Mass selective intensification

The analysis of experimental data revealed a correlation between the hydrodynamic mode of a tubular turbulent device and the interphase tension in the flow of the two-phase liquid-gas reaction system (Figure 2.52). This correlation confirms that the addition of surfactants is a reasonable solution for a reaction system with an interphase boundary. It leads to a decrease of bubble size and mass exchange intensification in the gas-liquid flow of fast chemical processes. In addition, the liquid-phase longitudinal mixing rate increases and the hydrodynamic mode of a process approaches perfect mixing conditions. Fast chemical processes, in two-phase systems, require consideration of the selective adsorption of feedstock reactants and reaction products on to the interphase boundary, and a change of the hydrodynamic motion structure of the continuous phase. A change in the work required to form the new surface is a typical phenomenon for all types of multiphase systems and depends on... 

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. 

The reactions are still most often carried out in batch and semi-batch reactors, which implies that time-dependent, dynamic models are required to obtain a realistic description of the process. Diffusion and reaction in porous catalyst layers play a central role. The ultimate goal of the modehng based on the principles of chemical reaction engineering is the intensification of the process by maximizing the yields and selectivities of the desired products and optimizing the conditions for mass transfer. 

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. 

Prefer because of intensification with 1.4 to 4 times the surface/volume. Possible to install in pipelines. Use when mass transfer affects selectivity or reactivity. Perhaps not for highly exothermic... 

In the example of the dehydrogenation reaction above, the mass flows are assumed invariant, their composition is not disturbed. Pressure and temperature are dictated by the thermodynamics of the system to attain a certain conversion of the feed. An alternative form of PI (process intensification) can be seen when one selectively removes one of the reaction products to shift the equilibrium and intensify the process. The combination of reaction and separation is a key example of PI. The literature aboimds with schemes to accomplish this. Its commercial use, however, is limited to a small number of cases. Following are examples, successful and not so successful, of this mode of operation ... 

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. 

Santos A, Bahamonde A, Schmid M, Avila P, Garcia-Ochoa F. Mass transfer influences on the design of selective catalytic reduction (SCR) monolithic reactors. Chemical Engineering and Processing Process Intensification 1998 37 117-124. 

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. 

Gas-liquid-solid processes are possible by using either waD-coated catalyst or mini-packed beds. Numerous applications have demonstrated process intensification in terms of selectivity, space-time yield, and safety by use of gas-liquid microreactors, including fluorinations, chlorination, hydrogenations, sulfonations, photooxidations, and so on. This is achieved by enhanced mass transfer via the interface and through formation of thin liquid layers, which also give better transfer and allow conducting photochemical reactions more efficiently. Operation is now possible in new process windows with very aggressive reactants such as elemental fluorine or even under explosive conditions. 

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