Micro residence time

Jacobs, T., Kutzner, C., Kropp, M., Brokmann, G., Lang, W., Steinke, A., Kienle, A., and Hauptmann, P. (2009) Novel impedimetric and perforated thermal flow sensor for inline chemical process analysis in micro residence time reactors. Sensors, 2009 IEEE, pp. 719-722. 

Mixing Models. The assumption of perfect or micro-mixing is frequently made for continuous stirred tank reactors and the ensuing reactor model used for design and optimization studies. For well-agitated reactors with moderate reaction rates and for reaction media which are not too viscous, this model is often justified. Micro-mixed reactors are characterized by uniform concentrations throughout the reactor and an exponential residence time distribution function. 

Suppose is a function of a alone and that neither dSt Ajda nor d Alda change sign over the range of concentrations encountered in the reactor. Then, for a system having a fixed residence time distribution. Equations (15.48) and (15.49) provide absolute bounds on the conversion of component A, the conversion in a real system necessarily falling within the bounds. If d S A/dc > 0, conversion is maximized by maximum mixedness and minimized by complete segregation. If d 0i A/da < 0, the converse is true. If cf- A/da = 0, micro-mixing has no effect on conversion. 

Actually, various efforts have been made to develop the compact and efficient microchannel PrOx reactor for portable PEMFC applications. Goerke et al. [2] reported micro PrOx reactor employing stainless steel microchannel foil and Cu/Ce02 catalyst. They showed more than 99% CO conversion at less than 150 C and residence time of 14ms while CO selectivity was about 20%. Chen et al. [3] also developed microchannel reactor made of... 

The catalytic reaction was carried out at 270°C and 101.3 kPa in a stainless steel tubular fixed-bed reactor. The premixed reaction solution, with a molar ratio catechol. methanol water of 1 1 6, was fed into the reactor using a micro-feed pump. To change the residence time in the reactor, the catechol molar inlet flow (Fio) and the catalyst mass (met) were varied in the range 10 < Fio <10 mol-h and 2-10 < met < 310 kg. The products were condensed at the reactor outlet and collected for analysis. The products distribution was determined quantitatively by HPLC (column Nucleosil 5Ci8, flow rate, 1 ml-min, operating pressure, 18 MPa, mobile phase, CH3CN H2O =1 9 molar ratio). 

Typical properties (Table 1.2) of these micro reactors preserved the basic characteristics of the existing processes, reduced the reactor volume with even slightly increased output and distinctively reduced residence time, thereby having a very large specific output [50]. In addition, the quality of the final product was improved, a high process flexibility was achieved, less raw material and catalyst were needed, specific expenses were reduced, and gains in economy of the process were achieved. Fewer staff members were needed for process operation. 

One of the most far reaching analyzes along these lines of thought was given by Commenge [114] in the context of gas-phase reactions in continuous-flow processes. Specifically, he analyzed four different aspects of micro reaction devices, namely the expenditure in mechanical energy, the residence-time distribution, safety in operation, and the potential for size reduction when the efficiency is kept fixed. 

The figure shows the ratio of the widths of initially delta-like concentration tracers at the reactor exits as a function of the micro-channel Peclet number for different values of the porosity. Taking a value of = 0.4 as standard, it becomes apparent that the dispersion in the micro-channel reactor is smaller than that in the fixed-bed reactor in a Peclet number range from 3 to 100. Minimum dispersion is achieved at a Peclet number of about 14, where the tracer width in the micro-channel reactor is reduced by about 40% compared with its fixed-bed counterpart. Hence the conclusion may be drawn that micro-channel reactors bear the potential of a narrower residence time than fixed-bed reactors, where again it should be stressed that reactors with equivalent chemical conversion were chosen for the comparison. 

During the time the process gas spends in a micro channel, heat is transferred from the gas to the channel walls or vice versa. If in an exothermic reaction the time-scale for heat conduction in the channel walls is larger than the residence time of the fluid, considerable temperature gradients will build up along the walls... 

Mass transfer of a solute dissolved in a fluid is not only the fundamental mechanism of mixing processes, it also determines the residence-time distribution in micro fluidic systems. As mentioned in Section 1.4, in many applications it is desir-... 

Catalysts and their carriers are provided in micro channels by various means and in various geometric forms. In a simple variant, the catalyst itself constitutes the micro-reactor construction material without need for any carrier [2-A], In this case, however, the catalyst surface area equals that of the reactor wall and hence is comparatively low. Accordingly, applications are typically restricted to either fast reactions or processing at low flow rates for slow reactions (to enhance the residence time). 

Figure 3.12 Residence time distribution in a micro reactor which is tightened by different means. ( ) Glued reactor without catalyst coating (X) glued reactor with catalyst coating ( ) reactor with graphite joints. Calculated curves for tubular reactors with the Bodenstein number Bo = 33 (solid line) and Bo = 70 (dashed line).

This is the first reactor reported where the aim was to form micro-channel-like conduits not by employing microfabrication, but rather using the void space of structured packing from smart, precise-sized conventional materials such as filaments (Figure 3.25). In this way, a structured catalytic packing was made from filaments of 3-10 pm size [8]. The inner diameter of the void space between such filaments lies in the range of typical micro channels, so ensuring laminar flow, a narrow residence time distribution and efficient mass transfer. 

The precise and, where needed, short setting of the residence time allows one to process oxidations at the kinetic limits. The residence time distributions are identical within various parallel micro channels in an array, at least in an ideal case. A further aspect relates to the flow profile within one micro channel. So far, work has only been aimed at the interplay between axial and radial dispersion and its consequences on the flow profile, i.e. changing from parabolic to more plug type. This effect waits to be further exploited. 

GP 3] [R 3b] At similar conversion and residence time, smaller micro channels (80 pm) have better selectivity than larger ones (200 pm) with a V205/P205/Ti02 catalyst (0.4 vol.-% 1-butene in air 0.1 MPa 400 °C) [103], The residence time was varied in these investigations. 

GP 3] [R 3b] The maximum selectivity of about 33% was the same for the best micro reactor and the fixed bed (Figure 3.34) at the same conversions from 73 to 85% with a V205/P205/Ti02 catalyst [103]. At stiU higher conversion, the fixed bed has a better performance. However, the residence times needed for comparable conversion are one order of magnitude shorter than in the fixed-bed reactor. 

GP 4] [R 11] For methanol oxidation over sputtered silver catalyst, conversion is higher when using micro channels of smaller diameter (8.5 vol.-% methanol balance oxygen 510 °C 4—27 ms sHghfly > 1 atm) [72]. For two channels of the same width, but different depths (70 pm, 130 pm), concentration differences of nearly 10% at the same residence time were detected, all other parameters being equal. 

A comparison of experimental findings and theoretical predictions is given in [72], Although qualitatively consistent, experiments confirm a weaker dependence on parameters such as residence time and micro-channel diameter. 

Based on this assumption, the needed measures were evident to reduce the hot spots and, possibly concurrently, the operating temperature and, at best, to reduce the residence time substantially An ideal combination of all these parameters is provided by operation in the micro reactor. As a result, a much better performance of the micro reactor than the conventional laboratory reactors was foimd (see the previous section for more details) no hot spot was found. 

All conventional reactors, tested before using the micro reactor (simply since micro reactors were hardly available at that time), only fulfilled the demands of one measure, at the expense of the other measures. For instance, a single-tube reactor can be operated nearly isothermally, but the performance of the oxidative dehydrogenation suffers from a too long residence time. A short shell-and-tube reactor provides much shorter residence times at improved heat transfer, which however is still not as good as in the micro reactor. 

The reaction was of a scouting nature, actually one of the most common investigations done concerning gas-phase reactions in micro reactors. Hence it addresses in a general way the investigation of general micro reactor properties such as mass and heat transport and residence time. 

Figure 3.42 Evolution of a pulse at the entrance of a micro channel for different diffusion coefficients. Calculated concentration profile (left) and cumulative residence time distribution curve (channel 300 pm x 300 pm x 20 mm flow velocity 1 m s f = 10 s) [27],...

The equilibrium values are not reached at a rhodium catalyst on a micro structured reactor within the limits of the experimental conditions and the constructional constraints [3]. As possible explanations post-catalytic reactions at lower temperatures or, more likely, insufficient catalyst activity concerning the short residence times are seen. 

Residence time by changing micro channel length... 

GP20][R9] The residence time in the micro heattransfermodules amounts to 10ms itis3mswhenreferredtothemicrochannelsonly[29,30],Theresidence... 

This class of hybrid components comprises chip micro-reactor devices, as described in Section 4.1.3, connected to conventional tubing. This may be H PLC tubing which sometimes has as small internals as micro channels themselves. The main function of the tubing is to provide longer residence times. Sometimes, flow through the tube produces characteristic flow patterns such as in slug-flow tube reactors. Chip-tube micro reactors are typical examples of multi-scale architecture (assembly of components of hybrid origin). 

For the application referred to, the interdgital micro mixers were used on their own, without tubing attached, as reactors. Especially at low flow rates, the internal flow-through chamber acts as delay loop for providing a sufficient residence time. 

D microfabricated micro mixers (see Section 4.1.5) may be connected to tubes for reasons of residence time prolongation, similar to chip-tube micro reactors (see Section 4.1.4). The tube may also have the function of creating distinct hydro-dynamic features (see Section 4.1.4). 

Interdigital micro mixers comprise feed channel arrays which lead to an alternating arrangement of feed streams generating multi-lamellae flows [39 2]. If processes have to be carried out with extended residence times (> 1 s) and/or at a temperature level different from the mixing step, tubes have to be attached to the interdigital micro mixers. Their internals comprise millimeter dimensions or below, if necessary. 

Micro channel volume 1.3 pi Typical residence time 21 s... 

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