Thermal conductivity, exothermic reaction

When analysing a large amount of sample at a slow scanning rate, a silver sample vessel is recommended owing to its high thermal conductivity. A reaction between the solvent and the inner surfaces of a sample vessel can cause unexpected features to be observed on the DSC curve. For example, water in a sample can react with aluminium and an exothermic peak is observed in the region of 400 K. The peak is due to the formation of aluminium hydroxide [AI(OH)3 ] on the inner surface of... 

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. 

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. 

Differential Thermal Analysis (DTA) A sample and inert reference material are heated at a controlled rate in a single heating block. This test is basically qualitative and can be used for identifying exothermic reactions. Like the DSC, it is also a screening test. Reported temperatures are not reliable enough to be able to make quantitative conclusions. If an exothermic reaction is observed, it is advisable to conduct tests in the ARC. 

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. 

Under isothermal decomposition, he states that it is difficult to maintain isothermal conditions in such strongly exothermic reactions as are involved in the thermal decomposition of explosives owing to their tendency for selfheating. One is also concerned with. the elimination (or minimization) of temperature transients in bringing the sample to the predetermined temp of the experiment. After a brief description of experiments of A.J.B. Robertson and of A.D. Yoffe, conducted in England, the quartz spring apparatus designed by M.A. Cook ... 

In one test, 0.5 g was sealed in a thermal stability bomb and the temp was raised 5°C/min. At ca 120°, a violent exothermic reaction occurred, rupturing the burst diaphragm of the app at 1800 psi. Another test conducted on a 0.5 g sample at 60°C resulted in sudden de-compn after 3 hrs at that temp. The sample temp rose sharply and exceeded the scale limit of 250°C. The same tests on the 2.6 Dichloro deriv (qv) showed similar, but less severe, instability. 

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. 

Thermal explosions may be expected to develop whenever the rate of heat liberation in an exothermic reaction exceeds the rate of heat dissipation by conduction and convection. (An endothermic reaction can never cause a thermal explosion.) Because of the exponential dependence of the reaction rate on temperature, the rate increases rapidly as the temperature rises, until an explosion results. There is little difference, therefore, in the temporal behavior prior to explosion, between explosions that develop as a result of a thermal acceleration of the reaction rate, or those that occur by virtue of a catastrophic build-up of reactive reaction intermediates. 

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. 

Oxidative Stability The fact that C02 cannot be further oxidized makes it an ideal candidate for carrying out catalytic oxidation chemistry. The enhanced thermal conductivity of sc C02 relative to organic solvents suggests that it could also act as an efficient solvent for buffering heat transfer (especially relative to gas-phase reactions), even for highly exothermic reactions. 

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