Ind . Eng . Chem. Res. 1989, 28, 967-976 967

Levenspiel, 0. Chemical Reaction Engineering, 2nd ed.; Wiley: New York, 1972.

Mc Ilvried, H. G.; Massoth, F. E. Effect of Particle Size Distribution on Gas-Solid Reaction Kinetics for Spherical Particles. Ind. Eng. Chem. Fundam. 1973, 12, 225-229.

Paul, I . C.; Curtin, D. Y. Reactions of Organic Crystals with Gases. Science 1975,187, 19-26.

Paul, I. C.; Curtin, D. Y. Gas-Solid Reactions and Polar Crystals. In Organic Solid State Chemistry; Desiraju, G. R., Ed.; Elsevier: Amsterdam, 1987; pp 331-370.

Perrin, R.; Lamartine, R.; Perrin, M.; Thozet, A. Solid State Chem-

istry of Phenols and Possible Industrial Applications. In Organic Solid State Chemistry; Desiraju, G. R., Ed.; Elsevier: Amsterdam,

Prout, E. G.; Tompkins, F. C. The Thermal Decomposition of Po- tassium Permanganate. Trans. Faraday SOC. 1944,40,488-499.

Tine, C. B. D. Simple Model for Non-catalytic Gas-Solid Reaction. Chem. Eng. Res. Deu. 1985, 63, 112-116.

1987; pp 271-329.

Received for review April 1, 1988 Revised manuscript received December 19, 1988

Accepted February 28, 1989

Modeling of Thermal Steam Cracking of an Atmospheric Gas Oil

Dominique Depeyre,*st Chantal Flicoteaux,' Ferechteh Arbabzadeh,t and Anastasia Zabaniotouf Laboratoire de GPnie et Informatique Chimiques, Ecole Centrale des Arts et Manufactures, F 92295 Chatenay-Malabry, France, and Chemical Engineering Department, University of Thessaloniki, 54006 Thessaloniki, Greece

Gas oil cracking experiments in the presence of steam were performed in a laboratory-scale tubular quartz or Inconel reactor. The effects of temperature, inlet steam t o gas oil ratio, and residence time on the major effluent products were investigated. The temperature, steam to gas oil weight ratio, and residence time were varied in the ranges 625-800 "C, 1-2 kg/kg, and 0.4-1.0 s, respectively. The best yield of ethylene, 27% by weight, was obtained in the quartz reactor at 770 "C, residence time of 0.6 s, and mass ratio of steam to gas oil equal to 1. Experiments combined with a simulation model allowed us t o predict the effluent products distribution as a function of temperature and residence time. Several kinetic models were attempted. The best one was a mechanistic radical and molecular model. Gas oil feedstock composition was simplified, taking into account one com- pound as representative of the principal hydrocarbon families. For this study, the model proposed consisted of 138 reactions, 18 species, and 24 radicals.

For a long time, on an industrial level, thermal cracking has been a very important production method. This is one of the most interesting techniques used to transform hy- drocarbon feedstocks into ethylene and propylene, which are intermediate products of the petrochemical industry. These steam crackers are more and more frequently fed with heavier distillated fractions such as atmospheric or vacuum gas oil, while naphtha is kept for petroleum manufacturing. The literature gives numerous studies on the pyrolysis of light, pure hydrocarbons and binary hy- drocarbon mixtures (Allara and Edelson, 1975; Froment et al., 1976; Sundaram and Froment, 1977a,b, 1978; Murata et al., 1974; Murata and Saito, 1975; Kunzru et al., 1973; Volkan and April, 1977). Only a few investigations on heavy petroleum cuts have been presented (Hirato et al., 1971; Kunzru and Kumar, 1985).

An experimental study on the effects of various param- eters on olefin production during gas oil cracking in a tubular reactor was performed by Hirato et al. (1971). They suggested a kinetic molecular model and assumed that the gas oil was a single component with a mean mo- lecular formula of C15,46H29,02.

The first step in the research on atmospheric gas oil cracking is to determine the operating conditions and yields of various products, particularly ethylene.

The aim of the present experimental study was to obtain data on the composition of effluent products of cracking and to investigate the effects of various parameters on the optimal production of ethylene.

* T o whom correspondence should be addressed.

t University of Thessaloniki. Ecole Centrale des Arts e t Manufactures.

0888-5885/89/2628-0967$01.50/0 8

Table I. Physicochemical Character is t ics of Atmospheric Gas Oil Feed

density ASTM distillation

IBP FBP

PONA anal. paraffins naphthenes aromatics C /H ratio

elemental anal. C H N 0 S

mean molec wt mean molec formula BMCI index

0.8596

384 o c 243 "C

39.0 wt 70 30.7 wt 90 22.5 wt 90 6.4

85.35 wt % 13.30 wt 9i 50.10 wt % 50.10 wt % 1.40 wt % 238.72 C17.19H32.43 34.5

The following parameters were considered: temperature from 625 to 800 "C; dilution by maintaining a constant gas oil flow rate and varying the steam flow rate; residence time by varying the gas oil and steam flow rates, while a steam to gas oil weight ratio of 1 was maintained.

Then, in order to predict product distribution as a function of temperature and residence time, an attempt was made to develop a kinetic model.

Experimental Section (a) Feed Characteristics. Atmospheric gas oil was

supplied by "TOTAL" (HarFleur, France). It was a dis- tilled fraction, 243 "C < bp < 384 "C, with a density of 0.8596. Physicochemical characteristics, composition, and

1989 Amer ican Chemica l Society

968 Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989

Table 111. Effluent Liauid Products Analysis Table 11. Chemical Composition of Atmospheric Gas Oil Feed

w t %

paraffins (n-paraffins + isoparaffins, Clz-Cl,) uncondensed naphthenes (1 saturated ring) condensed naphthenes (several saturated rings)

aromatics alkylbenzene, CnHzn+ indane + tetralin, C,H2,_8 indene, CnHzn..10 naphthalene, CnH2n-12 alkylnaphthalene, CnH2n-12 acenaphthene + diphenyl, CnH2n-14 acenaphthylene + fluorenes, CnHzn-16 phenanthrene + anthracene, CnH2n-18

sulfured products benzothiophene, C,Hz,_loS dibenzothiophene, C,H2,-,6S

mean no. of atoms of C in family saturated naphthenic aromatic

39.0 20.3 10.4 69.7 total

7.0 3.9 1.7 5.2 5.2 2.0 1.2 1.5 22.5 total

5.4 2.4 7.8 total

16.7 13.3 14.8

PRODUCTS

Figure 1. Inconel 600 reactor: circled 1, cracking zone; circled 2, gas oil preheating zone; circled three, steam preheating zone; 4, thermocouples.

mean molecular weight are presented in Tables I and 11. (b) Apparatus. The first reactor used was a quartz

tubular reactor already described by Depeyre et al. (1985b). The second reactor used was designed by Blouri et al. (1981, 1987) and is illustrated in Figure 1. It is a cracking coil Inconel 600 reactor, 4 m long with 4-mm internal diameter. I t consists of three parts: part 1, cracking zone; part 2, preheating gas oil zone; part 3, preheating steam zone. The coil is wound on a 18/8 stainless steel pipe, of 60/64-mm diameter. The reactor is heated internally by an electrical resistance coiled on a coralum H threaded tube, of 17/25-mm diameter. The electrical resistance is protected by a quartz tube, 30/33-mm diameter, 300 mm long. The reactor is insulated by a kerlane sheet. The inside temperatures of the three parts of the reactor are measured by means of seven thermocouples.

(c) Experimental Methods. Experiments were carried out for a range of temperatures from 650 to 800 "C. The flow rates of gas oil and steam used in the quartz reactor were respectively 33.7, 46.0, 54.0, 60.0 g h-' and 34.9, 46.6,

At 650 OC distillate 1 5200 "C 13.57 wt % 8.86% paraffins +

cycloparaffins; 4.71% aromatics + polyaromatics

naphthenes; 8.66% aromatics + polyaromatics

naphthenes; 31.88% aromatics + polyaromatics; 6.59% resins

At 750 "C: Most of the Liquid Products Consisted of Aromatics alkylbenzene 12.6 wt % indane + tetralin 8.0 wt 70 indene 8.2 wt % naphthalene 27.9 wt %

distillate 2 200-270 "C 22.25 wt % 13.59% paraffins +

residue 5270 O C 64.16 wt % 25.69% paraffins +

acenaphthylene 7.4 wt % acenaphthene 9.3 wt % phenanthrenes 9.9 wt % benzothiophenes 9.9 wt %

118.8 g h-I. Various residence times were obtained as described in the previous section. The characteristics of the cracking experiments are presented in Tables VI-VII.

(d) Analytical Instrumentation. Effluent gas product analysis was performed by gas chromatography as de- scribed by Depeyre et al. (1985b).

The analysis of two effluent liquid products cracked at 650 and 700 "C has been carried out by I.F.P. (Institut Franqais du PBtrole), Table 111. The various methods employed were Simulated distillation by gas chromatog- raphy, fractional distillation, and gas chromatography coupled with mass spectrometry. The results of the analysis have been reproducible with an accuracy higher than 5%.

From this analysis, the mean molecular weight of the liquid products has been determined. At 650 and 700 "C, the mean molecular weights were respectively 179 and 170. Due to the lack of appropriate analysis equipment, no analysis of the liquid products has been performed in our laboratory. In the results reported in Tables IV-VII, the carbon deposit yields have not been measured experi- mentally but have been calculated by difference (100% - gas % - liquids % 1.

dibenzothiophene 6.8 wt 70

Experimental Results and Discussion (a) Olefin Yields. The gaseous product consists of light

paraffins (CH,, C2H6, C3H8), olefins (CzH4, C3H6, C4H8, C5HI0), diolefin (C4H6), and hydrogen. The distributions of the products of steam cracking in the quartz reactor are given in Tables IV-VII. The optimal yields of C2H4 and C3H6, respectively 27% and 15% by weight, were obtained in a quartz reactor respectively a t 770 and 700 "C, for a steam to gas oil mass ratio equal to 1, using a residence time of 0.6 s.

(b) Temperature Influence. For all the inlet flow rates of gas oil and steam, the gaseous conversion increased with the temperature to the maximum obtained at about 750 "C. Gaseous conversion is the ratio of the total weight of gas produced per weight of gas oil injected. Olefin (CzH4, C3H6, C,H8, C5Hlo) and diolefin (C4H6) yields climbed to a maximum and decreased as the temperature increased. The yield of C3H6 increased when the tem- perature increased and reached a maximum (14%) at 700 "C. For temperatures higher than 700 "C, the yield of C3H6 declined as the conversion in gas declined. Acetylene first appeared near 770 "C, as shown in Table VI.

Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989 969

Table IV. Influence of Temperature and Inlet Flow Rate on Product Distribution of Atmospheric Gas Oil Cracked in a Quartz Reactor, for Mass Ratio Steam/Gas Oil = 1.03

mean temp of cracking zone, inlet gas oil flow rate, gh-' inlet steam flow rate, gh-' H,O/gas oil, wt/wt outlet gas flow rate, g-h-' outlet liquid flow rate, gh-' mass balance, wt 7 G C deposits, gh-' gas conversion, 7 G residence time, s yields. wt 70

"C 580 33.7 34.9 1.03 4.1 27.3 93.2 2.3 12.1 1.04

H2 0.06 CH4 1.4 C2H6 1.2 C2H4 3.7 C3Ha 0.0 C3H6 3.3 C4Ha 0.9 C4H6 0.8 C5H10 0.7 L (liq products + nonreacting gas oil) D (C deposits) 6.8

81.1

627 33.7 34.9 1.03 11.8 19.2 92.0 2.7 35.0 1.02

630 33.7 34.9 1.03 13.5 17.5 92.0 2.5 40.1 0.85

650 33.7 34.9 1.03 16.6 14.6 92.6 2.5 49.3 0.98

0.2 0.2 0.2 5.1 5.4 6.4 3.0 3.1 3.5 9.7 12.2 14.1 0.9 1.1 1.6 8.9 9.9 13.1 3.2 3.7 4.5 2.4 2.8 4.1 1.5 1.7 1.7 57.00 51.9 43.3 8.0 8.0 7.4

668 33.7 34.9 1.03 18.3 12.5 91.4 3.0 54.3 0.77

0.4 9.1 3.9 18.9 1.6 15.1 2.1 2.6 0.6 37.1 8.6

678 33.7 34.9 1.03 21.3 9.5 91.4 2.9 63.2 0.93

0.4 10.4 4.5 21.4 1.7 15.4 3.0 6.0 0.4 28.2 8.6

700 33.7 34.9 1.03 18.3 11.0 86.9 4.4 54.3 0.82

0.5 10.8 3.8 20.7 1.5 11.4 1.2 4.2 0.1 32.7 13.1

749 33.7 34.9 1.03 20.6 11.0 93.8 2.1 61.1 0.77

0.7 14.7 3.7 25.7 1.7 9.7 0.7 4.0 0.3 32.7 6.2

775 33.7 34.9 1.03 20.0 10.4 90.2 3.3 59.4 0.67

0.8 15.8 3.5 27.8 0.3 7.8 0.0 2.9 0.6 30.8 9.8

Table V. Influence of Temperature and Inlet Flow Rate on Product Distribution of Atmospheric Gas Oil Cracked in a Quartz Reactor, for Mass Ratio Steam/Gas Oil = 1.01

mean temp of cracking zone, "C inlet gas oil flow rate, gh-' inlet steam flow rate, gh-' H,O/gas oil, wt/wt outlet gas flow rate, geh-' outlet liquid flow rate, gh-' mass balance, wt 7~ C deposits, gh-' gas conversion, 70 residence time, s yields, wt 90

H2 CH4 C2H6

CSHlO L (liq products + nonreacting gas oil) D (C deposits)

576 46.0 46.6 1.01 7.2 34.5 90.7 4.3 15.7 0.82

0.1 2.3 1.5 4.0 0.4 4.1 1.7 0.8 0.8 75.0 9.3

623 46.0 46.6 1.01 13.7 29.0 92.8 3.3 29.8 0.68

0.1 3.9 2.9 8.3 0.8 7.8 2.7 1.8 1.4 63.0 7.2

647 670 46.0 46.0 46.6 46.6 1.01 1.01 17.5 22.7 23.3 20.5 88.7 93.9 5.2 2.8 38.0 49.3 0.62 0.57

0.2 0.3 4.8 5.6 2.5 3.0 11.8 15.7 0.9 1.0 10.5 13.0 3.9 4.4 3.2 4.1 0.2 2.2 50.7 44.6 11.3 6.1

702 46.0 46.6 1.01 26.7 18.7 98.7 0.6 58.0 0.55

0.5 9.0 4.0 18.5 1.3 15.2 2.5 7.0 0.0 40.6 1.3

732 750 770 799 46.0 46.0 46.0 46.0 46.6 46.6 46.6 46.6 1.01 1.01 1.01 1.01 27.9 28.0 26.9 23.9 15.4 16.1 13.1 14.7 94.1 95.9 87.0 83.9 2.7 1.9 6.0 7.4 60.7 60.8 58.4 52.0 0.51 0.51 0.51 0.47

0.7 11.4 3.6 23.5 1.5 13.5 1.2 5.3 0.0 33.4 5.9

0.8 13.8 3.8 26.2 0.0 11.3 0.5 4.2 0.1 35.0 4.1

1.0 1.3 13.3 15.1 3.6 2.1 24.6 27.8 0.9 0.1 9.9 3.9 0.5 0.1 4.6 1.6 0.0 0.0 28.6 31.9 13.0 16.1

Table VI. Influence of Temperature and Inlet Flow Rate on Product Distribution of Atmospheric Gas Oil Cracked in a Quartz Reactor, for Mass Ratio Steam/Gas Oil = 1.33

mean temp of cracking zone, "C inlet gas oil flow rate, gh-' inlet steam flow rate, g.h-' H,O/gas oil, wt/wt outlet gas flow rate, gh-' outlet liquid flow rate, gab-' mass balance, wt % C deposits, g k ' gas conversion, 70 residence time, s yields, wt %

H2 CH4

L-(l<q products + nonreacting gas oil) D (C deposits)

638 60.0 80.0 1.33 10.0 45.0 91.7 5.0 16.7 0.61

0.2 4.2 1.0 4.8 0.3 3.4 1.3 0.7 0.7 0.0 75.0 8.3

686 60.0 80.0 1.33 31.3 23.5 91.3 5.2 52.1 0.66

0.3 6.6 2.5 18.9 0.8 12.4 4.4 4.5 1.7 0.0 39.2 8.7

719 60.0 80.0 1.33 34.1 23.0 95.1 2.9 56.8 0.61

0.5 8.7 2.9 21.0 0.8 13.2 3.6 5.4 0.8 0.0 38.3 4.8

733 60.0 80.0 1.33 35.9 18.0 89.8 6.1 59.8 0.62

0.6 9.0 2.9 21.2 1.3 14.8 3.7 5.3 1.0 0.0 30.0 10.2

749 60.0 80.0 1.33 33.8 15.3 81.8 10.9 56.3 0.61

0.9 13.1 2.6 22.0 0.9 10.6 1.2 4.7 0.3 0.0 25.5 18.1

756 60.0 80.0 1.33 34.6 15.5 83.5 9.9 57.7 0.50

0.9 11.6 3.5 24.3 0.9 9.4 1.6 5.0 0.5 0.0 25.8 16.5

794 60.0 80.0 1.33 34.4 14.4 81.3 11.2 57.3 0.66

1.0 14.0 1.7 26.5 0.1 8.1 1.2 3.9 0.1 0.8 24.0 18.7

826 60.0 80.0 1.33 32.7 14.0 77.8 13.3 54.5 0.65

1.3 18.0 1.5 26.5 0.0 5.0 0.0 0.2 0.4 1.7 22.1 23.3

856 60.0 80:O 1.33 30.1 12.0 70.2 17.9 50.2 0.60

1.9 19.6 0.1 23.0 0.0 1.6 0.0 1.2 0.2 2.0 20.0 29.8

970 Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989

reactor mean temp of cracking zone, "C inlet gas oil flow rate, g.h-' inlet steam flow rate, g.h" H,O/gas oil, w t /wt outlet gas flow rate, geh-' outlet liquid flow rate, g.h-' mass balance, wt ?& C deposits, g-h-' gas conversion, 70 residence time, s yields, wt %

H* CH, C,H, C,H, C'3HU C,H, C',HU C,H, GHi, I, (liq products + nonreacting gas oil) D i C deposits)

quartz 730 54.0 117.0 2.0 31.4 17.6 90.7 5.0 58.1 0.44

0.6 11.3 4.4 20.6 0.5 12.9 2.0 4.9 0.9 32.6 9.3

Table VII. Influence of Temperature and Inlet Flow Rate on Product Distribution of Atmospheric Gas Oil Cracked in Quartz and Inconel Reactors, for Mass Ratio Steam/Gas Oil = 2

quartz Inconel Inconel Inconel ;52 58.9 118.6 2.0 33.3 13.4 79.3 12.2 56.5 0.40

0.7 12.7 2.9 22.7 0.5 11.1 1.2 4.5 0.3 22.8 20.7

730 46.5 91.0 2.0 30.1 7.0 79.8 9.4 64.7 0.34

1.1 15.6 2.2 27.8 0.0 9.8 1.4 6.0 0.8 15.1 20.2

760 43.0 80.0 1.9 28.4 6.8 81.9 7.8 66.0 0.37

1.6 14.2 2.3 30.8 0.0 10.2 0.4 5.5 1.1 15.9 18.1

790 50.0 117.0 2.3 26.8 6.0 65.6 17.2 53.6 0.25

2.7 14.7 1.1 26.9 0.0 5.4 0.0 0.0 2.7 12.0 34.4

The production of liquids declined as the temperature rose, as shown in Tables IV-VII. However, the production of carbon deposits increased with temperature.

(c) Steam Dilution Influence. Steam dilution re- duced the partial pressure of hydrocarbons and the resi- dence time as given in Tables VI and VI1 and slowed the production of aromatics, which were the precursors of carbon deposits, as shown for the n-nonane and n-hexa- decane pvrolyses by Depeyre et al. (1985a,b). When the dilution increased, CH4 and H2 yields slightly decreased, as given i n Table VI, column 6, and Table VII, column 3. The carbon deposits produced from the cracking of the gas oil were not measured experimentally.

(d) Residence Time Influence. Several residence times were obtained by varying the feed gas oil flow rates from 30 to 60 g h-' and inlet flow rates within the range 35-80 g h-l, while the steam to gas oil weight ratio was maintained within the range 1-1.33. The calculation of the residence time has already been described by Depeyre et al. (1985b). First, the temperature distribution in the tubular reactor was measured, and the equivalent reactor length was calculated. The mean pyrolysis temperature was calculated using the temperature indications of 14 points of the cracking zone. The maximum point of the temperature distribution was higher (1.5-3.7%) than the setting on the temperature regulator. The above-observed difference increased with the increase of the pyrolysis temperature.

For atmospheric gas oil, the molar outlet flow rate of the liquid hydrocarbons was estimated by taking into account the weight of the outlet flow rate of the liquids measured experimentally and the mean molecular weight of the liquid hydrocarbons calculated from I.F.P. analysis.

For a dilution ratio equal to 1 and temperatures lower than 750 "C, C2H, yields decreased with residence time, as shown in Figure 2.

(e) Comparison of the Results Obtained in an In- cone1 600 Reactor and in a Quartz Reactor. The dif- ference in the geometry and thermal history of the two reactors made the same operating conditions difficult to be achieved. The equivalent length of the quartz reactor is about 35-40 cm, while that of the Inconel one is 400 cm. This difference in the equivalent length affects the contact time and the conversion to gas and olefins. It has been observed that the longer the reactor, the larger the con- cersior. t r . gas.

20 i

0.4 0.6 0.8 1 0

RESIDENCE TIME ( 8 . )

Figure 2. Influence of temperature and residence time on product distribution of atmospheric gas oil cracked in a quartz reactor for a mass ratio H20/gas oil = 1. T = 650 O C (- - -), 770 "C (-).

The optimal C2H4 yield of 31 % by weight was obtained in the Inconel 600 reactor a t 760 "C for a residence time equal to 0.37 s and for a mass ratio of steam to gas oil equal to 1.86. For the same operating conditions, the residence time in the Inconel 600 reactor was 15% lower than that in the quartz reactor. Gaseous olefin conversion was 10% greater in the Inconel 600 than in the quartz reactor. Production of HP, CH,, and carbon deposits in the Inconel 600 reactor above 730 "C was greater than in the quartz reactor, as presented in Table VII. This difference in the yields of H2 and CH4 between the two reactors is caused by the secondary catalytic reactions on the metal walls of the Inconel 600 reactor, as reported by Blouri et al. (1981) and by Depeyre et al. (1985b).

For the above reason, the proposed model has to be modified in order to simulate the pyrolysis in the Inconel reactor, by adding the appropriate catalytic reactions.

(f) Comparison between Experimental Product Yields and Yields Reported in the Literature. It is difficult to compare our results with those of the literature because operating conditions and gas oil feed composition are different. The gas oil used by Hirato et al. (1971) is

Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989 971

Table VIII. Comparison between Experimental Product Yields and Yields Reported in the Literature for Other Gas Oils

Hirato et Dente and eas oil oresent work al.. 1971 Ranzi. 1983

sp gravity pressure, a tm temp, "C contact time, s steam dilution yields, wt 70

H2 C H 4

C2H2

C 3 H 4

C2H4

C2H6

C3H6

C3H6

C4H6

C4H8 CSHIO c6+

0.8596 1

794 0.66 1.33

1.0 14.0 0.8 26.5 1.7

8.1 0.1 3.9 1.2 0.1 42.6

0.838 1 800 0.4 0.7

10.0 13.0

25.0

8.0

3.0 1.0

46.5

0.83 5 970-1220 0.15

11.84 11.84 4.56 34.09 1.86 1.86 7.15 0.16 3.67 0.65 0.96 33.20

distillated from 185 to 320 "C, and our atmospheric gas oil is distillated from 243 to 384 "C. The product distri- bution of the gas oil cracked is greatly affected by the initial content of paraffins, isoparaffins, and naphthenics in the feed, but the PONA composition of the raw material gas oil used by Hirato et al. (1971) was not given.

Our experiments were performed for a dilution weight ratio of steam to hydrocarbons equal to or superior to 1; those of Hirato et al. (1971) were conducted for a dilution weight ratio equal to 0.7. However, the ratio C/H is sim- ilar.

The values deduced from the curves of Hirato et al. (1971) a t 800 "C show that our experimental results given in the Tables V and VI agree reasonably with the yields of the main gaseous products reported by the above au- thors.

A comparison between experimental product yields and yields reported in the literature for other gas oils is shown in Table VIII; the experimental product yields of the py- rolysis of the gas oil used for this work are compared with the experimental results reported by Hirato et al. (1971) and Dente and Ranzi (1983).

Kinetic Model of Thermal Cracking of an Atmospheric Gas Oil

Since atmospheric gas oil is not a single component substance but contains a large number of hydrocarbons (such as paraffins, isoparaffins, naphthenes, aromatics, and polyaromatics, whose number of carbon atoms ranges from 12 to 24), the development of a kinetic model is a very complex matter. Some models of gas oil were reported and based on molecular reactions. Gas oil was represented as a single pseudocomponent with a mean molecular formula (Hirato et al., 1971; Gossens et al., 1978).

The model proposed in the present study is a mecha- nistic model based on radical and molecular reactions and makes use of a simplified composition of gas oil. Each main family of hydrocarbons cited above was represented by one or more components. Some simulations were made with two components, n-hexadecane and n-dodecane, for the paraffinic family (Table IX), but the simulated results indicated a higher conversion, which .was opposite to trend of the experimental results. Others were based on one component only, representative of each family. Amongst a great number of radical reactions, the most significant was chosen. In fact, integration of the differential equa- tions associated with numerous radical reactions created

Table IX. Characteristics of Different Models Developed" model 1A 2A 3A 3 B 4A 5A 6 A no. of species 13 13 13 14 18 19 19 no. of radicals 21 23 24 24 24 24 24 no. of reactions 97 110 124 124 129 125 138 different types of 1 1 1 1 1 1 1

reactions used 2 2 2 2 2 2 2 3 3 3 3 3 3 3

4 4 4 4 4 4 5 5 5 5 5 5 5

6 6 7 7 8 8

A = one component representing the paraffinic family; B = two components representing the paraffinic family.

more difficulties and lengthened the time of calculation. The choice of the appropriate reactions was based on the product distribution obtained with the experimental data. The effluent olefins produced during cracking must be taken into account. In the development of the kinetic model to represent the atmospheric gas oil, the following simplified compositions were used:

paraffin n- hexadecane 26 mol % isoparaffin isohexadecane 7 mol % naphthenes C13H26 36 mol % aromatics C1SH34 31 mol %

To develop the set of reactions involved in the thermal cracking, a general scheme proposed by Gavalas (1966) and Woinsky (1968) was taken into account. In order to reduce the number of reactions involved in the model, the reac- tions that did not play a significant part in the system and all the olefins with carbon number greater than five and paraffins larger than propane were considered negligible.

The model presented below consists of the following reactions: 1. chain-initiation reaction

P - R1* + R2'

2. chain-propagation reactions

(a)

(b)

R1' - Rz' + 0

Pi + R1' - R2' + P2 3. chain-termination reaction

R1' + Rz' - P

4. secondary reactions

0 + R1' - Rq' + P

0 + R,' - Rz'

Rq' - 0 + R'

Rq' + Rq' - DO

Rq' - DO + R'

5. isomerization reaction

l-C6H11' - 2-C6H,,*

6. Diels-Alder molecular reaction C4H6 + 0 - aromatic + Hz

7 . molecular reaction that induces carbon deposits aromatics - carbon deposits + Hz

8. other molecular reactions, for example C4H8 - C4H6 + H,

972 Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989

Table X. Reaction Scheme for Gas Oil Thermal Steam Cracking

1 2 :I 4

6

8 9

10 I 1 12

7

I

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 3 3 3 4 3 5 36 3; 18 39 40 41 42 43 44

45 46 4; 48 49 ,5 0 5 1 5 2 5 .I 54 5 5 56 57 58 5 9 60 61 62 63 64 65 66 67 68 69 70 7 1

72 7,7

parameters from lit. parameters adopted no. reaction A' E" source A' E" ___-

1. Paraffin - Radical + Radical

C6H13' - C2H4 + C4Hg' CeH,,' - CTH, + l-CqH7' c,H;~. - C ~ H , + ~ z H 5 * CsH13' + C,H,o + CHJ' l-C6H11' -* 2-CsH11'

1-C6Hll* - CZH, + CIH;' C5Hll' + C2H4 + l-C,H,' C5H11' - C3H6 + C2H5'

2-C6Hll' - 1-C6Hll'

C5Hll* - C4H8 + CH3' CsHll* - C5Hlo + H' C4Hg' - C2H4 + C2H5'

C4H; - C4H8 + H'

1-C3H9' - C2H4 + CH3' 1-C3H7' - C3H6 + H' C2H5' - CZH4 + H' C,H-' - C4H6 + H'

CdHg' + C3H6 + CH,'

2-C3H9' - C3H6 + H'

1.44 x 1014 60 1.44 x 1014 60 1.44 x 1014 60 1.44 x 1014 60 1.44 x 1014 60 1.44 x 1014 60 1.44 x 1014 60 1.44 x 1014 60 1.44 X l O I 4 80 1.44 x 1014 60 1.44 X 10" 56 2.0 x 1014 61

2. Radical - Olefin + Radical 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013

1014 2.5 x 1013 1.6 x 1013 0.5 x 1013

2.5 x 1013 1.6 x 1013 0.5 x 1014

1014 2.5 x 1013 1.6 x 1013

1014

1014

3.2 x 1013 5.0 x 1013

2.5 x 1013 2.0 x 1013 2.0 x 1013 4.0 x 1013 2.0 x 1013 3.2 x 1013 1.2 x 1014

5 x 10'2

10'4

0.5 X 10"

3.2 X 10'' 5 x 10'0

4.0 X 10" 5.0 X 10"

1.6 X 10"

25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 29 33 28.8 28.3 29 32.5 29 28 29 32.5 28.8 28.3 29.1 32.5 11.7 16 30 28.7 29.1 31.5 36.6 28 31.9 39.8 38.7 32.6 38.4 40 49

3. Paraffin, + Radical, - Paraffinz + Radical? C16H34 + H' - C16H33' + H2 10" 10 f C,,Ho, + CH,' -+ C,cHn,' + CH. 4 0 x in11 8.3 P

0.5 x 1013 0.5 x 1013 0.5 x 1013 0.5 x 1013 0.5 x 1013 0.5 x 1013 0.5 x 1013 0.5 x 1013 0.5 x 1013 0.5 x 1013 1.5 x 1013 1.4 x 1013

1013 0.6 x 1013 0.6 x 1013 0.6 x 1013

1013 0.6 x 1013 0.6 x 1013 0.6 x 1013

1013 0.6 x 1013 0.6 x 1013 0.6 x 1013 1.0 x 1013 0.6 x 1013 0.6 x 1013 0.6 x 1013 1.0 x 1013 0.6 x 1013 0.6 x 1013 0.6 x 1013 1.0 x 1013 0.6 x 1013 0.6 x 1013 0.6 x 1013 1.0 x 1013 0.6 x 1013 0.6 x 1013 0.6 x 1013 2.5 x 1013 1.6 x 1013 0.5 x 1014 1.0 x 1013 2.5 x 1013 1.6 x 1013 0.5 x 1014 1.0 x 1013 2.95 x 1013 1.6 x 1013 0.5 x 1014 1.0 x 1013 2.5 x 1013 1.6 x 1013 0.5 x 1014 8.0 x 1013 0.22 x 1013 1.0 x 1014 1.0 x 1014

3.2 x 1013 5.0 x 1013

2.0 x 1013 2.0 x 1013 4.0 x 1013 2.0 x 1013 3.2 x 109 1.2 x 1014

4.0 X 10l2 5.0 X 10"

1.6 X 1OI2 2.5 X 10l2

0.1 x 10" 4.0 X 10"

60 60 60 60 60 60 60 60 80 60 56 61

25 26 26 30 25 26 26 30 25 26 26 30 25 26 26 30 25 26 26 30 25 26 26 90

25 26 26 30 25 25 29 33 29 28 29 28 29 28 29 28 29 28.3 29 33 0

30 30 29 29 31.5 36.6 28 32 40 39 33 38 40 49

10 8.3

Ind. Eng. Chem. Res., Vol. 28, NO. 7, 1989 973

Table X (Continued)

- no. reaction

parameters from lit. parameters adopted A' E" source A' E m

76 l-Ci6H34 + H' - 1-C1&33' + H2 10" 6 k 0.1 x 10" 6 77 l-Cl6H34 + CH3' + 1-Ci6H33' + CH4 4.0 X 10" 4.3 k 4.0 X 10" 4.3 78 1-C16H34 + C2H5' - 1-C16H33' + C2H6 4.0 X 10" 3 k 4.0 X 10" 3 79 1-C16H34 + l-C3H7' - 1.C16H33' + C3Hg 0.5 X 10" 4 k 0.5 X 10" 4 80 C3Hg + H' - l-C,H,* + H2 10" 9.7 e 0.7 X 10' 8.4 81 C3H8 + CH,' - 1-C&7' + CH4 3.4 x 10'0 11.5 e 0.49 x 104 8.2 81 C3H6 + C2H5' - l-C3H,' + C2H6 1.2 x 109 12.6 e 0.7 X lo2 10 83 84 85

C2H6 + H' - CZH,' + H2 C2H6 + CH,' - C2H5' + CH4 C2H6 + 1-C3Hi' -+ C2Hb' + C3H8

10" 9.7 e 0.54 x 105 9.1 3.8 X 10" 16.5 e 0.11 x 109 12 0.3 X 10' 12 a 0.3 X 10' 1 2

86 CH4 + H' - CH,' + H, 0.66 x 105 12 1 0.66 x 107 12 87 CHI + C2H5' - CH3' + C2H6 0.50 x 109 18 i 0.50 x 107 18 88 CH4 + l-C,H,' - CH3' + C3Hg 0.25 x 109 18 i 0.75 x 107 19 89 H2 + CH3' - H' + CH4 0.14 x 105 12 i 0.14 x 1015 1 2

91 H2 + l-C3H,' + H' + C3H8 0.30 x 105 0 i 0.30 x 1015 12 90 H2 + C2H5' - H' + C2H6 0.80 x 109 14 i 0.80 x 1015 14

92 93 94 95 96 97 98 99

100

101 102 103 104 105 106 107 108 109 110 111 112

113 114 115 116 117 118 119 120 121 122 123 124

125 126 127 128 129

130 131 132 133 134 135 136 137 138

4. Radical + Radical

4.0 X 1O'O

1.3 X 1O'O

10'0

3.2 x 109

1 0 2 4 10'0 2.0 x 10'0 1.0 x 10'0 1.3 X 10'O

- Paraffin 0 0 0 0 0 0 0 0 0

0.50 x 1014 0.50 x 1014 0.32 x 1013 1.30 x 1013 0.10 x 1024

1.00 x 1013 0.13 x 109

1.0 x 10" 2.00 x 10"

5. Secondary Reactions I: Olefin + Radical - Paraffin + Olefinic Radical; Olefinic Radical + Paraffin - Olefin + Radical CiHa + H' - CqH,' + H? 2.5 x 109 1.1

2.0 x 109 12.2 1.0 x 108 9.2 5.0 x 109 16 5.00 x 1010 3.9 1.0 x 108 7.3 1.0 x 1013 8.3 5.0 x 109 16 5.0 x 109 4.0

109 8.0 5.0 x 108 8.0 5.0 x 10' 16

6. Secondary Reactions I1 8.0 x 108 4.0

10'0 13.0 3.0 x 109 19.0

1010 1.5 2.0 x 108 7.9 1.5 x 10' 7.6 0.12 x 1012 49 5.0 x 107 7.0

1010 2.9 0.32 x 109 7.4 6.3 x 109 1.2

108 7.2

7. Molecular Reactions I 10'2 50 8.385 X 10' 34.56 9.74 x 108 35.64 6.4 x 1014 57.97 1.51 x 109 29.76

8. Molecular Reactions I1 2.0 x 1015 50.77 2.0 x 1015 50.77 2.0 x 1015 50.77 2.0 x 1015 50.77 3.75 x 1012 65 1.0 x 1014 60 4.69 X 1O'O 50 5.89 x 1010 51 7.386 X 10l2 64

e e e f e e a f f f f f

e e e e e e

f e e f e e

a

h h h h

h h h h h h h h h

2.5 x 109

5.0 x 109

1.0 x 1013 1.0 x 1013 5.0 x 104 5.0 x 109 1.0 x 1013 5.0 x 1013 5.0 x 109

0.8 x 109 0.1 x 10" 3.0 x 109 0.1 x 10'0 0.2 x 109 0.15 X lo8 0.12 x 10'2 5.0 x 107 0.10 x 10'0 0.32 x 109 0.47 X 10' 0.10 x 10'0

0.5 X 10l2 0.55 X lo8

2.0 x 10" 1.0 x 10"

1.0 x 10"

0.97 x 107 0.20 x 1014 0.15 x 107

0.85 X 10lz 0.85 X lo'* 0.85 X 10l2 0.85 X 10l2 0.37 X 10" 0.7 x 1013 0.5 x 1013 0.25 x 1014 0.74 x 1013

1.1 12.2 14.5 16 3.9 7.3 8.3

4.0 8.0 8.0

16

16

4.0 13.0 19.0 1.5 7.9 7.6

7.0 1.5 7.4 7.5 7.2

49

50 35 36 58 28

51 51 51 51 65 60 50 51 64

"Adjusted by the authors. bFabuss et al., 1964. 'Ranzi et al., 1983. dAllara and Shaw, 1980. eSundaram and Froment, 1978. 'Vermeulen, 1980. BGavalas, 1966. hKunzru and Kumar, 1985. 'MarBchal, 1977. jFabuss adjusted. kFabuss, Gavalas adjusted. 'In s-l or L mol-' s-'. mIn kcal/mol.

974 Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989

Table XI. Comparison of Simulated and Experimental Product Distributions of an Atmospheric Gas Oil Cracked at 650 OC in a Quartz Reactor

mol %

smcies

C3H6

C4H6

C4H*

C&,O n-C16H34

i-c16H34 naphthenes aromatics C deposit benzene toluene ethylbenzene styrene total L + D

650 "C, t , = 0.5 s,

gas oil = 60

steam = 60

ER SR 7.6 5.5

20.9 11.3 5.2 0.5

26.9 24.4 1.1 0.2

14.8 15.1 3.7 10.4 3.8 8.9 1.2 2.5

5.1 1.0 7.7 6.6 0.07 0.4 0.04 0.2 0.2

14.9 21.3

gsh-',

g*h-'

650 OC, t , = 0.62 s,

gas oil = 46

steam =

ER SR 6.1 7.0

18.2 11.8 4.9 0.7

25.5 25.0 1.2 0.2

15.2 15.7 4.2 9.9 3.6 8.9 0.2 2.5

4.2 0.7 6.5 5.5 0.09 0.6 0.05 0.3 0.3

21.0 18.2

g*h-'.

46.6 g*h-l

650 O C ,

t , = 0.98 s, gas oil =

steam = 34.9 ph-'

33.7 gab-',

ER SR 5.2 12.5

20.7 12.7 6.1 1.3

26.1 25.0 1.9 0.4

16.1 17.0 4.1 7.7 3.9 7.7 1.2 2.5

2.3 0.3 3.9 3.4 0.3 1.7 0.1 0.7 0.6

14.7 13.3

The model developed contains 138 reactions, 18 mo- lecular species, and 24 radical species, Table X. Steady-state conditions were assumed to determine the concentration of the radicals.

The cracking of gas oil was always followed by formation of carbon deposits which are a complex of carbon and hydrogen and may be produced either from the compo- nents of the feed or from the products of cracking. Blouri and Giraud (1977) suggested that the carbon deposits were the product of the olefin polymerization for temperatures lower than 750 "C and are the product of the dehydroge- nation of aromatics for temperatures higher than 750 "C.

Initially the kinetic parameters were provided by the literature (Kunzru et al., 1972; Sundaram and Froment 1977a,b, 1979; MarBchal, 1977; Allara and Shaw, 1980; Vermeulen, 1980; Ranzi et al., 1983; Kunzru and Kumar, 1985).

Then the kinetic parameters were adjusted so that the predicted product distribution matched the experimental data. A mathematical simulation program written by MarBchal (1977) was used for the prediction of the con- centration of the molecular and radical species as a function of temperature and residence time. To obtain the concentration of the species, two mathematical systems were used. The numerical methods employed were Raphson-Newton's method to solve the nonlinear alge- braical system, Gauss' method to resolve the linear system, and Euler's method to solve the differential system.

The simulated concentrations of benzene, toluene, ethylbenzene, and styrene were calculated from reactions 126-129 and the simulated concentrations of the carbon deposits were estimated from reactions 130-133, Table X.

Comparison of Simulated a n d Experimental Data The kinetic model used has enabled us to determine the

concentration of the cracking products within a range of temperatures of 650-750 "C. Experimental and simulated data are presented in Tables XI-XIII. The simulated results concerning C2H4 and C3H6 agree well with the ex-

Table XII. Comparison of Simulated and Experimental Product Distributions of an Atmospheric Gas Oil Cracked at 700 "C in a Quartz Reactor

mol 70 700 "C, 700 "C, 700 "C,

t , = 0.42 s, t , = 0.55 s, gas oil = 60 gas oil = gas oil =

steam = 60 steam = steam =

t , = 0.82 s.

g*h-', 46.0 g-h-', 33.7 gh-',

gh-' 46.6 g*h-' 34.9 gh-' species ER SR ER SR ER SR

n-C16H34

i-C16H34 naphthenes aromatics C deposit benzene toluene ethylbenzene styrene total L + D

11.9 14.0 10.4 16.9 10.1 19.3 28.0 14.8 23.2 15.4 27.5 17.0 4.6 2.6 5.4 3.6 5.1 4.7

30.1 25.8 27.4 24.5 30.0 23.0 1.1 0.4 1.2 0.5 1.4 0.6 6.8 19.5 14.9 20.4 11.0 21.0 2.5 6.8 1.8 5.1 0.8 3.7 4.2 4.5 5.4 2.9 3.2 1.4 0.3 2.4 0.0 2.1 0.04 1.6

1.0 0.5 0.2 0.08 0.03 0.0 2.0 1.3 0.7 1.9 1.2 0. 7 0.6 1.2 2.7 2.0 2.5 1.9 0.1 0.2 0.2 1.2 1.3 2.0 0.3 0.3 0.3

10.5 9.2 10.2 8.5 10.9 7.7

Table XIII. Comparison of Simulated and Experimental Product Distributions of an Atmospheric Gas Oil Cracked at 750 "C in a Quartz Reactor

mol 70 750 "C, 750 "C,

t , = 0.6 s, gas oil = 60

steam = 79

t , = 0.77 s , gas oil = 33.7

steam = 34.9

species ER SR ER SR

g*h-', g-h-',

gh-' g9h-l

H2 CH4

C3H*

C4H8

C5HIO

C2H6 C2H4

C3H6

C4H6

n-C16H34 i-C16H34 naphthenes aromatics C deposit benzene toluene ethylbenzene styrene total L + D

16.2 16.4 12.1 15.6 29.4 27.3 31.7 30.0

3.1 2.2 4.1 1.4 28.3 32.9 31.8 35.7 0.7 0.6 1.4 0.6 9.1 9.5 7.9 6.0 0.8 3.8 0.4 3.6 3.1 0.1 2.6 0.05 0.1 0.6 0.1 0.4

0.0 0.0 0.0 0.0 0.05 0.02 0.09 0.04 5.2 5.6 0.3 0.2 0.07 0.04 0.9 0.8 0.009 0.004

9.2 6.6 7.9 6.7

perimental data within a large range of temperatures. The optimum lies in a compromise solution between satisfac- tory concentrations for light paraffins, H, and CH,, and olefins, C2H4 and C3H6, and the deviation between simu- lated concentration and experimental data relative to C4H8, C5HI0, and C,H,. The global simulated concentrations of aromatics, nonreacting gas oil, and carbon deposits agree well with the experimental global concentrations of above components. A t 750 "C, the simulated and experimental global concentrations of the total (C4H8 + C4H6) are in agreement.

Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989 975

DO = diolefin PONA = paraffins + isoparaffins, olefins, naphthenics, and

aromatics

35

30

25

20

15

10

5

0 0 5 10 15 20 25 30 35

EXPERIMENTAL Figure 3. Scatter diagram showing the gaseous products distribu- tion for the cracking of gas oil. T = 650 OC (0), 700 OC (01,750 "C (0).

In Figure 3, a typical scatter diagram for products dis- tribution for the gas oil cracking is reported.

Conclusions

The aim of this study was to obtain experimental data on steam cracking of gas oil and to develop a kinetic model which allowed us to predict the concentration of products as a function of temperature and residence time. Exper- imental results indicated that the best yield of C2H4, 27 % by weight, was obtained in a quartz reactor at 770 OC, for a mass ratio of steam to gas oil equal to 1 and residence time of 0.6 s.

Simulation of experimental results indicated that it is possible to represent the initial gas oil composition by one model compound which is representative of the principal hydrocarbon families. In order to predict the distribution of products between 650 and 750 "C, it is necessary to use a kinetic model based not only on radical reactions for light paraffin models but also on molecular secondary reactions.

The molecular reactions are very important at temper- atures higher than 750 "C. Better results for the distri- bution of the products were obtained by using the mo- lecular Diels-Alder reactions, followed by dehydrogenation, producing the aromatics and precursors of the carbon deposits.

The use of a more complex formula for the carbon de- posits, such as C96H24 or (C4H), in reactions 130-133, in- stead of aromatics - C + Hz, used by Azcarate (1978) and Vermeulen (19801, has led to a better prediction of the carbon deposits.

Nomenclature

Pi = paraffin Rim = radical 0 = olefin Rq' = olefinic radical T = temperature, "C t , = residence time, s ER = experimental results SR = simulated results L = liquids D = carbon deposit

Registry No. CH4, 74-82-8; C@s, 74-84-0; C3Ha 74-98-6; C2H4, 74-85-1; C3H,3,115-07-1; CdHs, 25167-67-3; CbH10,25377-72-4; C4&, 106-99-0.

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Received for review March 21, 1988 Revised manuscript received October 25 , 1988

Accepted February 25, 1989

A General Model for Coal Dissolution Reactions

John M. Shaw* and Ernest Peters Metals and Materials Engineering Department, The University of British Columbia, Vancouver, British Columbia, Canada V 6 T 1 W5

Direct Coal Liquefaction has been treated as a purely kinetic process previously. Reaction rates have been related to intrinsic rates of product formation from specific coals. Process variables, such as solvent composition, the rate of interphase mass transfer, the intensity of turbulence, and catalysis are not included in these models, even though process variables have been shown to play an important role in determining the rates of liquefaction reactions, particularly with dense coal slurries. The impact of process variables on total coal conversion is quantified in the present model by subdividing the reaction scheme into an initial mass-transfer-controlled interval, followed by a second kinetically controlled one. This reaction scheme is consistent with experimental findings. Few coefficients are employed, and similar sets of coefficients describe the liquefaction behavior of different coals in a single solvent or a single coal in different solvents. The results from eight verification trials, involving six coals liquefied under a broad range of reaction conditions and in three different reactor types, are presented.

Coal undergoes a complex sequence of physical and chemical processes as it is dissolved and hydrogenated in Direct Coal Liquefaction reaction environments. Droege et al. (1981) observed that coal particles swell when initially exposed to a sovlent under pressure and swell at elevated temperatures. This swelling has been shown to be a con- sequence of coalsolvent interactions (Weinberg and Yen, 1980; Hombach, 1980; Shibaoka et al., 1979) and/or the onset of thermal decomposition (Habermehl et al., 1981). Both mechanisms of particle swelling lead to rapid solvent absorption and the initiation of liquefaction reactions. Since coal is not structurally homogeneous (Yarzab et al., 1980; Smith and Cook, 1980) and constituent macerals exhibit a broad range of reactivities (Shibaoka et al., 19791, a number of reactions occur simultaneously. Reactive macerals dissolve quickly. The reactions are substantially complete within 2-10 min or high-temperature contact time (Whitehurst et al., 1980; Thurgood et al., 1982). These reactions, frequently associated with the cleavage of ether linkages (Kuhlmann et al., 1981; Carson and Ig- nasiak, 1980) and the hydrogenation of the resultant free radicals (Curran et al., 1967; Vernon, 1980; Petrakis and Grandy, 1980; Franz and Camaioni, 1980), are highly exothermic. In poor hydrogen-donor solvents or in do- nor-depleted sovlents, polymerization reactions also occur. These reactions may involve free-radical solvent-molecule or radical-radical interactions (Whitehurst et al., 1980) and can lead to the formation of high molar mass liquid products or coke (Derbyshire and Whitehurst, 1981). These products can be re-hydrogenated subsequently (Shalabi et al., 1979) because coal radicals, stabilized by initial dissolution reactions, may possess additional reactive linkages (Mohan and Silla, 1981) but the space-time yields of light oil or naphtha products are reduced. Less reactive macerals dissolve slowly. These reactions are typically

* Current address: Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A4.

associated with methyl group cleavage, although some ether linkages have also been shown to have low reactivities (Carson and Ignasiak, 1980).

Dissolved molecular hydrogen (Vernon, 1980), hydro- gen-donor species present in the solvent (Abichandani et al., 1982; Cronauer et al., 1978), or molecular species hy- drogenated in situ (Derbyshire et al., 1982) can act as hydrogen sources for liquefaction reactions. Shaw and Peters (Shaw and Peters, 1984) show that molecular hy- drogen can be a more effective hydrogenation reagent than tetrahydronaphthalene for initial dissolution reactions. However, the rate of radical formation increases rapidly in the temperature range 350-450 "C (Ross and Blessing, 1979). At low temperatures within this range, radical formation occurs slowly and donor solvents can hydro- genate and stabilize radicals as they form. Liquefaction reactions, conducted under these conditions, are insensitive to the presence or pressure of molecular hydrogen (Der- byshire et al., 1982) and the solvent-to-coal ratio (Mochida et al., 1979). A t higher temperatures, the observed rates of liquefaction reactions are sensitive to solvent compo- sition, the rate of catalytic solvent rehydrogenation, and the pressure of molecular hydrogen (Derbyshire et al., 1982; Rudnick and Whitehurst, 1981), and polymerization re- actions become more significant (Mochida et al., 1979).

Polymerization/coking reactions have also been shown to occur at long mean residence times (Bickel and Thomas, 1982). These reactions can occur homogeneously or het- erogeneously (Painter et al., 1979). Shaw and Peters (1989) suggest that polymerization reactions arise primary in a persistent dispersed liquid phase.

The reaction models for coal dissolution, cited on Table I, vary in complexity but treat coal dissolution as a purely kinetic process. These models contain between 2 and 34 fitted constants, and the complexity of the predictions varies accordingly. There is little agreement on the dis- solution mechanism(s), and unrelated sets of parameters are required for each model depending on the reaction conditions. The parameter sets for the models proposed

0888-~885/89/2628-0976$01.50/0 0 1989 American Chemica l Society