Applied Catalysis, 22 (1986) 159-179 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

159

FLUID CATALYTIC CRACKING OF HEAVY (RESIDUAL) OIL FRACTIONS: A REVIEW

J.E. OTTERSTEDTa, S-6. GEVERTa, S.G. JxRfiSb and P.G. I.IENONa

aDepartment of Engineering Chemistry, School of Chemical Engineering, Chalmers

University of Technology, 412 96 Gbteborg, Sweden. b EKA AB, 445 01 Surte, Sweden.

(Received 16 July 1985, accepted 6 January 1986)

ABSTRACT

The oil crisis of recent times has caused a drastic decrease in the total con- sumption of oil and changed the demand pattern for the products of petroleum refinino. The demand for heavier fractions or residual oils has steadily decreased, making it imperative to convert these into gasoline, diesel and such lighter fractions. Fluid catalytic cracking (FCC) of these heavier fractions, however, poses several serious problems, caused mainly by their much higher hetero-atom concentration, metal contents and coking tendency, as compared to earlier feed- stocks. Several process and catalyst innovations have been made to tackle these problems. A new generation of FCC catalyst technology has emerged with tailor-made catalysts for higher structural stability and attrition strength, more complete CO combustion during regeneration, reducing SO, emissions from FCC stacks, en- hancing the gasoline octane number, passivating the harmful effects of metals like Ni and V accumulating on the catalyst, etc., These developments contain valuable lessons for the science and technology of catalysis.

INTRODUCTION

The arbitrary raising and controlling of crude oil prices since 1973 by OPEC

senia series of shock waves through the petroleum refining industry. On the one

hand desperate searches were launched to develop alternative energy sources as

also explore for and develop hitherto latent (often off-shore) oil and natural

gas reserves. On the other hand, the world has learned to live with less oil,

resulting in a radical change in the demand patterns for the various oil products.

Thus, the demand for heavier fractions and the so-called residual oils has steadily

decreased, while that for gasoline, diesel and such lighter fractions is increasing.

This shift in product pattern has become so compelling that refineries rjhich cannot

adjust and adapt quickly to this changing situation become uneconomic and are forced

to shut down (e.g. Texaco Refinery in Ghent and Chevron Refinery in Feluy, both

in a small country like Belgium).

is

In

One solution to the above change in the demand pattern for petroleum products

to crack more and more of heavier or resid fractions into lighter distillates.

the United States, the number of such resid-cracking operations doubled [I]

between February 1981 and October 1982. Furthermore, since the oil crisis of 1973,

the ratio between the cost of crude oil and the cost of refining has become high;

0166-9834/86/$03.50 0 1986 Elsevier Science Publishers B.V.

this has created a powerful incentive to develop processes for upgrading heavy

oils in general and for fluid catalytic cracking (FCC) and resid-cracking in

particular [Z]. Cracking of heavier fractions, however, poses numerous problems

for oil companies, design and engineering firms and catalyst manufacturers. Hence

these three groups are naturally developing several innovations to face these

urgent and stupendous problems. The purpose of this review is to draw attention

to these innovations because of their great impact and relevance for the science

and technology of catalysis. Quite recently, Maselli and Peters C3al have reviewed

the special processes adapted or newly developed for resid cracking; they have

also discussed in detail the large organo-metallic molecular structures which

complicate the resid-cracking operation, the special diffusion problems caused

by these giant molecules, etc., Many practical aspects of residue blending and

processing have been reviewed recently by Elvin [3b], and Campagna et al. [3c],

while the various grades of commercial catalysts available have been surveyed by

Aalund [3d]. More general process aspects of FCC have been described in a com-

prehensive review by Venuto and Habib [4a] and by Decroocq [4b] in 1984 in a

monograph in English, translated from its original French version published in

1978. Hence these process aspects will be only briefly mentioned, if at all, in

the present review. Greater attention is paid here to the disastrous consequences

of the accumulating metals on process selectivity and catalyst stability, the

recent breakthrough innovations to cope with the increased coke formation in an

existing or new catcracker, the separate roles of matrix and molecular sieve in

cracking heavier fractions, the new generation of FCC catalysts specially tailored

to give almost complete CO combustion, to reduce SOx emissions from FCC stacks, to

enhance the gasoline octane number, to passivate the harmful effects of Ni and V

accumulating on the catalyst, etc.

II. CHARACTERISTICS OF HEAVY OIL FRACTIONS

On the time scale of man, petroleum oils are stable substances. They do not

react with the rock formations which contain them nor do they separate or precipi-

tate with measurable rate. This pseudo-stable condition of petroleum oils implies

that there are only a limited number of functionalities defined in terms of

chemical bonds and heteroatoms in oil molecules. Furthermore, the distribution

of functionalities, but not the types of functionalities, in an oil fraction varies

.with the boiling point of the fraction. Hence the same compound types are present

in a low boiling fraction as in a high boiling fraction of vacuum gas oil and

indeed also as in the asphaltene fraction [4a]. The increasing portion of large

molecules containing heteroatoms and metal contaminants in fractions of increasing

boiling point accounts for the difficulties in processing of heavy oils [5,6].

Although the same type of functionalities are present in the asphaltene fraction

as in e.g. vacuum gas oil, the degree of association due to the sheer size of the

molecules and due to the presence of several functionalities in the same molecule

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TABLE 1

Contaminant variation in residues from 150 different crude sources

Residue type Ni/ppm V/ppm S/% N/% CCRa/% Density API

/kg crnm3 gravity

Atmospheric 0.5-94 0.3-274 O-2-4.5 0.2-0.6 0.1-15.6 840-1010 8.4-13.7

Vacuum 1.5-120 5-614 0.2-5.0 0.3-1.0 4.0-25.0 920-1050 3.4-21.5

aCCR = Conradson carbon residue.

is much higher in the asphaltene fraction. A major reason for the difficulties in

catalytic cracking of heavy feeds has therefore to do with the low volatility of

the large molecules and molecular clusters of the asphaltene and resin fractions.

This causes them to be present on or in the catalyst as liquids and be carried

over into the reactor where they will make coke and deposit metals onto the catalyst.

Table 1 shows typical concentration ranges in residues from atmospheric distillation

and vacuum distillation in typical refinery operations. Due to the hundreds of

cracking-regeneration cycles undergone by every catalyst particle in FCC, the metal

concentration on the catalyst can accumulate to hundreds or thousands of ppm.

A Scdlum n Vanadiur

FIGURE 1 Surface area as a function of metal impregnated in FCC catalyst (from

Zandona et al. [7]).

1. Metals

The metal content is considerably higher in heavy oil than in vacuum gas oil.

Nickel and vanadium are particularly important since they can be present in high

concentrations and will have detrimental effects on the cracking performance of

the catalyst. Nickel deposited on the catalyst causes nonselective cracking leading

162

to high hydrogen and C,-C4 gas production and high coke formation on the catalyst.

Vanadium will penetrate into the zeolite and react destructively with it (cf.

Section V). Figure 1 shows the change in specific surface of a catalyst when

contaminated with metals and subjected to a steaming test at 788°C for 5 hours [7].

Nickel and vanadium are usually present in porphyrine-like molecules and as

naphthenates. Sodium is often present in resids as a result of poor desalting.

Sodium neutralizes the acid sites which are so vital for the cracking activity

of the catalyst, it also leads to collapse of the crystalline structure of molecular

sieves at high temperatures.

2. Sulfur

Sulfur is present as mercaptans, sulfides, thiophenes and other organo-sulfur

compounds. The objective of American Petroleum Institute Research Project No. 48

was to characterize sulfur compounds in crude oil. The catalytic cracking of heavy

feeds increases the SOx content in the stack gas from the catalytic cracker. For

gas-oil cracking, as rule of thumb, 5% of the total S is transferred into the coke

on the catalyst and this appears as SO2 in the stack gas of a FCC unit; in other

words, 1 wt% S in the feed gives about 1000 ppm SO2 in the regenerator stack. For

resid cracking, the proportion of SO2 in the stack gas can be much higher than

these values C3al. Emissionshaveto be reduced below 60 kg S per 1000 barrels of

feed in California by January 1, 1987. Similar restrictions are being imposed or

contemplated in many other countries also.

3. Nitrogen and oxygen

The content of nitrogen is higher in heavier feeds than in vacuum gas oil.

Basic nitrogen compounds such as pyridines and quinolines are strongly adsorbed

at the acid sites of the catalyst resulting in reduced activity and higher rate

of coke formation. Neutral types of nitrogen compounds such as carbazoles, indols

and pyrrols are not as strongly adsorbed on the acid sites of the catalyst. Hence

they do not affect the cracking performance of the catalyst very much [Sl.

As in the case of nitrogen and sulfur, the content of oxygen also increases

with the boiling point of an oil fraction but relatively little attention has

been paid to the effect of oxygen on the performance of cracking catalysts.

4. Asphaltenes

Asphaltenes are aggregates of molecules containing polycyclic aromatics and

functionalities of several types. Their structures have been discussed quite

recently by Kaselli and Peters C3al and Bunger and Norman [9]. The size of

asphaltenes varies in the range 25-300 i. Since the pores of the zeolites are

too small to accept asphaltenes, catalysts with much larger pore structure than

zeolites are required to crack asphaltenes. Alternatively, asphaltenes can be

cracked on the matrix in which the regular zeolites used in cracking catalysts are

embedded. There are also commercial processes available for separating out

asphaltenes from crude oils [IO-121.

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PETROLEUM DESIRED TRANSPORTATION

Heavy 1 FUELS

RESIDUAL OILS \,_;_-__

/ * $

. .i

1.0 1.5 H/C ratio 2.0

FIGURE 2 H/C ratio for heavy and light petroleum feedstocks, residual oils, and

the desired end products.

5. Resins

Resins are polar molecules containing oxygen and nitrogen. Their molecular

weight is normally in the range 500-1000. They are slightly soluble in paraffins.

6. The H/C ratio

As shown in Figure 2, the H/C atomic ratio,for normal petroleum feedstocks

varies from 1.5 to 1.9, and for resids from 1.4 to 1.8. For all usable premium

transportation fuels this ratio should be in the range 1.8 to 2.1. All petroleum

refining operations in essence may be considered as efforts to raise the H/C

ratio. This can be achieved in two ways:

a) by rejecting the excess carbon as in processes like Delayed Coking [13], Flexi-

coking [14], Visbreaking [15], or Solvent Deasphalting [IO-121.

b) by introducing more hydrogen as in resid hydrotreating or hydrocracking.

These two options are widely practised and have been described in detail in

the literature. Hence they will not be discussed any further here, instead this

review will concentrate on the direct cracking of resids.

III. PROBLEMS FACED BY EXISTING CATCRACKERS IN PROCESSING HEAVY OIL FRACTIONS

Resids and blends of resids with gas oil have been processed in catcrackers

for more than 20 years [163. A survey conducted in 1983 showed [I71 that in the

United States already 47 refineries used resids as a part of their feedstock

mix for FCC. The blending of resids boiling at or above 500°C in the feedstocks

varied from 5% to even 50%. Most of this type of cracking has been done in con-

ventional or revamped FCC units originally designed for cracking only gas oil.

Such operations are possible only at the expense of unit capacity because of the

relatively high coke make per tonne of residual oil [18] and other problems,

peculiar to resid cracking, as briefly outlined below.

1. Problems of increased coke production

Introducing resid or blends containing resid to a FCC Unit causes an extra-

ordinary situation in several operational respects. Ordinary vacuum gas oil

contains less than 0.5% Conradson Carbon Residue (CCR). Catalytic cracking of

such conventional feed generally gives a coke-make of 4-6% based on the feed.

The combustion of this amount of coke will generate enough heat to satisfy the

various process energy requirements such as heat to vaporize the feed, heat to

carry out the endothermic cracking reactions and to heat the regeneration air

and the different kinds of process steam [19]. The FCC unit will generally adjust

itself to heat balance at regenerator temperatures in the range 670-720°C.

An atmospheric resid contains generally l-15% CCR, while a vacuum resid may

contain even up to 25% CCR (Table 1). If resid is introduced as part of the feed,

this will result in a higher coke production and more heat release in the re-

generator. The increased heat release is partially used in heating the increased

quantity of regeneration air needed, but the regenerator temperature will never-

theless rise considerably. This higher regenerator temperature causes several

types of problems in a FCC unit as described below:

a) Metallurgical problems: The regenerator in traditional FCC Units is generally

not designed for temperatures higher than 730-740°C. That limit is easily

exceeded when introducing even small amounts of resid in the feed, provided

the total throughput remains constant.

b) Deactivation of the catalyst: The rate of deactivation of present-day commercial

FCC-catalysts is mainly a function of contamination level on the catalyst,

regenerator temperature, partial pressure of water in the regenerator and time.

A higher regenerator temperature will deactivate the catalyst faster. Con-

sequently, a higher catalyst make-up rate is needed to keep a constant con-

version level in the unit.

c) Low catalyst/oil ratio: Since this ratio is set mainly by the heat balance

requirements in a FCC unit, a higher regenerator temperature will reduce the

catalyst circulation rate. This results in more thermal cracking and non-

selective catalytic cracking.

d) Inadequate regeneration: A higher coke make will increase the strain on the

regenerator air blower. If the air blower capacity is not the limiting factor,

another constraint can be the gas velocity in the regenerator or the flue-gas

cyclones. At higher velocities, more catalyst fines will be blown out of the

cyclones into the stack.

e) Environmental problems: If the regenerator air is not oxygen enriched, the gas

velocity through the regenerator must increase with increasing coke-make to

keep the coke on regenerated catalyst low. This will entrain more fines to

the environment, especially when accentuated by the fact that the proportion

of fresh catalyst in resid crackers generally is many times higher than in

normal operation. The increased temperature in the regenerator can also lead

to more fixation of atmospheric nitrogen and consequently more NOx in the stack

gases.

164

2. Problems with metal contaminants

Feed contaminants that are especially harmful from an operational

view are the metal organic compounds such as Ni- and V-porphyrines +

and salts of sodium, iron and copper. Heteroatoms such as sulfur and

165

point of

naphthenates

nitrogen

cause more environmental than operational problems. Basic nitrogen compounds,

however, can temporarily deactivate the acidic catalyst; since they are easily

adsorbed on the catalyst, this will also add some extra coke from the nitrogen-

containing molecules.

High metal and heteroatom levels will affect the catcracker performance in

the following respects:

a) Poor selectivity. The metals build up almost quantitatively on the Catalyst.

The selectivities change toward more H2- and coke production due to the strong

dehydrogenating function of nickel. The gas-make on a volume basis will increase

dramatically as the contamination level goes up. This often causes the dry gas

compressor to be the "through-put limiting" part when cracking resid blends. The

contaminant coke that is produced by the contaminant metals is an important part

of the coke problem described earlier.

b) Catalyst deactivations. The catalyst will mainly be deactivated by V and Na.

High levels of these metals have a destructive influence on the zeolite structure,

especially in combination with high regenerator temperatures. Catalyst make-up

must be increased to keep a reasonable activity level in the unit.

3. Interaction of metals with the catalyst

Some powerful physico-chemical characterization techniques have recently been

employed to get a better insight into the interactions of Ni and V with the

zeolitic cracking catalyst. Using secondary ion mass spectrometry (SIMS), Jsrfis

has shown [20] that, under cracking-regeneration conditions, V has a remarkable

tendency to get selectively enriched on the molecular sieve component in the

catalyst. No such preferential surface enrichment is exhibited by Ni, which remains

uniformly distributed on the sieve and matrix surfaces. But Ni is undoubtedly

the worst contaminant at low concentrations (<5000 ppm). At these low concentrations

the usual relation for calculating the total metal content of a feed is "equivalent

Ni": Equivalent Ni = Ni + 0.2 V + 0.1 Fe, all metals expressed in ppm by weight

as obtained from elemental analysis. Above 5000 ppm, Ni and V are additive.

ESCA studies by Anderson et al. [213 have shown quite recently that Ni may

exist as Ni 3+

or Ni 2+

and V as V 5+

after a typical regeneration. Up to a metal

concentration of 2%, Ni is homogeneously distributed throughout the catalyst,

but higher loadings of Ni lead to its surface enrichment. The segregation of V

in the catalyst has been found [211 to occur in three different ways: a) during

the cracking stage V is deposited on the outer part of the catalyst particle due

to the polar nature of the vanadium-porphyrin complex; b) during the regeneration

166

V migrates to the surface because of the low melting point of V2OB (690°C); c) V

also migrates from the matrix surface to the zeolite surface (as already shown

from SIMS studies [20])and destroys the crystal structure of the zeolite.

Pompe et al. [22] have applied DTA/TGA techniques to elucidate further the

destructive roles of the metal porphyrines and naphthenates on the FCC catalyst.

Ni- and V-porphyrines decompose fully only after about 30 min at 500°C under

cracking conditions, but the corresponding naphthenates are decomposed readily

and completely under those conditions. Under typical regenerator conditions, both

these classes of compounds decompose exothermally (could be combustion and/or

polymerization). The mechanism of the destruction of the rare-earth exchanged Y

sieve (REY) in the catalyst has been attributed to the attack of V205 on the rare-

earth component of REY forming a low-melting RE-vanadate phase. This vanadate

formation requires more oxygen per RE atom than what can be supplied by V205. Ttis

extra oxygen is apparently drawn from the zeolite lattice structure, thus leadi?+

to the ultimate collapse of the crystalline zeolite structure [ZZ]. i.

4. Environmental problems

High levels of S and N in the feed will give high SOx and NO, in the regenerator

flue gases. More coke deposition on the catalyst leads to higher regenerator

temperatures and relatively higher attrition loss (dust emissions) of the catalyst.

The high concentrations of CO (even up to 10%) in older-type regenerator flue

gases are objectionable both from the environmental and energy-conservation

standpoints. Finally, the increasing tendency to phase out lead from gasoline (both

from environmental considerations and also for use in automobiles with noble-metal

exhaust cleaning catalysts) dictates that higher octane numbers should be achieved

directly from the FCC operation than is possible at present.

The problems posed by the need to use heavier fractions as FCC feedstock are

a direct result of the oil crisis and the consequent energy crisis started in 1973.

These problems sometimes resemble closely those arising from environmental pressures,

as briefly enumerated above. Luckily, innovations in FCC process design and

catalyst improvement have sometimes been able to offer simultaneous solutions to

both these sets of problems. Typical examples for these will be discussed under

Section IV,

IV.. INNOVATIVE APPROACHES TO PROBLEMS OF RESIDCRACKING

1. Process and engineering adaptations

a) Modified Processes. The oil companies, design and engineering firms and

catalyst manufacturers have been quick to react to the problems posed by the in-

creasingly heavier feedstocks for FCC. The technical solutions developed involve

feedstock pretreatment or resid catalytic cracking or a combination of both.

Already in 1960, Kellogg installed a Heavy Oil Cracking (HOC) unit at a Phillips

refinery at Borger, Texas. Since then, a continuous development of the HOC process

167

has been taking place. The latest versions of HOC involve [23] a) a Vertical

riser to reduce catalyst-oil contact time, minimizing metal effects and over-

cracking, b) the riser ending at a 90"-turn into the riser cyclone for quick

separation of the catalyst c) multiple feed injectors and use of excess steam

to ensure good and instant mixing with the catalyst, d) a large regenerator and

regeneration in a counter-current mode to avoid excessive temperature and e)

internal cooling coils in the regenerator and external catalyst coolers.

Modifications introduced by Total Petroleum in 1982 consist [24-251 mainly

in conducting both the cracking and regeneration in two steps each. Here, an

instantaneous thermal cracking of the asphalt in the mixing zone is followed by

a riser cracking of the lighter portion of the product from the first step.

Similarly, the first stage of the regeneration is carried out at a lower tempera-

ture (600-750°C) in insufficient oxygen; steam is present at this stage, partly

from steam-stripping and partly from the combustion of the H-content of the coke

into H20. The CO-rich flue gas is then separated from the catalyst before the

catalyst undergoes the second-stage regeneration at 760-940°C in excess oxygen

to burn off only the carbon of the coke still remaining on the catalyst. Such

high temperatures are possible since the cyclones are located outside the re-

generator and lined with refractory bricks, hence there is no metallurgical

temperature limit. Since there is no steam present and the so-called CO after-

burning is avoided, the catalyst is protected against hydrothermal deactivation.

The RCC process [26] developed by Ashland Oil, jointly with UOP, incorporates

several improvements in process design like UOP's high efficiency regenerator

(see Section IV.2) and in catalysts like greater activity coupled with metals

passivation (see Section 1V.3~) and combustion promoters (see Section IV.3b)

added to the catalyst.

The Ashphalt Residual Treating (ART) process developed by Engelhard Corporation

[27,28] is essentially a feedstock pretreatment process. It is a low-conversion

process for removal of asphaltenes and metals from heavy oils by vaporization

of the hydrogen-rich components of the feed. Another process for resid conversion

is the new Hycon process of Shell C291, combining demetallization, desulfurization

and hydroconversion into one process. Such typical fixed-bed hydrogenative processes,

however, will not be discussed any further in this review of FCC of heavy oil

or resid fractions.

Since the conditions in no two refineries are identical, every refinery is in

a way forced to innovate and find its own solutions to its specific problems

encountered when processing heavier feedstocks. Many such solutions seldom find

their way to patent literature or scientific journals. Some of these solutions

may be applied in quite contradictory ways in different refineries. For instance,

some catcrackers will have to increase their regeneration efficiency if they want

to process heavier feeds. This is achieved by using catalysts containing the so-

168

called combustion promoters, or by using a high efficiency regenerator, or by

using oxygen-enriched air (see the following Sections). Some other catcrackers

may face just an opposite situation: the combustion of any additional coke may

lead to excessive heat generation which cannot be allowed because of metallurgical

constraints. In such cases, sometimes even a deliberate incomplete combustion

of coke will have to be resorted to. The resultant high CO production makes it

imperative to have a costly CO after-boiler if loss of CO as a valuable fuel gas

and environmental pollution with CO are to be avoided. Thus, the suitability and

acceptability for any particular catcracker of the recent process and catalyst

innovations will depend very much on the specific characteristics of that FCC

unit. No general panacea, applicable for all units, is available at present.

FIGURE 3 The high efficiency regenerator of UOP (from 1303).

disengaging vessel

combustion riser

b) UOP's high efficiency regenerator (HER). The HER is the result of a unique

application of the basic principles of chemical engineering in a very elegant and

advanced way to the problem of coke combustion on the catalyst [30].

In a conventional fluidized-bed regenerator in its bubbling-bed form, oxygen

escapes from the catalyst bed and reacts with CO at the top of the bed, causing

excessively high temperatures. The coke combustion, however, is far from complete

or optimum. If all the combustion took place in the catalyst bed, there would

not be any CO left for after-burning. All the heat generated can then be utilized

by the catalyst for the cracking proper. It is precisely for this purpose that

special combustion promoters (like ppm levels of Pt) are added to the catalyst by

catalyst manufacturers, but it increases the price of the catalyst by $100-200 per

tonne.

169

A closer examination from the chemical engineering standpoint by UOP revealed

the crux of this problem to be the bubbling bed leading to insufficient catalyst-

air mixing, break-through and escape of air (oxygen) and hence the after-burning.

Bubbling beds are notoriously sensitive to the design of the air distributor,

the catalyst bed level, and other internal geometries, and all of these problems

are magnified with increasing diameter of the regenerator. Even if the mixing

was perfect in this type of regenerator, the combustion of CO is slow at low

temperatures, hence CO escapes from the bed. If more air is added to complete the

combustion, it will increase bubble formation and loss of the catalyst.

The chemical engineering solution to this problem is an ideal plug-flow

regenerator. To reduce the coke content on the catalyst to the same extent, the

residence time required in an ideal plug-flow reactor system is only about one-

tenth of that in an ideal back-mixed reactor. This decrease in residence time

allows much smaller catalyst inventories of only about 50%. Decreasing the catalyst

inventory gives an operational advantage: the catcracker plant responds more

quickly to process control, the whole unit becomes easier and safer to operate.

The complete CO combustion achieved eliminates the need to have an expensive

CO after-boiler. All these advantages, inteA-related in a synergetic manner, are

achieved by incorporation of a riser tube in the regenerator (Figure 3). Most of

the coke burning occurs during the plug-flow movement of catalyst and air through

this riser tube, most of the CO also burns here. A maximum portion of the heat

from coke combustion and CO combustion is absorbed by the catalyst and thus becomes

available for cracking. This increases the throughput of the FCC plant, apart

from all the other gains mentioned above. In the 20 new FCC units built in Europe

during the last ten years, 13 units have the high-efficiency regenerator.

c) Oxygen enrichment of regeneration air. The trend and necessity to crack

heavier fractions, as discussed in Section III, bring invariably the problem of

more coke formation and the need to have more regeneration capacity. But expansion

of capacity, without substantial new capital investment, is not easy in most cat-

crackers, especially when many refineries in Europe and the United States are

running only at 50-70% of their installed capacity and some are even completely

shut down. So the present FCC units should themselves be made capable of cracking

heavier feedstock without any major capital investment. One way to increase

regeneration capacity, without any major mechanical alterations, is oxygen en-

richment of regeneration air (from 21% in air up to about 25%), proposed by Air

Products C311. Bringing in pure oxygen into a hydrocarbon plant of course increases

fire and explosion hazards. Hence special safety precautions will have to be in-

corporated along with this system.

170

2. Catalyst adaptations

As pointed out in Section III. 4, increasing energy costs, the need to process

heavier feedstocks, and the tightening environmental restrictions have led to

the creation of a new generation of catalyst technology for FCC. Some salient

features of this new development are briefly outlined below.

a) Reduce dust emissions. Concern of refiners on catalyst losses per day

and local restrictions on dust emissions through FCC stacks have forced catalyst

manufacturers to develop so-called high-attrition-strength catalysts. The problems

involved in improving the mechanical strength of colliding micron-size particles

at 500-700°C under cracking/regeneration/steam environments are still not

adequately understood. The improvements achieved are often based on ad hoc trials

or inspired guesses. The thermal stabilization of alumino-silicate catalysts by

rare-earth oxides is a valuable lesson drawn from solid-state and ceramic science.

b) Reduce CO emissions. Introduction of oxides like Cr203 and MnO2 and ppm

levels of Pt into FCC catalyst as combustion promoters has become quite common

in recent years [32]. This leads to an almost complete combustion of CO in the

regenerator itself, eliminating two expensive units: the feedstock preheater and

the CO after-boiler. Since Pt is also a powerful catalyst for the oxidation of

so2 to so3, the fixation of sulfur on the catalyst and hence reduced SOx emissions

from the stacks are also achieved by these combustion promoters. There is, however,

one complication here: the reduction of SOx emission requires excess oxygen in

the stack. A high level of oxygen enhances the emission of NOx due to oxidation

of any nitrogen in the coke or even by direct fixation of nitrogen in the re-

generation air.

esox transfer catalysts. Significant progress has been registered since

1980 in the removal of SOx from regenerator flue gas. Technology to reduce SOx

can, in principle, be based on [4a, 33 a,b]:

- feed selection - often not much choice here

- feed hydrotreating

I[

often need heavy investment and rather high

- feed-gas scrubbing operating costs

- catalyst adaptation - the cheapest alternative today, with no new capital

investment

According to McArthur et al. [34] the economic incentive for a purely catalytic

solution to the SO, problem is so strong that even if an SOx transfer catalyst

were priced at triple the present price for conventional catalysts, their incre-

mental cost would be lower than the operating cost for flue-gas scrubbing and

would also save the additional capital investment.

The chemical reactions involved in transfer of SO, via the catalyst are:

Regenerator: S (in coke) + 0 2

+ so2 + so3 (< 10%)

so2 + 302 + so3

Formation of metal sulfate: MO + SO3 + MSO4

171

TABLE 2

Active ingredients of SOx transfer catalysts

Company Active ingredient

Exxon Group IIA metal oxides (Mg on SiO,-MgO)

Chevron, Akzo-Ketjen Alumina; special clays

Arco, Katalistiks Oxides of Si, Zr, Mg, etc., Spinels

Amoco Na, SC, Ti, etc.,

Union Oil Bastnaesite (rare-earth ore)

Texaco Bi on alumina

Riser (cracker): MS04 + 4 H2 -+ MS + 4 H20

Stripper MS + H20 -+ MO + H2S

The HpS formed in the reactor section can be easily handled and removed

ultimately in the conventional Claus units and sulfur plant existing in any

modern refinery.

In the 1981-84 period, almost all FCC catalyst manufacturers have developed

and commercialized special SOx transfer catalysts or additives to combat excessive

SOx emissions from regenerator stacks. As additives or getters for SOx, the most

prominent ones developed by various companies are shown in Table 2. Other metal-

oxide combinations and detailed references to the recent patent literature are

given in a review by Habib [33a]. Of the above formulations, alumina and

lanthanides were already part of FCC catalysts even before SOx emissions became

an environmental issue. The characteristics of typical SOx getter materials have

been described by Baron and co-workers [34,35 a,bl, who have also reviewed C35bI

in 1985 the problem of regenerator emissions. One disturbing factor emerging

from all the above work is that often NOx emissions can increase with the use of

SOx transfer catalyst/additive technology. This is a serious problem to be tackled

by research in the near future.

d) Octane-boosting catalysts. Customer demand for higher engine performance

in automobiles and government-imposed limits on the use of lead as anti-knock

compounds have resulted in an urgency for higher-octane gasoline from all pro-

cessing operations in the refinery. One approach to enhance gasoline octane is

to use anti-knock additives not containing lead, e.g. ethanol, methyl tertiary-

butyl ether (MTBE), and manganese anti-knock agents. Since a significant fraction

of the gasoline pool (about 35% in the United States) comes from FCC units, it

is only natural that catalyst manufacturers and oil companies develop catalysts

tailored to enhance the octane number of FCC-derived gasoline.

The ultra-stable Y (USY) type of zeolites, developed by Mcdaniel and Maher

C361, was already being used by Davison in cracking catalysts since 1966. But it

was only in 1976 that the modern IJSY catalyst, containing even up to 25 wt%

172

zeolite, became commercially available as a typical octane-boosting catalyst.

The structural peculiarities of ultrastable zeolites fall outside the scope of

this review, except just to mention that, with the advent of Magic Angle Spinning

NMR, a fairly detailed picture of de-alumination and the formation of USY sieve

has evolved by following the 29 .

Sl NMR of faujasites after increasingly severe

thermal treatments. Klinowski [37] has reviewed this area in 1984.

The mechanism of octane improvement by USY catalysts, proposed by Magee et al.

[38-391 is as follows. In USY, the acid sites are more widely dispersed and are

of a stronger acidity than those in conventional rare-earth exchanged Y (REY)

zeolites. Consequently, cracking over USY yields a product significantly richer

in olefins at the expense of paraffins, and slightly richer in aromatics. Since

both these factors will minimize the well-known hydrogen-transfer reaction,

olefins + naphthenes + paraffins + aromatics

less paraffins are produced ultimately. Since the hydrogen transfer is impeded,

there is also less likelihood of aromatics condensing to form polynuclear aromatic

coke precursors and finally coke. Thus less coke and improved aromatics yield are

secondary benefits. Furthermore, the strong acid sites enhance cracking over hydro-

gen transfer. Hence large alkly aromatics would more likely dealkylate producing

aromatics and olefins in the gasoline or LPG range, rather than ending up as

high-molecular-weight components in the light cycle oil (LCO) range or polynuclear

aromatic coke precursors.

Another catalytic approach to enhance the octane number of FCC gasoline consists

of the use of ZSM-5 type of shape-selective sieves, developed by Mobil [40].

These sieves have pore-mouth openings < 6i. They catalyze secondary reactions

like re-cracking, double-bond and skeletal isomerization, polymerization and

hydrogen transfer, using the primary products from cracking on the X- or Y- sieve

component. The ease of reaction of straight chain molecules over that of branched

ones in the narrow pores lead to a preferential cracking away of the low-RON

straight-chain paraffins. At the same time, the narrow pores prevent the formation

of larger molecules like typical polyaromatic coke precursors. Hence the addition

of these shape-selective sieves (usually a few wt%) to the FCC catalyst produces

little or no extra coke over and above that produced by the faujasite component

and the matrix of the catalyst.

It is not difficult to predict that in the near future, octane-boosting catalysts

may try to combine both the above octane-booster additives, working by such entire-

ly different mechanisms. As Magee et al. C39] point out in 1985, "the mechanisms

by which these two families enhance cracked-product gasoline octane number are

reasonably well understood, but commercial exploitation of this understanding

has only just started".

173

3. Metals passivation

2) Nickel. One way to minimize the harmful effect of accumulating Ni on the

catalyst is to use the Phillips metal passivation procedure [411. Very low levels

of a soluble organometallic antimony compound are added continuously along with

the feed. ESCA studies have shown that Ni and Sb readily form an alloy in which

the Sb is highly enriched on the surface [42]. In this way the active Ni metal

area is effectively covered up by Sb under the process conditions. Boron, bismuth

and tin have also been claimed as potential metal passivators in recent patent

literature.

b) Vanadium. As discussed in Section III. 3, SIMS, ESCA, DTA/TGA and other

studies [20-221 in recent years have revealed the mechanism of selective destruction

of zeolites by V accumulating on the catalyst. This understanding, in turn, has

led to the development of three strategies to combat the effects of V:

- by selecting more optimum process conditions to render V harmless towards

zeolite

- by adding passivators for V and

- by providing a suitable dumping area or trap for the V on the catalyst.

Typical examples for these three different strategies are given below.

As Pompe et al. [22] have shown, in a reducing atmosphere, V205 in a REY/V205

mixture can be reduced in two steps to V204 (50-450°C) and V203 (450-750°C).

X-ray diffraction studies by the same authors show that the zeolite Y is not

affected by V204 or V203. Hence, one way to render V relatively harmless to the

zeolite in the FCC catalyst is to operate the regenerator as close as possible

to reducing conditions, or at least to ensure that there is no surplus oxygen.

Patent literature of the last few years indicate that oil companies seem to

favour the use of metal additives or passivators which can immobilize V, e.g.

formulations patented by Ashland Oil [43J, or tin additives and a Mg-containing

diluent, both developed by Gulf [44,453.

The strategy of catalyst manufacturers is aimed at providing a large dumping

area on the catalyst to accommodate the accumulating metals and preferably

separate the cracking function of zeolite from this metal sink. (This is apparent-

ly a valuable lesson learned from the development of hydratreating catalysts for

residual oils in the 1970's). Thus, the use of non-dispersive sepiolite as an

additive has been proposed by De Jong [46] of Akzo-Ketjen. Otterstedt et al. [47]

of Katalistiks suggest the use of two ranges of particle sizes: the larger

Particles in the range 80-100 u contain the zeolite component with the cracking

activity, while the smaller particles (30-60 u) comprise a matrix of kaolin and

amorphous silica-alumina, and possibly other additives for minimizing SO,

emissions and/or catalyzing CO oxidation. The principle here is that it will take

a longer time for V to penetrate the larger particles and reach the zeolite

crystallites embedded in them than if these were located in the smaller particles

or distributed uniformly in particles of all sizes in the catalyst. The idea of

using a vanadium sink with no catalytic activity has also been proposed by Wear

of W.R. Grace/Davison [48].

174

TABLE 3

Improvements in fluid catalytic cracking (from O'Dea [50]).

1951 1960's 1970's

designs zeolite high temp.

catalysts regeneration

Regenerator pressure/psig 9 20 33

Temperature/"C 607 677 732

Flue Enthalpy/Btu lb-' gas: 318 351 378

Available shaft energy/Btu lb-' 49 94 126

CO in flue gas/vol% IO 11 200 ppm

to 5%

C on regenerated catalyst/wt% 0.7 0.3 <O.l

Leuenberger [49] has quite recently provided some very useful correlations for

predicting the activity of vanadium-contaminated FCC catalysts.

4. Net improvements in FCC [50]

The mutually synergetic improvements in FCC process and catalysts over the

last 30 years have led to important gains in the process economy and energy con-

sumption, as summarized in Table 3.

V. THE ROLES OF MATRIX AND SIEVE IN RESIDCRACKING CATALYST

Catalysts for cracking heavy oils must have high hydrothermal stability in

order to maintain activity under severe regenerator conditions and selectivity

for lighter products such as gasoline and diesel. Normal FCC catalyst consists

of an alumino-silicate matrix of spherical particles of 20-80 u. The zeolite

crystallites, l-4 u size, are embedded in the matrix. The zeolite content may

vary from 5 to 25 wt% in the catalyst. It is IOO-10,000 times more active than

the matrix. This high activity has led to the present-day short-contact-time riser

crackers, which make the catalysts less sensitive to metal contaminants.

A matrix suitable for a heavy oil cracking catalyst should have an active

surface and larger pores for the following reasons: a) rapid transport of large

molecules, (since slow transport will lead to overcracking and coking); b) large

pores minimize pore collapse and sintering; c) small pores increase the carry-

over of heavy molecules present as liquids into the regenerator [51] where they

will decompose to coke; and d) large pores and low surface area matrix decrease

the dispersion of contaminant metals and thereby increase the metal resistance

[52]. The chemical composition of the matrix is also important, e.g. it may react

with Na and V and neutralize the detrimental effects of these metals. It has been

175

proposed [53] that a matrix of magnesium-alumino-phosphate will decrease the

yields of H2 and coke, compared to an alumina-silicate matrix. Free alumina,

silica or zirconia on the matrix will improve the metal resistance of catalysts

[54], but alumina will give more coke [55].

Resid cracking also requires higher thermal and hydrothermal stability of the

zeolitic active component. For this, the Na20 content in the zeolite has usually

to be below 0.5%. The rare-earth content is more important for hydrothermal than

for thermal stability [56]. Ultrastable Y which contains no rare earth and very

low sodium content has extremely high thermal stability but only moderate hydro-

thermal stability [57].

Different types of zeolites have different selectivity for lighter products

[58]. The distribution of the acid strength of the active sites decisively in-

fluences the selectivity [59]. By suitable ion-exchange, the desired selectivity

can be achieved. Very strong acid sites are undesirable since they cause secondary

cracking of the desired products into light gases and increased coke formation.

The acidic nature of the matrix surface is also important: thus a SiO*/MgO matrix

will for instance produce more gasoline with lower octane number and more diesel

than a Si02/A1203 matrix [58].

The specific surface area of catalysts decreases by ageing. This, in turn,

changes the distribution of acid sites and their acid strengths. The dehydration

at high temperatures converts Brtlnsted acid sites into Lewis acid sites, while

rehydration with steam in the stripper leads to the reverse reaction. Transfer

of catalyst from the reducing atmosphere in the cracker to the oxidizing atmosphere

in the regenerator may change the oxidation state of deposited metals. All such

changes can affect the activity, selectivity and stability of the catalyst.

VI. GENERAL DISCUSSION

The upgrading of heavy oil by catalytic cracking is a question of optimizing

the interactions in the three component system: Feedstock - Process- Catalyst.

The general aspects of these interactions in normal FCC have been discussed in

detail by Venuto and Habib [4a]. Hence only some special aspects pertaining to

heavy oil cracking will be discussed here.

1. Process - feedstock interactions

Much work has gone into the development of processes for heavy oils such as

vacuum gas oil with additions of atmospheric and vacuum resids, deep vacuum gas

oils, atmospheric resids and vacuum resids. Commercia7 processes for cracking of

such oils are the Kellogg process, the RCC process, the Total process, the ART

process. The Kellogg and Total processes can be used to upgrade the first two

types of oil and atmospheric resids of high to medium quality. The KC process

is claimed to be able to handle atmospheric resids in general. For atmospheric

resids of medium to poor quality and for vacuum resids, the ART process is

176

necessary as a pre-treatment before the feed enters one of the other processes

developed for heavy oil cracking. No systematic work on the crackability of oil

fractions in the boiling point region 300-750°C has been published.

2. Catalyst - feedstock interactions

The large molecules present in heavy oils have low volatility, which creates

special problems in the cracking operation. In most processes the temperature of

the regenerated catalyst does not exceed 750°C. The fraction of heavy oil boiling

above 750°C will not vaporize and will be present on the catalyst as a liquid

which will be difficult to strip from the catalyst. The high boiling material

will therefore be carried over into the regenerator where it will behave as coke.

Early studies of stripability did not show any correlation between pore structure

and ease of stripability. The situation is probably very different for large non-

volatile molecules and one can expect that a catalyst with an open large pore

structure will strip more readily.

The yield of heavy cycle oils will for a given catalyst increase as the oil

gets heavier. In order to crack the heavy cycle oil fraction, the catalyst must

have an active surface accessible to the large molecules of the HCO fraction. The

catalyst must either have an active large pore matrix or an active large pore

phase, e.g. crosslinked smectites [60].

Since most heavy oils contain more V than Ni, it is important to develop a

matrix-zeolite system which is resistant to attack by V or a passivating system

for V, analogous to Phillips system based on Sb for passivating Ni. FCC catalyst

manufacturers like Katalistiks, Ketjen and Davison have claimed the development

of such metal-resistant catalysts in recent years.

Sulfur in heavy feeds is more of an environmental problem than an operational

problem. ARC0 [61], Chevron [62] and Union Oil [63] have developed methods, based

on adsorption of SOx in the regenerator by an active ingredient, such as alumina

or bastnasite, which reduces the emission of SOx from the regenerator stack.

Here also, catalyst manufacturers claim the development of so-called SOx-catalysts.

Andersson et al. [64] have just pointed out that the presence of 02 and the

absence of CO are necessary in order to obtain sufficient adsorption of SO, on

alumina at 700°C. This is in accordance with the general experience of FCC units

that the use of y-alumina for SOx transfer requires the regenerator to be operated

in the total CO combustion mode.

3. Process - catalyst interaction

In general catalyst development has guided and stimulated the process develop-

ment [19]. The replacement of amorphous alumino-silicate catalysts with the more

active, stable and selective zeolite-based catalysts has led to very significant

changes in process design, e.g. riser cracking. Total's process for heavy oil

cracking, on the other hand, is an example where process development has stimulated

catalyst development, namely the development of catalysts with extremely high

thermal stability but not necessarily hydrothermal stability. The rapid development

of efficient catalysts for heavy oil cracking raises the question, however, if

present process designs can make full use of the properties of such catalysts.

4. Some unsolved problems

Much progress has been made in the development of processes and catalysts for

catalytic cracking of heavy feeds. Further development work will be stimulated

and assisted when more is known about:

a.

b.

C.

d.

e.

f.

9.

Characterization of heavy oil in terms of crackable and non-crackable fractions.

The crackability of fractions of heavy oil as function of increasing molecular

weight.

The state of the largest and most non-volatile molecules of heavy oils in the

mixing zone. Extent of vaporization. Possible separation of these in a pre-

treatment of the feedstock.

Differences between standard catalyst and special catalysts for heavy oil

cracking in terms of physical and catalytic properties.

The mechanism of metal ageing in the regenerator.

Steps in the process of catalytic cracking of heavy oils which are specially

sensitive or insensitive to catalyst performance.

How to prevent NOx emissions increasing when the catalyst and process are

adapted to reduce SOx emissions.

ACKNOWLEOGEMENT

The authors express their thanks to the Swedish Board for Technical Development

(STU) and the Swedish National Energy Administration (STEV) for financial support.

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