CATALYTIC HYDROCRACKING Mechanism of Hydrocracking of Lou: Temperature Coal Tar S. A . Q A D E R
A N D
G . R .
HILL
Department of Fuels Engineering, University of Utah, Salt Lake City, Utah 841 12 The apparent kinetics of hydrocracking of a low temperature tar fraction over a dual-functional catalyst a t a constant pressure of 1500 p.s.i. are reported. The rate constants for gasoline formation, desulfurization, deoxygenation, and denitrogenation were consistent with the following equations: 'gasoline
'*"If", 'oxygen 'nitrogen
= 0.1567 x 106e-'7r600/RT hr.-'
= 0.2134 x 105e-148500/RT hr.-' = 0.3612 x 105e-13~600/RThr. = 0.4738 x 105e-15~900/RThr.-'
Hydrocracking of low temperature tar proceeded through a mechanism involving simultaneous and consecutive cracking, hydrogenation, and isomerization reactions. Cracking reactions involving the breakage of C-C, C-S, C-0, and C-N bonds on the surface of the dual-functional catalyst were rate-controlling.
HYDROCRACKING has been applied
in recent years for the refining of coal-derived liquid fuels (Qader and Hill, 1969). The work reported so far is very limited and the fundamental aspects of hydrocracking of tar are not well understood. Much of the earlier work was done on the study of product distributions (Katsobashvili and Elbert, 1966) or catalyst performance (Zielke et al., 1966), although some data were reported on the mechanism of hydrocracking of pure hydrocarbons (Flinn et al., 1960). In the present communication, the results of hydrocracking of a low temperature coal tar fraction over a catalyst containing sulfides of nickel and tungsten, supported on silica-alumina, are discussed. Experimental
Materials. Tar prepared from a high volatile bituminous coal from Utah by carbonization a t 550" C. in a laboratory oven was distilled t o separate light oil boiling up to 200°C. The fraction boiling above 200°C. was washed with 10% sodium hydroxide and 2 0 5 sulfuric acid to remove tar acids and bases, respectively. The neutral tar fraction was further hydrorefined over a cobalt-molybdate catalyst a t 375°C. and a pressure of 1500 p.s.i. to remove sulfur along with the residual oxygen and nitrogen present. A tar fraction boiling between 200" and 360°C. was also prepared from the same tar by distilling off the light oil and residue. The fraction boiling from 200°C. and above is designated as tar and the fraction boiling between 200" and 360°C. is designated as tar fraction in this study. The tar, the neutral tar, the refined tar, and the tar fraction were used as feed materials (Table I). The catalyst used was the same as reported by Qader and Hill (1969). Equipment and Experimental Procedure. The equipment and experimental procedure described by Qader and Hill (1969) were employed for the experimental work and to evaluate the products. The yields of liquid plus gaseous product from different coal tar fractions were almost 100' by volume of the feed material (Table 11). The kinetic data were obtained a t a constant pressure of 1500 p.s.i. 456
I & E C PROCESS D E S I G N A N D DEVELOPMENT
Product Analysis. The methods described by Qader and Hill (1969) were employed for product analysis. The tar acids were liberated from the alkali extract with 205 hydrochloric acid and sodium chloride. The tar acids were 9 V b pure, containing 4 5 oil. The saturates of the neutral tar fraction were separated by sulfonation of bulk samples.
Table I. Properties of Feed Materials
Tar Sp. gr., 25' C. Tar acids, vol. 7; Tar bases, vol. 5 Sulfur, wt. 5 Oxygen, wt. cl> Nitrogen, wt. SC Distillation data I.B.P., C. Pitch point, C. Residue, vol. 5 Analysis of neutral oil boiling up to 360'C., vol. 'C Saturates Olefins Aromatics
Neutral Tar
0.9946 30.0 3.0 0.6984 6.7 0.4828
Refined Tar
0.9827 Si1 Nil 0.6798 0.84 0.2738
Tar Fraction
0.9648 Si1 Si1 0.0082 XI1 0.01
0.9632 29.0 3 .5 0.6884 6.5 0.4782
200 360 30.0
200 360 28.0
200 365 25.0
200 360 Nil
32.0 19.0 49.0
32.0 19.0 49.0
43.0 7.0 50.0
32.0 19.0 49.0
Table II. Product Yields Temperature, 450" C.; pressure, 1500 p.s.i.
Feed, cc. Hydrogen consumption, g. Product yield, vol. 'C Gasoline Diesel oil Residual oil, +360" C. Gas Water, cc.
Tar
Neutral Tar
100.0 3.0
100.0 2.0
56.0 24.0 13.0 7.0 3.0
55.0 28.0 11.0 7.0 0.75
Refined Tar Tar Fraction 100.0 1.8
iOO.O 2.7
57.0 27.0 9.0
59.0 24.0 10.0 7.0 2.5
- .aI
Nil
The aromatics were extracted by a mixture of dimethyl formamide and n-heptane (Qader and Vaidyeswaran, 1966). The extracted aromatics were 97' c pure, containing 3 ' ~saturates and olefins. Results and Discussion
1
I
I
1
475
500
f/P m -
Product Distribution. I n the hydrocracking of tar, the yield of gasoline and gas increased with temperature while diesel oil decreased. The residue remained almost the same (Figure 1). Tar acids and bases were removed completely a t 450'C. and 1500-p.s.i. pressure along with most of the sulfur, oxygen, and nitrogen (Table 111). The ratio of C ,+ C,gases to C,+ C ? gases decreased with reaction temperature (Figure 2 ) . Catalytic cracking reactions essentially produce C and C,gases; thermal cracking reactions,
v u
IO
400
425
450 Temperature,
O C
Figure 2. Effect of temperature on gas composition Pressure, 1500 p.s.i.
,ol
1 GASOLINE 2 DIESEL OIL
3 RESI3UE
C1 and C2 gases. Thus, the decrease in the ratio of C.% + C, to C1+ C L gases indicates that thermal reactions occur to some extent as the reaction temperature increases. Figure 2 indicates that the hydrocracking of the heteromolecules of tar produces predominantly C,and C, gases relative to C i and C4 gases, as shown by the lower gas ratios obtained with tar relative to neutral tar, which in turn gave lower ratios than refined tar. In the hydrocracking of the tar fraction, tar fraction free from tar acids, neutral tar fraction, and the saturates, aromatics, and tar acids of the tar fraction (Table I V ) , the results suggest that the hydrocracking of the hydrocarbons of the tar which produces predominantly C j and C4 gases proceeds essentially through a carbonium ion mechanism characteristic of catalytic cracking, and the hydrocracking of the heteromolecules of tar which produces mainly C , and C, gases proceeds through a radical mechanism characteristic of thermal cracking (Greensfelder et al., 1949). This is further evidenced by the higher iso-normal ratios of butanes and gasoline obtained in the hydrocracking of refined and neutral tar, compared to the tar itself
50
01 400
425
450
500
475
T EM PE RAT U R E,
1
"C
Figure 1. Effect of temperature on product distribution Pressure, 1500 p.s.i.
Table Ill. Influence of Temperature on Product Distribution
Pressure, 1500 p.s.i.
Temperature, C. 400
Yield, vol. c c Gasoline Diesel oil T a r acids Tar bases Residue Gas (including losses) Sulfur, wt Oxygen, wt [ c hitrogen, wt. ( c Composition of gasoline, vol ( c Aromatics Iaphthenes Olefms Isoparafins n-Paraffins Diesel index of diesel oil Isobutane/ n-butane ( (
45.0 35.0 2.0 1.0 12.0 4.5 0.0489 0.0342 0.0924
425
51.0 29.0 1.0 0.5 12.0 6.5 0.0210 0.0184 0.0442
450
56.0 24.0
...
13.0 7.0 0.0140 0.0084 0.0321
475
61.0 20.0
500
64.0 16.0
... 11.5 n -
i .5
0.0135 Nil 0.0201
12.0 8.0 0.0136 Si1 0.0163
34.0 10.0 2.0 25.0 29.0
35.0 9.0 3.0 26.0 27.0
33.0 10.0 2.0 28.0 27.0
32.0 9.0 2.0 30.0 27.0
33.0 10.0 2 .0 30.0 25.0
40.0 1.0
37.5 1.25
34.0 1.45
31.0 1.51
28.0 1.75
VOL. 8 N O . 4 OCTOBER 1 9 6 9
457
Table IV. Gaseous Products from Hydrocracking of Different Feedstocks Temperature, 450" C.; pressure, 1500 p.s.i.
c, + c, c, + c2
Feed Tar fraction Tar fraction without tar acids Zeutral tar fraction Aromatics of tar fraction Saturates o! tar fraction T a r acids of tar fraction
18 80 2' 23 25
8
(Figure 3), since isomerization is characteristic of only catalytic cracking. The increase in the iso-normal ratios (Figure 3 ) also indicates that isomerization increases with cracking and the former follows the latter. The gas yield and the iso-normal ratio in butanes are indicative of the extent of cracking taking place, leading t o the formation of gasoline, and the gasoline is mainly formed by the cracking of the diesel oil, affecting its yield and quality (Table 111). The composition of gasoline obtained a t different temperatures was almost the same and the aromatics of the gasoline were formed mainly by the dealkylation of alkyl aromatics and hydrocracking of hydroaromatics and heterocyclic compounds. The gasoline yield increased a t different rates with pressure (Figure 4 ) : high in the pressure range 1000 to 1500 p.s.i., slowing down from 1300 to 2300 p.s.i., and increasing again a t higher pressures, as observed by Qader and Hill (1969). The residue decreased rapidly from 1000 to 1500 p.s.i. but the decrease was small a t higher pressures. On the other hand, the gas yield and the iso-normal ratio in butanes remained almost constant up t o a pressure of 1300 p s i . and increased a t higher pressure (Figure
5 ) , indicating that appreciable cracking reactions do not occur in the pressure range 1000 to 1300 p.s.i. Pressure does not seem to have a marked influence on cracking reactions in the range 1000 to 1300 p.s.i., but the increase in the yield of gasoline appears to be mainly due t o the suppression of coke-forming reactions. Figure 6 indicates that the hydrocracking of tar a t high temperatures and low pressures produces significant amounts of residue, and higher pressures suppress the reactions responsible for the formation of residue. In the pressure range 1500 to 2300 p.s.i., partial hydrogenation of aromatics to the corresponding hydroaromatics takes place, followed by the cracking of the latter, which increases the yield of gasoline and its aromatic content (Figures 7 to 9) as shown by Qader and Hill (1969). A t higher pressures,
f
40k
70-
60 J
W
Z
5 40
R
"v 20
10
1000
1500
2500
2000
300
P R E S S U R E , PSI
Figure 4. Effect of pressure on product distribution
0'5 1.3
1 I
2 Neutral Tar 3. Refined T a r I
5c5c
I
I
400 'C
10
C -
500'C 450'C 40Q'C
1
n 1 425 450 475 500 5Q
0.7 400
3
Temperature,
OC
Figure 3. Effect of temperature on gasoline composition Pressure, 1500 p.5.i.
458
I&EC PROCESS DESIGN A N D DEVELOPMENT
1000
"I500
2530
2030 PRESSd?:,
30c0
PSI
Figure 5. Effect of pressure on product distribution
8o
2'
3
NAPHTHENES AROMATICS I S0PA R A FFI N S
5
1250 1500 Pressure, psi
1000
1750
ZLLi
n - PARAFFINS
I
Figure 6. Effect of reaction conditions on the formation of residue
1
7n
GASOLIhE
1000
2000 PRESSURE,
1500
3000
2500 PSI
Figure 9. Effect of pressure on yield of gasoline and hydrocarbon types Temperature, 500" C.
'000
15CC
2500
2000
3000
PQESSURE, D s t
aromatics are hydrogenated to the corresponding naphthenes and the naphthenes formed crack easily, increasing the yield of gasoline and its naphthenic content (Qader and Hill, 1969). High-aromatic gasolines were obtained in the pressure range 1750 to 2500 p.s.i. Yield of 77% of gasoline was maximum a t 500°C. and 3000-p.s.i. pressure from tar, but the highest quality product containing 60% aromatics and 13% isoparaffins was formed a t 450" C. and 2000-p.s.i. pressure (Table V). Kinetics and Mechanism
Figure 7. Effect of pressure on the yield of gasoline and hydrocarbon types Temperature, 400" C.
The results of hydrocracking of tar were analyzed by the first-order rate Equations 1 to 4, assuming first-order kinetics. Qader et al. (1968) showed that the hydrocracking reactions of tar are all first-order in the temperature range 400" to 500" C. a t 1500 p.s.i.
d (gasoline) = h, (tar) dt
(1)
-d (sulfur) = h, (sulfur) dt
->
40t
Y
-d (oxygen) = h, (oxygen) dt
(3)
-d (nitrogen) = h, (nitrogen) dt
(4)
Table V. Yield and Composition of Gasoline
Gasoline
01 1000
1500
2000 PRESSURE,
2500
3000
PSI
Figure 8. Effect of pressure on the yield of gasoline and hydrocarbon types
Yield, vol. ' 5 Composition, vol. Aromatics Isoparafins Olefins Naphthenes n-Paraffins
4 5 ~ C. " ami 9000 p.s.i.
500" C. and 3000 p.s.i.
60.0
77.0
60.0 13.0 1.o 12.0 14.0
26.0 26.0 2.0 34.0 12.0
'C
Temperature, 450" C.
VOL. 8 NO. 4 OCTOBER 1969
459
where h,, h,, h,, and h, are first-order rate constants for gasoline formation from tar, desulfurization, deoxygenation, and denitrogenation, respectively. There was no change in hydrogen concentration in the system during the course of the reaction, since the hydrogen pressure was maintained constant throughout a t 1500 p.s.i. Hydrogen atoms might have been involved in the reaction, but their concentration constitutes one of the constant factors in the rate constant term and does not show up in the rate equation. Figure 10 illustrates Arrhenius temperature dependence of first-order rate constants for hydrocracking experiments with tar a t 400" to 500°C. and 1500 p.s.i. The rate constants which represent the rates of hydrocracking are consistent with Equations 5 to 8.
I n gasoline formation from tar (Figure 11) a t 450°C. the hydrogen concentration has a marked effect on the reaction rate in the pressure ranges 500 to 1500 and 2500 to 3000 p s i . , but not in the range 1500 to 2500 p.s.i. The order with respect to hydrogen is not zero throughout. The following values of enthalpies and entropies of activation were calculated by the' Eyring equation, plotting log h / T us. 1 / T (Figure 1 2 ) . A@
= 16,200 cal./mole,
AS:
AH?
= 12,200 cal./mole, = 11,600 cal./mole, = 14,900 cal./mole,
AS? = -44.9 e.u. AS? = -45.0 e.u. AS? = -45.9 e.u.
AH? AH?
= -43.5 e.u.
I n general, heterogeneous catalytic processes involve
h, h, h, h,
0.1567 x = 0.2134 x = 0.3612 x = 0.4738 x =
106e-17Mhi " hr.-' 10' eC145uu hr.-' 105e-1J6"" " hr.-' 1 0 a e ~ 1 5" y uhr.-' u
(5) (6) (7) (8)
0.1 I
- 0O. 02 I Y Q-
0.4
-0
i
I Nitrogen 2. Oxygen 3. Sulfur 4. Gasoline
-,,I
\LI
I
-1 .ol
1.0
I
I
1.1
1.2
1/T x10
I
3
1
1.3
1.5
1.4
(1) diffusion of the reactant molecules from the bulk phase to the catalyst surface, (2) adsorption of the reactants
on the catalyst surface, (3) surface reactions of the adsorbed molecules to form products, (4) desorption of the products, and (5) diffusion of the desorbed products from the catalyst surface to the bulk phase. The enthalpies of activation obtained in this work indicated that chemical reactions but not physical processes control the reaction rate in the hydrocracking of low temperature tar, which excludes steps 1, 2, 4, and 5 from limiting reaction rate. The product distribution data indicated that the hydrocracking of tar proceeds through a reaction mechanism involving simultaneous and consecutive cracking, hydrogenation, and isomerization reactions on the surface of the dual-functional catalyst, leading to the formation of various products. The dual-functional catalyst contains two types of active sites: the acidic sites of silica which promote cracking reactions and the sites of cobaltmolybdate which promote hydrogenation reactions. The product distribution data have shown that cracking, isomerization, and hydrogenation are the principal reactions during the hydrocracking of tar. The cracking and isomerization reactions occur on the acidic sites of silica and the hydrogenation reactions on the sites of cobaltmolybdate. I t was inferred from the product distribution
Figure 10. Arrhenius plot for gasoline formation, desulfurization, deoxygenation, a n d denitrogenation
0.7I
1
I
I
k 0.5 -
-3.2
0.6
u
t-
0
+ O
c 0 C
*
0.4
0
-
-3.4
0)
-
0
o'2r
0.3
u
\
y
-
i i I. Nitrogen
2. Oxygen 3. Sulfur
0.1
01 500
I
I
IOQO
I
I500 2000 Pressure, psi
I
2500
! I
3000
Figure 1 1. Effect of hydrogen pressure on rate constant
for gasoline formation 460
I & E C PROCESS D E S I G N A N D DEVELOPMENT
-4
o
10
11
12 13 I/T x 103
14
15
Figure 12. Eyring plot for gasoline formation, desulfurization, deoxygenation, a n d denitrogenation
data that the cracking of hydrocarbon molecules of tar proceeds mainly through a carbonium ion mechanism, wherein they either lose a hydride ion or add a proton to form a carbonium ion and the heteromolecules by a radical mechanism characteristic of thermal cracking as proposed by Greensfelder et al. (1949). Thus, the cracking of hydrocarbon molecules of tar can be represented by steps i to vi,
CdHa + H + (acidic site) + CdH,, AR
+ H . (acidic site)
+
+
CeHk+ CIH2/- 2
-
(ii)
A + R - (carbonium ion) (iii) AN -+AN- + H LR1 + R,H H - + H - + H1 H - + R--+R H
(iv) (VI
(Vi)
where AR and A-N represent alkyl aromatic and hydroaromatic compounds, respectively, A represents fused aromatic rings, N represents fused naphthenic rings, and R , R , , and R Prepresent alkyl chains. Steps i to iv represent the formation of carbonium ions followed by the splitting of C-C bonds of the latter to form products. The cracking of carbonium ions is a chain reaction and the nature of the products depends upon the extent to which the chain reactions proceed. Steps v and vi represent the reaction of the hydride ions formed with the hydrogen ions of the catalyst and the carbonium ions formed in the reaction, respectively. The small carbonium ions formed may also react with parent molecules or other carbonium ions. The olefins formed will undergo skeletal rearrangement and hydrogenation, as can be represented by steps vii to ix.
C-0
bonds, and steps xiii to xviii represent hydrogenation reactions. These are the principal surface reactions that can occur and indicate that the over-all kinetics observed resulted from a sequence of this type. The three principal surface reactions during the catalytic hydrocracking of low temperature tar are the cracking reactions involving the rupture of C-C, C-S, C-0, and C--N bonds, isomerization reactions involving the skeletal rearrangement of olefins, and the hydrogenation reaction of the olefinic species. The product distribution data indicated that hydrogenation was complete under all reaction conditions, since the products contained negligible amounts of olefins. I n the presence of an excess of hydrogen, the reactions between olefinic species and hydrogen occur freely and rapidly. Steps viii, ix, and xiii to xviii are thus expected to be fast and cannot control the reaction rate. The effect of hydrogen pressure on the rate constant (Figure 11) indicates the occurrence of a reaction consisting of a primary step in which hydrogen is not involved and which is rate-controlling and a faster subsequent step involving hydrogen which may limit the rate only when the hydrogen concentration is very low. Therefore, the cracking reactions (steps i to iv and x to xii) and the isomerization reaction (step vii) might control the rate. This is in conformity with the results of Weisz and Prater (1957) and Keulemans and Voge (1959), who found that reactions occurring on the acidic sites of the dual-functional catalyst were rate-determining. The iso-normal ratio in the butanes indicated that more branched isomers than can be predicted by thermodynamic equilibrium were formed during the hydrocracking of low temperature tar. This type of isomerization can occur only if the olefinic fragments isomerize very rapidly and leave the catalyst surface without appreciable readsorption before attaining equilibrium conditions. Therefore, the isomerization reactions are expected to be very fast and cannot control the reaction rate. Hence, the cracking reactions involving the breakage of chemical bonds on the surface of the dual-functional catalyst must be ratelimiting in the hydrocracking of low temperature tar. Literature Cited
Isomerization
C,HzI
C ,Hi7 C,Hel + H2+ CyH2.x - 2 C,Ha + He C,Hz, 2
-
i~
(vii) (viii) (ix)
The removal of S,0, and N from tar proceeds through an accelerated radical mechanism characteristic of thermal cracking reactions, as can be represented by steps x to xviii,
RHeC - SR' + RHrC' + R'S' RlHaC - ? J R ; - +RIHgC' + R{N' ReHgC - OR; R2HzC' + RiO' R'S' + H?- R'H + H,S R;N' + He R;H + KH? RIO' + He RIH + HjO RHZC' + H,+ RIH RiHJC' + H, RAH RZHQC' + Hz+ RjH +
-
+
+
(XI
(Xi) (xii) (xiii) (xiv) (xv) (xvi) (xvii)
Flinn, R. A., Larson, O.A., Beuther, H., Ind. Eng. Chem. 52, 152 (1960). Greensfelder, B. S., Voge, H. H., Good, G. M., I n d . Eng. Chem. 41, 2573 (1949). Katsobashvili, Ya. R., Elbert, E . I., Coke and Chemistry USSR 1, 41 (1966). Keulemans, A. I. M., Voge, H. H., J . Phys. Chem. 63, 476 (1969). Qader, S. A . , Hill, G. R., IND.ENG.CHEM.PROC. DESIGN 8,450 (1969). DEVELOP. Qader, S. A., Vaidyeswaran, R., Indian J . Technol. 4, 128 (1966). Qader, S.A., Wiser, W. H., Hill, G. R., 155th Meeting, ACS, Division of Fuel Chemistry, Preprints, 12, No. 2, 28 (1968). Weisz, P. B., Prater, C. D., Aduan. Catalysis 9, 575 (1957). Zielke, C. W., Struck. R. T., Evans, J. M., Costanza, c . P., Gorin, E., IND. ENG. CHEM. PROCESS DESIGN DEVELOP. 5, 151, 158 (1966).
(xviii)
RECEIVED for review July 25, 1968 ACCEPTED May 24, 1969
where R represents a hydrocarbon chain or hydrogen atom. Steps x to xii represent breakage of C-S, C-X, and
Research work sponsored by the U. S. Office of Coal Research and the University of Utah. VOL. 8 NO. 4 OCTOBER 1969
461
