Pyrolysis process

Pyrolysis (or carbonization Chapters 13 and 16) is perhaps the oldest technique for obtaining liquid products directly from coal and involves heating the coal in the absence of air (or oxygen) to produce heavy and light oils, gases, and char. In the presence of hydrogen, the process is called 

The composition and relative amounts of the products formed are dependent on process parameters such as heating rate, pressure, coal type coal (and product) residence time, coal particle size, and reactor configuration. A major disadvantage of this type of process is the large yields of char (Table 18.2) that markedly reduce the yield of liquid products. 

On the other hand, pyrolysis and hydrocarbonization processes are relatively less complex than, say, liquid-phase hydrogenation processes. The operating pressures for pyrolysis processes are usually less than 100 psi (more often between 5 and 25 psi) but the hydrocarbonization processes require hydrogen pressures of the order of 300-1000 psi. In both categories of process, the operating temperature can be as high as 600°C (1110°F). 

There are three types of pyrolysis reactors that are of interest (1) a mechanically agitated reactor, (2) an entrained-flow reactor, and (3) a fluidized-bed reactor. 

Lurgi- Ruhrgas Lurgi- Ruhrgas Mechanical mixer 450-600 840-1110 15 20 s 55-45 15-25 30 

Radical reactions are of paramount importance in industrial aromatic chemistry, especially in the production of aromatics from coal and suitable petroleum fractions, as well as in the pyrolysis of aromatic mixtures to produce coke. The reaction temperature of such processes is generally well above 500 °C, so that a wide spectrum of products results from pyrolysis. 

The thermal dealkylation of toluene (or methylnaphthalenes) also follows a radical reaction mechanism. 

Kinetic studies have shown that thermal hydro-dealkylation is a first-order reaction with respect to the aromatics, and a half-order reaction in relation to hydrogen. 

A further important process which follows a radical mechanism is the catalytic dehydrogenation of ethylbenzene for the production of styrene (see chapter 5.1.3). 

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Pyrolysis gasoline Pyrolysis-gc Pyrolysis of coal Pyrolysis processes Pyrolysis yields... 

Mechanisms and Rates of Combustion. AH soHd fuels and wastes bum according to a general global mechanism (Fig. 2). The soHd particle is first heated. FoHowing heating, the particle dries as the moisture bound in the pore stmcture and on the surface of the particle evaporates. Only after moisture evolution does pyrolysis initiate to any great extent. The pyrolysis process is foHowed by char oxidation, which completes the process. 

Hoechst HTP Process. The two-stage HTP (high temperature pyrolysis) process was operated by Farbwerke Hoechst ia Germany. The cracking stock for the HTP process can be any suitable hydrocarbon. With hydrocarbons higher than methane, the ratio of acetylene to ethylene can be varied over a range of 70 30 to 30 70. Total acetylene and ethylene yields, as wt % of the feed, are noted ia Table 11. 

Table 11. High Temperature Pyrolysis Process Yields...

Regenerative pyrolysis processing is very versatile it can handle varied feedstocks and produce a range of ethylene to acetylene. The acetylene content of the cracked gases is high and this assists purification. On the other hand, the plant is relatively expensive and requires considerable maintenance because of the wear and tear on the refractory of cycHc operation. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

American Ligurian, Inc. (Stanford, Connecticut) is marketing a pyrolysis process developed in Italy, which generates steam for hot water, air heating, dryers, kilns, and similar installations. A modular plant produces 8000 kg/h of steam from 1 tih of tires. The pyrolysis process produces 0.9 t of fuel ok, 270 t of steel, and 54 t of ash annually. Gas emissions meet the strictest environmental standards (16). 

SoHd by-products include sludge from wastewater treatment, spent catalyst, and coke from the EDC pyrolysis process. These need to be disposed of in an environmentally sound manner, eg, by sludge digestion, incineration, landfill, etc. 

Outside the United States, coal pyrolysis is more important as a source of BTX. The proportions are about 70 20 10, but can vary greatiy depending on the coal and on the pyrolysis process used. Product quaUty is not as good as petroleum-derived BTX. This source could become more important again if petroleum costs escalate. Much higher yields of BTX from coal can be obtained by first hydrogenating the coal (22). 

Process development on fluidized-bed pyrolysis was also carried out by the ConsoHdation Coal Co., culminating in operation of a 32 t/d pilot plant (35). The CONSOL pyrolysis process incorporated a novel stirred carbonizer as the pyrolysis reactor, which made operation of the system feasible even using strongly agglomerating eastern U.S. biturninous coals. This allowed the process to bypass the normal pre-oxidation step that is often used with caking coals, and resulted in a nearly 50% increase in tar yield. Use of a sweep gas to rapidly remove volatiles from the pyrolysis reactor gave overall tar yields of nearly 25% for a coal that had Eischer assay tar yields of only 15%. 

Other large-scale coal pyrolysis process developments were carried out by the Tosco Corp., with its TOSCO AT, process (36). Essentially a direct copy of Tosco s rotating kiln technology that was developed for pyrolysis of oil shale, this slow heating scheme achieved tar yields at maximum temperatures of 482—521°C that were essentially identical to those obtained by a Eischer assay. 

Table 4. Product Distribution for the Occidental Flash Pyrolysis Process...

Ethylene. Where ethylene is ia short supply and fermentation ethanol is made economically feasible, such as ia India and Bra2il, ethylene is manufactured by the vapor-phase dehydration of ethanol. The production of ethylene [74-85-1] from ethanol usiag naturally renewable resources is an active and useful alternative to the pyrolysis process based on nonrenewable petroleum. This route may make ethanol a significant raw material source for produciag other chemicals. 

Typical pyrolysis yields and oil qualities for two bituminous coals, Utah A and lUinois No. 6, are presented in Table 27-15. The major problem with any pyrolysis process is the high yield of char. 

Small-angle X-ray scattering (SAXS) [19] has been widely used to investigate the inhomogeneous electron density in materials [20]. In carbonaceous materials, porosity is commonly encountered. The pores form and provide escape routes for gases produced during the pyrolysis process. 

The energy available in various forms of irradiation (ultraviolet, X-rays, 7-rays) may be sufficient to produce in the reactant effects comparable with those which result from mechanical treatment. A continuous exposure of the crystal to radiation of appropriate intensity will result in radiolysis [394] (or photolysis [29]). Shorter exposures can influence the kinetics of subsequent thermal decomposition since the products of the initial reaction can act as nuclei in the pyrolysis process. Irradiation during heating (co-irradiation [395,396]) may exert an appreciable effect on rate behaviour. The consequences of pre-irradiation can often be reduced or eliminated by annealing [397], If it is demonstrated that irradiation can produce or can destroy a particular defect structure (from EPR measurements [398], for example), and if decomposition of pre-irradiated material differs from the behaviour of untreated solid, then it is a reasonable supposition that the defect concerned participates in the normal decomposition mechanism. 

The performance of a novel microwave-induced pyrolysis process was evaluated by studying the degradation of HDPE and aluminiutn/polymer laminates in a semibatch bench-scale apparatus. The relationship between temperature, residence time of the pyrolytic products in the reactor, and the chemical composition of the hydrocarbon fraction produced was investigated. 28 refs. 

RECOVERY OF VALUABLE CHEMICAL FEEDSTOCKS FROM WASTE AUTOMOTIVE PLASTICS VIA PYROLYSIS PROCESSES... 

The application of a selective pyrolysis process to the recovery of chemicals from waste PU foam is described. The reaction conditions are controlled so that target products can be collected directly from the waste stream in high yields. Molecular beam mass spectrometry is used in small-scale experiments to analyse the reaction products in real time, enabling the effects of process parameters such as temperature, catalysts and co-reagents to be quickly screened. Fixed bed and fluidised bed reactors are used to provide products for conventional chemical analysis to determine material balances and to test the concept under larger scale conditions. Results are presented for the recycling of PU foams from vehicle seats and refrigerators. 12 refs. 

The use of pyrolysis for the recycling of mixed plastics is discussed and it is shown that fluidised bed pyrolysis is particularly advantageous. It is demonstrated that 25 to 45% of product gas with a high heating value and 30 to 50% of an oil rich in aromatics can be recovered. The oil is found to be comparable with that of a mixture of light benzene and bituminous coal tar. Up to 60% of ethylene and propylene can be produced by using mixed polyolefins as feedstock. It is suggested that, under appropriate conditions, the pyrolysis process could be successful commercially. 23 refs. 

Details are given of a pyrolysis process developed by BP Chemicals for the recovery of raw materials from plastics waste. 

Fast screening techniques, such as temperature-resolved in-source filament pyrolysis and laser-assisted pyrolysis, benefit from the high cycle time and mass accuracy of FUCR-MS [214]. An additional advantage of FUCR-MS in the study of pyrolysis processes is that MS can be readily used for structural identification of desorption and pyrolysis products. 

Montaudo and co-workers have used direct pyrolysis mass spectrometry (DPMS) to analyse the high-temperature (>500°C) pyrolysis compounds evolved from several condensation polymers, including poly(bisphenol-A-carbonate) [69], poly(ether sulfone) (PES) and poly(phenylene oxide) (PPO) [72] and poly(phenylene sulfide) (PPS) [73]. Additionally, in order to obtain data on the involatile charred residue formed during the isothermal pyrolysis process, the pyrolysis residue was subjected to aminolysis, and then the aminolyzed residue analysed using fast atom bombardment (FAB) MS. During the DPMS measurements, EI-MS scans were made every 3 s continuously over the mass range 10-1,000 Da with an interscan time of 3 s. 

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