Heat Effects of Mixing Processes

It gives tile enthalpy change when pure species are mixed at constant T and P to form one mole (or a unit mass) of solution. Data are most commonly available for binary systems, for wliichEq. (12.39) solved for H becomes  

This equation provides for tire calculation of the enthalpies of binary mixtures from entlialpy data for pure species 1 and 2 and from tire heats of mixing. Treatment is here restricted to binary systems. 

Data for heats of mixing are usually available for a very limited nttmberof temperatures. If the heat capacities of tire pttre species and of the nrixture are known, heats of mixing are calculated for otlier temperatures by a metliod analogous to the calculation of startdard heats of reactionat elevated temperaturesfrom tire value at 298.15 K (25°C). 

When solids or gases are dissolved in hquids, the heat effect is called a heat of solution, and is based on the dissolntionof 1 mol ofsolute. If species I is the solnte, then jci is the moles of solnte per mole of solution. Since AH is the heat effect per mole of solntion, IxHjxi is the 

Solutionprocessesareconvenientlyrepresentedby physical-change equations analogous to chemical-reactionequations. When 1 mol of LiCl(s) is mixed with 12 mol of H2O, the process is represented by  

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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. 

Heat Requirement of the Process. Heat is required for vaporization in the extractive distillation column, and for the reconcentration of magnesium nitrate solution. Overall thermal effects caused by the magnesium nitrate cancel out, and the heat demand for the complete process depends on the amount of water being removed, the reflux ratio employed, and the terminal (condenser) conditions in distillation and evaporation. The composition and temperature of the mixed feed to the still influence the relative heat demands of the evaporation and distillation sections. For the concentration of 60 wt% HNO3 to 99.5 wt% HNO3 using a still reflux ratio of 3 1, a still pressure of 760 mm Hg, and an evaporator pressure of 100 mm Hg, the theoretical overall heat requirement is 1,034 kcal/kg HNO3. 

The microanalytical methods of differential thermal analysis, differential scanning calorimetry, accelerating rate calorimetry, and thermomechanical analysis provide important information about chemical kinetics and thermodynamics but do not provide information about large-scale effects. Although a number of techniques are available for kinetics and heat-of-reaction analysis, a major advantage to heat flow calorimetry is that it better simulates the effects of real process conditions, such as degree of mixing or heat transfer coefficients. 

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. 

When two or more reactants are brought together in a single phase, where they will react with each other, it is often not possible to separate the effects of mixing from chemical reaction, either in space or in time. In exceptional cases it may be possible to mix the reactants at a sufficiently low temperature, where no reaction takes place, and heat the mixture until the reaction starts. This may be done in pure batch reactors, but these are rarely applied in industry. In most technical processes chemical reactions are carried out in such a way that at least one of the reactants is introduced into a mixture where the reaction is already taking place. In these situations the rate of the chemical reaction is in principle dependent on the rate of mixing. There are two well known examples of single phase reactors where this applies ... 

Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

Iron Browns. Iron browns are often prepared by blending red, yellow, and black synthetic iron oxides to the desired shade. The most effective mixing can be achieved by blending iron oxide pastes, rather than dry powders. After mixing, the paste has to be dried at temperatures around 100°C, as higher temperatures might result in the decomposition of the temperature-sensitive iron yellows and blacks. Iron browns can also be prepared directiy by heating hydrated ferric oxides in the presence of phosphoric acid, or alkaU phosphates, under atmospheric or increased pressure. The products of precipitation processes, ie, the yellows, blacks, and browns, can also be calcined to reds and browns. 

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