Heat effects of mixing

All of the important heat effects are illnstratedby this relatively simple chemical-manu-factnring process. In contrast to sensible heat effects, which are characterized by temperatnre changes, the heat effects of chemical reaction, phase transition, and the formationand separation of solntions are determined from experimental measnrements made at constant temperatnre. In this chapter we apply thermodynamics to the evalnation of most of the heat effects that accompany physical and chemical operations. However, the heat effects of mixing processes, which depend on the thermodynamic properties of mixtnres, are treated in Chap. 12. 

For amorphous separation systems with a UCST and L( ST, heat effects of mixing (dilution) are, as a rule, of an endothermic (Prigogine and Defay, 1954 Tager, 1978) and... 

The evaluation of the P terms involves the heat of vaporizing 1 mol of the component from the mixture. At moderate pressure this is equal to the latent heat of vaporization at the same temperature plus the heat effects of mixing the liquids at this temperature and bringing them to the total pressure. As an approximation the latent heat of vaporization of the pure liquids can be employed in Eq. (3-26), but it will not give satisfactory results if (1) the heat of mixing is large or (2)... 

Equation 7.27 says that the partial molar enthalpy of a species in an ideal solution is equal to the pure species molar enthalpy, or that a molar enthalpy-mol fraction diagram must have the form sketched in Figure 7.5. This may also be stated that there is no heat effect of mixing for any ideal solution. 

The enthalpy of a vapor mixture is obtained first, from zero-pressure heat capacities of the pure components and second, from corrections for the effects of mixing and pressure. 

Biyikli, S., and Chen, J. C., Effect of Mixed Particle Sizes on Local Heat Transfer Coefficients Around a Horizontal Tube in Fluidized Beds, 7th Int. Heat Transfer Conf, Munich, Germany (1982)... 

In contrast to the self-stability of hydrodynamic lubrication, the heating effects in mixed lubrication tend to destabilize the film. An increase in temperature reduces the viscosity, which in turn reduces the film thickness, leading to increased probability of intermittent contact. This further increases friction and the temperature rises even more. 

In the foregoing examples of reactions that occur at approximately 1 bar, we have tacitly assumed that the heat effects of reaction are the same whether gases are mixed or pure, an acceptable procedure for low pressures. For reactions at elevated pressures, this may not be the case, and it may be necessary to account for the effects of pressure and of mixing on the heat of reaction. However, these effects are usually small. 

The heat-transfer rate is found to be substantially higher under conditions of agitation. The heat transfer is usually said to occur by combined conductive and convective modes. A discussion and explanation are given by Holt [Chem. Eng., 69(1), 110 (1962)]. Prediction of [/ by Eq. (11-48) can be accomplished by replacing a by a, the effective thermal diffusivity of the bed. To date so httle work has been performed in evaluating the effect of mixing parameters that few predictions can be made. However, for agitated liquid-phase devices Eq. (18-19) is applicable. Holt (loc. cit.) shows that this equation can be converted for solids heat transfer to yield... 

Although normally straightforward for bulk process streams, it is important to select materials for skin rather than bulk temperatures in heat transfer equipment, and to evaluate the effects of mixing exotherms on local bulk fluid temperatures, such as that may occur in acid addition/dilution equipment. 

The statistical description of multiphase flow is developed based on the Boltzmann theory of gases [37, 121, 93, 11, 94, 58, 61]. The fundamental variable is the particle distribution function with an appropriate choice of internal coordinates relevant for the particular problem in question. Most of the multiphase flow modeling work performed so far has focused on isothermal, non-reactive mono-disperse mixtures. However, in chemical reactor engineering the industrial interest lies in multiphase systems that include multiple particle t3q)es and reactive flow mixtures, with their associated effects of mixing, segregation and heat transfer. 

Adsorption Work. In the adsorption work, glass beads were used as a blank to determine whether the heat of dilution of the surfactant or changes of flow pattern lead to a thermal effect. At both 25° and 30° C, the heat effect of flowing an increasing concentration from 0.1 to 2% in five steps was not detectable. Since the amount of adsorption to solid glass beads is small, it can be concluded that there is either very little mixing of the two solutions as the interface between them moves through the cell, or if there... 

To account for the effect of mixing between the jet and ambient crossflow and to obtain meaningful results for different applications, the local convective heat transfer coefficient is defined by... 

The recording of temperature requires a calibrated sensor but otherwise is relatively straightforward. There are only two aspects that require care. The first is the location of the sensor. This has to be such that the measured temperature is representative of the temperature at which the reaction is taking place. A misplaced sensor could read a temperature distorted by, for example, the proximity of heat transfer surfaces or by the effects of mixing. The second is that the measured temperature must remain constant throughout the duration of observations and throughout the volume of the reactor. These latter aspects can be distorted by improper temperature control or by inadequate attention to heat losses and/or inputs in various parts of a reactor. Multiple temperature sensors may have to be located throughout the reactive volume to make sure that these possibilities have been eliminated. 

The resistance rdc is determined by the effects of mixing forced by buoyant convection (as a result of surface heating of the ground and/or lower canopy) and by penetration of winds into canopies on the sides of hills. The resistance (insm-1) is estimated from... 

The sum of the enthalpies of the nitrating acids was —830,220 Btu. The heat evolution due to changes in acid concentration during the nitrating reaction, including any thermal effect of mixing of the acids, is, therefore, 1,870,859 minus 830,220, or 1,040,639 Btu,... 

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