Thermal conductivity, exothermic

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

When a gas reacts with a solid, heat will be transfened from the solid to the gas when the reaction is exothermic, and from gas to solid during an endothermic reaction. The energy which is generated will be distributed between the gas and solid phases according to the temperature difference between the two phases, and their respective thermal conductivities. If the surface temperature of the solid is T2 at any given instant, and that of the bulk of the gas phase is Ti, the rate of convective heat transfer from the solid to the gas may be represented by the equation... 

Treatment of thermal conductivity inside the catalyst can be done similarly to that for pore diffusion. The major difference is that while diffusion can occur in the pore volume only, heat can be conducted in both the fluid and solid phases. For strongly exothermic reactions and catalysts with poor heat conductivity, the internal overheating of the catalyst is a possibility. This can result in an effectiveness factor larger than unity. 

Fillers are used in tooling and casting application. Not only do they reduce cost but in diluting the resin content they also reduce curing shrinkage, lower the coefficient of expansion, reduce exotherms and may increase thermal conductivity. Sand is frequently used in inner cores whereas metal powders and metal oxide fillers are used in surface layers. Wire wool and asbestos are sometimes used to improve impact strength. 

Ya.B. ZeFdovich, FizGoreniyaVzryva 7 (4), 463-76 (1971) CA 77, 64194 (1972) The influence of turbulence and nonturbulence is examined relative to a proplnt burning in a gas flow. Equations indicate exptl methods for determining the magnitudes of the thermal conductivity and viscosity under turbulent flow, and permit a study of thermal flow distribution and temps in a gas wherein an exothermic chem reaction occurs. Equations for non turbulent conditions can be used to calculate the distance from the surface of the proplnt to the zone of intense chem reaction and establish the relation of bulk burning rate to the vol reaction rate. 

The transformation of straw and agrofood residues with high sulfur and ash content requires the development of materials for sulfur abatement at high temperature, tar cracking and as monolith for syngas production by exothermic or autothermal processes thanks to catalysts supported on materials with a high thermal conductivity. 

High thermal conductivity, heat capacity and heat of evaporation. High specific heat capacity means that exothermic reactions may be controlled effectively. 

On the other hand, it has been argued that the resistance to heat transfer is effectively within a thin gas film enveloping the catalyst particle [10]. Thus, for the whole practical range of heat transfer coefficients and thermal conductivities, the catalyst particle may be considered to be at a uniform temperature. Any temperature increases arising from the exothermic nature of a reaction would therefore be across the fluid film rather than in the pellet interior. 

The thermal properties (i.e., density, specific heat capacity, and thermal conductivity) have a particularly strong influence on the curing behavior. The exothermal peak temperature is one example It can differ significantly between a composite mold with low thermal mass and a metal mold [35], A more thorough discussion of pros and cons of different mold materials can be found in Morena [37]. 

Here Cp is a heat capacity, k the thermal conductivity, and Q the reaction exothermicity. The term A exp [ — E/RT is simply the Arrhenius form of the reaction rate constant. 

Foamed-in-place polyurethane is prepared by allowing a polyol [po y(ethy ene glycol), polyester alcohols, etc.] to react with a diisocyanate in the presence of an amine catalyst. The gas which creates the foam may be a dissolved material, such as a Freon, which volatilizes during the exothermic polymerization reaction.7 A second method involves the use of water in the reaction mixture this hydrolyzes part of the isocyanate to produce an amine and C02 gas. The Freon-formed material is preferred for the insulation of low-temperature apparatus because the thermal conductivity of the foam is greatly reduced at low temperatures by the condensation of the Freon in the cells. It is probable that the longterm effectiveness of this phenomenon must be maintained by surrounding the foamed plastic with an airtight enclosure which will prevent diffusion of air into and Freon out of the cells. 

Expls and proplnts decomp exothermally at every temp above absolute zero. If the mass of the material is such that the heat produced by the decompn cannot be dissipated as rapidly as it is produced, the mass will heat itself to expln. The lowest constant surface temp above which a thermal expln is initiated is a function of the size, thermal conductance, the heat of reaction, and the reaction kinetic parameters. 

---