QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES:

Q-30:

How do you maintain catalyst activity to optimize FCC Operation and Profitability ?

A-30:

                                Catalyst Management in FCC Units

 Introduction:

The zeolite  catalysts used in FCC is the heart of the whole operation. It is therefore necessary to understand the effects of the catalyst on the actual operation of the unit. Catalyst management basically involves recovery of the catalyst and minimizing losses, replacement of the spent catalyst from time to time, ensuring proper catalyst circulation between the reactor and regenerator and stripping hydrocarbons from it before regeneration. The importance of these operations is discussed in the following paragraphs

.

 Addition of fresh catalyst:

Regeneration of catalyst, no matter how efficient cannot restore its activity to the level of fresh catalyst. Hence it is imperative to add new catalyst from time to time. It is necessary to maintain proper activity and selectivity and compensate for catalyst losses due to some inefficiency in the recovery cyclones. Also the metals deposited on the catalyst cannot be allowed to increase beyond some level for which some of the catalyst must be withdrawn and replaced by fresh catalyst periodically. This will also depend on the quality of the feedstock. The amount of catalyst added is equal to the amount lost and withdrawn. But sometimes low activity catalyst may be provided as replacement in order to prevent excessive activity which in turn leads to coking. Catalyst replacement is also necessary to ensure that the particle size distribution in the FCC unit is properly maintained, as the particles tend to be reduced to fines in the harsh operating conditions of the FCC. Replacement does not always ensure the same levels of activity as was prevailing before the addition of new catalyst. Generally if the cyclone separators are working properly about 80% replacement efficiency is obtained

 Spent catalyst stripping:

Before the catalyst enters the regenerator it must be freed of the hydrocarbons remaining on it. Generally steam is used as the stripping agent. The optimum steam rate is governed by the mechanical design of the riser, the catalyst circulation rate, the porosity and the surface area of the equilibrium catalyst. The stripping steam rate is optimize control the losses it is necessary to monitor the performance of the cyclones. The size distribution of the equilibrium catalyst as well as the fines needs to be considered. While smaller particles lead to easy fluidization, the requirements for preventing excessive catalyst losses are quite the opposite. Hence there are optimum size ranges for the catalyst particles. The cyclone performance can be affected if it has suffered some kind of mechanical damage like the development of perforations in the dip legs or erosion of the internal lining. Sudden surges in the  vapour  rates in the reactor unit could lead to entrainment of even smaller particles. Too high or too low catalyst levels in the disengagement section of the reactor can also lead to the malfunctioning of the cyclones, thereby leading to catalyst Losses.

Preventing  Excessive Coking Tendencies:

Coke formation is an important part of the FCC operation and the heat balance depends on the exothermic reactions involving the combustion of the coke in the regenerator. But excessive coking diminishes catalyst life and in general affects the working of the unit drastically. Coking tendencies depend on the nature of the feed especially its metal content. To minimize these problems it is necessary to avoid having cold spots in the system, minimizing of heat losses from the transfer lines connecting the reactor and regenerator and improvement of the feed-catalyst mixing system by introducing high efficiency feed nozzles

QUESTIONS AND ANSWERS

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

B.CATALYST/ADDITIVES:

Q-29:

What are the typical reactions occur on  FCC Catalyst?

A-29:

Typical reactions occuring on FCC catalyst are as follows:



The Fluid Catalytic Cracking (FCC) process is the most important refinery process .
The reason for this lies in the ability of the FCC process to convert more of the crude barrel into fuel than any other process. The chemical composition of the feedstock to the FCC unit is the most important variable in determining the basic yield structure from the unit

Each type of hydrocarbon reacts under catalytic cracking conditions in certain definite ways. The major difference among hydrocarbons types is in their crackability or extent of conversion for a given set of operating conditions. In all cases, for each type of molecule, increasing the molecular weight or carbon number increases the crackability. A variety of primary and secondary reactions take place during catalytic cracking. These include chain rupture, isomerization, cyclization, dehydrogenation, polymerization, hydrogen transfer, and condensation. Hence, the result of cracking even a simple molecule such as normal paraffin is complex



                                                              FCC-CHEMISTRY

Normal paraffins

Crack mostly to olefins and paraffins and give fair yields of very light

gasoline . The normal paraffins are more difficult to crack than isoparaffins and naphthenes. The reaction rates and products of paraffin cracking are determined by the molecular size and structure.

Naphthenes and Isoparaffins

Tend to crack at about the same rate, but the product distributions are much

different. Naphthenes produce relatively little gas and give excellent yields of

gasoline. The gasoline is of better quality than that from paraffin cracking and

contains appreciable quantities of aromatics, resulting from dehydrogenation of

the naphthene rings

Aromatics

Crack in several ways. The benzene ring is practically impossible to crack.

Condensed-ring aromatics without side chains are converted to a limited extent,

but almost entirely to coke. Alkyl aromatics with side chains containing at

least three carbon atoms crack by the carbon located in the beta position to the

aromatic ring (beta fission) leaving one or more methyl radicals linked to the

ring. Monoaromatics dealkylate to give a high octane naphtha. With long side

chains, secondary reactions will occur, resulting in products similar to those

from the cracking of olefins and paraffins. Generally, more aromatic feeds give

poorer FCC yields. A contributing factor to this general trend is that, as the

number of ring structures in the feed increases, the likelihood increases that

dehydrogenation from contaminant metals will cause multi-ring aromatics to form,

leading to condensation and coking of the catalyst. The molecular structure of

the aromatic hydrocarbons is another important issue regarding their

crackability. The distribution of aromatics according to the degree of

condensation clearly affects the rate of cracking. As the number of rings in a

poly nuclear aromatic molecule increases, the rate of cracking decreases,

although the aromatic content appears to remain the same. The net result of the

catalytic cracking of aromatic hydrocarbons is moderate yields of gas, very

little gasoline, large quantities of very aromatic cycle stock, and high coke

yields

.

Olefins

Olefin seldom appear in catalytic cracking feeds, but their reactions are of

interest because they are the primary products of other cracking reactions

Olefins heavier than about C6  are extremely reactive. The products of olefin
cracking are primarily propylene and butenes, along with butanes from

secondary reactions. Some polymerization and cyclization takes place in olefin

cracking to produce a small amount of cycle stock and fairly high coke yields

Non-hydrocarbon contaminants

Such as nitrogen, iron, nickel, vanadium, and copper compounds, act as

poisons to cracking catalysts. Basic nitrogen, reacts with the acid centers on

the catalyst and reduces the catalyst activity. However, the basicity of

nitrogen compounds at cracking conditions can vary widely. The total nitrogen

content is considered as a sound indicator for cracking inhibition by basic

nitrogen compounds. The metals deposit on the catalyst and cause a reduction in

throughput by increasing coke formation. Finally, the sulfur content of an FCC
feed has no major effect on the crackability of the feed, but it strongly

affects the product distribution and quality

ZEOLITE-Y

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES:

Q-28:How the composition of FCC Catalyst influence its Properties ?

A-28:

Effects of FCC Catalyst Composition on its Properties

Introduction:

A wide variety of zeolite catalysts, having a range of useful properties are being used in different FCC units round the world. These catalysts are tailor-made for specific purposes like selective cracking of the feed stock to produce more olefins or more gasoline as per the requirements. Several catalysts have additives like ZSM-5 mixed with them to enhance the octane number of the gasoline produced or to control the emission of SOx, NOx or CO from the regenerator. In general, as the yield of gasoline increases its octane number decreases, and  this  makes it necessary to add more expensive octane boosting agents to it. As conversion of the feed stock rises, the yields of both gasoline and LCO pass through a maximum and then decrease. But coke formation and gas production show a rising trend  as conversion rises.

Effect of Zeolite Content of the Catalyst:

An increase in the zeolite content of the catalyst leads to an increase in the catalytic activity as expected. For a given conversion, as zeolite input increases the gasoline and LCO yields also increase while hydrogen, C3-C4 olefins and coke yields decrease. The decrease in the unit cell size of the catalyst arising out of the increase in the silica-to-alumina ratio leads to decrease of catalytic activity. Though the USY catalysts are less active compared to conventional  REY zeolites, they retain their activity under more severe operating conditions. For a higher silica-to-alumina ratio there is greater production of LPG fractions, lower formation of coke and lower gasoline selectivity . The unit cell size thus has a major role to play in dictating the selectivity exhibited by the zeolite. Apart from this the decrease in the unit cell size of the zeolite results in an increase in the RON and MON values of the gasoline. This is predominant at very low pore sizes in the region of highly acidic sites. The presence of Na+ ions in the de-aluminated  zeolite framework inhibits the formation of high octane gasoline. This is probably due to the neutralization of the acid sites by the residual free Na ions. Even the preparation method of the zeolites has been found to have an effect on the catalytic activity and hence the final product distribution from the FCC unit.

Effect of Additives:

Several kinds of additives are often added to the zeolite catalysts used in FCC units in order to change the product distribution or to comply with increasingly stringent environmental regulations. Some of these are discussed here.

ZSM- 5

This is a special kind of high silica zeolite which is a member of the pentasil  family. and it is

widely used as an octane boosting additive in FCC. It may be incorpoThis is a special kind of high silica zeolite which is a member of the pentasil  family. It is rated into the catalyst during the preparation or it may be added as separate particles. About 1-3 % of ZSM-5 added to the FCC catalyst can raise the RON of the gasoline produced by up to 3 units. The pore geometry of this additive is such that it prevents the formation of cyclic compounds which lead to subsequent coke formation.

CO Promoter:

This additive consists of metals from the platinum group, present in the concentration of 300 to 800 ppm and distributed over a support, and its use is to facilitate the conversion of CO produced during catalyst regeneration to CO2. Though more uniform burning of the coke is accomplished the additive tends to raise the temperatures during regeneration and also the production of  NOx. Typically 1 to 2.3 Kg of the promoter is added per ton of the fresh catalyst

SOx Additive:

The coke deposited on the FCC catalyst contains sulphur in the form of organic compounds and during regeneration it forms SO2 and SO3. These acidic gases are finally discharged into the atmosphere along with the flue gases. Keeping in mind the harmful environmental effects of the Sox it has become necessary to control their emission from the FCC units. The additive is usually a metal oxide which is directly mixed with the catalyst and it absorbs and bonds chemically with the SO3 in the regenerator. This stable sulphate species is carried to the riser section where it is reduced to H2S and the metal oxide. An excess of oxygen is needed in the regenerator so that the additive may provide the full benefits.

Metal  Passivators:

Metals like Nickel, Vanadium and Sodium are often present in the feed to the FCC unit and they have a detrimental effect on the FCC catalyst as they poison the active sites. To prevent  this metal passivators like Antimony are used. The passivators are injected into the feed and they form an alloy with the nickel. This greatly reduces the dehydrogenation reactions and the resultant production of dry gas and hydrogen.

QUESTIONS AND ANSWERS

I.FLUID  CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES

Q-27:Briefly summarize the Physical Characteristics of  FCC Catalysts ?

A_27:

 Major Physical Characteristics of FCC Catalysts

Introduction:

Several properties of zeolites have to be considered when selecting them as catalysts for FCC. Some of these properties govern the catalytic behaviour of the zeolites while some others affect the mechanical design and operation of the FCC unit itself . These properties are briefly discussed below:

Attrition Resistance:

During the operation of the FCC unit the high gas flow rates and the high temperature often cause the attrition of the catalyst particles and produce fines. Not all of the fines may be recovered by the cyclone separators before the products can be separated. This leads to catalyst losses and causes emission of  particulate matter into the environment . It also governs the rate at which fresh catalyst make-up must be provided to the unit. As the particle size distribution is affected by this, the conditions of fluidization itself might change.An increase in the zeolite content of the catalyst, reduction of the zeolite crystal size and better dispersion of the zeolite within the matrix leads to improved attrition resistance. The nature and quantity of the binder provided to the catalyst also affects the attrition resistance. For example the boehmite form of alumina is more effective in reducing attrition than gibbsite or bayerite. The type, particle size and themorphology of the clay used as filler has a role to play in the attrition of the catalyst. Kaolin has been found to increase the attrition resistance. The conditions maintained during spray drying of the catalyst affects its future attrition resistance.Catalyst attrition resistance is indicated by the value of the attrition index which is obtained from standard methods.

Pore Size Distribution and Pore Volumes:

The pore size distribution has a major role to play in the catalyst properties of zeolites. If the pores are too small, then they have a greater tendency to get clogged by coke and they also exhibit greater diffusional resistance. If the pores are too large they provide a lower surface area for a given volume and hence lower the efficacy of the catalyst and lead to enhanced attrition. The pore size distribution is also an important parameter for the catalyst matrix as well. The shape of the pores is the most important property governing the shape selectivity of the catalyst which is the most important characteristic used in many applications.

Surface Area

The surface area of the catalyst comes from the zeolite and also the matrix. It ranges from over 800Surface Area/gm for conventional Y zeolites and is around 600 m2/gm for USY zeolites. The reduction in surface area is due to the hydrothermal treatment given to the zeolites to enhance their stability which affects their crystallinity and brings about changes in their pore structure. During the steam stripping step carried out before regeneration of these catalysts the pore structure might collapse and thereby there is loss of surface area. Catalysts in which the matrix is also active generally have higher surface areas

Particle Size Distribution:

Most of the FCC catalysts have particle size ranging from 60 to 80 µm. The actual size distribution depends heavily on the conditions prevailing in the spray drying step of catalyst manufacture. Fines generated during the fluidized catalytic cracking often change the size distribution of the catalyst. This might call for changes in the gas velocities to enhance good fluidization in the riser section and might also make it necessary to make the operation of the cyclone separators somewhat flexible. Particulate emission might also result from the fines.

Thermal and hydrothermal stability

The chemical nature of the zeolites, especially the silica-to-alumina ratio, crystallinity, ion-exchangeand residual sodium ions in the zeolite affect its stability. Maintaining the catalyst stability is essential tomaintaining its activity and selectivity. Rare earth exchanged zeolites are exceptionally stable. Stable catalysts retain their pore structure during regeneration also.

Crystallinity

The crystalline nature of the FCC catalysts as determined from X-ray diffraction studies is an indication of the zeolite content. The X-ray studies of the fresh catalyst and the equilibrium catalyst can provide information about structural loss and the catalyst stability

Microactivity Test (MAT) for FCC Catalysts:

The standard Microactivity test of the FCC catalysts is a valuable tool to evaluate the properties of equilibrium catalysts from the FCC unit . Since the operation of FCC units are cyclic the catalysts undergo major changes in their properties from their fresh state and all these evaluations are carried out after they have stabilized i.e. they have become equilibrium catalysts. At the heart of this test lies a fixed bed reactor into which the hot gas oil sample is injected. The catalyst activity is reported as the conversion of the 221C material. The conversion depends on the catalyst-to-oil ratio, feed space velocity and other factors. The coke forming and gas forming tendencies of the catalyst as well as the above mentioned physico-chemical properties of the catalysts cam be obtained from this useful test. It helps in ascertaining the conditions to which the catalyst has been exposed. This test is simple and not much time consuming.

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST/ADDITIVES:

Q-26:Pl.Review the development of FCC Cataysts

A-26:

Development of FCC Catalysts 

At the heart of FCC units are the catalysts themselves. The development of active and
stable FCC catalysts went parallel with the FCC design  development .It was known, that
 for cracking of C-C bonds, the acid catalysts are needed. The first acid catalyst, tested for
 cracking of heavy petroleum fraction, was  aluminium  chloride. But the problems with
 corrosion and the waste treatment were greater than its positive action.

In the 1940’s, silica-alumina catalysts were created and greatly improved over the
natural clay catalysts. It was Houdry, who for the first time used acid-activated bentonite as
active acid catalyst for catalytic cracking. But the most significant advance came in 1962
when zeolite catalysts were incorporated into the silica-alumina structures. Advances in
catalysts have produced the greatest overall performance of FCC units over the last fifty
years.

After natural  alumino silicates,  synthetic aluminoosilicates were prepared with
outstanding cracking properties. Both natural and synthetic aluminosilicates (silica-alumina
catalysts) were known for their Lewis acid sites. The early synthetic amorphous alumosicate
catalysts contained about 13 % wt. of Al2O3 (low alumina), in about 1955 the content of
Al2O3 increased to about 25%. But when the zeolites were put into their structure, strong
Bronsted acid sites resulted, with very easily accessible Lewis acid sites also present. After
experimentation, it was found that these new catalysts possessed all of the properties required
of a successful catalyst: activity, stability, selectivity, correct pore size, resistance to fouling,
and low cost.

In 1962 a catalyst known as Zeolite-Y was added to the active alumina catalyst.
Researchers from Mobil Oil found that by adding small amounts of zeolite into the matrix of
the older silica-alumina catalyst structures, a new catalyst was produced which performed
notably better than any catalyst before. The zeolite catalyst vastly improved gasoline yield
and quality. The first commercial zeolite catalysts were introduced in 1964, and zeolite
catalysts are still in use today.

Not only quality of acid component of cracking catalysts has a great importance for
the use in FCC-process. The very important properties of the catalysts are size and shape of
catalyst particles. In Houdry´s fixed-bed catalytic cracking unit the catalyst – activated
bentonite was probably in the form of pellets. For  Thermofor  catalytic cracking unit the
catalysts were of spherical shape with the diameter of about 1-2 mm.
For FCC technology, the finely powdered catalyst was originally obtained by grinding
the catalyst material. In 1948, the first spray-dried catalyst was introduced, examples are in
The microspherical particles (50-100 μm) were produced with the similar particle-size
distribution as the ground catalysts before. However, the spherical particles showed both
improved fluidization properties as well as significant reduction of attrition losses.

Zeolites as acid component of FCC catalysts:

 Natural zeolites were known from 1765, when the first natural zeolite was discovered
by Swedish mineralogist  Croensted.  Since that time, more naturally-occuring  zeolites as
mineralogical rarities were discovered in volcanic rocks. Because of the very small quantities
of zeolite supply, their use was impossible. Only after the discovery of huge resources of
some types of natural zeolites in sedimentary rocks, great applications were opened in ionexchange
 and sorption areas. At the same time, their catalytic properties were studied, and the
great effort to prepare synthetic zeolites started.

As first prepared synthetic zeolite by Union Carbide in 1949 was A-type zeolite (LTA)
 having Si/Al=1 with very high ion-exchange capacity but it was not possible to convert
 it to acidic form,to remain stable  in the Process conditions of the FCC-technology. The
 next synthetic zeolite prepared in 1950 was zeolite X with faujasite structure (FAU), having
 Si/Al=1.2 is synthesized in Na-form, that is inactive in acid catalysis. It is impossible to
 ion-exchange sodium cation into ammonium cation and to calcine them to obtain H-form
, because such form is not stable and after such treatment the crystalline zeolite structure collapses
. To obtain acid catalytic activity of X-zeolite, the only possibility was the ion-exchange with
 multivalent cations, partially with calcium, but predominantly with rare earths, mainly
 Lantanum and/or Cerium (REX). The rare-earth cations increased the hydrothermal
stability of X-zeolite in FCC process, because the La3+ cations eliminated the negative charge
 of three aluminium atoms in zeolite structure at the same time.

 Very soon after the X-zeolite first synthesis, the next synthetic zeolite with faujasite
structure (FAU) was prepared: Y-zeolite with Si/Al=2.5-3. The higher Si/Al ratio was found
as more stable against acid and hydrothermal dealumination. It was possible by ion exchange
of NaY zeolite to prepare ammonium form, and by calcinations convert into H-form,
possessing strong Broensted acid sites.But, such prepared HY-zeolite was still not stable in the Operating  condtions in FCC - the presence of 100% steam at high temperature in riser, reactor and stripper about 550 °C, in regenerator more than 700 °C. At such “hydrothermal” conditions, the hydrolysis of framework aluminium occurs and the frameworks strongly dealuminates, causing partial and even total collapse of zeolite Y structure.

The first solution of the Y-zeolite instability was ion-exchange with rare-earths cations similarly as in the case of X-zeolites. REY zeolites were for long-time the most used zeolites in  FCC-catalysts. Later, the method of stabilitation of zeolite Y structure without RE-cations  was developed: mmonium-exchanged zeolite was treated in special conditions by 100% steam at  the temperature up to 800 °C,  causing partial dealumination of framework, creation of extraframework aluminium (weak Lewis acid centers), but in the framework still rest  strong  Broensted acid sites necessary for cracking. Such treated Y-zeolites were extremely stable, and were called as “ultrastable zeolites Y” – USY. This kind of Y-zeolites represented the new generation of acid component in FCC-catalysts.The great advantage of the USY zeolites was that during hydrothermal dealumination with steam at the high temperature the secondary mesoporous  structure was created in zeolite crystals.The created mesopores in Y.zeolites increase its cracking activity from the point of view of improving the diffusivity of greater molecules into zeolite crystals to acid catalytic centers.

 Generally, the philosophy of the FCC-catalyst preparation is: to have weak acid 

centers in macroporous part of catalyst particles to insure pre-crack the great molecules of 

residue to smaller molecules which could enter to the mesopores with stronger acidity. 

Product of cracking in mesopores could finally enter the zeolite micropores to crack over 

strongest zeolite acid centers into the smaller molecules, mainly gasoline fraction . 

 The mesopores created in zeolite crystals after hydrothermal dealumination improve 

the diffusivity of greater molecules to strong acid centers and increase the overall conversion 

into gasoline in comparison with zeolite without mesopores.

 The above-mentioned overall pore structure of FCC-catalyst is insured by a complex 

catalyst composition that is generally “know-how” of each company producing FCCcatalysts
. 

Generally, the FCC-catalyst contains the active macroporous matrix (maybe the 

activated clay), mesoporous synthetic silica-alumina, and zeolite. The activated clay after 

calcinations also plays role as a binder and gives to FCC spheres strong resistance against 

attrition. As zeolite component, REY, USY and also combination – REUSY are used. Beside 

of these components, FCC-catalyst could contain also so-called metal traps for the elimination 

of poisoning of acid centers by V and Ni from feed. Moreover, each FCC catalyst could be 

“taylor-made” for certain feed composition and desired products spectra. 

The future in the FCC-catalysts is in the looking for new materials, maybe special kind 

of microporous alumosilicates – zeolites, or even mesoporous molecular sieves with more 

opened pore structure allowing the diffusion even greater molecules into pores, but with 

geometry and strength of acid sites similar to the strongest acid sites known in zeolites. 

One of the results of the effort of Mobil researchers was the synthesis of so-called 

“mesoporous molecular sieves” of M41S family, that are marked as MCM-41 and MCM-48. 

These materials are prepared via mechanisms of the creation of hexagonal micelles 

structures from surfactants (e.g.trimethyl hexadecyl ammonium bromide). Depending on the 

alkyl chain of surfactant and the conditions of synthesis, the prepared mesoporous material 

could have regular uniform mesoporous structure with pore size 3-10 nm 

Such material could be very suitable as acid component of FCC catalyst under 

assumption to preserve its strong acidity, similar as in zeolites, even after steam treatment at 

elevated temperatures. Unfortunately, up to now, the effort for the preparation of enough

stable acid component of FCC catalyst from mesoporous molecular sieves was not successful; 

aluminium in its structure is not as stable as in Y-zeolite

 Special types of FCC catalyst is zeolite ZSM-5 containing catalyst, used as additive to
standard FCC catalyst to increase RON of produced gasoline and/or to increase the propylene
yield. This ZSM-5 zeolites has strong “reactant selectiviy” and crack predominantly n alkanes. Because the cracking over acid centers proceeds via cracking of beta C-C bond to,
created secondary (or terciary) carbenium ions, the main product of cracking of longer n-
alkanes is propene (or butene.)

. 

Conclusions

 FCC-technology represents one of the most expanded processes producing motor fuels 

from heavy distillates and residues. On the one side, the FCC technology from the 

engineering point of view is one of the most sophisticated equipment in the chemical industry. 

On the other side, the key factor in such technology is good active, stabile and selective 

“tailor-made” catalyst, converting specific feed of heavy distillates and residues into desired 

products. Market with FCC-catalysts is the greatest one in all kinds of catalysts, and at the 

same time it is the greatest consumer of zeolites (Y, partially ZSM-5) besides of zeolite A in 

detergents. There is still great effort to look for more active, selective and stabile materials as 

acid components of FCC catalysts to improve their desired performance

QUESTIONS AND ANSWERS--I.FCC/A.PROCESS/SUMMARY

                                       I. FLUID CATALYTIC CRACKING/A.PROCESS

                                                                 SUMMARY 

Fluid Catalytic Cracking is one of the most important conversion processes in a Petoleum Refinery.The process incorporates most phases of chemical engineering fundamentals,such as fluidization,heat/mass transfer,and distillation.The heart of the process is the Reactor-Regenerator,where most of the innovations have occurred.



.The objective of the FCC unit is to convert low-value,high-boiling-point feedstocks into more valuable products such as gasoline and diesel.FCC is an extremely efficient process.The cracking reactions result in deposition of coke on the catalyst.The catalyst activity is restored by burning off the coke with air.The catalyst -burning step supplies the heat for the reactions through circulation of catalyst between reactor and regenerator



The hot pooduct vapors from the reactor are recovered in the main fractionator and the gas plant.The primary function of the main fractionator is to separate the heaviest products,such as light-cycle and decanted oil,from the gasoline and the lighter products.The gas plant seperates the main fractionator overhead vapors into gasoline,C3's,C4's and fuel gas.These products contain sulfur compounds and need to be treated prior to being used. A combination of amine and caustic solutions are employed to sweeten these products


.
FCC catalyst can only be made to flow like a liquid if the pressure force is transmitted through the catalyst particles and not the vessel wall.This means that the catalyst must always remain in a fluidized state as it makes a loop around the reactor/regenerator.The only proper way to evaluate the performance of a cat cracker is by conducting material and heat balance tests.Predictions and evaluation of changes in feed stocks,catalyst,operating parameters,and any mechanical hardware can be made by carrying out these tests.

The material balance test run provides a standard and consistent approach for daily monitoring of a cat cracker.It allows for accurate analysis of yields and proper comparison of unit performance.The heat balance provides a tool for in -depth insight into the operation of a cat cracker.Heat balance surveys are used to determine catalyst circulation rate,delta coke,and heat of reaction.The pressure balance provides an insight into the hydraulics of the catalyst circulation.Performing pressure balance surveys will help the unit engineer to recognize circulation " pinch points" and also to get the most out of the common constraints:the air blower and the wet gas compressor.Finally,the process control features control schemes to operate the unit smoothly and safely.



                                                                       
                                                       

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING/A.PROCESS(Contd.)

Q-24:

When operating with one or more catalyst coolers on a regenerator,what control philosophy do you employ (e.g.constant heat duty,constant regenerator temperature,etc.)?.What are the advantages and disadvantages for each approach? How does operating in full or partial burn impact the control decision?

A-24:

The catalyst coolers are operated to minimize regenerator bed temperature to a limit,usually air or catalyst circulation.Catalyst cooler optimization is an advanced control application.The benefits for minimizing regenerator temperature include: maximize catalyst circulation,minimize dry gas production,minimize swings during feed changes,and reduce catalyst deactivation.

Q-25:

With the introduction of modern riser termination devices (RTD'S) ,what is your experience with the ash content of the main fractionator bottoms (MFB) product?Please describe the testing methodology utilized?

A-25:

Ash composition can vary greatly from one refinery to another.Though the primary ash components are inorganic(catalyst fines,metals,corrosion scale),heavier organic molecules can also be present which stabilize the ash in solution.Typical ash content data for various RTDs range from 0.005 to 0.2 wt% ash,with most in the 0.05 to 0.1 wt% range.The market price for saleable decant oil is strongly a function of ash content.Therefore ,refiners can realize significant economic benefit if the ash content of the slurry is below these specs
.
Testing of the ash content is done accordance with ASTM D-482.For units which are required to meet stringent ash content specifications,slurry settlers and filters have been uesd. All have been problematic and the filters require considerable maintenance.Filter plugging due to asphaltenes and waxes has been an issue at one location.The recycle stream generated by the backwash from these systems has caused feed system fouling and reactor coking.

One refiner is using settling aid to sell clarified slurry into the more lucrative "carbon black" market.

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING/A.PROCESS(Contd.)

Q-22:

Some refiners have installed gas injection in FCC secondary cyclone diplegs to increase  capacity and avoid defluidization problems.Please describe your experience operating with gas addition in the diplegs and any maintenance issues.What advice would you give to others considering this installation?

A-22:

Cyclone dipleg aeration is a technique used to improve cyclone dipleg fluidization.It involves adding a stream of purge gas,usually air or steam,to a location near the bottom of the cyclone dipleg ahead of the trickle valve.It assures that the cyclone dipleg does not defluidize and plug,This can be a problem for secondary cyclone diplegs that are lightly loaded.

The purge flows aee small and regulated via restriction orifice as per normal instrument purges.Purge piping for the coldwall Regenerators enters the top of the vessel and is routed along the diplegs.Purge piping for the hotwall Reactors enters the vessel near the bottom.The purges need to be commissioned before catalyst loading to avoid plugging the aeration point.Erosion at the injection points has not been an issue.The routing of the purge piping needs to consider thermal expansion as well as impingement.Failure of the internal piping is a concern.

Q-23:

Several refiners are considering continuous operation of the combustion air heater to maintain minimum regenerated temperature when processing light feed stocks.What control systems and general precautions should be considered?

A-23:

The combustion air heaters are not always models of reliability,either during ,or perhaps because of their infrequent use.Reliability issues and safety shutdown system  design  would be required before cinsidering use of these air heaters.

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING/A.PROCESS(Contd.)

Q-21:

What minimum nozzle velocities are required in air and steam distributors to prevent
Catalyst back flow and subsequent erosion? Please consider both upward and downward pointing nozzles?

A-21:

The FCCU  has many distributors,including combustion air,stripping steam,various fluffing distributors,and even the feed nozzle system.

Good distributor and nozzle design is about half the battle in good FCC operation. Many
Routine FCC problems are the result of poor distributor operation.This can include poor yields due to poor oil/catalyst contacting,poor stripping due to lack of catalyst/steam contacting,and excessive catalyst attrition due to  distributor damage.

Unfortunately distributor design is challenging in many FCC services,with too many design requirements and not enough degrees of freedom to satisfy all of them.

Distributor designs differ from designer to designer,but there are some common points:

1.The standard for FCC design is a system that incorporates dual diameter nozzles,meaning an orifice controlling the pressure drop to ensure even distribution and no catalyst back flow,and a nozzle diameter that sets an acceptable outlet velocity.

2.Sufficient distance from the orifice to the nozzle such that there is fully developed flow in the nozzle before it exits the nozzle.

3.Sufficient coverage of the process area to ensure good vapour/catalyst contacting,but this is done in two different ways.The more common manner is to have a lot of nozzles,as many as 1 per square foot of process area coverage,with appropriate sized nozzles and orifices to ensure even flow.The second manner is to have nozzles with high outlet velocity and rely on jet penetration for good process coverage.Typical distributor and nozzle designs include the following parameters:

--orifices sized to give pressure drop equal to 30% of the bed static height above the distributor.
--nozzle outlet velocity of 90 to 150 fps.
--minimum nozzle length of 5.2 times the difference between the nozzle diameter and  orifice diameter,with some safety factor applied.Engineering firms use a factor of 1.5-3 times this length to set an acceptable safety factor.
--Drains to allow wet media to discharge freely.

In terms of keeping catalyst from back flowing into nozzles,there are two separate answers,but they are related.

In the event of a single nozzle(like an emergency steam nozzle),the industry rule of thumb is to have 25 fps velocity in the orifice area of the nozzle.This results in low velocity,and should keep the orifice area of the nozzle clear,but catalyst will likely to be back mixing into the nozzle diameter area downstream of the orifice.This means the nozzle must be designed for catalyst temperature.

If there is a distributor (ring,pipe,grid,horse-shoe type designs),the minimum parameter used to prevent catalyst back flow is distributor delta P.Industry guidelines vary from a minimum of 10% bed static height upwards to 30% bed static height.A common industry belief is that 10% minimum bed static height for distributors with downward pointing nozzles,and 30% minimum bed static height for upward flowing nozzles.

The most likely way that we have direct experience with catalyst back flowing into distributors is by loss of pressure of the flowing media into the distributor.This can result in an instantaneous back flow into the distributor.If this happens,it may well plug nozzles until the next scheduled maintenance turnaround on the unit allows entry to the vessel to clear the distributor.