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Hydrocracking Heavy Hydrocarbon Feedstocks: Aspects Of Catalysis Related To Feedstock Coking Tendency
545
S. Kaliaguine and A. Mahay (Editors), Catalysis on the Energy Scene 1984 Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands
HYDROCRACKING HEAVY HYDROCARBON FEEDSTOCKS:
ASPECTS
OF CATALYSIS RELATED TO FEEDSTOCK COKING TENDENCY
J.F. KRIZ and M. TERNAN Energy Research Laboratories, CANMET, E.M.R. Canada, Ottawa (Canada) K1A OG1
ABSTRACT When hydrocrackinq heavy hydrocarbon feedstocks, reactions occur that involve fragmentation of high molecular components resulting in the formation of usable hydrocarbons of lower molecular mass. The choice of operating conditions in maximizing the yields of desirable products is limited because of competing reactions leading to formation of coke. Some solids, when added to the feedstock, tend to enhance hydrogen transfer at their surface and to suppress coke formation. Their addition resembles the beneficial effect of increased hydrogen pressure. The effectiveness of catalysts inhibiting coke formation was observed by determining the coking tendency of a mixture containing the feedstock and the catalyst. These observations were complemented by measurements of catalyst coke deposits. Effective lowering of the feedstock coking tendency by choice or through developing appropriate catalysts can lead to improved hydrocracking technologies.
INTRODUCTION Hydrocracking is one of the recognized conversion routes for the production of liquid fuels from bitumens or heavy and residual oil feedstocks.
Original
feedstocks are upgraded by raising their overall hydrogen:carbon ratio while reducing the average relative molecular mass.
The high-boiling fraction of the
original material, which is useless as a liquid fuel, is thereby converted to fractions boiling within an acceptable range.
Although this conversion may not
be complete, coke formation which accompanies pyrolytic processes does not occur during hydrocracking. The objective in hydrocracking is to maximize conversion which drives the operating severity to its limits. sive the hydrocracking.
The higher the temperature, the more exten-
However, high temperatures create undesirable side
reactions involving mesophase or unstable cracked intermediates, which can lead to unacceptable coke deposits in the reactor.
Rapid hydrogen transfer necess-
ary to prevent these side reactions demands high hydrogen pressures.
As the
546
cost of equipment and operation lncreases with the pressure ratings, alternative potential methods for rapid hydrogen transfer include the use of solid catalysts or additives (ref.1). Effects of operating conditions and catalyst properties on product quality and ease of operation are of fundamental importance when developing hydrocracking processes.
For upgradinq difficult feedstocks, a good knowledge of the
catalyst role is particularly desirable.
In this paper an attempt is made to
describe the influence of selected catalysts on undesirable formation of coke. The effects are shown graphically using the feedstock coking tendency and catalyst coke deposits as measured quantities. EXPERIMENTAL ARRANGEMENT The equipment and operating procedures employed during these experiments have been discussed previously (ref.2). Catalysts slurried with Athabasca oil-sand bitumen and Boscan (Venezuela) heavy oil feedstocks were used. 1.
The feedstock properties are given in Table
A method, unavailable for full disclosure at present, consisted of measur-
ing the tendency of the feedstock to form coke under predefined operating conditions.
This phenomenon, referred to as feedstock coking tendency, was
observed for the feedstock alone and for slurries with a number of solid catalysts.
The catalysts are identified in Table 2.
The catalysts prepared for
the slurried mixtures were in powder form (minus 100 U.S. mesh) and their concentration was ordinarily maintained at 5 wt % of the feedstock. In another set of experiments, catalysts were tested in a fix-bed arrangement using Athabasca bitumen described elsewhere (ref.3).
After 30 h on
stream, the used catalysts were Soxhlet extracted with toluene, to remove adhering oil, and oven dried.
Catalyst carbon contents were measured using a
Perkin Elmer 240 analyzer. RESULTS AND DISCUSSION Feedstock Coking Tendency Coke formation is a reaction competing with the conversion of high-boiling fractions, such as the conversion of 525°C+ pitch to lower-boiling fractions. Both conversion and feedstock coking tendency depend on operating conditions. Presumably, the conversion is primarily affected by temperature and space velocity, while the feedstock coking tendency is affected by temperature and hydrogen pressure.
Phenomena related to the feedstock coking tendency are shown in
Fig. l(a,b,c).
Relative severity is a function of the system operating condi-
tions.
For example, it increases with increasing temperature and decreasing
pressure and the relationship can be established experimentally for a range of operating conditions.
547
lABLE 1 Feedstock properties Property
Athabasca bitumen
Relative density Pitch 524°C+ (975°F+) Conradson carbon residue Pentane insolubles loluene insolubles Ash, 700°C
(15/15°C) (wI. ~~) (wt.
~~)
(wI. (wt
~~) ~~)
(wt ~n
Major components Carbon Hydrogen Sulphur Oxygen Nitrogen
(wI. ~n
Metals
(ppm)
Iron Vanadium Nickel
Boscan heavy o i I
1.009 51.5 13.3 16 0.7 0.6
1.016 66.7 16.7
83.4 10.5 4.5 1.0 0.4
82.4 10.4 5.7 0.5 0.8
358 213 67
23 1174 114
2Z
0.1 0.2
TABLE 2 Catalyst specifications Catalyst
Sped ficat ions
A
-alumina ~onohydrate area 191 m /g
B
Baker Silica Gel treated in a solution of ammonium hydrox~de and calcined at 800°C for a 6 h, spec. surface area 110 m /g
C
FeS04/Alz03, containing 12 wI. ~~ Fe, spec. surface area 240 mZ/g
D
FeS04. HzO
E
FeS04/SiOz-Cabosi~,
surface area 90 m /g
calcined at 500°C for 6 h, spec. surface
containing 12 wt % Fe, spec.
F
CoO-Mo0 3/SiOZ-Cabosil, ~ontaining spec. surface area 90 m /g
8.0 wI. % Mo and 2.4 wt % Co,
G
NazMo0 4/AI Z03, con~aining surface area 196 m /g
H
CoO-Mo0 3/AI Z03, Harshaw Hl-400 catalyst c2ntaining 10 wt % Mo and 2.4 wI. ~~ Co, spec. surface area 203 m /g
12 wt % NazMo0 4, spec.
548
As shown in Fig. la, the coking tendencies of the two feedstocks vary widely with the parameter of severity and significantly differ from each other as well.
Also, it appears that this approach provides a procedure for sensitive
experimental determination of the feedstock coking tendency.
It should be
noted that in Fig. la the feedstock coking tendency refers to feedstocks alone (without catalysts).
(he differences between the feedstock properties noted in
fable 1 may allude to observations shown in Fig. la.
(he most important
properties to note include Conradson carbon residue, pentane insolubles, relative density and pitch content.
Since there are chemical aspects common to all
these properties they cannot be viewed as independent of each other.
On the
other hand, none of these properties alone provides acceptable correlation with the feedstock coking tendency (when data for a number of different feedstocks are examined).
Quantitative estimates of the tendency would thus have to be
based on functions of several feedstock properties. The presence of catalysts strongly affects the feedstock coking tendency. The catalytic property providing stability during hydrocracking is of special relevance.
It is a hydrogenation function in principle, which is experiment-
ally observed to accelerate the hydrogen transfer to unstable intermediates formed by cracking.
In the absence of sufficient hydrogen transfer these
intermediates tend to polymerize, eventually forming coke.
Since increasingly
rapid hydrogen transfer is required for stabilization at high temperatures, this catalytic function effectively substitutes the need for increased hyrogen pressure.
In Fig. lb, an indicator of desirable catalytic performance for a
chosen severity is the extent to which the feedstock coking tendency is lowered. When catalyzed by solids, the hydrogen transfer becomes a surface reaction, the rate of which generally depends on surface availability.
Thus catalyst
surface geometry and concentration in the slurry are also variables to consider.
With some catalysts the feedstock coking tendency can be further low-
ered when higher-concentration slurries are used.
Depending on feedstock prop-
erties, an appropriate incorporation of catalysts may be the only way to hydrocrack at acceptable pressures. Catalyst Functions Two catalytic functions are of key importance for hydrocracking.
The
cracking function can be provided by acidic sites of variously treated catalyst supports while the hydrogenation function can be provided by a number of metal sulphides.
Typical multifunctional hydrotreating catalysts, which contain
either molybdenum or tungsten, accelerate both cracking and hydrogenation. These catalysts are especially active in cleaving carbon bonds with heteroatoms such as sulphur or nitrogen.
Heteroatom removal also enhances the extent of
549
0.5
o
o
o>z UJ
A
o
0 Z
UJ
t-
o Z ::s:::
0.5
0
o
::s:::
0
o
0
s t-
RELATIVE SEVERITY
UJ UJ
U.
0.5
o 0.4
0.6
0.8
1.0
Fig. 1. Diagrams showing the relationships between the feedstock coking tendency and the relative severity, when hydrocracking: (a) Feedstocks (as indicated) without catalysts. (b) Slurries of Boscan heavy oil with catalysts A, B, C, D and E. (c) Slurries of Boscan heavy oil with catalysts F, G and H. The broken line indicates the heavy oil alone in Fig. la.
550
hydrocracking.
Most of the published information on catalytic hydrocracking of
distillate oils relates to the latter phenomena (ref.4,5).
Knowledge of cata-
lytic effects on feedstock coking tendency should be helpful when dealing with heavy or residual oils. Information on catalysts derived from Fig. lb and Ie is largely experimental at this stage.
Fundamental considerations meet with extreme complexity inher-
ent to the environment and severity in which the catalysts must function. Effects of chemical (electronic) and physical surface properties combine, so that the results reflect a combination of phenomena such as surface acidity, mass transfer and steric hindrance.
Fig. lb indicates that surfaces of some
solids including aluminas, silicas or their combinations, e.g. catalyst A, actually retrograde the coking tendency of the feedstock alone.
50me improve-
ment was found after thermal sintering of the fresh catalyst (catalyst B). Certain unsupported metal compounds (catalyst D) are not expected to show the cracking activity of acidic surfaces but are effective in stabilizing the thermally-cracked intermediates.
Both positive (catalyst E) and negative
(catalyst C) changes can result from application of supports. In a hydrocracking reaction system, coke deposits can be formed by polymerization of free radicals generated during thermal (non-catalytic) reactions. This coke formation is eliminated when hydrogen reacts with the free radicals thus preventing their reacting with each other.
The hydrogen source can be
either gaseous hydrogen or a hydrogen transfer agent.
The catalyst can have
the role of hydrogen transfer agent and also may be indirectly involved by generating hydrocarbon molecules (such as tetralins) which act as hydrogen transfer agents.
Multifunctional catalysts may respond to increasing severity
in a more complicated manner.
For example, a commercial Co-Mo catalyst (cata-
lyst H) does not lower the coking tendency well at low severity (Fig. Ie); however, it appears to be very effective at high severity.
The addition of an
alkali element (catalyst G) can reduce some of this behaviour, so that the feedstock coking tendency pattern shifts towards a more monotonous shape. The complicated pattern associated with the Co-Mo catalyst in Fig. Ie could result from changing selectivities within the network of cracking and hydrogenation reactions.
One of the functions of these catalysts is brought about by
their tendency to form active sites by generating sulphur vacancies.
At high
severity, changes may also be caused by reduction of acidity through poisoning.
It would appear that these catalysts should be used with caution, since
the risk of coke formation is high at low severity.
Again, changes caused by
application of different supports can be significant (catalyst F). The correlation of Fig. 2 suggests that catalyst properties influence both the coking tendency of the feedstock oil and the coke formation on the catalyst itself.
The role of catalysts can be explained in terms of a previously
551
described mechanism (ref. 6).
Carbonaceous molecules are adsorbed by electron
acceptor states (Lewis acid sites on alumina) and hydrogen is adsorbed by electron holes (associ at ed wit h eit her excess sulphur anions in molybdenum sulphide or cobalt cati ons in the sulphide phase).
lhe alumina (c at a l ynt B) in Fig. 2
would adsorb carbonaceous molecules with its electron acceptor states.
Product.
molecules would be desorbed in a hydrogen deficient condition, since alumina does not. cont.ain any elect.ron holes capable of adsorbing hydrogen.
lhis could
increase the feedstock coking t.endency as found experimentally. The Mo-cont.aining cat.alysts have electron holes t.hat are capable of adsorbing hydrogen, in which case t.he product. molecules can be hydrogenat.ed prior t.o desorption from t.he cat.al yst sur face.
Once hydrogenat.ed, the molecules would
not. compete for hydrogen in t.he liquid phase and would not affect. negat.ively t.he feedst.ock coking t.endency.
In addit.ion, these cat.alyst.s may provide ext.ra
hydrogen to unst.able species coming from t.he liquid phase.
As point.ed out.
earlier, their capacity to provide effective hydroqen transfer changes wit.h operating conditions.
0.5
o 0.5
CARBON
0.6
ON
CATALYST
0.7
0.8
mg C / m2
Fig. 2. Relationship between the feedstock coking tendency for the Boscan heavy oil and the catalyst carbon deposit for Athabasca bitumen when using similar operating conditions and the catalysts indicated.
552
CONCLUSIONS The feedstock coking tendency when hydrocracking heavy or residual oils was measured to test the effectiveness of different catalysts.
Influence of
catalysts on the feedstock coking tendency was found to depend stronqly on catalyst functionality.
The catalytic property providing stability through
effective hydrogen transfer appears to be particularly important.
Information
on the catalytic lowering of feedstock coking tendencies should lead to improved hydrocracking technologies. REFERENCES 1 2 3 4 5 6
J.F. Kriz, M. Ternan and J.M. Denis, J. Can. Pet. Technol., 22 (1983) 29-34. J.F. Kriz and M. Ternan, Development of a Simulated Catalyst Aging Technique, Amer. Chern, Soc. Div. Fuel Chem. Prep. 25(2), 146 (1980). M. Ternan and J.F. Kriz, in B. Delmon and G.F. Froment (Eds.), Catalyst Deactivation, Elsevier, Amsterdam, 1980, pp. 283-293. N. Chaudhary and D.N. Saraf, Ind. Eng. Chem. Prod. Res. Dev. 14 (1975) 74-83. B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, New York, 1979, pp. 393. M. Ternan, Can. J. Chem. Eng., 61 (1983) 133-147.
S. Kaliaguine and A. Mahay (Editors), Catalysis on the Energy Scene 1984 Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands
HYDROCRACKING HEAVY HYDROCARBON FEEDSTOCKS:
ASPECTS
OF CATALYSIS RELATED TO FEEDSTOCK COKING TENDENCY
J.F. KRIZ and M. TERNAN Energy Research Laboratories, CANMET, E.M.R. Canada, Ottawa (Canada) K1A OG1
ABSTRACT When hydrocrackinq heavy hydrocarbon feedstocks, reactions occur that involve fragmentation of high molecular components resulting in the formation of usable hydrocarbons of lower molecular mass. The choice of operating conditions in maximizing the yields of desirable products is limited because of competing reactions leading to formation of coke. Some solids, when added to the feedstock, tend to enhance hydrogen transfer at their surface and to suppress coke formation. Their addition resembles the beneficial effect of increased hydrogen pressure. The effectiveness of catalysts inhibiting coke formation was observed by determining the coking tendency of a mixture containing the feedstock and the catalyst. These observations were complemented by measurements of catalyst coke deposits. Effective lowering of the feedstock coking tendency by choice or through developing appropriate catalysts can lead to improved hydrocracking technologies.
INTRODUCTION Hydrocracking is one of the recognized conversion routes for the production of liquid fuels from bitumens or heavy and residual oil feedstocks.
Original
feedstocks are upgraded by raising their overall hydrogen:carbon ratio while reducing the average relative molecular mass.
The high-boiling fraction of the
original material, which is useless as a liquid fuel, is thereby converted to fractions boiling within an acceptable range.
Although this conversion may not
be complete, coke formation which accompanies pyrolytic processes does not occur during hydrocracking. The objective in hydrocracking is to maximize conversion which drives the operating severity to its limits. sive the hydrocracking.
The higher the temperature, the more exten-
However, high temperatures create undesirable side
reactions involving mesophase or unstable cracked intermediates, which can lead to unacceptable coke deposits in the reactor.
Rapid hydrogen transfer necess-
ary to prevent these side reactions demands high hydrogen pressures.
As the
546
cost of equipment and operation lncreases with the pressure ratings, alternative potential methods for rapid hydrogen transfer include the use of solid catalysts or additives (ref.1). Effects of operating conditions and catalyst properties on product quality and ease of operation are of fundamental importance when developing hydrocracking processes.
For upgradinq difficult feedstocks, a good knowledge of the
catalyst role is particularly desirable.
In this paper an attempt is made to
describe the influence of selected catalysts on undesirable formation of coke. The effects are shown graphically using the feedstock coking tendency and catalyst coke deposits as measured quantities. EXPERIMENTAL ARRANGEMENT The equipment and operating procedures employed during these experiments have been discussed previously (ref.2). Catalysts slurried with Athabasca oil-sand bitumen and Boscan (Venezuela) heavy oil feedstocks were used. 1.
The feedstock properties are given in Table
A method, unavailable for full disclosure at present, consisted of measur-
ing the tendency of the feedstock to form coke under predefined operating conditions.
This phenomenon, referred to as feedstock coking tendency, was
observed for the feedstock alone and for slurries with a number of solid catalysts.
The catalysts are identified in Table 2.
The catalysts prepared for
the slurried mixtures were in powder form (minus 100 U.S. mesh) and their concentration was ordinarily maintained at 5 wt % of the feedstock. In another set of experiments, catalysts were tested in a fix-bed arrangement using Athabasca bitumen described elsewhere (ref.3).
After 30 h on
stream, the used catalysts were Soxhlet extracted with toluene, to remove adhering oil, and oven dried.
Catalyst carbon contents were measured using a
Perkin Elmer 240 analyzer. RESULTS AND DISCUSSION Feedstock Coking Tendency Coke formation is a reaction competing with the conversion of high-boiling fractions, such as the conversion of 525°C+ pitch to lower-boiling fractions. Both conversion and feedstock coking tendency depend on operating conditions. Presumably, the conversion is primarily affected by temperature and space velocity, while the feedstock coking tendency is affected by temperature and hydrogen pressure.
Phenomena related to the feedstock coking tendency are shown in
Fig. l(a,b,c).
Relative severity is a function of the system operating condi-
tions.
For example, it increases with increasing temperature and decreasing
pressure and the relationship can be established experimentally for a range of operating conditions.
547
lABLE 1 Feedstock properties Property
Athabasca bitumen
Relative density Pitch 524°C+ (975°F+) Conradson carbon residue Pentane insolubles loluene insolubles Ash, 700°C
(15/15°C) (wI. ~~) (wt.
~~)
(wI. (wt
~~) ~~)
(wt ~n
Major components Carbon Hydrogen Sulphur Oxygen Nitrogen
(wI. ~n
Metals
(ppm)
Iron Vanadium Nickel
Boscan heavy o i I
1.009 51.5 13.3 16 0.7 0.6
1.016 66.7 16.7
83.4 10.5 4.5 1.0 0.4
82.4 10.4 5.7 0.5 0.8
358 213 67
23 1174 114
2Z
0.1 0.2
TABLE 2 Catalyst specifications Catalyst
Sped ficat ions
A
-alumina ~onohydrate area 191 m /g
B
Baker Silica Gel treated in a solution of ammonium hydrox~de and calcined at 800°C for a 6 h, spec. surface area 110 m /g
C
FeS04/Alz03, containing 12 wI. ~~ Fe, spec. surface area 240 mZ/g
D
FeS04. HzO
E
FeS04/SiOz-Cabosi~,
surface area 90 m /g
calcined at 500°C for 6 h, spec. surface
containing 12 wt % Fe, spec.
F
CoO-Mo0 3/SiOZ-Cabosil, ~ontaining spec. surface area 90 m /g
8.0 wI. % Mo and 2.4 wt % Co,
G
NazMo0 4/AI Z03, con~aining surface area 196 m /g
H
CoO-Mo0 3/AI Z03, Harshaw Hl-400 catalyst c2ntaining 10 wt % Mo and 2.4 wI. ~~ Co, spec. surface area 203 m /g
12 wt % NazMo0 4, spec.
548
As shown in Fig. la, the coking tendencies of the two feedstocks vary widely with the parameter of severity and significantly differ from each other as well.
Also, it appears that this approach provides a procedure for sensitive
experimental determination of the feedstock coking tendency.
It should be
noted that in Fig. la the feedstock coking tendency refers to feedstocks alone (without catalysts).
(he differences between the feedstock properties noted in
fable 1 may allude to observations shown in Fig. la.
(he most important
properties to note include Conradson carbon residue, pentane insolubles, relative density and pitch content.
Since there are chemical aspects common to all
these properties they cannot be viewed as independent of each other.
On the
other hand, none of these properties alone provides acceptable correlation with the feedstock coking tendency (when data for a number of different feedstocks are examined).
Quantitative estimates of the tendency would thus have to be
based on functions of several feedstock properties. The presence of catalysts strongly affects the feedstock coking tendency. The catalytic property providing stability during hydrocracking is of special relevance.
It is a hydrogenation function in principle, which is experiment-
ally observed to accelerate the hydrogen transfer to unstable intermediates formed by cracking.
In the absence of sufficient hydrogen transfer these
intermediates tend to polymerize, eventually forming coke.
Since increasingly
rapid hydrogen transfer is required for stabilization at high temperatures, this catalytic function effectively substitutes the need for increased hyrogen pressure.
In Fig. lb, an indicator of desirable catalytic performance for a
chosen severity is the extent to which the feedstock coking tendency is lowered. When catalyzed by solids, the hydrogen transfer becomes a surface reaction, the rate of which generally depends on surface availability.
Thus catalyst
surface geometry and concentration in the slurry are also variables to consider.
With some catalysts the feedstock coking tendency can be further low-
ered when higher-concentration slurries are used.
Depending on feedstock prop-
erties, an appropriate incorporation of catalysts may be the only way to hydrocrack at acceptable pressures. Catalyst Functions Two catalytic functions are of key importance for hydrocracking.
The
cracking function can be provided by acidic sites of variously treated catalyst supports while the hydrogenation function can be provided by a number of metal sulphides.
Typical multifunctional hydrotreating catalysts, which contain
either molybdenum or tungsten, accelerate both cracking and hydrogenation. These catalysts are especially active in cleaving carbon bonds with heteroatoms such as sulphur or nitrogen.
Heteroatom removal also enhances the extent of
549
0.5
o
o
o>z UJ
A
o
0 Z
UJ
t-
o Z ::s:::
0.5
0
o
::s:::
0
o
0
s t-
RELATIVE SEVERITY
UJ UJ
U.
0.5
o 0.4
0.6
0.8
1.0
Fig. 1. Diagrams showing the relationships between the feedstock coking tendency and the relative severity, when hydrocracking: (a) Feedstocks (as indicated) without catalysts. (b) Slurries of Boscan heavy oil with catalysts A, B, C, D and E. (c) Slurries of Boscan heavy oil with catalysts F, G and H. The broken line indicates the heavy oil alone in Fig. la.
550
hydrocracking.
Most of the published information on catalytic hydrocracking of
distillate oils relates to the latter phenomena (ref.4,5).
Knowledge of cata-
lytic effects on feedstock coking tendency should be helpful when dealing with heavy or residual oils. Information on catalysts derived from Fig. lb and Ie is largely experimental at this stage.
Fundamental considerations meet with extreme complexity inher-
ent to the environment and severity in which the catalysts must function. Effects of chemical (electronic) and physical surface properties combine, so that the results reflect a combination of phenomena such as surface acidity, mass transfer and steric hindrance.
Fig. lb indicates that surfaces of some
solids including aluminas, silicas or their combinations, e.g. catalyst A, actually retrograde the coking tendency of the feedstock alone.
50me improve-
ment was found after thermal sintering of the fresh catalyst (catalyst B). Certain unsupported metal compounds (catalyst D) are not expected to show the cracking activity of acidic surfaces but are effective in stabilizing the thermally-cracked intermediates.
Both positive (catalyst E) and negative
(catalyst C) changes can result from application of supports. In a hydrocracking reaction system, coke deposits can be formed by polymerization of free radicals generated during thermal (non-catalytic) reactions. This coke formation is eliminated when hydrogen reacts with the free radicals thus preventing their reacting with each other.
The hydrogen source can be
either gaseous hydrogen or a hydrogen transfer agent.
The catalyst can have
the role of hydrogen transfer agent and also may be indirectly involved by generating hydrocarbon molecules (such as tetralins) which act as hydrogen transfer agents.
Multifunctional catalysts may respond to increasing severity
in a more complicated manner.
For example, a commercial Co-Mo catalyst (cata-
lyst H) does not lower the coking tendency well at low severity (Fig. Ie); however, it appears to be very effective at high severity.
The addition of an
alkali element (catalyst G) can reduce some of this behaviour, so that the feedstock coking tendency pattern shifts towards a more monotonous shape. The complicated pattern associated with the Co-Mo catalyst in Fig. Ie could result from changing selectivities within the network of cracking and hydrogenation reactions.
One of the functions of these catalysts is brought about by
their tendency to form active sites by generating sulphur vacancies.
At high
severity, changes may also be caused by reduction of acidity through poisoning.
It would appear that these catalysts should be used with caution, since
the risk of coke formation is high at low severity.
Again, changes caused by
application of different supports can be significant (catalyst F). The correlation of Fig. 2 suggests that catalyst properties influence both the coking tendency of the feedstock oil and the coke formation on the catalyst itself.
The role of catalysts can be explained in terms of a previously
551
described mechanism (ref. 6).
Carbonaceous molecules are adsorbed by electron
acceptor states (Lewis acid sites on alumina) and hydrogen is adsorbed by electron holes (associ at ed wit h eit her excess sulphur anions in molybdenum sulphide or cobalt cati ons in the sulphide phase).
lhe alumina (c at a l ynt B) in Fig. 2
would adsorb carbonaceous molecules with its electron acceptor states.
Product.
molecules would be desorbed in a hydrogen deficient condition, since alumina does not. cont.ain any elect.ron holes capable of adsorbing hydrogen.
lhis could
increase the feedstock coking t.endency as found experimentally. The Mo-cont.aining cat.alysts have electron holes t.hat are capable of adsorbing hydrogen, in which case t.he product. molecules can be hydrogenat.ed prior t.o desorption from t.he cat.al yst sur face.
Once hydrogenat.ed, the molecules would
not. compete for hydrogen in t.he liquid phase and would not affect. negat.ively t.he feedst.ock coking t.endency.
In addit.ion, these cat.alyst.s may provide ext.ra
hydrogen to unst.able species coming from t.he liquid phase.
As point.ed out.
earlier, their capacity to provide effective hydroqen transfer changes wit.h operating conditions.
0.5
o 0.5
CARBON
0.6
ON
CATALYST
0.7
0.8
mg C / m2
Fig. 2. Relationship between the feedstock coking tendency for the Boscan heavy oil and the catalyst carbon deposit for Athabasca bitumen when using similar operating conditions and the catalysts indicated.
552
CONCLUSIONS The feedstock coking tendency when hydrocracking heavy or residual oils was measured to test the effectiveness of different catalysts.
Influence of
catalysts on the feedstock coking tendency was found to depend stronqly on catalyst functionality.
The catalytic property providing stability through
effective hydrogen transfer appears to be particularly important.
Information
on the catalytic lowering of feedstock coking tendencies should lead to improved hydrocracking technologies. REFERENCES 1 2 3 4 5 6
J.F. Kriz, M. Ternan and J.M. Denis, J. Can. Pet. Technol., 22 (1983) 29-34. J.F. Kriz and M. Ternan, Development of a Simulated Catalyst Aging Technique, Amer. Chern, Soc. Div. Fuel Chem. Prep. 25(2), 146 (1980). M. Ternan and J.F. Kriz, in B. Delmon and G.F. Froment (Eds.), Catalyst Deactivation, Elsevier, Amsterdam, 1980, pp. 283-293. N. Chaudhary and D.N. Saraf, Ind. Eng. Chem. Prod. Res. Dev. 14 (1975) 74-83. B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, New York, 1979, pp. 393. M. Ternan, Can. J. Chem. Eng., 61 (1983) 133-147.