Introduction

Various multiphase reactors have been developed to meet the specific requirements of diverse multi-phase reactions existing in the chemical industry. The trickle bed reactor, for instance, is mainly developed for slower reactions requiring high catalyst loading (volume fraction more than 30%), particularly for the large-scale processes1. In addition, due to the flexibility and simplicity in operation, it has been widely used in chemical and petroleum processes2. In trickle bed reactors, the gas and liquid can flow cocurrently downward through the packed bed and conventionally the gas works as the continuous phase while the liquid trickles over particles in the form of films or rivulets3,4,5. The trickle bed reactors offer several advantages like plug flow operation, a high catalyst to liquid ratio, lower power requirements and so on6. However, it also has some intrinsic drawbacks like uneven liquid distribution, poor heat transfer rates, significant diffusion resistance7. Therefore, it is vital to balance different competing requirements for a particular application. For example, using larger catalyst particles can reduce the pressure drop, however, the intraparticle diffusion resistance will be enhanced8. In recent years, considering the specific requirements of different applications, new modifications of conventional trickle bed reactors like monolith reactors and micro-trickle bed reactors have been developed, which may minimize some of the inherent drawbacks of the traditional trickle bed and enlarge the application of trickle bed reactors significantly9,10,11. However, there is no modification for the strong exothermic reactions with rapid deactivation rate of catalysts12. The configuration of the moving bed has been proven to be a promising alternative by Iliuta in catalytic wet oxidation13 and ChevronTexaco14in hydrodemetallization of heavy oil, which can realize an online replacement of catalysts. Therefore, a novel moving bed reactor concept based on cocurrent downflow of gas, liquid, and solid phases has been proposed by our research team and scientist in ExxonMobil to supply a potential solution for the reactions with rapid deactivation rate of catalysts as well as high catalyst loading. This new reactor concept not only has the same characteristics as a conventional moving bed reactor with continuous catalyst regeneration capability but also has some of the advantages of the trickle bed. Moreover, it is further assumed that the complex particle movements may potentially enhance the local turbulent intensity and increase the heat and mass transfer rates.
In our previous work, we have investigated the pressure drop in a 3D three-phase moving bed15 and the flow regimes in a rectangular three-phase moving bed16. Results show that the pressure drop decreases almost linearly with the increasing solid flow rate within our operating limits and it can be correlated well by coupling the solid flow rate with the Clements’ correlation developed for the trickle bed. Besides, the typical flow regimes in trickle bed including the trickle flow, the pulse flow and the bubble flow are all observed in the rectangular three-phase moving bed, but the transition boundary between different flow regimes is significantly influenced by particle movement. More importantly, it shows from the aspect of flow regimes that the three-phase moving bed may be suitable for reactions that occur in pulse flow, which may increase the liquid loading significantly. However, although the flow regimes investigation in rectangular three-phase moving bed has supplied important information, it is not enough for the design and application of three-phase moving bed in the real process, since results in rectangular apparatus may be influenced by wall effects more or less. Therefore, this work will investigate the flow regimes in a 3D cylinder-shaped three-phase moving bed for the first time. Since results in the rectangular apparatus have shown that the flow regimes in three-phase moving bed have some similarity with those in trickle bed, we will also use the flow regime map in the 3D trickle bed as a basis. Besides, considering the large driving forces of particles on the liquid phase and the liquid distribution is significant for the development of flow regimes, this work will also investigate the liquid distribution in three-phase moving bed.
In a trickle bed, based on the nature of individual phase flow, four flow regimes can be distinguished, specifically the trickle, pulse, bubble and spray flow regime17,18,19. At low gas and liquid flow rates, the liquid trickles over the packed particles in the form of films or rivulets and the gas flows through the remaining interstices. Such a flow pattern is generally called as the trickle flow or low interaction regime in which both gas and liquid phases are continuous20. Weak gas-liquid interaction, low shear stresses at gas-liquid interfaces and gravity-driven liquid flow are characteristics of the low interaction regime21. The trickle flow regime region widens with the increase in particle size, decreases in liquid viscosity and surface tension22. Due to the low gas-liquid interaction, the pressure drop is low and fluctuates slightly23. As the gas and liquid flow rates increase, the gas-liquid interaction increases, the solid surface changes from partial wetting to complete wetting, and liquid pockets or plugs constantly block the entire cross-section, leading to the alternation of gas-rich and liquid-rich regions. The corresponding regime is classified as the pulse flow regime or high interaction regime in which both gas and liquid phases are semi-continuous24. The alternation of gas-rich and liquid-rich slugs results in significant pressure fluctuations. The pulse flow regime offers characteristic advantages in terms of effective wetting and utilization of catalyst, and higher heat and mass transfer rates25. At low gas flow rate and high liquid flow rate, the liquid phase becomes a continuous phase filling the entire bed, while the gas phase flows downward through the bed in the form of dispersed bubbles. This is known as the bubbling flow, in which the surface of the particles is completely wetted. Due to the high liquid holdup and complete wetting of the bed, bubbling flow is suitable for liquid phase restricted reaction and strong exothermic reaction2. At the high gas flow rate and low liquid flow rate, subjected to the high shear caused by the gas-liquid slip velocity, liquid phase loses its semi-continuity and turns into droplets, and gas phase becomes the continuous phase26. This regime is called as the spray flow regime.
Most of the trickle bed reactors are operated close to the boundary between trickle flow and pulse flow regime, taking both advantages of these two operating regimes. As a result, most of the experimental studies on the trickle bed were restricted to trickle and pulse flow regimes. Numerous methods have been developed to identify the flow regime transition from trickle flow to pulse flow, which includes the visual observations27, pressure drop fluctuation28, microelectrodes29, computed tomography (CT)30 and magnetic resonance imaging (MRI)31,32,33. The same to that in the trickle bed, this work also mainly focused on the transition between trickle flow and pulse flow in the 3D three-phase moving bed.
In summary, this work is organized as follows. Firstly, the new developed three-phase moving bed is worked as a trickle bed setting the solid flow rate as 0, and the flow regime map in it is established as a basis. Then, the transition between trickle flow and pulse flow is analyzed when particles start to move in the three-phase moving bed, based on the variation of pressure drop signals and the observations from the wall. According to the experimental results at different solid flow rates, the hydrodynamic parameters governing the transition between the trickle flow and the pulse flow are given. Meanwhile, a correlation for the transition boundary is established by relating the parameters governing the flow regime transition between trickle flow and pulse flow. Finally, the effect of particle movements on the radial liquid distribution is further analyzed.