(c) Typical flow regime map in the trickle bed
In order to verify the accuracy of experimental design in this work
and provide a basis for subsequent
analysis, the flow regimes in the trickle bed were investigated firstly.
The variation of the standard deviation of pressure drop and the
pressure drop hysteresis were first checked up for identification of
flow regime transition in the trickle bed reactors. Figure 2(a) shows
typical time series pressure drop
fluctuations for various liquid mass flow rates at a given gas mass flow
rate
(G= 0.088 kg·m-2·s-1), which are
different as the liquid mass flow rate increases and the flow regime
transforms from low to high interaction regime. When the flow regime
transforms from trickle flow to pulse flow and even reaches the bubble
flow regime, the pressure fluctuation increases firstly and then
decreases. This is because the gas-liquid interaction increases when the
alternation of gas-rich and liquid-rich regions appears, but decreases
when the liquid changes to continuous phase with the increasing liquid
flow rate. The standard deviation determined from the pressure drop
signals is used as a criterion to distinguish flow regimes, and it is
compared with the transition determined by the visual observations. The
typical images and videos used to identify the flow regime transition
are given in supporting information. At a constant gas mass flow rate,
the variations of pressure drop and the standard deviation of pressure
drop fluctuations during the increasing and decreasing branches were
measured and shown in Figure 2(b). It can be seen from the Figure 2(b)
that at the constant gas mass flow rate, the standard deviation of
pressure drop increases firstly and then decreases before levelling off
as the liquid mass flow rate increases. Indicating that at a constant
gas mass flow rate, the flow regime transforms from trickle flow to
pulse flow and further transforms to bubble flow as the liquid mass flow
rate increases, which is also proved in the work of Chou et
al.37. As for the pressure drop hysteresis, it can be
seen from Figure 2(b) that the difference of the pressure drop between
the increasing and decreasing branches exists in the trickle flow
regime, which is due to the different bed wetting characteristics while
increasing and decreasing the liquid mass flow rate. While the
difference of the pressure drop between the increasing and decreasing
branches disappears when the flow regime transforms to pulse flow. This
is in accordance with the experiment conducted by Gunjal et
al.5. In the increasing branch, the liquid flow
pattern transforms from rivulet to film in the trickle flow regime as
the liquid mass flow rate slowly increases from zero. The bed becomes
completely wet and the liquid flow changes into the film just before the
transition to a pulse flow regime occurs. While the liquid is always in
the form of film in the decreasing branch as the liquid flow rate
decreases slowly back to zero38. This effect is
negligible in pulse flow regime since the liquid flow pattern does not
change during the increasing and decreasing branches.
Based on the standard deviation of pressure drop mentioned above and the
visualization, a flow regime map in the trickle bed as a function of gas
and liquid mass flow rates was constructed and shown in Figure 2(c),
which is similar to the typical flow regime maps mentioned in the
literature by Sie39 and Gunjal5. It
can be seen that there are three flow regimes including the trickle,
pulse and bubble flow. At the constant gas mass flow rate, starting from
the low liquid mass flow rate, the first regime to be identified is the
trickle flow, characterized by independent and continuous flow channels
of gas and liquid. A further increase in the liquid mass flow rate leads
to a pulse flow pattern characterized by the alternation of gas-rich and
liquid-rich regions. Finally, the flow regime transforms to bubble flow
with the further increase of liquid mass flow rate, and the liquid flows
continuously through the bed while the gas flows as bubbles. This
provides a basis for the study of flow regimes in three-phase moving
beds.