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6 CATALYTIC CONVERSION PROCESSES - catalytic processes in the petrochemical industry, both in terms

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6

CATALYTICCONVERSIONPROCESSES

6.1.1 Principles and development

IntroductionAfter about 70 years of activity, at the beginning of

the new millennium catalytic cracking still remains theprincipal process used to convert heavy oil fractionsinto lighter products, especially gasoline.

Historically, the distinction between simple-cyclerefineries and conversion refineries is based on theabsence or presence of catalytic cracking in theproduction cycle.

The first true catalytic process in the refiningindustry, cracking is still one of the most importantcatalytic processes in the petrochemical industry, bothin terms of plantsize and the amount of catalyst.

The advent of cracking has significantlycontributed to the understanding of the acid catalysismechanisms that leads to the formation ofcarbocations starting from hydrocarbon molecules.

Compared to its predecessor (i.e. the thermalcracking process), catalytic cracking presentsnumerous advantages, including higher gasoline yields(50% in weight with respect to the feed), the higherquality of the gasoline produced and a lowerproduction of gaseous, liquid and solid by-products(coke). The gaseous fractions can be used as feed foralkylation processes (see Chapter 4.3), for theproduction of methyl tert-butyl ether or MTBE (seeChapter 4.4) and for polypropylene plants; heavyliquid fractions (cycle oil) are excellent feedstocks forthe production of carbon black (Fig. 1). The use ofalkylates and ethers has been encouraged, from the lastdecade of the Twentieth century onwards, by thereduction of the aromatics and benzene content ofcommercial gasolines.

Typical feedstocks for catalytic cracking are thehigh boiling distillates obtained from vacuum

distillation, and deasphalted or hydrogenatedresidues.

The most recent developments in the process alsoallows the partial feed of atmospheric residues, albeitmixed with the distilled feedstock, since the processtakes place in the vapour phase; moreover, residuesdeactivate the catalyst more rapidly.

Thanks to its versatility and capacity forcontinuous renewal and development, catalyticcracking has long withstood competition from otherconversion processes, especially hydrocracking (see Chapter 6.2).

It cannot be ruled out that crackingspredominant role among catalytic processes willbe downscaled, due to changing marketrequirements (lower demand for gasoline withrespect to other fuels), the need to obtain sulphur-free products directly, and to the establishment ofprocesses capable of converting residues directly.However, catalytic cracking will remain afundamental process in the refining industry formany years to come.

Development of the processesDespite some earlier attempts to improve the

thermal process with the addition of varioussubstances (which cannot always be described ascatalysts), it was only in the 1930s that catalyticcracking became commercially important, thanks tothe work of Eugne Houdry (see Chapter 1.1).

The first unit, equipped with three fixed bedreactors, came on line in the United States in 1936; thecatalyst consisted of a natural clay based onmontmorillonite. In the same year, the first plant wasbuilt to supply activated earths (with acid) to thecatalytic plants and, in 1940, the Houdry Corporationstarted up a plant for the production of syntheticaluminium-silicates.

247VOLUME II / REFINING AND PETROCHEMICALS

6.1

Catalytic cracking

The fixed bed process was difficult to manage atthat time since the three reactors alternated reactionphases with regeneration phases, with intermediatepurging. The process remained complex and demandingdespite an increase in the number of reactors (six) andthe introduction of electrical cycle timers to control theopening and closing of all the valves in the variouscircuits (oil, vacuum, air and steam). Motivated by thewar, experiments began to introduce moving bedplants (TCC, Thermofor Catalytic Cracking) and fluidbed plants (FCC, Fluid Catalytic Cracking), whichcame on line in the United States almostsimultaneously in the years 1942-43.

In moving bed reactors, the catalyst was initiallymoved using mechanical bucket elevators, andsubsequently with air; this allowed for the continuousregeneration of the catalyst, leading to improved yieldsand product quality. The same benefits were obtainedby fluidizing the bed with the vaporized feed (in thereactor) and with air (in the regenerator).

The catalyst consisted of spheres approximately 3 mm in diameter, and microspheres (powder) for theTCC and FCC processes, respectively.

After competing for several decades, the fluid bedtechnology supplanted the moving bed process so that,already at the end of the Twentieth century, TCC unitswere extremely rare.

The FCC process, in turn, has undergone continuousdevelopment over the years, maintaining it constantlyup-to-date. One of the most important developmentswas the introduction in the early 1960s of zeolitecatalysts (able to select the reacting molecules), whichconsiderably revitalized the process.

The improved efficiency and stability of catalystshas led to the elimination of the traditional reactor andthe introduction of the riser reactor; more efficientcatalyst regeneration systems have also been developed.A further step forward was made possible thanks tocatalysts, again zeolites (ZSM-5), able to improve theoctane number of the gasolines produced, and to theintroduction of a series of new additives and passivators.

Catalytic cracking reactionsThe relatively high process temperatures

(450C) lead to the formation of free radicals andto thermal reactions. These reactions have lowselectivity and produce light gas molecules, such asmethane and ethane, and lead to the formation ofolefins.

Although the latter may be precursors to theformation of carbocations, thermal reactions should belimited by operating at temperatures which are as lowas possible.

Catalytic cracking reactions includeisomerization, the b-scission of paraffins,dehydrogenation, hydrogen transfer and varioustypes of condensation reactions. The main reactions,according to the various classes of hydrocarbons, aresummarized in Table 1.

Catalysts of acid type promote the formation ofcarbocationic intermediates rather than free radicals,improving yields and selectivity.

Carbocations may form starting from an olefin,in the presence of Brnsted acid sites in thecatalyst, or by the protonation of a paraffin ornaphthene:

The first of these mechanisms is universallyaccepted and, in comparison with others, issignificantly faster. However, the hypothesis thatcarbocations may also be formed starting from

248 ENCYCLOPAEDIA OF HYDROCARBONS

CATALYTIC CONVERSION PROCESSES

carbon black

propylene

alkylatednaphtha(C7-C8)

MTBE(or other ether)

crackingnaphtha

heavydistillate

feed

CH3OH(or other alcohol)

aromatic oil

gas

Fig. 1. Processesdownstream ofcatalytic cracking.The most commonconfigurationinvolves alkylationimmediately aftercracking.

R1 C R1 C C+ R2C R2H

H H H

H H

R1 C C R2H+ R1 C C R2

H H H H

H

H

HH

H

R1 C C+ R2H2

H

H H

Lewis-type sites, present on the catalyst togetherwith Brnsted sites, is also commonly accepted:

Since they are deficient in electrons, Lewis-typesites can stabilize one of the hydrogens in the H

form, and form the complementary carbocation.The carbocations that form on the surface of the

catalyst tend to isomerize towards the more stableform (from a primary to secondary to tertiarycarbocation); in the latter state, the carbon containingthe charge is linked to three other carbon atoms. Withreference to a paraffin chain, after the formation of thecarbon ion, there are various possibilities. The first isisomerization towards a more stable form; the second,endothermic, involves the rupture of the CC bondin the b position with respect to the charge, forming anolefin and an unstable paraffinic carbocation, whichsubsequently isomerizes:

The probability of b-scission increases if theconfiguration of the original carbocation is favourable

(tertiary or secondary, rather than primary). There arealso other possibilities: the carbocation frees a protonand turns into an olefin, or saturates by taking a protonfrom the catalysts active site, or reacts with an olefinto alkylate it.

Olefins behave in a comparable way, with thedifference that they crack much faster, given theirhigher tendency to form carbocations; however, theymay also oligomerize and cyclize, contributing,alongside aromatics, to the formation of coke.

The b-scission mechanism leads to a preferentialrupture of the bonds inside the molecule; non-condensible gases such as methane, ethane andethylene, which would be formed by the rupture ofterminal bonds, are thus only present in smallquantities, in contrast to what occurs in thermalprocesses. The olefins that form have 3 or 4 carbonatoms and are excellent feedstocks for the processesdownstream (see again Fig. 1).

However, the formation of olefins is on averagelower than that predicted by the mechanisms describedabove. This is due to exothermic reactions involvingthe transfer of hydrogen from cycloalkane donormolecules to unsaturated molecules, with theformation of aromatic compounds and paraffins.

This reaction is probably as important as therupture of the naphthene ring, with the formation ofisoalkanes.

The reactivity of the naphthene ring increases withthe degree of substitution, in other words the potentialfor forming tertiary carbocatio

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