Micro heat-exchange time

The high heat transfer rates achievable in micro heat exchangers and reactors avoid unfavorable reaction conditions resulting from hot spots or thermal runaway effects. An optimum temperature or temperature profile for the reaction can be chosen with respect to spatial distribution and time. Thus, a fast-flowing fluid element can be cooled down or heated up very rapidly, in fractions of a millisecond. Because of the small thermal mass of microdevices, a periodic change of temperature of the reactor can be realized, with a typical time constant of some seconds. All these examples offer possibilities to improve yield and selectivity. 

For the design of micro heat exchangers, it has to be considered that both heat and mass transport time-scales are strongly correlated with the characteristic dimensions of the exchanger according to diffusion theory [8,9] ... 

In a few cases with high demands on mixing and heat transfer, typically for very fast reactions which are of the order of the mixing time (<1 s), customized integrated micromixer-micro heat exchanger systems were developed [2,4]. These systems are also demanded when dealing with explosive reactions where expansion of the irmer volume between the process units is forbidden. 

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. 

P 68] No detailed experimental protocol was given [61, 62,142,143]. Two reactant streams, the solution of the reactant in hexane and concentrated sulfuric acid, were fed separately in a specially designed micro reactor by pumping action. There, a bilayer was formed initially, potentially decomposed to a dispersion, and led to rapid mass transfer between the phases. From this point, temperature was controlled by counter-flow heat exchange between the reaction channel and surrounding heat-transfer channel. The reaction was typically carried out at temperatures from 0 to 50 °C and using residence times of only a few seconds. If needed, a delay loop of... 

Another interesting application of micro reactors is to use them as calorimeters. They may show excellent performance in terms of sensitivity [9-12]. Moreover, their performance in terms of heat exchange allows study of the kinetics of fast exothermal reactions under isothermal conditions. Such a development was realized by Schneider [13, 14], who studied such a reaction with a power of up to 160 kW kg-1. This type of calorimeter is simple to use and determines the reaction kinetics in a short time, with very small amounts of reaction mass, and without any hazard for the operator. 

Early work by Tonkovich et al. [46] dealt with a heat exchanger/reactor containing catalyst powder for the partial oxidation of methane for distributed hydrogen production. The intention was to run the reaction safely in a micro structured reactor owing to the short residence times applied and the improved heat removal avoiding hot-spot formation. 

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. 

Also, the number of micro cracks is ten times more significant in quartz vein than in the hydrothermalized facies and the main part of micro cracks appears during the first four hours of heat exchange (Figure 5). 

---