Preheat zone temperatures

Drawback 1 A reflective scale is generally formed when the preheat zone is held at temperatures at or above 2300 F (1260 C). The cause of the reflective scale is the normal softening of the scale above 2320 F (1271 C) and the lower conductivity of the surface. If a furnace has this problem, reducing the preheat zone temperatures and increasing the product discharge temperature will increase furnace productivity. 

A control method variation uses the output signal from a temperature control in a downstream zone as process variable for energy input in the next upstream zone, for example, soak zone temperature controls main heating zone input and/or heat zone temperature controls preheat zone temperature. Note that zones may sometimes be a series of closely spaced, separate catenary furnaces. If a very low setpoint for the output signal of the soak and/or heat zones is used to control the upstream zone, the soak time will be extended to allow the chrome carbides to dissolve into the strip and thereby produce a quality product. 

Q2. A five-zone slab heating furnace had a very high fuel rate because the operators believed it was necessary to maintain the top and bottom preheat zone temperature setpoints (with temperature measurements about 60% through the zone) the same at all production rates. What can be done to reduce fuel rates of such a furnace ... 

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. 

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]. 

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. 

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. 

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... 

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. 

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. 

A parametric study of moving bed behaviour has been undertaken. The solid pellets are assumed to be preheated to the appropriate reduction temperatures before entering the reaction zone of the reactor. Although this neglects the solids preheat zone, this can easily be included in the model if required. The present study therefore is focussed on the reaction zone itself where the important parameters of gas and solid flow rates, gas inlet temperature and gas mixture composition are considered. Reactor length is also of major importance but in the present paper this has been fixed at lm in order to obtain comparative data. 

In this paper, a tube of size 1/4" in diameter was considered with styrene monomer preheated to 135 C. The radial variations in temperature are minimal and good control over the concentration profile was possible. Some typical variations in conversion with radial position are shown in Figure 10. The zone temperatures for this example represent a sub-optimal case. However, it is readily seen that as we approach the optimal solution, the first zone temperature converges to an upper limit, while the second zone temperature goes to absolute zero. Figure 11 shows this trend. We also note that as the optimal temperatures are approached, there is a steady drop in the... 

The temperature profile for the preheating zone of the combustion wave can be derived readily by introducing the variable q= dT/dx for the heat flux, so that Eq. (14) rearranges as... 

Direct integration of Eq. (29) with BCs [Eq. (28)] yields the Michelson (1930) solution for the temperature profile in the preheating zone ... 

Fig. 71. Based on the measured temperature profile data (curve 1), the distribution of conversion along the combustion wave, riix) (curve 2), and the heat release function, (x) (curve 3), have been determined using Eq. (15). The characteristic length of the zones, is given by the size of the domain where < (jc) is nonzero. The preheating zone, xt, is defined as the sample length ahead of the front where...

---