr N2 30 1.8 21.0 1.0 0.1 5.5 0.4 1.4 2 PY r N2 120 3.2 25.9 0.9 1.0 5.2 2.5 0.8 36 PY r H2 120 1.9 1.1 1.1 3.8 31.5 6.4 3.5 0.7 4 PYr H2 120 2.4 35.9 0.5 1.3 0.4 2.2 7.2 1.3 5 PYr H2 30 2.3 22.8 8.1 1.0 0.9 1.6 4.9 3.0 6 ash H2 120 2.6 25.4 1.0 0.9 7 ash H2 120 1.3 5.5 2.9 6.3 34.6 2.0 0.4 11.1 9.3 I, liq N2 66 1.8 4.3 34.6 14.8 1.3 0.9 8.1 2.9 8.0 15 liq H2 110 "Run numbers are as in Table 11. Runs I1 and I6 are from Gioia and Murena (1988). bResults of two identical runs. Cpyr,pyrolysis; ash, with added ash; liq, liquefaction. dPartial pressure of the gas.
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1845 Runs I1 and Is are liquefaction runs described by Gioia and Murena (1988). Run I1 is in the presence of nitrogen (pNQ= 66 bar), while run I, is under a hydrogen atmosphere ( p H = 110 bar). h e c vs t data of runs 6 and 7 are analyzed with the same procedure used for the homogeneous runs 1-5. For the sake of conciseness, the figures of the data and fitting curves are not reported. The calculated kinetic constants are, however, indicated in Table 111. Runs I1 and I5 are also analyzed according to the network of Figure 4 and with the procedure previously described. However, only the data at times longer than 100 min are considered in order to minimize the effect of the reactions between the organic part of the coal and the tetralin. For those runs, as shown by Gioia and Murena (1988), it may be safely assumed that the reactions between tetralin and coal play a minor role with respect to the pyrolysis reactions about 50 min after the beginning of the liquefaction process. The calculated kinetic constants are reported in Table I11 under the headings I1 and I,. Inspection of Table I11 shows that both LTA and the residue have no significant influence on the value of the relevant kinetic constants. An effect, however, is shown by LTA as long as a catalytic activity on the rehydrogenation of naphthalene is considered. In fact, a better fitting of the data of run 6 (LTA) is obtained if the reverse reaction N + 2H2 T is introduced in the network (see Figure 4). The statistical analysis of the data of run 7 (residue) leads, on the contrary, to the exclusion of this reaction. In fact, inconsistent kinetic constants are calculated if the rehydrogenation reaction is maintained in the network. The reduced catalytic activity of the coal residue on the naphthalene hydrogenation reaction is probably due to the long time laps between the liquefaction runs and the pyrolysis run and to a poisoning of the active sites by the asphaltenes. The kinetic constants for runs I, and I5 (see Table 111) confirm the limited catalytic effect of the coal mineral matter on the pyrolysis reactions. (The larger value of k12 does not deserve much consideration because it is due to a residual reaction of tetralin and asphaltenes. As shown by Gioia and Murena (1988), a k12value of about 3 X lo4 min-I can be calculated to account for this reaction.) However, a catalytic activity for the naphthalene rehydrogenation reaction is shown. In fact, a better fitting of the data of run I, (under hydrogen atmosphere) is obtained if the naphthalene rehydrogenation reaction is introduced in the network. Finally, it seems appropriate to estimate the kinetic constant k2, of the naphthalene rehydrogenation reaction in order to confirm the consistency of the values of this constant reported in Table 111. On the hypothesis of equilibrium between gas and liquid phases, eq 3 may be written
-
r21
= k12CNPH,2/K12
(3)
where K12is the equilibrium constant of reaction a. Equation 3, which is certainly valid close to the equilibrium of reaction a, is assumed to hold for the whole range of concentrations. Therefore, the pseudo-first-order kinetic constant kP1is given by k21
= k1gH,3/K12
(4)
At 430 "C, as reported before, the equilibrium constant of reaction a is K12= 1.54 X lo3 bar2. Values of k2, consistent with those reported in Table I11 can therefore be
calculated with eq 4; e.g., for run 6 eq 4 gives kB1= 14.96 x IO4 min-'.
Conclusions The pyrolysis of tetralin has been explored at 430 "C in the presence of either a nitrogen or a hydrogen atmosphere and at different naphthalene initial concentrations. Runs in the presence of the mineral matter of coal (Illinois No. 6) have also been performed to demonstrate the catalytic effects on the network reactions. The investigation has been carried out on the assumption that tetralin is a model compound for studies aimed at the understanding of the fundamentals of the kinetics of hydrogen donation in the coal liquefaction process. Experimental results consisting of concentration vs time of the principal compounds produced during the pyrolysis process have been obtained. A statistical analysis of the experimental results has allowed the reaction network to be identified. The kinetic constants of the reactions involved have also been evaluated. The corresponding theoretical curves obtained by integrating the differential equations describing the network fit the experimental data satisfactorily. The results of the kinetic analysis indicate the following: (i) The majority of the network reactions have kinetics which are independent of the gas atmosphere. (ii) The catalytic effect of coal mineral matter is minor except for the naphthalene rehydrogenation reaction. (iii) The tetralin dehydrogenation reaction takes place by means of a mechanism which makes the rate of this reaction decrease with increasing hydrogen pressure. Acknowledgment This work was financed by research grants from "Minister0 della Pubblica Istruzione (progetto nazionale carbone)" and from the "Progetto finalizzato energetica-2, CNR-ENEA" Roma, Italy. We gratefully acknowledge Drs. E. Girardi and A. Del Bianco of Eniricerche for supplying the low-temperature ash (LTA) used in run 6.
Nomenclature K12= equilibrium constant of reaction a, bar2 kij = first-order kinetic constant of the reaction leading from compound i to product j (see Figure 4 for values of i and j ) , min-' P = total pressure, bar pH2= hydrogen partial pressure, bar pN, = nitrogen partial pressure, bar rij = rate of reaction (i and j as above),mol/(g of solution-min) t = time, min Registry No. BB, 104-51-8; MI, 767-58-8; T, 119-64-2; N, 91-20-3; I, 496-11-7; benzene, 71-43-2; toluene, 108-88-3;ethylbenzene, 100-41-4;o-xylene, 95-47-6; propylbenzene, 103-65-1; trans-decalin, 493-02-7; cis-decalin,493-01-6. Literature Cited Benjamin, B. M.; Hagman, E. W.; Raaen, V. F.; Collins, C. J. Pyrolysis of Tetralin. Fuel 1979, 58, 386-390. Curran, J. P.; Struck, R. T.; Gorin, E. Mechanism of the HydrogenTransfer Process to Coal and Coal Extract. Ind. Eng. Chem. Process Des. Dev. 1967, 6, 166-173. de Vlieger, J. J.; Kieboom, A. P. G.; van Bekkum, H. Behaviour of Tetralin in Coal Liquefaction. Examination in long-run batchautoclave experiments. Fuel 1984,63, 334-340. Gioia, F.; Murena, F. Kinetics of Hydrogen Donation in the Liquefaction of Coal. Znd. Eng. Chem. Res. 1988,27, 1978-1983. Hooper, R. J.; Battaerd, H. A. J.; Evans, D. G. Thermal Dissociation of Tetralin Between 300 and 450 O C . Fuel 1979,58, 132. Himmelblau, D. M.; Jones, C. R.; Bischoff, K. B. Determination of Rate Constants for Complex Kinetics Models. Znd. Eng. Chem. Fundam. 1967,6, 539-543.
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Ind. E n g . Chem. Res. 1990, 29, 1846-1855
Kara, M.; Sung, S.;Klinzing, G. E.; Chiang, S.H. Hydrogen Mass Transfer in Liquid Hydrocarbons at Elevated Temperatures and Pressures. Fuel 1983,62, 1492-1498. McPherson, W. P.; Foster, N. R.; Hastings, D. W.; Kalman, J. R.; Gilbert, T. D. Tetralin decomposition in short contact time coal liquefaction. Fuel 1985, 64, 457-460. Murena, F. Cinetica del process0 di donazione di idrogeno nella liquefazione del carbone. Ph.D. Dissertation, Universiti di Napoli, Italy, 1989.
Simnick, J. J.; Lawson, C. C.; Lin, H. M.; Chao, K. C. Vapor-Liquid Equilibrium of Hydrogen/Tetralin System at Elevated Temperatures and Pressures. AIChE J. 1977,23, 469-476. Whitehurst, D. D.; Mitchell, T. 0.;Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980; p 166. Receiued for reuiew November 1, 1988 Revised manuscript received April 23, 1990 Accepted May 14, 1990
Kinetics of the Pyrolysis of Almond Shells and Almond Shells Impregnated with CoC12in a Fluidized Bed Reactor and in a Pyroprobe 100 Rafael Font,* Antonio Marcilla, Emilio Verdii, and Joaquin Devesa Division of Chemical Engineering, University of Alicante, Apartado 99, Alicante, S p a i n
A fluidized bed reactor and a Pyroprobe 100 have been used in order to study the kinetics of the flash pyrolysis of almond shells and of almond shells impregnated with CoC1, (14.1 g of CoCl,/lOO total g). Assuming first-order reactions, a good fit of the yields of total gases, total liquids, and solids to the expressions deduced has been obtained for the four kinetic studies carried out. Nevertheless, on considering the components analyzed in each case, some discrepancies have been observed. Kinetic parameters obtained for the four kinetic studies have been compared, and their differences have been discussed. Introduction
pregnated with CoCl,, which causes an increase in the yield of 2-furaldehyde.
By biomass pyrolysis, different chemicals and fuels can be obtained. The kinetics of pyrolysis of lignocellulosic materials has been investigated by several researchers using different techniques: furnace reactors ( S t a ” , 1956; Hajaligol et al., 1980,1982; Antal et al., 1980; Thurner and Mann, 1981; Jegers and Klein, 1985; Nunn et al., 1985a,b), vacuum (Broadbury et al., 1979),DTA and TGA (Browne and Tang, 1963; Chatterjee and Conrad, 1966; Akita and Kase, 1967; Mack and Donaldson, 1967; Maa and Bailie, 1978; Leu, 1975; Tran and Rai, 1979; Antal et al., 1980; Urban and Antal, 1982; Bilbao et al., 1987a,b; Alves and Figueiredo, 1988),and fluidized bed reactor (Barooah and Long, 1976; Kosstrin, 1980; Liden et al., 1988; Scott et al., 1988). A wide variation in the kinetic parameters reported, activation energy and rate constants, can be observed in the literature. This fact is probably due to the diversity of raw materials investigated (cellulose, lignin, different types of wood, biomass), the particle size, the different schemes of reaction considered, the different operating conditions studied (temperature intervals, pressure, heating rate, atmosphere, residence time of the volatiles), the different types of reactors, and the variables considered and analyzed (yields of solids, liquids, tars, gases; yields of the different compounds; etc.). Thus, for example, the different values for the activation energy of the pyrolysis reactions of these types of materials, obtained by following the evolution of the total volatiles or condensable volatiles, range from 14.6 to 227 kJ/mol. Almond shells are an abundant and available agricultural byproduct in moderate climate zones as the Alicante area (southeastern Spain). The scope of the present investigation is to study the kinetics in two experimental equipments (fluidized bed reactor and Pyroprobe 100) with nonimpregnated almond shells and almond shells im0888-5885/90/ 2629-1846$02.50/0
Kinetic Model Different schemes of reaction have been suggested for the pyrolysis of biomass, each one leading to different expressions to correlate the kinetic data (Kosstrin, 1980; Thurner and Mann, 1981; Nunn et al., 1985a,b; Scott et al., 1988). From all of these, we have selected two models for the correlation and discussion of the experimental data obtained in the present work. Biomass decomposes at high temperatures according to a complex scheme of reactions, both in series and in parallel. Furthermore, mass diffusion and heat-transfer phenomena may affect the overall kinetics of the process. Assuming that for a solid the specific decomposition rate (mass reacting by time unit and mass unit) is constant, this rate may be written according to the following equation: -(l/B)(dB/dt) = constant (1) where B is the biomass present at any time (mass or mass fraction with respect to the initial mass). This equation corresponds to a first-order overall reaction for the decomposition of the solid material. A single reaction can be written as B aG + bL + cS (2) where G, L , and S are the different fractions of gases, liquids, and solids produced at a time t and a, b, and c are the yield coefficients, expressed as grams of gas, liquid, and solid per gram of reacted biomass. Applying the kinetic law expressed by eq 1, we can deduce dA/dt = k(A, - A ) (3)
-
where A is the yield of any fraction or compound and A , is the maximum value of A at time infinite. The kinetic 0 1990 American Chemical Society
