Self-cooling

Selbst-kosten, f.pl. cost of production, prime cost. -kUhlung, /. self-cooling, natural cooling. -laut, -lauter, m. vowel. 

The self cooling beer keg is based on the principle of closed sorption systems shown in chapter Y2 Figure 8. Figure 244 shows the beer keg from the outside and the inside. 

Figure 245. Temperature during the adsorption/discharging of the self cooling beer keg...

Figure 246. Self-cooling beer keg as an indirect heat storage...

Auto-oxidation is the process of slow oxidation with accompanying evolution of heat, sometimes leading to autoignition if the energy is not removed from the system. Liquids with relatively low volatility are particularly susceptible to this problem. Liquids with high volatility are less susceptible to autoignition because they self-cool as a result of evaporation. 

Fig. 9. Counter-current self-cooled catalytic reactor for NHj synthesis.

In high-pressure circuits, centrifugal pumps are used to overcome the pressures encountered in the pipeline and in the system, such that at lower pressure heads they are usually employed as single-stage pumps (Figures 6 and 6.1). At fluid temperatures of up to 180°C, self-cooling motors ofinsulation class H are employed. 

Reaction 15 is endothermic and the electrolyzed water will undergo self cooling unless external heat is supplied. The enthalpy balance and its related thermoneutral... 

Conditions of qsoiar > t ph0t can be shown to place specific restrictions on the photoabsorber. When Hi20 < Etneut, heat must flow to compensate for the self cooling which occurs at the electrolysis rate. That is, for an enthalpy balanced system any additional required heat must flow in a flux equivalent to iieai = imo, and at an average power dieat, such that ... 

Somorjai and Lester [40] detail some of the problems likely to be encountered in vaporization measurements. These include (i) the effects of variations of with crystal surface, which are particularly important in the use of polycrystalline samples (ii) the self-cooling resulting from the endothermic vaporization, which may cause temperature gradients in the sample, especially at high fluxes and (iii) the complications caused by vapour-vapour collisions when measurements are made in a significant partial pressure of vapour. 

Several instances of compensation behaviour have been described, typically for endothermic, reversible decompositions. Kinetic characteristics are sensitive to reaction conditions, most notably the effects of the presence of the volatile product and heat flow to and within the reaction zones undergoing self-cooling. The following are examples of reactions studied in detail. 

The delay, before the onset of the main reaction may include contributions from (i) the time, required for the sample to attain reaction temperature, (ii) a further delay, resulting from changes within the sample, e.g. water removal (endothermic and therefore self-cooling) from a hydrate, phase transitions, etc. and (iii) the time, required to complete the reaction steps preceding establishment of the main reaction, which is to be regarded as the true induction period. The values of 4, and 4 may vary differently with temperature so that the temperature dependence of 4(=/h + 4 + 4) may be complex and may be very different from that of the main reaction. The magnitude of 4 can be measured from the intercept on the t-axis of a g( r)-time plot. 

More quantitative measurements of the systematic variations of dehydration rates with/7(H20), referred to as Smith-Topley behaviour, could lead to support for one or more of the several theoretical explanations that have been proposed [2,21,49,54,63] based on recrystallization of sohd product, local self-cooling and/or diffusion (effects expected to occur in all dehydration reactions) and adsorption of the volatile product. Dehydrations may also involve the intervention of a zeolitic residue and/or an amorphous phase, the formation and reciystallization of one or more lower hydrates as intermediates, and diffusive esc e of water through various channels of the barrier layer of product may be slow. 

Kinetic data measured for the decomposition of calcium carbonate under isothermal and under programmed-temperature conditions [11] and varied reaction environments influencing the ease of removal of the CO2 product, show that the apparent values of the kinetic parameters k, A and may be influenced by sample heating rate, reactant self-cooling, sample mass, geometry and particle size, which determine the rate because of the reversible nature of the decomposition [12]. These effects can lead to compensation behaviour [13]. 

MgCOj has the calcite structure [2]. Britton et al. [30] concluded that the decomposition of magnesite (MgCOj) was an inter ce process, initiated at boundary surfaces and thereafter advancing inwards. The value of E found (150 kJ mol between 813 and 873 K) was appreciably greater than the enthalpy of dissociation (101 kJ mol ). They considered the possible influences on the reaction rates of factors such as self-cooling, the recombination process at the reaction interface, the restriction of escape of carbon dioxide, and the rate of the nucleation step. 

Self-Cooling or Self-Heating During Reaction of a Solid.175... 

Reactant temperature within the reaction zone may be appreciably different from that measured for the controlled furnace reaction vessel because of local self-cooling or self-heating as a consequence of the reaction enthalpy. The significance of self-cooling in dehydrations has been discussed by Bertrand et al. (19). L vov et al. (20) have developed a computer model to represent the effect with reference to the endothermic dehydration of Li2S04 H20. Not all research reports discuss the possible consequences of reaction enthalpy in influencing reactant temperature. 

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