Pyrolysis of Polycyclic Compounds Containing Sulfur

44 Pyrolysis of Polycyclic Compounds Containing Sulfur P. X. MASCIANTONIO and J. W. WALTER

Downloaded by CORNELL UNIV on October 24, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch044

Applied

Research Laboratory,

U. S. Steel Corporation,

Monroeville,

Pa.

The pyrolysis of polycyclic compounds has been of considerable interest in studying carbonization and the manufacture of carbon products. It is usually desirable to minimize the sulfur content of the carbonaceous residues resulting from pyrolysis of organic materials. To understand the disposition of sulfur during pyrolysis, a series of model polycyclic sulfur-containing compounds has been pyrolyzed at 625°C. A correlation between the sulfur distribution and the elemental composition of the model compounds has been observed. A mechanism of desulfurization that involves the probability of combining hydrogen and sulfur atoms during pyrolysis appears to fit the experimental data best. The study indicates that the volatile matter and oxygen content of the model compounds are also significant factors in desulfurization.

jy|ost of

the solid carbonaceous material available to industry is derived from the pyrolysis of petroleum residues, coal, and coal tar residues. Understanding the reactions occurring during pyrolysis would be beneficial i n conducting materials research on the manufacture of carbonaceous products. T h e pyrolysis of aromatic hydrocarbons has been reported to involve condensation and polymerization reactions that produce complex carbonaceous materials (I ). Interest i n the mechanism of pyrolysis of aromatic compounds is evidenced i n a recent study by Edstrom and L e w i s (2) on the differential thermal analysis of 84 model aromatic hydrocarbons. T h e study demonstrated that carbon formation was related to the molecular size of the compound and to energetic factors that could be estimated from ionization potentials. There has long been an interest i n eliminating sulfur during pyrolysis of organic substances ( 3 ) ; however, little fundamental data pertinent to formation of carbonaceous residues are available on the pyrolysis of sulfur com687 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

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pounds. T h e selection of materials suitable for conducting systematic studies presents a problem i n such an investigation. Because petroleum residues, coal, and coal tar residues are very complex i n chemical composition, it is difficult to study sulfur distribution during pyrolysis of such materials. Furthermore, the effects of mineral matter and other heteroatoms, such as oxygen a n d nitrogen, on pyrolysis and sulfur distribution are not well understood. T o study the effect of chemical composition on sulfur distribution during pyrolysis, it appeared desirable to study the pyrolysis of sulfur-containing model compounds of well-defined structure. T h e correlation of chemical composition and sulfur distribution during pyrolysis is the subject of this paper. Experimental A series of 10 polycyclic compounds and blends with starch, Bakélite, a n d hydrogenated creosote were employed as model substances to examine the effect of carbon, hydrogen, a n d oxygen on sulfur distribution during pyrolysis at a temperature of about 6 2 5 ° C . Materials. T o satisfy the requirements for models for pyrolysis studies, it was necessary to select compounds that were of high purity and well-defined structure w h i c h would produce carbonaceous residues when subjected to high temperatures. Commercially available organic dyestuffs proved to be a ready source of model compounds containing various heterocyclic structures. It was necessary to purify these dyes b y recrystallization and extraction to remove small quantities of organic impurities and residual inorganic contaminants. The identity, structure, and chemical composition of the model compounds can be seen i n Table I and Figure 1. Starch a n d Bakélite were obtained from stock supplies and were used as-received for pyrolysis studies of blends. Hydrogenated creosote was prepared i n the laboratory b y catalytically hydrotreating a 2 7 0 ° - 3 5 5 ° C . creosote fraction to remove a l l heterocyclic impurities and to saturate completely the ring systems. Apparatus. Pyrolysis experiments were conducted i n a quartz tube heated by an induction furnace. A continuous flow of argon was used to sweep out gaseous pyrolysis products. T h e materials to be pyrolyzed were contained i n alundum boats, which could be readily admitted and removed from the quartz pyrolysis tube. Table I.

Data on Pyrolysis

Composition, atom % Compound

C

Ciba Pink Β Sulfur Black 2B Hydron Blue G Flavon G C Alizarin Black Β Indanthrene Brown Blue-Green F F B Algol Orange R F Ciba Orange R Cibanone Yellow R

57.2 46.2 56.3 58.4 49.2 60.0 66.7 47.6 62.6 59.2

S 28.6 27.0 22.0 29.2 32.1 30.0 25.9 38.1 25.0 30.2

7.14 3.84 7.80 4.17 13.20 5.00 3.71 9.53 8.34 9.22

Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

7.14 11.50 7.80 4.17 1.89 5.00 3.71 4.76 4.17 1.32

44.

MASCIANTONIO AND WALTER

689

Pyrolysis of Polycyclics

Procedure. Experiments were conducted i n the following manner. A 3-gram sample of the material to be pyrolyzed was weighed into an alundum boat. T h e sample was then placed i n the cold furnace within the quartz tube, and argon was passed slowly through the system. T h e tube was heated to 6 2 5 ° C . over 2 hours a n d then allowed to cool to room temperature with the flow of argon continuing throughout the cycle. T h e carbonaceous residue was then accurately weighed, and its elemental composition was determined. Volatile matter was calculated from the quantity of residue. Data were obtained from replicate experiments on model compounds and single experiments on the blended materials.


Results and

Discussion

Experimental data were obtained on the carbonaceous residue (char), and sulfur distribution was calculated for the solid and gaseous products from the pyrolysis of model compounds. Sharp differences were observed i n the quantity of char and the sulfur distribution for the different substances studied. The quantity of volatile matter varied from 21 to 4 3 % . T h e sulfur retained by the char varied from 21 to 7 4 % of the total present i n the compound pyrolyzed (see Table I ) . T h e raw data show a possible relationship between the volatile matter and sulfur retention which indicates that as volatile matter decreases, sulfur retention generally increases (Table I ) . Neither structural features nor the molecular size of the various model compounds appear to have a significant relationship to sulfur distribution. In an attempt to correlate sulfur distribution with elemental composition, the mechanism of sulfur elimination was assumed to involve two processes: ( 1 ) molecular fragmentation to form volatile sulfur-containing compounds, and (2) reactions between hydrogen and sulfur to form H2S (see Figure 2 ) . Therefore, the loss of sulfur b y molecular fragmentation should be related to the volatile matter realized from the material pyrolyzed, and the formation of H2S should be related to the hydrogen a n d sulfur contents of the substance pyrolyzed. The sulfur ratio, defined as the ratio of weight percent sulfur i n the char to the weight percent sulfur i n the dye, provides an interesting parameter regarding sulfur volatilization. The three compounds w i t h sulfur ratios between of Model Compounds

Volatile Matter, wt % 43 42 31 29 35 34 21 37 39 25

Sulfur Ratio 0.38 0.40 0.43 0.56 0.64 0.74 0.70 0.91 0.97 0.98

Sulfur Retained, wt % 21 23 30 40 42 49 56 58 59 74

Pms

(W)

3.15 5.86 4.38 2.50 0.84 3.00 1.71 3.75 1.09 0.56

PT

(W)

1.35 2.46 1.36 0.73 0.22 1.02 0.36 1.39 0.43 0.14


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690


Cibanone Yellow R

Sulfur Black 2B

OH

0

OH

0

Ν

ÎV y

8-0-

I

Η

Alizarin Black Β 0

Algol Orange RF

Blue-Green FFB

Figure 1.

Sfrucfure

0.38 and 0.43 have a significant desulfurizatJon reaction occurring during pyrolysis whereas compounds that exhibit values greater than 0.90 apparently lose sulfur principally by molecular fragmentation. T o test this mechanism, the probability of H2S formation was calculated for each of the model com pounds using Equation 1. (S) Ρ . (H) (H-I) (1) (A) ( A — 1) (A — 2 )


44.


Pyrolysis of Polycyclks

ocxio

691

0 C i b a Pink Β

0

Ο

Ο

Ο


C i b a Orange R

Flavon GC

coxoo Indanthrene Brown

of model compounds Here, P H = the probability of H2S formation, H = total number of available hydrogen atoms, S = number of sulfur atoms, and A = total number of atoms. Because oxygen is a major component a n d since the quantity of hydrogen available for forming H j S is affected by the amount of hydrogen that reacts with oxygen atoms to form H2O, a correction was applied to the total hydrogen content of the pyrolyzed material to account for the effect of water formation. The expression H = H T — O r was used for correction, where H T and O r are j 8


COAL SCIENCE

692


(XO

Hydrosulfurization Figure 2.

Process for sulfur

elimination

the total hydrogen a n d oxygen, respectively. It was assumed that about onehalf the oxygen would be involved i n other reactions such as carbon monoxide formation. T h e values obtained for P H , S can be seen i n Table I. Since sulfur distribution also involves fragmentation, the total sulfur elimination during pyrolysis should include a factor for volatile matter ( V ) . T h e percentage weight loss during pyrolysis was taken as the value for V . A n empirical ex pression representing a sulfur elimination factor ( P r ) can then be expressed as PT =

P , .

;

S X V

(2)

Values of Ρτ calculated from Equation 2 also appear i n Table I, a n d the rela tionship between Ρτ a n d experimental values of sulfur retained is presented graphically i n Figure 3. T h e data indicate that sulfur retention b y the char increases as the value of Ρτ decreases. Because it was difficult to examine gradual changes i n composition with the available model compounds, experiments were conducted using blends of starch, Bakélite, and hydrogenated creosote to obtain more data on the effect of oxygen a n d hydrogen atoms on sulfur distribution. C i b a Orange R was selected as the model compound for the blending experiments. T h e materials were blended b y mixing together the solid components i n a crucible.


44.


Pyrolysis of Polycyclic*

693

T h e composition of the various blends a n d the data resulting from the pyrolysis of the blends appear i n Table II. Sulfur volatilization by fragmenta tion is not directly related to the volatile matter of the blend as was true w i t h model compounds. F o r the same model compound ( C i b a Orange R ) i n the various blends this factor is considered constant and has been neglected.

Δ Ο


-WYOMING CHAR

J0.5

L

1.0

1.5

2.0

BLENDS MODEL COMPOUNDS

2.5

3.0

SULFUR ELIMINATION FACTOR. P

Figure 3. Table II.

Sulfur elimination

T

3.5

* 10

4.0

9

during

pyrolysis

Data on Pyrolysis of Blends of Model Compounds Compositio η, atom % H S Ο

Volatile Matter, wt %

Sulfur Retained, wt % Ρτ (10 )

Blend

c

Dye Starch Dye Starch 1.0 Dye 2.0 Bakélite 0.1 Dye 2.9 Bakélite 1.0 Dye 2.0 Naphthene 0.1 Dye 2.9 Naphthene RobenaCoaP

37.7

41.7

19.6

1.13

67

67

0.53

29.4

47.5

23.4

0.11

73

100

0.06

53.8

37.4

7.55

1.14

37

67

0.98

47.3

45.7

6.72

0.10

37

76

0.15

30.5

66.7

1.8S"

0.91

70

13

3.76

35.8

64.2

0.13

0.07

67

3

0.28

53.4

42.6

3.64

0.28

29

46

0.12

58.5

31.1

0.18

53

85

0.04

62.6

25.0

4.17

39

59

0.43

1.0 2.0 0.1 2.9

e

3738-118-1

Wyoming Char 3940-64-2 Ciba Orange R

10.2

8.34

s

" Ciba Orange R Free of pyritic sulfur h



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COAL SCIENCE

Values of Ρτ were therefore calculated from Equation 1 wherein P H , * = Ρτ for blended materials. T h e calculated values can be seen i n Table II, and the graphical relationship between Ρτ and sulfur retained i n the char can be seen in Figure 3. It is again apparent that sulfur retention increases as Ρτ decreases. T h e curve obtained from the data on the model compounds a n d blended materials can be used to predict the sulfur retention from the analytical data of the material pyrolyzed. T o test the usefulness of the curve depicted i n Figure 3, samples of Robena coal a n d a char from W y o m i n g coal were examined. Pyrite was re moved from the coal sample b y extraction with molten caustic (4) before pyrolysis to eliminate interference from inorganic sulfur. T h e W y o m i n g char was prepared from W y o m i n g coal containing no detectable pyrite. Calculating Ρτ from analytical data on these materials (Table II) using Equation 2 gave a value of 0.12 Χ 10" for Robena coal and 0.04 X 10"* for W y o m i n g char. T h e pyrolysis residues from these samples were analyzed, and sulfur retentions of 46% and 85%, respectively were observed. These values have been included in Figure 3 and appear to fit i n a general way the data obtained from the model compounds. 3

Summary A series of model compounds has been examined to determine the sulfur distribution that occurs during pyrolysis at 625°C. T h e data indicate that there is a correlation between sulfur volatilization and the elemental composi tion of the material pyrolyzed. It is significant that volatile matter and oxygen content are important factors i n addition to the concentration of hydrogen a n d sulfur. T h e molecular size and structure of model compounds appear to have little influence on sulfur retention. T h e equation correlating elemental compo sition and sulfur retention during pyrolysis appears to be applicable i n a general way to experimental data from Robena coal a n d char from W y o m i n g coal. Acknowledgment The assistance of M . Katz and Genevieve Dudgeon i n supplying analytical data for this investigation is gratefully acknowledged. Literature

Cited

(1) Conroy, J. S., Kinney, C. R., Murphy, D., Slysh, R. S., "Proceedings of the Third Conference on Carbon," Pergamon Press, London, 1959. (2) Edstrom, T., Lewis, J. C . , Tech. Rept. No. W A D D T R 61-72, Wright-Patterson Air Force Base (1962). (3) Given, P. H . , Wyss, W . F., British Coal Utilization Research Association, Monthly Bulletin Vol. X X V , No. 5, May 1961. (4) Masciantonio, P. X., "Abstracts of Papers," 148th Meeting, A C S , Aug.-Sept. 1964, p. 7L. R E C E I V E D October 5, 1964.


Pyrolysis of Polycyclic Compounds Containing Sulfur

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