(165a) Experimental and CFD Studies of a New Continuous Process for Mixing of Complex Non-Newtonian Fluids

Experimental and CFD studies of a New
Continuous Process for Mixing of Complex Non-Newtonian Fluids

Simona Migliozzi¹, Robert Sochon²,
Luca Mazzei¹ and Panagiota Angeli¹

¹Department of Chemical Engineering,
University College London, Torrington Pl, London WC1E 6BTA, UK
²GlaxoSmithKline Consumer
Healthcare, St George's Avenue, Weybridge, KT13 0DE, UK

1.    Introduction

Mixing processes
involving non-Newtonian fluids are widely employed in several industrial
applications. Specifically, the design of new continuous mixing operations
poses many challenges, especially when dealing with highly non-Newtonian fluids.
Typically, batch processes are employed to perform blending in high viscosity
conditions. However, the large fluid volumes that characterize these operations
can lead to concentration and temperature gradients within the vessels and to
the establishment of dead mixing zones for more viscous fluids. New continuous
processes can prevent these problems and achieve better performance at lower
costs. In particular, static mixers are a promising alternative, especially for
blending highly-viscous fluids that can be processed exclusively in laminar
flow conditions. Indeed, in this flow regime, static mixers can enable radial
mixing via a periodic sequence of splitting and recombining of the fluid
streams that reduces progressively the thickness of each fluid layer,
eventually promoting diffusive mass transfer between the two phases [1].

In this work, our main
objective is investigating the mixing efficiency of two different static mixers
and the effects of the
rheological properties of the mixture using experimental and computational
fluid dynamics (CFD) methods. Our focus application is the manufacturing of a newly
formulated non-aqueous toothpaste. Therefore, a complete characterization of
the complex rheological behaviour of the specific working mixture was also carried out at different temperatures and compositions.

2.    Materials
and methods

The two liquid phases
employed in the process of interest are glycerol and a carbomer suspension in
polyethylene glycol. When mixed, the liquids start forming a gel. We studied
the structure evolution of the liquid mixture using time-resolved rheometry
[2-3] and estimated the gelling time at different temperatures and glycerol mass
fractions. Then, the flow curves of the final gels were obtained for the same
ranges of temperature and mass fractions. A TA Instruments Discovery Hybrid
Rheometer, equipped with a Peltier plate to precisely control the operating
temperature, was used to carry out all the rheological measurements.

To perform the
mixing studies, two different static mixers were selected: the Kenics, which
has been extensively studied in the literature, thus allowing a first
comparison with literature correlations [4-5], and the GFX, which is a
modification of the SMX (Sulzer technology). In both geometries all the
elements were detachable, thus allowing to easily change the overall length of
the mixer. Pressure drop measurements and Planar Laser Induced Fluorescence (PLIF)
techniques [6] were used for the experimental campaign. A schematic of the
whole experimental apparatus is shown in Figure 1. ANSYS Fluent 18.2 was used
to perform the simulations. Specifically, we selected the ’Species transport’
model, present in the software, given the miscibility of the two fluids.

Figure 1. Schematic of the experimental apparatus.

3.    Results

The flow properties of the activated mixtures show a clear dependence of the
mixture viscosity on both composition and temperature (Figure 2-3). Therefore,
a viscosity model that accounts for this dependence should be added in the
Species transport model of Fluent.

Figure 2. Flow curves of the activated mixtures for all mass
fractions of Carbopol suspension studied at a fixed temperature.

Figure 3. Flow curves of the activated
mixtures for all mass fractions of Carbopol suspension studied at a fixed mass
fraction of glycerol in polymer suspension.

Pressure drop values and concentration maps were obtained from the experimental
apparatus and compared with the CFD results. Thanks to the possibility to
remove the elements, we were able to compare the concentration fields at different
cross-sections of the computational geometry (Figure 4) with the results obtained from the PLIF
experiments. The main differences found between the experimental and
computational concentration profiles can be addressed tothe presence of
numerical diffusion in the CFD results, which is a non-physical diffusion phenomenon
that arises from the discretization of the nonlinear convective term in the
Navier-Stokes equations. Therefore an analysis of numerical diffusion was carried out
to understand the phenomenon [7].

Figure 4. Volume fraction evolution in the GFX static mixer geometry.

References

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