Enhancement of liquid–liquid micromixing performance in curved capillary microreactor by generation of Dean vortices

Abstract

Micromixing efficiency is an important parameter for evaluating the multiphase mass transfer performance and reaction efficiency of microreactors. In this work, the novel curved capillary reactor with different shapes was designed to generate Dean flow, which was used to enhance the liquid–liquid micromixing performance. The Villermaux–Dushman probe reaction was employed to characterize the micromixing performance in different curved capillary microreactors. The effects of experiment parameters such as liquid flow rate, inner diameter, tube length, and curve diameter on micromixing performance were systematically investigated. Under the optimal conditions, the minimum value of the segmentation factor XS was 0.008. It was worth noting that at the low Reynolds number (Re < 30), the change of curved shape on the capillary microreactor can significantly improve the micromixing performance with XS reduced by 37.5%. Further, the correlations of segment index XS with dimensionless factor such as Reynolds number or Dean number were developed, which can be used to predict the liquid–liquid micromixing performance in capillary microreactors.

1. Introduction

The common microreactor system in the laboratory, including a premixer and a straight capillary tube, is used to improve micromixing and liquid–liquid mass transfer efficiency [1–3]. The premixer is employed to enhance the premixing degree of different reactants, which determines the initial mixing performance between reactants [4]. However, when the reactants flow into the straight tube, the micromixing performance of premixed reactants decreases due to molecular diffusion, especially at low Reynolds numbers [5]. Strengthening the micromixing process of reactants in a capillary reactor is crucial for improving reaction efficiency.

In the past two decades, effectively enhancing the micromixing process between fluids in a microreactor system has been regarded as a great challenge and has attracted widespread attention. At present, there are several methods to improve micromixing performance, including changes in fluid flow patterns [6,7], injection of an immiscible fluid to enhance disturbance, and the static mixing process [8,9]. For example, introducing a gas phase or an immiscible fluid into liquid streams to form a segmented flow can significantly reduce the mixing length, but it requires the subsequent separation process and would reduce the liquid residence time [10]. Another effective strategy is creating different patterns on the walls of fluoropolymer tubes to improve the cross-streamwise mixing process [11,12]. However, the patterns created with complex structures are dependent on expensive processing equipment, leading to an increase in construction costs and time. Therefore, it is very important to develop an efficient and easy strategy to enhance liquid–liquid micromixing performance.

At present, the curved tube reactor, due to its advantages of simple process and low manufacturing cost, has been widely applied to intensify multiphase mixing, heat transfer and mass transfer processes [13,14]. Due to its advantages in fast mass and heat transfer processes, the curved tube reactor is ubiquitous in various fields such as gas absorption [15], polymerization [16], and the synthesis of nanomaterials [17]. In the curved tube, for the effects of centrifugal force and pressure on the fluid, the Dean vortices are generated in the flowing fluid, greatly improving the multiphase mixing process [18,19]. Zhu et al. [20] have demonstrated that the introduction of Dean vortices in curved tubes can significantly enhance the mixing process through experiments and computational fluid dynamics simulation methods. In the curved tube, as the curved diameter decreased, the radial mixing intensity was greater than that of the straight tube. In the same way, Vanka et al. [21] have implemented a numerical study to determine the mixing process rate in a curved square channel at low Reynolds numbers. The research results also indicated that fluid mixing efficiency in a curved channel was far more efficient than in a straight channel. Furthermore, Mandal et al. [22] investigated the mixing performances of different reactors by numerical simulation, including a novel coiled flow inverter.