Micromixing molecular scale

Establishing the process sensitivity with respect to the above-mentioned factors is crucial for further scale-up considerations. If the sensitivity is low, a direct volume scale-up is allowed and the use of standard batch reactor configurations is permitted. However, many reactions are characterized by a large thermal effect and many molecules are very sensitive to process conditions on molecular scale (pH, temperature, concentrations, etc.). Such processes are much more difficult to scale up. Mixing can then become a very important factor influencing reactor performance for reactions where mixing times and reaction times are comparable, micromixing also becomes important. 

Molecular-scale micromixing models, conditional scalar Laplacian,... 

Bourne, J. R. (1983). Mixing on the molecular scale (micromixing). Chemical Engineering Science 38, 5-8. 

In contrast to this, when a solution of a substance is diluted, the molecules of the solute mix on a molecular scale with fresh solvent and essentially lose their identity in the process. This is called micromixing. In the same way solutions of two different solutes can undergo micromixing. 

As explained in Chapters 10 and 11, the existence of pressure fluctuation and the promotion of micromixing are the major features of liquid-continuous impinging streams. For processes occurring on the molecular scale in liquid or multiphase systems with a liquid as the continuous phase, these features have important implications. One of their valuable applications is the promotion of process kinetics. 

Micromixing phenomena are the processes whereby different chemical species which are supposed to mix and react are coming into contact at the molecular scale. The result of imperfect micromixing is local unhomogeneity of the reacting mixture and this causes differences in the conversion and yield of chemical reactions, especially when portions of the fluid having reacted at different instants are mixed together. Fast reactions, combustions, precipitations, polymerizations may particularly be affected by these phenomena. 

When local micromixing is slow compared to the reaction time scale and the macromixing time scale is smaller than the process time scale, the performance of a reactive flow process is controlled only by the micromixing. In such cases, though there is no macroscopic segregation, reactants are not mixed on a molecular scale (see the right bottom case of Fig. 5.5). Several micromixing models have been developed to simulate such reactive flow processes. Some of the widely used models are ... 

Instead, the Kolmogorov micro-scales (1.328) to (1.330) characterize the dissipation scales of turbulence and might indicate the effectiveness of micromixing in the flow provided that the turbulent energy dissipation rate and the kinematic viscosity of the fluid is known. If the Kolmogorov micro length scale is much larger than the molecular scales, the molecules are not efficiently mixed by turbulent diffusion. 

The local linear velocity of shrinkage is m, which depends upon time and position in the layers . Equation (10.16) is applied to the fine-scale vortices, because only there are the layers sufficiently thin for molecular diffusion to effect contact between A and B on the molecular scale (micromixing). The initial thickness of the layers is about A ( and the initial vortex diameter about 12Ak. Deformation is completed during the mean life-time of such a fine-scale vortex, T , where... 

These test reactions exhibit several suitable features, for example, known kinetics, partial segregation at molecular scale, a product distribution characterizing the degree of micromixing, simple chemical analysis, and many degrees of freedom for the experimentahst. During the experiments, the product distribution is determined by GC analysis of either the residual ester or of the ethanol formed. 

In this chapter, as in the previous one, we stress that the notion of mixing remains at the turbulent dispersion level. Mixing at the molecular scale, which enables chemical reactions to occur, is not necessarily carried out We are thus dealing with macromixing , as opposed to micromixing , where mixing at the molecular level is carried out Micromixing is discussed in Chapters 10-12. 

Now that we know how to estimate the size fx of the smallest scales of turbulence, it is simple to conclude by summing up the principles that govern mixing and chemical reactions in a flow with homogenous turbirlence. Table 11.1 shows, for a chemical reaction, for micromixing (molecular diffusion), and for macromixing (dispersion at different scales), the characteristic times and the scales with which these various processes are associated. For a stirred reactor, the phenomenon involves six parameters of different nature ... 

Micromixing is concerned with all the processes whereby parts of a fluid initially unmixed come into contact and mix on a molecular scale. However, this definition has to be specified. Let p(C) be the local concentration distribution of a given component in an imperfect mixture. The average concentration is... 

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