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.