Molecular fluid catalytic cracking

Figure 4 Catalytic cracking (fluid catalytic cracking). Heavy fraction gas oils are cracked (broken down) into lower molecular weight fractions in the presence of finely powdered catalyst, handled as a fluid. (From Ref. 5.)...

Cracking reactions are carried out in order to reduce the molecular size and to produce more valuable transport fuel fractions (gasoline and diesel). Fluid catalytic cracking is acid catalyzed (zeolites) and a complex network of carbe-nium ion reactions occur leading to size reduction and isomerization (see Chapter 4, Section 4.4). Hydrogenation also takes place in hydrocracking, as well as cracking. 

The use of molecular sieve catalysts has also become more widespread in the past decade for the production and inter-conversion of olefins from feedstocks other than oxygenates. The addition of a modified ZSM-5 additive to the Y zeolite-based catalyst can substantially increase the amount of propylene produced in a conventional Fluid Catalytic Cracking (FCC) unit. This has become a very valuable modification, particularly in areas where propylene supplies are tight. More recently, a number of processes have been announced for the direct cracking of C4+ olefinic steams to propylene. These processes also use modified ZSM-5 based... 

The earliest applications of zeolites utilized the molecular sieving properties of small pore zeolites, e.g. zeolite A, in separation and purification processes such as drying and linear/branched alkane separation [33]. In 1962 Mobil Oil introduced the use of synthetic zeolite X, an FCC (fluid catalytic cracking) catalyst in oil refining. In the late sixties the W. R. Grace company introduced the "ultra-... 

Huge amounts of catalyst are consumed for refinery operations to convert crude oil into lower molecular-weight fractions (fluid catalytic cracking). Many of the catalyst compositions available contain lanthanides including cerium [13]. 

Microporous and, more recently, mesoporous solids comprise a class of materials with great relevance to catalysis (cf. Chapters 2 and 4). Because of the well-defined porous systems active sites can now be built in with molecular precision. The most important catalysts derived from these materials are the acid zeolites. The acid site is defined by the crystalline structure and exhibits great chemical and steric selectivities for catalytic conversions, such as fluid catalytic cracking and alkane isomerization (cf. Chapter 2). In Section 9.5 we discuss the synthesis of zeolites and, briefly, of mesoporous solids. 

FCC (fluid catalytic cracking) to develop novel molecular sieves which are comparable with or better than ZSM-5 in shape-selectivity of light olefins (C3=-C5=). 

Catalytic cracking is a very flexible process used to reduce the molecular weight of hydrocarbons. Today, fluid catalytic cracking (FCC) remains the dominant conversion process in petroleum refineries. Although FCC is sometimes considered to be a fully matured process, new challenges and opportunities in its application and a continuing stream of innovations in the process and catalyst field ensure that it will remain an important and dynamic process in the future of refining. 

The main task of fluid catalytic cracking is the conversion of a wide range of both virgin and cracked hydrocarbon residues into lower molecular weight and more valuable products. 

As catalysts, zeolites have found their most important application in petroleum refining processes. Their acid function is used in Fluid Catalytic Cracking (FCC), in hydroisomerisation of light alkane fraction as well as in ohgomerisation and isomerisation steps to upgrade the hquid fuels into gasohne and diesel. The combination of two different zeolites in the same industrial process is illustrated in the Shell-UOP TIP process an acidic zeolite, MOR, is used for isomerisation and the neutral LTA is used as molecular sieve for separation as shown in the scheme below (Figure 5.3). 

Molecular mechanics has also been used to study skeletal isomerization of 1-butene to isobutene (80), olefin selectivity in fluid catalytic cracking using ZSM-5, zeolite Y, mordenite and P (81), carbon-sulfur bond cleavage over zeolite Y (82), and the location of naphthalene and 2-methylnaphthalene in HZSM-5 (83). In all cases, a methodology similar to those described earlier were adopted (75,77). 

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