Heat selective intensification

Selective intensification of heat transfer with respect to reaction. The scale dependence of heat transfer and reactions being different, it is possible to accelerate heat transfer selectively compared with reactions. As a result, more homogeneous temperature conditions or even isothermal conditions can be reached. That enables one not only to control temperature conditions for determination of kinetic parameters, but also to control fast exothermic reactions and prevent thermal runaway. 

Selective intensification of mass transfer with respect to reaction. With a similar scale dependence to heat transfer, one can preferentially improve the selectivity of competing reactions, in either single- or multi-phase systems. For reasons of readability, the mixing times are not discussed in this chapter but would enter this category. 

There is no doubt that the ultimate development of process intensification leads to the novel field of microreaction technology (Figure 1) (7-9). Because of the small characteristic dimensions of microreaction devices, mass and heat transfer processes can be strongly enhanced, and, consequently, initial and boundary conditions as well as residence times can be precisely adjusted for optimizing yield and selectivity. Microreaction devices are evidently superior, due to their short response time, which simplifies the control of operation. In connection with the extremely small material holdup, nearly inherently safe plant concepts can be realized. Moreover, microreaction technology offers access to advanced approaches in plant design, like the concept of numbering-up instead of scale-up and, in particular, the possibility to utilize novel process routes not accessible with macroscopic devices. 

Process intensification is achieved by the superimposition of two or more processing fields (such as various types of flow, centrifugal, sonic, and electric fields), by operating at ultrahigh processing conditions (such as deformation rate and pressure), a combination of the two, or by providing selectivity or extended interfacial area or a capacity for transfer processes. In heat and mass transfer operations, drastic reduction in diffusion/conduction path results in equally impressive transfer rates. As the processing volume (such as reactor... 

In micro-process technology, micro-structured process components such as heat exchangers, mixers or reactors are being developed in which very intensive heat and mass transfer can be realized. In many cases, under defined conditions, this allows process intensification with drastically reduced residence times for the reacting components and simultaneously a considerable increase in selectivity and yield. Due to the low degree of hold-up, hazardous components can be handled safely, even under extreme pressure and temperature conditions. 

Intensification of catalytic processes involves innovative engineering of the MSR and the simultaneous development of the catalytically active material. The catalyst design should be closely integrated with the reactor design. Intrinsic reaction kinetics, mass and heat transfer, and energy supply or removal must all be considered to obtain a high selectivity and yield of the target product. 

The selection of a reactor, from those described above as an alternative to the STR, depends on the heat reaction, the nature of the phases involved and the rate of production. The safety, reliability, energy consumption and cost have to be considered. The volumetric heat- and mass-transfer coefficients can be viewed as measures of process intensification. The attainable coefficients in the reactors dictate the yield, selectivity and size reduction. 

To avoid mass and heat transfer resistances in practice, the characteristic transfer time should be roughly 1 order of magnitude smaller compared to the characteristic reaction time. As the mass and heat transfer performance in microstructured reactors (MSR) is up to 2 orders of magnitude higher compared to conventional tubular reactors, the reactor performance can be considerably increased leading to the desired intensification of the process. In addition, consecutive reactions can be efficiently suppressed because of a strict control of residence time and narrow residence time distribution (discussed in Chapter 3). Elimination of transport resistances allows the reaction to achieve its chemical potential in the optimal temperature and concentration window. Therefore, fast reactions carried out in MSR show higher product selectivity and yield. 

Future Trends in Reactor Technology The technical reactors introduced here so far are those used today in common industrial processes. Of course, research and development activities in past decades have led to new reactor concepts that may have advantages with respect to process intensification, higher selectivities, and safety and environmental aspects. Such novel developments in catalytic reactor technology are, for example, monolithic reactors for multiphase reactions, microreactors to improve mass and heat transfer, membrane reactors to overcome thermodynamic and kinetic constraints, or multifunctional reactors combining a chemical reaction with heat transfer or with the separation in one instead of two units. It is beyond the scope of this textbook to cover all the details of these new fascinating reactor concepts, but for those who are interested in a brief outline we summarize important aspects in Section 4.10.8. 

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