Enviromental Fluid Catalytic cracking Technology.pdf

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Environmental Fluid Catalytic CrackingTechnologyW.-C. CHENG a , G. KIM a , A. W. PETERS a , X. ZHAO a , K.

RAJAGOPALANa

, M. S. ZIEBARTHc

& C. J. PEREIRAd

a Grace Davision , Washington Research Center , 7500 Grace Drive,Columbia, Maryland , 21044b Research Division , W. R. Grace & Co.-Conn., Columbia, Marylandc Grace Davison , Washington Research Center , 7500 Grace Drive,Columbia, MD, 21044d Dupont Engineering , 1007 Market St., Wilmington, DE, 19898Published online: 15 Aug 2006.

To cite this article: W.-C. CHENG , G. KIM , A. W. PETERS , X. ZHAO , K. RAJAGOPALAN , M. S.ZIEBARTH & C. J. PEREIRA (1998) Environmental Fluid Catalytic Cracking Technology, CatalysisReviews: Science and Engineering, 40:1-2, 39-79, DOI: 10.1080/01614949808007105

To link to this article: http://dx.doi.org/10.1080/01614949808007105

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40 CHENG ET AL.

IV. CONTROL OF SULFUR OXIDES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56A. Mechanismof SO, Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57B. Developmentof SO, Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59C. Future Horizons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

V. CONTROLOF SULFUR IN GASOLINE ....................... 61

B. Reaction Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62C. Gasoline Sulfur Reduction Additives . . . . . . . . . . . . . . . . . . . . . . . 65D. Future Horizons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

VI. CONTROLOF NITROGEN OXIDES . . . . . . . . . . . . . . . . . . . . . . . . . . 68

A. Sulfur Species in FCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

A. Origin of Nitrogen Oxide Emissions . . . . . . . . . . . . . . . . . . . . . . . 68B. Mechanism of Nitrogen Oxides Formation . . . . . . . . . . . . . . . . . . 69C. Control Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

D. Future H orizons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

ABSTRACT

Th e fluid catalytic cracking(FCC) process converts heavy oil into valuablefuel products and petrochemical feedstocks. Environmental regulations are akey driving force for reducingFCC process air-pollutant emissions and forchanging the composition of fuel products. Environmental considerations areaffecting the design and o peration of the F C C and are providing opportunitiesfor the development of in-process additives. The present article reviewsde-velopments in these environmental technologies.

I. INTRODUCTION

The fluid catalytic cracking(FCC) process converts gas oil and resid-containing feedstoc ks, primarily in the 315-650°C boiling-point range, intolighter, more valuable gasoline and distillate fuel products. FCC gasolineaccounts for approximately 35% of the U.S. gasoline pool [l ] . Light gasesfrom the FCC are used as reactants for alkylation and for the production ofoxygenates, further increasing the contribution ofFCC products to the gas-oline pool.

Environmental regulations are affecting the design and operationof theFC C process. They are a key driving force for reducing process air-pollutantemissions and fo r changing the comp osition of products. Air emission regu-lations are directed at criteria pollutants and air toxic compounds. Criteria

pollutants include carbon monoxide (CO), ozone, nitrogen oxides(NO,), sul-fur oxides SOx), and particulates [2]. Atmospheric ozone, formed b y thereaction of volati le organic compounds(VOCs) and NO,, has been found to

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ENVIRONMENTAL FCC TECHNOLOGY 41

be detrimental to human health. NO, and SO, are precursors to acid rainprecipitation, which is harmful to the ecosystem [2,3].

At the federal level, air emissions are regulated by the Clean Air ActAmendments (CAAA) of 1970 and, more recently, by the CAAA of 1990.The latter law further limits site emission levels and addresses issues such asnonattainment for criteria pollutants (Title I), mobile source emissions (TitleII), air toxic emissions (Title 111), and acid rain (Title IV). The CAAA of1990 represents guidelines for air quality but does not set specific limits forunit operations. Regulations that follow the CAAA may set specific limits.Thus, the CAAA of 1990, by itself, does not set specific limits for a particularunit operation like FCC. The federal regulations overlay a complex set ofstate and local laws. Standards for a given unit depend on a variety of factors,including site status (e.g., new or retrofit) and location. For example, new

sources must often meet more stringent New Source Performance Standards(NSPS) than existing sources. State Implementation Plans (SIP) aimed atalleviating local nonattainment problems can vary from state to state and maycall for more stringent controls than required by the federal regulations.

The composition of products, regulated by the CAAA of 1990, affectsthe design and operation of the refinery in general and the FCC unit in par-ticular. For example, the law mandates the sale of “reformulated” gasolineto improve the environment. The need for oxygenates in reformulated gaso-line is creating an increased demand for light (C,) olefins. Restrictions on theconcentration of benzene and aromatics in reformulated gasoline are expectedto lower both reformer severity and the fraction of reformate in the gasolinepool. The sulfur content of gasoline and distillate fuels is being regulated.Lower gasoline sulfur levels will likely reduce CO and VOC (or unburnedhydrocarbon) emissions from automobiles (by improving the performance ofthe automobile catalytic converter) and reduce sulfur oxide emissions. Be-cause most of the sulfur in the gasoline pool comes from FCC gasoline, theFCC process and the catalyst are expected to play a major role in meetinggasoline sulfur specifications.

A. Process Description

The FCC process consists of a riser and a regenerator. The riser cracksthe feed into valuable fuel products. The regenerator burns off the coke de-posited on the catalyst during cracking in the riser. A simplified schematic ofthe process is shown in Fig. 1. Preheated feed injected at the bottom of theriser contacts the hot active catalyst. The feed vaporizes and flows upwardthrough the riser, together with the catalyst. During this time, the feed cracksand, in the process, the catalyst becomes deactivated by coke and feed metals.

The residence time in the riser is in the range 2-10 s [4]. The riser toptemperature is between 480°C and 570°C. Cracked products are separatedfrom the coked catalyst by cyclones at the top of the riser and sent to a

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42 CHENG ET AL.

REGENERATOR

COMBUSTION

AIR HEATER

OlSPERSlON -STEAM

REACTOR EFFLUENT

YCLONE VESSEL

JTRIP PERSTRIPPING STEAM

RISER REACTOR

STEAM

UPPER FEED NJECTION

LOWER FEE0 NJECTION

FIG. 1. Typical FCC unit.

fractionation column. The coked catalyst is stripped of entrapped and othervolatile hydrocarbons and sent to the regenerator.

The regenerator is a hot, dense fluidized bed in which the coke oncatalyst is burnt off. Typical regenerator temperatures range between 675°Cand 760°C. The typical catalyst residence time in the regenerator ranges fromabout 5 to 15 min. Combustion products and entrained catalyst are conveyedupward, out of the dense fluidized bed, into a dilute phase zone where cy-clones separate the catalyst, which is returned to the bed. A typical FCCregenerator operates in turbulent fluidization and there is considerable car-ryover of catalyst to the cyclones. In fact, the whole bed circulates throughthe cyclones every 5 min [ 5 ] .The hot, nearly coke-free, regenerated catalystis sent to the riser and contacts the feed, thereby completing the cycle. Thehot regenerated catalyst provides the heat required for the endothermic crack-ing reactions, thereby maintaining the FCC unit in heat balance. A moredetailed discussion of the FCC process is available in the work of Venutoand Habib [6].

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ENVIRONMENTALFCC TECHNOLOGY 43

B. Catalysts

Commercial FCC catalysts contain faujasite (or zeolite Y) dispersed in

an inorganic oxide matrix, schematically represented in Fig.2.

Although thezeolite provides most of the cracking activity of the FCC catalyst, the matrixfulfills both physical and catalytic functions. The matrix binds the zeolitecrystallites together in a microspheroidal particle with appropriate physicalrequirements to survive interparticle and reactor-wall collisions in the FCCunit. The matrix can also react with sodium and prevent the zeolite fromstructural damage due to sodium [7]. Acid sites in the matrix can also con-tribute significantly to the catalyst activity. Catalysts with an active matrix,as well as a nearly inert matrix, are commercially available. Due to the re-duced diffusional effect, a properly designed active matrix cracks the feed

components with a high boiling point (>475 C) more effectively than zeolite[8,9]. Additionally, the physical and chemical properties of the matrix canalso be tailored to provide increased tolerance of feed metals that accumulateon the catalyst with time [lo].

As with other types of industrial catalysts, the key performance attributesof FCC catalysts include activity, selectivity, durability, and attrition resis-tance. The property-function relationships have been discussed in a numberof references and are summarized in Table 1 [4,11].

Additives that impart specific product selectivity or environmental ben-efits may also be introduced into the FCC unit along with the cracking cat-alyst. Additives have physical characteristics that are similar to the crackingcatalyst. They are typically blended in at concentrations of less than 10 wt%.For example, an additive containing the zeolite ZSM-5 may be used to boostgasoline octane and increase the yield of light (mainly C,) olefins. The octane

FIG. 2. Schematic representation of a FCC catalyst. Zeolite particulates a)are dispersed in a porous amorphous matrix.

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44 CHENG ET AL.

TABLE 1

Property-Function Relationships for FCC Catalysts

Function Required properties Selected ingredients~

Gasoline selectivecracking

Large-porecracking

Binding

Filler

V tolerance

Ni tolerance

~~

Acidity (Brmsted, Lewis),hydrothermal stability

Acidity, hydrothermal stability ofpore structure

Inorganic polymer withoptimized chain-length

distributionAbility to provide

macroporosity; no adverseeffects on zeolite, activematrix

Solid base, surface area stability,large-pore diameter, resistanceto S poisoning

High surface area, capacity to

bind Ni, lower coke andhydrogen yield

USY, rare-earth exchangedY; ZSM-5 for lightolefins

Pseudoboehmite Al,O,;NaOH leached, calcinedKaolin; HC1 leached,calcined Kaolin

Al,(OH),Cl-based polymer;SiO,, Si0,-Al,O,-

based polymersKaolin clay

MgO, La,O,, CaO

Tailored matrix materials

(e.g., alumina, silica,and clay)

increase is usually accompanied by a gasoline yield penalty. The status ofenvironmental additives will be the subject of the present review.

C . Environmental Technologies

The schematic in Fig. 3 highlights some of the environmental consid-erations in FCC. Process emissions are mainly associated with the regeneratorexhaust. Typical pollutant concentration ranges are 50-200 vppm for NO,,300-600 vppm for SO,, and 0-5 vol% for CO. Particulate emissions due tocatalyst attrition and cyclone efficiency are approximately 1 pound of catalystfines per 1000 pounds of coke burned in the regenerator.

Pollutant produced in the riser include hydrogen sulfide (H2S), ammonia(NH,), and light hydrocarbons. H2S treatment is part of the refinery sulfurmanagement system. Typically, H2S is converted to elemental sulfur in aClaus plant. A small fraction of the H2S, together with product NH3, is ab-

sorbed in wastewater that condenses from the stripping steam. Most of thelight hydrocarbons are separated for use as petrochemical feeds, used as afuel (for cogeneration or producing process steam), or flared.

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c

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.I

-6

The challenge for environmental FCC technologies is to maintain overallcracking activity and selectivity while reducing process emissions and pro-viding products that meet environmental specifications (e.g., reformulated

gasoline). A further requirement is that control technology for one pollutantdoes not increase emission of another pollutant. Emissions of a particularpollutant may represent only a small fraction of the concentration of thatpollutant in the feed. This is illustrated for sulfur and nitrogen by the numbersin parentheses in Fig. 3. Only 5 of the feed sulfur accounts for regeneratorSO, emissions. About the same percentage of feed N is typically convertedto NO,. Approximately half of the feed sulfur and nitrogen species end up inthe light cycle oil (LCO) and bottoms fraction. Only about 5 of the feedsulfur and


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46 CHENG ET AL.

may be reduced by countercurrent regenerator designs. Solid addi-tives, when introduced in small concentrations along with the FCCcatalyst, may reduce CO, SO,, and NO, levels in the regenerator

exhaust. Such in-process approaches can minimize the need for tail-end controls and, at the same time, may also offer improved processeconomics.Tail-End Control: These devices may include CO boilers, bag housefilters, nitrogen oxides reduction catalysts, scrubbers for reducingSO, and particulates, and the like.

3.

This article reviews the present state of the art in environmental FCCtechnologies. The topics are ordered according to the maturity level of thetechnology. For example, particulate control, initially driven more by the need

to retain expensive catalyst in the unit than by environmental concerns, iscurrently in commercial practice. In contrast, reduction of sulfur in fuels andcontrol of nitrogen oxides are more recent environmentally driven areas.Technology in these areas is aimed more at elucidating the mechanistic as-pects of the problem, and the hardware and additives discussed are moredevelopmental in nature. Future horizons for developing new technologies ineach area are also discussed.

11. CONTROL OF PARTICULATE EMISSIONSThe FCC catalysts are spherical in shape and approximately 70 p m in

diameter. These catalysts continuously circulate between the riser and theregenerator. In the regenerator, catalyst particles are separated from the fluegas with the aid of cyclones. In the process, a fraction of the catalyst particlesin the unit get entrained with the flue gas. These catalyst particles leave theregenerator in the exhaust gas and contribute to particulate emissions.

Throughout the history of the FCC process, efforts have been made tocontrol particulate emissions from the FCC regenerator. Initially, the driving

force for reducing these emissions was economic-to reduce the loss ofactive, expensive catalysts from the unit. More recently, however, environ-mental regulations have become a driving force for controlling particulateemissions.

Particulate emissions are usually monitored by measuring stack opacity.Another quantitative measure commonly used is to determine the pounds ofparticulates emitted per 1000 pounds of coke burned in the regenerator. Dur-ing the earlier days of the FCC (1950s), stack opacity in FCC regeneratorsranged from 30% to 50% and particulate emissions amounted to 12 poundsper 1000 pounds of coke burned [12]. These emissions translated to a catalystloss of about 2.5 tons per day from a typical FCC unit. With improvementsin cyclone technology, the range of stack opacity was reduced to 20-30%

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during the 1970s. Particulate emissions represented 5 pounds per 1000 poundsof coke burned and typical FCC unit losses were 1.5-2 tons per day. Sincethe 1970s, catalyst manufacturers have made substantial improvements incatalyst attrition resistance. Today, stack opacity ranges from 10% to 20%,with particulate emissions ranging from 0.5 to 1 pound per 1000 pounds cokeburned [12]. There has been a 90% reduction in particulate emissions fromthe 1950s to the present time through a combination of improvements inhardware (e.g., cyclone design) and catalyst properties (e.g., attrition resis-tance). The New Source Performance Standard (NSPS), which represents theparticulate emissions control requirement for new or revamped FCC regen-erators, is 1 pound per 1000 pounds of coke burned.

The key physical properties of the FCC catalyst that can influence par-ticulate emissions are the following [13]: (1) average particle size and particle

size distribution, (2) apparent bulk density, and (3) attrition resistance. Theaverage particle sizes of most commercial catalysts range from 60 to 80 pm,and apparent bulk densities range from 0.65 to 0.9 g/cm3. These propertiesprimarily determine the fluidization properties of the catalyst. The attritionresistance typically depends on the sphercity and the hardness of the catalystparticle. Typically,


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48 CHENG ET AL.

be related to regenerator air velocity (VJ, particle density 8J, nd diametero,) y ~ 3 1

Because the FCC unit is in heat balance, the catalyst circulation rate (orthe catalyst-to-oil ratio) is a dependent variable. Catalyst activity, coke selec-tivity, and feed preheat temperature are some of the independent variablesthat influence catalyst circulation rate. In general, higher catalyst circulationrates increase the propensity for fines generation.

B. Control Options

Controlling the particle size distribution of the fresh FCC catalyst is oneof the methods of controlling particulate emissions. It is known that 0-40-Fm particles, while facilitating fluidization, can result in increased particulateemissions [18]. Thus, depending on the particle size distribution of the cir-culating inventory, the particle size distribution of the freshly added catalysthas to be optimized to manage this trade-off between emissions and fluidi-zation/operation of the unit. Catalyst manufacturers can supply most catalystgrades with coarse, medium, or fine particle size distribution to manage thistrade-off [13].

Improving the attrition resistance of the catalyst is another way to reduceparticulate emissions. The attrition resistance of the FCC catalyst is a functionof zeolite content, the type of matrix, and matrix content [19]. The attritionresistance of a catalyst typically decreases with increasing zeolite content,This is illustrated in Fig. 4 for the earlier silica alumina gel as well as the

new process

00 00 0 2 5 0 5 0 a75 1 Do 1 2 5

Relative zeoEte content

FIG. 4. Effect of zeolite content on attrition resistance.

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TABLE 2

Methods of Improving Attrition Resistance

Method

Increasing binder contentIncorporation of peptized aluminaUse of small-particle platelet KaolinThermaVhydrothermal treatment of catalystControl of zeolite morphologyHigh solids concentration in spray-dryer feedHigh outlet temperature in spray-dryerViscosity reducing additivesReduction of zeolite crystal sizeDry/wet milling of catalyst ingredients

Rigorous mixing of spray-dryer feed

Ref.

11129130131132

1111

1331115

15

modern silica hydrosol binder systems. A variety of catalyst preparation meth-ods can be employed to improve attrition resistance. A summary of thosemethods is provided in Table 2. The use of these methods has resulted indramatic improvements in catalyst attrition resistance, as measured by theDavison Index. As shown in Table 3, the catalyst DI index has dropped from30 to 40 in the 1960s to below 7 for today's commercial catalysts.

The design of efficient cyclones in the regenerator and appropriate re-

TABLE 3

Properties of FCC Matrices

Typical catalyst properties

Microactivity of ABD YearDescription steamed matrix DI' (crn-,) commercialized

Si0,-Al,O, gel(13% Al,O,) 35 40 0.50 1963

SiO,--AI,O, gel (25% Al,O,and clay; semisynthetic) 35 35 0.52 1965

Silica hydrosol and clay 10 6 0.77 1972Silica hydrosol with increased

matrix activity 25 6 0.76 1978Alumina sol and clay 25 5 0.80 1980Clay based, XP 58 4 0.73 1986Silica hydrosol with Ni trap 25 6 0.76 1990

Steamed 6 h 1400°F, 100% steam, 5 psig; microactivity test (MAT) conditions, 98OoF,

bAttrition, Davison Index.'Apparent bulk density.

WHSV = 30, C/O = 4, sour imported heavy gas oil feed.

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50 CHENG ET AL.

generator operation can also reduce particulate emissions. The design of acyclone is illustrated as Fig. 5 . The variables influencing cyclone efficiencyinclude gas velocity, cyclone diameter, outlet pipe diameter, barrel length,and cone length [20]. Typically, two cyclones (primary and secondary) areused in series in the regenerator to minimize emissions [21]. The key designchanges that improved the efficiency of cyclones in the 1970s were longerbarrels, the addition of dust bowls and flapper valves at the bottom of thediplegs, and the reduction of the size of the secondary diplegs to match thecatalyst flux. Additional capturing of fines can be achieved with a third-stagecyclone located external to the regenerator. Some FCC units employ powerrecovery turbines (PRTs) to recover the thermal energy of the flue gas. Theexternal third-stage cyclone can protect the PRT.

The location of the cyclone inlet within the regenerator is another pa-

rameter that can be used to minimize particle entrainment and emissions [22].The density of the regenerator dilute phase decreases as the distance abovethe bed surface increases. Eventually, a height above the surface is reachedwhere the amount of entrained catalyst particles reaches a minimum. Thisheight is known as the transport disengaging height (TDH). Positioning thefirst-stage cyclone inlet at a height greater than the TDH from the surface ofthe bed minimizes the amount of fines carried into the cyclone separator.With modern, efficient cyclones, much of the particulates emitted in the re-generator stack are less than 10 pm in diameter [12].

A few of the FCC units employ flue gas scrubbers. Although scrubbersare designed to control SO,, they can also remove over 80 of the catalystfines that leave the regenerator [lo].Due to the high capital and operatingcosts of flue gas scrubbers, this technology is not widely employed.

P L A N V I E W

1 s t StapeCyclone-

2 n d StageCyclone

U

ELEVATION VIEW

FIG. 5 . A two-stage cyclone system.

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An alternate method for further reducing particulate emissions is the useof electrostatic precipitators (ESPs) [23]. An ESP is a large rectangular steelshell which contains several wires called discharge electrodes and several flatcollecting plates. The wires and plates are hung vertically in alternating rows.An ESP can remove 80% of the fines emitted from the regenerator. With theavailability of attrition-resistant catalysts and hardware options, most FCCunits are able to meet the particulate emissions limits.

C. Future Horizons

Although large reductions in particulate emissions have been achievedthrough catalyst and hardware improvements, attrition resistance continues tobe an area of ongoing interest to catalyst manufacturers. Because the attrition

resistance of the catalyst particle depends on its physical and chemical prop-erties, the issue of attrition resistance has be revisited each time there is achange in catalyst formulation. For example, the need for matrix pores of>lo0 A is believed to be important for bottoms cracking. A change in porestructure is expected to change the catalyst attrition resistance.

If regulations require further reduction in particulate emissions, a baghouse can be used [12]. However, bag houses for FCC units are complex todesign and expensive to operate and are not commercially practiced at thepresent time.

111. CONTROL OF CARBON MONOXIDE EMISSIONS

Carbon monoxide (CO) is the product of incomplete combustion of thecoke-burning reactions in the FCC regenerator. The driving forces for thecomplete oxidation of CO include (a) the heat balance of the FCC unit, (b)the control of regenerator afterburn, and (c) environmental considerations.

As discussed in Section I, the heat liberated in the regenerator influencesthe catalyst circulation rate and thereby effects the process performance. Thetotal heat liberated in the regenerator includes the heat of combustion of thecoke to CO, the oxidation of the CO to CO,, and the formation of water fromoxidation of the hydrogen in the coke. As shown in Table 4, the combustionof coke or carbon to carbon monoxide provides about 111 kJ/mol, whereasthe combustion of the CO to CO, provides 283 kJ/mol, about 70% of thetotal heat obtained from burning the carbon in the coke [24]. It has beenfound that the COJCO ratio obtained from burning graphite is about unity[25,26]. The uncontrolled regenerator COJCO ratio at 1 excess oxygen istypically about 1 or a little more, in agreement with the kinetic model for theburning of CO developed by Prater et al. [27]. Consequently, only about 65%

of the heat available is being utilized.Another reason for completing the combustion of CO is to control af-terburn. Coke burning reactions take place in a dense fluidized catalyst bed

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52

TABLE 4

Thermodynamic Properties

CHENG ET AL.

Heat of formation Heat of combustion

(kJ/mol) (kT/mol)Carbon 0 394co -111 283CO, 94 0H2 0 242H2O 242 0

within the regenerator. Combustion gases disengage from the catalyst bed andenter a region called the dilute phase. The gases are separated from the cat-alyst in a series of cyclones and exhausted to the atmosphere. Gases in thedilute phase and in the cyclones contain both CO and oxygen. The excessoxygen continues to react with the CO, and the temperature can increasesharply, especially as there is little thermal mass of the catalyst to absorbheat. The temperature increase in the dilute bed and in the cyclones comparedto the temperature of the dense catalyst bed is called afterburn. The afterburntemperature has to be maintained below the maximum metallurgical temper-

ature constraint of the equipment. Afterburn can be a major problem: Theamount of coke burned in the regenerator is limited by the afterburn temper-ature. This can detrimentally impact the conversion and the throughput of theFCC process. The afterburn can be controlled by facilitating the completeconversion of CO to CO, in the regenerator dense bed.

Carbon monoxide emissions from the regenerator are also controlled forenvironmental reasons. CO is a criteria pollutant and a pollution problem ina number of urban areas in the United States [ 28 ] . As discussed earlier, COemissions from automobiles and cogeneration power plants are being feder-ally regulated. Individual states have regulations governing emissions of COas well as other pollutants. These regulations can vary considerably from stateto state and even within a state, from refinery to refinery, depending on localair quality and other factors. The New Source Performance Standard for COemission for FCC units is 500 ppm [29]. As mentioned in Section I, Title Iof the CAAA of 1990 does not set specific revised limits for CO emissionsfrom the FCC unit. This applies to new units and older units that are re-vamped. For older FCC units, CO emissions range from 500 to 2000 ppm.

Complete CO combustion can be achieved through the appropriate de-sign and operation of the regenerator or the use of an oxidation catalyst, alsoreferred to as a CO promoter. Using a combination of measures, CO emis-sions from FCC regenerators have been reduced 99 from 17,500 Ibs/h in1950 to less than 200 lbsb in 1990 [12].

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A. Control Options

The coke-burning efficiency depends to a large extent on the regenerator

design. Discussions of some of the basic design considerations such as mixingare available in the literature [30,31], as are a number of references on hard-ware design [22,32]. Modern designs attempt to optimize mixing with a smallcatalyst inventory and a short residence time.

Regenerators can be operated in a variety of different modes. In partial-burn operation, the oxygen present is insufficient for the complete combustionof the coke and CO to CO,. The intrinsic burning rate for coke on porousbeads has been measured and modeled by Weisz and Goodwin [33,34]. Theresults suggest that coke oxidation has an intrinsic rate independent of thesource or amount of the coke but depends strongly on the diffusion charac-

teristics of the particle. In a partial-burn mode, CO, and CO levels are typi-cally comparable. Even in an operation with as much as 1% excess oxygen,COJCO ratios are typically about 1 without special equipment such as COboilers or a CO oxidation catalyst.

During the 1960s, a CO boiler was added to the FCC unit to recoverthe energy available from CO oxidation in the flue gas. The CO boiler isplaced downstream of the FCC regenerator for complete conversion of CO.The temperature, residence time, and excess air in the CO boiler is controlledto burn additional CO. In the early seventies, Amoco demonstrated a processfor essentially complete combustion of CO as well as carbon on the catalyst[35,36].

The use of a small amount of platinum to control CO emissions wasinitially proposed by Mobil in a series of patents (e.g., Refs. 37 and 38).Platinum impregnated on the cracking catalyst at a level of about 1 ppm wasfound to increase the COJCO ratio from 1.1 to >47. Figure 6 shows anexample of the effect of the noble metal loading on the COJCO ratio. Mostnoble metals (especially platinum and iridium) were found to be effective.Because platinum is less expensive and more available than iridium, it waspreferred.

In the original patent, the noble metal was distributed uniformly on allof the catalysts in the inventory. In later Mobil patents, the use of a separateadditive particle containing a higher noble metal loading was discussed [39].Conversation of CO with time at 650°C in air was studied for a number ofcases in which the noble metal loading on the total catalyst inventory washeld constant at 5 ppm. As shown in Fig. 7, additives with S O - loo- and200-ppm platinum loadings were mixed with unpromoted catalysts and tested.Comparison of CO conversion performance with time showed that the ad-ditive concept was superior to the concept of distributing noble metals on theentire catalyst inventory. The additive particle containing 200 ppm platinumparticle performed the best.

Carbon monoxide oxidation promoters are typically prepared on an alu-mina or mixed metal oxide supports [40]. Platinum concentrations range from

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54 CHENG ET AL.

FIG. 6. Relative oxidation activity.

300 to 800 ppm. For a fixed loading of noble metal on the catalyst inventory,there is an optimum loading of platinum on the additive particle 1391. Abovethis loading, the volume fraction of the additive particles within the regen-erator bed is low and CO oxidation is limited by catalyst bypassing. Further,at higher noble metal loadings of >lo00 ppm, deactivation by sintering canreduce the effectiveness of the expensive noble metal. Additive addition ratesdepend strongly on regenerator design and operation. Typical addition ratesare 2-4 pounds of promoter per ton of catalyst, corresponding to about 0.5ppm noble metal on the total catalyst inventory.

Certain FCC units processing residual oil provide a high coke load tothe regenerator. In these units, the regenerator is intentionally operated in thepartial combustion mode (incomplete combustion of CO) to better control thetemperature of the regenerator. CO conversion is completed in a downstreamunit (e.g., CO boiler) for these operations. Residual oils contain smallamounts of nickel, vanadium, and iron. These metals build up on the catalystand can affect the COJCO ratio independently of the CO promoter. Contri-butions from Ashland have shown that iron and vanadium have a small effect,but nickel contamination can change the COJCO ratio by as much as a factorof 3. It appears that nickel catalyzes the oxidation of coke directly to CO,rather than via CO [41].

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CO CONVERSION DEACTIVATION 8'f AIR AT 1200 FlOOr I 1 1 I I +

8

-C0N

200-5 PPM -

-I

0PcT 3 0 - -

0

I75AIR EXPOSURE TIME IN MINUTES

FIG. 7 Effect of noble metals loading on additive performance.

Although combustion promoters are often needed to control the level ofafterburn and to reduce CO emissions, they typically increase NO, emissions[42,43]. To address this problem, CO combustion promoters that minimizethe increase in NO, are being developed. A n example of such an additive,tested in the Davison Recirculating Riser (DCR) pilot unit, is shown in Fig. 8.

-2 -1 0 1 2 3 4 5

Hours

FIG. 8. DCR testing of CO combustion promoters (gas oil feed, E-Cat, 0.5wt% additive, 1 excess oxygen).

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56 CHENG ET AL.

A commercial CO promoter increased the NO, concentration from 100 to 350ppm during an experiment carried out in the DCR. In contrast, the new pro-moter, XN0,-2, is able to provide adequate CO conversion without apprecia-bly increasing NO,. The reason for this behavior is under investigation. TheCO combustion promoter typically has little effect on SO, emission.

B. Future Horizons

Carbon monoxide oxidation chemistry is an old area of catalysis with anumber of literature reviews, particularly in auto exhaust catalysis. Somewhatolder reviews deal more specifically with the mechanism of CO oxidation onplatinum [44,45]; however, the kinetics and mechanism of CO oxidation hasnot been characterized under the high-temperature conditions of the FCCU

(FCC unit) regenerator. Although CO promoters are widely used, some as-pects of the operation of the FCCU regenerator in combination with a COpromotion catalyst are still not entirely understood. The interactions betweencoke burning, CO oxidation, and the level of NO., (and other pollutants) inthe regenerator warrant further investigation.

IV. CONTROL OF SULFUR OXIDES

Sulfur oxides (SO,) are a major atmospheric pollutant and precursorsfor acid rain. The Environmental Protection Agency (EPA) has regulationsfor the control of sulfur oxides from industrial and power plants. SO, emis-sions from the FCCU have been under federal and local regulations since1984. These regulations limit emissions from a revamped FCCU to 9.8 kgSO, per 1000 kg coke burned off or approximately 300 vppm [46]. Regula-tions are more stringent for new units.

Fifty to sixty percent of the sulfur in the FCC feed appears in the liquidproducts, and most of the rest, 3.5-45%, is released as H,S. The H,S istypically recovered downstream in a Claus plant. A small fraction of the feedsulfur, typically 2-S%, ends up in the coke [47]. The higher the thiophenicsulfur content in the feedstock, the higher the percentage of feed sulfur inthe coke. Up to 30% of feed sulfur has been reported in the coke, dependingon the nature of the feedstock [48]. Sulfur in the coke is burned to sulfuroxides, which are emitted from the regenerator.

Sulfur oxides control options in the FCCU include the following: (a)treatment to reduce sulfur on spent catalyst, (b) back-end treatment such asflue gas scrubbing, and (c) the introduction of in-process SO, transfer addi-tives, which transfer sulfur back into the riser and release as H,S. BecauseSO, is only a small fraction of the overall sulfur balance, the increase of H,S

due to the usage of SO, additive is typically very small. Option (a) includesboth front-end hydrodesulfurization or an additional intermediate process stepin which sulfur on the spent catalyst is reduced by stripping prior to regen-

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ENVIRONMENTAL,FCC TECHNOLOGY 57

eration [49]. Compared to options (a) and (b) that require a significant capitalinvestment, the use of additives requires almost no capital, except for the costof an additive loading system and the availability of a Claus plant.

A . Mechanism of SO, Transfer

The generally accepted mechanism [50-531 for SO, transfer is as fol-lows: (1) the oxidation of SO, to SO,, 2) the chemisorption and storage ofSO, in the additive, and (3) the reduction of sulfates to H,S. Steps 1 and 2occur in the FCC regenerator at approximately 700-730°C and under slightlyoxidizing conditions. Step 3 occurs in the FCC riser at approximately 520-530°C under reducing conditions. Because each of the steps occurs in series,overall SO, transfer performance can be limited by any one of these threesteps. The chemical reactions for SO, transfer additives together with the freeenergies of reaction are illustrated for MgO and MgAl,O, spinels in Table 5.

The first step in the mechanism of SO, transfer additives is the oxidationof SO,. Under FCCU regenerator conditions, SO, is favored over SO,. Themolar ratio of SO, to SO, is approximately 0.1. Thus, SO, transfer additivescontain catalytic ingredients that promote the oxidation of SO . Several ma-terials have been examined. The activity ranking reported for SO, oxidationat low temperatures is V > Cr > Fe > Ce [54]. The weight gain for metaloxides of these components has also been studied at 700°C using thermogra-vimetric analysis (TGA) [55]. The ranking based on weight gain per gram ofatom of the metal for samples with similar surface area and median porediameter was Ce > Cr > V > Fe (Table 6).

Desirable metal oxides for the chemisorption and storage of SO, arethose that possess a large adsorption capacity of SO, to form moderatelystable metal sulfates. Strong bases, such as CaO, BaO, SrO, and the oxidesof alkali metals, form highly stable IV-metal sulfates that are not reducible

TABLE 5

Gibbs' Free Energies of SO Transfer Reactions

AG'OWK 4.931 kJAG'OmK= -101.555 kJAGlomK= -62.754 kJAGlWOK= -38.801 kJ

soOK = -(?) k~AGsooK -197.374 W

AGsmK= -169.447 kJAG800K= -26.931 kJ

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TABLE 6

Catalyst Properties and Performance in Lab Tests

CHENG ET AL.

Metal oxide

None V,O, Cr,O, Fe,O, CeO,

Loading (wt%) 0 2.49 2.14 2.13 4.22Precursor compound Oxalate Acetate Oxalate AcetateSurface area N,) m2/g) 151 167 193 186 186Median pore diameter (nm) 14.5 8.3 6.6 7.2 6.9

Performance in lab testsWt% gain in TGA testb 3.0 5.3 9.8 4.5 11.3

Onset temp. ( C) 600 460 580 520 500Take-off at or below 530°C N o Yes No Yes No

H,S Release in Propane-TPR

Prepared by postimpregnation on calcined microspheres consistingof 37.4% MgO,

'Over a period of 15 min exposure to SO, oxidation at 700°C in flowingN, (225 ml/

'TPR test of presulfated catalysts in a soak (300 )-ramp (30 C/min)-soak (530°C)

18.9 La203, and 42.8%AI,O, by weight.

min) with 0.73%0 and 0.27% SO,.

mode, using propane as the reductant.

under FCC riser conditions. Such materials are therefore not useful for SO,

transfer. Basic oxides that form less stable sulfates, such as MgO and La203,are preferred.

The prevailing thinking has been that molecular hydrogen is responsiblefor the reduction of metal sulfates in the reducing riser environment. Recently,however, it has been proposed that hydrocarbons (HCs) in the FCCU risermay provide the hydrogen for reducing sulfates according to the reaction [54]

MSO, + HCS = MO + 3 H 2 0 + HzS + (HCS - 8 H)where M is the metal. At simulated riser conditions, the temperature for theonset of metal sulfate reduction was studied using propane. The ranking forthe metal oxides examined was found to be V > Ce > Fe > Cr (Table 5).This conclusion is confirmed by more recent results [ 5 6 ] These results showthat these metal oxides have the ability for both oxidation of SO, and for theformation of metal sulfates. Vanadium in combination with metal oxides suchas MgO and La,O, facilitate the reduction of metal sulfates and sulfides toH,S in the riser.

In addition to additive chemistry, overall SO, reduction performancedepends on FCCU operating conditions. The efficiency improves with in-creasing catalyst circulation rate, with increasing oxygen partial pressure (op-

erating in a full combustion mode is much better than in a partial combustionmode), with better catalyst mixing, and at higher SO, concentrations. Forexample, the performance of a commercial additive can vary widely between

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8 and 60 kg SO, removed per kg of additive, depending on the FCCU op-erating conditions [48,57].

B . Development of SO, Additives

Prior to 1985, SO, control additive technology was directed at “oxida-tive SO, absorption” [58-601. The assumption at that time was that thesulfates would thermally decompose, thereby regenerating the material. Be-tween 1977 and 1986, there were a number of patents and publications fromthe oil industry [57-59,61-661. The two most important SO, traps werebased on MgO and other rare-earth oxides all supported on alumina [64-671.Traps based on M@,04 spinel [62] and with the spinel containing excessMgO 163,641 were also found to be effective. The ingredients for promotingSO, oxidation included the platinum group metals, rare-earth oxides, and theoxides of nearly all of the transition metals [61,68,69].

In 1986, a SO, transfer catalyst containing vanadium, DESOX” KD-310, was introduced for the first time. The additive was a nonstoichiometricspinel, MgAI,O, with an excess mole of MgO. The relationship between thecomposition and performance of nonstoichiometric spinels of MgA1204 pinelcontaining either excess MgO or excess AI,O, has been extensively discussedin the literature [70]. Vanadium enhances both the rate of SO, oxidation andthat of sulfate reduction for the fast release of sulfur as H,S in the riser. SO,

transfer additives today, without exception, contain some vanadium in therange of about 1-2.5 wt%. The additive addition rate is quite low relative tothe FCC catalyst inventory in the regenerator, typically 1 n 200 parts.

Nonspinel additives, such as a ternary oxides consisting of MgO-La,O,-Al,O, or MgO-(La/Nd),O,-AI,O,, have been claimed to be just asefficient as the current commercial additive when Ce and V are included ascatalytic ingredients [71]. SO, additives derived from hydrotalcitelike com-pounds containing the oxides of Ce and V have also been claimed to beeffective [72]. This additive, upon exposure to temperatures in excess of450°C, undergoes a structural change, forming largely free MgO (periclase)and some stoichiometric MgA1204 spinel.

The performance of SO, additives has been evaluated in a recirculatingpilot plant such as the Davison Circulating Riser (DCR) unit. Typical resultsare presented in Fig. 9 [55]. The rate and the extent of the decrease in SO,concentration depend on the rate of SO, oxidation and on the SO, storagecapacity of the additive. As equilibrium storage capacity is reached, the rateof adsorption decreases and the concentration of SO, increases until thesteady-state value is reached. Type 1 materials in Fig. 9 represent additiveswith a high SO, oxidation activity and a large SO, adsorption capacity but

lower activity for the reduction of metal sulfates. Type 2 materials representthe performance of MgAI,O, with excess MgO (as discussed above) but with-out the vanadium. Type 3 is an additive that has a limited number of SO,

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60 CHENG ET AL.

I I I I

TIME ( h )0

FIG. 9. SO, transfer catalyst performance in a pilot plant.

trapping sites and storage capacity and a moderate activity for SO, oxidationand metal sulfate reduction. Additives, which are largely based on La-richrare-earth oxide dispersed on alumina, belong to this class [66].

The SO, additives undergo severe deactivation in the FCCU regenerator.Silica poisoning [73] and the hydrothermal sintering of both CeO, and spinelhave been discussed as deactivation mechanisms [56]. As CeO, sinters, thecrystallite size grows. This results in lower SO, oxidation activity and lowerrates of SO, adsorption (due to decreasing surface area and spinel sintering).Compared to the hydrothermal effect, the effect of silica poisoning-due tosilica buildup and the formation of some MgSiO, (fosterite)-was found tobe relatively minor [ 5 5 ] . Unlike CeO, (which tends to sinter rather readilydue to a lack of sufficient interaction between CeO, and any of the three,MgO, A1203 r MgA1204 spinel), V,O, at a low loading, well below itsmonolayer concentration, is believed to be relatively stable, presumably dueto the strong interaction between V,O, and MgO. It has been proposed thatthe vanadia i s in the form (MgO),V,O, [74]. This may explain why, in con-trast to spent FCC catalysts containing vanadium, there is practically no va-nadium transport during the hydrothermal aging of an FCC catalyst blendedwith a vanadium-containing SO, additive [ 5 5 ] . Basic metal oxides such asMgO, MgA1204, hydrotalcite [75,76], and MgO-(La/Nd),O,-Al,0, [77]are known to be effective vanadium traps which form stable compoundswith V.

C. Future Horizons

There is a need to develop a laboratory test for evaluating SO, additives.

A test method based on the microactivity test [78], with a modification toinclude all the three steps in repeated cycles, including even steam stripping[66], lacked reproducibility and did not correlate with the commercial per-

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formance. A more commonly employed method is based on TGA tests of therate and extent of weight change during the oxidation of SO, and 0, overthe catalyst sample at 7OO0C, followed by the reduction with pure or diluted

hydrogen at 650°C or higher temperatures [79-811. As discussed above, thereduction of presulfated additives with hydrocarbons instead of molecularhydrogen needs to be further explored [55].

There is also an ongoing need to develop improved additives. Ap-proaches include alternate chemical compositions (e.g., Ref. 82) that increaseSO, transfer or new concepts for SO, reduction. Improved additives will ob-viate the need for installing more expensive tail-end control technologies suchas flue gas scrubbers.

V. CONTROL OF SULFUR IN GASOLINE

High levels of sulfur in gasoline have been linked to poor performanceand shortened life of catalytic converters [83]. Accordingly, gasoline sulfurcontent is being restricted by legislation in both the United States and theEuropean Union [84]. Effective January 1995, refiners selling gasoline to theReformulated Gasoline market must maintain sulfur content at or below the1990 baseline under the EPA “Simple Model.” The Complex Model, whichwill become effective in January 1998, will require that refiners trade off

gasoline sulfur with other parameters, subject to a cap based on the 1990industry average. The more stringent CARB (California Air Resources Board)Phase I1 regulations, enacted in March 1996, requires the average gasolinesulfur content be below 40 ppm, with a cap of 80 ppm.

The average sulfur levels in the U.S. gasoline pool are 143 ppm sulfurfor premium fuel, 375 ppm sulfur for mid-grade fuel, and 384 ppm sulfur inregular gasoline. The sulfur in gasoline comes from straight-run naphtha,coker naphtha, and FCC gasoline. Straight-run and coker naphthas are highin sulfur, but as they account for less than 5% of the gasoline pool, theircontribution to total gasoline sulfur is only about 10%. FCC gasoline accountsfor about a third of a typical U.S. refinery gasoline pool, and about 90% ofthe total gasoline sulfur [85,86].

Reducing the sulfur content of FCC gasoline is one of the major con-cerns for refiners. Traditional means of reducing sulfur include feed hydro-treating, naphtha hydrofinishing, and lowering the end point of FCC naphtha.Hydrotreating and hydrofinishing are capital-intensive. Hydrofinishing alsosignificantly lowers the octane of FCC gasoline [87]. Lowering the gasolinecut point can significantly lower gasoline volume. Furthermore, it may bedifficult to find a home for the high-sulfur heavy gasoline, without further

processing. Thus, there is great incentive for refiners and catalyst manufac-turers to find ways to reduce gasoline sulfur directly in the FCC process.The objective of this section is to discuss sulfur chemistry, propose pos-

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62 CHENG ET AL.

sible ways of lowering FCC gasoline sulfur content, and review ongoingprogress on solid additives for gasoline sulfur reduction.

A . Sulfur Species in FCC

Sulfur compounds in FCC feed include mercaptans, sulfides, alkyl-sub-stituted thiophenes, thiophenols, benzothiophenes, and multiring aromaticthiophenes.

A number of analytical methods for characterizing sulfur species in thefeed have been discussed in the literature [88-901. Sulfur compounds tendto be concentrated in the aromatic and polar fractions of the feed. Massspectrometry (MS) gives quantitative information on benzothiophenes andmultiring thiophenes, but it lumps thiophenes with sulfides and mercaptans[91]. Combining x-ray photoelectron spectroscopy (XPS) and MS analyses,a semiquantitative distribution of sulfur species as aliphatic sulfur, substitutedthiophenes, substituted benzothiophenes, and multiring thiophenes can be ob-tained. The sulfur species in typical FCC feeds are shown in Table 7.

Sulfur compounds in gasoline can be identified using a gas chromato-graph (GC) equipped with a sulfur chemiluminescence detector (SCD) or anatomic emission detector (AED) [92,93]. A sulfur GCAED chromatogram(30-m Hewlett-Packard HP-1 column) of an FCC gasoline sample is shownin Fig. 10. Typical FCC gasoline contains mercaptans, thiophene, Cl-C4-

substituted thiophenes, thiophenol, C1 and C2 thiophenols, tetrahydrothio-phene, and benzothiophene. The boiling ranges for these compounds aregiven in Table 8. Figure 10 also shows that most of the sulfur species areconcentrated in the heavy end (375 F+ or 190°C+). Benzothiophene alonemay account for 30% or more of the sulfur in full-range gasoline. Thus,undercutting, or lowering the end boiling point of gasoline, is an effectiveway of lowering the sulfur content of gasoline.

B. Reaction Mechanism

In an FCC unit, typically about 35-45% of the feed sulfur is convertedto H,S, 40%, 5% in coke, 5% in gasoline, and the rest ends up in LCO andbottoms [47,94]. The reactivity of feed sulfur compounds and the mechanismby which they end up in gasoline is a subject of ongoing study. Mercaptansand sulfides are converted to H,S and do not significantly increase the amountof sulfur in gasoline. As shown in Fig. 11, H,S production from FCC cor-relates well with the amount of nonthiophenic sulfur in the feedstock [94].Huling et al. [95] and Nguyen and Skripek [87] have correlated sulfur in FCCgasoline with the sulfur species in FCC feed. Alkyl-thiophenes and aromatic

sulfides are believed to be the key contributors to gasoline-range sulfur.The concentration of sulfur in gasoline is only weakly dependent onconversion as a result of opposing effects [47,87]. Mercaptans and alkyl-

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TABLE 7

Sulfur Species in FCC Feeds

Feed A Feed B Feed C

Wt% in feed

API gravityK factorConradson carbonTotal nitrogenBasic nitrogen

Simulated distillation (“F)IBP50 vol%

FBPX P S analysis

Atomic % S as aliphatic SAtomic % S as thiophenic SAtomic % S as oxidic S

MS analysisAtomic % S as benzothiopenes and

Atomic % S as mercaptans, sulfides,multiring thiophenes

and thiophenes

Normalized overallMercaptans and sulfidesThiophenesBenzo and multiring thiophenesOxidic sulfur

2.77

20.211.65

1.230.080.02

495840

1171

789

4

31

69

76030

4

0.3

25.811.78

0.530.120.052

252790

1202

146718

20

80

12561715

2.59

22.511.52

0.250.0860.034

423755

1027

2770

3

30

70

264229

3

thiophenes are unstable with respect to desulfurization. The concentration ofthese species decreases with increasing contact time or as gasoline overcrack-ing occurs. However, benzothiophene, formed by dealkylation of higher-mo-lecular-weight sulfur compounds, is relatively stable with respect to desul-furization. In general, the concentration of benzothiophene increases withconversion. At constant conversion, the concentration of sulfur in gasolineincreases with riser temperature due to increased dealkylation reactions ofaromatic sulfur species. The sulfur content of gasoline increases as the gas-oline cut-point temperature is increased, because of the inclusion of benzo-thiophenes. These effects are shown on Fig. 12.

Catalysts with higher hydrogen transfer activity tend to produce less sulfur

in gasoline [47,87,96,97]. Catalysts containing REY (rare-earth-exchanged Yzeolite) and REUSY (rare-earth-exchanged ultrastabilized Y zeolite) made lowerfull-range gasoline and cut-gasoline sulfur than catalysts containing non-rare-

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64 CHENG ET AL.

thiophcne C2.thiophenoll

Cl -ihioplturmI tetmhydrothiophene

benzolhiophcncetmhydrothiophene

1s

0O 5 10 1s 20 s 3 3s

RetuitionTindmin

FIG. 10. Sulfur GCAED chromatogram (30-m Hewlett-Packard HP-1 column).

50.0

4 5 . 0

4 0 . 0

35.0

30.0

25.0

20.0

15.0

0.0 10.0 2 0 . 0 30.0 40 .0 5 0 . 070 n o n - t h i o p h e n i c S i n feed

FIG. 11.sulfur in feed.

Percentage of feed sulfur as H,S versus percentage of nonthiophenic

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ENVIRONMENTAL FCC TECHNOLOGY

I . . . . , . . . . , . . . . , . . . .I

Full range. T90 = 3x0 O F . cndpoinl = 430 F

-< - * -4:

0;-i

Cut, T90 = 300 F, endpoint = 340 O F

65

TABLE 8

Sulfur Species in FCC Gasoline

Sulfur compounds in Boiling rangeFCC gasoline ( F)

Mercaptans c150 (65.5 C)

C1 thiophenes,Thiophene 150-200 (65.5 -93°C)

tetrahydrothiophene 200-250 (93 - 121°C)C2 thiophenesC3 thiophenes, thiophenol

250-300 (121 - 149°C)300-375 (149- 190°C)

C4 thiophenes, C1 thiophenol 350+ (177 C+)Benzothiophene, C2 thiophenol 375 + (190 C+)

earth zeolites. This is illustrated in Fig. 13. A possible reason for this behavioris that catalysts with high hydrogen transfer activity promote the saturation ofthe thiophene ring, thereby leading to the desulfurization of alkyl-thiophenes.

C. Gasoline Sulfur Reduction Additives

Approaches for developing Gasoline Sulfur Reduction (GSRTM)addi-tives may include (a) direct desulfurization of thiophenes and thiophenols

5 0 0

450

4 0 0

350

300

250

200

150

100

FIG. 12. Sulfur in full-range and cut gasoline for different reactor tempera-tures versus conversion.

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66

s o o t

4 5 0

400 :

cn

300aa

250 :

.00 :

1 5 0 -

1 0 0 '

CHENG ET AL.

-Q- REUSY. : . * . : . .- +- U S Y- C4 SYlMATRlX

- _- ,r c +- . -*- ., - L - ; t - A--,

Full rang;. T90 = 380 O F , endpoint = 4

- - c 1 L - - 2 * - . *@ W

Cut. T90 300 OF. endpoint = 340 F

' . . ' . ' . ' ' .

60 6 4 68 72 76 80

conversion, wt . %

FIG. 13. Sulfur in full-range and cut gasoline for different catalyst types ver-sus conversion.

from the product gasoline, (b) adsorption of gasoline sulfur precursors, or (c)alkylation of gasoline sulfur precursors into a higher-boiling-range fraction.The best alternatives are obviously (a) and (b), as they do not transfer thesulfur to the other liquid products. In addition, it is desirable that the additiveconverts the sulfur compounds into H,S and hydrocarbon products rather thanto coke.

Lewis-acid materials (e.g., zinc aluminate) have recently been discussedas gasoline sulfur reduction additives in the literature [98,99]. A proprietaryadditive, GSR-1, when blended along with a commercial base catalyst at the10 wt% level provided gasoline sulfur reduction with no measurable changesin activity, product selectivity, or coke selectivity. A comparison of the sulfurdistribution in the products is shown in Table 9. The feed had 26.6 M I 11.59K-factor, 182 aniline point, and contained 1.05% S and 0.23% ConradsonCarbon. The overall sulfur reduction for the full-range gasoline was 22.4 .Sulfur in light gasoline, defined by cutting the gasoline at C2 thiophene, wasreduced by 45%; however, sulfur species in the heavy cut gasoline werereduced by only 7%. The reduced sulfur compounds were found to primarilyform H,S [43].

Harding et al. described the reaction kinetics of thiophenes, alkyl-thio-phenes, and tetrahydrothiophene (THT) under simulated FCC conditions us-ing standard cracking catalysts and cracking catalysts with GSR-1 [97]. They

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TABLE 9Sulfur Distribution: MAT Comparison of Base Catalyst and Catalyst Blended with

10% GSR-1 and GSR-2

Base catalyst Base catalystBase with 10% with 10%

catalyst GSR-1 GSR-2

Sulfur distribution (ppm)MercaptansThiopheneMethylthiopheneTetrahydrothiopheneEthylthiophenePropylthiophene plus thiophenol

Butylthiophene plus thiophenolBenzothiopheneTotal Gasoline SCut Gasoline S

33.836.6

105.824.2

118.476.11

82.6276.0

753.5477.5

19.124.362.2

5.361.861.9

84.9265.5

585.0319.5

22.526.566.0

6.066.647.9

40.5264.6

540.6276.0

% Sulfur conversionTotal gasoline 22.4 28.2Cut gasoline 33.1 42.2Light cut' gasoline 45.8 41.1Heavy cut' gasoline 7.5 44.3

Excluding benzothiophene.C2 thiophene minus.

'C3 to C4 thiophenes and thiophenols.

concluded that at FCC conditions with a typical cracking catalyst, the hydro-genation of thiophenes to THT, followed by the cracking of THT to H2S wasthe primary route for the removal of sulfur species from the gasoline range.Their results also indicated that the addition of GSR-1 enhances the rate ofremoval of THT from the gasoline and thereby decreases the amount of gas-

oline sulfur. The rate of THT removal is important because it readily recon-verts to thiophene by dehydrogenation reactions.

A second additive, GSR-2, that builds on the successful properties ofGSR-1 has also been developed [43]. Table 9 summarizes the sulfur distri-bution in products using this additive at the 10 wt% level. As before, GSR-2 does not impact product yields or selectivity relative to the base case.However, it is more effective than GSR-1 in reducing sulfur in heavy cutgasoline. As shown in Table 8 GSR-2 reduced the concentration of C3 thi-ophenes through C4 thiophenes by 44.3%. For the total gasoline, GSR-2reduced sulfur by 28.2%. Excluding benzothiophenes by lowering the gaso-line cut point increases sulfur reduction by GSR-2 to 42.2%. Process devel-opment and commercial testing is underway for this product.

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68

D. Future Horizons

CHENG ET AL.

More work aimed at improving GSR additives will likely continue inthe near future. New processes

forimproved desulfurization of feed and gas-

oline products are also being developed. An example is a separate processfor the adsorption or alkylation of sulfur compounds [loo].

VI. CONTROL OF NITROGEN OXIDES

Nitrogen oxides (NO,) are primarily emitted from the FCC process asnitric oxide (NO). Smaller amounts of nitrogen dioxide (NO,) and nitrousoxide (N20) can also be formed. NO converts to NO, in the atmosphere. NO,levels in the flue gas are typically in the range 50-200 ppm [ lo l l . Theseemissions are increasingly being controlled by various state and localregulations.

The FCC regenerator poses a challenging environment for controllingNO. In addition to NO, the high-temperature regenerator exhaust contains 0 2 ,CO, COz, H20, and other nitrogen or sulfur oxides. NO, control technologycannot interfere with the primary catalytic cracking and preferably should notdetrimentally impact the emissions of other pollutants.

This section reviews our present understanding of NO, chemistry, dis-cusses the options available for controlling NO, emissions, and identifies fu-

ture directions in FCC NO, control.

A . Origin of Nitrogen Oxide Emissions

Nitrogen levels in FCC feeds range from 0.05 wt% to 0.5 wt%. Nitrogencompounds are typically distinguished by their basicity. Much work has beenreported on the characterization of nitrogen compounds in gas oils [102-1051. Most of the nitrogen compounds fall into the following four groupswith decreasing basicity: amines, pyridine derivatives, pyrrole derivatives,and amides. Typically, one-third of the nitrogen is considered basic nitrogen.Some of these compounds adsorb on the acid sites of the FCC catalyst, inhibitcracking activity, and may be converted to coke. Some of the nonbasic nitro-gen compounds may also contribute to coke. Nitrogen compounds left in theliquid products are typically neutral or acidic (e.g., pyrrole derivatives) [106].The nitrogen concentration in light and middle distillates is typically at ppmlevels. The concentration increases significantly when the boiling point isgreater than 350°C.

To better understand NO, formation mechanisms, Peters et al. attemptedto obtain a nitrogen balance around the FCC unit [42]. The experiments were

conducted in the Davison Circulating Riser (DCR) together with equilibriumcatalyst [107,108]. In order to exclude the nitrogen from air, a mixture ofargon with 5 oxygen was used to regenerate the catalyst. Their results

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indicated that approximately half of the feed nitrogen appeared in the liquidproducts. Most of the nitrogen in the liquid products was concentrated in theheavier fractions. Feeds with higher concentrations of nitrogen had higher

nitrogen levels in the LCO and bottom products. Titration of the water con-densate (from steam stripping and feed atomization) and analysis of ammoniain the light gases showed that about 8% of the feed nitrogen is convertedinto ammonia. In these experiments, 30 wt% to 40 wt% of the feed nitrogenwas recovered as molecular nitrogen. Approximately 10- 15% of the nitrogenin the coke was converted to NO; the rest was molecular nitrogen. In com-mercial FCC regenerators, the fraction of nitrogen released as NO varies fromabout 3% to 25% depending on the regenerator design and operating condi-tions [109].

In contrast to sulfur, about 50% of the feed nitrogen is present in theliquid products, less than 10% of nitrogen appears as ammonia, and the restis present in the coke. As discussed above, very little of the nitrogen in thecoke is oxidized to NO; most of it undergoes reduction to N2 in the regen-erator. Although a smaller fraction of sulfur than nitrogen ends up in coke,the concentration of sulfur in coke is typically similar to that of nitrogen dueto the high concentration of sulfur in feed.

B. Mechanism of Nitrogen Oxides Formation

Regenerator NO, emissions may be formed by three mechanisms: (a)reaction of air-derived nitrogen and oxygen (thermal NO,), (b) oxidation ofnitrogen-containing compounds in the fuel (fuel NO,), and (c) reaction be-tween radicals in the combustion flame (prompt NO,). Contributions due tomechanism (c) are expected to be low under typical regenerator conditions.

It has been hypothesized that some of the NO, results from the thermaloxidation of air-derived nitrogen [1101. Thermodynamic calculations showthat the temperature needed for forming appreciable amounts of thermal NO,is much higher than the temperature of commercial regenerators. At equilib-

rium, less than 10 ppm NO is produced under typical regenerator conditionsof 1% excess 0, and temperatures between 730°C and 780°C. Consequently,the contribution of thermal NO, to the observed NO, in regenerator flue gasis minor,

The chemistry on how coke-bound nitrogen is converted to nitrogen andnitrogen oxides is not well understood. However, there are analogies amongregenerator coke combustion, fluidized-bed coal combustion [111,112], andon the formation of nitrogen compounds during the regeneration of spenthydroprocessing catalysts [113,114]. During coal combustion, fuel nitrogen

is converted to HCN and NH, intermediates which then form NO, (Fig. 14)[112]. A similar mechanism also prevails during the regeneration of spenthydroprocessing catalysts. Selectivity to HCN, NH,, and N2 is strongly af-

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70 CHENG ET AL.

r Pyrrolic-N HCN

700

FIG. 14. Schematic of the NO, formation mechanisms. (Adapted from Ref.13.

__........ ............... ....................................................... i .......................... : .......................... ........................... i.._ ......................i .

fected by the type of nitrogen originally in the feed or in the coke [113]. Thesubsequent destruction of HCN or NH, is more affected by operating con-

ditions. In coal combustion, there is evidence that N,O is formed mainly fromcyano species, whereas NH,-based compounds convert to NO.

At typical regenerator conditions of excess oxygen and in the presenceof catalysts/metals, most of the HCN and NH, are oxidized to NO, and N,O.Part of the NO, and N,O can then go through reduction or decomposition tonitrogen, respectively. It has been shown that ammonium compounds canconvert to HCN in a simulated partial combustion regenerator [115]. In lab-oratory tests, HCN is more readily oxidized than ammonia, although both arevery reactive and are readily oxidized to nitrogen and NO,. Figure 15 showsthe NO emission from a series of commercial units and the Davison Circu-lating Riser with feeds containing different levels of nitrogen. Higher nitrogenfeed typically produces more NO,, leading to the conclusion that regenerator

300-L

200

100

0

...................... Q ..................................................... i .......................... i..i o 0 ;

i........................... i .......................... i .....-

-- ................ _....................... :.......................... i .......................... ..........................: o ;

1

....

: : : : I : : : : I : : : : I : : : : I : : : : I : : : : I : : : :

FIG. 15. Correlation of NO emissions with feed nitrogen. FCCU data fromRef. 101.

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NO, emissions are largely a result of the fuel-bound nitrogen in the cokedcatalyst.

To explore the conversion of nitrogen in coke to NO,, the FCC regen-erator was simulated in a laboratory temperature-programmed oxidation ex-periment [42]. The results indicate that, initially, most of the carbon is burnedto C O , and very little NO is formed. At higher temperatures, when a signif-icant fraction of carbon has been oxidized, most of the NO is formed. Thecarbon and nitrogen contents of the spent catalyst at various stages of regen-eration are shown in Fig. 16. As the amount of carbon on the regeneratedcatalyst decreases, the nitrogen content of the remaining coke increases. Thismay be due to the strong interaction between the basic nitrogen species andthe acid sites on the catalyst.

Operating conditions and hardware design of the regenerator can sig-

nificantly influence regenerator NO, emissions [1161. Higher excess oxygenis known to correlate with higher NO, emissions. A set of commercial dataillustrating the relationship is shown in Fig. 17. Full combustion, comparedwith partial combustion, also has a similar effect of increasing NO,. Thisbehavior could be due to (a) increased conversion of the nitrogen in coke toNO, at higher oxygen levels or (b) decreased reaction of the formed NO, byreducing species. There is some evidence that mechanism (b) could be im-portant. The reaction of NO with carbon has been reported previously asbeing as rapid or perhaps more rapid than the reaction of coke with oxygen[22]. It is also well known that NO can react with CO readily in the presence

of various supported metal oxides or noble metals [117,118]. As discussed in

20

15

10

5

00 0.1 0.2 0.3 0.4 0.5 0 6 0.7 0.8

Coke on Catalyst, w t

FIG. 16. Nitrogen content in a partially regenerated catalyst.

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72 CHENG ET AL.

0 0.5 1 1 .5 2 2.5 3

Excess 02. mot

Excess oxygen versus NO, emissions in a commercial FCCU.IG. 17.

the CO control section, the fact that a small amount of CO combustion pro-moter substantially increases NO emission may be evidence of the importanceof NO reaction with CO and other reducing species in the regenerator.

C . Control Option s

Regenerator NO, control options include (a) reducing coke nitrogen bycatalytic hydrodenitrogenation of the feed, (b) reacting NO formed to N2 byincreasing the concentration of reducing agents, and (c) installing a tail-endprocess for controlling NO,. Option (a) is typically expensive and may requirethe installation of additional equipment and the increased use of hydrogen.Option (b) can be implemented by redesigning process hardware, operatingconditions, and the introduction of solid additives.

Injection of external reducing agents downstream into the flue gas canreduce the NO emissions [ 119,1201. For example, ammonia reacts selectivelywith NO, to form nitrogen and water. The molar ratio of ammonia to NO,has to be tightly controlled. Otherwise, excess ammonia, which itself is anair pollutant, may be released. The above reaction is referred to in the liter-ature as selective noncatalytic reduction (SNCR).

It has been reported that countercurrent regenerators reduce NO, emis-sions due to the presence of higher concentration of reducing agent in thetop of the regenerator catalyst bed [109,110]. A countercurrent regeneratordistributes the coked catalyst on the top of the catalyst bed in the regenerator.A commercially available countercurrent design reduces the percentage of

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nitrogen in coke being released as NO, to about 5 from 10-20% in otherregenerator designs. Another commercially available high-efficiency regen-erator design has been claimed to reduce both CO and NO, emissions [121],

mostly due the reduced usage of CO combustion promoter in such designs.The discussions so far have focused on complete combustion operation,or regeneration with excess oxygen. For partial combustion, the presence ofa large amount of carbon monoxide and coke on catalysts can substantiallyreduce the NO emissions. Most of the NO is reduced to nitrogen. However,reducing conditions in the dense bed can potentially result in the conversionof nitrogen-containing intermediates to NH, and HCN. Thermodynamic equi-librium concentrations of ammonia of over 1% were reported from a FCCregenerator simulation at a low-temperature partial combustion operation[52]. The presence of HCN in commercial regenerator flue gas has also been

reported. Because the combustion of nitrogen comes after the combustion ofcoke, partial combustion may also allow the accumulation of nitrogen on theregenerated catalysts and affect the cracking performance of the catalyst. Thiseffect may be particularly important for high-nitrogen feeds. CO boilers areoften used in partial combustion regenerators to facilitate the oxidation ofCO in the flue gas. In such cases, the NO, emissions from the CO boiler arean issue.

A number of materials aimed at NO, reduction have been discussed inthe patent literature. A perovskite/spinel-based additive has been found to beeffective [122,123]. The additive is a combination of a Cu/Mn-based perov-skite and a spinel such as MgAl,O,. The additive reduces NO,, in the presenceof SO, and oxygen, by catalyzing the reaction of NO, with reducing agentsto form N,. The effectiveness of the perovskite additive depends on the levelof reducing agent at the NO, reaction site. The NO, reduction activity isenhanced when a large excess of CO is present. The perovskite additive istherefore most effective when no (or only a small amount 09 Pt-based com-bustion promoter is utilized and when levels of excess oxygen in the regen-erator are relatively low. Commercially available SO additives have similarchemistry and have also been explored as NO, additives [56,124].

Lanthanum or yttrium oxides, or lanthanum titanate, have been foundto be effective in reducing NO, in laboratory fixed-fluidized-bed studies [1251.Zinc-based NO, reduction additives have been discussed [1261. Reduction ofNO to N, can also occur over a catalyst particle containing inorganic reducedcomponents such as ceria. A ceria-containing material is reduced in the riserand serves as a reducing agent to reduce NO to molecular nitrogen in theregenerator. Such additives require the presence of a substantial amount ofreducing agents in the regenerator to maintain the redox process of ceriumbetween Ce: and Ce,' [Sl]. A particle containing ZSM-5 zeolite in a matrixcontaining titania and zirconia impregnated with Cu and rare-earth cations

has been claimed as a NO, additive [127].Selective catalytic reduction (SCR) is also being considered as an optionfor NO, control. In this case, ammonia is mixed into regenerator effluent

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74 CHENG ET AL

gases, which are then passed through an SCR catalyst. In the presence of thecatalyst, ammonia selectively reacts with the NO, to form nitrogen and water.The catalyst is typically an extruded or coated monolith containing vanadia

supported on titania, which is the catalytic component. The horizons for tail-end NO, control technology have been recently discussed by Pereira andAmiridis [128].

D. Future Horizons

With NO, emissions being increasingly regulated, it is expected thatrefiners will demand more NO, control technology. Some of the new processhardware will help reduce NO, emissions. However, the majority of the re-fineries will have to rely on optimizing operating conditions or using

additives.Currently, there is a need for a clear understanding of the relationship

between coke combustion chemistry and the formation or destruction of NO,.Such an understanding will undoubtedly aid in the development of new orimproved technology. With the increasing numbers of units processing residfeeds, there probably will be an increase in the number of partial combustionoperation units. The understanding of nitrogen chemistry in this mode willbe important as well.

The simultaneous reduction of SO, and NO, is a challenge. Simultaneousreduction has opposite requirements: NO, control technologies rely on theavailability of reducing agents, whereas SO, additives work better in a moreoxidizing environment. Additives for the direct decomposition of NO, in thepresence of oxygen still represent the Holy Grail.

ACKNOWLEDGMENTS

The authors would like to thank Dr. R. H. Harding, Dr. G. Yaluris, andDr. R. F. Wormsbecher for helpful discussions. We would also like to thankthe many Grace Davison researchers that have contributed to the developmentand understanding of FCC catalysts over the years.

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