Heat Transfer with Reaction

When the heat of reaction is large, intrapellet temperature gradients ma have a larger effect on the rate per pellet than concentration gradient 

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Many operations in chemical engineering require the contact of two liquid phases between which mass and heat transfer with reaction occurs. Examples are hydrometallurgical solvent extraction, nitrations and halogenations of hydrocarbons, hydrodesulfurization of crude stocks, emulsion polymerizations, hydrocarbon fermentations for single-cell proteins, glycerolysis of fats, and phase-transfer catalytic reactions. A most common method of bringing about the contact of the two phases is to disperse droplets of one within the other by mechanical agitation. 

At first sight mathematically these problems are not completely new similar ones have been met in different fields and one can count one s weapons before attacking the formulation and add the necessary complements examples of diffusion of heat or matter (diffusion-absorptions) (diffusion-reactions) [51], heat transfer with reactions c r propagation of vivid heat ( chaleur vive ), diffusion of heat through media including... 

Applications One typical apphcation in heat transfer with batch operations is the heating of a reactor mix, maintaining temperature during a reaction period, and then cooling the products after the reaction is complete. This subsection is concerned with the heating and cooling of such systems in either unknown or specified periods. 

Some of the most common methods of heat transfer with catalytic reaction will be briefly reviewed. 

Optimized microfabrication and advanced assembly led to the use of thin platelets, in an original version 100 pm thick with a 80 pm micro channel depth, so that very thin walls (20 pm in the case sketched) remain for separating the fluids. Therefore, also the total inner reaction volume with respect to the total construction volume or the active internal surface area is very large. The latter surface amounts to 300 cm (for both the heat transfer and reaction sides) at a cubic volume of 1 cm. Indeed, the micro heat exchangers exhibited high heat transfer coefficients for gas [46] and liquid (Figure 3.10) [47, 48] flows. 

Our initial work on reaction thermal effects involved CFD simulations of fluid flow and heat transfer with temperature-dependent heat sinks inside spherical particles. These mimicked the heat effects caused by the endothermic steam reforming reaction. The steep activity profiles in the catalyst particles were approximated by a step change from full to zero activity at a point 5% of the sphere radius into the pellet. 

This is a unit where chemical reactions, and possibly heat transfer with the environment, take place. 

MODELLING OF SIMULTANEOUS MASS AND HEAT TRANSFER WITH CHEMICAL REACTION USING THE MAXWELL-STEFAN THEORY—I. MODEL DEVELOPMENT AND ISOTHERMAL STUDY... 

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 this thin-film machine, the small clearance between heated wall and rotor blade, together with the high peripheral blade velocity, results in high shear gradients, whereby the apparent viscosity in the film is considerably reduced. The resulting increased turbulence and better surface renewal improve heat transfer, increase reaction velocities, and aid the required forced product flow on the wall. On the basis of test... 

Figure 3.35 (See color insert following page 390.) X-ray CT imaging shows radial dissociation of a hydrate core. Image number 1 -8 (top number on each image) recorded over 0-245 min (bottom number on each image). The cell pressure was decreased from 4.65 to 3.0 MPa over 248 min. The hydrate core temperature decreased from 277 to 274 K with time, following the three-phase methane hydrate equilibrium line. (From Gupta, A., Methane Hydrate Dissociation Measurements andModeling The Role of Heat Transfer and Reaction Kinetics, Ph.D. Thesis Colorado School of Mines, Golden, CO (2007). With permission.)...

The heat balance on the bubble phase in Figure 4.24 that is assumed to be in plug flow mode and to have a negligible rate of reaction, as well as a negligible heat transfer with the heating/cooling coil, leads to... 

At such a scale and without stirring, the heat transfer with the surroundings, that is the jacket with hot water at 90 °C is very poor. Thus, adiabatic conditions should be assumed as a worst case approach. Under these circumstances, the heat released in the reaction mass serves to increase its temperature. Thus, we have to calculate the heat release rate. For a conversion of 1% per day, the heat release rate is... 

From a process simulation point of view, in addition to impingement mixing, there are two main problems (a) nonisothermal and transient flow with chemical reaction, prevalent during the filling stage of the process, and (b) conductive heat transfer with heat generation due to the polymerization reaction. We discuss these two problems next, using the case of... 

At the end of the filling stage, only heat transfer with chemical reaction occurs, which can be described by the following species and energy balance equations ... 

A continuation of this application of the second-law analyses is an examination of the various irreversibilities in the reformer process for potential improvements. The chief sources of thermodynamic irreversibilities (with the associated exergy destruction) are (1) frictional losses, (2) heat transfer with a finite temperature difference, (3) chemical reaction far from equilibrium, and (4) diffusion. 

The quantity (RA) can be further elaborated if interface mass and heat transfer coefficients are known, and with the theory of mass transfer with reaction inside porous catalysts as treated in Chapters 6 and 7 ... 

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