Micro-mixing and chemical reaction

As regards scale-up it must therefore be noted that mixing efficiency in small devices is more favorable than in large ones. This must also be taken into account in baffled tanks, although here fx.y.z/w is constant, but the circulation and macromixing times are longer This relationship wiU be considered further in connection with micro-mixing and chemical reactions (see [462] and Section 1.4.6.4). 

The volumetric mass transfer coefficient may be considered as the reciprocal value of a micro-mixing time. This coefficient can be used in combined models describing micro-mixing and chemical reaction, which will be treated in section 5.2.3. 

One might also try to establish a model describing micro-mixing and chemical reaction at the same time. This method is discussed in section 5.2. 

Bourne and coworkers [21-23] have developed the mathematics and applied their model to predict the effects of stoichiometric ratio, startup of a semi-batch reactor, effect of volumetric feed ratio, batch vs. continuous operation, etc. They have also experimentally demonstrated the use of reaction systems with well-characterized kinetics to determine the level of micro-mixing. Thus chemical reactions can be considered as molecular probes to be used to study segregation. Other... 

Dispersion, the tendency for ordered molecules to decrease gradients and local concentration, is caused by both molecular diffusion and nonuniform bulk liquid motion. High dispersion rates may be advantageous for mixing and chemical reactions, but are undesirable in separation and purification applications. For separations, minimizing dispersion improves resolution and sensitivity [3] and yields improved dynamics for concentration and purification [4]. As a consequence, the physical processes that lead to dispersion have been a subject of intense interest for more than a century. In recent years, the development of the concept of the micro-total analysis system (p,TAS) or labs on a chip has motivated further exploration of dispersion in microchannel flows. 

For single phase systems, the effects of micro and macro mixing cannot always be determined unambiguously. In section 5.2 the combined effects of mixing and chemical reaction will be dealt with. 

A detailed characterization of micro mixing and reaction performance (combined mixing and heat transfer) for various small-scale compact heat exchanger chemical reactors has been reported [27]. The superior performance, i.e. the process intensification, of these devices is evidenced and the devices themselves are benchmarked to each other. 

Various operations in the field of chemical engineering and in combustion can be characterized by the simultaneous interaction of two processes, namely the transfer of heat and mass and chemical reaction. However, the determination of mean reaction rates in turbulent flows requires detailed knowledge about fluctuations of scalar quantities such as species concentrations and enthalpy. Due to the non-linear character of chemical reaction the calculation of mean reaction rates based on mean values of temperature and species concentrations is only possible in special circumstances, such as practically infinitely fast micro-mixing-rates or very small fluctuations of the scalar variable around its mean value. 

With regard to applications, these include mixing, aero- and hydrodynamics, heat dissipation and chemical reactions. One could use the features to, for example, change the location of transition from laminar to turbulent flow, or change the drag characteristics of a surface. Curved protrusions could be used to create swirl, and the holes could function as micro-injectors - even for chemicals - or to increase cooling. Surfl-sculpt could also function as a mechanical interlock. Shape memory alloys could also be improved in their functionality. Many materials could be processed in this manner metals, polymers, ceramics and glass are all feasible. The time to process 5 cm of material is a few seconds, and the equipment needed includes an electron beam machine and a vacuum chamber. 

Within the chemical industry, micro-organisms and enzymes are often used as catalysts. It is possible for a unit operation in an essentially chemical production process to be a biochemically catalysed step giving rise to a mixed chemical/biochemical production process. The products of these reactions include organic chemicals, solvents, polymers, pharmaceuticals, and purfumes. Mixed chemical/biochemical production processes are continuously innovated and optimised, mainly for economical reasons. 

The methods of mechanical micromachining and micro EDM have been extensively applied to the fabrication of components such as micro heat exchangers, mixers, and reaction channels as well as chemical microsystems with integrated heat exchange, reaction, mixing, and distribution elements (Figure 10). 

Figure 1.100 Characterizing mixing by diffusion of a dye in an aqueous solution (left) and by a chemical reaction yielding a colored product (right) in the slit-shaped interidigital micro mixer viewing direction is the flow direction [37] (by courtesy ofAIChE).

A schematic diagram of the demonstrator chemical analysis system is given in Fig. 24. The MCB comprises three in/outlets, two micro-pumps, two flow sensors and an optical absorption detector module. The purpose is to measure chemical reaction products by detection of the (spectral) absorption intensity. Sample and reagent liquids are mixed in the appropriate amounts on-board (currently the actual mixing takes place during the propagation in channels) and the optical absorption is measured at the detector side. 

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