Pyrolysis and Catalytic Cracking

Catalysts speed up chemical processes they do not change the position of thermodynamic equilibrium, so all of the above comments on the relative thermodynamic position of feed and product molecules applies to catalytic processes. Because catalysts accelerate chemical processes (by lowering activation energies) they can be conducted at 

The first reaction involves interaction of a hydrocarbon with the catalyst surface. Hydride abstraction occurs to form a carbonium ion. Abstraction can be of any suitable hydrogen atom but if this results in a primary ion as shown, this will rapidly isomerise by hydrogen shift to the more thermodynamically stable secondary ion. This may be further isomerised by carbon shift to a tertiary ion. This contrasts with free radicals and although isomerisation occurs it is relatively slower. The carbonium ions can also undergo inter-molecular transfer (not shown) when a carbonium ion meets another hydrocarbon molecule. 

Olefins are formed from carbonium ions by P-scission reaction. This produces propylene from the secondary ion shown. Isobutene will 

Ethylene and methane cannot be produced by (3-scission and ethylene and methane are minor products that may be the consequence of some radical processes occurring within the spaces between catalyst particles. 

Today much of the propylene used in the world is produced by the catalytic hydrocarbon cracking in fluid cat-cracking and similar operations . 

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The principal olefins for the production of polymers and resins are ethylene and propylene. These are made by cracking larger molecules, which for the most part are paraffins. Two processes are involved -thermal cracking (pyrolysis) and catalytic cracking. Of these two types the former is the dominant process for the production of ethylene and propylene whilst the latter makes a significant contribution to the production of propylene. 

S. H. Ng, H. Seoud, M. Stanciulescu, and Y. Sugimoto, Conversion of Polyethylene to Transportation Fuels through Pyrolysis and Catalytic Cracking, Energy and Fuels, 9, 735-742 (1995). 

This chapter deals exclusively with tertiary recycling by pyrolysis and catalytic cracking of plastics waste alone and by coprocessing with petroleum residue or heavy oils to... 

In connection with thermal and catalytic processes such as coking, pyrolysis and catalytic cracking for the conversion of petroleum fractions, there is considerable interest in the mechanism of the transformation of various... 

Samolada, M.C., Baldauf, W., Vasalos, I.A., Production of bio-gasoline by upgrading biomass flash pyrolysis liquids via hydrogen processing and catalytic cracking, Fuel, 1998, 77, 1667. 

D. P. Serrano, J. Aguado, J. M. Escola, J. M. Rodriguez, L. Morselli, and R. Orsi, Thermal and catalytic cracking of a LDPE-EVA copolymer mixture, J. Anal. Appl. Pyrolysis 68-69, 481 (2003). 

The most widely used conventional chemical methods are pyrolysis [21-25] and catalytic cracking [13, 26-30], The latter yields products with a smaller range of carbon numbers and of a higher quality than products generated by the former method. Several types of solid acid catalysts, which are known to be effective for catalytic cracking (e.g. HZSM-5, HY and rare earth metal-exchanged Y-type (REY) zeolite and silica-alumina (SA)) were evaluated by catalyst screening tests and are listed in Table 6.1. The acidic... 

Pyrolysis is a tertiary or feedstock recycling technique capable of converting plastic waste into fuels, monomers, or other valuable materials by thermal and catalytic cracking processes. This method can be applied to transform both thermoplastics and thermosets in high-quality fuels and chemicals. Moreover it allows the treatment of mixed, unwashed plastic wastes. 

The results obtained can be explained by considering the reactions involved in the processes. We can assume that the main reactions in catalytic pyrolysis are catalytic cracking of tars and light hydrocarbons, which will explain the increase in gas yields when the catalyst is present in the reaction bed (18). Steam reforming of tars (reac. 1), methane (reac. 2) and Cj (reac. 3), and the water-gas shift reaction (reac. 4) can explain the final gas composition generated in catalytic steam gasification. 

A great many reactions in physics and chemistry proceed via chain mechanisms. This large family of mechanisms includes free radical and ionic polymerization, Fischer Tropsch synthesis, gas phase pyrolysis of hydrocarbons, and catalytic cracking. Nuclear reactions, of both the power generating and the explosive kind, are also chain processes. Notice that chemical chain reactions can be catalytic or non-catalytic, homogeneous or heterogeneous. One is almost tempted to say that chain reactions are the preferred route of conversion in nature. 

Other stocks which may be upgraded by hydrotreating are thennally and catalytic cracked naphthas, straight-run naphthas abnormally high in contaminants, and coker gasolines. The latter are produced by the high-temperature pyrolysis of reduced fuel to coke and distiUate fractions. Naphthas to be used as specialty solvents also are treated to obtain premium products with respect to color, odor, and stability. 

Manufacture of high-value coke (premium coke) with simultaneous production of minor amounts of liquid and gaseous hydrocarbons, i.e. the production of premium coke from highly-aromatic residues (coal-tar pitch, pyrolysis residues from naphtha cracking, residues from thermal and catalytic cracking). 

Pyrolysis is a major method of processing in the chemical industry. Petroleum is subjected to pyrolysis processes, known as cracking and catalytic cracking, to produce alkanes or alkenes that can be further used to produce fuels, such as gasoline. Pyrolysis has long been used to make charcoal from wood and other similar products such as shells. Pyrolysis occurs in our foods when we cook them—so when you take a bite of that golden brown apple... 

Catalytic Pyrolysis. This should not be confused with fluid catalytic cracking, which is used in petroleum refining (see Catalysts, regeneration). Catalytic pyrolysis is aimed at producing primarily ethylene. There are many patents and research articles covering the last 20 years (84—89). Catalytic research until 1988 has been summarized (86). Almost all catalysts produce higher amounts of CO and CO2 than normally obtained with conventional pyrolysis. This indicates that the water gas reaction is also very active with these catalysts, and usually this leads to some deterioration of the olefin yield. Significant amounts of coke have been found in these catalysts, and thus there is a further reduction in olefin yield with on-stream time. Most of these catalysts are based on low surface area alumina catalysts (86). A notable exception is the catalyst developed in the former USSR (89). This catalyst primarily contains vanadium as the active material on pumice (89), and is claimed to produce low levels of carbon oxides. 

Fuel industry is of increasing importance because of the rapidly growing energy needs worldwide. Many processes in fuel industry, e.g. fluidized catalytic cracking (FCC) [1], pyrolysis and hydrogenation of heavy oils [2], Fischer-Tropsch (FT) synthesis [3,4], methanol and dimethyl ether (DME) synthesis [5,6], are all carried out in multiphase reactors. The reactors for these processes are very large in scale. Unfortunately, they are complicated in design and their scale-up is very difflcult. Therefore, more and more attention has been paid to this field. The above mentioned chemical reactors, in which we are especially involved like deep catalytic pyrolysis and one-step synthesis of dimethyl ether, are focused on in this paper. 

This section concerns catalytic processes that transform chemicals from renewables by C-C bond breaking. Among these are thermochemical processes, such as pyrolysis and also gasification, catalytic reactions, such as catalytic cracking and different reforming reactions, and decarbonylation and decarboxylation reactions. Many of these reactions occur simultaneously, particularly in the thermochemical processes. Another technically important class of C-C bond breaking reactions is the fermentation processes, however, they will not be considered in this section since they do not involve heterogeneous catalysis. 

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