I n d . Eng. Chem. Res. 1988,27, 1767-1775 Elperin, I.; Tamir, A. “Method and Reactor for Effecting Interphase Processes”. Israeli Patent 66 162,1985. Herskowitz, D.; Herskowitz, V.; Tamir, A. “Desorption of Acetone in a Two-Impinging-Streams Spray Desorber”. Chem. Eng. Sci. 1987,42,2331-2337. Kitron, Y. “Performance of a Coaxial Gas-Solid Two-ImpingingStreams (TIS) Reactor: Hydrodynamics, Residence Time Distribution and Drying Heat Transfer”. BSc. Project, Chemical Engineering, Ben Gurion University of the Negev, Beer Sheva, Israel, 1987. Kitron, A.; Buchman, R.; Luzzatto, K.; Tamir, A. “Drying and Mixing of Solids and Particles’ RTD in Four-Impinging-Streams and Multistage Two-Impinging-Streams Reactors”. Ind. Eng. Chem. Res. 1987,26, 2454. Kunii, D.; Levenspiel, 0. Fluidization Engineering; Wiley: New York, 1969. Levelspiel, 0. Chemical Reaction Engineering, 2nd ed.; Wiley: New York, 1972. Luzzatto, K. “Investigation of a Two-Impinging-Streams Reactor”. Ph.D. Thesis, Chemical Engineering, Ben Gurion University of the Negev, Beer Sheva, Israel, 1987. Luzzatto, K.; Tamir, A.; Elperin, I. “A New Two-ImpingingStreams Reactor”. AZChE J. 1984,30,600-608. Mathur, K.B.;Epstein, N. Spouted Beds; Academic: New York, 1974. Meltser, V. L.; Pisarik, N. K. “Interphase Heat Transfer Upon Collision between Single and Two-Phase Flows”. Heat Transfer-Sou. Res. 1982,14, 130-133. Tamir, A. “Absorption of Acetone in a Two-Impinging-Streams Absorber”. Chem. Eng. Sci. 1986,41,3023-3030. Tamir, A,; Falk, 0. “Dissolution of Solids and Pressure Drop in Cyclone and Two-Impinging-Streams Semibatch Reactor”. Znd. Eng. Chem. Res. 1988,submitted.
1767
Tamir, A,; Grinholtz, M. ”Performance of a Continuous Solid-Liquid Two-Impinging-Streams (TIS) Reactor: Dissolution of Solids, Hydrodynamics, Mean Residence Time, and Holdup of the Particles”. Znd. Eng. Chem. Res. 1987,26,726-731. Tamir, A.; Herskowitz, D. ‘Absorption of COz in a New Two-Impinging-Streams Absorber”. Chem. Eng. Sci. 1985,40,2149-2151. Tamir, A.; Kitron, A. “Application of Impinging-Streams in Chemical Engineering Processes”. Chem. Eng. Commun. 1987,50,241-330. Tamir, A.; Luzzatto, K. “Solid-Solid and Gas-Gas Mixing Properties of a New Two-Impinging-Streams Mixer”. AIChE J. 1985,31, 781-787. Tamir, A.; Luzzatto, K. “Mixing of Solids in Impinging-Streams Reactors”. J. Powder Bulk Solids Technol. 1985,9,15-24. Tamir, A.; Shalmon, B. “Scale-up of Two-Impinging-Streams (TIS) Reactors“. Znd. Eng. Chem. Res. 1988,27,238. Tamir, A,; Sobhi, S. ‘A New Two-Impinging-Streams Emulsifier“. AZChE J . 1985,31,2089-2092. Tamir, A.; Elperin, I.; Luzzatto, K. “Drying in a Two-ImpingingStreams Reactor”. Chem. Eng. Sci. 1984,39,139-146. Tamir, A.; Luzzatto, K.; Artana, D.; Salomon, S. “A Correlation Based on the Physical Properties of the Solid Particles for the Evaluation of the Pressure Drop in the Two-Impinging-Streams Gas-Solid Reactor”. AZChE J. 1985,31,1744-1746. Zabrodsky, S. S. “Hydrodynamics and Heat Transfer in Fluidized Beds”. The M.I.T. Press: Cambridge, MA, 1963. Ziv, A.; Luzzatto, K.; Tamir, A. “Application of Free ImpingingStreams to the Combustion of Gas and Pulverized Coal”. Combust. Sci. Technol. 1988,in press.
Received for review November 24, 1987 Revised manuscript received May 16, 1988 Accepted June 2, 1988
Catalytic Hydroprocessing of SRC-I1 Heavy Distillate Fractions. 7. Kinetics of Hydrogenation, Hydrodesulfurization, and Hydrodeoxygenation of the Neutral Oils Determined by Analysis of Compound Classes and Individual Compounds Sanjeev S. Katti and Bruce C. Gates* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
David W. Grandy, Tim Youngless, and Leonidas Petrakis* Gulf Research and Development Company, Pittsburgh, Pennsylvania 15230
The catalytic hydroprocessing of the neutral oils fraction of a heavy liquid prepared from Powhatan No. 5 coal in the SRC-I1 hydroliquefaction process was characterized by conversion in a batch reactor a t 355 “C and 36 atm; the catalyst was sulfided Ni-Mo/y-A1203. Compound classes in the reactant and products were characterized by high- and low-resolution mass spectrometry and lH and 13C NMR spectroscopies. Individual compounds were determined by gas chromatography and gas chromatography/mass spectrometry. The results provide a detailed profile of the hydroprocessing reactions, namely, hydrodesulfurization, hydrogenation, and hydrodeoxygenation. The hydrodesulfurization reactions of the major organosulfur components (dibenzothiophene, methyldibenzothiophenes) are characterized by pseudo-first-order rate constants of the order of lo* L/ (g of catalyst-s),and the hydrogenation reactions of major aromatic hydrocarbons (e.g., phenanthrene and pyrene) and hydrodeoxygenation of dibenzofuran and related compounds are characterized by pseudo-first-order rate constants of the order of lo-” L/(g of catalyst-s). The use of coal-hydroliquefaction products and related heavy liquids as fuels requires their refining by catalytic hydroprocessing to remove sulfur, nitrogen, and oxygen and to increase the hydrogen-to-carbonratios. As part of a systematic investigation of the hydroprocessing reactions of a heavy liquid prepared from Powhatan No. 5 coal by the SRC-I1 hydroliquefaction process, we reported on the separation of 1kg of the liquid into nine chemically distinct
fractions by liquid chromatography (Petrakis et al., 1983a) and a detailed characterization of the fractions (Petrakis et al., 1983b), Katti et al. (1984) reported an investigation of the kinetics of catalytic hydrodesulfurization of the organosulfur compounds in a dilute solution of the neutral oils, the fraction accounting for 72.8 wt % of the coal liquid. The following report is a full description of the catalytic hydroprocessing reactions of this fraction, in-
0 1988 American Chemical Society OS8S-SSS5/88/2627-~~67~Ol.SO~~
1768 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988
cluding hydrogenation, hydrodeoxygenation, and hydrodesulfurization. The feed and products have been characterized by group-type methods based on elemental analysis, NMR spectroscopy, and mass spectrometry (Allen et al., 1984, Petrakis et al., 1983c), and individual reactant compounds have been determined by gas chromatography and gas chromatography/mass spectrometry. One of the goals was to compare the patterns of reactivity determined by the individual compound analyses with those determined by the group-type methods. This report is the first to allow such a comparison for a heavy fossil fuel mixture. The related hydroprocessing literature is summarized in Katti's thesis (1984) and in earlier publications in this series characterizing the reactivity of the acidic and basic fractions of the SRC-I1coal liquid (Katti et al., 1984,1986; Li et al., 1985; Grandy et al., 1986). The numerous reported investigations of the reactions of individual compounds provide a basis for understanding the reaction sequences and kinetics of the individual reactions, and data characterizing the reactions of mixtures of a few compounds provide information about competitive reaction and inhibition effects. These results, however, fall short of representing the complexity of real feedstocks. Our approach was to investigate the conversion of individual compounds as well as of compound classes in a feed of intermediate complexity, namely, the neutral oils fraction of the coal liquid. , An advantage of the fraction is its simplicity in comparison with that of the whole liquid; since the fraction consists of chemically similar compounds, far fewer in number than those constituting the whole liquid, the analytical chemistry gives a far more precise profile than can be obtained for the whole liquid. The neutral oils of the SRC-I1 coal liquid consist predominantly of polycyclic aromatic hydrocarbons (Petrakis et al., 1983a,b). The concentration of organonitrogen compounds is low (0.06 w t % N), but the concentrations of organosulfur and organooxygen compounds are significant, there being 0.62 wt % S and 0.72 wt % 0. The analyses (Petrakis et al., 1983b) indicate that typical heteroatom-containing constituents are dibenzothiophene and dibenzofuran; typical hydrocarbons are polycyclic aromatics such as phenanthrene and pyrene. In the experiments reported earlier (Katti et al., 1984), the concentration of neutral oils in cyclohexane solvent used as feed to a flow reactor was low (0.25 wt %) to allow use of a large excess of hydrogen as a dissolved coreactant. In industrial practice, one would expect to use undiluted coal liquids (or possibly those mixed with petroleum), which have reactivities different from those of highly diluted coal liquids. Here we report the results of experiments conceived to characterize the hydroprocessing of undiluted neutral oils. This experiment, carried out with a batch reactor, was possible with the neutral oils but not the other fractions of the coal liquid, since there were not sufficient quantities of the latter. An important advantage of the batch experiment over the flow experiment is the higher quality of the product analysis associated with the availability of relatively large amounts of undiluted product. Further, the experiment could be carried out to long reaction times, providing characterizations of the slower reactions, which were not investigated in the flow reactor experiments (Katti et al., 1984).
Experimental Methods Materials. The catalyst was a commercial NiOMo03/y-A1203(American Cyanamid, HDS 9A), crushed and sieved to 149-178-pm (80-100-mesh) particle size and sulf'ided in situ. Catalyst compositions are given elsewhere (Houalla et al., 1978). Hydrogen was obtained from Linde
as 3500 psi grade and treated to remove traces of moisture and oxygen with copper oxide and zeolite traps. H2S was supplied by Linde as a custom mixture with Hz, the concentration being 10 mol % H2S. The neutral oils fraction of the SRC-I1 coal liquid is described elsewhere (Petrakis et al., 1983a,b; Katti, 1984). Catalytic Reaction Experiment: Equipment and Procedure. The stirred autoclave reactor has been described in detail (Shih et al., 1977; Bhinde, 1979),and only a brief description and discussion of the procedure are given here. The reactor was a heated autoclave, with a volume of 1 L, equipped with a stirrer, cooling coils, a thermowell, and a glass liner. A catalyst charging vessel was located above the autoclave and separated from it by a ball valve. The required quantity of the neutral oils mixed with carbon disulfide (to keep the catalyst sulfided) was added to the autoclave, purged with hydrogen to remove air, and then brought to the desired temperature and pressure by addition of hydrogen. The catalyst charger was loaded with presulfided catalyst, care being taken to minimize its contact with air. About 25 g of neutral oils was added to the loader and pressurized with hydrogen. The resulting slurry of catalyst was injected from the charger into the autoclave to begin the reaction. With this start-up procedure, the pressure and temperature were stabilized within 5 min. Liquid samples were taken periodically during the experiment. Catalyst pretreatment (Katti et al., 1984; Katti, 1984) involved sulfiding the oxide form in a glass boat held in a glass tube in a temperature-controlled oven, with a continuous 30-60 cm3 (STP)/min flow of 10 mol % hydrogen sulfide in hydrogen. The oven was maintained at 400 "C for 2 h, after which the tube was removed and allowed to cool to room temperature. The catalyst was injected into the reactor almost immediately after it had been loaded into the charger. The catalytic reaction experiment was carried out at 355 "C, the same temperature used in the earlier flow reactor experiments (Katti et al., 1984). The pressure before heating of the reactant mixture was 21.4 atm; as the liquid was heated, the pressure increased to 36 atm (the desired pressure). The organic reactant mixture consisted of 280.9 g of the neutral oils, 1.037 g of carbon disulfide, and 1.311 g of n-decane (an internal standard for the GC analysis); this was saturated with hydrogen. The amount of catalyst (Ni-Mo/y-Al,O,) was 5.00 g. The conversion process was exothermic. The heater was off for the first 1.5 h, and cooling water flowed intermittently to maintain a temperature constant within f 3 "C. Subsequently, heating was required to maintain this temperature (355 "C). The pressure was kept constant at 36 f 1atm by repressurizing the reactor with hydrogen from time to time. A typical pressure drop in the second half of the experiment was about 2 atm/h. Seventeen samples were taken during the run of 25 h. Initially, the samples were taken at 5-minute intervals, and toward the end of the experiment, they were taken at intervals of 5 h. Analysis of Feed and Products. The following analytical techniques were used to characterize the feed and products: group-type analyses (high-resolution mass spectrometry, low-resolution mass spectrometry, NMR), elemental analyses, capillary column gas chromatography/mass spectrometry (GC/MS), with the mass spectrometer operated in the chemical ionization (CI) and electron impact (EI) modes, and capillary column GC. Routine analysis of the feed and products was carried out with capillary column GC. With dual detectors, two
Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 1769 simultaneous traces were recorded, one for all the compounds, obtained with the flame ionization detector (FID), and one for only organosulfur compounds, obtained with a Hall electrolytic conductivity detector. The details of this analysis have been published (Westerman et al., 1983). The sulfur compound trace had seven peaks, the identifications of which were described earlier (Katti et al., 1984); the three major organosulfur compounds (dibenzothiophene, 4-methyldibenzothiophene,and another isomer of methyldibenzothiophene) account for approximately 85% of the sulfur in the neutral oils. The CI spectrum of the neutral oils determined by GC/MS is shown by Katti (1984). The identification of the compounds in the neutral oils was carried out as follows: the CI spectrum, E1 spectrum, and the FID chromatogram were used to do peak matching. The E1 mass spectra were matched with reference spectra of likely pure compounds. The retention times in the GC were checked with those of available pure compounds. Sometimes boiling points were used to determine GC retention times of isomers, since the capillary column separates essentially by boiling point. Two product samples were also analyzed by GC/MS (E1 mode) to confirm the identification of compounds in the feed and to identify products. The GC/MS results confirm that the neutral oils contain largely hydrocarbons. Some of the major constituents are acenaphthene and its mono- and dimethyl derivatives, methyl- and dimethylbiphenyls, fluorene and methylfluorene, phenanthrene and methylphenanthrenes, fluoranthene, and pyrene. The oxygen-containing compounds include biphenyl ether and dibenzofuran. The relative concentrations of each of the three major organosulfur compounds mentioned above and of 14 other major compounds (chosen from the complex FID trace on the basis of the ease of quantification as well as the abundance) were monitored as a function of reaction time by GC analysis. A mixture of benzothiophene and tetralin in cyclohexane was used as the external standard for the quantitative analysis. Typically, three injections were made with each sample, and the reported GC results are average values. The elemental analyses of the samples were performed for C, H, N, 0, and S by Microanalysis, Inc., Wilmington, DE. The low-resolution group-type mass spectrometry was carried out to characterize the aromatic and hydroaromatic species in the neutral oils feed and products. The data reduction is essentially a matrix inversion to calculate the weight percent of each of the calibration groups from the experimentally obtained ion intensity matrix. The details of the procedure are reported elsewhere (Swansiger et al., 1974; Katti, 1984). The high-resolution mass spectrometry at 10000 dynamic resolving power was used to quantify the groups of compounds present in the neutral oils feed and products. The data reduction was based on principles similar to those underlying the low-resolution mass spectrometry grouptype analysis (Katti, 1984). The neutral oils feed and products were characterized by 'H and 13CNMR spectroscopies. The NMR data were reduced to give the average compositional parameters with computer algorithms developed at Gulf specifically for coal liquids. These are based on the average molecule parameters approach (reported by Clutter et al. (1972), Knight (1967), and Brown and Ladner (1960)).
Results Analyses were obtained for the neutral oils feed and for liquid products taken from the reactor at various times.
v'O
5
IO
20
15
25
T1ME.h
Figure 1. Evidence of hydrodesulfurization in hydroprocessing of the neutral oils in a batch reactor catalyzed by Ni-Mo/y-Alp03 a t 355 "C and 36 atm. The sulfur content was determined by elemental analysis.
TIME, h
Figure 2. Evidence of hydrogenation in hydroprocessing of the neutral oils in a batch reactor catalyzed by Ni-Mo/yAl2O8 at 355 "C and 36 atm. The data were determined by elemental analysis.
"'0
w 5
10
15
20
25
30
TIME, h
Figure 3. Pseudo-first-order kinetics of hydrodesulfurization of organosulfur compounds in the neutral oils a t 355 O C and 36 atm: (0) 4-methyldibenzothiophene;( 0 )dibenzothiophene; (A)methyldibenzothiophene (an unidentified isomer).
The elemental analyses are summarized in Table I. The analyses for C, H, 0, N, and S summed to 100 f 0.8% for each sample. The sulfur analysis was carried out in duplicate, and the results are plotted as a function of time in the pseudo-first-order kinetics plot of Figure 1. The H/C weight ratio was calculated from the elemental analyses and is plotted as a function of time in Figure 2; it shows a steady increase with reaction time. The nitrogen data (Table I) show that approximately 40% of the nitrogen had been removed by the end of the experiment. The sulfur data lend themselves to comparison with results determined by GC with the sulfur-specific Hall detector, establishing the conversions of each of the three major sulfur-containing compounds. These individual
1770 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 Table I. Elemental Analyses (Weight Percent) of Samoles of Neutral Oils Feed and Products time, h feed samde from storage hot feed before adding iatalyst feed immediately after adding catalyst
a
0.0 0.047 0.083 0.168 0.333 0.517 0.750 1.25 2.00 3.00 5.00 7.50 10.0 15.0 20.0 25.0
70 C % H 91.54 7.58 91.73 7.66 91.07 7.54
'70 0 % N 0.24 0.042 0.21 0.045 0.27 0.039
91.50 7.69
0.07 0.029
0.54
0.53
0.54
99.83
0.0840
1.01
91.50 7.90
0.00 0.033
0.54
0.51
0.53
99.96
0.0863
1.04
91.62 7.88
0.44 0.034
0.45
0.44
0.45
100.43
0.0860
1.03
0.039 0.019 0.023 0.027 0.000 0.023 0.024
0.30 0.33 0.36 0.26 0.23 0.18 0.17
0.38 0.36 0.31 0.24 0.21 0.19 0.17
0.34 0.35 0.34 0.25 0.22 0.19 0.17
99.95 99.85 100.75 100.49 99.71 100.30 100.38
0.0865 0.0899 0.0865 0.0906 0.0914 0.0920 0.0922
1.04 1.08 1.04 1.09 1.10 1.10
91.26 91.21 92.23 91.77 91.66 91.50 91.09
7.89 8.20 7.98 8.31 8.32 8.42 8.40
0.42 0.07 0.18 0.13 0.11 0.17 0.17
1.11
Sulfur analysis was done in duplicate.
Table 11. Pseudo-First-Order Rate Constants for Hydrodesulfurization of Components of Neutral Oils Catalyzed by Sulfided Ni-Mo/y-A1203 at 355 "C and 36 atm pseudo-first-order rate constant g of 106L/ slope," oil/(g of (g of reactant h-' catalyst-h) cata1yst.s) dibenzothiophene 0.216 12.1 3.97 4-methyldibenzothiophene 0.071 4.0 1.30 methyldibenzothiophene (an 0.253 14.2 4.64 undetermined isomer other than 4-methyldibenzothiophene) a
H/C H/C mass atomic % S (I)" % S (11)" % S, av total, % ratio ratio 0.67 0.56 0.62 100.02 0.0828 0.99 0.62 0.56 0.59 100.23 0.0835 1.00 0.56 0.61 0.59 99.51 0.0828 0.99
Slope of the pseudo-first-order kinetics plots (Figure 3).
I
I
a
a
'Phenanthrene
a LL
1
u t 5 15 20 25 33 :'-CI
i0
TIME,h
Figure 5. Pseudo-first-order plots for the hydrogenation of aromatic hydrocarbons during batch hydroprocessing of the neutral oils at 355 "C and 36 atm. Table 111. Pseudo-First-Order Rate Constants for Hydrogenation of Components of Neutral Oils Catalyzed by Sulfided Ni-Mo/y-A1203 at 355 "C and 36 atm pseudo-first-order rate constant g of oil/ i07L/ slope," (g of (g of comDd h-' catalvst-h) catalvstd fluorene 0.024 1.35 4.4 phenanthrene 0.04 2.42 7.9 methylphenanthrenes 0.035 1.97 6.0 pyrene 0.012 0.674 2.2
a LL 3
'Slopes of the pseudo-first-order kinetics plots.
t TIME,h
Figure 4. Batch hydroprocessing of the neutral oils at 355 "C and 36 atm. The weight percent sulfur was calculated from the analyses for the three major organosulfur compounds in the neutral oils (Figure 3). The curve is a sum accounting for the first-order reaction of each of these three compounds (Table 11).
compound data are plotted in Figure 3; the results demonstrate that pseudo-first-order kinetics provides a good description of the conversion of each of the sulfur-containing compounds. The pseudo-first-order rate constants calculated from the slopes of the lines are summarized in Table 11. The total sulfur content in each sample was approximated by adding the three major components determined with the Hall detector; the data are shown as experimental points in Figure 4. The curve shows the values predicted from the pseudo-first-order rate constants
given in Table 11. It is clear from the comparison of Figures 1 and 4 that most of the sulfur is contained in these three major compounds and that the remaining (unidentified) organosulfur compounds (likely substituted dibenzothiophenes and substituted benzonaphthothiophenes) have lower reactivities. Data characterizing the hydrocarbon hydrogenation reactions were obtained from the FID traces of the product samples. [Traces are shown in Katti's thesis (1984) and in the paper by Westerman et al. (1983).] The results presented next are selected from those presented in Katti's thesis to illustrate typical behavior of individual hydrocarbon compounds. Pseudo-first-order kinetics plots for the removal of pyrene and phenanthrene are shown in Figure 5 . Similar linear plots were obtained from methylphenanthrene and fluorene (Katti, 1984). The rate constants calculated from the slopes of these plots are summarized in Table 111.
Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 1771 Table IV. Low-Resolution, Group-Type Mass Spectrometry Data Showing Compositions (Weight Percent) for Samples from the Batch Experiment with Neutral Oils reaction time, h group type decahydrobenzochrysenes octahydrobenzochrysenes hexahydrobenzochrysenes tetrahydrobenzochrysenes benzochrysenes decahydrobenzopyrenes octahydrobenzopyrenes hexahydrobenzopyrenes tetrahydrobenzopyrenes binaphthyls benzopyrenes dodecahydrochrysenes octahydrochrysenes hexahydrochrysenes chrysenes decahydropyrenes hexahydropyrenes tetrahydrofluoranthenes pyrenes/ fluoranthenes phenylnaphthalenes octahydrophenanthrenes hexahydrophenanthrenes tetrahydrophenanthrenes phenanthrenes fluorenes biphenyls/acenaphthenes tetralins tetrahydroacenaphthenes naphthalenes benzenes total
0
0.517
3.00
7.50
15.0
25.0
~~
0.6 0.6 0.4 0.6 0.5 0.8 0.8 1.0 0.8 0.7 1.7 1.7 0.6 1.0 1.7 2.5 5.3 2.6 10.5 6.3 2.0 1.6 9.6 11.4 9.4 10.1 0.0 0.8 5.3 3.1 94.0
0.6 0.6 0.5 0.6 0.5 0.7 0.8 1.0 0.8 0.7 1.6 1.6 0.7 1.0 1.5 2.7 5.2 2.7 10.6 6.5 1.5 1.6 9.6 11.4 9.4 10.1 0.0 1.0 5.4 2.9 94.0
0.1 0.3 0.3 0.3 0.4 0.6 0.9 1.2 0.8 0.7 1.4 1.7 1.0 1.2 1.1 2.8 5.1 3.0 10.0 7.1 1.4 1.4 9.5 11.0 10.3 10.0 0.0 1.7 5.3 2.8 94.0
0.3 0.3 0.2 0.3 0.3 0.6 0.8 0.9 0.6 0.5 1.0 1.3 0.8 1.1 0.9 2.8 5.8 3.1 9.3 6.8 1.6 1.8 10.1 11.0 10.6 10.6 0.1 1.8 5.7 2.9 94.0
TIME, h
Figure 6. Disappearance of dimethylnaphthalene during hydroprocessing of the neutral oils at 355 O C and 36 atm.
Data represented in a similar way for dimethylnaphthalene (Figure 6) and other compounds (not shown) are nonlinear. This nonlinearity often indicates equilibrium limitations in the hydrogenation reaction networks. More detail concerning this behavior for hydrogenation reactions is given elsewhere with data characterizing some reaction products and intermediates (Katti, 1984). Typical data characterizing the feed and products by low-resolution mass spectrometry are shown in Table IV. Data obtained by high-resolution mass spectrometry and by 'H NMR spectroscopy are summarized in Tables V and VI, respectively. The 'H NMR data indicate that hydrogenation takes place throughout the course of the reaction. The percentage of alkyl carbon increased from 30% in the feed to 38% in the final product. The average molecular formula of the product also changed with time, as shown in Table VI; the H/C atomic ratio varied from 0.98 in the feed to 1.04 in the final product. The NMR data allow characterization of the hydrogenation reactions in terms of the concentrations of mono-,
0.3 0.3 0.2 0.2 0.4 0.7 0.7 0.8 0.5 0.4 1.0 1.3 1.0 1.3 0.7 2.9 6.8 3.2 8.7 6.5 3.9 1.6 9.7 10.0 10.7 10.0 0.4 2.5 4.4 2.7 94.0
0.4 0.5 0.4 0.4 0.4 0.9 0.9 1.1 0.8 0.6 1.4 1.5 1.0 1.3 0.9 2.8 6.7 3.1 9.1 6.7 3.8 1.3 9.2 9.6 10.5 9.5 0.2 2.4 4.1 2.6 94.0
0.3 0.3 0.4 0.3 0.4 0.8 0.7 0.9 0.6 0.5 1.1 1.4 1.1 1.4 0.8 2.7 7.1 3.1 8.8 6.1 5.3 1.1 9.8 9.0 10.3 9.9 0.6 2.7 3.7 2.9 94.0
0.3 0.5 0.4 0.4 0.4 0.8 0.8 0.9 0.7 0.5 1.2 1.4 1.1 1.4 0.8 2.7 7.1 3.1 8.9 6.1 5.3 1.0 9.6 8.9 10.2 9.9 0.5 2.7 3.7 2.9 94.0
0.3 0.3 0.3 0.3 0.3 0.7 0.6 0.8 0.5 0.4 0.9 1.2 1.0 1.3 0.7 2.5 6.9 2.9 8.2 5.4 6.4 0.9 10.3 8.1 9.9 10.9 1.1 3.2 3.9 3.8 94.0
0.2 0.4 0.3 0.3 0.4 0.7 0.7 0.8 0.5 0.4 0.9 1.3 1.1 1.3 0.8 2.5 6.9 2.9 8.6 5.6 6.3 0.8 10.1 8.1 9.8 10.7 1.0 3.0 3.8 3.7 94.0
0.1 0.3 0.2 0.2 0.3 0.7 0.6 0.7 0.4 0.4 0.9 1.2 1.0 1.2 0.7 2.3 6.8 2.8 7.9 5.2 6.9 0.8 10.5 7.8 9.6 11.4 1.4 3.2 4.1 4.3 94.0
0.1 0.4 0.4 0.4 0.4 0.9 0.8 0.9 0.7 0.5 1.3 1.6 1.0 1.2 0.8 2.3 6.7 2.7 8.5 5.5 6.6 0.5 10.0 7.5 9.5 10.9 1.0 3.0 3.7 4.0 94.0
di-, and triaromatic ring compounds in the samples as a function of time. The increase in the percentage of monoaromatics at the expense of di- and triaromatics shows the trend to formation of smaller aromatic units and lower aromaticity. As a consequence of the definitions underlying the average molecule calculations, there is not a one-for-one stoichiometry indicated among these three groups as hydrogenation occurs; for example, the average molecule calculation treats 9,lO-dihydrophenanthreneas being two monoaromatic species. Thus, with little added hydrogen, a triaromatic compound becomes classified as two monoaromatics. As will be seen in the following summary of the mass spectrometry data, the concentration of monoaromatics as calculated from the NMR data does not agree well with the concentration of substituted benzenes or tetralin compounds as measured by mass spectrometry. This discrepancy is related to the problem of counting aromatic rings in a molecule by NMR when the rings are not fused. The naphthenic and alkyl carbon data as determined by NMR (Table VI) further imply that the aromatic structures underwent hydrogenation. Naphthenic carbon comprises about 50% of the alkyl carbon in the feed and increases to about 75% in the 15-h sample. The decrease in the naphthenic-to-alkyl carbon ratio in the last 10 h of the run, coupled with an increase in overall alkyl carbon, suggests that naphthenic rings were opening but that alkyl chains were not being removed. The low-resolution group-type mass spectrometry data (Table IV) provide additional evidence of hydrogenation and characterizations of the molecular types being hydrogenated. The increase in the concentration of the hydroaromatics and the decrease in the concentration of the purely aromatic compounds took place largely in the first 15 h of reaction. Little additional hydrogenation took place in the last 10 h of reaction. The percentage of two- and
1772 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 Table V. Analysis of the Neutral Oils Feed by High-Resolution Mass Spectrometry" Z hydrocarbon type av C no. monoaromatics 7.10 -6 alkylbenzenes 14.88 -8 indans -10 indenes 14.34
wt %
3.76 0.69 1.16 1.91
13.93 14.65 15.29
36.14 8.36 16.03 11.75
15.31 17.43
25.61 19.16 6.45
tetraaromatics -22 pyrenes -24 chrysenes
17.25 19.29
20.54 16.43 4.11
pentaaromatics -26 chrysocycloparaffins -28 benzpyrenes -30 dibenzanthracenes
20.48 21.09 21.73
4.58 1.87 2.00 0.71
polyaromatics -32 -34 -36
22.64 22.00 0.00
0.97 0.88 0.10 0.00
thiophenes -10 benzothiophenes -12 -14 -16 dibenzothiophenes -18 -20 -22 benzonaphthothiophenes -24 -26 -28
0.00 0.00 0.00 13.18 0.00 14.00 16.49 0.00 0.00 0.00
1.81 0.00 0.00 0.00 1.59 0.00 0.03 0.19 0.00 0.00 0.00
0.00 147.34 15.62 17.27
6.59 0.00 5.03 0.92 0.64
diaromatics -12 naphthalenes -14 biphenyls -16 fluorenes triaromatics -18 phenanthrenes -20 phenanthrocycloparaffins
phenols/ furans -6 phenols -16 dibenzofurans -18 -22 benzonaphthofurans
"Average carbon number = 15.89; average 2 = -17.78; average H / C (atomic) = 0.881.
three-ring hydroaromatic compounds increased with reaction time, the four-ring compounds increased in concentration for about 7 h and then decreased, and the five-ring hydroaromatics decreased in concentration monotonically with time. All aromatic types appear to have been hydrogenated to some extent, but most of the hydrogenation occurred
0
01
I
IO
0
20
TIME, h
Figure 7. Pseudo-first-order kinetics of hydrodesulfurization of thiophenes and hydrodeoxygenation of furans in the neutral oils, determined by high-resolution mass spectrometry. Reaction took place a t 355 "C and 36 atm in a batch reactor.
in the three- and four-ring aromatics. The data of Table IV indicate that octahydrophenanthrene was the predominant three-ring hydrogenation product. The decrease in hexahydrophenanthrene concentration with time appears to contradict the observed trend to increased concentrations of three-ring hydroaromatics with increased time. However, dibenzothiophene and methyldibenzothiophene, which contribute to the concentration of the hexahydrophenanthrene group as indicated by low-resolution mass spectrometry, are removed by the hydrodesulfurization reaction, causing an apparent decrease in the concentration of hexahydrophenanthrene. Additional evidence of the hydrogenation reactions is given by the high-resolution mass spectrometry data. The average 2 number [a parameter indicating the degree of unsaturation, calculated by taking the difference between the number of hydrogen atoms in a molecule and two times the number of carbon atoms in a molecule (for reference, cyclohexane has a Z number of 12- (6 X 2), or 0; benzene, -6; naphthalene, -12; etc.)] varied from -17.78 in the feed to -16.22 in the 25-h product. This result indicates that the average molecule gained 1.5 hydrogen atoms during the 25-h period of reaction. The progress of hydrodesulfurization and hydrodeoxygenation reactions can be inferred from the highresolution mass spectrometry data, since these data show the percentage of thiophenes (including benzothiophenes, dibenzothiophenes,and benzonaphthothiophenes) and the percentage of phenols/furans (including phenols, dibenzofurans, and benzonaphthofurans). The concentration of oxygenated compounds decreased only slightly during the 25-h period of reaction; however, there was a large
Table VI. Batch Hydroprocessing of Neutral Oils: Average Compositional Parameters Obtained reaction time, h 0 0.5 3 7.5 av molecular formula C14.6H14.3 C14.2H14.7 C15.1H15.4 C15.2H15.1 189.5 184.5 196.8 198.6 average molecular wt 1.0 H/C atomic ratio 1.0 1.0 1.0 0.082 H/C mass ratio 0.086 0.085 0.084 0.69 aromaticity 0.70 0.66 0.64 2.0 aromatic rings/molecule 2.0 2.0 1.9 10.1 aromatic ring carbons/molecule 9.9 10.0 9.7 19.7 70monoaromatics 32.5 23.5 25.0 57.6 % diaromatics 37.8 53.2 56.8 22.7 % triaromatics 29.7 23.3 18.3 9'0 alkyl carbon 30.6 33.8 30.1 36.2 alkyl substituents/molecule 1.9 1.6 2.1 2.0 no. carbons/alkyl substituent 2.3 2.6 2.5 2.6 nonbridge aromatic carbon/molecule 8.0 8.1 7.9 7.9 55.3 % nonbridge aromatic ring carbons 52.9 56.1 51.6 naphthenic rings/molecule 0.8 1.0 0.7 0.6 % naphthenic carbon 18.4 24.0 16.7 15.4
from 'H NMR Spectroscopy 15 25 C14.9H16.9
194.6 1.1
0.088 0.65 1.9 9.7 28.8 50.4 20.8 35.0 1.7 3.1 7.8 52.7 1.1
25.5
C14.8H16.1
192.7 1.0 0.087 0.62 1.8 9.1 35.3 51.3 13.4 38.2 2.3 2.5 7.6 51.3 1.o
22.7
Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 1773 reduction in the concentration of the organosulfur compounds, consistent with the results of the individual compound analyses shown above. Figure 7 is a pseudo-firstorder kinetics plot for the class of thiophenes and the class of furans as determined by the high-resolution mass spectrometry. The pseudo-first-order rate constants calculated for the thiophenes (essentially dibenzothiophene plus substituted dibenzothiophenes with some benzonaphthothiophenes) and the furans (essentially dibenzofuran plus substituted dibenzofurans with some benzonaphthofurans) are 2.3 X lo4 and 3.0 X lo-' L/(g of catalyst-s), respectively.
Discussion The plots of Figure 3 demonstrate that the individual hydrodesulfurization reactions are well represented by pseudo-first-order kinetics, in agreement with the results of the flow reactor experiments carried out with the diluted neutral oils (Katti et al., 1984);the pseudo-first-order rate constants measured in the batch reactor experiment are typically an order of magnitude less than those measured in the flow reactor experiments (Katti et al., 1984). This difference is explained by the differences in reactant concentrations in the two kinds of experiments in the flow reactor experiments, a 0.25 wt % solution of the neutral oils in cyclohexane was used as the feed, whereas in the batch experiment the undiluted neutral oils fraction was the reactant. The rates of the hydrodesulfurization reactions of dibenzothiophene and methyldibenzothiophenes are known to decrease with increasing concentration of the organosulfur reactants because of inhibition by these reactants (Houalla et al., 1980), whereas the rates increase roughly in proportion to the concentration of hydrogen [consistent with the observed Langmuir-Hinshelwood kinetics for hydrodesulfurization of dibenzothiophene (Broderick and Gates, 1981)]. The initial concentration of each organic reactant in the batch experiment was about 400 times the concentration in the feed to the flow reactor. The hydrogen concentration in the feed to the flow reactor experiment was about 0.14 mol/L; the exact hydrogen concentration in the batch reactor experiment could not be measured, but thermodynamics estimates (Katti, 1984) show that the fugacity of hydrogen in the feed to the flow reactor was 13 atm, whereas the fugacity of hydrogen in the liquid in the batch reactor was 37 atm, assuming equilibrium between the gas and liquid phases. The results (Table 11) show that the effects of methyl substituents on dibenzothiophene hydrodesulfurization are similar to those observed in the flow reactor experiments and in the literature generally (Houalla et al., 1980). The results presented in Table I1 show that substitution in the 4 position reduced the pseudo-first-order rate constant by a factor of 3.05 (compared to 3.2 in the flow reactor experiments), whereas substitution in another (undetermined) position increased it by a factor of 1.17 [compared to 1.8 in the flow reactor experiments for the same (undetermined) methyl-substituted dibenzothiophene]. The agreement is surprisingly good in view of the difference in the reactant concentrations; these data are suggested to have some general validity as a basis for predicting substituent effects in catalytic hydrodesulfurization. The total sulfur data (obtained by adding the concentrations of the three major organosulfur compounds determined by the Hall detector) presented in Figure 4 show higher than first-order (but not quite second-order) kinetics for total sulfur removal. Hydrodesulfurization of complex feeds such as petroleum residua (Gates et al., 1979), SRC-I1 recycle solvent, and H-coal distillate (Stein
et al., 1977) is often approximated as second-order in total sulfur, consistent with the parallel reaction of several organosulfur compounds, each by first-order kinetics; the total sulfur data determined from elemental analysis (Figure 1)compare well with data characterizing the hydrodesulfurization of complex feeds. The total sulfur data obtained from high-resolution mass spectrometry (Figure 7) indicate a pseudo-fmt-order dependence (data for times longer than 15 h were not obtained). The plots of Figure 5 [and similar plots (not shown) (Katti, 1984)] demonstrate that the individual hydrogenation reactions of many aromatic compounds are also well represented by pseudo-first-order kinetics for irreversible reactions. However, for the lighter (two-ring aromatic) compounds (e.g., dimethylnaphthalene, Figure 6), the equilibrium limitations are important. The pseudo-first-order rate constants for the hydrogenation reactions (Table 111) are typically an order of magnitude less than those for hydrodesulfurization (Table 11). This result confirms that the Ni-Mo/y-A120, catalyst is highly selective for the hydrodesulfurization reactions (Gates et al., 1979). The extent of hydrogenation is characterized by the H/C ratio. Figure 2 shows that the H / C weight ratio determined by elemental analysis increased steadily from 0.083 in the feed to 0.092 in the product at the longest time (25 h). NMR (Table VI) and mass spectrometry (Table VII) also provide H/C ratios confirming the increasing trend with reaction time. It is the trend that is significant rather than the absolute values determined by the latter two methods. The increases in the H/C ratio result not only from the hydrogenation reactions but also from hydrodesulfurization and other heteroatom removal reactions. For example, conversion of dibenzothiophene to biphenyl leads to an increase in H/C weight ratio from 0.0558 to 0.0692, whereas hydrogenation of phenanthrene to di-, tetra-, and octahydrophenanthrene leads to an increase in H/C weight ratio from 0.0595 to 0.0714,0.0833, and 0.1072, respectively. The pseudo-first-order rate constants for the hydrodesulfurization of thiophenes and hydrodeoxygenation of furans (as determined by high-resolution mass spectrometry), respectively, are 2.3 X lo4 and 3.0 X lo-' L/(g of catalyst-s). The ratio of these rate constants (7.8) is in good agreement with the results reported by Rollmann (1977) (who observed a ratio of 9 with a Co-Mo/y-A1203 catalyst at 344 "C and 49 atm) and with the results of Krishnamurthy et al. (1981) (who observed a ratio of 10 with a Ni-Mo/y-Al,03 catalyst at 365 OC and 106 atm). Li et al. (1984) reported kinetics of the hydrodeoxygenation of the acidic fraction of the SRC-I1 heavy distillate. The pseudo-first-order rate constant for the removal of total organooxygen as determined by infrared spectra of the weak-acid fraction was reported to be 1.56 X 10" L/(g of catalyst-s) at 350 "C and 120 atm, as determined in flow reactor experiments. The pseudo-firstorder rate constants for hydrodeoxygenation of individual phenolic compounds in the weak acid fraction are of the order of L/ (g of cata1yst.s). The pseudo-first-order rate constant for overall hydrodeoxygenation of the neutral oils under similar conditions is estimated from the batch reactor data (using the ratio of hydrodesulfurization rate constants for dibenzothiophene in flow and batch experiments as a correction factor) to be roughly 7 X lo4 L/(g of catalyst-s). Thus, nonpolar oxygen-containing compounds in the neutral oils (typified by furans) are roughly 20 times less reactive than the phenolic oxygen compounds in the acidic fractions.
1774 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 Table VII. Batch Hydroprocessing of Neutral Oils: Characterization of Feed and Products by High-Resolution Mass SDectrometrs reaction time, h av C no. av 2 no. H/C atomic ratio 0.0 0.517 3.00 7.50 15.0 25.0
15.89 16.07 15.8 15.77 15.65 15.55
-17.78 -17.53 -17.30 -16.74 -16.89 -16.22
0.881 0.909 0.905 0.938 0.921 0.957
There is little evidence of cracking during the hydroprocessing experiment, as would have been indicated by a reduction of the molecular weight of the neutral oils. The high-resolution mass spectrometry data show only a slight reduction in the average carbon number of the molecular species found in the neutral oils, from 16.0 in the feed to 15.6 in the 25-h product. The average molecular weight as calculated by NMR is higher in the 25-h sample than in the feed, presumably as a consequence of hydrogenation of the neutral oils. As another check on the possibility of cracking reactions occurring in the experiment, all compounds identified in the low-resolution mass spectrometry data were grouped by the numbers of rings, both aromatic and hydroaromatic, and plotted as a function of time (Katti, 1984). These data show little change with time; however, the concentration of four-ring compounds did decrease by a few percent. This change can be explained by naphthenic ring opening without loss of carbon atoms as gas or light molecules. Supporting this explanation is the fact that molecular weight, average carbon number of the molecules in the samples, and average carbon number of any of the classes of molecules identified by high-resolution mass spectrometry did not change significantly with reaction time. The NMR results also suggest the lack of significant removal of alkyl side chains by cracking; the decrease in the naphthenic-balky1 carbon ratio in the final 10 h of the conversion is due to the opening of naphthenic rings, as indicated by the increase in the overall alkyl carbon. Summary
The data presented here illustrate the determination of quantitative kinetics of hydrodesulfurization and hydrogenation of several of the individual compounds found in the highest concentration in the neutral oils fraction of a heavy coal liquid. The pseudo-first-order rate constants for hydrogenation (Table 111) and hydrodeoxygenation are typically an order of magnitude less than those for hydrodesulfurization (Table 11). The reactivities of representative classes of compounds in the neutral oils determined from group-type analyses (high- and low-resolution mass spectrometry and proton NMR) are in good agreement with the individual compound data. Together, these results represent one of the most complete profiles of the conversion of a heavy fossil fuel mixture. With readily available analytical techniques, the methods of combined group-type analysis and individual compound analysis can be applied to even more complex fossil fuels. Acknowledgment
We thank C. Koch, G. Hancock, and R. Minard of the Department of Chemistry of The Pennsylvania State University for help with the GC/MS analyses. This research was supported by the US.Department of Energy. Registry No. Ni, 7440-02-0; Mo, 7439-98-7; dibenzothiophene, 7372-88-5; methyldibenzothiophene, 30995-64-3; fluorene, 86-73-7;
phenanthrene, 85-01-8; methylphenanthrene, 31711-53-2; pyrene, 129-00-0.
Literature Cited Allen, D. T.; Petrakis, L.; Grandy, D. W.; Gavalas, G. R.; Gates, B. C. “Determination of Functional Groups of Coal-Derived Liquids by N.M.R. and Elemental Analysis”. Fuel 1984, 63, 803-809. Bhinde, M. V. “Quinoline Hydrodenitrogenation Kinetics and Reaction Inhibition”. Ph.D. Dissertation, University of Delaware, Newark, 1979. Broderick, D. H.; Gates, B. C. “Hydrogenolysis and Hydrogenation of Dibenzothiophene Catalyzed by Sulfided Co0-Mo03/~-A1,0,: The Reaction Kinetics”. A I C h E J. 1981, 27, 663-673. Brown, J. K.; Ladner, W. R. “A Study of the Hydrogen Distribution in Coal-like Materials by High-resolution Nuclear Magnetic Resonance Spectroscopy 11-A Comparison with Infra-red Measurement and the Conversion to Carbon Structure”. Fuel 1960, 39, 87-96.
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Received for review November 9, 1987 Accepted May 23, 1988
Transport Model with Radiative Heat Transfer for Rapid Cellulose Pyrolysis Lee J. Curtis and Dennis J. Miller* Department of Chemical Engineering, Michigan State University, East Lansing, Michigan 48824-1226
A mathematical model is presented which describes mass and energy transport during rapid pyrolysis of fibrous cellulose particles. Radiative heat transfer within porous cellulose is modeled by using the method of zones. The kinetic model for pyrolysis developed by Bradbury e t al. is extended to include secondary decomposition of condensible liquids (tars) formed. Solution of the governing equations shows that both mass- and heat-transfer resistances influence product composition from pyrolysis even for cellulose particles as small as 0.5 mm in diameter. Heating rate has little influence on product composition, but increasing the total pressure results in a decreased condensible product yield. Radiative heat transfer plays a minor role within the solid for the conditions simulated. The model is useful for identifying critical parameters and conditions in pyrolysis and for predicting trends in product yields.
I. Introduction Since biomass materials are abundant, inexpensive, and renewable resources, their conversion to synthetic fuels and chemical feedstocks appears attractive. For these end uses, rapid pyrolysis is a potential route since it requires less energy input than steam gasification, requires no additional reagents, and produces very little char. Pyrolysis does have one important drawback, however: it is difficult to selectively produce high yields of valuable products. Therefore, the focus of much present pyrolysis research is on understanding pyrolysis chemistry and physics, both via experimentation and modeling, so that desired product yields might be enhanced. The need for this work is clearly outlined in the Kona workshop on thermochemical biomass conversion (Antal, 1984). Major products from rapid pyrolysis of cellulose are gases and a condensible fraction containing anhydrosugars and organics. A number of reaction models have been developed from experimental data to describe product formation pathways (Shafizadeh, 1968; Madorsky, 1964; Mok and Antal, 1983a); kinetic constants have been determined for several reaction steps (Bradbury et al., 1979; Hajaligol et al., 1982; Broido, 1976). The influences of heat- and mass-transfer resistances have been experimentally investigated, but unfortunately most studies only give qualitative information. The primary effect of mass-transfer resistance appears to be a decrease in condensible tar yield, as secondary decomposition to gases becomes more important with increasing residence time in the reacting region. Mok and Antal (1983b) and Shafizadeh and Fu (1973) both report a decrease in tar yield as pressure is increased, and Hajaligol et al. (1982) report a similar decrease as sample thickness is increased. Others have shown similar trends (Scott and Piskorz, 1982; Shafizadeh et al., 1979). Heat-transfer resistances result both from endothermicity of the primary reactions and the low thermal conductivity of cellulose. The thermal resistance becomes *To whom all correspondence should be addressed.
even more pronounced as the high porosity char layer is formed during pyrolysis. Several models have been developed based on the assumption that heat transfer limits pyrolysis rate, and the models consider only heat-transfer resistances (Kung, 1972; Kansa et al., 1977). In extreme cases, a shrinking core mode of pyrolysis is assumed (Chan et al., 1985; Kanury, 1972). Models including both mass- and heat-transport effects have been developed primarily for predicting the rate of wood pyrolysis and combustion. Fan et al. (1978) have developed a general pyrolysis model which includes generation, reaction, and diffusion of the gas-phase components within the solid. However, convective flow and solid structure changes are not included in their model. Antal (1985) has set forth “general” transport equations for the pyrolysis of cellulose, and Kothari and Antal (1985) have presented numerical solutions to some simplified forms of the equations. In these model equations, however, two important phenomena were not explicitly included: the effects of secondary decomposition reactions on overall product yield, and the contribution of radiation to overall heat transfer within the cellulose particle. These phenomena may play an important role in determining overall pyrolysis behavior, especially at the high temperatures and heat fluxes encountered in flash pyrolysis (Kansa et al., 1977). Chan et al. (1985) included both secondary reactions and a diffusive radiation term as a correction to the thermal conductivity in their model for slow pyrolysis, but to date, no detailed treatment of radiation has been conducted for cellulose. This paper presents a model for flash pyrolysis which includes a semirigorous treatment of radiation within a “gray” cellulose solid and the secondary decomposition of pyrolysis tars. The objectives of developing the model are to investigate the importance of radiative energy transport within the porous solid and the effects of transport resistances on overall product yield. 11. Model Development Cellulose is modeled as a one-dimensional porous slab (half-thickness = L ) of randomly oriented fibers located
0888-5885/88/2627-17~5~0~.50/0 0 1988 American Chemical Society
