Trickle Flow Regime Research Articles

Hydrodynamics of gas–liquid counter-current flow in solid foam packings

Solid foam materials combine high voidage and high surface area. These two properties are advantageous for use in chemical reactors due to the low frictional pressure drop and relatively high surface area that may be used for catalyst deposition. Hydrodynamic parameters such as liquid holdup, pressure drop, and flow regimes similar to those for packed beds, have been obtained for the gas and liquid flows through these solid foam packings. The open-celled solid foam packings used were in the range of 5–40 pores per linear inch (ppi). The regimes studied are two high liquid holdup regimes and a low liquid holdup regime (trickle flow regime). Also the flooding points for counter-current flow have been determined.

Catalytic wet air oxidation of substituted phenols using activated carbon as catalyst

Continuous catalytic wet air oxidation (CWAO) was investigated as a suitable precursor for the biological treatment of industrial wastewater that contained phenols (phenol, o-cresol, 2-chlorophenol and p-nitrophenol), aniline, sulfolane, nitrobenzene or sodium dodecylbenzene sulfonate (DBS). Seventy-two-hour tests were carried out in a fixed bed reactor in trickle flow regime, using a commercial activated carbon (AC) as catalyst. The temperature and total pressure were 140°C and 13.1bar, respectively. The influence of hydroxyl-, methyl-, chloride-, nitro-, sulfo- and sulfonic-substituents on the oxidation mechanism of aromatic compounds, the occurrence of oxidative coupling reactions over the AC, and the catalytic activity (in terms of substrate elimination) were established. The results show that the AC without any supported active metal behaves bifunctionally as adsorbent and catalyst, and is active enough to oxidate phenol, o-cresol, 2-chlorophenol and DBS, giving conversions between 30 and 55% at the conditions tested. The selectivity to the production of carbon dioxide was considerable with total organic carbon (TOC) abatement between 15 and 50%. The chemical oxygen demand (COD) reduction was between 12 and 45%. In turn, aniline, sulfolane, p-nitrophenol and nitrobenzene conversions were below 5% and there was almost no TOC abatement or COD reduction, which shows the refractory nature of these compounds.

New insights to trickle and pulse flow hydrodynamics in trickle-bed reactors using MRI

Ultra-fast Magnetic Resonance Imaging (MRI) is used to image liquid distribution during air–water, co-current downflow through a fixed bed of cylindrical porous pellets of length and diameter 3 mm, packed within a 43 mm internal diameter column in both the trickle- and pulsing-flow regimes. Two different data acquisition times are used, 20 ms and 6.4 s, giving two-dimensional images of in-plane spatial resolution 1.4 mm × 2.8 mm and 351 μ m × 351 μ m , respectively. Superficial gas velocities in the range 27 – 275 mm s - 1 ( 0.033 – 0.338 kg m - 2 s - 1 ) and superficial liquid velocities in the range 0.4 – 13.3 mm s - 1 ( 0.4 – 13.3 kg m - 2 s - 1 ) were used. This work reports images of local pulsing events within the bed occurring during the trickle-to-pulse flow transition. The evolution of the local instabilities is studied as a function of increasing and decreasing liquid velocity, at constant gas velocity. Temporal fluctuations in local liquid holdup were seen even in the trickle flow regime; these fluctuations had much shorter time periods than the pore-scale liquid pulses, and are perhaps associated with fluctuations in local surface wetting of the packing elements. A preliminary assessment of the effect of periodic operation on liquid distribution within the bed is also reported.

Lagrange–Euler–Euler CFD approach for modeling deep-bed filtration in trickle flow reactors

A Lagrange–Euler–Euler computational fluid dynamic approach was developed to represent the evolution of two-phase pressure gradients in trickle-bed reactors undergoing deposition of colloidal/non-colloidal fines under deep-bed filtration conditions. The ballistic trajectory equation to analyze fines interception was extended to two-phase flows using the inclined slit as the basic collector geometry. New equations for the collection efficiencies and filter coefficients were established for the various filtration stages encountered in single-phase flow and trickle flow regime both for monolayer and multilayer collection mechanisms. These slit-based expressions were established for non-polar and non-electrolytic petroleum-like liquids where there is a need for the refining industry to model deep-bed filtration in multiphase flow. By embedding the new collection efficiency and filter coefficient expressions in an unsteady-state multidimensional computational fluid dynamics code, the deep-bed filtration process in trickle flow reactors was simulated and the increase of pressure drop during plugging was explained by increasing local specific surface area and decreasing local porosity due to fines deposition. The simulations were benchmarked using the experimental pressure drop data and observations of [M.R. Gray, N. Srinivasan, J.H. Masliyah, Pressure build-up in gas–liquid flow through packed beds due to deposition of fine particles, Can. J. Chem. Eng. 80 (2002) 346] in their study of kaolinite–kerosene–air flows in trickle-bed reactors.

Onset of pulsing in gas–liquid trickle bed filtration

When liquid suspensions containing low concentration of fine solids are treated in catalytic packed bed gas–liquid–solid reactors, which are operated in trickle flow or near the transition between trickle and pulse flow, plugging develops and increases the resistance to two-phase flow. Also due to obstruction, such accumulation of fines in the catalyst bed shifts progressively the flow pattern from trickling to pulsing flow. The progressive onset of pulsing flow along the packed bed was estimated using a sequential approach based on combining a “large time-scale” unsteady-state filtration solution of two-phase flow with a “short time-scale” solution of a linear stability analysis of two-phase flow. Space–time evolution and two-phase flow of the deposition of fines in trickle bed reactors under trickle flow regime was described using a one-dimensional two-fluid model based on the volume-average mass and momentum balance equations and volume-average species balance equation for the fines. The model hypothesized that plugging occurred via deep-bed filtration and incorporated physical effects of porosity and effective specific surface area changes due to the capture of fines, inertial effects of phases, and coupling effects between the fines filter rate equation and the interfacial momentum exchange force terms. The transition between trickle flow and pulse flow regimes was described from a stability analysis of the solution of the transient two-fluid model around an equilibrium state of trickle flow under pseudo steady state conditions. The impact of liquid superficial velocity, viscosity and surface tension, gas superficial velocity and density, feed fines concentration, and fines diameter on the transition between trickle and pulse flows in the presence of fines deposition was analyzed.

Computation of effectiveness factors for partially wetted catalyst pellets using the method of fundamental solution

Trickle bed reactors are widely used in many process industries. The catalyst particles are often incompletely wetted especially in the trickling flow regime and hence to design these reactors, the effectiveness factor of partially wetted catalyst needs to be calculated accurately. Numerical solutions by traditional methods are time consuming and not very accurate, especially for some commonly used complex catalyst shapes such as trilobes, quadrilobes etc. The paper presents a novel numerical solution for these problems based on the method of fundamental solutions. The advantage of the method is that it involves only boundary collocation and can be applied to catalysts of any shape. Further the method provides an accurate estimate of the gradient of the concentration profiles and this information can be related directly to the effectiveness factor. This accuracy of the method is demonstrated for two dimensional (2-D) and axisymmetric problems for a linear kinetics. Illustrative results are presented for some complex shapes under partial wetting conditions.

Effects of Induced Pulsing Flow on Trickle-Bed Reactor Performance

The benefits of trickle-bed reactor operation under the induced pulsing flow regime are investigated using experiments and modeling. Under these conditions, by cycling the liquid feed, trickling and pulsing flow regimes can be made to alternate during the cycle period under time-averaged conditions corresponding to the trickling flow regime. For the hydrogenation of phenylacetylene over Pt/γ-Al2O3 catalyst, experimental results obtained in a laboratory-scale reactor operating under mild gas-limiting conditions indicate better performance for steady flow, as opposed to induced pulsing flow. The model predictions compare well with the experimental data. Further, simulations of a trickle-bed reactor over a wide range of initial reactant concentrations and pressures predict up to 45% improvement in styrene selectivity for induced pulsing flow under liquid-limited conditions. The findings suggest that enhancements in reactor performance due to induced pulsing can be expected for liquid-limited systems, which g...

Noble metals supported on carbon black composites as catalysts for the wet-air oxidation of phenol

Pt, Pd and Ru catalysts supported on carbon black composites (CBC) were characterized in the wet air oxidation of phenol solution using a fixed-bed reactor working in a trickle-flow regime under relatively mild conditions: temperature, 393–433 K; pressure, 50–80 bar; liquid hourly space velocity (LHSV), 0.5–6 h −1. The activity of the catalysts decreases in the following order: Pt/CBC>Pd/CBC≈Ru/CBC≫CBC. The physicochemical properties of the CBC are affected by its reaction with oxygen during the oxidation process. Combustion of the CBC material in the aqueous phase proceeds at a lower temperature than that in the gas phase; its surface properties change according the same rules as during low temperature oxidation by gaseous air.

Magnetohydrodynamics of trickle bed reactors: Mechanistic model, experimental validation and simulations

An isothermal one-dimensional two-fluid magnetohydrodynamic (MHD) model based on the volume average mass, charge, and momentum balance equations and the Maxwell's equations coupled via the Lorentz force and Ohm's law was developed for the prediction of the two-phase pressure drop and the total liquid holdup in trickle bed reactors experiencing a homogeneous transverse magnetic field. The slit model approximation and the drift flux Kozeny–Carman approach were extended for the derivation of appropriate drag force closures required in the conservation equations, respectively, in the trickle flow regime and in the dispersed bubble flow regime. The expression of liquid–solid drag was adapted to take into account the influence of the magnetic field on the laminar term and the damping of turbulent/inertial term via the Hartman number and the liquid-to-bed electrical resistance ratio. Associating these drag forces with the proposed model resulted in a fully predictive MHD approach for trickle beds. Several model limiting formulations were derived for an electrically conducting fluid flowing downwards with a stagnant gas (pure trickle flow) to yield liquid holdup, as well as for single-phase upward conditions to yield the single-phase pressure drops.

Nature and characteristics of pulsing flow in trickle-bed reactors

Pulsing flow is well known for its advantages in terms of an increase in mass and heat transfer rates, complete catalyst wetting and a decrease in axial dispersion compared to trickle flow. The operation of a trickle-bed reactor in the pulsing flow regime is favorable in terms of a capacity increase and the elimination of hot spots. Extending the knowledge on the hydrodynamic nature and characteristics of pulsing flow stands at the basis of further exploitation of the effects of this flow regime on reactor performance. An analysis of the hydrodynamics of pulsing flow reveals that pulse properties as liquid holdup, velocity and duration, are invariant to the superficial liquid velocity at a constant gas flow rate. The pulse frequency, however, increases with increasing superficial liquid velocity. The relative contribution of the pulses and the parts of the bed in between pulses to an average measured property can thus be obtained. By applying this concept it is shown that the linear liquid velocity inside the pulses varies between 0.1 and 0.2 m s −1 . The linear liquid velocity in between pulses, however, is invariant to gas and liquid flow rates and packing properties and equal to about 0.05 m s −1 . This suggests that a linear liquid velocity of about 0.05 m s −1 is the maximum velocity possible in the bed to maintain the trickle flow regime. All liquid in excess is transported as pulses. The liquid holdup in the parts of the bed in between pulses equals the liquid holdup at the transition to pulsing flow at all gas flow rates. The same trend holds for the linear liquid velocity in between pulses. Pulsing flow then is a hybrid of two transition states. The pulses reside at the transition to bubble flow, while the parts of the bed in between pulses reside at the transition to trickle flow. The enhanced particle-liquid heat transfer coefficient inside the pulses is mainly the result of the high linear liquid velocity inside the pulses. Particle-liquid heat transfer rates in between pulses are constant due to the constant linear liquid velocity.

Hydrodynamic Continuum Model for Gas–Liquid Fixed Beds Operating in Trickle Flow and Pre-Loading Regimes

A one-dimensional two-fluid continuum model based on the volume-average mass and momentum balance equations was developed for the prediction of the two-phase pressure drop and the total liquid hold-up in co-current down-flow and counter-current fixed-bed reactors operating respectively in the trickle flow regime and in the pre-loading zone. The slit model drag force closures, as well as the drag closures derived from the modified Kozeny–Carman theory based on a two-dimensional tortuosity model, were adapted. The inertial term in the drag force expressions was corrected to predict satisfactorily the two-phase pressure drops at high pressures. Wall effect functions were incorporated into the drag force equations. Associating the drag forces based on the modified Kozeny–Carman theory and the presently developed continuum model resulted in a fully predictive tool with no adjustable parameter both in co-current down-flow and counter-current flow.

Advantages of Forced Non-steady Operated Trickle-Bed Reactors

Trickle-bed reactors are usually operated in the steady state trickle flow regime. Uneven liquid distribution and the formation of hot spots are the most serious problems experienced during trickle flow operation. In this paper, we advocate the use of non-steady state operation of trickle-bed reactors. Based on a square-wave cycled liquid feed, several operation modes are developed that involve the artificial induction of natural pulses and control of the catalyst wetting efficiency over longer times. The operation modes aim at increasing the mass transfer rate of the limiting reactant and simultaneous prevention of flow maldistribution and hot spot formation. The operation modes are distinguished by a relatively fast and slow cycling of the liquid feed. The potential advantages of the developed feed strategies on reactor performance are evaluated.

Hydrodynamics of power-law fluids in trickle-flow reactors: Mechanistic model, experimental verification and simulations

A fully predictive one-dimensional mechanistic model was developed for describing the hydrodynamics of power-law fluids in trickle-bed reactors. The model is a generalization of the slit approach to the case of non-Newtonian fluids obeying Ostwald–deWaele rheological behavior. Without recourse to adjustable parameters, the proposed model enabled prediction of the experimental values of (i) total two-phase total pressure drop and total liquid holdup in the trickle flow regime, (ii) frictional pressure drop in single-phase flows through packed beds, and (iii) total liquid holdup in gravity driven liquid downflow and stagnant gas through packed beds. Parametric simulations guided by knowledge of the behavior of highly viscous Newtonian liquids in trickle beds highlighted the capability of the model in the simulation and design of trickle flow operation using power-law fluids.

CFD of multiphase flow in packed‐bed reactors: II. Results and applications

AbstractNumerical simulations of multiphase flow using the k‐fluid CFD model described in Part I of this issue are presented for packed beds at various operating conditions. Both steady‐state and unsteady‐state (e.g., periodic operation) feed conditions were studied numerically. Predictions of the k‐fluid CFD model are comparable with the experimental data in the literature for liquid upflow in a cylindrical packed bed. In addition to the mean porosity and the longitudinally averaged radial porosity profile, the variance of the porosity distribution is needed for predicting the probability density function of the sectional flow velocity. In the trickling flow regime, the k‐fluid CFD model provides reasonable predictions of the global liquid saturation and the pressure gradient. Relevant applications of the k‐fluid CFD model are identified in quantifying the relationship between bed structure and flow distribution in various‐scale packed beds. The combined flow‐reaction modeling scheme is proposed through the “mixing‐cell” network concept, in which the k‐fluid CFD simulation can provide the information on sectional flow distribution.

Hydrodynamics of Trickle-Flow Reactors: Updated Slip Functions for the Slit Models

A new set of velocity ( f v ) and shear ( fs ) slip correlations was proposed for improving the prediction, by the extended and the double-slit trickle-bed mechanistic models, of the liquid hold-up, the total pressure drop, the wetting efficiency, and the gas-liquid interfacial area in the trickle flow regime. Amendment of the original f s and f v slip correlations 3 ’ 6 was motivated by the following incentives: (i) a much wider trickle flow regime database currently at disposal with in excess of 5,100 experimental data; (ii) application of a traceablity protocol to guarantee no corruption and normalization of the compiled data in the database; (iii) contrary to previous formulations, reduction to one single set of f s and f v slip functions applicable to both the extended and the double-slit models. The slip functions were correlated in terms of gas Reynolds ( Re L ) , liquid Reynolds ( Re L ) , Froude ( Fr L ) , Weber ( We L ) , Lockhart-Martinelli ( X L ) , and Stokes ( St L ) numbers. Implementation of the slip functions in the extended slit model yields liquid hold-up and pressure drop with an average absolute relative error (AARE) of 17% and 19.5%, respectively. Similarly, the double-slit model predicts wetting efficiency, gas-liquid interfacial area, liquid hold-up, and total pressure drop with AARE = 11%, 34.7%, 20%, and 18%, respectively.

Measurement of liquid flow distribution in trickle bed reactor of large diameter with a new gamma-ray tomographic system

In the petrochemical industry area, many processes are carried out using fixed bed reactors with cocurrent upward or downward gas/liquid flows. One of the main problems to overcome during the scale-up of such reactors is to optimize the gas/liquid flow distribution across the reactor catalytic bed section. To accurately characterize the gas/liquid flow distribution inside a real catalytic bed, a new computed gamma-ray tomographic system has been developed to investigate two-phase flow through cold mock-ups of large scale ( 60 cm in diameter). In the case of developing a new reactor technology for processes working with fixed bed in the trickling flow regime, this type of measurement is quite important. This system, unique for such a large size catalyst bed, includes a series of 32 BGO detectors and a Cs 137 source. The performances of this experimental system have been fully evaluated through a validation including reconstruction of physical phantoms in static conditions. Several tests have been performed to measure the gas/liquid flow distribution in the catalyst bed for different inlet distributor configurations. It is shown, in the experiments performed for the trickle flow regime, that the instrumental technique has a very good sensitivity to liquid retention measurement. When the liquid retention measurements are compared with a hydrodynamic model based on Kozeny–Carman formalism a quantitative agreement is observed.

Extending the F-function concept to two-phase flow in trickle beds

The F-function approach is a class of two-fluid phenomenological models which has been recently proposed to conceptualize two-phase flows through fractured porous media in connection with petroleum and nuclear engineering. This approach takes into account inertial effects during two-phase flow. It is based on the generalization of the single-phase Forchheimer equation by multiplying the superficial velocity of each fluid by a function F that depends only on the fluid saturation. In this study, we propose to extend the F-function concept to the chemical engineering circle, especially for high-pressure gas–liquid co-current downflow trickle-bed reactors. Reclassification of trickle flow regime is attempted by distinguishing viscous, transitional and inertial regimes. The approach is tested on experimental data and carried out in the pressure range 0.3– 5.1 MPa using water/nitrogen, ethanol/nitrogen and ethylene glycol/nitrogen systems. Finally, the F-function concept performance is compared to most popular trickle-bed correlations.

Prediction of liquid - solid wetting efficiency in trickle flow reactors

A comprehensive database of catalyst wetting efficiency measurements in trickle flow regime was gathered from 14 independent studies to conduct a thorough evaluation of the performances of current correlations for the prediction of wetting efficiency during vertical gas-liquid co-current down-flow in randomly packed fixed bed reactors. Cross-examined with the database, several shortcomings arising from a low level of accuracy or a lack in generalization revealed the weakness of existing estimation methods. An approach relying on the combination of artificial neural network computing and dimensional analysis (ANNDA approach) helped to derive a highly accurate correlation for wetting efficiency in trickle flow regime. This correlation yielded an absolute average relative error of 8% and a standard deviation of 10%. The five dimensionless groups intervening in the proposed correlation were a composite two-phase low Reynolds, Re g, liquid Stokes (St), Froude (Fr) and Galileo (Ga) groups and a bed correction factor (S b).

Induced Pulsing Flow in Trickle Beds — Characteristics and Attenuation of Pulses

Gas/liquid down-flow in packed beds is studied, under periodic liquid feeding (at sufficiently high frequencies to be classified as “ fast” mode of pulsing), in a range of mean liquid and gas flow rates within the steady “trickling flow regime”. The aim is to identify periodic feeding conditions resulting in improved fluid-mechanical characteristics (e.g. uniform fluids distribution) and possibly enhanced transport rates in this flow regime, which is common in industrial processes. From instantaneous, cross-sectionally averaged holdup measurements, at various locations along the packed bed, quantitative information is obtained on the axial propagation and attenuation of induced pulses. A phenomenological treatment of the pulse decay process facilitates data interpretation and leads to the determination of a characteristic attenuation factor for the various conditions tested. Key parameters of the process studied include, in addition to dynamic holdup, pressure drop, pulse celerity and intensity, as a function of fluid feed rates (G, L) and liquid cyclic frequency. Under the conditions of these tests, and for fixed mean rates G, L, the time averaged holdup and the pulse celerity are practically constant along the bed; furthermore, these quantities as well as the pressure drop do not seem to be affected by the imposed cyclic liquid feeding frequency. An expression to tentatively correlate pulse celerity data is recommended. The computed attenuation factors indicate that there is a rather narrow band of mean gas and liquid rates (along the so-called “pseudotransition” boundary to pulsing flow) where pulse decay is at a minimum. Based on these results as well as on pulse intensity vs. bed length data, recommendations are made on preferred conditions for induced pulsing (from the fluid-mechanical standpoint) which would maximize expected benefits. 2005 Elsevier Ltd. All rights reserved.

Catalytic wet air oxidation of Kraft bleach plant effluents in a trickle-bed reactor over a Ru/TiO2 catalyst

Catalytic wet air oxidation of two acidic and alkaline Kraft bleach plant effluents (total organic carbon (TOC) content 1138 and 1331mgl−1, respectively) was investigated in a trickle-bed reactor at T=463K and oxygen partial pressure of 8bar. The reactor packed with titania or titania-supported ruthenium catalyst was operated in a low-interaction (LIR) trickle-flow regime either in continuous-flow or batch-recycle mode. In the off-gas, no carbon monoxide was detected in any of the runs. When the catalytic bed was composed of TiO2 particles, moderate abatement of organic compounds from the bleach plant effluents was observed (TOC conversions up to 46 and 26%, respectively). The removal of parent organic material was further enhanced by deposition of metallic ruthenium (3wt.%) on the titanium oxide support. In that case, the once-through oxidation produced decolorized outlet streams with TOC conversions as high as 89 and 88%, respectively. These values further increased to 98 and 95%, respectively, by running the trickle-bed reactor in the batch-recycle mode of operation; the residual carbon content in treated effluents was found in the form of acetic acid. No leaching of Ru or Ti was detected by ICP-AES analysis to the detection limits of 0.2 and 0.1mgl−1, respectively.