Pyrolysis of gas oils

Lumping constituents is clearly necessary when the reactant mixture is of such a complexity that it can only be characterized, for practical (e.g. analytical) reasons, by global properties that is notably the case for the pyrolysis of gas oils and crude oils. Examples of lumped constituent models can be found in refs. 35 and 36. In what follows, a few typical models will be briefly described, to bring to light the underlying principles of model building. 

The pyrolysis of gas oils for ethylene manufacture can yield four fractions ... 

The coumarone-indene resins have been important commercial polymers produced in the sulfuric acid-catalyzed polymerization of mixed concentrates of these two compounds from coal tar naphthas. These and other unsaturated cyclics are also produced in the pyrolysis of petroleum oils for the production of manufactured gas, and the use of such petroleum-derived unsaturates is becoming of increasing significance in the production of this type of resin. 

Table 11.3 shows the yield of gas, oil/wax and char from the fixed bed pyrolysis of the main polyalkene plastics, polyethylene and polypropylene, found in municipal solid waste [7-15], Table 11.4 shows the gas, oil/wax and char from the pyrolysis of other plastics [7-9, 14-17], All of the plastics produced an oil/wax and gas and in some cases a char. The product yield related directly to the type of plastic, the reactor type... 

Table 11.4 shows the product yield of gas, oil/wax and char from the pyrolysis of other single plastics, including thermoplastics and thermoset plastics. Pyrolysis of polystyrene under moderate temperatures of between 500 and 600°C produces high levels of oil. Even at higher temperamres above 700°C, there is a high conversion of the polymer to oil. In fact the oil mainly consists mainly of the monomer styrene [8, 9, 23, 24]. 

Research on the pyrolysis of thermoset plastics is less common than thermoplastic pyrolysis research. Thermosets are most often used in composite materials which contain many different components, mainly fibre reinforcement, fillers and the thermoset or polymer, which is the matrix or continuous phase. There has been interest in the application of the technology of pyrolysis to recycle composite plastics [25, 26]. Product yields of gas, oil/wax and char are complicated and misleading because of the wide variety of formulations used in the production of the composite. For example, a high amount of filler and fibre reinforcement results in a high solid residue and inevitably a reduced gas and oiFwax yield. Similarly, in many cases, the polymeric resin is a mixture of different thermosets and thermoplastics and for real-world samples, the formulation is proprietary information. Table 11.4 shows the product yield for the pyrolysis of polyurethane, polyester, polyamide and polycarbonate in a fluidized-bed pyrolysis reactor [9]. 

The composition of the pyrolysis products is primarily determined by the means of disintegration of the macromolecule to the molecules of gas, oil, and solid residue. Thus to anticipate the pyrolysis oil composition of a plastic material, the chemical composition and structure of the polymer and its thermal decomposition reactions should be consistently considered. Typical thermal decomposition pathways of the various polymer kinds are abundantly treated in the relevant scientific literature [18-20]. Thermal decomposition of the polymer component of a plastic material is expected to begin at the weakest chemical bonds of the macromolecule. However, there are decomposition pathways which require lower energy than the direct breakage of the bonds, when rearrangement over four or six neighbouring atoms leads to the elimination of a volatile compound or to the scission of the macromolecular chain. 

In summary, pyrolysis is a tertiary recycling process that is used to break down large polymer molecules. In this process, the polymer samples are heated in an inert atmosphere, which causes the carbon-carbon bonds to break along the polymer backbone. This depolymerization step results in monomers (short-chained compounds) being formed. Generally, three types of products are formed from pyrolysis reactions gas, oil and char. All three have the potential to be nsed as a fnel or chemical feedstock. Depending on the feed polymer and the reaction conditions, different products can be obtained. The pyrolysis oil can either be used directly or can be nsed as a raw material for the petroleum industry [1-5]. 

Witt, R. Wall, F. Theoretical Analysis of Gas Oil Pyrolysis and Product... 

Witt, R.H. Wall, F.M. Theoretical analysis of gas oil pyrolysis and product yield correlation. Paper Presented at AIChE 70th Annual Meeting, New York, 1977. 

Naphthalene is present, in varying amounts, in all petroleum-derived pyrolysis products which have been exposed to a temperature of over 500 °C or to catalytic processes under relatively severe conditions. It can thus be found in the appropriate distillation fractions of the liquid products of steam cracking for the production of ethylene, in by-products of crude oil cracking and in residues of catalytic gasoline reforming as well as catalytic cracking of gas oils. The extraction of kerosene fractions provides a further source of naphthalene, after the separation of ali-phatics. 

Benzene was first isolated by Faraday (1825) from cylinders of compressed illuminating gas obtained from the pyrolysis of whale oil. In 1845, benzene was found in coal-tar by Hofmann. 

In both cases the pyrolysis gasoline/gas oil feed enters at the top of the first reactor. A recycle stream of hydrotreated gas oil is injected with the feed. The recycle streams serve as a reactant diluent, a heat sink to aid in reactor temperature control and as a solvent for polymer removal. Multiple catalyst beds are employed in the reactors to aid in temperature control. Quench oil is injected between the beds for reaction heat control. The first stage is operated at temperatures in the range of 107-177 C and at hydrogen partial pressures of the order of 48-68 atmospheres. 

Figure 2. Flow schematic of three-stage pyrolysis gasoline/gas oil hydrotreater for selective hydrogenation of gasoline...

PROPERTIES OF HYDROTREATED GASOLINE FROM COMBINED PROCESSING OF PYROLYSIS GASOLINE/GAS OIL... 

Carbon blacks [44] are a form of carbon produced by controlled pyrolysis of hydrocarbon oil or gas. They are the most important filler type for use in rubber as they are the main agents for providing high-strength compounds. These materials also have a pronoimced effect on the processing behaviour of rubbers. 

Petroleum. Thermal cracking (pyrolysis) of petroleum or fractions thereof was an important method for producing gas in the years following its use for increasing the heat content of water gas. Many water gas sets operations were converted into oil-gasification units (55). Some of these have been used for base-load city gas supply, but most find use for peak-load situations in the winter. 

Kerogen Decomposition. The thermal decomposition of oil shale, ie, pyrolysis or retorting, yields Hquid, gaseous, and soHd products. The amounts of oil, gas, and coke which ultimately are formed depend on the heating rate of the oil shale and the temperature—time history of the Hberated oil. There is Htde effect of shale richness on these relative product yields under fixed pyrolysis conditions, as is shown in Table 5 (10). 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

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