Radial temperature gradient, effect

An important effect in the design of a tubular flow reactor is the development of a radial temperature gradient in a highly exothermic reaction with wall cooling. The temperatures near the tube axis are... 

Airway surfaces, like skin, are continually exposed to the ambient environment. In contrast to skin submucosal vessels, however, w hich shed excess heat by vasodilating when heated and conserve heat by vasoconstricting when chilled, it is unclear how the airway vasculature responds to temperature extremes. Inspiring cold air poses two challenges to conducting airway tissues the risk of tissue injury should inadequate heat reach the airway surface and excessive body heat loss due to increasing the radial temperature gradient. Vasodilation would protect airway tissue but increase heat loss, while vasoconstriction would produce the opposite effect. 

The temperature with large columns may not be homogenous. A mathematical model of the effect of a radial temperature gradient has been developed and validated on octadecyl-packed columns of 11-15 cm diameter... 

Because the thermal diffusivity of SC water is comparable to that of many high quality insulation materials, gross radial temperature gradients can easily exist in a flow reactor. As shown in Figure 2, radial temperature gradients within the annular flow reactor are negligible. A computer program, which accurately accounts for the effects of the various fluid (solvent, solvent and solute, air) compressibilities on flow measurements, calculates mass and elemental balances for each experiment. A typical experiment evidences mass and elemental balances of 1.00+0.05. 

The one-dimensional homogeneous model is useful for first-order estimates and when lab (or pilot-plant) data for the same diameter tube are available. This simple model does not provide information on the effect of the tube diameter on the effective radial temperature gradients. 

Effect of Tube Diameter. For 1" tubes,the radial profiles of activity are parabola -like functions with minimum value at the center of the tube. For tubes with higher values of the diameter, e.g., 2" tubes, the picture can be rather different. High radial temperature gradients result also in large gradients of benzene and poison. For a deactivation process with high E, a minimum on the activity profile can occur between the reactor axis and wall, see Figure 8. Blaum ( 3) observed radial hot spots of temperature between the reactor axis and wall for a very rapid deactivation. For slow deactivation,these hot spots are not likely. 

Lateral transport of heat and reactants across the diameter of the reactor This feature is important when the reaction has a high heat effect or when insufficient mixing of the reactants would have a strong adverse effect on the performance. Examples are selective oxidation processes (highly exothermic), where lateral or radial temperature gradients would decrease the selectivity of the conversion. Another example is the selective catalytic reduction process of nitric oxide with ammonia, where mixing of ammonia with the flue gas is often a point of great concern. Because the void space in a BSR is continuous... 

Although the preparation of continuous beds on a small scale is easy, the preparation of large-size monoliths is quite difficult. The unstirred nature of the polymerization within the mold leads to a low capacity to effectively dissipate the exothermic reaction heat. The appearance of radial temperature gradient across the reaction mixture also results in the formation of inhomogeneities in the pore structure of the obtained monolith. This is the reason why most of the work reported in the last decades focused on the apphcation of smaU-size monoliths in chromatographic processes. 

Radial temperature gradients are more critical than axial gradients. In order to limit the effects of heat transport limitations on the observed rates to 5% it is sufficient that for a single reaction with the following kinetics ... 

Lastly, practically in any kinetic experiment there are some uncertainties in its conditions and results. This concerns such factors as mixing of reactants, regimes of gas flow in different zones of the reaction system, wall effects, axial and radial temperature gradients, spatial distribution of reaction rates, concentrations, pressure, etc. A lot of effort must be applied even to measure the reactant conversion and the main product distributions at the exit of the reactor, needless to say anything about concentrations of active intermediates and concentration distributions inside the reactor. Moreover, up to now there is no clear understanding about which measured parameters are the most informative and should be preferred. 

We see that the Ts depends on the rate of heat release q and that the overall effect of convection is to reduce the surface temperature and make it nonuniform, with the surface temperature being highest at q = 1 (i.e., the downstream stagnation point of the sphere) and lower at the front, q = — 1. The asymmetry is due to the fact that the radial temperature gradient is slightly increased at the front relative to the back and thus requires a slightly lower surface temperature to sustain the heat flux q, compared with the surface temperature that is required at the back. 

Limit the height of the bed to keep temperature increase <50°C to minimize effects of radial temperature gradients. Bed can be shallow and wide. Quench can include injection of cold reactants, internal or external heat exchangers. 

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