QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:               www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.):

Q-39: What's the difference between a high Al2O3 FCC catalyst and a low Al2O3 one?

A-39:

High and low alumina are generally applied to older style amorphous catalysts. The first synthetic FCC catalyst contained about 12% alumina. High alumina catalysts were introduced in 1955 and were the dominant catalyst until the introduction of zeolite catalysts in 1964. Today, virtually all FCC's run on zeolite catalysts.

Some of these catalysts do, however, contain an active alumina in addition to the zeolite. These may be what you are referring to as high Al2O3 catalysts. The added alumina is intended to improve the conversion of the highest boiling fraction of the FCC feed. For this reason, these catalysts are often referred to as "bottoms cracking" catalysts.

In addition to increasing the bottoms conversion, the addition of active alumina to zeolite based catalysts also tends to increase the catalsyt delta coke (and thus the regenerator temperature). The alumina can also increase the dispersion of nickel deposited on the catalyst and thus, increase its dehydrogenation activity.

                   

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST / ADDITIVES(Contd.)

Q-38:  Discuss briefly "Real time FCC Catalyst Optimisation?

A-38:


                                            Real-time FCC catalyst optimisation 


The FCCU has long been considered the heart of a high conversion modern refinery due to its unique capability of processing a wide range of feedstocks under an equally wide range of process conditions. The unit has matured in terms of control systems over the past six decades, with  most units now being operated under advanced distributed control systems. Additionally, many units are equipped with multiple on-line analysers and kinetic-based simulators, all designed to integrate with the control systems to maximise unit profitability. One striking exception to this trend is found in an independent variable, which has remained elusive in terms of on-line, real-time optimisation: the composition of the FCC catalyst circulating inventory.

For very good reasons, many refiners generally limit catalyst evaluations to once every two to three years. Thorough testing and selection of catalyst technology is typically a medium- to long-term process, requiring dedicated laboratory equipment and six months or more before decisions can be finalised. The FCCU, however, responds to feedstock variation and operating changes in a matter of hours. The ease of catalyst additions combined with the equilibrium nature of the circulating inventory make the unit unique among all refining processes in terms of real-time optimisation. However, the ability to actually optimise the circulating inventory composition remains an elusive independent variable potentially providing an additional degree of freedom in the maximisation of FCC profitability.

In addition to feedstock variation, today’s FCC operator faces an increasingly dynamic market some-times subject to significant volatility. Many fuel markets are observing a transition in which diesel is being favoured over gasoline. Propylene markets continue to evolve seemingly from week to week. The refiner who blends ZSM-5 additive into their base catalyst will often be at a competitive disadvantage to the competitive refiner who injects on demand as the market demands. The ability to alter product slates with dexterity may determine, in the long run, which refineries are profitable vs marginal. The ability to control the circulating catalyst inventory in real-time will be an additional enabling factor, allowing rapid response to market demands.

Two factors have eliminated the possibility for most refiners to optimise their circulating catalyst inventory composition in real-time: the first limitation being hardware related (multi-component addition systems and catalyst hoppers) and the second limitation being the catalytic components themselves. Most operating units today are equipped with one fresh catalyst hopper and possibly one additive addition system added to the unit subsequent to startup. Significant advances in catalyst and additive addition technology, plus catalytic technology have been achieved, which now permit the real-time, on-line optimisation of the FCC circulating inventory.

Overcoming existing technology barriers

The two barriers preventing the implementation of real-time FCC catalyst circulating inventory control are related to hardware and the long lead times necessary for catalyst selection. These two barriers are to be described in further detail within the context of:

• Catalyst and additive loader technology
• Catalyst evaluation lag time.

Catalyst and additive loader technology

Intercat has developed a full line of automated, catalyst and additive addition systems. More than 250 of these additive systems are currently in use around the world, designed to ensure the exact amount of catalyst or additive that is targeted is actually added to the units in small, consistent shots throughout the day. These units are equipped with feedback control systems to ensure the targeted addition levels are actually achieved. The hallmark of these systems has been their high accuracy, reliability and low maintenance . The standard precision observed with these loaders is typically a greater than 99% approach to target. This is observed for both fresh catalyst and additive addition systems.

Multiple compartment catalyst and additive hoppers have been developed that are capable of accurately injecting three separate materials into an FCCU simultaneously . Hopper capacities range from 1–120 tonnes. These systems deliver to the refiner the ultimate level of flexibility and control with respect to catalyst additions.

These loaders present multiple advantages for the innovative refiner choosing to eliminate bottlenecks limiting maximum operating flexibility. The most obvious of these include the capability to begin optimising FCC circulating inventory in real-time. These loaders provide the capability to add an additional degree of freedom to the operation of their units. Specifically, the circulating inventory can be manipulated with the same degree of precision as preheat or riser outlet temperatures. Furthermore, refiners are capable of achieving precision addition rates from a wide range of containers, such as tote bins, super sacks and drums, enabling multi-component catalytic system trials without the necessity to first invest in hopper capacity.

The end result will be a system designed to deliver both the capacity and capability to begin manipulating circulating inventory compositions. This can be achieved via a combination of a wide range of selective additive technologies designed to maximise refinery profitability together with state-of-the-art catalyst and additive addition system technology.

Catalyst evaluation lag time

The second significant barrier to the implementation of on-line, real-time FCC catalyst circulating inventory control is the long evaluation time required for the minimum risk catalyst selection procedures currently in place among most refiners.   For most refineries the catalyst selection procedure can require up to 50 weeks from the time of issuing bid requests until the new catalyst formulation selectivities have been achieved.






·        

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:                 www.wissenschaftler-avh.in
B.CATALYST/ADDITIVES(Contd.)

Q.37     What tools are being used to monitor the FCC performance? What are the key performance indicators (KPIs) and expectations?

A-37

There are many tools that are available to monitor FCC unit performance. Most FCC operators monitor in process, or “as produced”yields and operating conditions, and additionally they complete unit mass balances at routine intervals (often weekly or more frequently).

Of primary importance is the net product value produced by running the FCC. This is most often examined using mass balanced data from the unit, in conjunction with refinery specific product and feed values (pricing). In many refineries, FCC profitability is driven by overall volume gain, so this is an important KPI.

Unit reliability is also a very important profitability parameter, and (along with a strong routine maintenance program), many refiners monitor in process operating parameters to ensure that limits are not being exceeded. Some of these parameters are directly measured in the unit, and some are calculated using in-process or mass balanced data.

Measured parameters include unit temperatures (at various points in the unit, including reactor outlet, regenerator bed, reactor and regenerator dilute phases, and others), unit pressures (reactor & regenerator), wet gas compressor suction and power, regenerator emissions, and slide valve / standpipe differential pressures.

Calculated parameters, which are important to monitor, include yield selectivities (yields / conversion), coke make, superficial velocities (especially in cyclones), and catalyst circulation (or cat-to-oil ratio).The recommended ranges for all of these parameters are specific to the FCC unit’s configuration, feed and catalyst type, and operating strategy.

 Equilibrium catalyst analysis reports can be a critical tool to monitor FCC unit performance, and to troubleshoot the various problems that can arise in typical FCC unit operation. They are most effectively used when they are incorporated into  routine operating reviews with the catalyst supplier. During the reviews, recommendations are often made to adjust operating strategies or fresh catalyst formulation to address future operations or issues that are expected/anticipated for the refinery. These routine reviews are a critical component of the successful operation of the FCC.

The catalyst KPI’s include cracking data (ACE or MAT), physical properties of the catalyst, and chemical analysis of the catalyst. As with yield and operating KPI’s the expectation for these KPI’s varies greatly depending on the feed processed, unit design, catalyst type,and operating strategy of the refinery.In addition, equilibrium catalyst data can be used to benchmark unit performance against similar units, feed types, catalyst types, or a number of other variables. Advanced analytical tools and methods have been developed by catalyst suppliers to understand how age distribution, cyclone performance, contaminant metals, and other variables may impact the performance of the FCC catalyst.

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:               www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.)

Q-36: What are the COMMON TOOLS and ANALYSES which are useful for the maintenance of 
Catalyst properties in FCC?

A-36:



                                    Maintenance  of  Catalyst  Properties

Maintenance of catalyst activity is critical to optimized FCC operation and profitability.
The loss of yield, conversion and selectivity can be acute if catalyst contaminants and
Microactivity  are not sustained at appropriate levels. Effective catalyst management must
be practiced to minimize delta coke and dry gas yields.

Fresh catalyst  microactivity (MAT) is determined by the proprietary components and
formulations of the supply companies. In the U.S., FCC catalyst is primarily supplied by
3 vendors; Grace Davison, Engelhard and Akzo. Each can custom formulate a catalyst
to meet the specific processing goals of the refiner. In general, this falls into three
categories; maximum gasoline, maximum octane or maximum octane-barrels of gasoline.
However, each unique catalyst’s performance is negatively affected by the accumulation
of contaminants absorbed from the feed oil in the Reaction Riser. The primary
contaminants are nickel and vanadium and to a lesser degree, sodium and iron.
Increasing concentrations of these additive contaminants function to shift the reaction
mechanisms  to those less desirable to the FCC processing  goals

 The main negative aspects of nickel accumulation are:

• Deposits on catalyst surface with minimal migration

• Catalyzes undesirable dehydrogenation reactions and coke

• Slightly decreases equilibrium MAT

• Increases delta coke and decreases catalyst circulation rate and cat-to-oil ratio,
leading to lower conversion, lower gasoline yield, lower gasoline selectivity, and
lower total liquid yield. Fuel gas yield and olefinicity of liquid products will be
increased.

The main negative aspects of vanadium accumulation are: 

• Migrates to the active zeolite components and destroys them

• Decreases equilibrium MAT

• Slightly catalyzes undesirable dehydrogenation reactions and coke

• Decreased MAT leads to lower gasoline and LPG liquid yields, lower gasoline
selectivity, and lower total liquid yield.

Overall, high catalyst contaminant concentrations are always unfavorable as they result in
gasoline and LPG product yield decreases with corresponding increases in Light Cycle
Oil (LCO), Slurry, Dry Gas and Coke. Whether by reaction mechanism or zeolite
destruction, delta coke increases with a resultant increase in Regenerator temperatures,
loss of catalyst circulation rate and loss of cat-to-oil. Stripper operation may also be
affected due to increased heavier, hydrocarbon undercarry.


Although each FCC is unique, certain common tools and analyses are useful
 to maintain the proper activity and contaminant levels. These include

• Routine laboratory analysis of feed contaminant concentrations to allow variation
of catalyst addition/withdrawal strategies

• Weekly Equilibrium Catalyst analysis by the FCC supplier to determine trends in
Contaminant  concentrations and impact on MAT and surface areas

• Survey and review of Regenerator temperatures to assess shifts related to
Changing  MAT or contaminants Solve the Five Most Common FCC Problems

• Frequent computation and review of heat and material balances and key
Performance  indicators. These would include coke yield, delta coke, air-to-coke
ratio, catalyst circulation rate, cat-to-oil ratio, conversion, gasoline yield, gasoline
selectivity,  gasoline octane or olefinicity,  total liquid yield, total dry gas yield, dry
gas SCFB, dry gas hydrogen concentration, and dry gas hydrogen to methane
ratio
.
• Injection of nickel or vanadium  passivation  additives to counteract metals effects.
Increasing steam or injection of sour lift gas into the Riser may also have a slight
Benefit  to decreasing nickel and vanadium effects

• Optimized catalyst addition and withdrawal program to maintain a desirable range
of contaminant concentrations.  The necessary fresh catalyst additions are based on
 feed total  Ni+V concentration to arrive at an equilibrium metals  concentration on the
 catalyst in the inventory.

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:                  www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.)

Q-35:  How do you  predict  FCCU  Performance with Laboratory Testing?

A-35:

  Predict  FCCU Performance with Laboratory Testing

Introduction

 Conducting testing before commercial implementation reduces risk for a refiner

 Examples of questions that pilot testing can answer include:

• What will be the effect of a potential feedstock change on yields?
• How will a new catalyst technology perform?
• Which catalyst technology is best for my operating goals?
• What effect will an additive have on my yield structure?

 On a lab scale, the goal is to match the complex processes occurring in a commercial FCC unit. In the unit, catalyst deactivates over a period of many weeks due to temperature, steam, and contaminant metals. Commercial deactivation conditions are too slow to be practically copied in the lab, so an accelerated lab deactivation is done to generate a simulated Ecat to match the chemical and physical properties of the commercial Ecat. Bench scale (ACE or MAT) or pilot-scale (DCR) test equipment is then used to simulate the reaction conditions in the FCC unit  and react catalyst and feed to produce products.

Laboratory Deactivation

Approaches

When studies are being done for feedstock selection or process development, commercial Ecat is usually used and no lab deactivation is needed. However, catalyst selection studies and catalyst R&D start with fresh catalyst. Fresh FCC catalysts need to be deactivated before testing because fresh catalysts are too active and the selectivities seen in fresh catalysts do not represent  Ecats in the FCC unit. Temperature, metals and steam are therefore used to turn fresh catalyst into simulated Ecat.

 Commercial Ecat properties that we want to match with simulated Ecat include: surface area, unit cell size (UCS), metals concentration, metals oxidation state, and metals distribution.
Accelerated conditions to simulate hydrothermal deactivation of zeolite typically involve times of 2 to 50 hours, temperatures between 1400°F and 1525°F, and steam concentrations between 50 and 100%

. At temperatures below 1400°F, it may be impossible to match the equilibrium unit cell size, and at temperatures above 1525°F unrealistic zeolite sintering can be encountered.Contaminant metals such as nickel and vanadium accelerate catalyst deactivation and have dehydrogenation activity that increasescoke and hydrogen. It is important to test catalysts with contaminant metals in order to realistically assess the performance of catalysts with metals trapping and passivation technologies. The best way to simulate the contaminant metals is to apply the same metals level to the fresh catalyst that is present on the Ecat, and then deactivate all the fresh catalyst samples in a study under the same conditions.

Deactivation methods that simulate poisoning by contaminant metals include the Mitchell method (MM), cyclic metals impregnation(CMI) and cyclic propylene steaming (CPS)

 The Mitchell method involves impregnation of fresh catalyst with organic Ni and V naphthenates followed by steam deactivation for 4 to 20 hours.

 The CMI method involves multiple cycles of cracking with metals spiked feedstock and regeneration, resulting in a deactivation time of more than 50 hours.

 The CPS method involves impregnation of fresh catalyst with organic Ni and V compounds, followed by aging in a cyclic redox environment for ~20 hours. The reducing atmosphere (which simulates the riser) is a blend of steam, nitrogen and propylene, and the oxidizing atmosphere (which simulates the regenerator) is a blend of steam, air and SO2.. Grace developed the CPS deactivation method

 The CPS method provides a good match to Ecat properties. The CPS method has been adopted by many labs around the world and can be easily fine-tuned to match the severity and specific deactivation conditions of different commercial units by adjusting the temperature. In deactivating catalyst.with contaminant metals, it is important to include the effect of sulfur competition by using SO2 as part of the simulated regenerator conditions. Under commercial regenerator conditions, calcium oxide and barium based metals traps are rapidly poisoned by sulfur and lose their vanadium trapping ability. This sulfur poisoning does not happen with rare earth based vanadium traps. Testing of vanadium traps in the laboratory without simulating the SO2.present in a commercial regenerator can give a false prediction of catalyst metals trapping ability.

Commercial FCC units differ in their catalyst turnover rates. When it is desired to very closely match lab simulated Ecat to Ecat from a specific refinery, age distribution deactivation can be used. Commercial Ecat consists of catalyst particles with varying age, surface area,unit cell size (UCS), metals level, activity, and selectivity. Sink/Float experiments that separated refinery Ecat into age fractions have determined that activity and selectivity are dominated by the youngest fraction of the catalyst. Typically, the youngest 20% of the inventory contains less than 10% of the contaminant metals and contributes about 50% of the overall activity. For a specific unit, the metals and activity distribution will depend on catalyst addition rate, deactivation rate and catalyst activity.

In summary
 fresh FCC catalysts need to be deactivated before testing. Cyclic propylene steaming (CPS) is a rapid method to match Ecat properties and yields. To better match a specific refinery’s Ecat, CPS with age distribution can be used, but it is more time consuming.

FCC Catalyst Testing

Performance testing of FCC catalysts can be done by either bench scale testing or pilot plant scale testing. Examples of bench scale testing equipment include fixed bed microactivity  testing (MAT) and fixed  fluidized bed testing, one example of which is the ACE ® (Advanced Cracking Evaluation) instrument marketed by Kayser Technology. Several pilot plant designs are in operation throughout the world and include both once through and circulating designs. The most common is the Grace developed Davison Circulating Riser(DCR) .MAT and ACE testing have the advantages that they are easy to set up and require small amounts of material. The DCR pilot plant has the advantage that it mimics all the processes present in commercial operation and it can operate at the same hydrocarbon partial pressure.. Both the regenerator and the stripper are equipped with slide values for control of catalyst circulation rate.The DCR is typically operated in adiabatic mode, where changing feed preheat or regenerator  temperature will result in a change in catalyst circulation to maintain reactor outlet temperature, the same process control strategy used in many commercial FCC units. Due to the similarity between the DCR riser and the commercial FCCU riser, yields obtained from the DCR simulate commercial FCCU. The DCR is a highly flexible pilot plant and has been used to successfully evaluate many different feedstocks including resids, naphthas, gases, and feeds from non-petroleum sources such as vegetable oils and pyrolysis oils.

             

QUESTIONS AND ANSWERS

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

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES:

Q-33

what processes are currently available to recover precious metals from spent catalysts?

A-33

There are two proven methods for the recovery of precious metals from spent catalysts

. Hydro-metallurgy is generally used as the best application resulting in highest precious metals yields when high surface alumina catalysts are being recovered. Typically, more than 50% of these catalysts contain the high value metal rhenium (Re) as well, which can only be recovered using this method. The use of either the alkaline or sulfuric acid leach process depends mainly on the infrastructure to dispose off Al by-products. At Heraeus the alkaline pressure leach is used in Hanau, Germany, to initiate the recovery process while sulfuric acid leach is used in Santa Fe Springs, California. Both methods produce similar results.

Pyro-metallurgy is generally used when zeolyte substrate catalysts are being recovered. This employs traditional ‘smelting’ technology. This method is also preferred when a gamma-based catalyst has been subjected to extreme temperatures during operation and subsequently suffered a major alumina phase change, rendering the substrate highly insoluble.

    

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES:

Q-32:

What conditions or contaminants will deactivate ZSM-5 additive? What is the half-life of ZSM-5 in clean feed operation? Will contaminants such as vanadium, sodium or other metals adversely affect the propylene selectivity of ZSM-5? What is the best way to monitor the effectiveness of the ZSM-5?

A-32:

 The deactivation mechanism for FCC catalyst is primarily related to unit cell size reduction and, eventually, collapse or sintering of the zeolite crystal. The mechanism of ZSM-5 deactivation is quite different.

The deactivation mechanism is simply the dealumination of the ZSM-5 crystal, and activity is lost through the loss of active aluminum sites. The crystal structure does not collapse. The activity retention and half-life of the ZSM-5 additive in the circulating inventory is strongly affected by hydrothermal conditions within the regenerator with temperature being the dominant variable.
ZSM-5 additive activity is less affected by contaminant metals than is FCC catalyst due to the fact that heavy feed molecules containing contaminant metals, such as vanadium, are less likely to crack on ZSM-5. ZSM-5 will therefore maintain its activity longer than  the FCC catalyst.

It is worth pointing out that a unit experiencing high equilibrium vanadium levels will likely experience a loss in conversion, which will reduce LPG yields. This loss may give the appearance of a ZSM-5 effect. The propylene selectivity will likely remain unaffected.

The activity retention difference between ZSM-5 and FCC catalyst will increase as the equilibrium metals level increases. Intercat has evaluated ZSM-5 additive half-lives for several units and found a typical half-life of about 18 days, with a minimum of 2 days and a maximum of 36 days.

ZSM-5 additive activity in an operating unit is strongly affected by the catalyst replacement rate. Units having a very high replacement rate present a higher average ZSM-5 activity than units with very low changeout rate. A paper presented at the 2000 American Chemical Society conference investigated the subject of LPG selectivity differences in detail. This study reviewed additives having different ZSM-5 crystal content, different levels of additive additions in the FCC, additives from different manufacturers, additives with different silica-to-alumina ratios, and additives steamed at different severities.

The results of the study   demonstrate that if one additive were more selective than another, the propylene yield would fall on a different line, which did not occur. All additives tested at all concentrations fell on the same line. We also found that the propylene yield increases faster than butylene yield and that higher delta LPG yield leads to higher propylene yields
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Additive zeolite content, type, method of manufacture, and steaming severity have no effect on the selectivity of the final LPG product. The ratio of propylene to butylene in the final product depends only on how much LPG is made. The conclusion is that ZSM-5 additive selectivity is determined by the zeolite structure alone. Therefore, measuring activity differences is more important than looking for selectivity differences with a standard ZSM-5 additive. (Please note that these results apply only to standard ZSM-5 technology.)

While propylene selectivities are determined by the ZSM-5 crystal structure, the activity and stability of the various additives are determined by the crystal stabilization technology employed plus the interaction of the crystal with the matrix. An additive containing properly stabilized ZSM-5 crystal, combined with a strong matrix, will result in excellent activity retention with superior propylene yields when compared to other technologies.

Intercat and other suppliers have invested significantly in development of ZSM-5 additive technology, which is reflected in our broad product portfolio. Intercat possesses an extensive range of ZSM-5 additives maximizing propylene, butylene, and octanes. Additionally, Intercat produces ZSM-5 additives that minimize LPG increase for wet gas compressor-limited operations.

There are several selectivity-based ratios that can be used in monitoring ZSM-5 performance. These include: propane olefinicity, propylene yield vs. LPG, propylene vs. conversion, propylene vs. butylene, and propylene vs. gasoline. The most important of these ratios are the propane olefinicity and the propylene-to-LPG.

   

QUESTIONS AND ANSWERS

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B.CATALYST/ADDITIVES(Contd):


Q-31: Troubleshoot the most frequently encountered FCC Catalyst Losses

A-31:

Introduction

Catalyst losses from the FCC unit are of great concern to the refiner due to their impact on the environment, unit operation and catalyst cost. As a consequence, the losses are held down to the minimum practical level. In a well designed and operated unit losses below 0.1 pounds per barrel of fresh feed (0.1 Lbs/Bbl-FF) are achieved. However, problems with excessive catalyst losses (>0.2 Lbs/Bbl - FF) are not uncommon as there are numerous possibilities for creating the trouble. These include changes in operating conditions, mechanical problems (cyclone, air grid, etc., damage) or poor catalyst retention   properties .The following discussion will assist the refiner in trouble-shooting the most frequently encountered loss problems.

Increased Loss Determination

Catalyst loss problems are easily determined by the refiner when they observe any increases in either:

- Ash content of the slurry oil is Basic Sediment & Water (BS&W)

- Stack opacity

- Fines collection from the precipitator or tertiary recovery system

- Catalyst additions in order to hold unit inventory constant.

Loss Patterns Provide Clues to Cause

Observing the catalyst loss patterns provides clues as to the possible cause of the problem

Two typical patterns are:

Decrease In 0-40 Micron Fraction

The first of these patterns occurs when a catalyst loss problem is accompanied by a reduction in the 0-40 micron fraction of the circulating inventory and increasing average particle size (APS). Additionally, the increased losses are limited either to the reactor or regenerator side of the unit. These patterns indicate that the cyclone system was experiencing problems due to:

- excessive dip leg catalyst backup

- holes in the cyclone  or internal plenum

- spalled coke or refractory lodged in the dip leg

- flapper valve  problems (coke build-up on the counter weight, counter weight improperly set or valve lodged open with coke)

- trickle valve   problems (erosion of the valve seat or the valve immersed in an  unfluidized  zone)

- false  bed or stripper levels which interfere with flapper or trickle valve operation. (  generally,   flapper valves do not perform well if covered, while trickle valves perform best when submerged).

The catalyst losses will increase disproportionately from the vessel (side) containing the malfunctioning / damaged cyclone. However, losses from the unaffected side might also increase if the fresh catalyst make-up rate is raised to maintain inventory.

No Decrease In 0-40 Micron Fraction

A simultaneous increase in catalyst losses on both the regenerator and reactor sides, without a decrease in the 0-40 micron fraction, indicates an increase in catalyst breakage

The following are possible causes:

- catalyst attrition by high velocity gas jets. These could be from a damaged feed nozzle, aeration nozzle, stripping steam distributor or open blast taps

- air distributor damage that results in impingement of catalyst against the vessel walls or other hardware

- damaged torch oil nozzle that results in poor atomization

- use of fresh catalyst that is "soft" (non-attrition grade), possesses low bulk density or contains excessive amounts of 0-40 micron material.

Particle size distribution and microscopic examination of the catalyst fines are useful in determining the cause of the loss problem. Average particle size of fines from the main fractionators  tower bottoms or the fines leaving in the overflow of the secondary regenerator cyclone can serve as a guide to the cause of the problem. High average particle size (APS) and the presence of extremely coarse particles indicate a hole or a malfunctioning cyclone. Low APS of these fines  suggests a catalyst related problem or the presence of an "attritor" in the unit.

Operating Conditions Affect Losses

Operating conditions of the FCC unit can contribute to increased catalyst losses regardless of the cyclone problems discussed earlier. Operating variables that affect catalyst entrainment and inlet velocity to the cyclones as well as fines generation in the FCC unit will  effect catalyst losses. Some of the major variables affecting these are discussed below.

Catalyst Circulation Rate

Catalyst attrition in the unit is directly related to the catalyst circulation rate. At a given feed rate, increasing catalyst circulation will increase fines generation. The increased losses would be observed on both sides of the unit

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Superficial Gas Velocity

Increasing superficial gas velocity in a vessel will increase catalyst entrainment to the cyclones and result in increased catalyst losses. In most instances, cyclone efficiency improves with higher gas throughput but not sufficiently to offset the increase in solids loading.

Superficial velocity will be increased on the reactor side of the unit by:

- increasing feed rate

- increasing reactor temperature and unit conversion

- reducing operating pressure

- using or increasing dilution water/steam to the riser/ reactor.

The superficial velocity on the regenerator side will be increased by:

- increasing air rate

- increasing regenerator temperature

- reducing operating pressure

- increasing quench steam.

- Increasing the superficial velocity will increase cyclone inlet velocity and increase cyclone pressure drop. The dip leg exit is sealed against the pressure difference by the static head of the catalyst in the dip leg. Increased pressure drop will result in increased catalyst back-up in the cyclone dip legs.Normally, a margin of several feet is allowed between the top and the maximum operating level in the dip leg to allow for fluctuations in catalyst unit inventory. Excessively high levels of catalyst back-up in the dip leg will result in re-entrainment of catalyst inside the cyclone with subsequent loss.- Occasionally, a unit will experience increased catalyst losses when the feed rate and consequently the air rate is reduced substantially below the design level. In this case, the catalyst entrainment is reduced but not sufficiently to over come the decrease in the cyclone efficiency.In order to minimize the problem, the superficial velocities should be maintained artificially high by using additional steam or air on the reactor and regenerator sides of the unit respectively.If the throughput reduction is for a substantial length of time, some refiners have taken a few cyclones out of service in order to improve operating efficiency of the remaining cyclones. In this instance, the total solids entrainment remains the same but losses are reduced due to improved cyclone efficiency as a result of higher inlet velocities.

Corrective Actions

Depending on the nature of the loss problem, the refiner should consider the following solution.

Use Harder and Denser Catalyst

A denser, more attrition resistant catalyst will reduce fines production, lower catalyst loading and will be retained better in the unit. As a consequence, the stack opacity and slurry BS&W will be reduced. A catalyst switch should be contemplated if current unit retention is inadequate due to increased capacity or severe attrition conditions.

Shutdown to Repair Mechanical Problems

Mechanical problems generally require a unit shut down to correct. As an interim measure, many refiners increase the amount of purchased equilibrium catalyst in the FCC make up in order to compensate for increased losses while maintaining desired catalyst activity in the unit.

Reduce Cyclone Loading

A reduction in the cyclone loading by reducing the feed rate, dilution steam/water, catalyst circulation, etc. sometimes is beneficial in reducing losses. A switch to a denser more attrition resistant catalyst may also improve unit operation and retention.

Install Redesigned Dip Legs

Dip leg sizing should be checked. If the size is inadequate for the new sustained operating conditions, then the dip legs should be replaced. An undersized dip leg will choke and reentrain the fines back into the gas flow while an oversized dip leg may allow the catalyst to deaerate and plug.

Guidelines for estimating entrainment, pressure drop, dip leg sizing and cyclone efficiency are contained in Chapter 13 of the API Publication 931. Cyclone manufacturers should be consulted when vigorous evaluation of cyclones are required.

Preventive Measures

There are several things that the refiner can do as preventative measures to ensure proper operation of the cyclones. These should include thorough cyclone inspection for holes, refractory loss or coke build up prior to start up.During the start up the coke build up in the reactor cyclones can be minimized by using a light feedstock and by ensuring that the reactor is at operating temperature prior to introducing feed.

Summary

There are numerous reasons for increased catalyst losses from an operating FCC unit. In determining the cause, the refiner should review in great detail recent operating conditions and the physical properties of equilibrium/fresh catalysts. Additionally, list current mechanical problems in the unit. Compare these rate  to the period when the losses were normal. The review will help to pin-point the reasons for the problem.