Micro homogeneous reaction

For fast reactions Da becomes large. Based on that assumption and standard correlations for mass transfer inside the micro channels, both the model for the micro-channel reactor and the model for the fixed bed can be reformulated in terms of pseudo-homogeneous reaction kinetics. Finally, the concentration profile along the axial direction can be obtained as the solution of an ordinary differential equation. 

In the following, the impact of the micro-channel diameter on the temperature rise due an exothermic gas-phase reaction is investigated. For simplicity, a homogeneous reaction A —> B of order n with kinetic constant k is considered. Inside the micro channel, the time evolution of the radially averaged species concentration c and temperature T is governed by the equations... 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Thus there is an essential difference between classical homogeneous reactions in organic chemistry and reactions such as those in which catenanes and knots are formed. In the latter, there are heterogeneities on the micro scale. Thus supramolecular chemistry lies also in the border area between classical organic chemistry and surface chemistry. 

Alternatively, carbon dioxide was added as an inhibitor to avoid the homogeneous reactions. This problem could likely be obviated when applying micro structures, as they act as flame arrestors (see Section 2.5.1). 

A variety of other continuous flow systems has also been proposed, for example, a microwave-transparent PFA (polyfluoroalkyl Teflon) coil in a micro-wave oven (Cablewski et al., 1994), and a more recent one suitable for homogeneous reactions and also for heterogeneous catalytic reactions (Chemat et al., 1995, 1998). 

Micro-batch reactor for catching intermediates and monitoring kinetics of rapid and exothermic homogeneous reactions. Chem. Eng. /, 214, 149-156. 

Since 2004, many articles on preparation of zeolite MMs have been published, on such areas as MFI or Sil-1 zeolite etched on the Si substrate for gas separation applications, and MMRs for KCR and fine chemical synthesis (Coronas and Santamaria, 2004 Kwan et al, 2010 Wan et al., 2001 Yeung et al, 2005). Coronas and Santamaria (2004) have reported on the use of zeolite films and interfaces in micro-scale and portable applications, including the removal of volatile organic compounds from indoor air, recovery of catalysts in homogeneous reactions, zeolitic microreactors and microseparators, for example. Moreover, zeolite coated microreactors and microseparators exhibit high surface-to-volume ratio, and are capable of high productivity as a result of the good contact between reactants and catalyst wall. 

Several heterogeneous reactions, especially performed in the gas phase with risk of explosion or thermal runaway were also safely performed by micro process technology. But this is not discussed here, since the chapter deals only with homogeneous reactions. 

A detailed description of AA, BB, CC step-growth copolymerization with phase separation is an involved task. Generally, the system we are attempting to model is a polymerization which proceeds homogeneously until some critical point when phase separation occurs into what we will call hard and soft domains. Each chemical species present is assumed to distribute itself between the two phases at the instant of phase separation as dictated by equilibrium thermodynamics. The polymerization proceeds now in the separate domains, perhaps at differen-rates. The monomers continue to distribute themselves between the phases, according to thermodynamic dictates, insofar as the time scales of diffusion and reaction will allow. Newly-formed polymer goes to one or the other phase, also dictated by the thermodynamic preference of its built-in chain micro — architecture. 

Another exception to the known mechanisms of conventional chemistry may arise when dominance of surface reactions is achieved in micro reactors. This holds for all catalytic reactions on solid contacts. Beyond that, it was shown that some formerly homogeneous bulk reactions may become heterogeneous when carried out in a micro reactor owing to the very large surface-to-volume ratio [155,171,172],... 

GP 11] ]R 19] The suppression of explosive homogeneous gas-phase reactions is not due simply to thermal quenching as a result of the heat losses from a micro reactor, but rather to radical quenching [9]. The micro reactor will therefore be safe even when heat losses from the reaction micro channel are reduced by design modifications. 

The performance of the Sonogashira reaction is claimed to be the first example of a homogeneously metal-catalyzed reaction conducted in a micro reactor [120], Since the reaction involves multi-phase postprocessing which is needed for the separation of products and catalysts, continuous recycling technology is of interest for an efficient production process. Micro flow systems with micro mixers are one way to realize such processing. 

Micro reaction systems may help to overcome or at least reduce some of the above-mentioned limitations [69]. Electrochemical micro reactors with miniature flow cells where electrodes approach to micrometer distances should have much improved field homogeneity. As a second result of confined space processing, the addition of a conducting salt may be substantially reduced. In addition, benefits from a uniform flow distribution and efficient heat transfer may be utilized. 

The homogeneously catalyzed oxidation of butyraldehyde to butyric acid is a well-characterized gas/Hquid reaction for which kinetic data are available. It thus serves as a model reaction to evaluate mass transfer and reactor performance in general for new gas/liquid micro reactors to be tested. This reaction was particularly used to validate a reactor model for a micro reactor [9, 10]. 

The E/Z ratio of the isomers formed over a supported Pd catalyst differed from that obtained with the homogeneous counterpart. The activity of the catalyst under the action of microwave and conventional conditions was comparable, but micro-wave irradiation improved yields and reduced reaction times. 

The high surface-to-volume ratio can also significantly improve both thermal and mass transfer conditions within micro-channels in two ways firstly, the convective heat and mass transfers, which take place at the multi-phase interface, are improved via a significant increase in heat and mass transfer area per unit volume. Secondly, heat and mass transfers within a small volume of fluid take a relatively short time to occur, enabling a thermally and diffusively homogeneous state to be reached quickly. The improvement in heat and mass transfer can certainly influence overall reaction rates and, in some cases, product selectivity. Perhaps one of the more profound effects of the efficient heat and mass transfer property of micro-reactors is the ability to carry potentially explosive or highly exothermic reactions in a safe way, due to the relatively small thermal mass and rapid dissipation of heat. 

---