Pyrolysis of coal liquids

Pyrolysis of coal liquids S. Krishnamurthy”,

Y. T. Shah and G. J. Stiegalt

Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA tProcess Sciences Division, Pittsburgh Energy Technology Center, US Department of Energy, Pittsburgh, Pennsylvania 15236, USA (Received 25 October 1979; revised 30 April 1980)

The pyrolysis of process recycle solvent derived from Western Kentucky coal via the SRC-II coal liquefaction process was investigated to ascertain the effect of residence time and temperature on the production of olefins. The study was made using an alonized transfer line reactor operating at temperatures of 650 and 73O”C, essentially atmospheric pressure, and residence times up to 0.13 s. A comparison is made with previously published results for the pyrolysis of hydrotreated COED light and heavy coal liquids (subsequently referred to as COED light and heavy oils, respectively) derived from Western Kentucky coal and steam pyrolysis of a hydrogenated fraction of SYNTHOIL derived from Western Kentucky coal. Results indicate that in each case the preferential pyrolysis of the saturate fraction occurs under convential pyrolysis conditions. Ethylene, propylene, and methane were the dominant gas products in all cases. The liquid pyrolysatesfrom the COED oilsand SRC-II recycle solvent had lower H/C ratios and heating values than their respective feedstocks. Mass spectroscopic analysis of the liquid pyrolysates in each case revealed the presence of polycyclic aromatics that were not present in the individual feedstocks. This trend which increased with temperature is indicative of cyclization and/or recombination of free radicals during pyrolysis. It is therefore surmized that the yields of light olefins from primary coal liquefaction products can be improved by partially hydrogenating them prior to pyrolysis. Alternatively, sufficient hydrogen can be provided in the vicinity of cracking to suppress retrogressive reactions which lead to the formation of coke. The pyrolysis of COED oils and SRC- II recycle solvent was found to follow first-order irreversible kinetics. The activation energy for the pyrolysis of the COED light and heavy oil was found to be 76.1 and 70.85 kJ g-mol-‘, respectively.

Olefins are valuable feedstocks for the petrochemical industry. Hitherto, olefins have been derived by the thermal cracking of petroleum mid-distillates. With declining reserves of world oil and the uncertainty of oil imports, alternative sources of olefins have to be investigated. The present study examines the feasibility of producing olefins by the pyrolysis of an unhydrotreated SRC-II recycle solvent derived from Western Kentucky coal, at high temperatures and short contact times, using a transfer line reactor. A comparison is also made with the results reported earlier for the pyrolysis of hydrotreated COED light and heavy oils derived from Western Kentucky coal (Krishnamurthy et al.ls2) and steam pyrolysis of a hydrogenated fraction of SYNTHOIL derived from Western Kentucky coal (Korosi et ~1.~).

EXPERIMENTAL

gases were

separated from the solids in the separator tank, cooled in a condenser, and then sampled along with the product oil and alumina. To eliminate undesirable reactions promoted by the stainless steel, the internal surfaces of both the reactor and separator tank were alonized. The alonizing process consists of treating the stainless-steel surface with aluminum which results in an aluminum-rich surface. This technique has been successfully employed by Frech et aL4 and Albright et a1.5 for passivating stainless-steel surfaces. Alonized surfaces have been shown to withstand temperatures up to 1000°F without degradation. In this study a cross-sectional sample of the reactor was examined after 100 h of discontinuous operation, and the alonizing was found to be intact, as shown by the photomicrographs in Figure 2 (Weinbaum6).

Apparatus

The experimental apparatus was the same as that used in earlier investigations and is described in detail by Krishnamurthy et al.‘. The unit, shown schematically in Figure 1, employed at stainless-steel transfer line reactor with appropriate connections for feeding the coal liquid, fluidizing gas and alumina powder. Details of the reactor are given in Table 1. The gas, liquid, and solid were preheated before being fed to the reactor. The alumina powder was used to provide part of the heat required for the pyrolysis reactions and to remove any coke which may be formed and deposited on the reactor wall. The product *

Present address: Mobil Research Paulsboro, New Jersey 08066, USA

and

Development

0016-2361/80/11073849.%2.00 @ 1980 IPC Business Press

738

FUEL,

1980,

Vol 59, November

NCN.

Corporation, Figure

1

Experimental

apparatus

Pyrolysis

of coal liquids:

et al.

S. Krishnamurthy

Procedure

The primary variables investigated were residence time and temperature. The composition and physical properties of the SRC-II recycle solvent are given in Tables 2 and 3. It was fed to the reactor at a rate of 120 cm3 h- ‘. The alumina powder had an average particle diameter of 0.01 cm and a BET area of 235 m2 g-l and was fed to the reactor at a rate of 60 g h- ‘. Details of the product distribution from each run are given in Tables 3 and 4. The reaction temperatures were 650 and 730°C. Considerable effort was expended to maintain an isothermal reactor during the course of an experimental run. At each temperature, several runs were made by varying the residence times. The residence time for each run was calculated from the following equation: Residence time = Reactor volume (Total flow rate of N, + Noncondensibles)

b

at reactor conditions

The residence time thus calculated is not the actual time since the flow rate of the condensible products at reactor conditions is not accounted for; however, the error can be shown to be negligible. The above procedure was followed because no means of measuring the reactor effluent prior to condensation was readily available. Each experimental run was performed by feeding the coal liquid into the reactor after the test section had

Tab/e 1

Design details of the transfer

line reactors Daimeter

COED light oil COED heavy oil SRC-II recycle solvent

(cm)

Length (cm)

0.61 0.61 0.94

90 76 77 figure 2 (b) Used

Table 2

Comparison

Photomicrographs

of reactor cross-sections.

of feedstocks Hydrotreated Western Kentucky

COED

Kentucky SRC II recycle solvent

Hydrotreated Western Kentucky SYNTHOI L

na

na

ia98 2.5 115-497

:a895

87.2 9.0 2.5 1 .o 0.2 1.24

87.14 12.76 0.07 0.03 0.001 1.76 na na

Tvpe

Light oil

Heavy oil

Flash point (ASTM D92) (‘C) Fire point (ASTM 092) (“C) Specific gravity @ 60°/600F (ASTM D287) Kinematic viscosity @ 140°F (ASTM D2170)

25 naC 0.818

74 91 0.906

0.78 13-500

2.5 loo-51

84.7

85.5

12.5 2.7 GO.1 GO.1 1.77 10717 112

1 1.6 2.8 0.0 0.0 1.63 10531 215

9664 209

G2 26.1 Gl 71 95

0 48 61 52 61

npd np np np np

fcSt)a

Boiling range (‘C) Ultimate analysis fwt %I Carbon Hydrogen Oxygen (by difference) Nitrogen Sulphur H/C ratio Heating value (kcal kg-‘@ Molecular weight @ 27OC FIA analysis (wt %) Heterocyclics Aromatics (assuming diolefins absent) Olefins Saturates Per cent recovery a b z

(a) New;

0

k-380

;1;.8 if.1 na

1 cSt=10-6mZs-1 1 kcal kg-’ = 4.19 kJ kg-’ na, data not available np, data not available because analysis was not possible

FUEL, 1980, Vol 59, November

739

Pyrolysis Tab/e 3

of coal liquids: Summary

of operating

S. Krishnamurthy conditions

Reactor temperature 1°C) Nitrogen feed rate (at reactor conditions) Superficial velocity Residence time (s)

et al.

and product

distributions

X 10’)

6.5

fm 5-t)

9.32 0.082

0.0013 0.0185 tr -

Feed 5.06 4.23 10.0 22.69 9.76 17.17 10.94 3.05

5.29 0.82 15.29 0.353 4.47 1.53 2.47 4.0 0.47

0.9954 3.04 1.23 9664

1980,

Vol 59, November

652 4.93 7.08 0.11

651 4.23 6.07 0.13

0.0134 0.0134

0.0063 0.0088

ot;47 -

o&94 0.0427 -

3.0

2:;2

2.37

5.19 5.6 8.66 21.39 11.31 14.57 10.9 3.06

4.09 5.32 7.89 18.34 9.31 14.06 8.55 2.66

4.88 5.38 8.86 20.7 8.66 13.44 10.36 2.99

4.38 0.815 11.0 -

3.61 0.67 12.54 1.14 2.57 0.57 1.43 2.28 -

3.98

-

reached a predetermined temperature. The condensibles were collected after steady-state conditions were achieved. Steady-state was determined during preliminary tests by collecting gas samples at 15 min intervals and comparing the analyses for consistency. It was found that steady-state operation was always achieved within 30 min from the start of a run. During steady-state operation, gas samples were collected at 30 min intervals and analyses were always consistent. Upon terminating each run, the solids in the separator were collected and the coke deposited in the reactor was burned off by passing air through it for one hour at 500°C. At the end of this period the reactor was purged for onehalf hour with nitrogen. All gas, liquid and solid samples were analysed by the Chemical and Instrumental Analysis Division of the Pittsburgh Energy Technology Center. The procedures employed were either standard ASTM methods or techniques developed at the Pittsburgh Energy Technology Center. The gas samples were analysed by gas chromatography and the liquid samples by gas chromatographic simulated distillation, mass spectroscopy, specific gravity, kinematic viscosity and ultimate analysis, The alumina powder from the reactor was subjected to proximate and ultimate analyses to ascertain the extent of coke deposition and to determine if any hydrocarbons had adsorbed on the alumina surface. FIA (Fluorescent Indicator Absorption) analysis of the SRC-II feedstock and liquid pyrolysates was not possible.

FUEL,

recycle solvent at 6W’C 656

fm3 s-t

Product analysis Gas analysis (g/g of feed) Hydrogen Ethylene Propylene Butylene Straight-chain butane Oxygen H/C atomic ratio Liquid analysis fg h-‘1 Benzenes BTX fm78) lndenes f=116) lndansltetralins f=l16) Naphthalenes f=128) Acenaphthalene/fluorenes f-166) Acenaphthene/biphenyI (~154) Phenanthrene/anthracene Methylene phenanthrenelphenyl naphthalene f=192-204) 4-ring peri (i.e. Pyrenes) (=202) 4-ring cata (i.e. Chrysene) Phenols lndenols Acetophenonelindanols Dihydrophenols fm264) Phenylphenols Dibenzofuran Biphenols Properties of pyrolysates Specific gravity @ 60°/600F Viscosity @ 140°F icSti H/C atomic ratio Heating value fkcal kg-‘)

740

for SRC-II

2.04 0.92 1.63

0.9988 2.52 1.22 9172

1 .o 2.63 1.204 na

0.5 11.95 1.29 2.39 0.9 1.59 1.59 -

0.996 2.59 1.216 9384

This technique (ASTM 1319) is used to estimate the quantities of aromatics, saturates, olefins, and heterocyclies constituting the sample, and involves adding the sample to the top of a column containing a silica gel and a dye and allowing it to flow to the bottom during which time each hydrocarbon fraction extracts specific components from the dye which imparts a characteristic colour to the fraction. The coloured fractions separate and the quantities of each are noted. This technique is limited to samples of low viscosity which are not dark, so as not to obscure the demarcation line between the colours. The first attempts to analyse the SRC-II feedstack and the liquid products derived from it failed because of very poor separation and recovery. In further attempts to perform this analysis, the samples were diluted with lighter organic components such as cyclohexane and benzene to provide more mobility to the various fractions and to enhance recovery; however, these attempts also failed. The specific reason for the failure of this analysis remains unknown; however, the lack of light components and/or the presence of heterocyclic components in the coal liquid which are known to separate poorly in some samples, are possibilities. As a result, no mass spectrometric data on the saturate and aromatic fractions were obtained on the product samples. DISCUSSION The analysis of the SRC-II recycle solvent feedstock is presented in Table 2. The distillation curve for this

Pyrolysis Tab/e 4

Summary

of operating

conditions

Reactor temperature (‘Cl Nitrogen feed rate (at reactor conditions) Superficial velocity (m s-‘) Residence

and product

(m

s-l

distributions

I

I

728 4.76 6.82

733 6.34

X 104)

9.09

Product analysis Gas analysis, g/g of feed Hydrogen Methane Ethylene Ethane Propylene Butylene Straight-chain butane Carbon dioxide Oxygen H/C atomic ratio Liquid analysis (g h-l) Benzenes, BTX (~78) lndenes (~116) Indans/tetralins (=I161 Naphthalenes t-128) Acenaphthalenes/fluorenes (=I661 Acenaphthenes/biphenyl (-159) Phenanthrene/anthracene Methylene phenanthrene/phenyl naphthalene (=192-204) 4-ring pefi (i.e. Pyrenes) (=202) 4-ring cafa (i.e. Chrysenes) Phenols lndenols Naphthols Acetophenonelindanols Dihydrophenols (~264) Phenylphenols Dibenzofurans Binaphthyls Biphenols Properties of pyrolysates Specific gravity @ 60°/600F Viscosity @ 14O’F (cSt) H/C atomic ratio Heating value (kcal kg-‘)

I

S. Krishnamurthy

et al.

for SRC-I I recycle solvent at 730%

time is.1

500 -

of coal liquids:

0.1 1

0.12

0.0023 0.012 0.043 trace 0.0 166 _

0.004 0.024 0.0743 trace 0.027 trace trace trace trace 2.8

0.0052 0.032 0.086 trace 0.028 -

2.76 4.44 7.34 4.64 21.15 12.27 12.75 12.27 3.09

4.47 1.53 2.47 4.0 -

3.4 5.32 2.87 20.65 11.42 9.76 11.94 3.14

4.46 0.84 8.09 1.58 1.3 1.02 _

4.79 1.22 7.23 1.22 -

-

1.57 0.17 0.78 _

0.37 -

0.17 _

1.045 2.73 1.013 9401

1.05

0.74

0.47 1.034 2.69 1.05 9377

0.9954 3.04 1.23 9664

trace trace trace 2.9 1

4.0 5.86 3.53 23.06 11.72 11.16 12.18 3.26

4.25 0.772 9.08 0.87 0.39 1.835 0.39 1.062 _

5.29 0.82 15.29 0.353 -

6.32

0.084

trace Feed 5.06 4.23 10.0 22.69 9.76 17.17 10.94 3.06

732 4.4 1

2.42 1.017 9329

feedstock is shown in Fipre 3 along with the COED SYNTHOIL coal liquids which are shown comparison.

I

Material

and for

balance

Elemental carbon and hydrogen material balances were performed for each run using the analytical results obtained for the gas, liquid and solid products. All material balances were consistant to within five per cent.

0

I 20

I 40 Per

cent

I 60

I 80

distilled

Figure3 Distillation curves for COED light I- -1 and heavy c-.-J oils, SCR-II recycle solvent (---I and SYNTHOIL t-.,-j

100

Gus unulllsis The gaseous products formed during pyrolysis of SRCII recycle solvent are presented in Tuhles 3 and 4 and Figures 447. The amount of gaseous products formed at 730’C is higher than at 650°C. However, the formation of gaseous products exhibits an unusual trend with residence time, for the amounts of ethylene and hydrogen formed per gram of feed decreases with increasing residence time at 650°C. At the same time, propylene and butylene are formed in the gas phase, possibly owing to recombination of some of the ethyl radicals formed under mild pyrolysis conditions. With increasing temperature of pyrolysis the

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Pyrolysis

of coal liquids:

I

I

et al.

S. Krishnamurthy

I

I

I

I

I

I

1 I..

0

0.02

0.04

0.06 Residence

0.08

0.10

time

0.12

0 .lL

I s)

Figure 4 Hydrogen yield. 0, COED light oil, 74O’C; . . COED light oil, 76O’C; 0, COED heavy oil, 690%; 0, COED heavy oil, 750%; *, SRC-II, 650%; *, SRC-II, 730%

2

P

I

I

I

I

I

I

1

and the existence of substituted side-chains is not precluded. It is evident from Tables 3 and 4 that the SRC-II recycle solvent is composed primarily of aromatics, some of which are partially saturated. From the variation in the quantity of each compound with residence time, certain major trends are discernible at both 650 and 730°C. Partially hydrogenated aromatics such as indans, tetralins, acenaphthenes and indanols appear to dehydrogenate and revert to their fully aromatic state. Insofar as bond breaking reactions are concerned, the bonds that are not included in the aromatic ring appear to break easily. This is due to the stability of the benzene ring and is confirmed by the marginal change observed in the benzenes in contrast to a significant decrease in phenols. For dihydrophenols, phenylphenols and biphenols a net decrease could be due to dehydrogenation or scission of the hydroxyl group or both. Dibenzofuran, an organooxygen compound, is extremely susceptible to cracking and the bond scission in such a reaction commences probably at the -C-O- bond. Another interesting side reaction is the formation of binaphthyls which could be

I

I

L

I

I

I

I

I 0.12

-I

i lOOI 0

I

I

0.02

O,OL

I 0.06

Residence

I 0.08 time

I

I

0 .lO

0.12

0%

I s)

Figure 5 Methane yield. 0, COED light oil, 740%; n, COED light oil, 760%; 0, COED heavy oil, 690°C; 0, COED heavy oil, 75O’C; 4 SRC-II, 730°c 100

formation of the ethyl radicals could surpass its recombination to heavier products thereby resulting in the net increase in the yield of ethylene observed at 730°C. Methane and propylene are also observed in the reaction products at 730°C and their formation increases with increasing residence time. Ethylene is the most significant hydrocarbon formed during pyrolysis of the SRC-II recycle solvent, and its formation is favoured by increasing temperatures and residence times. The highest yield observed was 0.085 g/g of feed at a residence time of 0.12 s. Methane and propylene are the other important hydrocarbons formed from the SRC-II recycle solvent with their formation also being favoured by increasing temperatures and residence times. A similar observation was made from studies on the pyrolysis of a COED light and heavy oils (Krishnamurthy et ~1.l.~). Liquid

I

I

I

0.06

0.08

0.10

I

PO2

O.OL

Residence

time

_I 0.14

Is)

Figure 6 Ethylene yield. 0, COED lighe oil, 615%; 0, COED light oil, 74O’C; n, COED light oil, 780°C; 0, COED heavy oil, 69O’C; 0, COED heavy oil, 750°C; 4 SRC-II, 650°C;A, SRC-II, 73O’C

2

n 0 x

I

I

I

I

I

0.06

0.08

I

I

analysis

Mass spectrometric analyses of the liquid pyrolysates are presented in Tables 3 and 4. This data represents the fraction that each component contributes to the total ionization of the sample and may not be an absolute indication of the composition of the pyrolysates. Moreover, these compounds represent the basic structure

742

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FUEL, 1980, Vol 59, November

O.OL

Residence

Inme

0.10

0.12

0.U

0.16

Is1

Figure 7 Propylene yield. q # COED light oil, 740°C; n, COED light oil, 790°C; 0, COED heavy oil, 690%; 0, COED heavy oil, 750%; + SRC-II, 730%

Pyrolysis Tab/e 5

Summary

tions for hydrotreated Steam/hydrocarbon

of operating SYNTHOIL

conditions at 875’C

and product

weight ratio

1 .o

Reactor exit temperature (‘C) Reactor exit pressure (bars abs)a Residence time (s) Products (g/g feed) Hydrogen Methane Acetylene Ethylene Ethane Propadiene, propyne Propylene Propane 1,3 Butadiene Butenes Cs olefins, diolefins Benzene Toluene C8 aromatics Other Cg, C,, C8 C; to 205’C Pyrolysis fuel oil (92O!FC) a

1 bar=

distribu-

(Korosi etaL3)

875 1.85 0.3 0.0103 0.1166 0.0049 0.1883 0.024 0.0058 0.0783 0.0021 0.0468 0.0163 0.0256 0.1215 0.0676 0.0343 0.0048 0.0282 0.2246

lo5 Pa

due to polymerization of naphthyl radicals owing to lack of hydrogen in the vicinity of cracking. Polyaromatics such as anthracenes, phenanthrenes, pyrenes and chrysenes do not undergo any significant change. Interpretations of the mass spectroscopic data are at best qualitative and detailed studies have to be made on individual compounds to determine the kinetics and mechanisms of such reactions. In T&es 3 and 4, data are also presented for the viscosity, specific gravity and H/C atomic ratio of the pyrolysates under the various reaction conditions. At the lower temperature, little change in the H/C ratio was evident, whereas at the higher temperature, a considerable decrease was observed. The specific gravity was seen to increase with temperature and residence time. These phenomena possibly are due to dehydrogenation and polymerization reactions occurring during pyrolysis. COMPARISON SYNTHOIL A summary of the distributions for the reported elsewhere SYNTHOIL given

WITH

COED

OILS

AND

operating conditions and product pyrolysis of the COED oils have been (Krishnamurthy et ~l.‘*~), those for in Table 5.

Feedstocks Results of the analysis of the SRC-II recycle solvent, the hydrotreated COED light and heavy oils reported by Krishnamurthy et ~1.‘~~ and the analyses of a hydrogenated fraction of SYNTHOIL obtained by Korosi et 01.~ are presented in Tuble 2. All four feedstocks contain approximately the same percentage of carbon; however, the unhydrotreated SRC-II recycle solvent has a considerably lower hydrogen content than the other coal liquids. SYNTHOIL contains very little oxygen, nitrogen and sulphur whereas the two COED oils contain a substantial quantity of oxygen. SRC-II recycle solvent contains significant amounts of both oxygen and nitrogen; however, the sulphur content is low. From the data in Figure 3 it can be seen that the hydrotreated COED heavy oil, SRC-II recycle solvent

of coal liquids:

S. Krishnamurthy

et al.

and SYNTHOIL have similar distillation curves. Results of the mass spectroscopic analysis of the COED heavy oils (Krishnamurthy et ~1.l.~) and the SRC-II indicate that the two coal liquids are composed of only a few similar components. The distillation curve for the COED light oil is considerably lower than those of the other feedstocks. The FIA analyses presented in Tmble 2 for the COED light and heavy oils and the SYNTHOIL fraction indicate a high percentage of saturates. The SYNTHOIL feedstack has a saturate content similar to that of the COED light oil. However, the distillation curve of SYNTHOIL is higher than that of the COED light oil indicating that the latter contains more lower boiling saturates. Gas analysis Figures 447 present the yields of hydrogen, methane, ethylene, and propylene as a function of residence time. In general, the formation of these compounds, in the cases of the COED and SRC-II coal liquids, is favoured by increasing temperatures. Increasing residence times also favour the formation of these compounds in all cases except that of the SRC-II recycle solvent at 65O’C. The trend for the SYNTHOIL could not be ascertained as the data was available only at a temperature of 875’C and a residence time of 0.3 s. The yield of hydrogen as a function of residence time is shown in Figure 4. For a given temperature and residence time, its formation decreases in the order: COED light oil, COED heavy oil, SRC-II. The trends exhibited by methane, ethylene and propylene are, within limits of experimental error, similar to those described for hydrogen and are shown in Figures $6, and 7, respectively. The COED light oil has a higher saturate content than the COED heavy oil. Since saturates are preferentially cracked over aromatics, this could explain the higher yields of gaseous hydrocarbons for the COED light oil. SRC-II recycle solvent was not hydrotreated prior to pyrolysis, which probably explains the low gas yields obtained. The yield of the hydrocarbon gases from SYNTHOIL at 875°C and a residence time of 0.3 s is similar to the yield of the same gases from the COED light oil at 780°C and a residence time of 0.086 s. As discussed previously, the SYNTHOIL liquid has higher boiling saturates than the COED light oil and therefore would probably require more severe operating conditions to produce similar yields of the various components. Other gaseous compounds formed were propane and butylene for the COED light oil, butane and ethane for the COED heavy oil, traces of ethane, butane and butylene for the SRC-II recycle solvent, and acetylene, ethane and propylene for SYNTHOIL. Total gas production always increased with increasing temperature and residence time. The gases produced at a given temperature and residence time increased in the order: COED light oil, COED heavy oil. SRC-II. Liquid analysis The mass spectrometric analysis of the COED light and heavy oils are given elsewhere (Krishnamurthy et ~1.l.~). The recovery of the COED heavy oil during FIA analysis ranged between 72-95% and therefore, the discussion on the heavy oil is mainly qualitative. The COED light oil had a high saturate content which was preferentially pyrolysed at both 740 and 780°C. The

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Pyrolysis

of coal liquids:

S. Krishnamurthy

et al.

aromatic fraction was composed of substituted benzenes, indenes, tetralins and naphthalenes. The indenes are readily cracked at both temperatures. However, there is an increase in the benzene, naphthalene and tetralin content of the pyrolysate with increasing temperature and residence time. A net increase in the aromatic content of the pyrolysates is also observed. In addition, at 740 and 780°C a number of polyaromatic compounds appear in the pyrolysates which were not structurally represented in the feed. It is speculated that this could be due to a combination of cyclization reactions and/or reactions involving recombination of free radicals as discussed in detail by Krishnamurthy et al.‘. The extent of such molecular rearrangements was extremely small at 615°C. The saturate content of the COED heavy oil is lower than that of the COED light oil. In addition, there are significant amounts of 2-ring, 3-ring, and 4-ring naphthenes which were not detected in the light oil. Here again, the saturates are preferentially cracked at both 690 and 750°C but not at 600°C. Although COED heavy oil contains a wider variety of aromatic compounds, other aromatic components which were not structurally represented in the feed were detected in the pyrolysates; also, a net increase in the aromatics content of the pyrolysates was observed with increasing temperature and residence time. Both these phenomena were found also for the COED light oil. These observations indicate that reactions involving free radicals which yield aromatic products were occurring. Such rearrangements, however, were not occurring at 600°C. In all cases, the H/C atomic ratio of the pyrolysates decreased with increasing temperature and residence time. The heating value of the COED oils showed a similar reduction. Although heating values were not obtained for all the SRC-II pyrolysates, the trend appears to be similar.

Light fraction _k,

(a& gas tind coke

(2)

where: a,, a2 and a3, are stoichiometric coefficients; and k, and k,, are the rate constants. As indicated in reactions (1) and (2), reversibility is included if polymerization or cyclization reactions are significant. In analysing the data it was found that reactions (1) and (2) did not adequately model the data from the three feedstocks. Subsequently, a simple nth order kinetic model for the overall cracking of the COED oils and SRC-II recycle solvent, which did not employ the light and heavy fractions defined previously, was postulated: k, Coal liquid -(al)

gas and coke

(3)

The rate expression for the nth order irreversible kinetic model is given by

rate=

-dC dt

----lc=k,C:

with initial conditions C,=l

at t=O

(44

where: C,, is the concentration of coal liquid (where x = L, H, or S); t, is the residence time in seconds; and k,, is the nth order rate constant, mol’ -“/l’ -” s. This model with n = 1 (i.e. first-order kinetics) was found to fit the data for the COED oils and SRC-II recycle solvent reasonably well. For the case of IZ= 1, Equation (4) reduces to dC x=k,C, d

Solids

The solids were subjected to proximate and ultimate analysis, giving the necessary information required for material balance calculations. The H/C ratio on the solids was usually low and did not exhibit any trend, indicating that little coke deposition had occurred on the solids.

which upon integration yields

KINETIC

Figures 8,10 and 12 show plots of -In C, (where x = L, H, or S) versus t to be rectilinear in the temperature range of

ANALYSIS

An attempt was made to analyse the results obtained for the pyrolysis of the COED light and heavy oils (Krishnamurthy et ul.1-2) and the SRC-II recycle solvent using simple kinetic models. Initially, a reaction mechanism similar to that used by Weekman and Nate’ to model the catalytic cracking of gas oil was postulated, based on experimental observations. The model proposed requires the use of three distinct fractions: (1) a light liquid fraction boiling in the range 255250°C; (2) a heavy liquid fraction boiling in the range 250-550°C; and (3) a gas plus coke fraction. The relation amounts of each fraction were estimated from the simulated distillation curves of the various products. Based on the assumption of these fractions the proposed model takes the following forms: Heavy fraction +k,‘ar)

light fraction

+(a& gas and coke

744

FUEL, 1980, Vol 59, November

(1)

C,(O) = 1

(5a)

-lnC,=k,t

(6)

615-780°C for the COED light oil, 5OO-750°C for the COED heavy oil, and 65&73o”C for the SRC-II recycle solvent. The first-order rate constants for the various coal liquids are provided in Table 6. The temperature dependence of the rate constants for the COED light and heavy oils was determined by the Arrhenius plots shown in Figures 9 and 11. The rates of cracking of the COED oils are given by the following equation: t-,=5.8x r,=3.1

lO”exp(-18

170/RT)C,

x 104exp(-16974/RT)C,

(7)

(8)

where: R is the gas constant in cal K-i g-mol- ‘; and T is the temperature of pyrolysis in K. A similar expression for the SRC-II recycle solvent could not be established owing to insufficient data at various temperatures. From Table 6 it is seen that the rate of cracking of the COED light and heavy oils are comparable in the

Pyrolysis

of coal liquids:

et al.

S. Krishnamurthy

formation is favoured at higher temperatures and longer residence times; (3) Dehydrogenation of partially saturated aromatics and cracking of bonds without the resonance-stabilized benzene rings are observed. A comparison of the pyrolysis products of the COED oils, SRC-II recycle solvent and SYNTHOIL suggest the following conclusions: (1) the total gas production generally increases with temperature and residence time. Irrespective of the nature of the feed, ethylene, methane,

0

0.01,

0.02

Residence

0.06

0.08

time

0 ,lO

Figure8 oil.

Plot of -In C versus residence time for the COED n, Feed; 0,615OC: 6 ,740’C; 0,780’C

2.8

I

I

0.6

0.8

0.12

t s) light

propylene and hydrogen are the dominant gas phase products. Higher temperatures and longer residence times favour their formation: (2) in the absence of saturates, as in the SRC-II recycle solvent, the primary reactions are those involving dehydrogenation of partially saturated aromatics and scission of substituted side-chains (dealkylation) attached to the condensed rings. However, in the presence of saturates, as in the COED oils, the primary

reactions appear to be those involving cracking of naphthenes to yield desirable olelins. For both COED oils, cyclization reactions and reactions involving recombination of free radicals are observed: (3) the H/C atomic ratio and the heating value of the liquid pyrolysates decrease with increasing temperature and residence times while the specific gravity shows the opposite trend.

2.4 2.0 &

1'6

5

l-2 0.8

0

0.1

Iif Figure 9 mol-’

Arrhenius

1-o

x lo3

plot for the COED

1.2

(K-l)

light oil.

E = 18 170 cal g

temperature range studied, probably because similar components are pyrolysed in both cases. The rate of cracking of the SRC-II recycle solvent is considerably lower because its composition is different from that of the COED oils. A possible reason is that the COED oils were derived from coal by a pyrolytic process and were subsequently hydrotreated, whereas the SRC-II recycle solvent was derived using the donor solvent method and was not hydrotreated further. Table 7 presents the activation energies for the pyrolysis of the COED light and heavy oils. Within the limits of experimental error the activation energies for the COED oils are comparable, once again indicating that similar type components are reacting in both cases. CONCLUSIONS For the SRC-II recycle solvent it was yield of gaseous products is low at probably owing to the highly aromatic (2) ethylene is the most significant

found that: (1) the all temperatures, nature of the feed; product and its

0.02

0.04 Residence

0.06

0.08

time

0.10

Is)

Plot of -In CH versus residence time for the COED Figure 10 oil. ? Feed; 0,600’C; A, 69O’C; 0, 75O’C

Tab/e6

Kinetic

constants

heavy oils and SRC-II

for the pyrolysis

of COED

light and

recycle solvent Rate constants

Temperature (W

heavy

COED light oil _

600 615 650 690 730 740 750

6.69 -

780

9.671

1.906 -

(s-l)

COED heavy oil 1.897 _ 3.702 _ _ 8.366 _

FUEL, 1980, Vol 59, November

SRC-I I _ 1.488 2.359 _ -

745

Pyrolysis

of coal liquids:

I

I

et al.

S. Krishnamurthy

I

I

I

I

Tab/e 7

I

Activation

energy for the pyrolysis of light and heavy oils

Feedstock COED COED

0

0 Figure I I

0.01 Arrhenius

0.12

0.00

Residence

time 1s)

plot for the COED

heavy oil.

0 a16

light oil heavy oil

Activation energy (cal g-mol--lJ

Range of validity

18 170

615-780 600-750

16947

(W

750°C (SRC-II recycle solvent). The activation energy for the cracking of COED light and heavy oil were found to be 18 170 cal g-mall ’ and 16 947 cal g-mol-i, respectively. In summary, the maximum yield of olelins is obtained by pyrolysing highly saturated coal liquids. Partially saturated aromatics release hydrogen and revert to their stable aromatic state. Therefore, it may be beneficial to hydrogenate primary coal liquefaction products or to remove the aromatic fraction prior to pyrolysis. Alternatively, polymerization of free radicals during pyrolysis can be minimized by providing hydrogen in the vicinity of cracking.

E = 16 547 cal g

mole1

ACKNOWLEDGEMENTS The financial support received from the Pittsburgh Energy Technology Center under DOE Contract No. EY-77-S-02-4083.*0 and the assistance of the Chemical and Instrumental Analysis Division in analysing product samples are gratefully acknowledged. Also, two of the authors (S. Krishnamurthy and Y. T. Shah) were partially supported under the above contract. This paper was presented in part at the joint meeting of the American Chemical Society and Chemical Society of Japan held between April 2-6, 1979, Honolulu, Hawaii.

3

2 it 1

REFERENCES Krishnamurthy,

0

I

I

1

0.3

0.6

0.9

1.2

1.5

I/T x lo3 (K-‘I Figure 12

Plot of -In CS versus residence time for the SRC-I recycle solvent. m, Feed; 4 650%; 0, 73O*C

I

Temperature has the greatest effect on these properties; (4) the cracking of the COED oils and SRC-II recycle solvent can be satisfactorily modelled by first-order irreversible kinetics in the following temperature range: 615-780°C (COED light oils); 600-750°C (COED heavy oil); and 63@-

746

FUEL,

1980,

Vol 59, November

S., Shah, Y. T. and Stiegel, G. J. Ind. Eng. Chem.

Process Des. Dee. 1979, 18 (3), 466

I

Krishnamurthy,

S., Shah, Y. T. and Stiegel, G.

J. Ind. Eng. Chem.

Process Des. Den 1979, 18 (3), 466

Korosi, A., Woebcke, H. N. and Virk, P. S. Am. Chem. Sot. Div. Fuel Chem. Preprints 1976 21 (6), 190 Frech, K. J., Hoppstock, F. H. and Hutchings, D. A. ‘Industrial and Laboratory Pyrolysis,’ ACS Symposium Series 32 (Eds. L. F. Albright and B. L. Crynes) American Chemical Society, Washington, DC, 1976, pp. 197-217 Albright, L. F., Yu. Y. C. and Welther, K. 85th National Meeting of AICHe Philadelphia, PA, paper 151, 1978 Weinbaum, M. J. Alon Processing Inc., personal communication (1979) Weekman, V. W. and Nate, D. M. AIChE J. 1970, 16, 397