Preheat zone

SL/RN Process. In the SL/RN process (Fig. 4), sized iron ore, coal, and dolomite are fed to the rotary kiln wherein the coal is gasified and the iron ore is reduced. The endothermic heat of reduction and the sensible energy that is required to heat the reactants is provided by combustion of volatiles and carbon monoxide leaving the bed with air introduced into the free space above the bed. The temperature profile in the kiln is controlled by radial air ports in the preheat zone and axial air ports in the reduction zone. Part of the coal is injected through the centerline of the kiln at the discharge end. The hot reduced iron and char is discharged into an indirect rotary dmm cooler. The cooled product is screened and magnetically separated to remove char and ash. 

Heat is produced by chemical reaction in a reaction zone. The heat is transported, mainly by conduction and molecular diffusion, ahead of the reaction zone into a preheating zone in which the mixture is heated, that is, preconditioned for reaction. Since molecular diffusion is a relatively slow process, laminar flame propagation is slow. Table 3.1 gives an overview of laminar burning velocities of some of the most common hydrocarbons and hydrogen. 

For isothermal measurements, it is advisable to use a furnace of low thermal capacity unless suitable arrangements can be made to transport the sample into a preheated zone. The Curie point method [132] of temperature calibration is ideally suited for microbalance studies with a small furnace. A unijunction transistor relaxation oscillator, with a thermistor as the resistive part with completion of the circuit through the balance suspension, has been suggested for temperature measurements within the limited range 298—433 K [133]. 

As the concentration gradient further increases (regime C), the radius of curvafure of premixed wings l cur decreases. When if becomes comparable wifh, for example, the preheat zone thickness Sj, typically O (1mm), one or both of fhe premixed flame wings can be merged to the trailing diffusion flame by having a bibrachial or cotton-bud shaped structure. 

It is presumed that the global-quenching criteria of premixed flames can be characterized by turbulent shaining (effect of Ka), equivalence ratio (effect of 4>), and heat-loss effects. Based on these aforemenhoned data, it is obvious that the lean methane flames (Le < 1) are much more difficult to be quenched globally by turbulence than the rich methane flames (Le > 1). This may be explained by the premixed flame shucture proposed by Peters [13], for which the premixed flame consisted of a chemically inert preheat zone, a chemically reacting inner layer, and an oxidation layer. Rich methane flames have only the inert preheat layer and the inner layer without the oxidation layers, while the lean methane flames have all the three layers. Since the behavior of the inner layer is responsible for the fuel consumption that... 

A pulse reactor system similar to that described by Brazdll, et al( ) was used to obtain the kinetic data. The reactor was a stainless-steel U-tube, composed of a l/S" x 6 preheat zone and a 3/8" X 6 reactor zone with a maximum catalyst volume of about 5.0 cm. The reactor was Immersed In a temperature controlled molten salt bath. 

The thickness of the flame zone can be estimated in a manner similar to that used for the premixed flame. A control volume is selected between the condensed phase surface and the point just before the reaction zone. This is the preheat zone , which is heated to 7j. 

Results of the model for two parameters, i.e., the spatial temperature profile and the mass flux into the reaction zone as a function of gas mass flux are presented in Fig. 8.7. The temperature profile of the solid fuel flame (Fig. 8.7, left) is similar to that of a premixed laminar flame it consists of a preheat zone and a reaction zone. (The spatial profile of the reaction source term, which is not depicted here, further supports this conclusion.) The temperature in the burnt region (i.e., for large x) increases with the gas mass flux. The solid mass flux (Fig. 8.7, right) initially increases with an increase of the air flow, until a maximum is reached. For higher air flows, it decreases again until the flame is extinguished. 

Building on the foundation of the hydrocarbon oxidation mechanisms developed earlier, it is possible to characterize the flame as consisting of three zones [1] a preheat zone, a reaction zone, and a recombination zone. The general structure of the reaction zone is made up of early pyrolysis reactions and a zone in which the intermediates, CO and H2, are consumed. For a very stable... 

Figures 4.6—4.8 are the results for the stoichiometric propane-air flame. Figure 4.6 reports the variance of the major species, temperature, and heat release Figure 4.7 reports the major stable propane fragment distribution due to the proceeding reactions and Figure 4.8 shows the radical and formaldehyde distributions—all as a function of a spatial distance through the flame wave. As stated, the total wave thickness is chosen from the point at which one of the reactant mole fractions begins to decay to the point at which the heat release rate begins to taper off sharply. Since the point of initial reactant decay corresponds closely to the initial perceptive rise in temperature, the initial thermoneutral period is quite short. The heat release rate curve would ordinarily drop to zero sharply except that the recombination of the radicals in the burned gas zone contribute some energy. The choice of the position that separates the preheat zone and the reaction zone has been made to account for the slight exothermicity of the fuel attack reactions by radicals which have diffused into...

If the same criteria are applied to the analysis of the H2-air results in Figs. 4.1H12, some initially surprising conclusions are reached. At best, it can be concluded that the flame thickness is approximately 0.5 mm. At most, if any preheat zone exists, it is only 0.1 mm. In essence, then, because of the formation of large H atom concentrations, there is extensive upstream H atom diffusion that causes the sharp rise in H02. This H02 reacts with the H2 fuel to form H atoms and H202, which immediately dissociates into OH radicals. Furthermore, even at these low temperatures, the OH reacts with the H2 to form water and an abundance of H atoms. This reaction is about 50kJ exothermic. What appears as a rise in the 02 is indeed only a rise in mole fraction and not in mass. 

As discussed in an earlier section, <5L is the characteristic length of the flame and includes the thermal preheat region and that associated with the zone of rapid chemical reaction. This reaction zone is the rapid heat release flame segment at the high-temperature end of the flame. The earlier discussion of flame structure from detailed chemical kinetic mechanisms revealed that the heat release zone need not be narrow compared to the preheat zone. Nevertheless, the magnitude of <5L does not change, no matter what the analysis of the flame structure is. It is then possible to specify the characteristic time of the chemical reaction in this context to be... 

The conversion process occurs both on macro- and micro-scale, that is, on single particle level and on bed level. In other words, the conversion process has both a macroscopic and microscopic propagation front. One example of the macroscopic process structure is shown in Figure 10. The conversion front is defined by the process front closest to the preheat zone, whereas the ignition front is synonymous with the char combustion front. 

To approach the analysis of, and to be able to comprehend, the complex phenomena of thermochemical conversion of solid fuels some idealization has to be made. For a simplified one-dimensional analysis, there is an analogy between gas-phase combustion and thermochemichal conversion of solid fuels, which is illustrated in Figure 41. Both the gas-phase combustion and the thermochemical conversion is governed by a exothermic reaction which causes a propagating reaction front to move towards the gas fuel and solid fuel, respectively. However, there are also some major differences between the conversion zone and the combustion zone. The conversion front is defined by the thermochemical process closest to the preheat zone, which is not necessarily the char combustion zone, whereas for the flame front is defined by the ignition front. In practice, many times the conversion zone is so thin that the ignition front and the conversion front can not be separated. 

The burning lance (or similar firework) reactions can be pictured as occurring in five zones as shown in Figure 10.4. The bottom zone (No. 1) is essentially the preheat zone where the lance composition is being heated to the NaNOs melting point. Based on the thermal conductivity of Mg/NaNOs /binder composition it is possible to estimate the thickness of this zone to be about 1 mm. 

Vapor Phase Hydrogenation of Acetic Anhydride Acetic anhydride was pumped into an evaporator where it was mixed with hydrogen. The temperature of anhydride-hydrogen mixture was raised to the reaction temperature in a preheater zone, made of a 2 feet bed packed with 2 mm glass beads. The reaction took place in a 2 feet catalyst bed packed with 1 m.m. alpha-alumina coated with 0.5% Pd. The effluent was condensed and analyzed by G.C. 

Note that the temperature profile differs from the total heat profile because the heat per unit volume depends not only on the local temperature but also on the local density of the flame gas. In the preheat zone the profile of conducted heat coincides with the profile of total heat, whereas in the reaction zone the conducted heat gradually drops to zero. The difference between total heat and conducted heat represents the heat gained by chemical reaction. The volume integral of the total heat is approximately H, and the volume integral of the conducted heat is approximately H"... 

If the same amount of source energy were delivered by, say, an electric current over a time larger than the time of development of a minimal flame, the temperature at the core would drop below the flame temperature, the heat liberation in the reaction zone would not attain a balance with the outflow of heat into the preheat zone, and the flame would become extinct. On the other hand, if the current flow were continued for a longer period, the temperature profile ultimately would become sufficiently broad, and the temperature in the core sufficiently high, so that heat liberation within the reaction zone overbalances the outflow of heat and ignition occurs... 

This non-uniformity in the distribution of the atoms in the flame arises because the flame has a distinct structure. Figure 2.4 shows the structure of a typical premixed flame. Premixed gases are heated in the preheating zone, where their temperature is raised exponentially until it reaches the ignition temperature. Surrounding the preheating zone is the primary reaction zone, where the most energetic reactions take place. 

The desolvation of the droplets is usually completed in the preheating zone. The mist of salt clotlets then fuses and evaporates or sublimes. This is critically dependent on the size and number of the particles, their composition and the flame mixture. As the absolute concentration of analyte in the flame is very small (< lO- atm), the saturated vapour pressure may not be exceeded even at temperatures below the melting point. 

The dry heat tunnel is connected directly after the washing machine. Starting at this point, the vials will be processed under class 100 laminar flow areas. First, the washed vials are loaded directly to the preheating zone of the tunnel, which is covered by HEPA filtered laminar flow. The vials are heated and dried properly before passing to the second stage, sterilization and depyrogenation zones. Finally, the vials will be cooled down to room temperature at the cooling zone. 

Vial Size Particle Size Preheating Zone Cooling Zone Sterilization Zone... 

Application of SFE necessitates a CO2 source, a pump to pressurize the fluid, an oven containing the extraction vessel, a restrictor to maintain a high pressure in the extraction line, an analyte collection vessel, and an overall system controller. CO2 is drawn from the bottom of the tank with a dip tube because the liquid is the more dense of the two phases. The substantial vapor pressure of the CO2 at ambient temperature helps to displace the liquid into the pump. CO2 remains a liquid throughout the pumping or compression zones and passes through small-diameter metal tubing as it approaches the extraction vessel. A preheating zone in front of the extraction vessel allows supercritical temperature, pressure, and density conditions to be applied immediately to the analyte matrix in the vessel. 

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