Heat flux by chemical reaction

Fig. 5.25 Temperature gradient, conductive heat flux, convective heat flux, and heat flux by chemical reaction as a function of distance from the burning surface at 3 MPa (initial temperatures 243 K and 343 K) for BAMO/ THF = 60 40 copolymer.

The parameter j3 represents the maximum temperature difference that could exist in the particle relative to the temperature at the exterior surface. For if it is recognised that, in the steady state, the heat flux within an elementary thickness of the particle is balanced by the heat generated by chemical reaction then... 

For an adiabatic coating (no heat flux at r = t see Figure 8.1), the following expressions relate mass/heat interphase transfer with the observed consumption of reactant and heat generation by chemical reaction ... 

Many practical mass transfer problems involve the diffusion of a species through a plane-parallel medium that does not involve any homogeneous chemical reactions under one-dimensional steady conditions. Such mass transfer problems are analogous to the steady one-dimensioiial heat conduction problems in a plane wall with no heal generation and can be analyzed similarly. In fact, many of the relations developed in Chapter 3 can be used for mass transfer by replacing temperature by mass (or molar) fraction, thermal conductivity by pD g (or CD ), and heat flux by mass (or molar) flux (Table 14-8). 

The condensed phase density p, specific heat C, thermal conductivity A c, and radiation absorption coefficient Ka are assumed to be constant. The species-A equation includes only advective transport and depletion of species-A (generation of species-B) by chemical reaction. The species-B balance equation is redundant in this binary system since the total mass equation, m = constant, has been included the mass fraction of B is 1-T. The energy equation includes advective transport, thermal diffusion, chemical reaction, and in-depth absorption of radiation. Species diffusion d Y/cbfl term) and mass/energy transport by turbulence or multi-phase advection (bubbling) which might potentially be important in a sufficiently thick liquid layer are neglected. The radiant flux term qr... 

Hyder et al. (2006) studied the surface and bulk properties of hydrophilic PVA membranes subjected to PV. The PVA membranes were cross-linked in two ways by heating at 125°C or by chemical reaction with glutaraldehyde at room temperature. These membranes were used for the dehydration of EtOH-water mixtures over a range of EtOH concentrations (10%-70%) in feed solution and at varied temperatures (from 25°C to 50°C). The PV results showed that the thermally cross-linked membrane was more hydrophilic than the chemically cross-linked membranes, and this helped transport water at a higher flux through the membrane. However, the selectivity of the thermally cross-linked membrane was lower and water flux through the membrane became higher when compared with the chemically cross-linked membranes. The dehydration results were correlated with the results of the physiochemical measurements of the membranes. 

The continuity equation (6.1 or 6.7) and momentum equations in x direction (6.40), y direction (6.41), and z direction (6.42) are mechanical equations and are not affected by chemical reactions. The energy equation for chemically nonreacting flows with included heat flux due to thermal conduction and thermal radiation is usually sufficient. But in chemically reacting flows, as common in chemical engineering applications, the energy equation with the heat flux due to additional diffusion of species has to be included. 

Deposit control is important because porous deposits, under the influence of heat flux, can induce the development of high concentrations of boiler water solutes far above their normally beneficial bulk values with correspondingly increased corrosion rates. This becomes an increasingly important feature with increase in boiler saturation temperature. In addition, deposits can cause overheating owing to loss of heat transfer. Finally, carryover of boiler water solutes, which can be either mechanical or chemical, can lead to consequential corrosion in the circuit, either on-load or off-load. Material so transported can result in corrosion reactions far from its point of origin, with costly penalties. It is therefore preferably dealt with by a policy of prevention rather than cure. 

DISCUSSION AND CONCLUSIONS In this study a general applicable model has been developed which can predict mass and heat transfer fluxes through a vapour/gas-liquid interface in case a chemical reaction occurs in the liquid phase. In this model the Maxwell-Stefan theory has been used to describe the transport of mass and heat. A film model has been adopted which postulates the existence of a well-mixed bulk and stagnant zones where the principal mass and heat transfer resistances are situated. Due to the mathematical complexity the equations have been solved numerically by a finite-difference technique. In this paper (Part I) the Maxwell-Stefan theory has been compared with the classical theory due to Pick for isothermal absorption of a pure gas A in a solvent containing component B. Component A is allowed to react by a unimolecular chemical reaction or by a bimolecular chemical reaction with... 

The heat transfer in the gas phase and in the condensed phase can be viewed schematically as shown in Fig. 3.11. The heat flux transferred from the high-temperature zone, i. e., the flame zone, to the condensed phase through the burning surface is determined by the sum of the heat produced by the conductive heat d/dx(kdT/dx), by the convective heat - prc dT/dx, and by the chemical reaction (x>Q. 

The heat flux feedback from the gas phase to the burning surface is also determined by the chemical reaction rate in the gas phase. The reachon rate in the gas phase is altered by the addihon of catalysts. The catalysts act either on the decomposition reaction of the condensed phase or on the reaction in the gas phase of the gaseous decomposihon products. There are two types of catalysts posihve catalysts that increase the burning rate and negative catalysts that decrease the burning rate. 

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