Latent heat of reaction

The form of the curve in Eig. 5.12, as well as the position and height of maximum depends, to a certain extent, on the value of p and d which display the contribution of the latent heat of reaction and the Joule heat, but the qualitative form of the diagram remains unchanged. The region of stationary stabihty is limited by the curve and the x-axis, whereas the shaded part of this region corresponds to an absolutely stable defect, where the condition of initial stability Eq. (5.64) holds... 

The heat balance of a reacdor is made up of three terms Heat of reaction -I- Heat transfer = Gain of sensible and latent heats by the mixture. This estabhshes the temperature as a function of the composition... 

The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water, and tap water at 10°C can - and usually does - contain 0.0030 wt% of air. In order to follow what this air does when we make an ice cube, we need to look at the phase diagram for the HjO-air system (Fig. 4.9). As we cool our liquid solution of water -i- air the first change takes place at about -0.002°C when the composition line hits the liquidus line. At this temperature ice crystals will begin to form and, as the temperature is lowered still further, they will grow. By the time we reach the eutectic three-phase horizontal at -0.0024°C we will have 20 wt% ice (called primary ice) in our two-phase mixture, leaving 80 wt% liquid (Fig. 4.9). This liquid will contain the maximum possible amount of dissolved air (0.0038 wt%). As latent heat of freezing is removed at -0.0024°C the three-phase eutectic reaction of... 

Condensation is the process of reduction of matter into a denser form, as in the liquefaction of vapor or steam. Condensation is the result of the reduction of temperature by the removal of the latent heat of evaporation. The removal of heat shrinks the volume of the vapor and decreases the velocity of, and the distance between, molecules. The process can also be thought of as a reaction involving the union of atoms in molecules. The process often leads to the elimination of a simple molecule to form a new and more complex compound. 

The integral terms representing AH and AH can be computed if molal heat capacity data Cp(T) are available for each of the reactants (i) and products (j). When phase transitions occur between T and Tj for any of the species, proper accounting must be made by including the appropriate latent heats of phase transformations for those species in the evaluation of AHj, and AH terms. In the absence of phase changes, let Cp(T) = a + bT + cT describe the variation of (cal/g-mole °K) with absolute temperature T (°K). Assuming that constants a, b, and c are known for each species involved in the reaction, we can write... 

The difference between the two heats of reaction is the latent heat of the water formed. 

Heat of reaction and latent heat, taken to be values at datum temperature of 298 K. There is no need to convert to kJ/kmol, providing quantities are expressed in kg. For the purposes of this example the dissociation of C02 and H20 at 1500 K is ignored. 

FOR EACH REACTION OR PHASE CHANGE INPUT THE HEAT OF REACTION OR THE LATENT HEAT, kJ/kmol AND QUANTITY INVOLVED kmol/h REMEMBER HEAT ENVOLVED POSITIVE HEAT ABSORBED NEGATIVE... 

A phenomenon that arises particularly with continuous stirred reactors is the occurrence of more than one steady state. This becomes apparent from the heat and material balances. "Heat generation" is made up of the heat of reaction plus any heat transfer, and the "heat removal" is the sensible and latent heat change of the reaction products. In problem P4.10.13, for instance, both the heat generation and the heat removal are plotted against the temperature. The two lines intersect at three points which represent the steady states. A point at which the slope of the heat generation line is... 

The variation of efficiencies is due to interaction phenomena caused by the simultaneous diffusional transport of several components. From a fundamental point of view one should therefore take these interaction phenomena explicitly into account in the description of the elementary processes (i.e. mass and heat transfer with chemical reaction). In literature this approach has been used within the non-equilibrium stage model (Sivasubramanian and Boston, 1990). Sawistowski (1983) and Sawistowski and Pilavakis (1979) have developed a model describing reactive distillation in a packed column. Their model incorporates a simple representation of the prevailing mass and heat transfer processes supplemented with a rate equation for chemical reaction, allowing chemical enhancement of mass transfer. They assumed elementary reaction kinetics, equal binary diffusion coefficients and equal molar latent heat of evaporation for each component. 

In combustion of liquid oils such as heavy diesel fuels, the fuel is sprayed through nozzles into air. After ignition, a flame forms around the evaporating drops, driven by the vaporization of the drop in the hot boundary layer. The latent heat of vaporization strongly affects the temperature of the drop so that the heat release by the reaction is compensated by the heat absorbed in vaporizing the fuel and the heat lost to the cooler gas in the air. The composition profiles we might expect are shown in Figure 12-11, but the temperature will also vary around the particle. 

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