FCC Technology Upgrades a Commercial Example

7/30/2019 FCC Technology Upgrades a Commercial Example

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Authors:

S.J. McCarthy - Mobil Technology Company

M.F. Raterman - Mobil Technology Company

C.G. Smalley - Mobil Technology Company

J.F. Sodomin - Mobil Oil Corporation

Rik B. Miller - The M.W. Kellogg Technology Company

Publication / Presented:AM-97-10

Date:

FCC TECHNOLOGY UPGRADES:

A COMMERCIAL EXAMPLE


2/12

by

S.J. McCarthy, M.F. Raterman, C.G. SmalleyMobil Technology CompanyPaulsboro, New J ersey USA

and

J.F. SodominMobil Oil Corporation

Paulsboro, New J ersey USA

and

Rik B. MillerThe M.W. Kellogg Technology Company

Houston, Texas USA

FCC TECHNOLOGY UPGRADES:

A COMMERCIAL EXAMPLE


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FCC Technology Upgrades: A Commercial Example

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I. ABSTRACT

The refining industry has become exceedingly competitive over the past 15 years. The net resultof competitive pressures has been significant downsizing and rationalization of less profitablerefineries. To survive, the remaining refineries must continue to improve efficiency, reduceoperating costs, and adopt flexibility to respond quickly to changes in market demands. Also,refineries must implement selective technology upgrades to ensure continued profitability. FluidCatalytic Cracking is one of the most flexible and profitable refinery upgrading processes.

Therefore, a properly equipped Fluid Catalytic Cracking unit has significant potential to ensurerefineries remain competitive.

Mobil and Kellogg have developed advanced FCC process technologies that are provenmechanically reliable and profitable. This paper highlights selective FCC technology upgradesaimed at debottlenecking an existing FCC unit and improving financial performance. Thesetechnology upgrades include: Closed Cyclones to reduce post-riser thermal cracking, stripper

modifications to reduce coke make, ATOMAX feed nozzles to improve feed atomization, andradial injection to improve feed and catalyst mixing. Mobils Paulsboro refinery is provided as an

example of how selective FCC technology upgrades can improve overall refinery profitability.

II. INTRODUCTION

Fluid Catalytic Cracking (FCC) is one of the most flexible and profitable refinery upgradingprocesses. While commercialized more than 50 years ago, FCC has maintained a preeminentposition by evolving to meet changing product, environmental and operational demands quickly andprofitably. Several recent hardware developments are aimed at debottlenecking unit constraints toimprove financial performance. Specific developments include elimination of post-riser thermalcracking, stripper modifications to reduce coke make, and high efficiency feed and catalyst mixing.

Successful commercial demonstration of new hardware developments is critical before widespreadimplementation. Reliability and operability are vital for a refinery to realize the full benefits of newtechnology. The FCC alliance of Mobil and M.W. Kellogg combines the strength of both companiesto ensure the reliability and operability of new technology. Mobil provides both traditional FCCresearch and development and commercial demonstration of new technologies. Kellogg providesstrong process and mechanical engineering capabilities.

This paper highlights specific FCC technology upgrades which have proven profitable for bothexisting and grassroots units. Specific technology upgrades discussed include closed reactorcyclones, new baffle design for high flux strippers, ATOMAXTM feed nozzles, and radial feedinjection. Mobils Paulsboro refinery provides an excellent example of how these selectivetechnology upgrades can improve overall refinery profitability. These FCC technology

improvements have also been successfully demonstrated at other Mobil and licensee refineries.


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III. BACKGROUND

Mobils Paulsboro refinery provides an excellent case study of how strategic technology upgradescan improve the FCC units contribution to overall refinery profitability. The FCC is a UOP highefficiency unit (Figure 1) built in 1980 and originally designed to process 30 TBD. The FCC

feedstock is a mixture of indigenous VGO, purchased VGO, lube plant byproducts, and coker gasoils. These feedstocks are not hydrotreated by the refinery and are a daily challenge to upgrade.Main products are fuel gas, C3 and C4 LPG, gasoline, light cycle oil and slurry oil. Operatingstrategy is for maximum conversion and is typically constrained by coke burning rate and wet gascompression capacity. Advanced control packages are in use on the FCC, main fractionator, andunsaturated gas plant.

The unit was revamped to include Closed Cyclone technology in 1988. Then, in 1995, Paulsboro

installed ATOMAX feed nozzles and proprietary FLUX TUBE stripper baffle technology that

reduced regeneration air requirements and improved yield selectivity. ATOMAX and FLUX

TUBE technologies were jointly developed by Mobil and Kellogg. The FCC unit now processesmore than 46 TBD at conversion levels well above 70 vol%. Yield surveys were performed before

and after the turnaround to quantify the benefits of the hardware improvements. The impact of eachof these technologies is discussed below.

IV. MOBIL / KELLOGG FCC TECHNOLOGY IMPROVEMENTS

a. Closed Cyclones

Cracking temperatures above 1000F are common for FCC units operating to produce light olefinsand high octane gasoline. At these temperatures, the contribution from non-selective thermalcracking in the reactor vessel becomes significant. The main advantage of catalytic cracking overthermal cracking is lower coke and higher gasoline selectivities. Therefore, it has become

imperative to redesign product and catalyst separation systems to eliminate thermal cracking.

Many designs to reduce non-selective post-riser cracking have been investigated, developed andcommercialized over the past 20 years. They all share the same basic concepts of quicklyseparating spent catalyst from cracked products, introducing the catalyst into the stripping zone, andminimizing the time product vapors are held at high temperatures.

The evolution of FCC riser termination devices is depicted in Figure 2. Early designs favored cruderough cut separators on top of the riser followed by reactor cyclones. In later designs, rough cutseparators were replaced by riser cyclones. Although they are more efficient than theirpredecessors, both rough cut separators and riser cyclones discharge the products into thereactor/disengager. The large volume in this region allows significant thermal cracking to occur at

essentially riser top temperature. Analysis shows that about 50% of the hydrocarbons that exit theriser cyclones are backmixed into the reactor vessel. Rough cut separators result in even higherlevels of backmixing. The hydrocarbons that backmix undergo unselective thermal crackingresulting in a loss of gasoline and distillate yield and increase in light gas and heavy fuel oil make.


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Closed Cyclones essentially eliminate the backmixing in the reactor vessel. This is accomplishedby directly connecting the outlet of the riser cyclones with the inlet of the reactor cyclones. WithClosed Cyclones only about 3 wt% of the riser products are backmixed in the reactor vessel, mostlydue to entrainment of hydrocarbons down the riser cyclone diplegs. Converting unducted riser

cyclones to Closed Cyclones typically increases gasoline and distillate yield by about 2.5 wt % whilereducing dry gas yield by about 1 wt %. This represents a reduction of 40% in the sulfur-free, drygas make. Even greater benefits have obtained when revamping units that previously used crudeinertial separators1.

A refinery may take advantage of the reduction in gas make by increasing cracking severity andconversion. Other refineries may choose to increase feed rate or resid content within a wet gasconstraint. Another benefit of Closed Cyclones is the reduction in unwanted byproducts. One suchthermal byproduct, butadiene, increases acid consumption in downstream alkylation. ClosedCyclones typically reduce butadiene yield by about 50%.

Closed Cyclones were installed on the FCC unit at Mobils Paulsboro refinery in 1988. Tracer tests

showed that Closed Cyclones reduced hydrocarbon backmixing in the FCC reactor vessel fromabout 40 wt % to about 3 wt %. Butadiene measurements after the turnaround were approximately45 to 50% lower than the pre-turnaround values, demonstrating a significant reduction in thermal

cracking. Also, reactor vessel temperatures ranged from 40 to 70F lower than the riser toptemperature, again confirming reduced backmixing.

Yield surveys before and after the turnaround show that gasoline and distillate yield increased by2.4 wt % at the expense of light gas and heavy fuel oil (Table 1). At that time, Paulsboros FCCoperation was constrained by the refinery fuel gas balance. Therefore, the refinery took advantageof the drop in dry gas make by increasing severity. Despite increasing severity to maximum risertop temperature, the fuel gas yield was still less than without Closed Cyclones. This increase inseverity further decreased bottoms yield and increased gasoline octane. Overall, Closed Cyclones

provided about a $0.30/bbl upl ift.

b. Feed Injection and Catalyst Mixing

The feed injection system is one of the keys to yield optimization and profitability. Catalytic crackingreactions occur in the vapor phase on the active catalyst surface. Therefore, a well designed feedinjection system must provide quick vaporization and intimate contact between catalyst and oil.Poor feed vaporization and mixing create localized regions of high and low catalyst-to-oil ratios andinduce backmixing which increases coke. Improvements in feed injection can reduce coke and lightgas production, resulting in higher conversion and improved selectivity toward premium products.Gasoline yield increases as high as 5 vol% and conversion increases of over 4 vol% have been

observed commercially with the Kellogg/Mobil ATOMAX feed injection system.

Rapid feed vaporization requires atomization into small droplets with a narrow size distribution.Smaller droplets vaporize faster than larger droplets. The feed injection system must also efficientlyuse the available liquid pressure drop to reduce operating costs or provide optimum atomizationwithin fixed feed supply pressure constraints. The spray pattern from the nozzle must evenlydistribute the oil within the riser to provide rapid and uniform oil and catalyst mixing. Oil should besprayed into a dense phase of catalyst to ensure high local catalyst-to-oil ratios. The feed musthave sufficient momentum to penetrate the flowing catalyst without causing erosion of the riser wallor excessive catalyst attrition.


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ATOMAX nozzles have been installed on most of Mobils FCC units and have been licensed to

more than 32 FCC units to provide superior feed atomization and mixing. The ATOMAX nozzle,shown in Figure 3, was developed jointly by Mobil and Kellogg. Its key features are that steampressure provides most of the energy for atomization and mixing and, thereby, avoids expensivepressuring of the liquid feed. Simple and reliable internals and a unique discharge cap providesuperior atomization, riser coverage and penetration without erosion of the nozzles or riser wall.Atomization is not significantly effected by large changes in feed rate and this compliments variousFCC operating strategies.

ATOMAXTM nozzles were installed at Mobils Paulsboro refinery during the 1995 turnaround.Paulsboros original feed nozzles were an atomizing tip-orifice type, installed axially at the base

of the riser. The new feed injection system consisted of ATOMAX nozzles oriented radially aroundthe circumference of the riser. The new nozzles were designed for higher feed rate and efficientlyutilized available feed pressure drop.

Yield surveys before and after the turnaround show Paulsboro achieved a 1.3 wt % increase inconversion and a 1.4 wt % increase in gasoline yield with the installation of ATOMAXTM nozzles

(Table 1). Improved feed vaporization and oil and catalyst mixing reduced thermal cracking, whichreduced the coke and dry gas selectivities and improved the gasoline selectivity. Reduced cokeselectivity allowed higher conversion within the coke burning limitations of the unit. Overall,ATOMAXTM nozzles provided a $0.34/bbl uplift , representing about a 3-month payout.

c. Stripper Baffle Design

The performance of the FCC reactor stripper also has a substantial impact on yields and profitability.Ideal stripper design provides adequate catalyst residence time and low catalyst fluxes to allowsteam to flow upward through the stripper. Good stripper baffle and steam distributor design arealso essential to good stripper performance. Poor stripper performance results in hydrocarbonvapors being burned in the regenerator. This increases both regenerator temperature and steam

partial pressure. Poor stripping also reduces feed rate or conversion in FCC units constrained byair blower capacity.

Mobils Paulsboro FCC stripper is an annular design (Figure 4) which operates at relatively highcatalyst fluxes. Higher than design feed rates and operating severities have increased catalystfluxes, resulting in flooding and poor stripping efficiency. Figure 5 depicts the problems encounteredin high flux annular strippers. This type of behavior has been observed in cold flow strippermodeling and confirmed in commercial tests.

A survey in August 1994 confirmed the stripper problems and, depending on conditions, unstrippedhydrocarbon vapor accounted for 10 to 15 wt% of the regenerator coke load. An efficient strippertypically achieves less than 6 wt% unstripped hydrocarbon vapor as a percentage of total coke. Low

spent catalyst standpipe density also indicates poor stripper performance because a highpercentage of the steam added to the stripper is being entrained with the spent catalyst to theregenerator.

In 1995, the Paulsboro refinery installed a new proprietary stripper baffle technology that reduces

flooding and improves stripping efficiency. Known as FLUX TUBE baffles, this technology wasjointly developed by Mobil and Kellogg. Compared with previous baffles designs, they are extremelyefficient in high flux strippers and they significantly increase the operating range without requiring

modification of the stripper shell. The performance of conventional and FLUX TUBE baffles iscompared in Figure 6.


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FLUX TUBES have been successfully installed at two other Mobil locations and in one licensedunit. The low-cost modifications did not require enlargement of the outside shell of the reactorvessel and did not extend the turnaround duration.

Paulsboro FCC Stripper Performance Before and After FLUX TUBE Installation

Pre-Turnaround Post Turnaround

Hydrocarbon Vapor as a % of Total Coke 10 - 15 3 - 5

Relative Stripper Efficiency 69 - 78 89 - 93

Steam to Reactor, wt% 35 - 50 68 - 78

Steam to Regenerator, wt% 50 - 65 22 - 32

Regenerator Temp., F 1365 1335

A survey conducted following the turnaround showed about a 65% reduction in unstrippedhydrocarbon vapor entrained to the regenerator at constant stripping severity. The reduced

regenerator coke load lowered regenerator temperature by about 30F and provided the flexibilityto operate the FCC unit at higher feed rates and higher severity. The lower regeneratortemperature, accompanied by a lower steam partial pressure, reduced catalyst hydrothermaldeactivation. Equilibrium catalyst activity increased by 3 MAT at constant catalyst makeup rate.

Yield surveys before and after the stripper modification show Paulsboro was able to achieve a 1.2wt% increase in conversion at constant feed rate because of the stripper modifications (Table 1).Correspondingly, gasoline and alkylation unit feed increased, predominantly at the expense of lightcycle oil and bottoms. There was a slight increase in dry gas as a result of improved stripperperformance. As expected, coke yield increased slightly due to the lower hydrogen content of the

coke, with better stripping, which reduced the heat of combustion per pound of coke.

Overall, FLUX TUBE stripper baffles prov ided about a $0.12/Bbl up lift. Also, the operatingenvelope of the Paulsboro stripper has been sufficiently extended to allow a much wider range ofoperating conditions.

V. SUMMARY

The Paulsboro Refinery has benefited from selective FCC technology upgrades all of which camefrom development programs jointly conducted by Mobil and Kellogg and which are available forlicense from Kellogg. Most of the FCC technology improvements have also been successfully

implemented at other Mobil locations as part of our program to remain competitive. As describedabove, these technologies have helped Mobils Refineries remain profitable and viable in anexceedingly competitive environment.

VI. REFERENCES

1. "FCC Reactor Product-Catalyst Separation --- Ten Years of Commercial Experience withClosed Cyclones; R.B. Miller, T.E. J ohnson, C.R. Santner, A.A. Avidan, and D.L. J ohnson;1995 NPRA Annual Meeting, San Francisco, CA; 20 March 1995.


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TABLE 1

Paulsboro FCC Yield Improvements: Post Revamp

Yields, wt% Closed Cyclones ATOMAX FLUX TUBES

H2S 0.0 0.0 0.0

Dry Gas -0.4 -0.3 0.2

Propane -0.1 0.1 0.1

Propene 0.0 -0.1 0.2

n-Butane 0.3 0.1 0.1

i-Butane 0.3 0.1 0.2

Butenes 0.0 0.0 0.1

Gasoline 0.8 1.4 0.2

Light Cycle Oil 1.6 -0.9 -0.8

Bottoms -2.6 -0.4 -0.4

Coke 0.1 0.0 0.1

Conversion 1.0 1.3 1.2

Uplift, $/bbl 0.30 0.34 0.12


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Mobil's Paulsboro FCC Unit

UOP high efficiency unit or iginally designedto proc ess 30 TBD

Feedstocks: Non-hydrotreated

Indigenous and imported VGO

Lube byproducts and coker gas oils

Products

Fuel gas, C3 and C4 LPG, and gasoline

Light cycle and slurry oils

Strategy: Maximum conversion

Constraints: Air blower and wet gascompressor capacities

Figure 1

Evolut ion of Riser Termination Devices

Inertial Separator

Poor catalyst/vapor separation

Much dilute phase cat cracking

High vapor residence time(post-riser thermal cracking)

Riser Cyclones

Good catalyst/vapor separation

Little dilute phase cat cracking

High vapor residence time(post-riser thermal cracking)

Closed Cyclones

Excellent catalyst/vapor separation

No dilute phase cat cracking

Very low vapor residence time(no post-riser thermal cracking)

Figure 2


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Original Mobil Paulsboro feed nozzles were anatomizing " tip-orifice" type, installed axially

ATOMAX Feed Injec tion System

now installed on most Mobil FCC units

small, uniform droplets provide rapidfeed vaporization

efficient use of feed pressure drop

uniform spray pattern that penetrates

flowing catalyst

catalyst lifted to injection zone in dense phase

for improved hydrodynamics and heat transfer

Higher conversion and improved produc tselectivities

ATOMAX Feed Injection System

ATOMAX Feed Injection Technology

Figure 3

Figure 4


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Figure 5

FLUX TUBES Outperform Conventional Baffles

Superficial Mass Flux (lb/ft/s)

S

tripperEfficiency(%)

CRITICAL FLUX

FLUX TUBE Baffle

Baffle "A"

Baffle "B"

Figure 6

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FCC Technology Upgrades a Commercial Example