Figure 4. Transition boundary between the trickle flow and the pulse flow at different solid flow rates in the three-phase moving bed
The transition boundary shown in Figure 4 indicates that the pulse flow in the trickle bed will transform to the trickle flow under the effects of the particle moving. In the following part, we will analyze the reason for this transition. As mentioned by Zhao33, the pulse flow is caused by the alternation of liquid-rich and gas-rich zones. Two conditions need to be satisfied for the generation of the pulse flow. Firstly, the local liquid blockages should generate between the particles. Secondly, the driving force from the momentum of the gas phase should be greater than the resistance force to push the liquid blockages downward. When the bed operates in the pulse flow regime, the gas mass flow rate is high enough to meet the requirement that the momentum of gas phase is much higher than the resistance for the given mass flow rates of the gas and liquid. The second condition is already satisfied for the given mass flow rates of the gas and liquid. Therefore, it can be deduced that at constant gas and liquid mass flow rates, the transition from the trickle flow to the pulse flow is dominated by the formation of the local liquid blockage. As the particles start to move and the bed is switched from fixed to moving bed, the flow pattern transforms from the pulse flow to the trickle flow, which means that the movement of particles has a significant impact on the formation of the local liquid blockage.
To get a better understanding of the transition phenomenon in the three-phase moving bed, we have also measured the time series pressure drop fluctuation and the dynamic liquid holdup under the operating conditions near the flow regime boundary. At the given gas and liquid mass flow rates (G = 0.126 kg·m-2·s-1, L = 15 kg·m-2·s-1), the flow pattern was pulse flow in the trickle bed, but when the particles started to move, it changed to the trickle flow. Under these conditions, the time-series pressure fluctuations at various solid flow rates are shown in Figure 5(a), which shows the flow behaviors from macro-scope. When the solid flow rate was 0 mm/s, the liquid pockets or plugs constantly blocked the entire cross-section, leading to the alternation of gas-rich and liquid-rich regions. The alternation of gas-rich and liquid-rich slugs resulted in significant pressure fluctuation, as shown in Figure 5(a). As the particles started to move, the bed was switched from fixed to moving bed, the fluctuation of pressure drop changed to be relatively stable compared to that in the trickle bed (us = 0 mm/s). This indicated that, at the given gas and liquid mass flow rates, the flow pattern transformed from the pulse flow to the trickle flow after particles started to move. The variations of pressure drop and standard deviation of pressure drop under different solid flow rates are shown in Figure 5(b). In the operation of moving bed, it can be seen from Figure 5(b) that the pressure drop decreases with the increasing solid flow rate at the given gas and liquid mass flow rates, which is caused by an increase in voidage with the increasing solid flow rate. Such a trend is in agreement with the observations of our previous work15. As the voidage increases, the transition from the trickle flow to the pulse flow gets delayed37. Meanwhile, it can also be seen from Figure 5(b) that at the constant gas and liquid mass flow rates, as the solid flow rate increases, the standard deviation of pressure drop increases, which may be due to the gas-liquid interaction becomes more and more intense with the increasing solid flow rate. However, even the solid flow rate is up to 6 mm/s, the standard deviation of pressure drop is still much lower than that in the trickle bed, which proves that the flow regime is always maintained in the trickle flow.