Adiabatic zone

Given that, in the snbsnrface, we are dealing with an open system, the fundamental eqnation may be applied only when the macroscopic system is decoupled in isolated, well-defined systems. As an example, we can consider that an adiabatic zone of the snbsnrface solid phase is in contact with an aqneous solntion throngh a rigid barrier, snrronnded by an insnlating wall. 

Operating temperatures are also critical. The listed reactions are endothermic. The best yields occur along isothermal reaction zones, but are difficult to achieve. Instead, the reaction beds are separated into a number of adiabatic zones operating at 500-1000°F with heaters between stages to supply the necessary energy to promote reaction of heat and hold the overall train near or at a constant... 

Figure 4. Schematic of triangular grooves (evaporator, adiabatic zone and condenser) in MHP, [25].

A zone with Qt > 0 is called a (net) radiation source, as it emits more radiation than it absorbs. A zone with Qt < 0 is a (net) radiation receiver, that absorbs more radiation than it emits. An adiabatic zone (Q = 0) with respect to the outside is known as a reradiating wall. Its temperature is such that it emits just as much radiation as it absorbs from radiation incident upon it (radiative equilibrium). 

Directional solidification of metals and alloys is often performed with Bridgman type furnaces shown schematically in Figure 34.23. Such a furnace consists of a hot and a cold zone separated by an adiabatic zone. The temperature profile in a sample has a typical S-shaped profile (see Figure 34.25, right). The sample, being in a cartridge, is withdrawn with a fixed speed fi om the hot into the cold zone. 

Bridgman furnace A furnace with at least three zones being at different temperamres. A hot zone and a cold zone are separated by an adiabatic zone. Directional solidification or crystal growth is achieved by either pulling the sample through the furnace from hot to cold or by moving the furnace... 

The reactor section consists of a vertical electrically-heated furnace in which the reactor tube is placed. The furnace has separately controlled zones with appropriate temperature and heat input measurement facilities for each zone. Fluid temperature measurements are made along the length of the reactor with calibrated couples located in adiabatic zones and temperature profiles can be varied. Reactor pressures are continuously monitored and pressure drop (AP) and pressure profile across the reactor can be controlled. Since each reactor represents a specific commercial coil, multiple reactors have been designed to cover the commercial contact time range of interest. In these studies a reactor designed for operation between 0.01 and 0.10 seconds was employed. 

In the film, there are apparently two zones(59) the adiabatic zone and the diffusive zone (Figure 8). In the adiabatic zone, the precursor thickness is a function of the horizontal distance X to the macroscopic drop Z(X) decreases as between its maximum value = a/0 at the drop and the limit thickness e (Figure 8). 

Schematic of a Bridgman-Stockbarger furnace with an adiabatic zone. Heat flows into the melt from the hot zone, through the solidification interface in the adiabatic zone, and is extracted through the solid in the cold zone as indicated by the arrows. The growth ampoule is slowly lowered at the desired solidification velocity. In order to maintain a near-planar solidification front, the solidification interface is kept in the adiabatic zone where the isotherms are nearly perpendicular to the walls.

Treatment of Refractory Walls Partially Enclosing a Radiating Gas Another modification of the results in Table 5-10 becomes important when one of the surface zones is radiatively adiabatic the need to find its temperature can be eliminated. If surface A9, now called A, is radiatively adiabatic, its net radiative exchange with Aj must equal its net exchange with the gas. 

An equation representing an energy balance on a combustion chamber of two surface zones, a heat sink Ai at temperature T, and a refractory surface A assumed radiatively adiabatic at Tr, inmost simply solved if the total enthalpy input H is expressed as rhCJYTv rh is the mass rate of fuel plus air and Tp is a pseudoadiabatic flame temperature based on a mean specific heat from base temperature up to the gas exit temperature Te rather than up to Tp/The heat transfer rate out of the gas is then H— — T ) or rhCp(T f — Te). The... 

This type of equipment can also be used for apphcations in which the only heat removed is that required for adiabatic cooling of the incoming feed solution. When this is done and the fines-destriiction feature is to be employed, a stream of hqiiid must be withdrawn from the settling zone of the ciystaUizer and the fine ciystals must be separated or destroyed by some means other than heat addition—for example, either dUiition or thickening and plwsical separation. 

Flame Temperature The heat released by the chemical reaction of fuel and oxidant heats the POC. Heat is transferred from the POC, primarily by radiation and convection, to the surroundings, and the resulting temperature in the reaction zone is the flame temperature. If there is no heat transfer to the surroundings, the flame temperature equals the theoretical, or adiabatic, flame temperature. 

The experimental data show that the magnitude of the heat capacity (or similarly of the specific heat) under adiabatic conditions decreases regularly with the increase of filler content. This phenomenon was explained by the fact that the macromolecules, appertaining to the mesophase layers, are totally or partly excluded to participate in the cooperative process, taking place in the glass-transition zone, due to their interactions with the surfaces of the solid inclusions. 

A reactor is run adiabatically when no heat is exchanged between the reaction zone and the surroundings. The reaction temperature can then only be controlled by quenching with a cold stream of the reaction mixture or by inter-stage heat exchangers. For highly thermally sensitive large molecules treated in the fine chemicals sector this is a very impractical mode of operation. Therefore, adiabatic reactors will not be discussed here. 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

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