I.FLUID CATALYTIC CRACKING:
B.CATALYST/ADDITIVES:
Q-34: Briefly describe the manufacture of FCC Catalyst bringing out the importance of Spray Drying/Atomisation during the preparation ?
A-34:
FCC catalyst manufacture
Four major categories of manufacturing routes are documented: 1) in-situ formation,2) gel based matrix method, 3) sol based matrix method, and 4) core and shell technology These production processes involve at least the mixing of components followed by spray drying.For the experimental work, the sol based matrix method is applied to prepare the FCC catalyst feed. This method replaced the older gel matrix method, due to improved attrition.
FCC catalyst feed preparation
. Sulphuric acid is mixed with sodium silicate at low pH (1.8 – 3.0) to form a silica sol. At lower pH, a sol of poor quality is formed, and at higher pH, thickening and gelling will occur faster. A dispersed aluminium source may be added to the silica sol. Then clay is added. Because zeolite is alkali, it must be added to a liquid with a pH in the range of 3.0 – 4.5. The spray drier feed should have a pH in the range of 2.8 – 4.0, because at pH lower than 2.8 the zeolite will be destructed and above 4.0 the slurry will be too thick. Rigorous mixing during feed preparation is essential to obtain a hard, dense and homogeneous catalyst. Insufficient mixing can influence attrition resistance, density and morphology as well as activity and selectivity of the FCC catalyst. Further, it is common to add fines to the feed before spray drying. The solids content of the feed affects the particle diameter: a higher solids concentration increases the particle diameter . A higher viscosity of the feed also increases the particle diameter, due to increased energy necessary for atomisation.
Spray drying
Spray drying is the transformation of a liquid containing suspended and/or dissolved solids into a powder by atomising the liquid into a hot drying medium (usually air). The principle of spray drying is extensive contacting of liquid with air. The air transfers heat to the droplet and takes up the evaporated water. The spray drying process is complex due to the simultaneous exchange of momentum, heat and mass. In addition, the properties of the dried material depend strongly on temperature and water content. Furthermore the spray drying process is large and expensive. The spray drying process is said to control the fluidisation and circulation properties of the catalyst in an FCC unit.
The spray drying process is usually divided in four stages: 1) atomisation of the feed into a spray of droplets, 2) dispersion of the spray in the air (spray-air contact), 3) drying of the droplets, and 4) product-air separation. Together with the feed properties, each of these stages determine the drying history of the droplet and hence the final properties.
.
Atomisation
Atomisation is the process that transforms the liquid feed into droplets with a high specific surface area. Commonly used atomisers are pressure nozzles, rotating disc atomisers and two-fluid nozzles. In this work a pressure nozzle is used for Spraying. Pressure forces the liquid through a core and a small orifice. The core swirls the liquid and the after the orifice, a thin conical sheet is formed that disintegrates into small droplets. The spray angle, droplet size distribution and droplet velocity are determined by feed properties (concentration,temperature, viscosity) and operating parameters (pressure and the nozzle type).
Spray-air contact
The liquid sheet and atomised droplets come in contact with the air and the droplets,which have a high velocity, are dispersed. The airflow direction with respect to the droplet motion can be co-current, counter-current or mixed-flow.
Drying
The drying process in a spray drier is complex because millions of individual droplets experience different drying histories. The properties of the dried material are made up of the properties of these dried particles. The description of the drying process varies from simplified, engineering models to sophisticated theoretical models and numerical methods
Product air separation
Product that hits the wall of the drying chamber usually slides down and is collected via a valve at the bottom of the drier. Sometimes the accumulated powder has to be removed from the wall: the used spray drier has two hammers at the conical part of the drying chamber. The product is separated from the exhaust air by cyclones.
Spray driers are built with different drying chamber shapes, air inlet geometries and flow direction. The spray drier used in this work has a co-current airflow, supplied by two ventilators, one in the inlet and one in the outlet duct. The pressure nozzle atomiser is placed in the centre of the inlet airflow.
FCC catalyst characterisation and evaluation methods
The choice of a suitable FCC catalyst is an important factor for the performance of a specific FCC unit. In the industry, the micro activity test is widely used to specify the catalysts cracking quality and attrition tests are used to evaluate the strength.
.
In this work. the particle size distribution is measured with laser scattering and the particle shape by electron microscopy. The skeletal density is measured with helium picnometry and the bulk density with a Stampf volumeter (tapping a certain amount of powder). The surface area,porosity and pore size distribution are measured with incipient wetness impregnation,nitrogen sorption and mercury porosimetry. Pore sizes can be divided into three regions:micro-pores with diameter < 2·10-9 m, meso-pores with a diameter between 2·10-9 and 5·10-8 m and macro-pores, with a diameter > 5·10-8 m.
Experimental
FCC catalyst formulation and feed preparation
The same FCC catalyst formulation has been used in all experiments. The FCC catalyst formulation is a mixture of 24 wt.% zeolite Y, 47 wt.% clay, 15 wt.% alumina and 14 wt.% silica. Dry solids contents are based on 48 hours drying at 200 °C. A silica sol was prepared by vigorously mixing sodium silicate, water and sulphuric acid in a vessel at constant pH. Kaolin clay was added gradually to the sol. subsequently zeolite Y and dispersed alumina were added. For standard experiments the water content of the feed, was 3.6 kgw/kgs (solids content 21.8 wt.%). Other experiments had initial water contents of 4.6 (17.9 wt.%) and 7.3 kgw/kgs (12.0 wt.%). The preparation of the silica sol and the dispersion of alumina results in the presence of salts in the feed. These salts will crystallise during the drying process. The amount of salts is 10.2 wt.% of the dry catalyst weight, which corresponds to 9.3 wt.% of total dry solids. In the manufacture of FCC catalyst the salts are washed out.
Spray drier
The feed is pumped to a pilot plant scale co-current spray drier with a three-plunger pump. The feed is atomised by a Spraying Systems SK pressure nozzle, producing a hollow conical spray with a spray angle of ca. 80°. The diameter of the drying chamber is 2.20 m and the total height is 3.70 m with a conical angle of 60° at 2.0 m from the top. . The electrically heated air enters the drying chamber via an annulus and the nozzle is placed in the centre. The air exits the drier via a horizontal pipe, which is placed about 0.5 m above the bottom and has a downward opening in the centre of the chamber. The airflow then passes a cyclone where particles are separated. Product was both collected from a valve at the bottom of the drying chamber (tower product) and a valve at the bottom of the cyclone (cyclone product).
Drying conditions
The feed flow is mass flow controlled and can be varied between 30 and 80 kg/hr,with corresponding nozzle pressures between 10 and 59 bars.
Evaluation
The central question that initiated this work was how spray drying conditions influence fluid catalytic cracking (FCC) catalyst properties. In order to give an answer to this question some fundamental aspects of spray drying FCC catalyst have been studied.The drying behaviour of FCC catalyst has been studied on a laboratory scale and spray drying experiments have been done on a pilot-plant scale.
Conclusions
The study of the drying kinetics provided insight in the phenomenological aspects of the drying process and gave rules to predict the sorption isotherm and diffusion coefficient with the properties of the single components. The shrinkage behaviour during spray drying is probably exemplary: FCC catalyst shrinks uniformly until the shrinkage limit is reached and then stops shrinking. The major part of the drying process is in the ‘constant activity period’, although the silica binder influences the ‘activity’. The formation of small pores at the interface decreases the activity and hence the drying rate is slightly decreased. The transition to the ‘falling rate period’ depends on the initial drying rate for layer drying experiments, though may coincide with the shrinkage limit in the spray drying process. When shrinkage has stopped, the liquid probably retreats in the pores, leaving the surface dry. This process takes place in the ‘falling rate period’ and can be described with the penetration period and the regular regime.
The experiments with the pilot-scale spray drier showed that the influence of the drying conditions on the studied properties is small. The largest influence is found on the morphology of the particles, as cracks manifest and particles break for high drying rates and wrinkles and agglomerates appear for very low drying rates. Using high drying rates increases the pore volume and surface area only a few percent. The particle size distribution is mainly controlled with the nozzle configuration and the solids content of the feed
. The time between preparation and spray drying of the feed has two effects on the FCC catalyst. A short time will decrease the amount of wrinkled surfaces, which is very likely positive for the attrition resistance. On the other hand, waiting a longer time increases the binder particle size and results in more segregation at the surface, which is also positive for the attrition resistance. This subject would be interesting for further study.
For the design of a spray drying process to manufacture FCC catalysts, the ‘falling rate period’ may not be so important. When shrinkage has stopped and the surface is dry, the particles will not be sticky and the morphology of the FCC catalyst is already largely determined. However, it is not quite certain how the binder segregates after the shrinkage has stopped.The models, which describe the spray drying of slurries with and without binder material, gave insight in a possible mechanism for crust formation and compaction during spray drying. In addition, the segregation of binder is described for a deformable crust. The main conclusion is that to be able to control the segregation, one should control the binder diffusion. It will be of interest to refine these models by comparison to more detailed experimental measurements, and to investigate the applicability in practice
Summary
The FCC process is a major technology in oil refining and produces about 40% of the total gasoline. FCC catalyst is a fine porous powder of spherical particles with an average diameter of 60 micron. FCC catalyst consists of different components that are held together by a binder material. In this work a typical
FCC catalyst is used that consists of zeolite Y, a matrix of clay and alumina and a silica binder. The central point of the study was the influence of process conditions on FCC catalyst properties .
With a pilot-plant scale spray drying experiments have been done to investigate the influence of the process conditions on the properties of FCC catalyst. The FCC catalyst has been analysed on moisture content, particle size distribution, bulk density, surface area, porosity and pore size distribution. The morphology has been studied with electron microscopy. The varied process conditions are the air temperature, airflow and droplet size distribution (nozzle configuration). The varied material properties are the feed water content and the batch aging time (time between preparation of the feed and spray drying).
A major result is that, for a standard formulation of FCC catalyst, the properties remain almost constant within the applied spray drying process conditions. Apparently, the silica binder drying behaviour does not influence the porosity of spray dried FCC catalyst. The average particle size can be calculated from the water content of the feed and the in (average) droplet size. The latter depends largely on the choice of the nozzle configuration
B.CATALYST/ADDITIVES:
Q-34: Briefly describe the manufacture of FCC Catalyst bringing out the importance of Spray Drying/Atomisation during the preparation ?
A-34:
FCC catalyst manufacture
Four major categories of manufacturing routes are documented: 1) in-situ formation,2) gel based matrix method, 3) sol based matrix method, and 4) core and shell technology These production processes involve at least the mixing of components followed by spray drying.For the experimental work, the sol based matrix method is applied to prepare the FCC catalyst feed. This method replaced the older gel matrix method, due to improved attrition.
FCC catalyst feed preparation
. Sulphuric acid is mixed with sodium silicate at low pH (1.8 – 3.0) to form a silica sol. At lower pH, a sol of poor quality is formed, and at higher pH, thickening and gelling will occur faster. A dispersed aluminium source may be added to the silica sol. Then clay is added. Because zeolite is alkali, it must be added to a liquid with a pH in the range of 3.0 – 4.5. The spray drier feed should have a pH in the range of 2.8 – 4.0, because at pH lower than 2.8 the zeolite will be destructed and above 4.0 the slurry will be too thick. Rigorous mixing during feed preparation is essential to obtain a hard, dense and homogeneous catalyst. Insufficient mixing can influence attrition resistance, density and morphology as well as activity and selectivity of the FCC catalyst. Further, it is common to add fines to the feed before spray drying. The solids content of the feed affects the particle diameter: a higher solids concentration increases the particle diameter . A higher viscosity of the feed also increases the particle diameter, due to increased energy necessary for atomisation.
Spray drying
Spray drying is the transformation of a liquid containing suspended and/or dissolved solids into a powder by atomising the liquid into a hot drying medium (usually air). The principle of spray drying is extensive contacting of liquid with air. The air transfers heat to the droplet and takes up the evaporated water. The spray drying process is complex due to the simultaneous exchange of momentum, heat and mass. In addition, the properties of the dried material depend strongly on temperature and water content. Furthermore the spray drying process is large and expensive. The spray drying process is said to control the fluidisation and circulation properties of the catalyst in an FCC unit.
The spray drying process is usually divided in four stages: 1) atomisation of the feed into a spray of droplets, 2) dispersion of the spray in the air (spray-air contact), 3) drying of the droplets, and 4) product-air separation. Together with the feed properties, each of these stages determine the drying history of the droplet and hence the final properties.
.
Atomisation
Atomisation is the process that transforms the liquid feed into droplets with a high specific surface area. Commonly used atomisers are pressure nozzles, rotating disc atomisers and two-fluid nozzles. In this work a pressure nozzle is used for Spraying. Pressure forces the liquid through a core and a small orifice. The core swirls the liquid and the after the orifice, a thin conical sheet is formed that disintegrates into small droplets. The spray angle, droplet size distribution and droplet velocity are determined by feed properties (concentration,temperature, viscosity) and operating parameters (pressure and the nozzle type).
Spray-air contact
The liquid sheet and atomised droplets come in contact with the air and the droplets,which have a high velocity, are dispersed. The airflow direction with respect to the droplet motion can be co-current, counter-current or mixed-flow.
Drying
The drying process in a spray drier is complex because millions of individual droplets experience different drying histories. The properties of the dried material are made up of the properties of these dried particles. The description of the drying process varies from simplified, engineering models to sophisticated theoretical models and numerical methods
Product air separation
Product that hits the wall of the drying chamber usually slides down and is collected via a valve at the bottom of the drier. Sometimes the accumulated powder has to be removed from the wall: the used spray drier has two hammers at the conical part of the drying chamber. The product is separated from the exhaust air by cyclones.
Spray driers are built with different drying chamber shapes, air inlet geometries and flow direction. The spray drier used in this work has a co-current airflow, supplied by two ventilators, one in the inlet and one in the outlet duct. The pressure nozzle atomiser is placed in the centre of the inlet airflow.
FCC catalyst characterisation and evaluation methods
The choice of a suitable FCC catalyst is an important factor for the performance of a specific FCC unit. In the industry, the micro activity test is widely used to specify the catalysts cracking quality and attrition tests are used to evaluate the strength.
.
In this work. the particle size distribution is measured with laser scattering and the particle shape by electron microscopy. The skeletal density is measured with helium picnometry and the bulk density with a Stampf volumeter (tapping a certain amount of powder). The surface area,porosity and pore size distribution are measured with incipient wetness impregnation,nitrogen sorption and mercury porosimetry. Pore sizes can be divided into three regions:micro-pores with diameter < 2·10-9 m, meso-pores with a diameter between 2·10-9 and 5·10-8 m and macro-pores, with a diameter > 5·10-8 m.
Experimental
FCC catalyst formulation and feed preparation
The same FCC catalyst formulation has been used in all experiments. The FCC catalyst formulation is a mixture of 24 wt.% zeolite Y, 47 wt.% clay, 15 wt.% alumina and 14 wt.% silica. Dry solids contents are based on 48 hours drying at 200 °C. A silica sol was prepared by vigorously mixing sodium silicate, water and sulphuric acid in a vessel at constant pH. Kaolin clay was added gradually to the sol. subsequently zeolite Y and dispersed alumina were added. For standard experiments the water content of the feed, was 3.6 kgw/kgs (solids content 21.8 wt.%). Other experiments had initial water contents of 4.6 (17.9 wt.%) and 7.3 kgw/kgs (12.0 wt.%). The preparation of the silica sol and the dispersion of alumina results in the presence of salts in the feed. These salts will crystallise during the drying process. The amount of salts is 10.2 wt.% of the dry catalyst weight, which corresponds to 9.3 wt.% of total dry solids. In the manufacture of FCC catalyst the salts are washed out.
Spray drier
The feed is pumped to a pilot plant scale co-current spray drier with a three-plunger pump. The feed is atomised by a Spraying Systems SK pressure nozzle, producing a hollow conical spray with a spray angle of ca. 80°. The diameter of the drying chamber is 2.20 m and the total height is 3.70 m with a conical angle of 60° at 2.0 m from the top. . The electrically heated air enters the drying chamber via an annulus and the nozzle is placed in the centre. The air exits the drier via a horizontal pipe, which is placed about 0.5 m above the bottom and has a downward opening in the centre of the chamber. The airflow then passes a cyclone where particles are separated. Product was both collected from a valve at the bottom of the drying chamber (tower product) and a valve at the bottom of the cyclone (cyclone product).
Drying conditions
The feed flow is mass flow controlled and can be varied between 30 and 80 kg/hr,with corresponding nozzle pressures between 10 and 59 bars.
Evaluation
The central question that initiated this work was how spray drying conditions influence fluid catalytic cracking (FCC) catalyst properties. In order to give an answer to this question some fundamental aspects of spray drying FCC catalyst have been studied.The drying behaviour of FCC catalyst has been studied on a laboratory scale and spray drying experiments have been done on a pilot-plant scale.
Conclusions
The study of the drying kinetics provided insight in the phenomenological aspects of the drying process and gave rules to predict the sorption isotherm and diffusion coefficient with the properties of the single components. The shrinkage behaviour during spray drying is probably exemplary: FCC catalyst shrinks uniformly until the shrinkage limit is reached and then stops shrinking. The major part of the drying process is in the ‘constant activity period’, although the silica binder influences the ‘activity’. The formation of small pores at the interface decreases the activity and hence the drying rate is slightly decreased. The transition to the ‘falling rate period’ depends on the initial drying rate for layer drying experiments, though may coincide with the shrinkage limit in the spray drying process. When shrinkage has stopped, the liquid probably retreats in the pores, leaving the surface dry. This process takes place in the ‘falling rate period’ and can be described with the penetration period and the regular regime.
The experiments with the pilot-scale spray drier showed that the influence of the drying conditions on the studied properties is small. The largest influence is found on the morphology of the particles, as cracks manifest and particles break for high drying rates and wrinkles and agglomerates appear for very low drying rates. Using high drying rates increases the pore volume and surface area only a few percent. The particle size distribution is mainly controlled with the nozzle configuration and the solids content of the feed
. The time between preparation and spray drying of the feed has two effects on the FCC catalyst. A short time will decrease the amount of wrinkled surfaces, which is very likely positive for the attrition resistance. On the other hand, waiting a longer time increases the binder particle size and results in more segregation at the surface, which is also positive for the attrition resistance. This subject would be interesting for further study.
For the design of a spray drying process to manufacture FCC catalysts, the ‘falling rate period’ may not be so important. When shrinkage has stopped and the surface is dry, the particles will not be sticky and the morphology of the FCC catalyst is already largely determined. However, it is not quite certain how the binder segregates after the shrinkage has stopped.The models, which describe the spray drying of slurries with and without binder material, gave insight in a possible mechanism for crust formation and compaction during spray drying. In addition, the segregation of binder is described for a deformable crust. The main conclusion is that to be able to control the segregation, one should control the binder diffusion. It will be of interest to refine these models by comparison to more detailed experimental measurements, and to investigate the applicability in practice
Summary
The FCC process is a major technology in oil refining and produces about 40% of the total gasoline. FCC catalyst is a fine porous powder of spherical particles with an average diameter of 60 micron. FCC catalyst consists of different components that are held together by a binder material. In this work a typical
FCC catalyst is used that consists of zeolite Y, a matrix of clay and alumina and a silica binder. The central point of the study was the influence of process conditions on FCC catalyst properties .
With a pilot-plant scale spray drying experiments have been done to investigate the influence of the process conditions on the properties of FCC catalyst. The FCC catalyst has been analysed on moisture content, particle size distribution, bulk density, surface area, porosity and pore size distribution. The morphology has been studied with electron microscopy. The varied process conditions are the air temperature, airflow and droplet size distribution (nozzle configuration). The varied material properties are the feed water content and the batch aging time (time between preparation of the feed and spray drying).
A major result is that, for a standard formulation of FCC catalyst, the properties remain almost constant within the applied spray drying process conditions. Apparently, the silica binder drying behaviour does not influence the porosity of spray dried FCC catalyst. The average particle size can be calculated from the water content of the feed and the in (average) droplet size. The latter depends largely on the choice of the nozzle configuration
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