Micro-channels characteristic dimension

On shrinking the size of micro-channel reactors by reducing the channel dimensions, a number of characteristic quantities such as pressure drop and the degree of chemical conversion are affected. In order to permit a meaningful comparison of the reactor geometry with a scaled geometry, it is important to keep one or a few... 

In some cases, a simple increase in characteristic dimensions is sufficient to suppress micro-channel plugging [9, 10]. In the first euphoria about the new technical capabilities, miniaturization of processing equipment was possibly flogged to death. The motto therefore is as small as (beneficially) needed, but not as small as possible. 

One of the most interesting theorems of Worz et al. is that they see a serious potential for micro reactors to permit small-scale production of some different sort [110-112]. Micro channels serve as an ultra-precise measuring tool, whereas production is done in channels about 10 to 100 times larger, i.e. miUimeter-sized channels. The limit of tube diameter of industrial production reactors is reported to be 2 cm hence any new reactor of smaller characteristic dimensions bears some potential for improvement. Worz et al. conclude with the remark that the above strategy could be the most important result of their studies [110-112]. 

Compared with laboratory fixed-bed reactors or conventional extruded monoliths, such a microstructured monolith is smaller in characteristic dimensions, lower in pressure loss by optimized fluid guiding and constructed from the catalytic material solely [3]. The latter aspect also leads to enhanced heat distribution within the micro channels, giving more uniform temperature profiles. 

Figure 1.135 Characteristic dimensions of a microfluidic mixing element with Y-type contactor attached to a zig-zag channel w, width of the micro channel s, linear length of the periodic step L, linear length of the zig-zag micro channel [59] (by courtesy of ACS).

An emerging class of emulsification methods is not based on imposing an overall flow field, but rather by making individual droplets on the mouths of membrane pores or micro-engineered channels. Characteristic is that the flow fields applied are much milder energy consumption is much lower, while the droplet sizes are strongly dependent on the shape and dimensions of the pores or micro-channels. A number of processes belong to this class ... 

The choice of the characteristic dimension can be nevertheless limited by other factors such as allowable pressure drop, occurrence of clogging/fouling, deviations from uniform flow distribution to individual channels, and maximization of productivity metrics based on product flow rates. For certain applications (e.g., synthesis of pharmaceuticals), larger channel diameters are also favored, since this facilitates cleaning. Beretta et al. [If] compare micropacked beds and annular microchannel reactors for intrinsic kinetic measurements at high temperature and high space velocities. The lower pressure drop and better temperature control in the coated micro-channel favored the use of the structured reactor. 

Table 1.6 Characteristic quantities to be considered for micro-reactor dimensioning and layout. Steps 1, 2, and 3 correspond to the dimensioning of the channel diameter, channel length and channel walls, respectively. Symbols appearing in these expressions not previously defined are the effective axial diffusion coefficient D, the density thermal conductivity specific heat Cp and total cross-sectional area S, of the wall material, the total process gas mass flow m, and the reactant concentration Cg [114].

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