catalytic cracking reference material

8/2/2019 Catalytic Cracking Reference Material

1/21

INTRODUCTION

Catalytic cracking is today the most widely used process for increasing the ratio of light to heavy

products from the refinery. The origin of the process actually goes back into the 1920's when

thermal cracking was the principal process used for this purpose.

In the mid-1920's, an engineer named Houdry found that an acid activated clay was an effective

catalyst for cracking heavy oil to lighter product, including high octane gasoline. In 1931, in

partnership with SOCONY-Vacuum (now Mobil), he founded the Houdry Process Company to

exploit the Houdry fixed bed catalytic cracking process. This was a cyclic process, alternating

conversion and regeneration steps. It was first commercialized in 1937 and, by 1940, there were

14 such units built.

The next advance was Mobil's Thermofor Catalytic Cracking, which employed a moving catalyst

bed. In this process, catalyst flowed by gravity down through a reaction zone, then to a

regeneration zone. Regenerated catalyst was moved back to the top of the reactor to repeat the

cycle. At first, this was done by a bucket elevator system, later by pneumatic lift. Units were

built using the TCC process up to the early 1950's.

Fluid Catalytic Cracking development started in the 1930's following the discovery that, under

proper conditions, finely divided solids could be made to flow like liquids. Such small particles

offered advantages in heat transfer and mass diffusion over the large catalyst pellets used in other

processes.

In 1942, the first FCC unit was started up. Due to heavy wartime requirements for motor and

aviation fuels, and the capability of this process, 34 new units were put on stream between 1942

and 1945. Installed capacity was over 500,000 barrels per day.

The Fluid Catalytic Cracking Process (FCC) is a process for conversion of straight-run

atmospheric gas oil, vacuum gas oils, certain atmospheric residues, and heavy stocks recovered

from other operations into high octane gasoline, light fuel oils and olefin-rich light gases.

Features of the process are a relatively low investment conversion process, reliable long-run


2/21

operations, and an operating versatility that enables the refiner to produce a variety of yield

patterns by simple adjustment of operating parameters. The product gasoline has an excellent

front-end octane number and good overall octane characteristics. Further, FCC gasoline is

complemented by the characteristics of alkylate produced from the olefinic by-products because

alkylate has superior mid-range octane and excellent sensitivity.

The process employs a catalyst in the form of very small particles, which behave as a fluid when

aerated. The fluidized catalyst is continuously circulated from a reaction zone, where the

cracking reactions occur, to a regeneration zone where the catalyst is reactivated. In addition to

providing the catalytic action, the catalyst is also the vehicle for the transfer of heat from the

regeneration to the reaction zone. These two zones are located in separate vessels called the

reactor and the regenerator.

PROCESS APPLICATION

One of the strengths of the FCC process is its versatility to produce a wide variety of yield

patterns by adjusting the basic operating parameters. While most units have been designed for

gasoline production, units have been designed for each of the three major operational modes.

Gasoline Mode

The most common mode of operation of the FCC unit is aimed at the maximum production of

gasoline. This condition requires careful control of reaction severity, which must be high enough

to convert a substantial portion of the feed, but not so high as to destroy the gasoline that has

been produced. This balance normally is achieved by using a very active and selective catalyst

and enough reaction temperature to produce the desired octane. The catalyst-circulation rate is

limited, and reaction time is confined to a very short exposure. Since this severity is carefully

controlled, there is normally no need for any recycle of unconverted components.


3/21

High-Severity Mode

If reaction severity is increased, an operation producing additional light olefins and a higher-

octane gasoline will result. Severity may be increased by increasing reactor temperature,

catalyst/oil ratio, or both. This case is sometimes described as a liquefied petroleum gas (LPG)

mode or as a petrochemical FCC because of the increased quantity of light material that is

produced and the increased aromatics in the gasoline product. If isobutane is available to

alkylate the light olefins, or if they are polymerized into the gasoline boiling range, very high

total gasoline yields and octanes can be produced.

Distillate Mode

If the reaction severity is strictly limited, the FCC unit can then be used for the production of

distillates. By a change in operating conditions, a shift from the normally gasoline-oriented yield

distribution to one with a more nearly equal ratio of gasoline-to-cycle oil can be accomplished.

Additional distillates can be produced at the expense of gasoline by reducing the end point of the

gasoline and dropping the additional material into the light-cycle-oil product. The usual

limitation in this step is reached when the resulting cycle oil reaches a particular flash-point

specification.

PROCESS DESCRIPTION

For the following descriptions, refer to the FCC Unit process flow diagram (Figure VI-1).

Reactor/Regenerator Section

Pumped raw oil charge is preheated by the fractionation column (main column) bottoms and side

stream products. Preheated raw oil and recycle streams are introduced into the bottom of the

reactor riser, together with a controlled amount of regenerated catalyst plus lift gas. Lift gas is a

mixture of steam plus dry gas from the refinery gas plant. The catalyst flow is controlled to

maintain a desired reactor temperature. Hot regenerated catalyst vaporizes the feed, and the

resultant vapors carry the catalyst upward through the riser. Cracking occurs as the hydrocarbon

vapors and catalyst travel up the riser. At the top of the riser, the desired cracking reactions have

been completed and catalyst is quickly separated from the hydrocarbon vapors to minimize


4/21

further cracking reactions. The catalyst-hydrocarbon mixture is discharged into the reactor

vessel through disengaging arms.

Final separation of catalyst and product vapor is accomplished in the cyclones. The reactor

product vapors flow through the reactor vapor line to the main column where they are condensed

and fractionated into gaseous co-products, gasoline, cycle oil products, and a heavy residual

bottoms material.

During the cracking reaction, a carbonaceous by-product called coke is deposited on the

circulating catalyst. This catalyst, referred to as spent catalyst, drops from the reactor chamber

into the stripping section where a counter-current flow of steam removes some adsorbed

hydrocarbon vapors. The stripped catalyst flows from the reactor stripper through the reactor

standpipe to the regenerator where the coke is continuously burned off. The catalyst flow

through the reactor standpipe is controlled to balance the circulating catalyst flow by maintaining

a constant reactor catalyst level.

In the regenerator, the heat of combustion raises the catalyst temperature to the 1200-1375F

(650-750C) range. The purpose of this regeneration is to reactivate the spent catalyst so that

when catalyst is returned to the reactor riser, it is in the optimum condition to perform its

cracking function. The regenerator also serves to burn the coke from the catalyst particles and

transfer heat to the circulating catalyst. Energy carried by the hot regenerated catalyst is used to

vaporize and heat the oil vapor to the desired reaction temperature in the riser, and provides the

heat of reaction necessary to crack the feedstock to the desired conversion level. Combustion

gases from the coke burn in the regenerator may be conducted through a CO boiler for steam

regeneration.

The regenerator is normally operated at conditions that achieve complete combustion of CO to

CO2. However, the combustion temperature can be varied for partial CO combustion if

processing conditions allow a lower level of heat generation. The regenerator is equipped with a

catalyst recirculation standpipe, which supplies hot regenerated catalyst from the upper to the

lower regenerator to provide additional heat for combustion. The recirculation catalyst flow is

normally controlled to maintain the lower regenerator temperature. The regenerator is also


5/21

provided with a combustion air startup air heater which is used to supply heat to the system until

the catalyst temperature is raised sufficiently for auto regeneration.

Figure XIII-1

FCC Process Flow

The sensible heat of the hot flue gas is recovered in a flue gas cooler steam generator. Flue gas

exits through cyclone separators to minimize catalyst entrainment prior to discharge from the

regenerator.

In order to maintain the activity of the working catalyst inventory at the desired level, and to

make up for any catalyst lost from the system, fresh catalyst is introduced into the circulating

catalyst system from a fresh catalyst storage hopper. An equilibrium catalyst storage hopper is

provided to hold regenerated catalyst withdrawn from the circulating system as necessary to

maintain the desired working activity. *

Fractionation Section


6/21

Reactor product vapors flow to the main column where gasoline and gaseous, olefin rich co-

products are vaporized overhead. The gasoline fraction is condensed from the overhead material

in the overhead condenser. Vapor and liquid are then routed to the vapor recovery unit from the

main column receiver. The naphtha and light and heavy cycle oils are recovered as side-cut

products with the net yield of these materials being stripped for removal of light ends and sent to

further processing or storage. Net column bottoms is clarified in the slurry settler and the

pumped clarified oil exchanges heat with the raw oil before flowing to storage. Heavy slurried

material is recycled to the reactor riser.

Maximum usage is made of the heat carried to the main column by the hot reactor effluent vapor.

Circulating light and heavy cycle oils, and main column bottoms streams are utilized in the vapor

recovery unit for heat exchange purposes. Additional main column bottoms streams are used for

preheating raw oil feed and for steam generation.

Sour water resulting from steam additions is collected and sent to the refinery sour water stripper

for acid gas elimination and for water recovery.

PROCESS CHEMISTRY

Feedstocks for the FCC process are complex mixtures of hydrocarbons of various types, from

small quantities of gasoline up to large molecules of 60 carbon atoms. There is a relatively small

content of contaminant materials such as organic sulfur, nitrogen compounds, and

organometallic compounds. The relative proportions of all these materials vary with the

geographic origin of the crude and the particular boiling range of the FCC feedstock. It is

possible, however, to rank feedstocks in terms of their crackabilities, or the ease with which

they can be converted in an FCC unit. Crackability is a function of the relative proportions of

paraffinic, naphthenic, and aromatic species in the feed. Generally, one can correlate

crackability of FCC feedstocks with the UOP characterization factor K:

where TR is the molal average boiling point of the feedstock, R; and s is its specific gravity. A

large amount of experimental and commercial data can be classified as follows:


7/21

Range of K Relative Crackability Feedstock Type

>12.0 High Paraffinic

11.5-11.6 Intermediate Naphthenic


8/21

in catalytic cracking are in the range where thermal cracking can also occur. Alternatively, the

ion could be formed by the interaction of the hydrocarbon molecule with an acid site on the

catalyst. The exact mechanism is not well understood. Once formed in the feed, the ions can

react in several ways.

Crack to smaller molecules React with other molecules Isomerize to a different form React with the catalyst to stop the chainThe subject of catalytic coke formation by cracking catalysts, especially its chemical nature and

how it is formed, is also a complex topic for which many theories have been proposed. The

formation of coke on the catalyst, an unavoidable situation in catalytic cracking, is likely due to

dehydrogenation (degradation reactions) and condensation reactions of polynuclear aromatics or

olefins on the catalyst surface. As coke is produced through these mechanisms, the H/C ratio of

the coke increases until it becomes nonvolatile and eventually blocks the active acid sites and

catalyst pores. The only recourse is to regenerate the catalyst to retain its activity by burning the

coke to CO and CO2. This is done in the FCC regenerator. As will be seen, this coke

combustion becomes an important factor in the operation of the modern FCC.

PROCESS VARIABLES REACTOR

The reactor and regenerator operate together as an integrated unit, but it is convenient to discuss

the process variables in each section independently, even though it is impossible to make a

process change in one section without affecting the other.

The reactor section variables are adjusted for optimum reactor severity. A measure of the

severity is conversion, which is defined as the liquid volume percent of raw oil charge cracked to

gasoline and lighter products. The conversion should be corrected for any gasoline contained in

the raw oil, and it is conventional to correct the yield of cracked cycle oils to that which would

be produced if the 90% point of the gasoline distillation occurred at 380F (190C).


9/21

Conversion and product properties change with reaction severity and product demand is the

major consideration in selecting reaction severity. A high severity mode of operation,

approximately 85-90 LV-% conversion, will yield large amounts of LPG, while a low severity

mode, approximately 45-55 LV-% conversion, will produce more distillate. The most common

mode of operation is the gasoline mode, which operates at about 75-80 LV-% conversion.

Conversion and product yields are also affected by charge stock properties.

It is important to note that gasoline yield does not always increase with reaction severity. If

severity is raised too high, overcracking will occur with the result that more LPG will be

produced at the expense of gasoline yield.

Catalyst Management

Good management of the catalyst inventory is important to a smooth operation. Fresh catalyst

must be added to the unit in order to maintain the desired level of activity and to make up for

physical losses. Because of the unit's large catalyst inventory, it will take a significant amount of

time for the effects of a catalyst replacement program to become fully apparent. Results can also

be clouded by changing feed stocks and operating conditions which affect product yields.

Fresh catalyst addition should be made as continuously and evenly as possible. Batchwise

addition tends to effect conversion, and there is evidence that this also causes increased losses.

An accurate method for accounting for catalyst additions and losses is also necessary.

Catalyst/Oil Ratio

Reference is frequently made to the catalyst/oil ratio, C/O, which is the ratio of lb/hr of catalyst

circulated to lb/h of fresh feed. C/O is not an independent variable and will increase with an

increase in reactor temperature and decrease with higher regenerator or combined feed

temperatures. When process conditions are changed so an increase in C/O occurs, an increase in

conversion and coke yield will also be observed. An increase in catalyst circulation at a constant

reactor temperature will:

Increase conversion Increase light gas yield


10/21

Increase C3 and C4 yield Decrease C3 and C4 olefinicities Increase gasoline aromatic content Decrease gasoline olefin content Increase aromatic content of the LCO Increase coke yieldCharge Rate

The unit will accommodate quite wide variations in charge rate at constant conversion. During

turndowns or when lighter feedstocks are being processed, a decrease in coke production will

cause a decrease in regenerator temperature. If the decrease in temperature is large enough to

effect stable operation, it will be necessary to add main column bottoms heavy cycle oil recycle

to the riser to increase coke production in order to help the unit heat balance. During turndowns,

steam to the riser may also be necessary to improve catalyst fluidization.

Combined Feed Temperature

The combined feed temperature is adjusted through the raw oil preheat and the recycle flow

temperatures. The effect of a change in the combined feed preheat can be predicted from the

energy balance around the reactor and regenerator. An increased preheat temperature at a

constant reactor temperature will:

Decrease the catalyst circulation Lower the coke production Increase delta coke (lb coke/lb catalyst) Slightly lower conversion Increase the regenerator temperatureAlthough coke production will decrease with an increase in preheat temperature, the delta coke

will increase and drive the regenerator temperature up. Delta coke is the major variable of the

two affecting the regenerator temperature.

Reactor Pressure and Pressure Balance


11/21

Reactor PressureThe pressure in the reactor is normally held constant. There is a trade-off, however, since a

higher reactor pressure would reduce the vapor recovery unit gas compressor horsepower

requirements, but would also increase the main air blower horsepower. Higher pressure

would also reduce the size of the vessels. Olefin content of the products will decrease with

an increase in hydrocarbon partial pressure; conversion will increase somewhat. Coke

formation will increase slightly, an effect that may be offset by adding steam or inert gas to

reduce the hydrocarbon partial pressure. This may, however, defeat the original purpose of

raising the reactor pressure.

Reactor pressure normally varies slightly with changes in feed rate and loading in the main

column. The operator has some element of control, but pressure must be kept within narrow

limits around the design value to avoid problems with riser and cyclone velocities. Normal

first-stage cyclone inlet velocities are in the 70 ft/s range. Higher velocities are better for

cyclone efficiency, but worse with respect to the greater amount of catalyst carried up to

them as the vessel superficial velocity increases.

Pressure BalanceCatalyst circulation through the catalyst section depends on small pressure differentials

across the slide valves. Flow from the regenerator to the reactor riser is due to the

regenerator pressure being slightly greater than the reactor pressure. An increase in the

regenerator level will increase the differential pressure across the regenerated catalyst slide

valve. This occurs because the additional catalyst head causes a higher pressure on the up-

stream side of the valve. An increase in reactor pressure would produce the opposite effect.

Catalyst flows from the reactor to the regenerator against the slight pressure difference

between the two vessels. The pressure due to the catalyst head in the reactor standpipe and

stripper must be great enough to overcome this differential. Like the regenerator, an increase

in reactor level will increase the spent catalyst slide valve differential pressure while an

increase in regenerator pressure will decrease it.


12/21

Reactor pressure is controlled indirectly by the main column overhead receiver pressure. The

reactor pressure will be dependent on the amount of material going through the main column

and the overhead system. Even if the receiver pressure is constant, as more material passes

through the main column, the pressure drop will increase. This will result in a higher reactor

pressure. Therefore, the main column receiver pressure must be set to allow for the pressure

drop between the reactor and the receiver.

The regenerator pressure is directly controlled by a pressure differential controller, which

operates the flue gas valves at the orifice chamber inlet. This pressure must be adjusted to

balance the spent and regenerated catalyst and slide valve differential pressures. As the

regenerator pressure is increased, the spent catalyst slide valve pressure differential will

decrease and the regenerated slide valve pressure differential will increase. While steady

valve differential pressures are needed to keep the catalyst circulating, the effects that

changes in these pressures have on other process conditions are also important.

Because of the importance of steady pressure conditions, special precautions must be taken to

avoid any large pressure surges. These could occur if water enters the reactor, or the wet gas

compressor fails. A main column overhead receiver over pressure controller is provided to

vent gas to the flare immediately in the event of an emergency. These systems are designed

to prevent abnormally high pressures, which can lead to negative slide valve differential

pressures and reversal of catalyst flow.


13/21

Reactor Temperature

Reactor temperature is the prime control of reactor severity. An increase in reactor temperature

at a constant catalyst circulation rate will:

Increase conversion Increase light gas yield Increase C3 and C4 olefinicities Increase gasoline research octane number (RON) Increase gasoline aromatic and olefin content Decrease light cycle oil aromatic contentCoke yield may also increase slightly, but this will depend on other conditions as well.

The following table presents typical commercial data showing how conversion and yield vary

with reactor temperature. Case A is for a highly paraffinic feedstock, while the feed for Case B

is more aromatic.

REACTOR TEMPERATURE COMPARISONS

Case A Case B

Reactor Temperature, FC

935502

1001538

984529

1013545

Feed Temperature, FC

655346

708376

624329

653345

Catalyst/Oil, Wt Ratio 6.6 6.9 12.9 12.6

Reactor Pressure, psigkg/cm

2Gauge

25.91.82

26.81.88

21.31.50

21.61.52

Conversion, LV-% 77.5 89.6 80.1 82.7

Gasoline, LV-% (C5-380F @ 90%)

RONMON

67.1

87.878.7

70.2

91.680.6

56.4

94.480.9

53.2

95.681.6

Butylenes, LV-% 6.8 9.5 10.2 11.5

Propylene, LV-% 5.9 9.5 9.5 11.2

Recycle Rate


14/21

The recycle rate determines the combined feed ratio, CFR, which is defined as:

where: recycle = total of recycle streams (volume)

feed = raw oil charge rate (volume)

Normal operation is typical with a CFR of 1.0 because recycle is not normally used. During

turndowns or when lighter feedstocks are being processed, it may be necessary to add recycle to

the riser to help the unit heat balance.

An increase in the recycle rate will:

Increase the catalyst circulation rate Increase the coke production Increase the regenerator temperature Increase the gas make

Riser Steam

During normal operation, it may be advantageous to inject some steam with the charge to the

riser to assure optimum mixing of catalyst and oil in the riser, and to lower catalyst deactivation

from metals contamination. Small amounts of steam, typically 1-2 wt-% of the raw oil charge,

will:

Decrease the hydrocarbon partial pressure in the riser Decrease the catalyst delta coke (weight coke/weight catalyst) Decrease the regenerator temperature Decrease the light gas makeRiser steam is used during start-up to establish catalyst circulation, during operation at low

charge rates, or during shutdown. One to two wt-% of steam, based on raw oil charge, injected


15/21

into the riser does not cause any noticeable catalyst deactivation, but the catalyst is subjected to

breakage and deactivation if contacted by large amounts of steam over an extended period.

Steam should be used if necessary, but its use should be kept to a minimum if there is not a large

quantity of oil with it.

Stripping Steam Rate

The quantity of steam required to strip the oil vapors from the spaces between the catalyst

particles is dependent upon the catalyst circulation rate. The stripping steam rate is generally

about 2-5 pounds per ton of catalyst circulated. Alternatively, the optimum rate can be

determined by observing the response of the regenerator temperature to slow stepwise decreases

in the stripping steam rate. A relatively large increase in regenerator temperature will occur

when the stripping steam rate is reduced below the minimum required and combustion of the un-

stripped hydrocarbons begins. For routine operation, the steam rate should then be increased by

about 10% above this minimum. Using excessive stripping steam, however, is detrimental as it

contributes to catalyst deactivation.

During normal operation, stripping steam rates will need to be increased whenever there is a

process change which results in an increase in the catalyst circulation rate, such as:

An increase in the raw oil charge rate An increase in the recycle rate A decrease in the combined feed temperature An increase in the reactor temperature


16/21

PROCESS VARIABLES REGENERATOR

The function of the regenerator is to burn the coke off the spent catalyst transferred from the

reactor. Heat generated from this combustion provides the heat necessary for the operation of

the unit. Regenerator operation influences reactor performance, as partially regenerated catalyst

does not provide adequate conversion and product distribution will be affected. More important

is the effect that variations in the regenerator dense phase temperature have on the catalyst/oil

ratio. A low upper regenerator dense phase temperature results in a high catalyst circulation rate

which increases both the conversion and the coke yield, but could also limit the capacity of the

unit.

Air Distribution

Even air distribution is essential to good regenerator operation. If more air is passing through one

section of the bed than another, catalyst regeneration may not be complete. Uneven temperature

profiles is a sign of poor distribution. This can be caused by a damaged distributor or operation

at an air rate substantially lower than the designed air rate. Low air rates result in grid pressure

drops below design values which can cause distributor erosion problems. Perforated grid type

distributors are designed for pressure drops of 0.7-1.0 psi.

Catalyst Condition

Regenerator operation is not greatly affected by nominal changes in catalyst properties.

However, a substantial loss of fines from the catalyst inventory will result in poor fluidization in

the regenerator and the carbon content of the regenerated catalyst will increase. For this reason,

withdrawal of equilibrium catalyst from the regenerator should be done regularly so that there

will not be a shift of catalyst particle size distribution toward larger size particles.

Occasionally, catalyst becomes sintered by exposure to high temperatures or as a result of

sodium contamination. Sintering occurs when the catalyst melts just sufficiently to close some

pores. If these pores contain coke, this coke cannot be regenerated since it is shielded from the

oxygen. If catalyst is still gray in color after laboratory regeneration in the routine carbon

determination, this is probably due to coke trapped in sintered pores and indicates the catalyst

pore structure has been permanently damaged resulting in its loss of activity.


17/21

Coke Combustion

Coke is a mixture of carbon and hydrogen which, when burned in the regenerator, can produce

carbon dioxide, carbon monoxide, and water. The carbon burning may be complete (to CO2) or

it may only be partial (to a CO and CO2 mixture). Some CO may, in turn, burn to CO2 and

achieve maximum combustion:

Complete Combustion: C + O2 CO2 + Heat

Partial Combustion: C + 1/2 O2 CO + Heat

CO Combustion: CO + 1/2 O2 CO2 + Heat

All of these reactions release heat. Complete combustion of carbon yields (14,150 Btu/lb) of

carbon, while partial burning to CO yields only 3,960 Btu/lb of carbon. Thus, CO combustionreleases roughly 72% of the total energy available per pound of carbon. If the unit is operated

for total combustion, the maximum amount of heat is released resulting in the highest catalyst

temperature. This in turn will lower the catalyst circulation rate and the total coke production for

a given energy requirement. The catalyst will be cleaner and more active which offsets most of

the potential conversion loss because of the lower catalyst circulation. The lower coke

production also means a higher liquid yield.

Total combustion may be initiated by using a catalyst promoter to catalyze the conversion of CO,

or in some cases by simply adding excess air. The promoter, usually a noble metal in very small

quantities,


18/21

Maintaining the normal regenerator temperature pattern Periodically checking that coke grayness is not increasing, indicating an accumulation of

coke

Anticipating changes in process conditions which increase coke production, e.g., chargestock changes, decreases in combined feed temperature, etc.

Carefully controlling the excess air levelIf the regenerator is operated with an excessive quantity of air, the regenerator efficiency will

decrease and cool the regenerator.

Dense Phase Temperature

The upper regenerator dense phase temperature is not directly controlled but is dependent upon

reactor conditions, combined feed properties, and air rate. The regenerator temperature is the

heat balancing mechanism of the regeneration process. Those changes in process conditions

which tend to produce more coke also cause an increase in regenerator temperature. The

increased temperature reduces the catalyst/oil ratio which, in turn, reduces the coke production

and restores the balance.

Process changes causing an increase in the regenerator dense phase temperature are:

An increase in charge specific gravity, average boiling point, or carbon residue A decrease in charge characterization factor (K) Addition or an increase in slurry bottoms recycle rate An increase in combined feed temperature An increase in reactor temperature or pressureRecirculation Catalyst Rate


19/21

A condition known as afterburning occurs when uncombusted CO does not begin to burn to CO2

until the flue gases reach the upper regenerator. In this area, there is very little catalyst to absorb

the heat. This causes extremely high temperatures that may seriously damage the cyclones or the

flue gas line.

In a high efficiency regenerator, the carbon burn should take place in the combustion riser to

maximize the heat transfer to regenerated catalyst in the upper regenerator dense phase.

If the lower regenerator temperature is insufficient to burn all the carbon in the combustion riser,

burning will take place in the upper regenerator, decreasing regenerator efficiency.

An indication of this type of afterburn is observed when the upper regenerator dilute phase

temperatures are greater than the dense phase temperatures. This problem can be corrected by

re-circulating more hot catalyst from the upper regenerator, which will increase the coke burning

rate in the combustion riser.

Regenerator Level

The upper section of the regenerator is the catalyst surge vessel of the unit. This level will vary

slightly with operating conditions but is maintained by adjusting the rate of catalyst addition or

withdrawal. Generally, fresh catalyst is added continually to maintain activity since the catalyst

will deactivate at a certain minimum rate, regardless of charge rate and composition. Usually

there will be a minimum daily catalyst addition rate of 1-2% of the catalyst inventory. The rate

necessary to maintain constant activity will tend to be less if catalyst inventory is small. This

normally results in an increasing catalyst level since this rate should be greater than any loss rate.

This requires periodic batchwise withdrawal of equilibrium catalyst when the upper regenerator

level gets high.

Operating with a low regenerator level should be avoided due to the decreased unit stability that

could arise. A low level will not provide the ability to absorb changes in catalyst density and

circulation. A large regenerator catalyst inventory will absorb the effects of minor upsets in

operating conditions since the change in regenerator temperature and catalyst circulation rate will

be smaller.


20/21

Regenerator Pressure

An increase in the regenerator pressure will improve the catalyst regeneration, though this

variable is almost never used for that purpose. The effect of regenerator pressure on slide valve

differential pressures, main air blower power consumption, catalyst entrainment, and cyclone

efficiency is more important. The regenerator pressure will be governed by that which gives the

optimum operation of the power recovery system expander. Refer also to the reactor-regenerator

pressure balance discussion included in the reactor process variable section.

Lowering the regenerator pressure will:

Increase the spent catalyst slide valve differential pressure Decrease the regenerated catalyst slide valve differential pressure Decrease the main air blower power consumption Slightly improve air distribution Increase catalyst entrainment to the cyclonesTorch Oil

The torch oil nozzles permit oil to be sprayed into the lower regenerator during start-ups as an

aid in heating up the catalyst inventory. Raw oil or circulating HCO is used since either is free

from metallic contaminants. They both have an IBP over 400F (205C) which eliminates the

danger of torch oil vaporizing before ignition when it is used during startup.

Torch oil should not be used during normal operations because its excessive heat can sinter the

catalyst resulting in deactivation. When it is necessary to use torch oil, care should be taken to

make sure the oil is properly atomized. A high concentration of the oil in a small area can result

in localized areas being hotter than the temperature indicated by the regenerator's temperature

indicators.

Flue Gas Quench

Temperature controlled, steam-atomized quench water sprays are provided at the regenerator

plenum. These sprays are used to protect the down stream power recovery equipment where


21/21

existing from excessive temperature because of the 1300F (705C) design limitation of the

expander. The flue gas temperature controller ensures that atomizing steam is present before the

water quench. In order to obtain good dispersion of the water, the atomizing steam control value

should be opened to its maximum before water is injected. During normal operation, steam

purges to the nozzles must be unblocked to maintain steam flow and keep the nozzles clear of

catalyst.

catalytic cracking reference material

Documents