(583j) Flow Regimes and Particle Residence Time Distribution in Horizontal and Vertical Gas-Liquid-Solid Slurry Taylor Flow

In multiphase micro-reaction
technology the solid catalyst is usually immobilized on the reactor walls,
which not only leads to rather small catalyst amounts per unit volume but more
seriously impinges on its flexibility. The catalyst removal in case of
deactivation or change of operation is difficult, even impossible without
damage to the reactor wall. Furthermore the coating process itself is specific
for each catalyst and requires therefore additional development time.

A new approach to join
beneficial properties of Taylor-flow with the operational flexibility of
conventional slurry reactors is the slurry-Taylor flow where catalyst particles
are suspended and kept in motion by the internal circulations in the liquid
slugs.

The concept of a three phase
slurry micro-reactor was first applied by Enache et
al. [1] for gas-liquid-solid vertical flow. In contrast to other recent studies
[2, 3, 4] on liquid-liquid horizontal flow where solid
particles are placed in the dispersed liquid phase, we investigate gas-liquid
horizontal and vertical flow with the solid localised (in general) in the
continuous phase.

We were able to show that the
performance of this new contact mode is comparable to a laboratory stirred tank vessel under semi-batch conditions [5] and we concentrate
now on hydrodynamics and mass-transfer properties.

Here we present the different
flow regimes obtained in horizontal and vertical flow, discuss the involved
forces and introduce our method to study the residence time distribution of
solid particles.

We identified different flow
regimes by varying the fluids flow rates, solid charge and flow direction. An
interesting distinction exists between both horizontal and vertical flow: while
in horizontal flow a certain total velocity is necessary to pass from particles
circulating only in the lower recirculation loop to a complete homogeneous
distribution over the entire slug height; in vertical flow, for the lowest
studied velocities, particles are already homogeneously distributed but for
higher flow rates the distribution pattern becomes rather heterogeneous, with
particles localised mostly in the centre of the slug (figure 1).

 SHAPE  \* MERGEFORMAT

Figure
1) Examples for some typical flow patterns for horizontal (A, C) and vertical
(B, D) flow. Materials : gas phase: N₂,
liquid phase : EtOH, solid phase: SiO₂,
40-76 µm, impregnated with NiO₂, solid charge 5g/L (C, D) and 10 g/L
(A,B). PFA tube, d_tube=1.65mm. Total velocity 47 mm/s (A, B) and 150 mm/s (C, D).

In horizontal flow a certain
amount of particles remains usually in the liquid film. We therefore search to
quantify the amount of particles trapped in the liquid film and investigate the
residence time distribution of the solid phase. The main challenge thereby is
to handle the tracer injection without disturbing or altering the Taylor-flow. A possible
solution is the use of photoactivatable push-pull chromophores. In our approach, a nonfluorescent,
blue-shifted azide-π-acceptor fluorogen
precursor is photoconverted to a fluorescent, redshifted amine-π-acceptor fluorophore. In this way,
functionalised silica particles can be activated using pulsed UV-light and
imaged using visible light. A tracer injection is thus not necessary and the
stimulus signal can be controlled by the activating tracer impulse.

REFERENCES

[1] D.I. Enache,
G.J. Hutchings, S.H. Taylor, R.Natividad, S.Raymahasay, J.M. Winterbottom,
E.H. Stitt, Experimental evaluation of a three-phase downflow capillary reactor, Ind. Eng. Chem. Res. 44 (2005)
6295-6303.

[2] A. Ufer, D. Sudhoff,
A. Mescher, D. W. Agar, Suspension catalysts in a
liquid-liquid capillary microreactor, Chem. Eng. J.
167 (2011) 468-474.

[3] K. Olivon,
F. Sarrazin, Heterogeneous reaction with solid
catalyst in droplet-flow millifluidic device, Chem.
Eng. J. in press, corrected proof.

[4] G. K. Kurup,
A. S. Basu, Field-free particle focusing in microfluidic plugs, Biomicrofluidics
6 (2012) 22008-2200810.

5] A.-K. Liedtke,
F. Bornette, R. Philippe, C. de Bellefon,
Gas?liquid?solid
??slurry Taylor''
flow: Experimental evaluation through the catalytic hydrogenation of
3-methyl-1-pentyn-3-ol, Chem. Eng. J. in press, corrected proof.

[6] F. M. Raymo,
Photoactivatable Synthetic Dyes for Fluorescence
Imaging at the Nanoscale, J. Phys. Chem. Lett. 2012, 3, 2379-2385.