Monolith Reactors for Intensified Processing in Green Chemistry - Process Intensification for Green Chemistry - page 178

air-water and air-water/isopropanol mixtures, which may be used to guide the selection
of gas and liquid velocities in order to achieve the desired flow regime within the
monolith. As mentioned in Section 6.1, Taylor flow is a desirable regime in which to
operate in order to maximize the transfer of gas such as hydrogen to the catalyst surface
within the three-phase reactor, owing to the thin liquid films formed at the sides of the
gas bubble.

6.3.2 Mixing and Mass Transfer

In order to design and optimize the monolith reactor in three-phase operation, it is
necessary to understand the different possible modes of mass transfer: (1) gas-liquid-
solid (GS) via the liquid film; (2) gas-liquid (GL) via the gas bubble caps; and (3) liquid-
solid (LS) within the liquid slugs (Figure 6.4). In this section, flow studies of the liquid
slugs are first reviewed, followed by mass-transfer characteristics of the three-phase
monolith reactor.

6.3.2.1 Recirculation in Liquid Slugs

Within the liquid slugs that occur in Taylor flow, mass-transfer processes (2) GL and (3) LS
occur and it is therefore important to understand the mechanisms of mixing which assist
these. Thulasidas et al. [21] used particle image velocimetry (PIV) to study the velocity
distributions in liquid slugs in Taylor flow operated in 'upflow' mode, and showed that
recirculating patterns occur with a high degree of mixing. The detailed behaviour is a
function of the capillary number, where:

¼ m

V
s

Ca (6.1)

Depending on the capillary number of the flow, counter-rotating vortices or a complete
bypass flow inside the liquid slug may be observed. Such recirculation patterns can play an
important role in transporting dissolved gas from the bubble cap to the catalyst coated upon
the channel walls. Tsoligkas et al. [37] also used PIV to study flow in liquid slugs, but
in downflow mode. They found that short slugs (slug length less than the tube hydraulic
diameter) led to a relatively flat velocity profile, where the axial velocity was only a
function of the position in the tube cross-section, as shown in Figure 6.5. By contrast, in

Figure 6.4 Recirculation patterns and mass-transfer processes within liquid slugs during
Taylor flow: (1) gas-solid, (2) gas-liquid, (3) liquid-solid mass-transfer steps.