Heat transfer process, influencing

Heat transfer and mass transfer occur simultaneously whenever a transfer operation involves a change in phase or a chemical reaction. Of these two situations, only the first is considered herein because in reacting systems the complications of chemical reaction mechanisms and pathways are usually primary (see HeaT-EXCHANGETECHNOLOGy). Even in processes involving phase changes, design is frequendy based on the heat-transfer process alone mass transfer is presumed to add no compHcations. But in fact mass transfer effects do influence and can even limit the process rate. 

The rates at which chemical transformations take place are in some circumstances strongly influenced by mass and heat transfer processes (see Sections 12.3 to 12.5). In the design of heterogeneous catalytic reactors, it is essential to utilize a rate expression that takes into account the influence of physical transport processes on the rate at which reactants are converted to products. Smith (93) has popularized the use of the term global reaction rate to characterize the overall rate of transformation of reactants to... 

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

A homogeneous, constant-volume chemical reaction taking place in a tubular reactor is influenced by mass and heat transfer processes. The flow condition is described by... 

It is well known that a hot plate of metal will cool faster when placed in front of a fan than when exposed to still air. We say that the heat is convected away, and we call the process convection heat transfer. The term convection provides the reader with an intuitive notion concerning the heat-transfer process however, this intuitive notion must be expanded to enable one to arrive at anything like an adequate analytical treatment of the problem. For example, we know that the velocity at which the air blows over the hot plate obviously influences the heat-transfer rate. But does it influence the cooling in a linear way i.e., if the velocity is doubled, will the heat-transfer rate double We should suspect that the heat-transfer rate might be different if we cooled the plate with water instead of air, but, again, how much difference would there be These questions may be answered with the aid of some rather basic analyses presented in later chapters. For now, we sketch the physical mechan-... 

A problem which may be easily solved with the network method is that of two flat surfaces exchanging heat with one other but connected by a third surface which does not exchange heat, i.e., one which is perfectly insulated. This third surface nevertheless influences the heat-transfer process because it absorbs and re-radiates energy to the other two surfaces which exchange heat. The network for this system is shown in Fig. 8-28. Notice that node is not connected to a radiation surface resistance because surface 3 does not exchange energy. Notice also that the values for the space resistances have been written... 

In Chapter 1.3 we defined a detonation as the propagation of a chemical reaction through an energetic material under the influence of a shock-wave at speeds faster than the speed of sound in the material. The velocity at which the energetic material decomposes is therefore only dependent on the velocity of the shock-wave. It is not determined by a heat-transfer process as it is the case for deflagration or combustion. 

The fin efficiency is always less than one. It depends on both the conduction processes in the fin and the convective heat transfer, which influence each other. Therefore, the geometry of the fin, the thermal conductivity Af, and the heat transfer coefficient a play a role in the calculation of the fin efficiency. This will... 

Proper selection of an HTF is crucial to process engineers for a variety of reasons. Different properties of HTFs will impact the design, size, and, ultimately, the cost of the system. Following are some examples of how HTFs can influence a heat transfer process ... 

The presence of temperature gradients in a multicomponent system introduces an additional complication in the analysis of the mass transfer process such gradients influence the values of physical, thermodynamic, and transport properties, such as the diffusion coefficients. These property variations may be taken care of by introducing temperature dependent property functions or by using average values of the properties (as is done here). The consequence of this simplification is that the basic mass transfer analysis remains essentially unchanged from those in Chapters 8-10 and we need only consider the effect of mass transfer on the heat transfer process. 

The term e/(ee — 1), which appears in equations 1 and 2, was first developed to account for the sensible heat transferred by the diffusing vapor (1). The quantity 8 represents the group M4-C 4 / hg, the ratio of total transported energy to convective heat transfer. Thus it may be thought of as the fractional influence of mass transfer on the heat-transfer process. The last term of equation 3 is the latent heat contributed to the gas phase by the fog formation. The vapor loss from the gas phase through both surface and gas-phase condensation can be related to the partial pressure of the condensing vapor by using Dalton s law and a differential material balance. 

Physical transport processes can play an especially important role in heterogeneous catalysis. Besides film diffusion on the gas/liquid boundary there can also be diffison of the reactants (products) through a boundary layer to (from) the external surface of the solid material and additionally diffusion of them through the porous interior to from the active catalyst sites. Heat and mass transfer processes influence the observed catalytic rates. For instance, as discussed previously the intrinsic rates of catalytic processes follow the Arrhenius... 

In the framework of process development it is often only possible to determine the macrokinetics (chemical kinetics superimposed by external mass- and heat-transfer processes). In this case it must be ensured that the laboratory reactor is hydrodyna-mically similar to the later industrial reactor (especially the length to diameter ratio), so that the transport influences are approximately the same in both. This is particularly easy to achieve in the case of tube-bundle reactors, as are often used for partial oxidations (e.g., production of phthalic acid, acrylic acid, and ethylene oxide). Here the macrokinetics can be determined in a single tube, since the subsequent hydrodynamic conditions are identical (scale-up factor = 1). 

A prerequisite for the design and operation of ehemical reactors is knowledge of the dependence of the reaction rate r on the process parameters. It has proved useful to make a distinction between micro- and maerokinetics. Whereas the true reaction rate (mierokmetics) depends only on the concentration of the reactants, the temperature, and the catalyst, the maerokinetics in industrial systems are additionally influenced by mass- and heat-transfer processes in the reactor. 

Mixing flows in an extruder influence the heat transfer process, and conversely, the flow of a viscous fluid generates heat according to Eq. (5.8). The resulting rise in the melt temperature reduces the apparent melt viscosity... 

With active systems, the controls are mostly automatic, heat flows are well contained, and heat transfer processes are accrrrately predictable. With passive systems, heat flows are more diffuse and complex interchanges occttr, which can be grossly influenced by user interference. Often the appropriate adjustment of controls (e.g., of shading devices or of Trombe-wall vents) by the user at the right time is reqtrired to achieve optimttm performance of the system. Both the competence and the attitude of the user can drastically influence the resrrlts. 

The first step in heterogeneous catalytic processes is the transfer of the reactant from the bulk phase to the external surface of the catalyst pellet. If a nonporous catalyst is used, only external mass and heat transfer can influence the effective rate of reaction. The same situation will occur for very fast reactions, where the reactants are completely exhausted at the external catalyst surface. As no internal mass and heat transfer resistances are considered, the overall catalyst effectiveness factor corresponds to the external effectiveness factor,... 

An increase of the circnlar flow rate (w = const = 130 smVs) in diffuser-confusor devices leads to an increase of the heat transfer coefficient (Figure 2.44). The influence of the u> parameter on the efficiency of convective heat exchange is levelled out in a cylindrical device. As heat transfer coefficients are determined hy the smallest of the heat emission coefficients, there is an opportnnity of heat exchange intensification in a diffuser-confusor reactor by the increase of the cooling agent flow rate. This opportunity is another tool to control heat transfer processes. 

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