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Shuaiba Refinery Experiences with H-Oil Unit
D.L. Trimm et al. (Editors), Catalysts in Petroleum Refining 1989 0 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
165
SHUAIBA REFINERY EXPERIENCES WITH H-OIL UNIT MUSTAFA EMBABY Kuwait National Petroleum Company, Shuaiba Refinery,Kuwait ABSTRACT The 11-Oil unit is licensed by HRI, and KNPC h a s operated this unit since 1968. Being the first commercial plant in operation, several mechanical and process problems have been experienced
during these years. During the early stages of operation at design conversion, severe compatibility, coking andd some mechanical problems were experienced during leading to very low onstream factor below 50%. A historical review of the plant performance indicated some mechanical and process problems. The mechanical problems were related mainly t o corrosion and to selection of materials, in addition,to some design limitations which were corrected whenever possible. A task force was formed to study these problems. As a result of the extensive inhouse work done and with the help of the process licensor, most of the problems were solved. A scientific analysis of the operating problems and the corrective actions taken to resolve them are discussed. INTRODUCTION The H-Oil process is a very complex hydroconversion process utilizing ebullating bed technology to allow for the severe conditions for conversion of asphaltene and heavy resins to distillates. The process is a thermally dependent hydrocracking process where thermalcracking, hydrogenation, desulfurization, denitrification and demetallization reactions are taking place. Some undesired side reactions are taking place such as, recombination of unsaturated products to form complex molecules, which is the main reason for coke precursors formation. Besides the chemical reaction equilibrium required, there are some physical equilibrium to keep the catalyst bed expansion and homogeneity. These physical equilibrium are also affected by the chemical reactions, which affect the viscosity and density of the reactor mass contents. PROCESS DESCRIPTION Before proceeding in discussing KNPC experience with H-oil unit, it is essential to have an idea about the process. As shown in Figure No.1 the hydrogen rich gas and the oil are heated to the required temperatures and then mixed in specially designed mixer, before being introduced to the bottom of the reactor through specially designed spargers. The reactor is a back mixed isothermal reactor designed to overcome problems of fixed bed catalytic reactors. At the bottom of the reactor, the feed and gas mixture is mixed with the ebullating fluid, which acts as a heat sink to bring the whole mixture to the reaction temperature and controls the temperature rise across the reactor. The mixture is then passed through a grid into the reactor chamber, which contains the catalyst. The reactor effluent is diverted to the hot separator. The vapours are cooled in steps and the condensed liquids are separated, while the hot liquid at high pressure and temperature
166
H2Makeup
=
I
Recycle Gas
1
Fractionator
J
A Kero.
.--)
Diesel
d
E.P.
Hydrogen Gas Heater
H. E.
= Heat Exchanger;
T Feed
Feed
Heater
E. P. = Ebullating Pump; V = Vap./Liq. Separator.
Figure 1: H-Oil process schematic flow diagram is let down in steps until it reaches the strippers. The separated vapours are cooled and the condensate is separated and collected to be fractionated to Naphtha, Kero, Diesel, etc. The heavy liquid from the strippers and fractionator bottoms are then vacuum distilled to produce gas oil and vacuum bottom. The vacuum bottoms are then fixed with cutter stocks and send to the fuel oil pool. KNPC H-OIL PLANT HISTORY
KNPC's H-Oil unit consist of two identical H-oil trains, each designed for 14800 BPSD of vacuum residue and a conversion level of 75% utilizing recycle stream. The conversion is defined by the disappearance of 975+"F material during the process. During the early stages of operation at sign conversion, severe compatibility, and coking problems and some mechanical problems were experienced leading to very low onstream factor below 50 %. A historical review of the plant's performance indicated some mechanical and process problems. The mechanical problems were related mainly to corrosion, and to selection of materials, in addition to some equipment design limitations which were corrected whenever possible. A KNPC task force was formed to study those problems. As a result of the extensive inhouse work done and with the help of the process licensor, most of the problems were solved. The products compatability was improved and an onstream factor of about 75% was reached at conversion level of 65% and fresh feed of 26,000 BPD per train.
167
The coking, plugging and corrosion problems were reduced but still exist, affecting the unit’s operation. A second task force was formed to improve the unit performance and it was possible to achieve over 85% onstream efficiency. PLANT PROCESS PROBLEMS
The process problems can not be dealt with individually, as they are inter related, but can be summarized as follows: 1. Coke Formation in t h e Reactor
Normally the equilibrium catalyst with drawn from the reactor did not have much coke deposition. The coke laydown percentage is much lower than the metal laydown on the catalyst. No measures were taken to estimate the deactivation rates as the reactors are operated at constant temperature and the deviation in conversion, desulfurization etc., is hardly enough to build a data bank for activity conclusions. However, massive coke formation was experienced in the reactors several times, which can be attributed to: a. T e m p e r a t u r e Excursion. Feed pump or ebullating pump failure are the major reasons for temperature excursions due to lack of cooling medium and increase in residence time.
b. Unbalanced t e m p e r a t u r e profile. Due to improper mixing and poor flow distribution, channeling can occur leading to localized temperature increases, which were improved by modifying the mixers, sparger and grids. c. High conversion operation. In order to achieve high conversion, high reactor temperature is required. At such high temperature, reaction rates are expected to increase and more hydrogen is consumed leading to low hydrogen partial pressure. This problem was improved by increasing the hydrogen partial pressures and reducing the conversion. d. Low catalyst activity. At low catalyst activity, the saturation reactions are suppressed allowing the unsaturated products to recombine forming coke precursors (3). The conditions were improved by increasing the catalyst addition rate. 2. Coke Deposition in t h e Reactor Downstream Vessels
Coke formation in reactor’s downstream vessels and pipelines, was experienced and can be attributed to: a. T h e r m a l cracking. High temperature, in those vessels, at low hydrogen partial pressure and high residence time due to some equipment limitation, promotes further thermal cracking. b. Coke precursors. Coke precursors formed in the reactor can recombine in those vessels, due to collision, changes in direction and velocity. c. Asphaltene precipitation. As a result of either the reduction of the asphaltene solvent and/or recombination of unsaturated compounds complex compounds can be formed that behave as asphaltenes.
168 3. Heat Exchangers Fouling
The reactor section heat exchangers fouling was one of the problems limiting the unit's performance. Although the materials in both sides of the heat exchangers, are vapoun and distillates, the fouling material was found to be of asphaltenic nature. This can be attributed to: a.
Carry-over of heavy liquid from the separators due to failure of level indication.
b. Carry-over of heavy liquid by entrainment of heavy oil droplets with the vapour. c.
Foaming of the system.
d.
Polymerization taking place on the tubes.
4. Bed Expansion
Due to some limitation in the reactor design, the KNPC plant is usually operated at lower than the design bed expansion, affecting the liquid hourly space velocity and at the same time, increasing the space and time for thermal cracking to take place at the top part of the reactor in the absence of the catalyst. 5. Foaming P r o b l e m s
Foaming was experienced several times. The nature and reasons for such phenomenon has not yet been solved. It was observed that increasing the catalyst addition rate, did decrease the foam. ANALYSIS OF THE M A J O R O P E R A T I N G P R O B L E M S
A scientific analysis for the operating problems, was essential to understand the mechanism of the reactions taking place and the conditions promoting both the desired main reactions and the undesired side reactions. Most of the operating problems resulted from off setting the balance between the different reactions taking place and the catalyst role within these reactions.
1. Conversion of 975+"F Material The conversion of 975+'F material believed to be caused by thermal cracking. The published data about thermal cracking and visbreaking indicate that a conversion of 50% is possible at the temperature and residence time, similar to that of the H-Oil unit. The results obtained from KNPC plant, together with pilot plant tests conducted by catalyst manufacturers, indicated that the conversion at a certain temperature and space velocity, is almost constant regardless of the catalyst activity or catalyst type used. As a result of thermal cracking, high concentration of heavy unsaturated compounds are expected to be produced. These compounds will react with the polynuclear aromatic, leading to coke precursors formation which might not be deposited in the reactor, but will accumulate in downstream vessels. The thermal cracking is not expected to stop at the reactor's outlet but will continue throughout the .P. separators, where the temperature is still well above 750F at much lower hydrogen partial pressure, favoring coke formation.
169
2. Hydrofining Hydrofining is meant here as catalytic saturation of the cracked products, in addition t desulfurization etc. The most important reaction to avoid coke formation is the saturation of the heavy unsaturated compounds to avoid further polymerization reactions. The hydrofining reactions of such large molecules are believed to be diffusion restricted reactions and will require special catalyst with suitable large pore volume. At the prevailing L.H.S.V., high conversion will produce more unsaturated materials beyond the capability of the catalyst to handle. High conversion can be achieved at lower temperature but at much lower L.H.S.V. which allows more contact time of the products with the catalyst for further hydrofining reactions.
3. Asphaltenes/Solvent Balance Asphaltene precipitation, together with coke precursors formation, are the major problems of plugging, exchanger fouling and product incompatibility. Asphaltenes are existing in he feed and kept in solution by the presence of resinous materials acting as solvents. Both the asphaltene and the solvent materials, are subjected to either thermal or catalytic cracking at the H-oil operating conditions. The rate of cracking of each differs, with temperature and catalyst (1). It is believed that at higher temperature, the rate of solvent cracking is higher than that of asphaltenes, which offsets the ratio of asphaltene to solvent (2 & 3) and will lead to asphaltene precipitation. This was verified by some test work conducted by one of the catalyst vendors, and it was also clear from the plant data, at 65% and 55% yield comparison. The heavy gas oil product yields are the most relatively affected yield ( Table No. 1). Table 1: H-Oil Products Yield Fkom Vacuum Residue Processing
64% Conversion 100%
3.1%
100% 3.3% 103.3%
7.1% 4.2% 9.4% 23.6%
9.2% 5.9% 11.4% 22.7%
Light Gasoil
5.1%
5.2%
Heavy Gasoil
4.2%
2.5%
49.5%
46.4%
Hydrogen Total Gases & Losses Naphtha Kero Diesel
Residue
4. Feed Quality It was observed that, when processing vacuum bottoms containing Heavy Crude over 5%, the rate of coke formation and fouling did increase. The factors expected to upset the unit
170
are visualized to be as follows: a. Asphaltene content. The heavy crude did not contain more asphaltenes than the regular crude. The increase in asphaltenes offset the balance between asphaltene and solvent and allowed for more asphaltene precipitation.
b. Salt content. High salt content in the desalter outlet was observed with the increase of the heavy crude. Salt precipitation was experienced in the vacuum unit heater and is expected to reach the H-Oil unit. The presence of salt might be one of the reasons affecting asphaltene conversion, sediment and coke formation causing accelerating fouling rate and might be responsible for foaming.
Corrective Actions In view of the above, the corrective actions taken to improve the KNPC H-Oil unit performance:
1. Reduction of conversion and setting a maximum reactor temperature in order to keep the balance between asphaltenes and solvent. Another advantage of temperature reduction is to keep the temperature in the reactor downstream vessels under control to reduce the thermal cracking. 2. Increase of hydrogen partial pressure by increasing the pressure to the maximum allowable limit by the system pressure and increase of hydrogen purity to the maximum limited by compressor performance. The high hydrogen partial pressure will promote the hydrogenation reactions and reduce the coke precursors formation.
3. Increase of catalyst addition rate to maintain higher catalyst activity to saturate the produced heavy unsaturated compounds.
4. Maintain a certain minimum gas oil rate, to insure the presence of enough amount of solvent to keep the asphaltenes in solution.
All the above variables were considered in the new Texaco-convent plant design, which is being operated at much lower temperature, better temperature control facilities, lower L.H.S.V., better gas oil rate and higher catalyst addition rate than the KNPC unit and achieved a conversion level of over 60% without apparent problems.
The H-Oil Catalyst As previously concluded, the 975+"F material conversion ia mainly a thermally dependent hydrocracking process. The role of the catalyst is to treat the products simultaneously as they are formed in order to reduce the concentration of the heavy unsaturates, in addition to promote desulfurization,denitrification and demetallization reactions. The first version of H-Oil catalyst was a CoMo catalyst with high metal content > 15% Mo. The new version of catalyst is Nimo catalyst with > 11.5%molybdenum. CoMo catalyst with reduced metal content is also proven in the process. The bulk density of the new catalyst was altered to suit the ebullating bed requirement. For the time being there are 3 suppliers of the catalyst, but the specification of each catalyst is not the same except for the metal content, size and bulk density. All should meet a requirement of surface area.
171
The H-Oil reactor should be well dynamically balanced to achieve a certain catalytic reaction equilibrium and the catalyst should meet some special physical and chemical properties. It is believed that the optimum H-Oil catalyst should meet the following physical and chemical properties.
A. Physical Properties. The regular H-oil catalyst is of extrudate type. The behaviour of the catalyst in the ebullated bed depends, to a great extent, on the following physical properties. 1. Catalyst size and size distribution. The catalyst bed homogeneity is one of the goals for a balanced reactor. The pellet fluidization is proportional to effective length and catalyst size distribution, which should be in a reasonable range to achieve maximum bed homogeneity. Significant amount of short extradite will tend to float and bed stratification will be reached, in addition to catalyst carry-over, This was observed in one of the catalyst batches and the temperature, spread in the reactor went out of order. 2. Catalyst bulk density.
The bed ebullation is a function of the catalyst density, which should be balanced with the ebulalting fluid flow for a certain bed expansion. Catalyst of low density, will require less ebullating flow, which might be less than the necessary fluid flow to sustain the reactions temperature profile. On the other hand, catalyst of high density will require high flow, which might be beyond the ebullating pump capacity.
3. Crushing s t r e n g t h a n d a t t r i t i o n resistance. In the ebullating bed, the catalyst is subjected to severe dynamic conditions, such as friction and collision. Those conditions, will tend to break the extrudates and produce some fines which will be carried-over to the down-stream vessels leading to catalyst losses, in addition to its effect in promoting further cracking reactions and causing system pluggage. 4. Surface area.
The surface area, is an important parameter for any catalytic reaction. High surface area catalyst is favorable, but without jeopardizing the other parameters, which might be of greater importance, such as bulk density and crush strength. 5 . P o r e volume a n d P o r e size distribution.
Dealing with such large molecules, the catalyst should have an adequate pore size and size distribution, so as to allow such molecules to get in contact with th catalyst active sites. Catalyst with relatively small pore size will be rapidly plugged and lose activity. Non-active catalyst will leave those large unsaturated molecules to condense and form coke precursors.
B. Chemical Composition The H-Oil catalyst is whether CoMo or NiMo, on alumina carrier. In both the plant and pilot plants it was found that the NiMo catalyst is more active towards hydrogenation
172
reactions but less towards desulfurization. The metals content in the currently used catalyst is much less than the early breed of catalyst without affecting conversion.
Figure 2: Asphaltene removal in train -4 and train B
--
so-
8 Y
I:
0 ,-
30-
bl
L
c
Figure 3: Conversion in train A and train B However, a new catalyst, with high metal content was tested in the plant and indicated better desulfurization and asphaltene conversion. The data collected was not enough to reach a positive conclusion to justify extra cost for the catalyst. The lower asphaltene
173
content could be a result of actual asphaltene conversion on the catalyst or due to better saturation reactions taking place, so that recombination and formation of complex molecules, are reduced (Figures 2 and 3). The catalyst manufacturers are working on developing an improved catalyst, by adding promoters and/or changing the pore size distribution. The results have indicated some improvements in hot filtration tests and sediments at the front end of the catalyst life, but the difference appears marginal by aging towards the actual unit catalyst life. Correlations are still needed to simulate the actual plant equilibrium.
Conclusion The economic incentive of the H-Oil unit is very attractive and it is considered the backbone of KNPC’s Shuaiba Refinery. KNPC is still working on improving the H-Oil unit performance to achieve higher onstream efficiency and higher conversion. The work is being done by exchanging the information with the Licensors, another plant in the United States, in addition to a research program being conducted with the Kuwait Institute for Scientific Research.
References [l] SAssoka et al., S. Nakata, Y. Shirota and C. Takeuchi “Asphaltene cracking in catalytic hydrotreating of heavy oils” ,Ind. Engg. Chemical Process, 1983.
[2] M. A Sin & J Menill, ”Test unit evaluates catalyst for ebullated and expanded bed process”. Ketjen Catalyst. June 1986. [3] Martin HUS, ”Visbreaking process has strong revival OGJ, April 13, 1981.
165
SHUAIBA REFINERY EXPERIENCES WITH H-OIL UNIT MUSTAFA EMBABY Kuwait National Petroleum Company, Shuaiba Refinery,Kuwait ABSTRACT The 11-Oil unit is licensed by HRI, and KNPC h a s operated this unit since 1968. Being the first commercial plant in operation, several mechanical and process problems have been experienced
during these years. During the early stages of operation at design conversion, severe compatibility, coking andd some mechanical problems were experienced during leading to very low onstream factor below 50%. A historical review of the plant performance indicated some mechanical and process problems. The mechanical problems were related mainly t o corrosion and to selection of materials, in addition,to some design limitations which were corrected whenever possible. A task force was formed to study these problems. As a result of the extensive inhouse work done and with the help of the process licensor, most of the problems were solved. A scientific analysis of the operating problems and the corrective actions taken to resolve them are discussed. INTRODUCTION The H-Oil process is a very complex hydroconversion process utilizing ebullating bed technology to allow for the severe conditions for conversion of asphaltene and heavy resins to distillates. The process is a thermally dependent hydrocracking process where thermalcracking, hydrogenation, desulfurization, denitrification and demetallization reactions are taking place. Some undesired side reactions are taking place such as, recombination of unsaturated products to form complex molecules, which is the main reason for coke precursors formation. Besides the chemical reaction equilibrium required, there are some physical equilibrium to keep the catalyst bed expansion and homogeneity. These physical equilibrium are also affected by the chemical reactions, which affect the viscosity and density of the reactor mass contents. PROCESS DESCRIPTION Before proceeding in discussing KNPC experience with H-oil unit, it is essential to have an idea about the process. As shown in Figure No.1 the hydrogen rich gas and the oil are heated to the required temperatures and then mixed in specially designed mixer, before being introduced to the bottom of the reactor through specially designed spargers. The reactor is a back mixed isothermal reactor designed to overcome problems of fixed bed catalytic reactors. At the bottom of the reactor, the feed and gas mixture is mixed with the ebullating fluid, which acts as a heat sink to bring the whole mixture to the reaction temperature and controls the temperature rise across the reactor. The mixture is then passed through a grid into the reactor chamber, which contains the catalyst. The reactor effluent is diverted to the hot separator. The vapours are cooled in steps and the condensed liquids are separated, while the hot liquid at high pressure and temperature
166
H2Makeup
=
I
Recycle Gas
1
Fractionator
J
A Kero.
.--)
Diesel
d
E.P.
Hydrogen Gas Heater
H. E.
= Heat Exchanger;
T Feed
Feed
Heater
E. P. = Ebullating Pump; V = Vap./Liq. Separator.
Figure 1: H-Oil process schematic flow diagram is let down in steps until it reaches the strippers. The separated vapours are cooled and the condensate is separated and collected to be fractionated to Naphtha, Kero, Diesel, etc. The heavy liquid from the strippers and fractionator bottoms are then vacuum distilled to produce gas oil and vacuum bottom. The vacuum bottoms are then fixed with cutter stocks and send to the fuel oil pool. KNPC H-OIL PLANT HISTORY
KNPC's H-Oil unit consist of two identical H-oil trains, each designed for 14800 BPSD of vacuum residue and a conversion level of 75% utilizing recycle stream. The conversion is defined by the disappearance of 975+"F material during the process. During the early stages of operation at sign conversion, severe compatibility, and coking problems and some mechanical problems were experienced leading to very low onstream factor below 50 %. A historical review of the plant's performance indicated some mechanical and process problems. The mechanical problems were related mainly to corrosion, and to selection of materials, in addition to some equipment design limitations which were corrected whenever possible. A KNPC task force was formed to study those problems. As a result of the extensive inhouse work done and with the help of the process licensor, most of the problems were solved. The products compatability was improved and an onstream factor of about 75% was reached at conversion level of 65% and fresh feed of 26,000 BPD per train.
167
The coking, plugging and corrosion problems were reduced but still exist, affecting the unit’s operation. A second task force was formed to improve the unit performance and it was possible to achieve over 85% onstream efficiency. PLANT PROCESS PROBLEMS
The process problems can not be dealt with individually, as they are inter related, but can be summarized as follows: 1. Coke Formation in t h e Reactor
Normally the equilibrium catalyst with drawn from the reactor did not have much coke deposition. The coke laydown percentage is much lower than the metal laydown on the catalyst. No measures were taken to estimate the deactivation rates as the reactors are operated at constant temperature and the deviation in conversion, desulfurization etc., is hardly enough to build a data bank for activity conclusions. However, massive coke formation was experienced in the reactors several times, which can be attributed to: a. T e m p e r a t u r e Excursion. Feed pump or ebullating pump failure are the major reasons for temperature excursions due to lack of cooling medium and increase in residence time.
b. Unbalanced t e m p e r a t u r e profile. Due to improper mixing and poor flow distribution, channeling can occur leading to localized temperature increases, which were improved by modifying the mixers, sparger and grids. c. High conversion operation. In order to achieve high conversion, high reactor temperature is required. At such high temperature, reaction rates are expected to increase and more hydrogen is consumed leading to low hydrogen partial pressure. This problem was improved by increasing the hydrogen partial pressures and reducing the conversion. d. Low catalyst activity. At low catalyst activity, the saturation reactions are suppressed allowing the unsaturated products to recombine forming coke precursors (3). The conditions were improved by increasing the catalyst addition rate. 2. Coke Deposition in t h e Reactor Downstream Vessels
Coke formation in reactor’s downstream vessels and pipelines, was experienced and can be attributed to: a. T h e r m a l cracking. High temperature, in those vessels, at low hydrogen partial pressure and high residence time due to some equipment limitation, promotes further thermal cracking. b. Coke precursors. Coke precursors formed in the reactor can recombine in those vessels, due to collision, changes in direction and velocity. c. Asphaltene precipitation. As a result of either the reduction of the asphaltene solvent and/or recombination of unsaturated compounds complex compounds can be formed that behave as asphaltenes.
168 3. Heat Exchangers Fouling
The reactor section heat exchangers fouling was one of the problems limiting the unit's performance. Although the materials in both sides of the heat exchangers, are vapoun and distillates, the fouling material was found to be of asphaltenic nature. This can be attributed to: a.
Carry-over of heavy liquid from the separators due to failure of level indication.
b. Carry-over of heavy liquid by entrainment of heavy oil droplets with the vapour. c.
Foaming of the system.
d.
Polymerization taking place on the tubes.
4. Bed Expansion
Due to some limitation in the reactor design, the KNPC plant is usually operated at lower than the design bed expansion, affecting the liquid hourly space velocity and at the same time, increasing the space and time for thermal cracking to take place at the top part of the reactor in the absence of the catalyst. 5. Foaming P r o b l e m s
Foaming was experienced several times. The nature and reasons for such phenomenon has not yet been solved. It was observed that increasing the catalyst addition rate, did decrease the foam. ANALYSIS OF THE M A J O R O P E R A T I N G P R O B L E M S
A scientific analysis for the operating problems, was essential to understand the mechanism of the reactions taking place and the conditions promoting both the desired main reactions and the undesired side reactions. Most of the operating problems resulted from off setting the balance between the different reactions taking place and the catalyst role within these reactions.
1. Conversion of 975+"F Material The conversion of 975+'F material believed to be caused by thermal cracking. The published data about thermal cracking and visbreaking indicate that a conversion of 50% is possible at the temperature and residence time, similar to that of the H-Oil unit. The results obtained from KNPC plant, together with pilot plant tests conducted by catalyst manufacturers, indicated that the conversion at a certain temperature and space velocity, is almost constant regardless of the catalyst activity or catalyst type used. As a result of thermal cracking, high concentration of heavy unsaturated compounds are expected to be produced. These compounds will react with the polynuclear aromatic, leading to coke precursors formation which might not be deposited in the reactor, but will accumulate in downstream vessels. The thermal cracking is not expected to stop at the reactor's outlet but will continue throughout the .P. separators, where the temperature is still well above 750F at much lower hydrogen partial pressure, favoring coke formation.
169
2. Hydrofining Hydrofining is meant here as catalytic saturation of the cracked products, in addition t desulfurization etc. The most important reaction to avoid coke formation is the saturation of the heavy unsaturated compounds to avoid further polymerization reactions. The hydrofining reactions of such large molecules are believed to be diffusion restricted reactions and will require special catalyst with suitable large pore volume. At the prevailing L.H.S.V., high conversion will produce more unsaturated materials beyond the capability of the catalyst to handle. High conversion can be achieved at lower temperature but at much lower L.H.S.V. which allows more contact time of the products with the catalyst for further hydrofining reactions.
3. Asphaltenes/Solvent Balance Asphaltene precipitation, together with coke precursors formation, are the major problems of plugging, exchanger fouling and product incompatibility. Asphaltenes are existing in he feed and kept in solution by the presence of resinous materials acting as solvents. Both the asphaltene and the solvent materials, are subjected to either thermal or catalytic cracking at the H-oil operating conditions. The rate of cracking of each differs, with temperature and catalyst (1). It is believed that at higher temperature, the rate of solvent cracking is higher than that of asphaltenes, which offsets the ratio of asphaltene to solvent (2 & 3) and will lead to asphaltene precipitation. This was verified by some test work conducted by one of the catalyst vendors, and it was also clear from the plant data, at 65% and 55% yield comparison. The heavy gas oil product yields are the most relatively affected yield ( Table No. 1). Table 1: H-Oil Products Yield Fkom Vacuum Residue Processing
64% Conversion 100%
3.1%
100% 3.3% 103.3%
7.1% 4.2% 9.4% 23.6%
9.2% 5.9% 11.4% 22.7%
Light Gasoil
5.1%
5.2%
Heavy Gasoil
4.2%
2.5%
49.5%
46.4%
Hydrogen Total Gases & Losses Naphtha Kero Diesel
Residue
4. Feed Quality It was observed that, when processing vacuum bottoms containing Heavy Crude over 5%, the rate of coke formation and fouling did increase. The factors expected to upset the unit
170
are visualized to be as follows: a. Asphaltene content. The heavy crude did not contain more asphaltenes than the regular crude. The increase in asphaltenes offset the balance between asphaltene and solvent and allowed for more asphaltene precipitation.
b. Salt content. High salt content in the desalter outlet was observed with the increase of the heavy crude. Salt precipitation was experienced in the vacuum unit heater and is expected to reach the H-Oil unit. The presence of salt might be one of the reasons affecting asphaltene conversion, sediment and coke formation causing accelerating fouling rate and might be responsible for foaming.
Corrective Actions In view of the above, the corrective actions taken to improve the KNPC H-Oil unit performance:
1. Reduction of conversion and setting a maximum reactor temperature in order to keep the balance between asphaltenes and solvent. Another advantage of temperature reduction is to keep the temperature in the reactor downstream vessels under control to reduce the thermal cracking. 2. Increase of hydrogen partial pressure by increasing the pressure to the maximum allowable limit by the system pressure and increase of hydrogen purity to the maximum limited by compressor performance. The high hydrogen partial pressure will promote the hydrogenation reactions and reduce the coke precursors formation.
3. Increase of catalyst addition rate to maintain higher catalyst activity to saturate the produced heavy unsaturated compounds.
4. Maintain a certain minimum gas oil rate, to insure the presence of enough amount of solvent to keep the asphaltenes in solution.
All the above variables were considered in the new Texaco-convent plant design, which is being operated at much lower temperature, better temperature control facilities, lower L.H.S.V., better gas oil rate and higher catalyst addition rate than the KNPC unit and achieved a conversion level of over 60% without apparent problems.
The H-Oil Catalyst As previously concluded, the 975+"F material conversion ia mainly a thermally dependent hydrocracking process. The role of the catalyst is to treat the products simultaneously as they are formed in order to reduce the concentration of the heavy unsaturates, in addition to promote desulfurization,denitrification and demetallization reactions. The first version of H-Oil catalyst was a CoMo catalyst with high metal content > 15% Mo. The new version of catalyst is Nimo catalyst with > 11.5%molybdenum. CoMo catalyst with reduced metal content is also proven in the process. The bulk density of the new catalyst was altered to suit the ebullating bed requirement. For the time being there are 3 suppliers of the catalyst, but the specification of each catalyst is not the same except for the metal content, size and bulk density. All should meet a requirement of surface area.
171
The H-Oil reactor should be well dynamically balanced to achieve a certain catalytic reaction equilibrium and the catalyst should meet some special physical and chemical properties. It is believed that the optimum H-Oil catalyst should meet the following physical and chemical properties.
A. Physical Properties. The regular H-oil catalyst is of extrudate type. The behaviour of the catalyst in the ebullated bed depends, to a great extent, on the following physical properties. 1. Catalyst size and size distribution. The catalyst bed homogeneity is one of the goals for a balanced reactor. The pellet fluidization is proportional to effective length and catalyst size distribution, which should be in a reasonable range to achieve maximum bed homogeneity. Significant amount of short extradite will tend to float and bed stratification will be reached, in addition to catalyst carry-over, This was observed in one of the catalyst batches and the temperature, spread in the reactor went out of order. 2. Catalyst bulk density.
The bed ebullation is a function of the catalyst density, which should be balanced with the ebulalting fluid flow for a certain bed expansion. Catalyst of low density, will require less ebullating flow, which might be less than the necessary fluid flow to sustain the reactions temperature profile. On the other hand, catalyst of high density will require high flow, which might be beyond the ebullating pump capacity.
3. Crushing s t r e n g t h a n d a t t r i t i o n resistance. In the ebullating bed, the catalyst is subjected to severe dynamic conditions, such as friction and collision. Those conditions, will tend to break the extrudates and produce some fines which will be carried-over to the down-stream vessels leading to catalyst losses, in addition to its effect in promoting further cracking reactions and causing system pluggage. 4. Surface area.
The surface area, is an important parameter for any catalytic reaction. High surface area catalyst is favorable, but without jeopardizing the other parameters, which might be of greater importance, such as bulk density and crush strength. 5 . P o r e volume a n d P o r e size distribution.
Dealing with such large molecules, the catalyst should have an adequate pore size and size distribution, so as to allow such molecules to get in contact with th catalyst active sites. Catalyst with relatively small pore size will be rapidly plugged and lose activity. Non-active catalyst will leave those large unsaturated molecules to condense and form coke precursors.
B. Chemical Composition The H-Oil catalyst is whether CoMo or NiMo, on alumina carrier. In both the plant and pilot plants it was found that the NiMo catalyst is more active towards hydrogenation
172
reactions but less towards desulfurization. The metals content in the currently used catalyst is much less than the early breed of catalyst without affecting conversion.
Figure 2: Asphaltene removal in train -4 and train B
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Figure 3: Conversion in train A and train B However, a new catalyst, with high metal content was tested in the plant and indicated better desulfurization and asphaltene conversion. The data collected was not enough to reach a positive conclusion to justify extra cost for the catalyst. The lower asphaltene
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content could be a result of actual asphaltene conversion on the catalyst or due to better saturation reactions taking place, so that recombination and formation of complex molecules, are reduced (Figures 2 and 3). The catalyst manufacturers are working on developing an improved catalyst, by adding promoters and/or changing the pore size distribution. The results have indicated some improvements in hot filtration tests and sediments at the front end of the catalyst life, but the difference appears marginal by aging towards the actual unit catalyst life. Correlations are still needed to simulate the actual plant equilibrium.
Conclusion The economic incentive of the H-Oil unit is very attractive and it is considered the backbone of KNPC’s Shuaiba Refinery. KNPC is still working on improving the H-Oil unit performance to achieve higher onstream efficiency and higher conversion. The work is being done by exchanging the information with the Licensors, another plant in the United States, in addition to a research program being conducted with the Kuwait Institute for Scientific Research.
References [l] SAssoka et al., S. Nakata, Y. Shirota and C. Takeuchi “Asphaltene cracking in catalytic hydrotreating of heavy oils” ,Ind. Engg. Chemical Process, 1983.
[2] M. A Sin & J Menill, ”Test unit evaluates catalyst for ebullated and expanded bed process”. Ketjen Catalyst. June 1986. [3] Martin HUS, ”Visbreaking process has strong revival OGJ, April 13, 1981.