274496 Gas-Liquid and Liquid-Liquid Hydrodynamics in an Advanced-Flow Reactor (AFR)
Gas-Liquid and Liquid-Liquid Hydrodynamics in an Advanced-Flow Reactor (AFR)
María José Nieves Remacha1, Amol A. Kulkarni1,2, and Klavs F. Jensen1,*
1 Dept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
2 Chemical Engineering and Process Development Div., National Chemical Laboratory, Pune 411008, India
Microfluidic systems are a relatively new technology that has been proven to have many advantages over conventional process technologies for the synthesis of chemical compounds. High heat and mass transfer rates, rapid mixing, and higher selectivities and conversions can be achieved in these microdevices. Microreactors are very useful in the laboratory scale to perform kinetic studies, elucidate reaction mechanisms, optimize reaction conditions, and catalyst screening. The interest in applying this technology for commercial purposes is growing significantly. However, microreactors by themselves provide very small throughputs, and direct parallelization of thousands microreactors becomes very expensive and the overall performance being dependent on achieving identical reaction conditions and flow uniformity in each microunit. Having an intermediate scale device with larger channel dimensions that can be parallelized with a reasonable number of reactor units to achieve significant production levels is the approach followed here. The challenge is to keep the mass and heat transfer performance while increasing the throughput. The Advanced-Flow Reactor (AFR) manufactured by Corning Inc. ® compensates for the loss in transport rates caused by the increase in channel size having a special design with the flow path composed by rows of heart-shaped cells in series. The ultimate objective is to scale-up specific multiphase reactions from the micro scale to the milli scale.
High speed imaging has been used to study the hydrodynamics of two-phase flow for carbon dioxide-water and hexane-water systems at ambient conditions for flow rates of each phase ranging from 10 ml/min to 80 ml/min. Bubble/drop size distributions, phase hold-up, specific interfacial area, and overall mass transfer coefficients have been determined for these systems from the flow visualization experiments. The performance of the AFR in terms of overall mass transfer coefficients vs. power consumption has been compared to other conventional gas-liquid and liquid-liquid contactors.
An image of the flow for carbon dioxide-water system is shown in Figure 1. It is observed that the bubble size and number decreases from the inlet of the reactor (bottom right in Figure 1) to the outlet (top right in Figure 1) due to the absorption of carbon dioxide into water. Interfacial areas on the range 200-1000 m-1 and mass transfer coefficients on the order of 1 s-1 were obtained for this system at the flow rates tested.
Figure 1: Hydrodynamics for carbon dioxide-water in the AFR. Bubble size decreases from the inlet to the outlet due to the absorption of carbon dioxide into water. Qwater = 60 ml/min
Three characteristic images of the flow for hexane-water are shown in Figure 2. Hexane resulted to be the dispersed phase regardless of the flow rates of each phase, due to the presence of wall effects which result in water wetting the hydrophilic glass walls of the reactor. The range of average drop sizes for the entire AFR for the flow rates tested from 0.3 to 1.3 mm, specific interfacial areas of 1,000 to 10,000 m-1, and overall mass transfer coefficients of 1.9-41 s-1.
Figure 2: Hydrodynamics for hexane-water in the AFR (hexane is the dyed phase). A) Qwater = 10 ml/min, Qhexane = 10 ml/min; B) Qwater = 40 ml/min, Qhexane = 10 ml/min; C) Qwater = 40 ml/min, Qhexane = 40 ml/min
Further studies including different gas/liquid and liquid/liquid systems are to be performed in order to study the effect of fluid properties on the flow characteristics and mass transfer rates. After characterization of the hydrodynamics of the system, demonstration of the scale-up process for a specific reaction is also performed. Ozonolysis is a challenging reaction from the heat and mass transfer viewpoint. It is usually performed at very low temperatures (-78 °C) and it is an instantaneous reaction where the overall reaction rate depends on mass transfer limitations. Therefore, we want to study this reaction at the micro scale and further scale-up to the Advanced Flow Reactor.
The development of a computational tool capable of predicting the flow characteristics for any system in the AFR is essential for reactor optimization and scaling processes. Simulations are performed using the open source software OpenFOAM ®, and more specifically, a volume of fluid approach for biphasic flow. After validation of the simulation model in simple geometries and determination of variables that guarantee successful predictions, the study is to be extended to the more complex geometry of the AFR.