(249a) Millisecond Mixing of Two Liquid Sheets in a Jet Nozzle

The chemical processes with fast parallel-competing or consecutive-competing reactions are often encountered in industry, such as the manufacture of fine chemicals, pharmaceuticals, polymers and so on. For the intermediate product is always desired, stringent demand is imposed on the mixer (or reactor) design to facilitate millisecond physical mixing of the liquid reactants at molecular scale followed by a plug flow of the bulk fluid. The time scale and the non-uniformity in space of the mixing process can lead to significant variance in reaction progress and accordingly the selectivity. Traditionally, such processes have relied on stirred tank reactors. However, a particularly effective mixing of liquids can be hardly achieved in milliseconds in stirred tanks, especially being harder as the vessel gets larger. Meanwhile, the mixing and reactions are also uncertain in scaling from the laboratory to full scale. In recent years, increasing interest is to apply process design through use of small flow channels to reduce the dimension towards the scale of micro-mixing, leading to process intensification, simplified scaling and hence shorter development times. Micron-sized flow channels could be an alternative in such applications, but they are subject to the difficulty for the mass production of certain chemicals. The so-called numbering-up rule for scale-up is in fact not easy to be implemented in practice.

This work aims to develop a novel design of jet nozzle as the mixer/reactor, which facilitates the fast mixing and reactions of the liquid reactants in millimeter channels. The basic idea is to intensify the process by reducing the distance for species mixing together with the help of strong turbulent energy dissipation, which is estimated as 100 to 1000 times larger than a stirred tank. Since the liquid streams contact each other at high velocity, the volume of the mixer becomes very small. For example, such a jet nozzle mixer or reactor just has an outside diameter of 200 to 300 mm for an annual production of a liquid product at ~100,000 tons. The approach to scaling up the jet nozzle is simply to ensure the local mixing behavior controlled by the thickness of liquid sheets, momentum ratio, cross-flow angle between the two streams.

As the channels are quite narrow, the Reynolds numbers are not high so that the bulk flow is not far from the transition of laminar flow to turbulence. This makes the flow and mixing behavior sensitive to the flow conditions. In some cases, periodic wavy movement of species vortex was observed in the experiments. For reliable predictions of the flow field, experimental validation is necessary. Therefore, experiments with the liquid flow rate at the pilot scale of industrial test were carried out in a simplified jet mixer, i.e., a localized element cut from the jet nozzle. Planar Laser Induced Fluorescence (PLIF) technique was used to visualize the mixing phenomenon. A small quantity of Rhodamine B dye was added to the working fluid, and a Nd:YAG laser was used for illumination (532 nm) to excite the fluorescence. The laser sheet was 0.5 mm thick. A high-resolution digital camera was used for PLIF, with the spatial resolution of 60 micron. Hence, meso-scale mixing phenomenon can be revealed. A dynamic sub-grid large eddy simulation (LES) approach was applied to study the fluids flow and mixing in more details. By optimizing the parameters of the jet nozzle, the mixing time of two liquid sheets is in millisecond(s).