Influence of Hydrodynamic Conditions on Micromixing in Microreactors with Free Impinging Jets

Rufat Sh Abiev,Alexey A Sirotkin

doi:10.3390/fluids5040179

Abstract

An experimental study and mathematical modeling of micromixing in a microreactor with free impinging jets (MRFIJ) with a diameter of 1 mm was carried out. In the experimental part, the iodide-iodate technique was used (involving parallel competing Villermaux–Dushman reactions with the formation of I3−). Theoretical assessment revealed that more than 50% of the introduced energy is dissipated in the jets collision region. Through the use of differentiated sampling, an uneven quality distribution of micro mixing in the central and peripheral zones of the reactor was found: at moderate flow rates (700–1000 mL/min, jets velocity of 15–21 m/s) the micromixing in the central part of reactor is up to 12 times better than that in the periphery. Furthermore, the weight fraction of the probes in the central zones of MRFIJ is reduced with increasing jet velocity; this effect is attributed to a more intense formation of ligaments and droplets upon collision of jets and their secondary mixing on the walls of the apparatus. In terms of the weighted average concentration, the best quality of micromixing in the samples is achieved at a flow rate of 300 mL/min. With an increase in the flow rate (and velocity) of the jets, the dependence of the I3− concentration on the flow rate has a nonmonotonic character, which is explained by a change in the nature of the flow in the collision zone of the jets: the transition from the formation of a liquid sheet to the intensive formation of ligaments and drops and secondary mixing of the liquid film formed on the walls of the reactor. The effect of “freshness” of solutions on the concentration of reaction products was studied.

Highlights

The development of methods for the intensification of various processes of chemical technologies (Process Intensification) is one of the key directions in the development of modern chemistry, both in large-scale industries, and in “fine”chemistry—this includes fine organic synthesis and production of nanosized materials [1].Miniaturization of chemical engineering equipment allows for a qualitative leap in the space-time scales of objects in which the transformation of matter takes place [2]
The weight fraction of the probes in the central zones of microreactor with free impinging jets (MRFIJ) is reduced with increasing jet velocity; this effect is attributed to a more intense formation of ligaments and droplets upon collision of jets and their secondary mixing on the walls of the apparatus
With an increase in the flow rate of the jets, the dependence of the I3 − concentration on the flow rate has a nonmonotonic character, which is explained by a change in the nature of the flow in the collision zone of the jets: the transition from the formation of a liquid sheet to the intensive formation of ligaments and drops and secondary mixing of the liquid film formed on the walls of the reactor

Summary

Introduction

The development of methods for the intensification of various processes of chemical technologies (Process Intensification) is one of the key directions in the development of modern chemistry, both in large-scale industries (typical examples are oil refining, petrochemistry, and metallurgy), and in “fine”chemistry—this includes fine organic synthesis and production of nanosized materials [1].Miniaturization of chemical engineering equipment allows for a qualitative leap in the space-time scales of objects in which the transformation of matter takes place [2]. As a result of the extremely high density of the kinetic energy of the flow, dissipated in extremely small volumes of mixed solutions (on the order of microliters or even nanoliters), an unprecedented high quality of micromixing is achieved [3,4] This leads to the production of particles (crystallite) with a size of 20–50 nm, and in some cases (when several products may be formed) to the production of a pure product in accordance with the stoichiometry employed (for example, formation of a pure bismuth orthoferrite BiFeO3 in a microreactor synthesis without the formation of mullite Bi2 Fe4 O9 or sillenite Bi25 FeO39 ; the latter two products are formed during the hydrothermal synthesis [3,4]). It should be noted that the level of energy consumption in microreactors does not exceed units or tens of watts.

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