Residence time distribution microreactor

An important advantage of the use of EOF to pump liquids in a micro-channel network is that the velocity over the microchannel cross section is constant, in contrast to pressure-driven (Poisseuille) flow, which exhibits a parabolic velocity profile. EOF-based microreactors therefore are nearly ideal plug-flow reactors, with corresponding narrow residence time distribution, which improves reaction selectivity. 

Fig. 3.6 Construction of a microreactor support for extreme control of residence time distribution. (Adapted from Refs. [46, 47. )...

Polymerizations as part of liquid-phase organic reactions are also influenced by mass and heat transfer and residence time distribuhon [37, 48]. This was first shown with largely heat-releasing radical polymerizations such as for butyl acrylate (evident already at dilute concentration) [49]. Here, a clear influence of microreactor operation on the polydispersity index was determined. Issues of mass transfer and residence time distribution in particular come into play when the soluhon becomes much more viscous during the reachon. Polymerizahons change viscosities by orders of magnitude when carried out at high concentration or even in the bulk. The heat released is then even more of an issue, since tremendous hot spots may arise locally and lead to thermal runaway, known in polymer science as the Norrish-Tromsdorff effect. 

Wibel, W, Wenka, A., Brandner, J.J., and Dittmeyer, R. (2013) Measuring and modeling the residence time distribution of gas flows in multichannel microreactors. Chem. Eng. 215-216, 449 -460. 

The drawback of randomly packed microreactors is the high pressure drop. In multitubular micro fixed beds, each channel must be packed identically or supplementary flow resistances must be introduced to avoid flow maldistribution between the channels, which leads to a broad residence time distribution in the reactor system. Initial developments led to structured catalytic micro-beds based on fibrous materials [8-10]. This concept is based on a structured catalytic bed arranged with parallel filaments giving identical flow characteristics to multichannel microreactors. The channels formed by filaments have an equivalent hydraulic diameter in the range of a few microns ensuring laminar flow and short diffusion times in the radial direction [10]. 

Tonomura O, Noda M, Kano M and Hasebe S (2004a), Optimal design approach for microreactors with uniform residence time distribution ,The 10th Asian Pacific Confederation of Chemical Engineering (APCChE),4B-07,Kitakyushu, Japan, Oct. 17-21. 

J.M. Commenge, T. Obein, G. Genin, X. Framboisier, S. Rode, V. Schanen, R. Pitiot M. Madosz, Gas-phase residence time distribution in a falling-film microreactor, Chem. Eng. Sci., 2006, 61, 597-604. 

The influence of residence time distribution (RTD) on performance, selectivity and yield is the same in microreactors as in conventional reactors. Therefore, the eflfects are well understood. Nonetheless, the demonstration of this at the microscale has hardly been reported so far. However, some experimental techniques have been developed to measure RTDs in microchannel flows which allow comparison between different types of flows or flows run at different parameters so that at least optimal flow conditions with regard to RTD can be found. 

Although the flow in the microchannels is laminar, a uniform radial concentration profile and consequently a narrow residence time distribution were obtained. Depending on the method used for manufacturing the microchannels, the Boden-stein number was found to be Bo = ud/Dj 70 and consequently the microreactor behaves almost like a plug-flow reactor. The catalytic coating had no influence on this distribution, indicating a uniform deposition of the catalyst within the microchannels (see Figure 14.3). 

Figure 14.3 Residence time distribution in different microreactors , glued reactor without coating X, glued reactorwith coating , reactor with graphite joints. Theoretical RTD curves for tubular reactors with (solid line) Bo = 33 and (dotted line)...

K. Golbig, A. Kursawe, M. Hohmaim, S. Taghavi-Moghadam, T. Schwalbe, Designing microreactors in chemical synthesis - residence-time distribution of microchannel devices. Chem. Eng. Commun., 2005, 192, 620-629. 

S. Lohse, B. T. Kohnen, D. Janasek, P. S. Dittrich, J. Franzke, D. W. Agar, A novel method for determining residence time distribution in intricately structured microreactors. Lab Chip, 2008, 8,431-438. 

Residence time distribution can be an important issue in the selection process. Microreactors usually operate at Reynolds numbers lower than 200. In this regime, laminar flow prevails and mass transfer is dominated by molecular diffusion. An injected substance in the channel will dissipate caused by the flow profile in the channel. Hence the input signal will be broadened until it reaches the exit of the channel (Figure 3.2). The extent of such a distribution depends on the channel design. In microchannels the mixing process can then be described by the Fourier number (no axial diffusion, dominating radial diffusion D ). A high Fourier Po number leads to a narrow residence time distribution ... 

To combine the advantages of packed-bed and catalytic wall microreactors, catalytic bed microreactors were proposed recently. In this novel reactor design, the catalyst is applied on metallic filaments or wires which are incorporated in a microreactor, leading to a low pressure drop and a nanow residence time distribution [87-89]. By insertion of metallic wires a uniform gas distribution and a reduced risk of temperature gradients is obtained. However, similarly to catalytic wall microreactors, an increase in the specific surface area of the grid or wire is required. In addition to metallic wires and grids, modified ceramic tapes can also be used as a catalyst support [90]. 

This criterion is important to consider with respect to the kinetics of the desired reaction. Slow reactions require longer residence times in the reactor and not every reactor provides the same contact time of the gaseous educts with the catalyst Packed-bed microreactors offer the longest residence times, since the gaseous molecules have to pass through a region of closely packed catalyst particles. Unfortunately, this also negatively effects the residence time distribution. 

It is known that microchannel reactors, due to their small dimension and well defined structure have many advantages compared to conventional fixed bed reactors. The main ones are an efficient temperature control and well defined flow patterns. As the channel diameters are in the order of micrometers, microreactors operate under laminar flow conditions resulting in a parabolic velocity profile. But, due to the short radial diffiision times the radial concentration profile is flat, resulting in a narrow residence time distribution of the reactant. The latter characteristic is of crucial importance in the actual sbufy. Only reactors with an uniform residence time can be used to get meaningful kinetics information under periodic operation at short cycle periods [9]. 

The catalytic dehydration of isopropanol was studied under tiansirat conditimis in a catalytic microreactor. The reaction is characterised by educt inhibition and shows a pronounced stop-effect . Therefore, the average productivity under forced poiodic operation can be considerably higher compared to the maximal productivity obtainable at steady state. For high rates of the sorption processes and surface reactions involved, the timal cycle time for the forced concentration variations lies in the order of seconds. As microreactors are characterized by low mass storage capacity and narrow residence time distribution, they are particularly suitable for periodic operation at relatively high fiequencies. Tis could be demonstrated in the present study. 

Figure 4.10..80 Typical residence time distribution in a microreactor (with a liquid phase) and a laminar tube without influence of molecular diffusion. Comparison with Figure 4.10.54 shows that the Bo number for the microreactor is >80 and, thus, we have almost plug flow. Adapted from Emig and Klemm (2005).

Because the droplets are formed directly in the microchannel, very small droplet microreactors with sizes in the range Ipl to 100 nl can be formed in an in-channel continuous platform The confined environment removes the residence time distribution due to the nonuniform... 

For heterogeneous catalysts, tandem reactor technology also relies on the fact that each polymer particle is in fact a microreactor operated in semibatch mode, into which monomers and chain-transfer agents are fed continually, while the polymer formed never leaves the microreactor. In this way, polymer populations with different average properties are produced in each reactor and accumulate in the polymer particle microreactor, as illustrated in Figure 8.37. In theory, an optimal balance does exist between the fractions of these different populations to meet certain performance criteria. This creates a truly fascinating reactor and product design problem because the fractions of the different polymer populations per particle will be a function of the residence time distribution in the individual reactors in the reactor train. 

A polyacrylate mesh microreactor was also used for analysis of residence time distribution and phase holdup [61,62]. Existence of hysteresis due to wetting characteristic of the mesh is observed and the liquid operating range was hmited by the area of individual mesh types [61]. Uniform flow and residence time distributions are achievable through an analytical resistance network model [62]. 

What are the reasons that microreactors in many cases produce better results than conventional reactors In order to provide an optimal progress of a chemical reaction, different conditions must be fulfilled in the reactor First, a nearly ideal mixing of the reactants should be ensured, linked with the generation of an extended phase interface in multiphase reactions. Afterward, the required response time must be guaranteed by a residence time with preferentially narrow residence time distribution. Finally, the reactor heat necessary for the reaction must be supplied or carried off. In this connection, control of temperature, pressure, time of reaction, and flow velocity in reactors with small volume is carried out much... 

Miniaturized packed-bed microreactors follow the paths of classical engineering by enabling trickle-bed or packed bubble column operation. The hydrodynamic characteristics of three-phase reactors, such as pressure drop and residence time distribution, can be determined similar to those for fluid-solid and fluid-fluid reactors. 

Microreactors create well-defined multiphase environments for the nanomaterial synthesis. The nanocrystal size and size distribution depend on the residence time and residence distribution. The axial dispersion effects assodated with the parabolic flow profile of typical microchannel contribute to a broader residence time distribution. A segmented-flow microreactor or droplet reactor offers an opportunity to solve this dispersion issue. Both gas-liquid and liquid-liquid segmented-flow microreactors have been reported [98]. Khan et al. [50] compared... 

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