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High pressure hydrocracking of vacuum gas oil to middle distillates
Physica 139 & 140B (1986) 725-728 North-Holland, Amsterdam
HIGH PRESSURE H Y D R O C R A C K I N G OF VACUUM GAS OIL TO MIDDLE DISTILLATES
C.R. LAHIRI and Dipa BISWAS Department of Chemical Technology, Calcutta University, Calcutta 700 009, India
Hydrocracking of heavier petroleum fractions into lighter ones is of increasing importance today to meet the huge demand, particularly for gasoline and middle distillates. Much work on hydrocracking of a gas oil range feed stock to mainly gasoline using modified zeolite catalyst-base exchanged with metals (namely Ni, Pd, Mo, etc.) has been reported. In India, however, present demand is for a maximum amount of middle distillate. The present investigation was therefore aimed to maximize the yield of middle distillate (140-270°C boiling range) by hydrocracking a vacuum gas oil (365-450°C boiling range) fraction from an Indian Refinery at high hydrogen pressure and temperature. A zeolite catalyst-base exchanged with 4.5% Ni was chosen for the reaction. A high pressure batch reactor with a rocking arrangement was used for the study. No pretreatment of the feed stock for sulphur removal applied as the total sulphur in the feed was less than 2%. The process variables studied for the maximum yield of the middle distillate were temperature 300-450°C, pressure 100-200 bar and residence period 1-3 h at the feed to catalyst ratio of 9.3 (wt/wt). The optimum conditions for the maximum yield of 36% middle distillate of the product were: temperature 400°C, pressure 34.5 bar (initially) and residence period 2 h. A carbon balance of 90-92% was found for each run.
1. Introduction
Middle distillate is the vital cut of the crude oil, in the light of Indian market demand. This leads India to import this fraction which will be 26% of the total demand at the end of VIIth plan period. The Indian economy, therefore, dictates the conversion of undesirable heavier fractions into desirable lighter ones.
temperature between 450 and 475°C decreases the activity of the catalyst. Low severity hydrocracking was reported by Pollitzer [4], Ward [9] and others. In a patent [10] a new two-stage process was reported using a Gr VIII metal deposited on zeolite in the first stage and nickel catalyst in the second stage.
3. Present work 2. Literature review
A thorough survey of the literature reveals that the product pattern controls the choice of catalyst and process parameters. A dual functional catalyst having both hydrogenation and cracking activities is needed. Flinn and Larson [1] studied the effect of different catalysts on hydrocracking and found that 2% NiO o n S I O 2 - A 1 2 0 3 , 10% NiS on SIO2-A120 3 and a cobalt-molybdena alumina catalyst were the best. Recent studies on hydrocracking by Liden and May [2] Conway [3], Abiko et al. [5] and others [6-8, 11], were based on base-exchanged zeolite catalysts, which were more active than an amorphous silica-alumina catalyst. Abidova also found that an increase in
The present investigation studied the effect of different process parameters, namely temperature, pressure and residence period on the hydrocracking reaction of vacuum gas oil (boiling range 365-450°C, S-content <2%, N-content <200 ppm.) aimed to maximize the middle distillate (140-270°C cut).
4. Experimental
4. i. Apparatus The major apparatus in which the hydrocracking reaction had been carried out was a high pressure reactor. The auxiliary apparatus con-
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.R. Lahiri and D. Biswas / Hydrocracking o f a vacuum gas oil
726
sisted of (i) an ASTM distillation apparatus for product analysis, and (ii) a bomb calorimeter for determination of the sulfur content. The details of the high pressure reactor are given in fig. 1. The reactor assembly is shown in fig. 2. The high pressure reactor, with a capacity of 1 litre, was of the rocking type (40 oscillations per minute). The external diameter was 100 mm and the internal diameter 60 mm. It was designed to withstand a pressure up to 300 bar and a temperature up to 500°C. Temperature control was by a variac. Closure of the autoclave was of the screw type with a plug. A silver gasket was used for the
~3QO L ~8o--
_j'[
PRESSURE GAUGE ~ , ~ REACTOR y
~
T~ OUTLET x
INLET
j ~ ~ /~D/CATOR
Fig. 2. Reactor assembly.
r 8NOS. M12 Sq.
.VER SKET
pressure-tight joints. The materials of construction of the reactor and other parts were: Reactor bomb: 316 stainless steel Bolts: 316 stainless steel Seal ring: High carbon steel Thermocouple well: 316 stainless steel Outside casing: mild steel.
4.2. Preparation of catalysts Two solutions were prepared. Solution A contained 465 ml of water glass (0.20 g SiOz/ml. ) and 900 ml of water and solution B contained 67 ml of HC1 (4.0 N), 140 ml of A12(504) 3 solution (0.50 g AI203/ml) and 400 ml of water. After cooling, the solutions t o about 5°C, solution A was added quickly to solution B and stirred. The gel formed was aged for 48 h at 60°C. The dried gel was then base-exchanged with NH 4 ions, followed by calcination at 450-500°C for 2 h. The calcined mass was then base-exchanged with Ni ions by impregnation to produce finally a catalyst containing 5 . 8 w t % of NiO. The mass was then calcined at 450-500°C for 3 h.
4.3. Process
SCOle
In mm
Fig. 1. Details of reactor bomb.
The reactor bomb was charged with 250 ml of gas oil and 25 g of catalyst (feed : catalyst - 9.3 wt/ wt). The reactor was closed and placed in position. Hydrogen gas was introduced (see fig. 2) until the desired pressure was attained. The temperature was then gradually increased by adjusting
C.R. Lahiri and D. Biswas / Hydrocracking o f a vacuum gas oil
the variac. At the desired temperature, rocking was allowed to start. The reaction was then allowed to take place for a given residence period. The gas was then released and the liquid product was collected, weighed and subjected to ASTM distillation. The distillation yield was then converted into a true boiling point vs. distillation curve from which the required percentage yield of middle distillate was found.
5. Results and discussion
5.1. Effect of temperature The temperature effect on hydrocracking of a vacuum gas oil to a middle distillate is given in
727
table I. The results of the table reveal that a temperature below 300°C is not favourable for hydrocracking and that if the temperature is increased to 400°C the middle distillate yield is increased. This can be attributed to the rate enhancement of the cracking and hydrogenation reaction. Also, the endothermic cracking reaction, which actually splits the higher molecules into lower ones via the olefin intermediates, is favourable at higher temperatures. However, the hydrogenation reaction of the olefinic and aromatic compounds becomes exothermic above 500°C and 300°C, respectively ( A F ° > 0). This effect is however compensated by the use of hydrogen under pressure which makes AF ° negative at high temperature, according to the equation AF °= - 3 R T log P. Thus, a high temperature together
Table I Effect of temperature: Feedstock amount, 250 ml; catalyst amount, 25 g; initial pressure, 34.5 bar; residence time, 2 h; catalyst type, zeolite-base nickel catalyst Reaction temperature Items
300°C
350°C
400°C
(1) Reaction pressure (bar) (2) Products: (a) % Yield of liquid product, by volume (b) Yield of light Naphtha (C5-90°C cut), by volume (c) % Yield of Naphtha (90-140°C), by volume (d) % Yield of middle distillate (140-270°C cut), by volume
48.2
62.0
75.8
78
84
72
1.5
6.5
13
2
9
17.5
2
25
36
Table II Effect of pressure: Feedstock amount, 250ml; catalyst amount, 25g; initial temperature, 400°C; residence time, 2 h; catalyst type, zeolite-base nickel catalyst Initial pressure Items
34.5 bar 51.7 bar 86.2 bar
(1) Reaction pressure (bar) (2) Products: (a) % Yield of liquid product, by volume (b) % Yield of light Naphtha (C5-90°C cut), by volume (c) % Yield of Naphtha (90-140°C cut), by volume (d) % Yield of middle distillate (140-270°C cut), by volume
75.8
96.5
72
68
13
9.5
162.0
64 12.5
17.5
14
16.5
36
33.5
26
728
C.R. Lahiri and D. Biswas / Hydrocracking of a vacuum gas oil
Table III Effect of residence period: Feedstock amount, 250ml; catalyst amount, 25 g;initial pressure, 34.5 bar; reaction temperature, 400°C; catalyst type, zeolite-base nickel catalyst Residence period Items
1h
2h
3h
(1) Reaction pressure (bar) (2) Products (a) % Yield of liquid product by volume (b) % Yield of light Naphtha (C~-90°C cut), by volume (c) % Yield of Naphtha (90-140°C cut), by volume (d) % Yield of middle distillate (140-270°C), by volume
89.6
75.8
I20.6
64
72
62
13.5
13
17.5
16.5
17.5
16.5
30.5
36
26.5
with a high pressure favours the hydrocracking reactions.
5.2. Effect of pressure The effect of pressure on the hydrocracking of a vacuum gas oil to a middle distillate is given in table II, which represents the percent yield of middle distillate (140-270°C) versus pressure. Table II also shows that an increase in the hydrogen pressure has an inverse effect both on the yield of the liquid product and of the middle distillate, while for gasoline the yield at first decreases, but then increases with increasing pressure. This may be due to the fact that lower pressures favour the cracking process from the equilibrium point of view. Hence, the splitting of larger molecules is greatest at lower pressures, thereby producing maximum yields of middle distillate.
5.3. Effect of residence period The effect of the residence period on the hydrocracking of a vacuum gas oil to a middle distillate is summarized in table III. It is found that the 2 h residence period gives the highest yield of liquid product and of middle distillate. This is probably due to the fact that a residence period less than 2 h is not sufficient to complete the reaction, while a longer residence period
(greater than 2 h) results in undesirable side reactions, such as partial polymerization and condensation, thus producing a smaller amount of middle distillate. 6. Conclusions
The above results show that the conditions for a maximum yield of middle distillate (36%) from the hydrocracking of a vacuum gas oil (from Haldia Refinery) are obtained in a residence period of 2 h at a temperature of 400°C, and at a pressure of 75.8 bar. Higher temperatures favour more coke deposition. Lower pressures give rise to a larger yield of middle distillate. References [1] R.A. Flinn and O.A. Larson, lndust. Eng. Chem. 4 (1957) 49. [2] T.M. Liden and J.E. May, U.S. 3,816,297 (1974). [31 3.E. Conway, Ger. often 2,341,854 (1974). [4] E.L. Pollitzer, Brit 1,364,470 (1974). [5] S. Abiko, K. Yohrihiro, T. Sugihar and M. Hanawa, Ger. often W, 411,351 (1975). [6] R.C, Hansford, U.S. 3,836,454 (1975). [71 D.A. Young, U.S. 3,860,533 (1975). [8] J.W. Ward, U.S. 3,838,040 (1975). [9] J.W. Ward, U.S. 3,867,277 (1975). [10] Texaco Development Corporation, Ft. Demande 2,240,947 (1975). [11] M.M. Abidova and 13. Abdazimov, Khim Zh. (1976).
HIGH PRESSURE H Y D R O C R A C K I N G OF VACUUM GAS OIL TO MIDDLE DISTILLATES
C.R. LAHIRI and Dipa BISWAS Department of Chemical Technology, Calcutta University, Calcutta 700 009, India
Hydrocracking of heavier petroleum fractions into lighter ones is of increasing importance today to meet the huge demand, particularly for gasoline and middle distillates. Much work on hydrocracking of a gas oil range feed stock to mainly gasoline using modified zeolite catalyst-base exchanged with metals (namely Ni, Pd, Mo, etc.) has been reported. In India, however, present demand is for a maximum amount of middle distillate. The present investigation was therefore aimed to maximize the yield of middle distillate (140-270°C boiling range) by hydrocracking a vacuum gas oil (365-450°C boiling range) fraction from an Indian Refinery at high hydrogen pressure and temperature. A zeolite catalyst-base exchanged with 4.5% Ni was chosen for the reaction. A high pressure batch reactor with a rocking arrangement was used for the study. No pretreatment of the feed stock for sulphur removal applied as the total sulphur in the feed was less than 2%. The process variables studied for the maximum yield of the middle distillate were temperature 300-450°C, pressure 100-200 bar and residence period 1-3 h at the feed to catalyst ratio of 9.3 (wt/wt). The optimum conditions for the maximum yield of 36% middle distillate of the product were: temperature 400°C, pressure 34.5 bar (initially) and residence period 2 h. A carbon balance of 90-92% was found for each run.
1. Introduction
Middle distillate is the vital cut of the crude oil, in the light of Indian market demand. This leads India to import this fraction which will be 26% of the total demand at the end of VIIth plan period. The Indian economy, therefore, dictates the conversion of undesirable heavier fractions into desirable lighter ones.
temperature between 450 and 475°C decreases the activity of the catalyst. Low severity hydrocracking was reported by Pollitzer [4], Ward [9] and others. In a patent [10] a new two-stage process was reported using a Gr VIII metal deposited on zeolite in the first stage and nickel catalyst in the second stage.
3. Present work 2. Literature review
A thorough survey of the literature reveals that the product pattern controls the choice of catalyst and process parameters. A dual functional catalyst having both hydrogenation and cracking activities is needed. Flinn and Larson [1] studied the effect of different catalysts on hydrocracking and found that 2% NiO o n S I O 2 - A 1 2 0 3 , 10% NiS on SIO2-A120 3 and a cobalt-molybdena alumina catalyst were the best. Recent studies on hydrocracking by Liden and May [2] Conway [3], Abiko et al. [5] and others [6-8, 11], were based on base-exchanged zeolite catalysts, which were more active than an amorphous silica-alumina catalyst. Abidova also found that an increase in
The present investigation studied the effect of different process parameters, namely temperature, pressure and residence period on the hydrocracking reaction of vacuum gas oil (boiling range 365-450°C, S-content <2%, N-content <200 ppm.) aimed to maximize the middle distillate (140-270°C cut).
4. Experimental
4. i. Apparatus The major apparatus in which the hydrocracking reaction had been carried out was a high pressure reactor. The auxiliary apparatus con-
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.R. Lahiri and D. Biswas / Hydrocracking o f a vacuum gas oil
726
sisted of (i) an ASTM distillation apparatus for product analysis, and (ii) a bomb calorimeter for determination of the sulfur content. The details of the high pressure reactor are given in fig. 1. The reactor assembly is shown in fig. 2. The high pressure reactor, with a capacity of 1 litre, was of the rocking type (40 oscillations per minute). The external diameter was 100 mm and the internal diameter 60 mm. It was designed to withstand a pressure up to 300 bar and a temperature up to 500°C. Temperature control was by a variac. Closure of the autoclave was of the screw type with a plug. A silver gasket was used for the
~3QO L ~8o--
_j'[
PRESSURE GAUGE ~ , ~ REACTOR y
~
T~ OUTLET x
INLET
j ~ ~ /~D/CATOR
Fig. 2. Reactor assembly.
r 8NOS. M12 Sq.
.VER SKET
pressure-tight joints. The materials of construction of the reactor and other parts were: Reactor bomb: 316 stainless steel Bolts: 316 stainless steel Seal ring: High carbon steel Thermocouple well: 316 stainless steel Outside casing: mild steel.
4.2. Preparation of catalysts Two solutions were prepared. Solution A contained 465 ml of water glass (0.20 g SiOz/ml. ) and 900 ml of water and solution B contained 67 ml of HC1 (4.0 N), 140 ml of A12(504) 3 solution (0.50 g AI203/ml) and 400 ml of water. After cooling, the solutions t o about 5°C, solution A was added quickly to solution B and stirred. The gel formed was aged for 48 h at 60°C. The dried gel was then base-exchanged with NH 4 ions, followed by calcination at 450-500°C for 2 h. The calcined mass was then base-exchanged with Ni ions by impregnation to produce finally a catalyst containing 5 . 8 w t % of NiO. The mass was then calcined at 450-500°C for 3 h.
4.3. Process
SCOle
In mm
Fig. 1. Details of reactor bomb.
The reactor bomb was charged with 250 ml of gas oil and 25 g of catalyst (feed : catalyst - 9.3 wt/ wt). The reactor was closed and placed in position. Hydrogen gas was introduced (see fig. 2) until the desired pressure was attained. The temperature was then gradually increased by adjusting
C.R. Lahiri and D. Biswas / Hydrocracking o f a vacuum gas oil
the variac. At the desired temperature, rocking was allowed to start. The reaction was then allowed to take place for a given residence period. The gas was then released and the liquid product was collected, weighed and subjected to ASTM distillation. The distillation yield was then converted into a true boiling point vs. distillation curve from which the required percentage yield of middle distillate was found.
5. Results and discussion
5.1. Effect of temperature The temperature effect on hydrocracking of a vacuum gas oil to a middle distillate is given in
727
table I. The results of the table reveal that a temperature below 300°C is not favourable for hydrocracking and that if the temperature is increased to 400°C the middle distillate yield is increased. This can be attributed to the rate enhancement of the cracking and hydrogenation reaction. Also, the endothermic cracking reaction, which actually splits the higher molecules into lower ones via the olefin intermediates, is favourable at higher temperatures. However, the hydrogenation reaction of the olefinic and aromatic compounds becomes exothermic above 500°C and 300°C, respectively ( A F ° > 0). This effect is however compensated by the use of hydrogen under pressure which makes AF ° negative at high temperature, according to the equation AF °= - 3 R T log P. Thus, a high temperature together
Table I Effect of temperature: Feedstock amount, 250 ml; catalyst amount, 25 g; initial pressure, 34.5 bar; residence time, 2 h; catalyst type, zeolite-base nickel catalyst Reaction temperature Items
300°C
350°C
400°C
(1) Reaction pressure (bar) (2) Products: (a) % Yield of liquid product, by volume (b) Yield of light Naphtha (C5-90°C cut), by volume (c) % Yield of Naphtha (90-140°C), by volume (d) % Yield of middle distillate (140-270°C cut), by volume
48.2
62.0
75.8
78
84
72
1.5
6.5
13
2
9
17.5
2
25
36
Table II Effect of pressure: Feedstock amount, 250ml; catalyst amount, 25g; initial temperature, 400°C; residence time, 2 h; catalyst type, zeolite-base nickel catalyst Initial pressure Items
34.5 bar 51.7 bar 86.2 bar
(1) Reaction pressure (bar) (2) Products: (a) % Yield of liquid product, by volume (b) % Yield of light Naphtha (C5-90°C cut), by volume (c) % Yield of Naphtha (90-140°C cut), by volume (d) % Yield of middle distillate (140-270°C cut), by volume
75.8
96.5
72
68
13
9.5
162.0
64 12.5
17.5
14
16.5
36
33.5
26
728
C.R. Lahiri and D. Biswas / Hydrocracking of a vacuum gas oil
Table III Effect of residence period: Feedstock amount, 250ml; catalyst amount, 25 g;initial pressure, 34.5 bar; reaction temperature, 400°C; catalyst type, zeolite-base nickel catalyst Residence period Items
1h
2h
3h
(1) Reaction pressure (bar) (2) Products (a) % Yield of liquid product by volume (b) % Yield of light Naphtha (C~-90°C cut), by volume (c) % Yield of Naphtha (90-140°C cut), by volume (d) % Yield of middle distillate (140-270°C), by volume
89.6
75.8
I20.6
64
72
62
13.5
13
17.5
16.5
17.5
16.5
30.5
36
26.5
with a high pressure favours the hydrocracking reactions.
5.2. Effect of pressure The effect of pressure on the hydrocracking of a vacuum gas oil to a middle distillate is given in table II, which represents the percent yield of middle distillate (140-270°C) versus pressure. Table II also shows that an increase in the hydrogen pressure has an inverse effect both on the yield of the liquid product and of the middle distillate, while for gasoline the yield at first decreases, but then increases with increasing pressure. This may be due to the fact that lower pressures favour the cracking process from the equilibrium point of view. Hence, the splitting of larger molecules is greatest at lower pressures, thereby producing maximum yields of middle distillate.
5.3. Effect of residence period The effect of the residence period on the hydrocracking of a vacuum gas oil to a middle distillate is summarized in table III. It is found that the 2 h residence period gives the highest yield of liquid product and of middle distillate. This is probably due to the fact that a residence period less than 2 h is not sufficient to complete the reaction, while a longer residence period
(greater than 2 h) results in undesirable side reactions, such as partial polymerization and condensation, thus producing a smaller amount of middle distillate. 6. Conclusions
The above results show that the conditions for a maximum yield of middle distillate (36%) from the hydrocracking of a vacuum gas oil (from Haldia Refinery) are obtained in a residence period of 2 h at a temperature of 400°C, and at a pressure of 75.8 bar. Higher temperatures favour more coke deposition. Lower pressures give rise to a larger yield of middle distillate. References [1] R.A. Flinn and O.A. Larson, lndust. Eng. Chem. 4 (1957) 49. [2] T.M. Liden and J.E. May, U.S. 3,816,297 (1974). [31 3.E. Conway, Ger. often 2,341,854 (1974). [4] E.L. Pollitzer, Brit 1,364,470 (1974). [5] S. Abiko, K. Yohrihiro, T. Sugihar and M. Hanawa, Ger. often W, 411,351 (1975). [6] R.C, Hansford, U.S. 3,836,454 (1975). [71 D.A. Young, U.S. 3,860,533 (1975). [8] J.W. Ward, U.S. 3,838,040 (1975). [9] J.W. Ward, U.S. 3,867,277 (1975). [10] Texaco Development Corporation, Ft. Demande 2,240,947 (1975). [11] M.M. Abidova and 13. Abdazimov, Khim Zh. (1976).