Microstructured reactors chemical reaction time

The characteristic features of microsystems stem from the small size of the space in the microstructures. Therefore, microsystems are not necessarily small systems in total size. They can be large in total size as long as they contain microstructures that can be used for chemical reactions. This sharply contrasts with the concept of a lab-on-a-chip, which should be small in total size. It is also important to note that microsystems are normally set up as flow-type reactors with a constant flow of solutions through a microstructured reaction chamber or channel. Although the reactor s capacity at any one time is small, total production capacity over time is much greater than may be imagined. Therefore, microflow systems are not necessarily used solely to produce small quantities of chemical substances. In fact, a microfluidic device has been developed that fits in the palm of the hand but can produce several tons of a product per year (see Chapter 10). 

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. 

The results confirm the beneficial use of segmented flow for realizing plug flow behavior in microstructured reactors. Narrow RTD can be obtained even for short residence times, allowing high performance and product yields for fast chemical reactions. 

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

The possibility of easily cleaning a microstructured device is essential for any development use on the laboratory scale. During the process development of chemical reactions, it is necessary to go to extreme conditions, at which dissociation or polymerization of educts and/or products takes place. Often this is connected with fouling in the reactor, which sometimes cannot be easily cleaned by rinsing. Then it is most important to have an easy, time-saving way to open the reactor and to clean the structure. 

As the specific surface area (Equation 2.24) and the heat transfer coefficient (Equation 2.21) increase with decreasing diameters, it follows that microstructured channel reactors are characterized by very short cooling times, thus improving temperature control and reducing the risk of reactor runaway. As a consequence, microstructured reactors can be operated under harsh reaction conditions such as high temperature and pressure. The chemical kinetics is speeded up drastically reducing the characteristic reaction time. This concept is often called novel process windows [20]. 

The economic benefit is one of the dominant problems if a micro structured reactor plant is used for chemical production. Without any doubt, an overall flow rate through a micro structured device can be achieved that is comparable to that with a conventional batch process. However, the residence time is very short because of the dimensions of a microstructured device. If the reaction kinetics are slow, an additional device is necessary to increase a dwell time. Hence, much effort should be devoted to increasing the reaction rate instead of transferring the standard protocol to a micro structured reactor [13]. 

---