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.     

Cyclic operation strategy for extending cycle life of trickle beds under gas-liquid filtration

The hydrodynamics of a trickle-bed reactor subjected to fines-contaminated feed flows was studied under cyclic operation to assess whether or not periodic flows are able to reduce fines deposition and hence to extend reactor operational life under filtration conditions. The bed was subjected to single-pass kaolin+kerosene suspension and air flows to study liquid-, gas- and alternating liquid-gas cyclic operation policies. It was found that fast- and slow-mode liquid cyclic operation policies were not be able to decrease either the pressure drop or the specific deposit; however, with adjusting carefully the parameters of liquid cyclic operation it would be possible to prolong the trickle-bed cycle life. The alternating gas-liquid cyclic feed was also found useful to reduce the levels of pressure drop and bed specific deposit and exhibited the same efficiency for both short and tall beds.     

Flow regime transition in a trickle bed with structured packing examined with conductimetric probes

Methods for identifying flow regime transitions in a trickle bed with structured packing based on the response of conductimetric probes are proposed. Time series of conductance measurements, obtained with an array of eight probes flushed to the reactor wall, are analyzed using tools borrowed from the theory of non-linear dynamics and symbolic dynamics. For the same range of gas and liquid velocities, the local hydrodynamics is inspected within a channel of the structured packing in a non-intrusive manner with a set of conductimetric probes imprinted on one of the corrugated plastic sheets that conforms a packing element. The local hydrodynamics within the channel is explored to interpret the trends of characteristic numbers calculated from the time series measured at the wall. The gas-liquid pattern shows a slugging behavior within the channel that is reflected in the analysis of the flushed probes response.     

Onset of pulsing in trickle beds: Evaluation of current tools and state-of-the-art correlation

The transition between trickle flow and pulse flow regimes in cocurrent trickle-bed reactors is not properly predicted by existing phenomenological, semi-theoretical and empirical tools. Based on the most complete flow regime transition data base (700 measurements, 30 gas–liquid systems, 18 columns diameters, 38 packing materials, high pressure conditions, coalescing, non-coalescing and pseudoplastic non-Newtonian aqueous and organic liquids), a state-of-the-art explicit correlation of trickle-to-pulse flow changeover was derived relying on neural network modeling. Robustness of the proposed correlation was verified, and the limitations of the literature correlations and models were demonstrated through systematic statistical testing over the constructed data base. The overall result was a net improvement in predicting the trickle-to-pulse flow regime transition.     

Flow regimes in trickle beds using magnetic emulation of micro/macrogravity

Strong inhomogeneous magnetic fields in atmospheric-bore superconducting solenoid magnets were used to investigate the hydrodynamics in bore-fitted trickle beds which undergo emulated earth-bound artificial micro/macrogravity. This environment was able to modify the apparent gravity for both diamagnetic and paramagnetic materials by means of magnetization body force densities. Body force vectors can be co-linear or antiparallel to the cocurrent two-phase downflow in trickle beds depending on material magnetic susceptibilities, magnetic field gradient and direction of magnetic field. Trickle-to-pulse flow transition was experimentally studied in microgravity, macrogravity and beyond-levitation conditions for the air-water and the phenylacetylene-kerosene/hydrogen systems. Magnetic fields were found to displace the transition boundary from trickle to pulse flow. This was rationalized in terms of an equivalent artificial gravity effect by formally commuting magnetization forces into an equivalent gravitational acceleration. A theoretical analysis, using a modified Grosser et al. [1988. Onset of pulsing in two-phase cocurrent downflow through a packed bed. A.I.Ch.E. Journal 34, 1850-1860] “artificial gravity” transition model, was carried out and model predictions were found to follow qualitatively the experimental findings.     

Experimental Visualization and Investigation of Multiphase Flow Regime Transitions in Two-Dimensional Trickle Bed Reactors

Different flow regimes are known to occur in the interaction of multiphase gas–liquid flows over packed beds of solid particles, such as those observed in trickle bed reactors (TBRs). There are four major flow regimes that are known to occur in downward cocurrent flow in TBRs, namely: trickle, pulse, bubble, and mist flow regimes. In this work, the focus is on macro-scale experimental visualizations and investigations of the flow regimes in a two-dimensional TBR.

Experimental observations are made to investigate the development and transition of these flow regimes over a wide range of liquid and gas velocities. Cylindrical particles are placed between two glass plates that are sealed on the sides, and water and air are injected over them using an injection manifold to simulate multiphase flow in a TBR. A diffused light emitting device (LED) light table is used to illuminate the experimental window, while real time images are obtained using a high-speed camera. Flow maps are reported depicting all four regimes and the transition regions between them. Transition regions occur where the characteristics of more than one flow regime coexist.

The 2D experimental results are then compared with the existing literature data of three dimensional results and found to be in good agreement. Emphasis is placed on the transition between the trickle and pulse regimes, since that is the most important mode of operation in industrial TBRs. It is observed that the change in diameters of the cylindrical particles in a two-dimensional TBR has little effect on the transition between the flow regimes when the porosity of the bed is kept constant.     

Onset of pulsing in trickle beds with non-Newtonian liquids at elevated temperature and pressure—Modeling and experimental verification

The onset of pulse flow in trickle-bed reactors involving gas-non-Newtonian liquid systems was predicted from a stability analysis of the solutions around equilibrium steady-state trickle flow of a transient two-fluid model based on the volume-average mass and momentum balance equations. The model was developed for the versatile Herschel-Bulkley constitutive rheological equation from which special solutions for plastic Bingham fluids, power-law shear-thinning and thickening fluids, as well as Newtonian fluids were derived. The impact of yield stress, consistency and power-law indices, and temperature and reactor pressure on the trickle-to-pulse flow transition was analyzed theoretically. Model predictions of the trickle-to-pulse transition for gas-non-Newtonian liquid systems were confronted with elevated temperature and pressure experimental transition data obtained for air-0.25 and 0.5 mass(carboxymethylcellulose) CMC solution systems measured by means of an electrical conductivity technique. In addition the model version offspring corresponding to the Newton case (n=1,k=μ_?,τ₀=0), confronted with measured high temperature/pressure-transition data from this work and high-pressure transition data from Wammes et al. [1990. The transition between trickle flow and pulse flow in a cocurrent gas-liquid trickle-bed reactor at elevated pressure. Chemical Engineering Science 45, 3149; 1991. Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures. A.I.Ch.E. J. 37, 1849] and Burghardt et al. [2002. Hydrodynamics of a tree-phase fixed-bed reactor operating in the pulsing flow regime at an elevated pressure. Chemical Engineering Science 57, 4855] proved equally successful.     

Lateral mixing in trickle bed reactors

Knowledge of lateral mixing is essential to understand heat and momentum transfer parameters in both single-phase liquid and two-phase gas-liquid co-current down flow through packed bed columns. The reactors through which gas and liquid concurrently flow downwards through a bed of catalytic packing are called trickle bed reactors. Experimental data on lateral mixing coefficients from both the heat transfer and radial liquid distribution studies are obtained over a wide range of flow rates of gas and liquid using glass spheres (4.05 and 6.75 mm), ceramic spheres (2.59 mm), and ceramic raschig rings (4 and 6.75 mm) as packing materials covering trickle flow, pulse flow, and dispersed bubble flow regimes. In the present work, an expression for estimation of lateral mixing coefficient (αβ)_L is derived using the data on radial liquid distribution studies. The agreement between the values of (αβ)_L obtained from heat transfer studies and from radial liquid distribution studies using the experimental data shows that there exists an analogy between the heat transfer and radial liquid distribution in packed beds. Since (αβ)_L is an important variable for estimation of various heat and mass transfer parameters, a correlation for (αβ)_L based on present heat transfer study is proposed. The agreement between the (αβ)_L values estimated from the proposed correlation and experimental values is satisfactory with a standard deviation (s.d.) of 0.119.     

Effect of partial wetting on liquid/solid mass transfer in trickle bed reactors

The wetting efficiency of liquid trickle flow over a fixed bed reactor has been measured for a wide range of parameters including operating conditions, bed structure and physico-chemistry of liquid/solid phases. This data bank has been used to develop a new correlation for averaged wetting efficiency based on five different non-dimensional numbers. Finally liquid/solid mass transfer has been determined in partial wetting conditions to analyse what are the respective effects of wetting and liquid/gas flow turbulence. These effects appear to be separated: wetting being acting on liquid/solid interfacial area while the liquid/solid mass transfer coefficient is mainly connected to flow turbulence through the interstitial liquid velocity. A correlation has been proposed for liquid/solid mass transfer coefficient at very low liquid flow rate.     

涓流床反应器中流区过渡的气相渗透率表征

由于Ergun方程可适用于气液间无相互作用的两相流动压降计算,并且由气相单相和气液两相并流下的气相压降比值可计算气相相对渗透率,因此,Ergun方程可用于涓流床中不同流区过渡和气液相互作用程度的表征。为检验这一方法的有效性,实验测定了空气-水体系在内径140mm有机玻璃塔中不同粒径玻璃珠(1.9、3.6、5.2、9.3mm)组成的床层压降和持液量。由于采用了压力传感器和电容层析成像仪,因此可测定脉冲流状态下的瞬态数据。通过压降的实验值与理论值比较,发现Ergun方程的适用范围有限,在没有进入脉冲流前先已失效,说明此时气液间作用已经相当显著。鉴于此,改用气液两相压降实验值代替理论值进行了气体渗透率的计算,发现不同气液流速和颗粒直径下出现脉冲流时的气体渗透率均低于0.08。     

Characterization of dynamic behavior of a spout-fluid bed with Shannon entropy analysis

Differential pressure fluctuation time series were obtained at different locations in a two-dimensional spout-fluid bed with a cross section of 300 × 30 mm and height 2000 mm. Shannon entropy analysis of differential pressure fluctuations was developed to characterize the dynamic behavior. Effects of two important operating parameters (spouting gas velocity and fluidizing gas flow rate) on the Shannon entropy were examined. It was demonstrated that a spout-fluid bed at a high spouting gas velocity or fluidizing gas flow rate was a deterministic chaos system since the Shannon entropies at all bed locations increased sharply and asymmetric unstable flows occurred. Shannon entropies were found to be significantly different at various bed locations. Shannon entropies of different flow regimes were distinct, so they were used to identify the flow regimes. The results show that the Shannon entropy helps to grasp the complex characteristics of dynamic behavior in spout-fluid beds.     

On flow regime transition in trickle bed: Development of a novel deep-learning-assisted image analysis method

An image analysis method was developed based on deep-learning algorithms to extract phase fractions quantitatively in a rectangular trickle bed, and the average identification error was lower than 5%. Furthermore, the flow regime transition in the trickle bed was studied. In trickle-to-pulse flow transition, the trickle flow could be further classified into the stable trickle flow and accelerated one. The SD of liquid fractions and the peak width at half-height of the probability density curve of liquid fractions were close to zero in stable trickle flow, increased rapidly in accelerated trickle flow, and remained approximately constant in pulse flow. In bubble-to-pulse flow transition, dispersed bubbles in bubble flow induced the outliers outside the upper boundary of the boxplot of gas fraction, while alternative appearance of gas-rich zone and liquid-rich zone in pulse flow induced outliers outside both the upper and lower boundaries of the boxplot of gas fraction.     

An investigation of liquid maldistribution in trickle beds

The influence of liquid maldistribution at the top of the packing on flow characteristics in packed beds of gas and liquid cocurrent downflow (trickle beds) is experimentally investigated. Particular attention is paid to the effect of gas and liquid flow rates on flow development. Tests are made in the trickling and pulsing flow regimes. A uniform, a half-blocked and a quarter-blocked liquid distributor is tested. Packings of various sizes and shapes are employed. Data are presented on pressure drop and liquid holdup as well as trickling to pulsing flow transition. Diagnosis of radial and axial liquid distribution is made by means of conductance probes. The effects of liquid foaming, bed pre-wetting, top-bed material, and blockage midway the bed on liquid distribution are also examined. Overall, liquid waves in the pulsing flow regime have a beneficial effect, promoting uniform liquid distribution in the bed cross section.     

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:

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Non-prewetted.

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

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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).

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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).

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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.     

Simulating simultaneous fines deposition under catalytic hydrodesulfurization in hydrotreating trickle beds—does bed plugging affect HDS performance?

The deposition of fine particles under chemical reaction conditions in a high pressure/temperature trickle bed reactor was analyzed theoretically using a dynamic multiphase flow deep-bed filtration model coupled with heat and species balance equations in the liquid, gas and solid (catalyst+deposit) phases. The hydrodesulfurization process in the presence of sulfided Co-Mo/γ-Al₂O₃ catalyst was considered as a case study. The deep-bed filtration model incorporates the physical effects of porosity and effective specific surface area changes due to fines deposition/detachment, gas and suspension inertial effects, and coupling effects between the filtration parameters and the interfacial momentum exchange force terms. The detachment of the fine particles from the collector surface was assumed to be induced by the colloidal forces in the case of Brownian particles or by the hydrodynamic forces in the case of non-Brownian particles. The three-phase heterogeneous model developed to simulate the trickle bed performance incorporates the concentration gradients inside the catalyst particle and solid deposit. An important finding of the work is that fine particles deposition does not influence appreciably trickle bed reactor performance. Thus, the only undesirable effect of the fine particles deposition process is the bed plugging and the increase of the resistance to two-phase flow.     

Application of Magnetic Resonance Imaging (MRI) for investigation of fluid dynamics in trickle bed reactors and of droplet separation kinetics in packed beds

Magnetic Resonance Imaging technique was used to investigate the fluid dynamics of multiphase flow in two different applications: (a) stationary two-phase flow in trickle beds, and (b) time-dependent droplet separation in granular bed filters. The experiments were carried out with different gas/liquid systems at either atmospheric conditions or at elevated pressure and temperature. Two-dimensional and three-dimensional image data were then evaluated to quantify the porosity profiles and gas/liquid distribution in packed beds. The results compare well with data from integral measurements.     

Liquid flow texture analysis in trickle bed reactors using high-resolution gamma ray tomography

Trickle bed reactor performance and safety may suffer from radial and axial liquid maldistribution and thus from non-uniform utilization of the catalyst packing. Therefore, experimental analysis and fluid dynamic simulation of liquid–gas flow in trickle bed reactors is an important topic in chemical engineering. In the present study for the first time a truly high-resolution gamma ray tomography technique was applied to the quantitative analysis of the liquid flow texture in a laboratory cold flow trickle bed reactor of 90 mm diameter. The objective of this study was to present the comparative analysis of the liquid flow dynamics for two different initial liquid distributions and two different types of reactor configurations. Thus, the hydrodynamic behavior of a glass bead packing was compared to a porous Al₂O₃ catalyst particle packing using inlet flow from a commercial spray nozzle (uniform initial liquid distribution) and inlet flow from a central point source (strongly non-uniform initial liquid distribution), respectively. The column was operated in downflow mode at a gas flow rate of 180 L h⁻¹ and at liquid flow rates of 15 and 25 L h⁻¹.     

Role of gas phase in the deposition dynamics of fine particles in trickle-bed reactors

Experiments were conducted to study the role of gas velocity in the capture of fine particles from non-aqueous suspensions circulated in co-current down-flow trickle flow reactors. The rate of filtration and pressure drop in the trickle bed were investigated using surfactant-stabilized kaolin-containing kerosene suspensions. It was determined that the filter coefficient was sensitive to liquid holdup and specific deposit. The initial collection efficiencies were compared with predictions based on existing theories. Agreement was generally not good with the exception for the limit of low superficial gas velocity. A general correlation establishing the relationship between the filtration rate and the liquid holdup in trickle beds was proposed to reconcile the experimental data with existing filtration theories.