Horizontal and Vertical Gas-Liquid-Solid Slurry-Taylor Flow: Liquid-Solid Mass Transfer Measurements Via Ion Exchange Resin

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.

We identified different flow regimes by varying the fluids flow rates, solid charge and flow direction (figure 1).

Here we discuss our first results regarding the liquid-solid mass transfer capacity of slurry-Taylor flow. We chose to work with ion exchange resins as suspended particles [6] and apply and compare two different measurement strategies to follow the neutralization of caustic soda and thereby the ion exchange rate: we use a pH color indicator (figure 2) which allows not only to estimate the time required to reach a specific pH-value but also to detect the region of transfer positioned in the liquid slug which is especially interesting for heterogeneous flow regimes. Also we measure the change of conductivity with electrodes consisting of two platinum wires, connected to an alternating current source. Several of these electrodes are placed along the tube length which allows us to follow the sodium concentration online. Both methods are used to calculate liquid-solid mass transfer coefficients for different fluid flow rates, particle diameters and charges with the aim to propose a correlation law.

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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).

Figure 2) Example of liquid-solid mass transfer measurement using a pH color indicator: gas phase: N2, liquid phase: H₂O, initially 0.01 mol/l NaOH, 0.1 g/L cresol red, solid phase: Dowex 50WX8, 200-400 mesh; PFA tube, d_tube=1.65mm; total velocity 120 mm/s; complete color change and thus due to the usage of cresol red neutralization can be detected after a residence time of ≈40 seconds.

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] V. G. Pangarkar, A. A. Yawalkar, M. M. Sharma, A. A. C. M. Beenackers, Particle-liquid mass transfer coefficient in two-/ three-phase stirred tank reactors, Ind. Eng. Chem. Res. 41 (2002) 4141-4167.