(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.