Thermal process

Thermal decomposition of polymers can be considered as a depolymerization process in only a few cases. Thus, polystyrene and polymethylmethacrylate are examples of polymers that can be thermally degraded with the formation of high yields of the corresponding monomer. However, for most polymers thermal decomposition leads to a complex mixture of products, containing low monomer concentrations. The type and distribution of products derived from the thermal degradation of each polymer depends on a number of factors the polymer itself, the reaction conditions, the type and operation mode of the reactor, etc. 

Some confusion is found in the literature over the terms used to describe the thermal treatment of polymers depolymerization, cracking, thermolysis, pyrolysis, etc. In this book, we shall use the term pyrolysis to refer to the thermal decomposition of polymeric materials at high temperatures (above 600 °C) whereas, when the degradation takes place at lower temperatures, we shall refer to it mainly as thermal cracking. However, in some cases it is difficult to assign a process to one of these categories, as is the case in thermogravimetric analysis where the temperature is continuously varied. 

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

Magnesium metal is produced primarily by either thermal or electrochemical processes. The thermal process operates at temperatures over 1200°C and utilizes a metallothermic reduction in which magnesium metal volatilizes from MgO and is condensed to recover the metal. The electrochemical process is based on the electrolysis of fused anhydrous magnesium chloride. 

In the thermal reduction process, reaction (14.4), magnesium oxide (as a component of dolime) reacts with a metal such as silicon, which is present in ferrosilicon, to produce magnesium metal. The two thermal methods in operation today are the Pidgeon and Magnetherm processes. The Pidgeon process is a batch process in which dolime and silicon (usually ferrosilicon) are briquetted and fed into a gas-fired or electrical heat retorts. The retort is 

In the Magnetherm process, alumina is added to melt the dicalcium silicate slag that forms at about 1500°C see reaction (14.5)  

SR of hydrocarbons, in particular of Natural Gas (NG), is still today the major industrial process for the manufacture of hydrogen [6-8]. 

This process was introduced in Germany at the beginning of the twentieth century to produce hydrogen for ammonia synthesis, and it was further developed 

A simplified scheme of methane SR is shown in Fig. 2.2, which includes all main process steps involved in hydrogen production plants based on the SR reaction [8]. 

Two units remove the sulphur concentrations (ppm), added to natural gas as an odorant for safety detection, or present in higher hydrocarbon feedstocks, to protect downstream catalysts (sulphur is a poison for SR catalysts) and process equipment. In particular, the organo-sulphur species are converted to H2S at pressures exceeding about 500 psig and temperatures higher than 350°C by catalytic hydrodesulphurisation (HDS unit), and Co and Mo alumina-based particulates are used as catalysts. This step is not required for methanol but would be necessary for any sulphur-containing petroleum-based fuels. A second unit permits the H2S produced in the first step to be removed by a particulate bed of ZnO. When necessary a further step for chloride removal should be included (not reported in Fig. 2.2). 

The third step is the heart of the process (steam reformer). Ni-based (Ni-Al2O3) catalysts, loaded in tubular reactors, favour the advancement of the following reactions  

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Like steam injection, in-situ combustion is a thermal process designed to reduce oil viscosity and hence improve flow performance. Combustion of the lighter fractions of the oil in the reservoir is sustained by continuous air injection. Though there have been some economic successes claimed using this method, it has not been widely employed. Under the right conditions, combustion can be initiated spontaneously by injecting air into an oil reservoir. However a number of projects have also experienced explosions in surface compressors and injection wells. 

Calibration of devices for residual austenite control in elements after thermal processing... 

The determining of sorting limits of steel parts after thermal processing in order to eliminate these, which indicate exceeded allowed content of residual austenite, requires elements of identical shape and dimensions, as the studied parts, and with known content of residual austenite. Such elements serve to define the sorting thresold, during manual control as well as automatic... 

To define a steel, it would be necessary to know its chemical composition, its physicochemical constitution, its metallurgical state (aimealed, hammered) and other parameters (superficial and chemical processing,. ..). The set of structural characters of a metallic alloy is consequently function of the chemical composition, the elaboration processing, the thermal processing, the temperature, etc. 

The obtained graph is the basis for evaluating the stress while applying the probe to controlled elements made of the same material and subjected to identical thermal processing as the reference sample... 

Computer Model of Thermal Processes in a Cement Kiln for Application in IR Defectoscopy. 

The given computer model of thermal processes in a cement kiln allows to calculate temperature pattern both at a surface and inside a kiln body. 

Laser ionization. Occurs when a sample is irradiated with a laser beam. In the irradiation of gaseous samples, ionization occurs via a single- or multiphoton process. In the case of solid samples, ionization occurs via a thermal process. 

In other work, the impact of thermal processing on linewidth variation was examined and interpreted in terms of how the resist s varying viscoelastic properties influence acid diffusion (105). The authors observed two distinct behaviors, above and below the resist film s glass transition. For example, a plot of the rate of deprotection as a function of post-exposure processing temperature show a change in slope very close to the T of the resist. Process latitude was improved and linewidth variation was naininiized when the temperature of post-exposure processing was below the film s T. 

Control of sonochemical reactions is subject to the same limitation that any thermal process has the Boltzmann energy distribution means that the energy per individual molecule wiU vary widely. One does have easy control, however, over the energetics of cavitation through the parameters of acoustic intensity, temperature, ambient gas, and solvent choice. The thermal conductivity of the ambient gas (eg, a variable He/Ar atmosphere) and the overaU solvent vapor pressure provide easy methods for the experimental control of the peak temperatures generated during the cavitational coUapse. 

The estabhshment of safe thermal processes for preserving food in hermetically sealed containers depends on the slowest heating volume of the container. Heat-treated foods are called commercially sterile. Small numbers of viable, very heat-resistant thermophylic spores may be present even after heat treatment. Thermophylic spores do not germinate at normal storage temperatures. 

CannedFoods, Principles of Thermal Process Control, Acidification, and Container Closure Evaluation, 5th ed.. The Food Processors Institute, Washington, D.C., 1988. D. M. Considine and G. D. Considine, eds.. Foods and Food Production Fnyclopedia, Van Nostrand Reinhold Co., New York, 1982. 

Research Needsfor Thermal Gasification of Biomass, Compiled by StudsvikAB Thermal Processes, for International Energy Agency, IGT, Chicago, Mar. 1992,... 

J. Diebold and co-workers, in E. Hogan and co-workers, eds.. Biomass Thermal Processing, The Chameleon Press Ltd., London, 1992, pp. 101—108. 

Activation Parameters. Thermal processes are commonly used to break labile initiator bonds in order to form radicals. The amount of thermal energy necessary varies with the environment, but absolute temperature, T, is usually the dominant factor. The energy barrier, the minimum amount of energy that must be suppHed, is called the activation energy, E. A third important factor, known as the frequency factor, is a measure of bond motion freedom (translational, rotational, and vibrational) in the activated complex or transition state. The relationships of yi, E and T to the initiator decomposition rate (kJ) are expressed by the Arrhenius first-order rate equation (eq. 16) where R is the gas constant, and and E are known as the activation parameters. 

Manufacture of P-Silicon Carbide. A commercially utilized appHcation of polysdanes is the conversion of some homopolymers and copolymers to siHcon carbide (130). For example, polydimethyl silane is converted to the ceramic in a series of thermal processing steps. SiHcon carbide fibers is commercialized by the Nippon Carbon Co. under the trade name Nicalon (see Refractory fibers). 

The UCB collection and refining technology (owned by BP Chemicals (122,153—155)) also depends on partial condensation of maleic anhydride and scmbbing with water to recover the maleic anhydride present in the reaction off-gas. The UCB process departs significantly from the Scientific Design process when the maleic acid is dehydrated to maleic anhydride. In the UCB process the water in the maleic acid solution is evaporated to concentrate the acid solution. The concentrated acid solution and condensed cmde maleic anhydride is converted to maleic anhydride by a thermal process in a specially designed reactor. The resulting cmde maleic anhydride is then purified by distillation. 

Thermal Process. In the manufacture of phosphoric acid from elemental phosphoms, white (yellow) phosphoms is burned in excess air, the resulting phosphoms pentoxide is hydrated, heats of combustion and hydration are removed, and the phosphoric acid mist collected. Within limits, the concentration of the product acid is controlled by the quantity of water added and the cooling capabiUties. Various process schemes deal with the problems of high combustion-zone temperatures, the reactivity of hot phosphoms pentoxide, the corrosive nature of hot phosphoric acid, and the difficulty of collecting fine phosphoric acid mist. The principal process types (Fig. 3) include the wetted-waH, water-cooled, or air-cooled combustion chamber, depending on the method used to protect the combustion chamber wall. 

Commercial condensed phosphoric acids are mixtures of linear polyphosphoric acids made by the thermal process either direcdy or as a by-product of heat recovery. Wet-process acid may also be concentrated to - 70% P2O5 by evaporation. Liaear phosphoric acids are strongly hygroscopic and undergo viscosity changes and hydrolysis to less complex forms when exposed to moist air. Upon dissolution ia excess water, hydrolytic degradation to phosphoric acid occurs the hydrolysis rate is highly temperature-dependent. At 25°C, the half-life for the formation of phosphoric acid from the condensed forms is several days, whereas at 100°C the half-life is a matter of minutes. 

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