CONTROLLING FCC YIELDS AND EMISSIONS UOP TECHNOLOGY FOR A CHANGING ENVIRONMENT

Keith A. Couch, Kelly D. Seibert and Pete J. Van Opdorp UOP LLC

Des Plaines, Illinois, USA

INTRODUCTION

Optimizing a fluid catalytic cracking (FCC) unit requires the balancing of many operating variables. However, the refiner is required to operate within certain limits when it comes to process manipulation. These limits can include feedstock quality, equipment capacity, system design, and environmental constraints. Over the past 20 years, FCC technology has seen significant improvement in catalyst, equipment, and process design. Catalyst companies have not only advanced basic catalyst design, but have developed emission and yield selective additives. Technology licensors have continued to make advancements in feed distribution systems, riser termination devices, and spent catalyst stripping. Each has contributed to improved operability and product selectivity through essentially the same objective: lowering the delta coke (weight percentage of coke on catalyst per pass through the regenerator) to achieve better yields and higher throughput. In practical application, lower delta coke (∆Coke) in the FCC unit manifests itself as a cooler regenerator dense-bed temperature. Traditionally, a lower regenerator temperature has permitted greater flexibility to increase unit conversion, and thereby improve unit profitability. Refiners have progressively revamped their FCC units toward this goal. As improved equipment

© 2004 UOP LLC. All rights reserved. AM-04-45

Page 1

technology and catalyst offerings result in compounded decreases in ∆Coke, hydraulic limits can be reached in the catalyst circulation between the reactor and regenerator. While it can be tempting to relieve one aspect of a hydraulic constraint through a slide valve or standpipe modification, not paying attention to the overall “system” design can result in moving a problem to another location and effectively accepting greater reliability risk. With the on-set of clean fuels initiatives, these issues can be further strained by an increase in the percentage of hydrotreated feed to the FCC unit. While this presents problems that the refiner must overcome, it also presents some excellent financial and environmental opportunities. This paper will highlight the UOP technology improvements that provide for higher conversion, better product selectivity, and lower flue gas emissions from the FCC unit, while providing the refiner with greater reliability and flexibility to maximize margins in a competitive marketplace.

REACTOR HARDWARE FOR IMPROVED PERFORMANCE

The path for improved FCC performance typically begins with the reactor side of the unit. The feed distributor system, riser termination device, and spent catalyst stripper are the three common areas where substantial improvements can be achieved.

FEED INJECTION SYSTEMS When a refiner decides to upgrade equipment technology on an FCC unit, quite often they start with the reactor riser and feed distribution system. The reasons for this selection are straight forward:

Well defined and commercially proven yield and process benefits

High return on investment

Lowest CAPEX project possible for a major technology upgrade With each generation of feed distributor technology, the catalyst and oil contacting has been improved to achieve lower coke and dry gas yields with improved product selectivities. This is a progression that started in the early 1970s and continues today. The benefit that a refiner can realize through an upgrade in FCC feed distribution technology is mostly dependent on the vintage of the existing feed injection system. The older the existing technology, the greater the benefit will be. Significant benefit can also be realized through technology upgrades within each vintage of feed injection. A review of the history behind modern feed distributor systems is useful to understanding the current state of the technology.

AM-04-45 Page 2

Feed Injection History In early FCC units, raw oil feed was injected into a moving catalyst stream that disengaged into a fairly dense bed reactor. Long contact times were required due to low activity amorphous catalysts. In the 1960s, high-activity zeolitic catalysts became the standard and greatly increased the conversion potential of the FCC unit. To improve product selectivities, modifications to the reactor were made to decrease the contact time between the catalyst and the oil. At that point, existing reactor systems started to be revamped to include a riser termination device through which the catalyst and hydrocarbon were disengaged and separated at the top of the riser. Within the original dense-bed reactor systems, riser feed injection was through an open ended pipe “bayonet” inserted into the base of the catalyst riser. “Bayonet” feed distributors were quickly replaced with “showerhead” distributors that incorporated multiple nozzle jets to achieve better oil distribution across the diameter of the riser. In the late 1970s and throughout the 1980s, UOP focused its effort on improved oil and steam mixing inside the distributor for better dispersion into the flowing catalyst stream. The result of this effort was the “Wye Premix” distributor in which the steam and oil passed through a tortuous path of mechanical internals to create a pseudo-emulsion phase for better distribution into the flowing catalyst. It was recognized that optimal feed injection in the area around the Wye section of the riser was hindered by the large mass flux of catalyst changing direction to move up the riser. This turbulent environment included significant back-mixing and density gradients, which contributed to uneven oil–catalyst contacting. The single feed distributor at the base of the riser was eventually replaced with multiple, elevated distributors spaced around the circumference of the riser. During the late 1980s and early 1990s, elevated feed injection became the norm throughout the FCC industry. To optimize the elevated feed injection into an FCC unit, UOP considered the entire feed distribution system, not just a single piece of equipment. UOP accomplished this by expanding its vision beyond just the feed distributor toward the dynamic balance between the physical properties of the oil, steam, and flowing solids in the riser. This led to the concept of properly conditioning the catalyst phase prior to feed injection. The acceleration zone is the vertical section between the base of the Wye and the elevated feed distributors (Figure 1). Proper riser design and operation of the acceleration zone result in a more even catalyst flow distribution and lower slip factor. “Slip” is defined as the ratio between the gas-phase velocity and the catalyst-particle velocity. A riser operating with a high slip factor has the gas rising significantly faster than the catalyst. Uneven catalyst flow, back-mixing, and high

AM-04-45 Page 3

slip in the riser can lead to localized areas of high temperature and feed over-cracking which result in higher coke and dry gas yield, and reduced selectivity to desired products.

Figure 1

Reactor Riser Dynamics

Riser Density

Wye Injection

Reaction Zone(Minimum Slip /

Plug Flow)

Initial ContactingZone

Acceleration(High Slip/Back

Mixing)Ris

er L

engt

h

RegeneratedCatalyst

ElevatedFeed

Injection

AccelerationZone

Riser

With the functionality of the lower riser better understood, efforts were shifted to the feed distributor. The first task with the feed distributors was to optimize the degree of feed atomization and dispersion to achieve an even distribution and penetration into the riser. One problem associated with atomizing oil with steam is the inherent nature of these materials to separate. Steam injection into the oil phase is ineffectual if the oil is allowed to re-coalesce prior to exiting the feed distributor tip. This could be overcome in large part through higher steam injection. However, over-atomizing the oil to alleviate this problem not only increases operating cost through increased steam consumption and sour water production, but can also force unit operations above recommended or hydraulic constraints. The objective should be to develop an atomized spray of uniform droplet size, with proper mass and velocity to achieve controlled penetration into the flowing catalyst phase.

OPTIMIX™ FEED DISTRIBUTION SYSTEM The primary result of all this work was the Optimix feed distribution system. The Optimix system incorporates the feed distributor (Figure 2), the riser acceleration zone, and the feed preheat process. In this system, the riser section below the feed distributor is designed to achieve a moderate and uniform catalyst density for optimum feed dispersion into the riser. The feed distributors are designed with three stages of atomization to achieve efficient use of energy with minimized oil re-coalescing.

AM-04-45 Page 4

Figure 2

Optimix Feed Distibutor

SteamInlet

CAU

T

OUP

NGG

AT

PR

INST

A

NO

DISA

SB

DUR O LOK™Couplings

OilFeedInlet

Riser NozzleFlange

321

Atomizing Stages

The Optimix feed distributor was introduced to the market in 1994. Since that time the distributors have gone through several stages of mechanical design improvements. The distributor tip design has been improved to control erosion and meet the demands of refiners wanting to achieve turnaround cycles greater than five years. Today’s Optimix feed distributors are designed to achieve two operating cycles, or 10 year service life, without replacement. Another major mechanical advancement to Optimix feed distributor designs is the inclusion of DUR O LOK couplings on both the inner oil gun assembly, and the outer distributor barrel assembly (Figure 2). In previous generations of feed distributors, if the refiner wanted to either significantly change throughput capacity, optimize performance with a dramatically different feed slate (VGO vs. resid), or plan for 10 year maintenance spares, complete replacement of the feed distributors would have been recommended. Through application of the DUR O LOK couplings, UOP has lowered the cost to the refiner by limiting the maintenance requirement to periodic replacement of only the outer tip assembly. In applications of throughput increase or feed slate change, both the internal oil gun and outer tip assembly can be easily changed-out.

AM-04-45 Page 5

UOP offers several variations of the Optimix feed distributor in diameters from 3 to 8 inches that include:

Standard Optimix

Optimix LS – for low steam consumption

Optimix RF – for primarily residue feed processing The results of these efforts are a feed distribution system that can be optimized for any FCC application. Optimix feed distributor experience as of February 2004:

80 unit installations world wide

Complete application range

─ Hydrotreated VGO through 100% residue operation

─ Conradson Carbon up to 7.5 wt-%

─ Riser diameter application range: 1.9 ft ⇒ 9.0 ft

Typical revamp benefit: elevated distributors to elevated Optimix distributors

─ Conversion: 1.0 – 2.0 lv-% increase

─ Gasoline yield: 0.7 – 1.5 lv-% increase

REACTOR RISER TERMINATION DEVICES Replacement of the riser termination device (RTD) with modern technology typically involves removal of the reactor head, and replacement with a new head, plenum chamber, and cyclone assembly. As such, upgrading the RTD is often associated with a throughput increase or maintenance turn-around in which the reactor cyclones have been identified for replacement.

RTD History

UOP riser termination devices have included:

Tee Disengager

Down-Turned Arm

Vented Riser: 16 placed into operation, 6 since modified to VSS RTD

Direct-Connected Cyclones: 7 placed into operation, 2 since revamped to VSS RTD

Vortex Separation Technology (VSS and VDS): 36 placed into operation

AM-04-45 Page 6

Figure 3

Progression of Riser Termination Technology

“Down-TurnedArm”

“Tee” “DCC-SCSS”“Vented Riser”

With the introduction of riser cracking, modern reactors could more properly be called a disengaging vessel. Down-turned arms replaced tee disengagers starting in the 1980’s. This provided improved separation efficiency and a reduction in catalyst loss from the reactor. Additional benefits of this technology upgrade were reduced catalyst fines generation and reduced mechanical erosion to the disengager as a result of less abrupt directional change in catalyst flow. The vented riser termination system was commercialized in 1983 and represented a step-change advancement in RTD technology. It was the first UOP RTD system to offer “hydrocarbon containment.” The next significant improvement in riser termination devices was the direct-connected cyclone (DCC) system. UOP in conjunction with Mobil Oil commissioned the first DCC in 1988. The DCC RTD further increased hydrocarbon containment and improved yield selectivities. It did, however, suffer from sensitivity to pressure upsets which made it a bit “temperamental,” especially during startups. Because DCC represented a major reduction in post-riser cracking, it was it was also very sensitive to riser residence time. Conversion loss had to be compensated with higher severity in

AM-04-45 Page 7

the other process variables (reactor temperature, catalyst activity, catalyst-to-oil ratio). Although the DCC pushed yield selectivity to the highest level achieved, it suffered from some serious operating issues that needed to be addressed. In the early 1990s UOP commercialized the Suspended Catalyst Separation System (SCSS). While it was similar to the DCC, it incorporated a pressure relief system at the top of the riser that made it much less sensitive to pressure upsets. In both the DCC and the SCSS, the cyclones are connected directly to the riser which provided for 100% of the catalyst circulation and reaction products to flow directly into the cyclones. With the high primary cyclone catalyst loading associated with DCC systems, the downward velocity of catalyst through the dipleg entrains hydrocarbon with the catalyst circulation and discharges the mixture into the reactor vessel where it is subject to post riser cracking. The flow rate is actually high enough to account for an effective 6% loss in hydrocarbon containment.

VSS and VDS Riser Termination Devices

To address the hydrocarbon entrainment in the catalyst discharge from the diplegs, for improved yield selectivities, and make the RTD more operator friendly, UOP developed vortex separation technology (Figure 4). This includes the vortex separation system (VSS) RTD for internal risers and the vortex disengager stripper (VDS) RTD for external risers. Since its first installation in 1991, 30 VSS RTD’s and 6 VDS RTD’s have been placed into operation. Applications range from 8,000 BPSD to 184,000 BPSD. In VSS / VDS RTD’s, catalyst is centrifugally discharged from the top of the reactor riser in the horizontal plane, swirls downward along the wall of the vortex chamber, and accumulates in the pre-stripping section. The small portion of catalyst that is carried to the cyclones exits into the reactor vapor space and accumulates as a bed on the outside of the vortex chamber, thus sealing the stripper from the reactor vapor space. The level in the system is controlled so that the catalyst entering the stripper from outside the vortex chamber does not drop below the top of the “window” openings into the stripper. The catalyst in the pre-stripping section counter-currently contacts stripping steam and hydrocarbon rising out the top of the integral stripper. The net superficial velocity of the catalyst as it flows into the stripper is well below the hydrocarbon bubble rise velocity, allowing the hydrocarbon to be “pre-stripped” from the catalyst. This minimizes hydrocarbon entrainment into the stripper and resultant bed cracking reactions that preferentially form dry gas and coke. The vapor outlet from the RTD is directly connected to a single stage of conventional cyclones. The swirl arm/chamber configuration effectively separates 95% of the catalyst from the hydrocarbon stream. Since only 5% of the catalyst enters the cyclone, the hydrocarbon underflow to the reactor vessel is essentially 5% of the DCC system. The net result is an improvement in hydrocarbon containment from 94 to 99.7%; meaning very little hydrocarbon is allowed to over-crack to dry gas and coke.

AM-04-45 Page 8

Figure 4

Vortex Separation Technology

VSSDisengager

VSSChamber

SwirlArms

“VSS”1995

“VDS”1991

To help quantify the potential benefits of increased hydrocarbon containment, UOP commissioned an independent laboratory to sample a reactor equipped with a DCC riser termination device (see Figure 5). The reaction mix sampling was conducted on the vapor line (Point 1) and the annular area (Point 2). The results are presented in Figure 5. The sampling of the cyclone outlets (Point 1) shows the more desirable products of a highly contained system. The sampling of the reactor vessel open area (Point 2) represents the effects of post-riser cracking of un-contained hydrocarbon and high residence time. Although the high LPG yield associated with the un-contained sample might appear attractive to some refiners, the olefinicty of the LPG is decreased by ~80% due to hydrogen transfer reactions. The long residence time also produces significant increases in dry gas and coke. The net effect is that whatever hydrocarbon escapes from a highly contained RTD will be dramatically changed to a lower economic value before it finally exits the system.

AM-04-45 Page 9

Figure 5

Hydrocarbon Containment

Yields

C2-, wt-% C3, lv-%C4, lv-% Gasoline, lv-% 54.2

Reactor Vessel

46.6

2.78.9

14.9

(Point 2)

15.617.220.4

(Point 1)

Point 2Point 2

Cyclone Outlet

LCO, lv-%Slurry, lv-%

20.98.1

10.05.8

Point 1Point 1 Effect on FCC Reaction Products

In addition to “best of class” hydrocarbon containment, with only a single stage of cyclones required, VSS / VDS RTD’s excel in de-bottlenecking existing reactor vessels by making the most efficient use of reactor cross-sectional area of any high containment RTD available. A recent VSS RTD revamp design accommodated an increase of >50% in throughput while accommodating a higher conversion than the original design and maintaining the existing reactor shell. Even with the excellent performance already achieved, UOP continues to optimize the VSS / VDS RTD’s for improved operability, reliability, and higher containment. While some hydrocarbon will always flow down the cyclone diplegs with the catalyst discharge, UOP has recently designed an improvement that reduces the volume into which the cyclone diplegs discharge by 50% to further “contain” the hydrocarbon. This improvement also provides a 10% reduction in active catalyst inventory and fresh catalyst make-up requirements.

SPENT CATALYST STRIPPING The primary objective of the FCC stripper is to remove the “strippable” hydrocarbon from the spent catalyst. Early stripper designs incorporated anywhere from one to nine stripping stages. As catalyst and hardware changes achieved lower ∆Coke operations, catalyst circulation rates were pushed progressively higher and a decline in stripper efficiency was observed. Refiners lost valuable products to the regenerator and had to increase stripping steam rates, or reached a hydraulic bottleneck in the unit. The loss of stripper efficiency offset a portion of the benefit

AM-04-45 Page 10

gained from the lower ∆Coke operations achieved through the previous revamps, i.e., upgraded feed distribution system and/or riser termination device. Improved stripping of hydrocarbon from the spent catalyst can provide significant processing and yield advantages. To maintain the unit heat balance, the coke component that is removed from the regenerator by more efficient stripping can be shifted to catalytic coke to produce higher conversion.

AF Stripper Technology

In early 1997, UOP launched an intensive review of FCC stripper technology in which we challenged much of what the industry thought to be true about hydrocarbon displacement in the FCC stripper. The bulk of the work focused on an extensive cold flow modeling program in which 15 different styles of stripper internals (both UOP and competitive) were tested. UOP constructed two large scale models, one “wedge” and one “cylinder”, which would accurately simulate commercial sized stripper hydraulics. The “wedge” model was developed to simulate the actual spatial conditions of a commercial 8 ft diameter annular stripper. The model could generate commercial tray pressure drops across an eight-tray configuration, and allowed for jet and bubble dynamics and catalyst flow to fully develop. The model was able to support studies up to a catalyst flux of 140,000 lb/ft2/hr (2,333 lb/ft2/min). Within this model, UOP tested various historical and developmental tray configurations, including high flux bypass tubes. The “cylinder” model was constructed to test non-annular tray configurations, including various types of full cross-section trays, gratings, and structured packing. The first development from the stripper improvement program was the AF tray technology. The AF tray was commissioned in 1998 and as of February 2004, there are 27 applications. An additional 21 AF tray strippers are in various stages of design and construction. These stripper installations have resulted in an average ∆Coke reduction of 0.04 wt-% at a typical steam consumption of 2 lbsteam / 1000 lbCatCirc. The 0.04 wt-% ∆Coke reduction equates to an average 22°F reduction in regenerator temperature, a 6% increase in catalyst-to-oil ratio, and all of the associated conversion, yield, and selectivity benefits. The benefits of the AF stripper technology are realized through more efficient contacting between steam and catalyst with improved hydrocarbon displacement. For a given unit design capacity, the dimensions of an AF stripper are ~10 ft shorter in length with equivalent diameters to those of an equal capacity FCC unit designed in the 1990s. In addition to improved stripping efficiency, a primary objective of the improvement program was to increase the catalyst flux capacity of stripper designs. The success in this area has been

AM-04-45 Page 11

tremendous. One particular application of AF stripper technology has been operating routinely at a catalyst flux rate of 120,000 lb/ft2/hr (2,000 lb/ft2/min) with stable operations. This represents a 20% capacity increase over the pre-revamp operation without any changes in stripper cross-sectional area. The second and third developments from the stripper improvement program were the AF grid and AF packing designs. The first unit with grid style stripper internals was commissioned in 2002. As of February 2004, five grid style AF strippers have been placed into operation. This brings UOP’s commercial experience with AF stripper technology to 32 operating units. The first AF packing design has been awarded and is currently in engineering.

THE ∆COKE CHALLENGE

As improved equipment technology and catalyst offerings have resulted in progressive decreases in ∆Coke, many refiners have continued to de-bottleneck unit constraints within the capacity of existing major equipment, often with the constraint of not replacing main vessels or large rotating equipment, i.e., main air blower (MAB), wet gas compressor (WGC), regenerator shell, reactor shell, stripper shell, and catalyst circulation standpipes. Maintaining this philosophy through several technology upgrades can result in several operational and reliability concerns.

1. Excessive catalyst loading to regenerator cyclones – Cyclone erosion, fines generation

2. High catalyst circulation and catalyst flux – Insufficient regenerator & stripper residence time – Hydraulic instability or limitation in the catalyst standpipes

3. Excessive catalyst fines to the flue gas system – Increased particulate matter (PM) emissions

Improvements in FCC technology to achieve lower ∆Coke operations, not only present the concerns listed above, but also cause refiners to face several dilemmas between “wants” and often conflicting “also wants” (see Table 1). Optimum product selectivities, conversion, and throughput are traditionally opposite to lower coke yield, optimum coke combustion, and retaining existing equipment.

AM-04-45 Page 12

Table 1

Conflicting “Wants” “ALSO WANTS”

BUT

Retain existing equipment – Min CAPEXMAB, WGC, standpipes

BUTHigher Throughput

Sufficient regenerated catalyst Temperature for

Improved coke burn kinetics

BUTLess Dry Gas

Lower coke yield forImproved selectivity andLower CO2

Higher Conversion

Sufficient regenerator temperature forImproved coke burn kinetics

BUTOptimum Product Selectivities

“WANTS”

COMBINED TECHNOLOGY EFFECTS Figure 6 shows the expected benefits to be obtained from a stew-wise improvement in FCC reactor technology. The individual steps include:

1) Replaced elevated Premix distributor with elevated Optimix feed distribution system 2) Replaced reactor stripper with an AF stripper technology 3) Replaced a tee RTD with a VSS RTD

With each progression, the refiner gains the yield and selectivity benefits inherent of increased catalyst-to-oil and lower ∆Coke. The combined technology improvements result in a 23% increase in catalyst circulation and a regenerator dense-bed temperature reduction to 1260°F. The technology benefits are summarized in Table 2.

AM-04-45 Page 13

Figure 6

Technology Upgrade Effects

1250

1270

1290

1310

1330

ElevatedPremix

ElevatedOptimix

+ AFStripper

+ VSSRTD

Reg

ener

ator

Den

se B

ed, °

F

6

8

10

12

14

16

Cat

/Oil

Regen Temp

Cat/Oil

( 0 )

( 2 )

( 1 )

( 3 )

Table 2

Technology Upgrade Effects

Cat-to-Oil Regen Temp, °FConversion, lv-%

(90% at 380°F)

Gasoline, lv-%(90% at 380°F)

PlusVSS

(Point 3)

10.141260+0.7

+1.8

Coke, wt-%∆Coke

5.60.55

8.271324Base

Base

Base CasePremix FeedDistributor(Point 0)

5.60.68

As hydraulic limits are reached in the catalyst circulation between the reactor and regenerator it can be tempting to relieve one aspect of a hydraulic constraint through a slide valve or standpipe modification. However, not paying attention to the overall design can simply result in the relocation of the constraint and the introduction of greater risk to the system reliability. With the on-set of clean fuels initiatives, these issues can be further strained by increasing the percentage

AM-04-45 Page 14

of hydrotreated feed to the unit and further decreasing the ∆Coke and regenerator temperature. Refiners moving towards more hydrotreated feeds are often placed in the position of opting for less coke selective catalyst, firing the air heater, or firing torch oil to add heat to the regenerator. The use of less coke selective catalyst (more coke produced) negates yield and selectivity benefits previously gained from equipment upgrades. Firing the direct fired air heater for long durations can lead to erosion and failure of the air distributor as the distributor jets are pushed beyond design exit velocities. These high exit velocities can also lead to excessive catalyst fines generation. Firing torch oil to keep the regenerator hot essentially burns high value feedstock as fuel, while at the same time damaging the catalyst activity. The burning of any fuel in the regenerator apart from coke on the circulating catalyst inventory is an economic loss. While the above issues present problems that the refiner must overcome, they also presents some excellent operational, financial, and environmental opportunities.

THE RXCAT SOLUTION The problems of low ∆Coke operations (low regenerator temperature) have created an opportunity that UOP has uniquely addressed. Traditionally the catalyst from the bottom of the reactor stripper has commonly been referred to as “spent”. However, modern catalyst systems can accumulate appreciable quantities of coke and still maintain a significant amount of activity. UOP has adopted the term “carbonized” to describe this catalyst. The activity characteristics of carbonized catalyst are not only usable, but in certain cases preferred. Coke deposition preferentially attenuates the strongest catalytic sites providing for more selective cracking with carbonized catalyst. To take advantage of the selectivity benefits of conditioned catalyst, UOP developed RxCat technology. In a traditional FCC unit, increasing the catalyst-to-oil ratio to increase conversion also increases the coke yield and catalyst circulation to the regenerator. RxCat technology provides the ability to increase both conversion and selectivity by recycling a portion of the carbonized catalyst back to the base of the reactor riser (Figure 7). The carbonized catalyst circulated from the stripper back to the base of the riser is effectively at the same temperature as the reactor. Since the recycle catalyst adds no heat to the system, the recycle is heat-balance neutral. For the first time, the catalyst circulation up the riser can be varied independently from the catalyst circulation rate to the regenerator and is de-coupled form the unit heat balance. With RxCat technology, a portion of the stripped catalyst (~1000°F) is directed through the recycle catalyst standpipe to the MxR chamber where it is combined with the hot regenerated catalyst (~1300°F). The lower contact temperature between the combined catalyst and raw oil feed results in higher product selectivity with less dry gas and coke production, and a substantial increase in conversion due to the higher riser catalyst-to-oil ratio. Similar to a conventional FCC unit, the balance of the carbonized catalyst that is not recycled travels through the stripper to the

AM-04-45 Page 15

regenerator where the coke is burned off before it returns to the base of the riser. The catalyst flowing to the regenerator carries a higher coke content, which in turn raises the regenerator temperature and enables the easing of constraints in the system.

Figure 7

RxCat Technology

MxR Chamber

CarbonizedCatalystRecycle

VSS RiserTermination

Device

Combustor StyleHigh-Efficiency

Regenerator

Spent CatalystStandpipe

RegeneratedCatalyst Standpipe

CarbonizedCatalyst

Slide Valve

The ability to control the catalyst flow up the riser independently of the heat-balance adds increased flexibility to the FCC unit to more easily handle changes in feed quality and shifting product slates. This aspect of the technology is particularly useful in units that periodically switch from gasoline to olefin or distillate mode, throughout the year. In a conventional FCC unit a shift in operating mode is accomplished by a change in reactor temperature and a change in the rate or activity of the catalyst make-up. With RxCat technology, a change in catalyst activity in the riser can be accomplished by merely changing the amount of carbonized-catalyst recycle and as such, the change from gasoline mode to/from olefin mode can be rapid. This application has an even greater impact for refiners that use LPG olefins additives, i.e., ZSM-5 or Supra ST-5,

AM-04-45 Page 16

and have traditionally had to shift their catalyst inventory over several weeks to reap the full financial benefits of a change in product slate. As of February 2004, three RxCat designs have been delivered to customers. Two of these units are now under construction, and the first unit is expected to go on-stream in mid-2004. Integration of RxCat technology provides numerous benefits in both revamp and new unit applications, providing the refiner with the ability to accomplish many of the following:

Increased conversion

Increased gasoline yield

Decreased dry gas yield

Increased propylene yield with additive use

Decreased coke yield at constant conversion

Increased regenerator dense bed temperature

Increased regenerator residence time

Lower regenerated and spent catalyst standpipe flux

Lower regenerator emissions

Two of the largest benefits listed above are described briefly below. Increased conversion - The substantial increase in riser catalyst-to-oil ratio results in a significant increase in conversion. At a 1:1 blend of carbonized to regenerated catalyst, the catalyst-to-oil ratio in the riser will typically increase 3 to 4 numbers. At a constant reactor temperature and catalyst activity, conversion can be increased 3 to 5 lv-%. Alternatively, the large increase in the catalyst-to-oil ratio with its corresponding increase in conversion allows the reactor temperature (and hence thermal cracking) to be reduced while still maintaining or exceeding the original conversion level. Depending on the severity of the operation and the catalyst quality, most of this conversion would be directed towards increased gasoline yield. Decreased dry gas yield – A major portion of the thermally cracked (dry gas) products formed in conventional FCC units occurs at the initial point of catalyst-feed contact due to the hot catalyst temperature. The catalyst blend from the MxR chamber results in a mix-zone temperature typically 150°F lower compared to a conventional FCC unit. This large reduction in catalyst temperature results in a large reduction in C2

- yield which could be very valuable in units limited by dry gas production.

AM-04-45 Page 17

In addition to the yield and selectivity benefits, RxCat technology can be used to effectively de-bottleneck unit constraints. Approximately half of the catalyst that would traditionally circulate from the reactor to the regenerator is recycled to the base of the riser. As such, the catalyst circulation through the spent and regenerated catalyst standpipes is actually less than the “base case” operations before any technology upgrades were initiated. As opposed to a purely mechanical modification which utilizes CAPEX (new reactor, stripper, standpipes, etc.) for only capacity benefits, RxCat technology pays back with both increased throughput and product yields. The expected benefits for the previously presented case study with the implementation of RxCat technology is shown below in Figure 8 and Table 3.

Figure 8

Effect of RxCat Technology on Unit Performance

1250

1270

1290

1310

1330

ElevatedPremix

ElevatedOptimix

+ AFStripper

+ VSSRDT

+ RxCat

Reg

ener

ator

Den

se B

ed, °

F

6

8

10

12

14

16

Cat

/Oil

Regen Temp

Cat/Oil

( 0 )

Riser Cat/Oil

( 4 )

Regen S. P.Cat/Oil

Base

AM-04-45 Page 18

Table 3

Effects of RxCat Technology on Unit Performance

Cat-to-Oil Regen Temp, °FConversion, lv-%

(90% at 380°F)

Gasoline, lv-%(90% at 380°F)

15.61312

Coke, wt-%∆Coke 0.35

8.271324Base

Base

Base CasePremix FeedDistributor(Point 0)

5.60.68

Cumulativeto RxCat

Technology(Point 4)

Riser

7.8

+3.8

+4.9

5.40.70

Regen. Standpipe

The ability of RxCat technology to improve conversion and selectivity provides the refiner with a tool to achieve improved yield targets with less coke make. The reduced coke make translates to reduced air blower demand as well as a reduction in CO2 emissions.

EMISSIONS CONTROL

In addition to the desires for increased throughput, better yields and product selectivities, refiners are facing increasing demands to reduce environmental emissions. Proper definition of specific emission limits, today and future, and identifying the proper engineering controls to best address these emissions can be a daunting task. UOP has some very effective solutions to meet refiner’s needs.

PARTICULATE MATTER CONTROL WITH THE UOP THIRD STAGE SEPARATOR Since the advent of the FCC third stage separator (TSS), the primary focus of the TSS was to protect downstream power recovery system turbo-expanders. While power recovery system applications have been in use for decades, UOP wanted to improve TSS technology to a point where it could be used to effectively meet FCC flue gas stack PM emission standards. UOP has been designing power recovery systems since 1973. Since that time, UOP has licensed 31 TSS’s with 20 placed into operation. The original units were designed by UOP under license

AM-04-45 Page 19

from Shell. Over the years, UOP improved upon the original design by implementing several modifications. Even with these modifications incorporated into the base design, very little had actually changed in the overall design of the TSS in 25 years. These TSS designs still suffered from the limitations imposed by radial flow gas distribution and reverse flow in the cyclone elements. In 1996, UOP launched a development program to design and offer a smaller, more economic, high efficiency TSS that could not only be utilized in power recovery installations, but also be a viable alternative to electrostatic precipitators and wet gas scrubbers for environmental applications. The cold flow modeling (CFM) test program extended over 2 years, during which both dimensional variables and process flow variables were studied. Based on a thorough understanding of cyclone theory, and drawing on other sources of cyclone expertise, the UOP program investigated the contribution of many variables on catalyst separation efficiency. These variables included:

Cyclone diameter and geometry

Inlet velocity

Length to diameter ratio

Outlet velocity

Catalyst loading

Gas distribution Over 200 individual tests were conducted on single and multiple cyclone models to determine the highest efficiency and highest capacity design cyclone. The tests were conducted with commercial FCC catalyst fines. Computational fluid dynamic (CFD) computer modeling was used to validate and benchmark the CFM work, and to quickly investigate potential improvements and guide the physical modeling program. The development work culminated in the new UOP TSS design (see Figure 9). The most significant improvement in the design is that the UOP TSS utilizes axial flow for catalyst/gas separation. The flue gas flow is maintained essentially in one direction - in the top and out the bottom of the unit. Axial flow distribution minimizes the potential for solids re-entrainment resulting from gas flow direction change and resultant eddy current formation. The older style TSS utilizes radial flow distribution in which the flue gas is distributed from the centerline of the TSS, radially outward between the two tube-sheets. As such, the inner tubes see a higher gas and dust loading than the outer tubes. The mal-distribution of flue gas and fines inherent in this design results in varying efficiency across the older style TSS.

AM-04-45 Page 20

The new UOP TSS is about 40% smaller than other TSS offerings for the same capacity; making it less expensive to fabricate, easier to install, and better suited where plot space is a premium.

Figure 9

Third Stage Separator Equal Capacity Comparison

11' 6" OD48 Tubes

23'

70 Tubes 19' 3" OD

29'

New Style TSSOld Style TSS

The first UOP TSS was commercialized in April 2002. Performance testing on the unit was performed twice in 2002, following the unit startup in April and again in December. The initial test showed that the UOP TSS discharged between 36-50 mg/Nm3 of particulates, depending on flue gas rate. The NSPS compliance testing resulted in a particulate matter emission of 0.6 lbs/1000 lbs of coke burn, only 67% of that allowed by NSPS standards. After nearly two years of continuous operations no performance degradation has been observed (Table 4).

Table 4

Commercial Unit Stack Data

384741

PM lb/M lbCoke

0.690.600.57

mg/Nm3

Sept. 2002Dec. 2002Oct. 2003

PM lb/hr

3.545.124.79

Date

AM-04-45 Page 21

This performance shows that the UOP TSS can replace more traditional and costly means (electrostatic precipitators and wet gas scrubbers) of controlling particulates exiting the flue gas stack. In addition to the on-stream UOP TSS, there are currently four more in various stages of engineering and construction with the second unit scheduled for startup in 2004.

COMBUSTION KINETICS In the RxCat technology analysis (Table 3), we showed how coke yield could be decreased coincident with increased conversion and improved yields through the advantages of decoupling the catalyst circulation up the riser from the FCC heat balance. The benefit of lower coke production can be utilized by the refiner either through increased capacity, increased severity, or lower CO2 emissions. Catalyst regeneration, as a carbon removal process, is widely accepted as being first order with respect to carbon concentration and oxygen partial pressure. The change in coke concentration on a catalyst particle with respect to time fits the following equation:

– dC / dt = K0e

Where:K0 = Frequency constant, (hr-1atm-1)C = Carbon on Catalyst, wt-fraction

Po2 = Oxygen Partial Pressure, (atm)∆E = Activation Energy, BTU/lb-mol

R = Gas Constant, BTU/lb-mol/°RT = Temperature, °R

CPo2RT–∆E

Given a higher regenerator temperature, the coke burn rate is accelerated due to the increasing rate constant. This relationship for the rate of carbon burning from FCC catalysts holds over a wide range of temperatures with diffusional limitation not controlling. The important relationships to note are that the rate of coke combustion increases with:

Higher coke content of catalyst

Higher oxygen partial pressure

Higher regenerator temperature

AM-04-45 Page 22

The application of RxCat technology to the FCC unit can effectively increase all three variables. Being able to manipulate each of these coke combustion drivers provides significant flexibility to the FCC operator.

NOx Control and UOP’s High Efficiency Regenerator

In an effort to better understand the chemistry and variables that affect NOx production in an FCC regenerator, UOP has embarked on a thorough investigative and development effort. Feedstock characteristics and quality, excess O2 in the regenerator, use of CO promoter, regenerated catalyst temperature, and regenerator design are among many variables evaluated. While the industry has not observed a model-predictable relationship between feed nitrogen and NOx, one particular FCC unit data set did show a substantial NOx response to a shift in the type of feed processed. The unit response is closely related to two feed characteristics:

Degree of residue processing Degree of feed pre-treating

In a refinery with a fixed infrastructure, (for example, always processing virgin gas oil with no residue in the FCC), different feeds with varying amounts of basic or total nitrogen have shown no strong correlation with NOx. However, when residue is blended into the feed, or if the feed is hydrotreated, then a noticeable change in NOx may be observed. Depending on simultaneous changes in other operating variables; e.g., regenerated catalyst temperature, flue gas excess oxygen, and use of CO promoter to name a few, the magnitude or actual direction of the shift may change from unit to unit. Figure 10 shows a noticeable effect on regenerator NOx emissions when 10-20% residue is blended into the feed, and also the relationship of lower NOx formation with lower O2 in the regenerator. This data set is from a single FCC unit equipped with a bubbling bed regenerator that processes both clean VGO and residue blended feedstocks. This data set is also consistent with those from other units, showing the relationship of lower NOx formation with lower excess oxygen in the flue gas.

AM-04-45 Page 23

Figure 10

Unit “F” - NOx Response on Feed Composition and Excess O2

00.0 1.0 2.0 3.0 4.0 5.0 6.0

Flue Gas Excess Oxygen, mol-%

0100200300400500600700800

Flue

Gas

NO

x, p

pmv

NOx without Resid Feed

NOx with Resid Feed

IncreasingAir Dilution

One of the challenges in minimizing excess oxygen in the flue gas to achieve lower NOx emissions is the apparent effect of CO promoter to increase the formation of NOx (Figure 11). In this graph, NOx emissions are plotted against platinum in the circulating catalyst inventory. The relationship is quite clear: as platinum increases with higher CO promoter use, NOx emissions increase. This same relationship has been observed through pilot plant studies and in many commercial FCC operations. This reproducible response is strong evidence that CO combines with NO to form N2 by the reaction: 2CO + 2 NO ⇒ N2 + 2 CO2. It is believed that when the CO combustion rate is increased by the CO promoter (2CO + O2 ⇒ 2CO2), the reduction of NOx to diatomic nitrogen is suppressed, promoting the release of NOx from the regenerator. While a hotter catalyst temperature kinetically provides the capability for lower excess O2 operation in the regenerator, tending towards lower NOx formation, achieving a hotter regenerator through the use of CO promoter can negate the benefits and drive NOx emissions higher.

AM-04-45 Page 24

Figure 11

Unit “A” – Impact of CO Promoter on NOx

020406080

100120140160

0.3 0.4 0.5 0.6 0.7 0.8 0.9E-Cat Platinum, ppmw

Flue

Gas

NO

x, p

pmv

Increasing CO Promoter

020406080

100120140160

0.3 0.4 0.5 0.6 0.7 0.8 0.9E-Cat Platinum, ppmw

Flue

Gas

NO

x, p

pmv

Increasing CO Promoter

Simultaneous control of CO and NOx in a complete combustion regenerator poses a challenge as CO is eliminated through oxidation while NO is eliminated through reduction. UOP’s high-efficiency combustor style regenerator excels in controlling these simultaneous reactions with typical emissions of CO less than 100 ppmvd and NOx less than 40 ppmvd. The differences between bubbling bed (turbulent regime) regenerators and combustor regenerators are shown in Figure 12. In the bubbling bed regenerator, the combustion air flows from the grid up through a dense bed of catalyst where the carbon is burned off. The catalyst enters the vessel from the spent catalyst standpipe, and is ideally distributed evenly across the bed.

AM-04-45 Page 25

Figure 12

Bubbling Bed Regenerator Combustor Regenerator

R

CatalystR

Catalyst

DilutePhaseDilutePhase

T

SpentCatalyst

T

SpentCatalyst

RegeneratedCatalyst

Main Air

DensePhase

Flue Gas

SpentCatalyst

RegeneratedCatalyst

Main Air

ecirc.

DilutePhase

DensePhase

Flue Gas

SpentCatalyst

RegeneratedCatalyst

Main Air

ecirc.

DilutePhase

DensePhase

Flue Gas

The high-efficiency combustor regenerator approximates a plug flow burning profile for the catalyst as opposed to the back-mix regime of the standard bubbling bed design. The spent catalyst from the reactor mixes with the blower air and roughly an equal amount of recirculation catalyst at the bottom of the regenerator (combustor). The coke burns off the catalyst as it travels up the combustor vessel with the air. There is a rough separation at the top of the combustor riser through a “tee” shaped outlet. The flue gas enters a two-stage cyclone before leaving the regenerator vessel. The catalyst discharge from the cyclone diplegs is returned to a dense phase catalyst bed. From there, the flow splits, part of the catalyst going to the base of the reactor riser, and the rest back to the combustor. To validate the superior performance of UOP’s combustor regenerators on NOx emissions, data was collected from many operating units. The effects of feedstock quality, type of feed being processed, presence or absence of CO promoter, O2 concentration, catalyst bed temperature, etc., can all drive NOx emissions and need to be screened for proper data validation. However, when

AM-04-45 Page 26

we plot a comparison of eight different combustor style regenerators against fifteen bubbling bed regenerators, the data are very clear: at any given excess oxygen level, combustors result in much lower NOx emissions (see Figure 13).

Figure 13

Comparative Regenerator Performance Analysis

0

40

80

120

160

200

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Excess Oxygen, mol-%

Flue

Gas

NO

x, p

pmv

8 Combustor Regenerators

15 Bubbling Bed Regenerators

SUMMARY

Proper control and optimization of an FCC unit requires careful balancing of many variables. This is particularly challenging in the changing refinery environment, where profitability revamps and the environmental control revamps compete for capital investment. UOP has a growing “toolbox” of process improvements in Optimix feed distributor systems, VSS and VDS riser termination devices, AF stripper technology, and RxCat technology to help refiners maximize profitability from their FCC unit. The cumulative benefits of these yield-improving technologies together with the new UOP third stage separator and UOP’s high-efficiency combustor regenerator are available to help refiners meet the challenges of maximizing return on investment while simultaneously providing a cleaner environment.

AM-04-45 Page 27

REFERENCES

1. L. L. Upson, and E. Cole Nelson, “RxCat Technology for More FCC Gasoline,” 1998 2. B. W. Hedrick, J. P. Koebel, and I. Cetinkaya, “Improved Catalyst Stripping from Cold Flow

Modeling,” 2001 3. D. A. Kauff, and B. W. Hedrick, “FCC Process Technology for the 1990s,” AM-92-06,

NPRA 1992 Annual Meeting 4. L. L. Upson, and D. A. Wegerer, “Rapid Disengager Techniques in Riser Design,” 1993 5. L. A. Lacijan, and M. W. Schnaith, “Refinery Profitability Drives FCC Revamps,” 2003 6. M. W. Schnaith, A. T. Gilbert, D. A. Lomas and D. N. Myers, “Advances in FCC Reactor

Technology,” AM-95-36, NPRA 1995 Annual Meeting 7. V. J. Memmott, and B. Dodds, “Innovative Technology Meets Processing and Environmental

Goals: Flying J Commissions New MSCC and TSS,” AM-03-13, NPRA 2003 Annual Meeting

8. J. A. Montgomery, “Guide to Fluid Catalytic Cracking Part One,” W.R. Grace & Co., CT,

1993 9. S. B. Reddy Karri, T. Balteau and T.M. Knowlton, “PSRI Desktop Design Manual – Edition

1,” June 1994

uopUOP LLC25 East Algonquin Road Des Plaines, IL 60017-5017 © 2004 UOP LLC. All rights reserved.

UOP 4285B

AM-04-45 Page 28