Coking reaction in hydrogenation

Coking

reaction

Koji Ouchi,

Masataka

in hydrogenation

Makabe,

Faculty of Engineering, Hokkaido (Received 19 July 1983)

lsao Yoshimoto

University,

Sapporo,

and Hironori

ltoh

060 Japan

Asphaltene prepared from a Japanese coal (Akabira, 81.2 wt% C) and coal tar pitch were heat treated under nitrogen or hydrogen. Under nitrogen the initial thermal decomposition produced radicals which abstracted hydrogen from other molecules to stabilize and to produce smaller molecules and gas. The molecules from which hydrogen was abstracted as well as other radicals polycondensed to produce heavier solvent-insoluble fractions. Under hydrogen the radicals were stabilized by hydrogen gas to produce smaller molecules avoiding the production of a heavier fraction. The higher the hydrogen pressure, the smaller was the yield of heavier fraction and the larger the yield of lighter fraction. Higher temperature accelerated the production of the heavier fraction. Donor solvents could reduce the production of the heavier fraction. (Keywords: coking; hydrogenation; asphaltene, coal tar pitch)

Coking is a serious problem in hydroliquefaction of coal. Generally the decrease in conversion with increasing reaction time in hydrogenation is ascribed to coking. Whitehurst et al.’ pointed out that the unreacted coal and the char product could be distinguished by optical microscopy. They postulated some of the reaction schemes which probably take place. Shibaoka and Bodily’ have suggested that coking and hydrogenation reaction always occur simultaneously and that when the hydrogen supply is sufficient, the reaction tends toward hydrogenation, but when the hydrogen supply is deficient, the coking reaction is dominant. Such a scheme is generally accepted qualitatively, although quantitative experiments have not previously been carried out. This paper aims to elucidate quantitatively the coking phenomena which occur during the hydrogenation process. EXPERIMENTAL Samples

Asphaltene was prepared from Akabira coal (Japan, 81.2 wt% C, 6.0 wt% H, 11.1 wt% 0 by diff., daf). 250 g of Akabira coal ground to pass a 100 Tyler mesh (< 149 urn) were charged to a 5 dm3 rotary autoclave with 1 dm3 of tetralin and 10 stainless steel balls to effectuate stirring. The atmosphere was replaced with 10 MPa of hydrogen and the autoclave was heated to 400°C and soaked at this temperature for 1 h. After cooling, the product was extracted with 40 times its weight of benzene and n-hexane by shaking for 10 h at room temperature. The yield of asphaltene was 20 wt%. Appropriate analytical data and properties are shown in Table I, together with those of the benzene soluble fraction of coal tar pitch (77.1 wt% of the pitch) which was also used in this study. Treatment For hydrogenation

of the asphaltene,

0016-2361/84/040449~4$3.00 @ 1984 Butterworth & Co. (Publishers)

Ltd

3 g of asphaltene

and 0.3 g of red mud* plus 0.03 g of sulphur as catalyst were charged to a 50 cm3 autoclave with a magnetic stirrer and the atmosphere was replaced by 10 MPa of hydrogen or by a mixture of 3 MPa of hydrogen and 7 MPa of nitrogen. The mixture of hydrogen and nitrogen was used to avoid the simple effect of pressure, which influences conversion by altering the degree of vaporization of the volatile materia13. The autoclave was then heated to 390,420 or 450°C at about 12 K min-’ and maintained at this temperature for O-20 h. After cooling, the gas was analysed by gas chromatography and the product was extracted with benzene to separate the catalyst and the polycondensation products benzeneinsolubles (BI). The amount of polycondensation products was calculated by subtracting the catalyst weight from the weight of the residue after benzene extraction. The benzene-solubles were then extracted with n-hexane. For heat treatment and hydrogenation of coal tar pitch, 1 g of sample was charged to a glass tube with nearly the same diameter as the inner diameter of the autoclave. The glass tube was sealed under high vacuum and placed in a 50cm3 autoclave which was then heated to 430°C at a heating rate of 24 K min - ‘. After holding at this temperature for the desired period and cooling, the gas was analysed by gas chromatography. The yield of gas was calculated from the difference between the original weight and the product weight. The product was then extracted with pyridine, benzene and n-hexane. Hydrogenation of pitch in 2 g portions was carried out under various hydrogen partial pressures for 1 h at 460°C keeping the total initial pressure at 12 MPa by addition of nitrogen. The product was simply extracted with pyridine to obtain the pyridine-insoluble yield (PI). The effect of additives in reducing the amount of PI was examined under 3 MPa initial nitrogen pressure at 460°C * By-product oxide content

sludge from aluminium

ore processing,

with a high iron

FUEL, 1984, Vol 63, April

449

Coking reaction in hydrogenation: Table

1 Analytical

data and properties Ultimate

Asphaltene Coal tar pitch

K. Ouchi et al.

of asphaltene and benzene soluble fraction

of coal tar pitch

analysis (wt% daf) H

0 (diff .)

fa (1)

Hau/Ca (2)

;31

cal (4)

Raus (5)

Rnus 161

MW

C 85.7 92.3

7.0 4.9

7.3 2.8

0.65 0.95

0.74 0.66

4.1 1 .2

0.3 0.15

2.6 4.2

2.1 0.7

1115 320

(7)

(1 1Aromaticity, Ca/C (2) H/C ratio of hypothetical unsubstituted aromatic molecule (3) Number of structural unit (cluster) in a molecule (4) Substitution index of alkyl groups to aromatic nucleus (5) Number of aromatic rings in a structural unit (cluster) (6) Number of naphthenic rings in a structural unit (cluster) (7) Molecular weight measured by VP0 in pyridine solution

for 5 h, using 2 g of pitch and 1 or 0.2 g of additives. products were extracted with pyridine.

The

Determination of properties Molecular weights were determined by vapour-phase osmometry in pyridine solution at various sample concentrations with extrapolation of the values to zero concentration. ‘H n.m.r. spectra were obtained in Dpyridine solution at 2.5% concentration, using a 200 MHz apparatus, and structural analysis was carried out using modified Brown-Ladner equations3. The properties and analyses of the starting materials are shown in Table 1. RESULTS

AND

DISCUSSION

Asphaltene The variations in the yields of gas, oil, asphaltene and polycondensed benzene-insoluble fraction (BI) for the hydrogenation of asphaltene are given in Figure 1. Under 10 MPa of hydrogen the oil and gas yields increased with increasing temperature. At 450°C a small amount of BI was produced by prolonged reaction, but at 420 or 390°C no polycondensation reaction took place. Under 3 MPa of hydrogen plus 7 MPa nitrogen, the oil yield had maxima at 450 and 420°C. The oil yield at long reaction time appeared to decrease with increasing temperature.

laMPa

3MPaH,.77Pa

HI

N.

4wC

42UC

40.

_p_-v-m-

20.

dYX .~_-r__~.-;_*~:__-_-~r

“0 Reaction

Figure 1 asphaltene:

450

time

5

10

15

Yields of reaction products from the hydrogenation LL gas; 0, oil; 0, asphaltene; m, BI

FUEL, 1984, Vol 63, April

:

h

of

Gas production was significantly greater than that under 10 MPa hydrogen pressure at 420 or 450°C. The most notable feature was the remarkable increase in BI yield at each temperature. Even at 390°C slight BI formation was observed. All such results can be explained by a reaction scheme in which the initial step is thermal splitting of bridge linkages and the radicals produced abstract hydrogen atoms from hydrogen gas or other molecules to stabilize to oil, while the other radicals and the molecules from which hydrogen was abstracted polycondense with each other to form the BI fraction. Therefore when the hydrogen gas pressure is sufficient, the oil yield is large and the BI yield is small. On the other hand when the hydrogen pressure is inadequate, more of the radicals stabilize through hydrogen abstraction from other molecules and the oil yield decreases while BI formation increases significantly. In this case the total pressure including nitrogen is high (10 MPa initially), so the oil produced is difficult to vaporize and stays in the liquid phase. This again participates in the successive reactions, reducing the oil yield as the process is prolonged and therefore increasing the BI or gas yield. Coal tar pitch The results of heat treatment under vacuum, but in a closed system, are presented in Figure 2. They are similar to those for asphaltene under 3 MPa of hydrogen plus 7 MPa of nitrogen, namely, the gas and PI yields increased with increasing reaction time and the n-hexane soluble (HS) or pyridine soluble-benzene insoluble (PS-BI) yield passed through a maximum. Although the initial pressure was 0.01 Pa, the whole system was enclosed in a glass tube and thus the HS or PS-BI, once produced, participates again in a chain reaction and decreases by conversion to gas or PI. The reaction scheme is probably similar to that for asphaltene under low hydrogen partial pressure, but in this case all the hydrogen was supplied by that liberated from the polycondensation reactions of other molecules. The results of hydrogenation of the pitch are shown in Figure 3. As the hydrogen partial pressure increased, the gas and HS yields increased and the PI decreased. The increase in the yield of gas with increasing hydrogen pressure was opposite to the behaviour of asphaltene. The benzene soluble-n-hexane insoluble (BS-HI) and PS-BI yields showed maxima. As the initial pitch all dissolved in benzene, the production of PS-BI and PI means that polycondensation reactions were taking place. A higher hydrogen partial pressure can effectively prevent this polycondensation reaction as in the case of asphaltene. The structural analysis of the PS fraction is shown in

Coking

reaction

in hydrogenation:

K. Ouchi et al.

Phenol, acetone and diethylether. Phenol is not a donor reagent but has a high reactivity for addition; stabilization of the radicals would therefore by expected. However, this was not observed. Acetone and diethylether increased the PI yield slightly. These compounds thermally decompose at relatively low temperature, and some of the radicals produced probably alkylated the pitch aromatics, but others may have abstracted hydrogen from pitch molecules. This may have contributed to the slight increase in PI. Pyridine. This was expected to stabilize the radicals by addition reactions but was ineffective. disulphide

Carbon (chloroform).

trichloromethane

and

These compounds significantly increased the PI yield. They readily decompose thermally, producing S or Cl radicals. These abstracted hydrogen from the pitch molecules, and the molecules affected polycondensed with each other to produce a large amount of PI. In agreement with widely held views, it can be concluded that the compounds which have high hydrogen donor properties can effectively inhibit polycondensation reactions. Thus, in the hydrogenation of asphaltene or coal tar pitch, the coking reactions are important at >45O”C even under 10 MPa initial hydrogen pressure. The lower the hydrogen pressure, the more significant these reactions .

w23456789K Reaction Figure 2 Yields of reaction pitch under vacuum

products

time

.

. ,

.

.

h

from heat treated

coal tar

4. The aromaticity fa and the aromatic ring number per structural unit (cluster) R,,, decreased and the naphthenic ring number per structural unit (cluster) R,,, increased with increasing hydrogen partial pressure. This indicates that the aromatic rings were hydrogenated. Such hydrogenated aromatic rings probably decompose easily to hydrocarbon gases. This is the reason why at a higher hydrogen pressure the gas yield increased. It is significant that even at 12 MPa initial hydrogen pressure (the working pressure was nearly 20 MPa), the polycondensation (coking) reaction still takes place at such a high temperature (460°C). Figure

Effect of additives to prevent coking reactions

The results of this part of the work are presented in Figure 5. The additives used can be classified into different

301

I

I

I

.

sot

201

I

groups. Hydroaromatic compounds. Tetralin is known to be an

excellent hydrogen donor, and was indeed very effective in reducing the formation of PI. Alcohols. Methanol had no effect. The effectiveness of ethanol, isopropanol and set-butanol was nearly proportional to the hydrogen donor activity of these compounds, isopropanol having the highest activity and set-butanol the lowest. Ethanol decomposes to ethylene, isopropanol to propylene and set-butanol to isobutylene, so these reagents gave high yields of these gases.

30. 20. 100 0 Figure 3 tar pitch

2

6 8 10 4 H=Particd pressure

Yields of reaction

products

12 MPa

from hydrogenation

FUEL, 1984, Vol 63, April

of coal

451

Coking

reaction

in hydrogenation:

K. Ouch;

et al.

100

80

20

C Amount 5Ao

*

4.0.

1

3.0

0.5. 0

l.OO*

1

--v .

I

I

8

10

12

L 2

4

6

pressure

Structural indices of hydrogenated MW, molecular weight; f,, aromaticity

M Pa PS fraction of coal (CJC); H,,/C,,

H/C ratio of hypothetical unsubstituted aromatrc molecule; 0, rings in a structural substitution index; R,,,, number of aromatic rings in a structural unit (cluster); R,,,, number of naphthenic unit (cluster); n, number of structural units in a molecule

452

%

FUEL, 1984, Vol 63, April

Effect of additives on the production of PI during heat treatment: 1, tetralin; 2, isopropanol; 3, ethanol; 4. sec.butanol; 5, pyridine; 6, phenol; 7, methanol; 8, acetone; 9. diethyl ether; 10, carbon dlsulphide; 11, trichloromethane; 12, without additives

become, taking place even at 390°C under 3 MPa initial hydrogen partial pressure. As 3 MPa initial hydrogen pressure corresponds almost to the SRC working pressure, the operation must be conducted carefully when no donor solvent is used. Coking reactions can be suppressed if donor solvents are used.

I

Hz P art ial Figure 4

I

o-0-H

0

tar pitch.

of aditive

Figure 5

ACKNOWLEDGEMENT The authors wish to express their sincere thanks for the support of Sunshine Project, Ministry of International Trade and Industry, Japanese Government. REFERENCES Whitehurst, D. D., Mitchell, T. 0. and Farcasiu, M. ‘Coal Liquefaction’, Academic Press, New York, 1980, p, 207 Shibaoka, M., Russell, N. J. and Bodily, D. M. Fuel 1982,61, 201 Ouchi, K., Chikada, T. and Itoh, H. Fuel 1979,58, 37