Flow pattern and break-up of liquid film in single-channel falling film microreactors

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Abstract

This paper concerns an experimental investigation into the flow pattern transition and break-up mechanism of liquid film in single-channel falling film microreactors. Three major flow patterns were observed to be ‘corner rivulet flow’, ‘falling film flow with dry patches’, and ‘complete falling film flow’. The critical flow rate associated with the transition between each flow pattern was determined. Hysteresis was found, as the critical flow rate was higher when the flow pattern shifted from ‘falling film flow with dry patches’ to ‘complete falling film flow’ than it was when the flow pattern shifted in the opposite direction. There existed a minimum wetting flow rate (MWF) in order to the complete falling film flow pattern to be present. MWF was observed to increase with the width or depth of microchannel and could not be well predicted by traditional falling film correlations. Based on the obtained data, an empirical correlation has been proposed for the prediction of MWF in falling film microreactors, where the influence of fluid physical properties and channel dimension is revealed.

Introduction

Falling film microreactors have potential applications for chemical processes due to high mass and heat transfer efficiency in a very thin liquid film. For example, falling film microreactors have been demonstrated to benefit a variety of gas–liquid operations including direct fluorination [1], catalytic hydrogenation [2], photochemical chlorination [3], and ozonolysis reaction [4]. It is well known that the completeness and fluctuation characteristics of the liquid film will have a significant influence on its heat and mass transfer performance [5], [6], [7]. To further improve the transfer rate and conversion of reactant in the liquid film, one possible way is to reduce the film thickness by decreasing the liquid flow rate. However, the complete falling film in the microchannel will breakup if the film thickness is sufficiently thin, which is obviously disadvantageous to the heat and mass transfer process [8]. To improve the reaction efficiency and optimize the operational condition, it is thus necessary to investigate liquid film break-up mechanism in microchannels.

Flow patterns in falling film microreactors have been shown to be somewhat different from their macroscale counterparts. In a traditional large-sized falling film reactor, the flow pattern usually displays as finger flow, or falling film with dry patches [9]. But due to the micron-sized geometry, the flow pattern in falling film microreactors turns to be corner rivulet flow, falling film flow with dry patches, and complete falling film flow [8]. The complete falling film flow is the most favorable flow pattern for chemical reaction, which is only present when liquid flow rate is kept above a certain rate (defined as the minimum wetting flow rate, MWF). In traditional falling film reactors, there are two available criterions to predict MWF, as suggested by Hartely and Murgatroyd [10]. The first one is the force balance criterion, which is that the surface tension force balances the fluid pressure on the top point of a stable dry patch, and then MWF and the minimum liquid film thickness can be obtained according to the Nusselt falling film velocity distribution. The second one is the minimum total energy criterion stated as the film will breakup if the total energy in a given streamwise length of a liquid rivulet is minimum. Based on these two criteria, some researchers have proposed their empirical correlations (cf. Table 1). Nevertheless, the validity of these correlations in falling film microreactors still remains unclear. This work presents an experimental observation of the flow pattern and break-up mechanism of the liquid film in a set of single-channel falling film microreactors (SFFMR) by using a high-speed CCD camera. The minimum wetting flow rate in microchannels was measured and compared with the prediction of the traditional falling film correlations. It has been found that MWF tended to increase with the width or depth of microchannel and could not be well predicted by the existing correlations. Therefore, new empirical correlations have been proposed for the prediction of MWF in falling film microreactors based on the experimental data, where the influence of fluid physical properties and channel dimension is revealed to a large extent.

Section snippets

Falling film microreactor design

Five single-channel falling film microreactors (denoted as SFFMR I–V, respectively) with different dimensions were milled on the substrate of polymethyl methacrylate (PMMA) using a precision machine (KPC-30a). The reactor configuration and their geometry dimension were shown in Fig. 1 and Table 2, respectively. The inlet of SFFMR was formed by sticking a transparent adhesive tape onto the plate (the shadow area shown in Fig. 1). Before each run, the microreactor was cleaned by ultrasonic method

Flow pattern and wetting behavior in microchannels

Flow patterns observed in the microchannel of SFFMRs were similar to those we have observed previously in multi-channel falling film microreactors [8]. They were identified as ‘corner rivulet flow’, ‘falling film flow with dry patches’ and ‘complete falling film flow’, as shown in Fig. 3. For ‘corner rivulet flow’, liquid flows as two wedges in the corners with dry strip in the middle of the microchannel, and the length of the dry strip is almost the same as the microchannel. As the second flow

Conclusions

This paper reports an experimental investigation of flow pattern and liquid film break-up in single-channel falling film microreactors by using different fluids including deionized water, 110 ppm SLS, 200 ppm SLS, 12–50 wt% EG.

Three flow patterns were observed as ‘corner rivulet flow’, ‘falling film with dry patches’, and ‘complete falling film flow’. The transition flow rate between each flow pattern was determined, where it was found that DWCF was about 8–14 times higher than WDCF due to the

Acknowledgements

This work has been supported financially by National Natural Science Foundation of China (nos. 20676129 and 20911130358), the Ministry of Science and Technology of China (no. 2009CB219903), Fund of Dalian Institute of Chemical Physics, CAS (no. K2009D01).

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