Multiple hydrodynamic states in trickle flow: Quantifying the extent of pressure drop, liquid holdup and gas-liquid mass transfer variation

It is well established that pressure drop and liquid holdup under trickle flow conditions are functions of the flow history. However, the extent of possible variation of these and other critical hydrodynamic parameters has not been fully quantified. In this study, specifically defined prewetting procedures are used as limiting cases for hydrodynamic hysteresis. These are:

•

Non-prewetted.

•

Levec prewetted: the bed is flooded and drained and after residual holdup stabilisation the gas and liquid flows are introduced.

•

Kan_L prewetted: the bed is operated in the pulse flow regime (by increasing liquid velocity) after which liquid flow rate is reduced to the desired set point (all at the desired gas flow rate).

•

Kan_G prewetted: the bed is operated in the pulse flow regime (by increasing gas velocity) after which gas flow rate is reduced to the desired set point (all at the desired liquid flow rate).

•

Super prewetted: the bed is flooded and gas and liquid flows are introduced once draining commences.

It is shown that the upper limiting case for pressure drop is the Kan_L mode of operation. The lower limiting cases are the non-prewetted and Levec prewetted modes (these coincide). Pressure drop may vary by as much as 700% even for prewetted beds. Liquid holdup is different in all five prewetting modes. The upper limiting case is the Kan_G mode of operation, while the lower limiting case is the non-prewetted mode (Kan_G holdup is approximately 160% that of non-prewetted mode holdup at ). At low gas velocities the Kan_L holdup can be 400% of that of the non-prewetted beds. Importantly, the lower limiting case for prewetted beds is the Levec mode. Holdup in the Kan_G mode may be as much as 130% of the holdup in the Levec mode (at ).The effect of hydrodynamic multiplicity of the volumetric mass transfer coefficient is measured by the desorption of oxygen from water into nitrogen. In this case the different prewetting procedures result in three distinct regions, the upper region being the Kan and Super prewetted beds, the intermediate region being the Levec prewetted bed and the lower region being the dry bed. Mass transfer coefficients in the upper region can be as much as 600% of that of the lower region and 250% of that of the intermediate region. Evidently, prewetting (and even pulsing flow prewetting) does not guarantee that the bed is operating at the maximum values of pressure drop, holdup and mass transfer coefficient. Evidence of operation in between the limiting cases is presented. These non-limiting cases can be reached in multiple ways.     

Trickle flow liquid–solid mass transfer and wetting efficiency in small diameter columns

A novel method to simultaneously measure liquid solid mass transfer and external wetting efficiency was employed at column to particle ratios of 10, 6 and 3. Two prewetting procedures representing the upper (Kan) and lower (Levec) hysteresis branches were used. For a multipoint distributor on a Kan prewetted bed wetting efficiency and the specific mass transfer coefficient were almost unaffected by column diameter. The multipoint distributor on a Levec prewetted bed exhibited a decrease in specific mass transfer with decreasing column diameter. Point source experiments resulted in significantly lower wetting and mass transfer measurements with an increasing trend with respect to decreasing column diameter. The results indicate that with proper distribution and prewetting, the effect of column diameter on averaged wetting and liquid–solid mass transfer is almost negligible, a powerful result considering the importance of these parameters on reaction experiments. © 2012 Canadian Society for Chemical Engineering     

Pressure drop hysteresis in trickle bed reactors

An understanding of the hydrodynamics of trickle bed reactors (TBR) is essential for their design and prediction of their performance. Flow variables, packing characteristics, physical properties of fluids and operation modes influence the behavior of the TBR. The existence of multiple hydrodynamic states or hysteresis (pressure drop, liquid holdup, catalyst wetting, gas-liquid mass transfer) is due to the different flow structures in the packed bed and can be attained by a set of different operating procedures. Experiments were performed to study the effect of liquid and gas velocity, liquid surface tension, liquid viscosity and the particle diameter of the packing on two-phase pressure drop hysteresis. The parallel zone model for pressure drop hysteresis in the trickling flow was used for analysis of experimental data and flow structure. Theoretically predicted pressure drop hysteresis loop is in satisfactory agreement with experimental data.     

Trickle bed reactor diluted with fine particles and coiled tubular flow-type reactor for kinetic measurements without external effects

Gas and liquid velocities in laboratory scale trickle bed reactors are one or two orders of magnitude lower than those in commercial reactors. Then, the kinetic data may include the external effects. This shortcoming of laboratory scale trickle bed reactor can be resolved by diluting the catalyst bed with fine inert particles. The catalyst bed dilution increases dynamic liquid holdup, pressure drop, gas–liquid mass transfer coefficient. Hydrogenation of 2-phenylpropene on Pd/Al₂O₃ was performed with the trickle bed reactor diluted with fine inert particles and the coiled tubular flow-type reactor to compare the kinetics with that of the basket type batch reactor. The trickle bed reactor diluted with fine inert particles is suitable to obtain the reaction rate without external effects even if the liquid velocity is low. The coiled tubular flow-type reactor should be used at high gas velocities.     

The Relationship Between Hysteresis and Liquid Flow Distribution inTrickle Beds

Experiments were conducted on a trickle bed with 0.283 m ID to eluddate the relation-ship between hysteretic phenomena and liquid distribution. The hysteresis of pressure drop and the variance of radial liquid distribution were observed simultaneously. Residence time dlstribu-tion (KTD), holdup and mean residence time (RT) of liquid phase were also found to demonmtrate hystereels of the same nature. RTD, liquid holdup and mean RT calculated with a simple model from the distribution of liquid flow rate show chaxacteristic consistant with the experlmeataJ data, suggesting that the hyteretic phenomena originate from the multiplicity and nonuniformity of liquid flow distribution.     

Numerical and Experimental Study of Catalyst Loading and Body Effects on a Gas‐Liquid Trickle‐Flow Bed

The influence of tortuosity and fluid volume fractions on trickle‐flow bed performance was analyzed. Hydrodynamics of the gas‐liquid downward flow through trickle beds, filled with industrial trilobe catalysts, were investigated experimentally and numerically. The pressure drop and liquid holdup were measured at different gas and liquid velocities and in two different loading methods, namely, sock and dense catalyst loading. The effect of sharp corners on hydrodynamic parameters was considered in a bed with rectangular cross section. The reactor was simulated, considering a three‐phase model, appropriate porosity function, and interfacial forces based on the Eulerian‐Eulerian approach. Computational fluid dynamics (CFD) simulation results for pressure drop and liquid holdup agreed well with experimental data. Finally, the velocity distribution in two types of loading and the effect of bed geometry in CFD results demonstrated that pressure drop and liquid holdup were reduced compared to a cylindrical one due to high voidage at sharp corners.     

Correlation between hysteresis of gas-liquid mass transfer and liquid distribution in a trickle bed

The hysteresis of gas-liquid mass transfer rate and the corresponding radial liquiddistribution in a trickle bed reactor are measured to provide evidence for the correlation between thesetwo behaviors.Experimental results indicate that the hysteresis of gas-liquid mass transfer originatesfrom the nonuniformity of the hydrodynamic state of gas-liquid flow and the radial maldistributionof local k_(gia) corresponds very well to the radial maldistribution of liquid flow in the bed.The localliquid flow rate is also found to be nonuniform in the azimuthal direction.In view of maldistributedliquid flow even in the pulsing flow regime,the conventional plug flow model seems oversimplifiedfor describing the behavior of a trickle bed.     

Hydrodynamics and mass transfer of gas–liquid flow in micropacked bed reactors with metal foam packing

Hydrodynamics and mass transfer of gas–liquid flow such as pressure drop, liquid holdup, and gas–liquid mass-transfer coefficient in micropacked bed reactors (μPBRs) with metal foam packing are investigated with an automated platform. Parametric studies are conducted varying gas and liquid superficial velocities, pore diameters of foam packing, and liquid physical properties. Experimental results show that μPBRs with foam packing have comparative mass transfer rate and 10 times lower pressure drop compared to the microparticles. The values of mass-transfer coefficient for three types of foam packing in μPBRs are 1–2 orders of magnitude larger than those in large-scale trickle bed reactors with foam packing. Furthermore, empirical correlations of pressure drop, liquid holdup, and gas–liquid mass-transfer coefficient in μPBRs with foam packing are proposed and the predicted values are found to be in good agreement with the experimental values.     

Effect of Forced Periodical Perturbation on Stability of Hydrodynamic States and Gas-Liquid Mass Transfer in a Trickle Bed

The stability of the hydrodynamic states corresponding to the operation on two branches of the hysteresis loop of pressure drop was examined for a trickle bed. The effect of nonsteady operation on the pressure drop and the gas-liquid mass transfer rate was investigated with several types of periodical perturbation on gas flow rate. It is found that the pressure drop and mass transfer coefficient obtained during operation under the conditions of the lower branch can be increased up to or even beyond those obtained by following the upper branch of the loop without necessarily traversing along the loop to pass through the pulsing point. These results shed light on the mechanism of performance enhancement by periodical operation of trickle beds and might provide a possible way to improve the operation of trickle bed reactors.     

Effect of Forced Periodical Perturbation on Stability of Hydrodynamic States and Gas-Liquid Mass Transfer in a Trickle Bed

The stability of the hydrodynamic states corresponding to the operation on two branches of the hysteresis loop of pressure drop was examined for a trickle bed. The effect of nonsteady operation on the pressure drop and the gas-liquid mass transfer rate was investigated with several types of periodical perturbation on gas flow rate. It is found that the pressure drop and mass transfer coefficient obtained during operation under the conditions of the lower branch can be increased up to or even beyond those obtained by following the upper branch of the loop without necessarily traversing along the loop to pass through the pulsing point. These results shed light on the mechanism of performance enhancement by periodical operation of trickle beds and might provide a possible way to improve the operation of trickle bed reactors.     

Trickle flow hydrodynamic multiplicity: Experimental observations and pore-scale capillary mechanism

Trickle bed reactors are encountered throughout the process industry. Considerable attention has been given to the study of the hydrodynamics of this type reactor. It has been identified that, in the trickle flow regime, the hydrodynamic parameters (e.g. pressure drop and liquid holdup) are not unique functions of the operating and system conditions, but depend on the flow history. This study reviews the experimental trends identified in literature on the basis of a limiting cases framework and then evaluates the three-dimensional pore-scale liquid distribution using computed tomography (CT) data. This leads to the identification of 20 phenomenological trends that characterize hydrodynamic multiplicity, including hydrodynamic flow hysteresis as well as the effects of pre-wetting. The CT study yields additional experimental insight into the role of capillary pressure and ultimately leads to the proposal of a capillary gate mechanism based on contact angle hysteresis as the root cause of multiplicity. The mechanism is incorporated into a simple pore-network model. It is shown that the qualitative performance of the model corresponds closely to the majority of phenomenological trends and is capable of explaining the observed experimental behaviour.     

Hydrodynamics in a pilot‐scale cocurrent trickle‐bed reactor at low gas velocities

Hydrodynamic data obtained from laboratory‐scale trickle‐beds often fail to accurately represent industrial‐scale systems with high packing aspect ratios and column‐to‐particle diameter ratios. In this study, pressure drop, liquid holdup, and flow regime transition were investigated in a pilot‐scale trickle‐bed column of 33 cm ID and 2.45 m bed height packed with 1.6 mm × 8.4 ± 1.4 mm cylindrical extrudates for air‐water mass superficial velocities of 0.0023 – 0.094 kg/m²s and 4.5 – 45 kg/m²s, respectively, at atmospheric pressure. Significant deviation was observed from pressure drop and liquid holdup correlations at low liquid flows rates, corresponding to gravity‐driven flow limit. Likewise, liquid saturation is overestimated by correlations at high liquid flow rates, owing to significantly reduced wall effects. Lastly, trickle‐to‐dispersed bubble flow and trickle‐to‐pulsing flow regime transitions are reported using a combination of visual observations and analysis of the magnitude of local pressure fluctuations within the column. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2560–2569, 2018     

Volume‐of‐fluid‐based model for multiphase flow in high‐pressure trickle‐bed reactor: Optimization of numerical parameters

Aiming to understand the effect of various parameters such as liquid velocity, surface tension, and wetting phenomena, a Volume‐of‐Fluid (VOF) model was developed to simulate the multiphase flow in high‐pressure trickle‐bed reactor (TBR). As the accuracy of the simulation is largely dependent on mesh density, different mesh sizes were compared for the hydrodynamic validation of the multiphase flow model. Several model solution parameters comprising different time steps, convergence criteria and discretization schemes were examined to establish model parametric independency results. High‐order differencing schemes were found to agree better with the experimental data from the literature given that its formulation includes inherently the minimization of artificial numerical dissipation. The optimum values for the numerical solution parameters were then used to evaluate the hydrodynamic predictions at high‐pressure demonstrating the significant influence of the gas flow rate mainly on liquid holdup rather than on two‐phase pressure drop and exhibiting hysteresis in both hydrodynamic parameters. Afterwards, the VOF model was applied to evaluate successive radial planes of liquid volume fraction at different packed bed cross‐sections. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Parallel hydrogenation for the quantification of wetting efficiency and liquid–solid mass transfer in a trickle‐bed reactor

A novel method for the measurement of wetting efficiency in a trickle‐bed reactor under reaction conditions is introduced. The method exploits reaction rate differences of two first‐order liquid‐limited reactions occurring in parallel, to infer wetting efficiencies without any other knowledge of the reaction kinetics or external mass transfer characteristics. Using the hydrogenation of linear‐ and isooctenes, wetting efficiency is measured in a 50‐mm internal diameter, high‐pressure trickle‐bed reactor. Liquid–solid mass transfer coefficients are also estimated from the experimental conversion data. Measurements were performed for upflow operation and two literature‐defined boundaries of hydrodynamic multiplicity in trickle flow. Hydrodynamic multiplicity in trickle flow gave rise to as much as 10% variation in wetting efficiency, and 10–20% variation in the specific liquid–solid mass transfer coefficient. Conversions for upflow operation were significantly higher in trickle‐flow operation, because of complete wetting and better liquid–solid mass transfer characteristics. © 2010 American Institute of Chemical Engineers AIChE J, 2011.     

Local hydrodynamics of gas–liquid-nanoparticles three-phase fluidization

The laser Doppler anemometer (LDA) and conductivity probes were used for measuring the local hydrodynamic performances such as gas holdup and liquid velocity in a lab-scale gas–liquid–TiO₂ nanoparticles three-phase bubble column. Effects of operating parameters on the local gas holdup and liquid velocity were investigated systematically. Experimental results showed that local averaged axial liquid velocity and local averaged gas holdup increased with increasing superficial gas velocity but decreased with increasing TiO₂ nanoparticles loading and the axial distance from the bottom of the bubble column. A three-dimensional computational fluid dynamic (CFD) model was developed in this paper to simulate the structure of gas–liquid–TiO₂ nanoparticles three-phase flow in the bubble column. The time-averaged and time-dependent predictions were compared with experimental data for model validation. A successful prediction of instantaneous local gas holdup, gas velocity, and liquid velocity were also presented.     

滴流床反应器流体力学的研究进展

系统综述了滴流床反应器的流体力学研究现状，分析了流型的转变、床层压降和持液量的关联结果，总结了主要的反应器模型和压力对床层压降及持液量的影响，并详细讨论了操作方式对滴流床反应器流体力学的影响。     

Hydrodynamic behaviour of a draft tube gas-liquid-solid spouted bed

Experiments were conducted using various types of solid particles to investigate the hydrodynamic properties of a gas-liquid-solid spouted bed with a draft tube. The hydrodynamic properties under study include flow modes, pressure profile and pressure drop, bubble penetration depth, overall gas holdup, apparent liquid circulation rate and bubble size distribution. Three flow modes were classified: a packed bed mode, a fluidized bed mode and a circulated bed mode. It was found that the friction factor accounting for the friction loss in the bed varies linearly on a logarithmic scale with the Reynolds number defined based on the apparent liquid circulation rate. The bubble penetration depth in the annular region, overall gas holdup and apparent liquid circulation rate increase with an increase in gas or liquid velocity. At high gas flow conditions an optimal solids loading exists which yields a maximum apparent liquid circulation rate. A model was proposed to describe the liquid circulation behaviour in the draft tube three-phas spouted bed. The average bubble size in the draft tube region is higher than that in the annular region for both the dispersed bubble regime and the coalesced bubble regime in the draft tube region.     

Hydrodynamics and flow regime transition study of trickle bed reactor at elevated temperature and pressure

The hydrodynamics in a trickle bed reactor (TBR) in non-ambient conditions are studied for air-water and air-acetone (pure organic liquid of low surface tension) systems. A flow map experiments for air-water and air-acetone systems are performed in a pilot plant reactor of 0.05 m i.d. and 1.25 m height. It has been demonstrated from the experimental results that the pressure drop tends to increase with increasing superficial gas and liquid velocity and reactor pressure, while it tends to decrease with increasing bed temperature. The results also show that the dynamic liquid holdup increases with increasing liquid velocity and decreases with increasing superficial gas velocity, reactor pressure and bed temperature. The dynamic liquid holdup and pressure drop values are obviously higher than those measured for air-water system at the same fluid fluxes, reactor pressure and bed temperature due to the surface tension effects. For higher reactor pressure and temperature, the trickle to pulse transition boundary shifts towered higher superficial velocities of both gas and liquid.     

Hydrodynamics and mass transfer enhancement of gas–liquid flow in micropacked bed reactors: Effect of contact angle

Pressure drop, residence time distribution, dispersive behavior, liquid holdup, and mass transfer performance of gas–liquid flow in micropacked bed reactors (μPBRs) with different contact angles (CA) of particles are studied. The value of pressure drop for three types of beads can be obtained: copper beads (CA = 88.1°) > stainless steel beads (CA = 70.2°) > glass beads (CA = 47.1°). The liquid axial dispersion coefficient is 1.58 × 10⁻⁶ to 1.07 × 10⁻⁵ m²/s for glass beads and copper beads, which is smaller than those of trickle bed reactors. The liquid holdup of 400 μm copper beads is larger than that of 400 μm glass beads. The ratio of effective interfacial area enhancement is evaluated up to 55% for big contact angle beads compared with the hydrophilic glass beads. In addition, correlations of pressure drop, liquid holdup, and effective interfacial area in μPBRs with different wettability beads are developed and predicted values are in agreement with the experimental data.     

纤维填充滴流反应器的流体力学

<正> 1 引言 近期的研究表明,无机纤维催化剂(玻璃纤维、碳纤维、氧化铝纤维等)具有较高的活性、选择性和稳定性,如氧化铝纤维为载体的裂解汽油加氢。但到目前为止,有关纤维滴流床的化工数据几乎未见报道。本工作的目的是对纤维滴流床进行流体力学研究,考察操作条件、填料性质、填充方式变化时流体流动性质如压降、持液量等的变化规律,以求建立计算压降和持液量的关联式,为纤维滴流床反应器的设计和放大提供基础数据。