(573b) 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. Kulkarni^1,2, and Klavs F.
Jensen^1,*

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. Q_water = 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.

A) B)  C)

Figure 2: Hydrodynamics for hexane-water in the AFR (hexane is
the dyed phase). A) Q_water = 10 ml/min, Q_hexane = 10
ml/min; B) Q_water = 40 ml/min, Q_hexane = 10 ml/min; C) Q_water
= 40 ml/min, Q_hexane = 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.