(476a) Computational Fluid Dynamic Simulations of Gas-Liquid and Liquid-Liquid Flow in An Advanced-Flow Reactor (AFR)

Computational
Fluid Dynamic Simulations of Gas-Liquid and Liquid-Liquid Flow in an
Advanced-Flow Reactor (AFR)

María José Nieves Remacha¹
and Klavs F. Jensen^1,*

¹ Dept. of Chemical
Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Microreactor technology has been demonstrated to be very useful
in the laboratory scale to perform kinetic studies, optimization of reaction
conditions, and catalyst screening with reduced waste generation, high mass and
heat transfer rates, and automated operation, among other benefits. However, microreactors do not provide sufficient throughput for
production purposes. Thus, the alternative is to have an intermediate-scale
device that still holds microreactor technology
advantages (such as mass and heat transfer performance) and can be parallelized
with a reasonable number of units to achieve the production rate according to
the demand.

One alternative is the Advanced-Flow
Reactor (AFR) manufactured by Corning Inc. The mixing module is devoted for
multiphase reactions and has a special design with several rows of heart-shaped
cells in series that produce large specific interfacial areas (1,000 ? 10,000 m^-1)
and overall mass transfer coefficients (0.1 ? 10 s^-1).¹

The
hydrodynamic characteristics of the AFR have been studied experimentally for
carbon dioxide-water and hexane-water.
Experiments can be tedious, expensive, and consume materials. Modeling allows
us to study the effect of fluid properties and operating conditions
(temperature, pressure, flow rates) on bubble/drop size distribution, specific
interfacial areas, phase holdup, and mass transfer coefficients without the
need of experiments. Computational fluid dynamic (CFD) simulations help us
understanding the local behavior of the flow in the reactor, propose new
designs, and select the best operating conditions and materials based on the
CFD results.  

Here
we present results of CFD simulations using the open source software OpenFOAM® for steady-state single-phase (Figure 1 shows an
example of the velocity profile obtained in the AFR) and transient two-phase
flow (gas-liquid and liquid-liquid). For two-phase simulations, the interface
capturing volume-of-fluid (VOF) approach was used, which corresponds to the interFoam solver included in OpenFOAM
®. Slight modifications to the code were performed in order to improve the
results and these are also presented here.

Figure 1:
Velocity profile of CFD simulation of single-phase flow through the AFR using OpenFOAM®.

A
first validation of the VOF method in simple geometries was performed to
determine variables that are critical to guarantee a successful prediction of
the biphasic flow characteristics. Some examples such as the prediction of
two-phase flow in a T-junction are included in Figure 2 ². Results
simulated with OpenFOAM® are compared with
experiments and simulations with Fluent available in the literature ^2-4.

A)     
 B)
 C)

Figure 2: CFD Simulation of two-phase flow in
a T-junction using OpenFOAM ®: A) Effect of inlet
channel width on slug length; B) Effect of contact angle on flow
characteristics; C) Effect of inlet channel length on slug size.

CFD
simulations for hexane-water and carbon dioxide-water in the AFR are included
in Figures 3 and 4 for different combinations of flow rates and orientations of
the AFR. Fair agreement between simulation results and experiments based on the
images of the flow obtained by high speed imaging was observed. Greater
discrepancies were encountered when including gravity in simulations for
liquid/liquid flow (as shown in Figures 3A) and 3C)), and better agreement
between experiments and simulations for gas/liquid flow was observed (Figures
4A) and B)). Current analysis of the CFD results include measurement of
bubble/drop size distribution, specific surface area, dispersed phase holdup,
and mass transfer coefficients, and comparison with the experimental values. Future
CFD simulations will include different fluid properties to study their effect
on the hydrodynamic characteristics of biphasic flows. The current interFoam solver only solves the transient
dynamics of the biphasic flow without mass transfer of components between
phases or reaction terms. The ultimate goal is to include mass transfer and
reaction so that prediction of conversion and selectivity for specific
multiphase reactions can be performed. 

Figure 3: CFD Simulation of hexane/water flow
using VOF in OpenFOAM® for vertical orientation of
the AFR:

A) Q_w = 10 ml/min, Q_h
= 10 ml/min; B) Q_w = 20 ml/min, Q_h = 20 ml/min; C) Q_w
= 10 ml/min, Q_h = 40 ml/min; D) Q_w = 40 ml/min, Q_h
= 10 ml/min

A)     

B)
  

Figure 4: CFD Simulation of carbon dioxide/water flow using VOF in OpenFOAM® for Q_L = 10 ml/min: A) Vertical
orientation, Q_g = 13.3 ml/min B)
Horizontal orientation, Q_g = 20.8 ml/min

References:

[1]  
Nieves-Remacha, M. J.;
Kulkarni, A. A.; Jensen, K. F. Hydrodynamics of
Liquid-Liquid Dispersion in an Advanced-Flow Reactor. Ind. Eng. Chem. Res., 51 (50), 2012, pp 16251?16262

[2]  
Qian, D; Lawal, A. Numerical study on gas and liquid slugs for
Taylor flow in a T-junction microchannel. Chem. Eng. Sci. 61 (23) 2006 pp 7609?7625

[3]  
Santos,
R.; Kawaji, M. Numerical modeling and experimental
investigation of gas?liquid slug formation in a microchannel
T-junction. International Journal of Multiphase Flow 36, 4 2010 pp
314-323

[4]  
Vandu,
C.O.; Liu, H; Krishna, R. Mass transfer from Taylor bubbles rising
in single capillaries. Chem. Eng.
Sci., 60 2005 pp
6430 ?
6437