Gas-liquid Cocurrent Downflow Research Articles

Assessment of the Resilience against Liquid Maldistribution of Monolith Packings under Offshore Floating Conditions

The study provides insights for adapting commercially available packings to work on floating vessels in the marine context aiming at preserving performances similar to land-based packed-bed reactors. Specifically, liquid distribution for cocurrent gas-liquid downflow (Ug = 0.0065-0.131 m/s and Ul = 0.001-0.0078 m/s) inside monolith packings embarked on a hexapod emulator was compared against glass-bead containing trickle bed and open-cell foam packings using wire-mesh sensors under vertical, inclined and rolling column conditions. Sensitivity of the tested internals with regard to liquid maldistribution was diagnosed and analyzed in terms of a maldistribution factor for the vertical/inclined and rolling columns. Liquid maldistribution was found to prevail at high gas and low liquid superficial velocities for the tested three packings regardless of column conditions. Monolith packed beds demonstrated superior robustness to counteract liquid maldistribution induced by bed static inclinations and roll motion...

Dynamic Liquid–Solid Mass Transfer in Solid Foam Packed Reactors at Trickle and Pulse Flow

The effective liquid–solid mass transfer (LSMT) has been investigated in solid foam packed reactors under cocurrent gas–liquid downflow. Based on a comprehensive literature analysis, the limiting current technique was adapted successfully to solid foam packings enabling measurements of dynamic mass transfer coefficients at different axial packing positions. The flexible reactor setup was used to analyze the appropriateness of various setup configurations. The LSMT coefficients are presented for two different solid foam pore densities for a wide range of gas and liquid velocities covering trickling and pulsing flow regime. To illustrate the difference in the LSMT dynamics in terms of electric current compared to the liquid holdup at pulse flow, a compartment model is developed. Excellent agreement is achieved between simulations and measurement data. Eventually, the effective mass transfer data are compared with conventional random and structured packings.

Process intensification of gas–liquid downflow and upflow packed beds by a new low‐shear rotating reactor concept

In the present work, a new low‐shear rotating reactor concept was introduced for process intensification of heterogeneous catalytic reactions in cocurrent gas–liquid downflow and upflow packed‐bed reactors. To properly assess potential advantages of this new reactor concept, exhaustive hydrodynamic experiments were carried out using embedded low‐intrusive wire mesh sensors. The effect of the rotational velocity on liquid flow patterns in the bed cross‐section, liquid saturation, pressure drop, and regime transition was investigated. Furthermore, liquid residence time and Péclet number estimated by a stimulus‐response technique and a macro‐mixing model were presented and discussed with respect to the prevailing flow patterns. The results revealed that the column rotation induces different flow patterns in the cross‐section of the packed bed operating in a concurrent downflow or upflow mode. Moreover, the new reactor concept exhibits a more flexible adjustment of pressure drop, liquid saturation, liquid residence time, and back‐mixing at constant flow rates. © 2016 American Institute of Chemical Engineers AIChE J, 63: 283–294, 2017

Hydrodynamics of gas–liquid cocurrent downflow in floating packed beds

An innovative experimental setup combining a robot with six-degree-of-freedom motions (translational: surge, sway, heave, and rotational: roll, pitch, yaw) and an embarked column instrumented with a capacitance wire mesh sensor was built to investigate experimentally the two-phase flow hydrodynamics in porous media within inclined stationary and oscillating packed beds under gas–liquid cocurrent descending flows. The hydrodynamic deviations with respect to the stationary straight column were thoroughly analyzed in terms of bed pressure drop, liquid saturation, regime changeover, and pulse flow properties. Non-tilting oscillations had marginal effects on the packed bed hydrodynamics which remained practically similar to the one prevailing in stationary vertical trickle beds. Conversely, tilting oscillations profoundly influenced the flow regime inside the packed bed as a result of the time-dependent nature of the gravitational and inertial (acceleration/deceleration) forces brought about by the hexapod rotational oscillations which caused important secondary transverse displacements of the fluids in the bed. Using moment analysis of the probability density functions of the registered liquid saturation time series, it was found that due to the periodic gravity-driven stratification, the transition to pulse flow shifted towards higher fluid velocities as the oscillation periods of roll or roll+pitch motions were increased.

A novel inclined rotating tubular fixed bed reactor concept for enhancement of reaction rates and adjustment of flow regimes

The inclined rotating tubular fixed bed reactor has been introduced recently as a novel reactor concept for multiphase processes, especially for heterogeneously catalyzed gas–liquid reactions with a mass transfer limitation of the gas phase (Härting et al., 2015; DOI: http://dx.doi.org/10.1016/j.ces.2015.02.008). It is based on the adjustment of a beneficial gas–liquid distribution in the cross section of the fixed bed that allows for complete utilization of the fixed bed accompanied by periodic wetting and draining of the catalyst.The hydrodynamics in gas–liquid co-current downflow are studied by applying a compact gamma-ray computed tomography system for different organic liquids and gas phase properties as well as various fixed bed packing materials. Four different flow regimes with stratified, sickle, annular and dispersed flow patterns are identified. Pressure drop and liquid saturation are presented as a function of reactor inclination and rotation.Inclination of the reactor is applied to force phase separation and the superimposed rotation of the clamped fixed bed results in a favorable wetting intermittency via periodic catalyst immersion. A significant rate enhancement of the hydrogenation of alpha-methylstyrene to cumene under severe limitations of the mass transfer of the gas phase is observed in the novel reactor concept at separated flow conditions. In addition, the potential for an efficient quenching of hotspots for exothermic reactions is demonstrated.

Liquid backmixing in an inclined rotating tubular fixed bed reactor – Augmenting liquid residence time via flow regime adjustment

The liquid residence time and the backmixing in an inclined rotating tubular fixed bed reactor operated with gas–liquid co-current downflow are studied experimentally. This novel reactor concept is introduced to extent the process intensification strategies of chemical multiphase reactors. The intermittent catalyst immersion due to rotation induces a continuous refreshment of the liquid at the catalyst surface and enhances the access of the gas phase to the catalyst in the drained section of the fixed bed. Depending on inclination angle and rotational velocity, different flow regimes are observed. In particular, the flow regimes with stratified gas–liquid flow can be utilized to enhance the performance of the reactor for heterogeneous catalytic reactions.The backmixing study is based on the method of the imperfect tracer pulse and the propagation of the tracer is measured by low-intrusive wire-mesh sensors. Compared to conventional trickle bed reactors, liquid residence time and axial dispersion are increased by the inclination and rotation. The effects of reactor inclination angle and rotational velocity as well as of particle size and liquid superficial velocity on the liquid backmixing in the inclined rotating tubular fixed bed reactor are shown in detail.

Hydrodynamics of co-current two-phase flow in an inclined rotating tubular fixed bed reactor — Wetting intermittency via periodic catalyst immersion

The hydrodynamics of an inclined rotating tubular fixed bed reactor operated with gas–liquid co-current downflow are studied. Reactor inclination is applied to force phase segregation, while the superimposed rotation of the reactor results in a wetting intermittency via periodic catalyst immersion. The fixed bed is clamped to avoid abrasion of the catalyst. The inclined rotating reactor is presented as a new reactor concept for process intensification of heterogeneous catalytic reactions requiring enhanced mass transfer of the gaseous phase and partial catalyst wetting. Four different flow regimes with stratified, sickle, annular and dispersed flow patterns are determined experimentally by applying a compact gamma-ray computed tomography system. The effects of (i) gas and liquid superficial velocities, (ii) inclination angle and rotational velocity of the reactor and (iii) physico-chemical properties of the liquid phase on the occurrence of the flow regimes are investigated. The results of these investigations are illustrated with flow maps. In addition, pressure drop and liquid saturation depending on the operating conditions are shown.

Assessment of mass transfer in the stagnant liquid regions in trickle-bed reactors

A critical revision has been carried out of experimental investigations to quantify the size of the liquid stagnant region in fixed beds with co-current gas–liquid down-flow, and its ability to exchange mass with the dynamic region and with the particle surface. The revision has exposed that a high level of uncertainty arise to estimate, particularly, the mass exchange rate from the available evidence. Therefore, a second purpose is to introduce a geometric model to represent the stagnant region for spherical particles from which, assuming that diffusion is the only mass transport mechanism, it becomes possible to estimate the mass exchange rates with the dynamic region and with the particle surface. Finally, it was discussed why to expect that the geometric model is able to provide a lower bound to mass exchange rate with the dynamic region and a tight lower bound to mass exchange rate with the catalyst surface.

Purification Processes –biodegradation of Vinyl Acetate from Waste Air in a Trickle-bed Bioreactor (tbb).

The removal of vinyl acetate from waste gas in co-current gas-liquid downflow trickle – bed bioreactor inoculated with Pseudomonas fluorescens PCM 2123 strain was studied experimentally. Due to the lack of literature data, it was necessary to experimentally determine a rate of the partial stages of the process, which are: the rate of mass transfer process in the gas and liquid phases and the rate of biological reaction. The results of the experiments carried out in TBB were compared with the values obtained from mathematical model. The good compatibility of the calculated and experimental data was obtained.

Trickle bed mechanistic model for (non-)Newtonian power-law foaming liquids

Physical models for the description of the hydrodynamics of trickle-bed reactors for foaming Newtonian and power-law liquids are virtually inexistent in the literature. An attempt is made in this work to develop a hydrodynamic model for cocurrent gas–liquid downflow involving foaming (non-)Newtonian power-law liquids through packed beds at non-ambient temperatures and moderate pressures. The model was an extension of the well-trodden slit model variants proposed for (non-)Newtonian power-law coalescing-liquid systems. Its validation using recently published experimental data [Aydin, B., and Larachi, F., 2008. Trickle bed hydrodynamics for (non-)Newtonian foaming liquids in non-ambient conditions. Chem. Eng. J. 143, 236–243.] allowed a quantitative representation of foaming flow hydrodynamics in terms of the two-phase pressure gradient and the liquid holdup up to 90 °C and 0.7 MPa. The model, requiring no arbitrary adjustable parameters, in addition related foam stability to quantitative descriptors such as the liquid-film-borne bubble holdup and diameter, and free (or headspace)-gas holdup. The model also exhibited logical and fully interpretable trends with regard to foam stability, reactor temperature and pressure, liquid and gas superficial velocities.

Experimental investigation of gas–liquid distribution in monolith reactors

The present work investigates the influence of gas and liquid flow rates on inlet liquid distribution across monoliths operating in gas–liquid cocurrent downflow mode. Gas and liquid superficial velocities range from 0 to 68 and 1.4 to 8.5 cm/s, respectively. Gas–liquid distribution was studied using a packed bed liquid distributor and a 1 2 in pipe distributor for the aforementioned range of operating conditions. To determine the liquid distribution over the monolith, gravimetric, time-averaged liquid collection method was applied using a customized collector apparatus. Quantification of the distribution is reported using a suitably defined maldistribution factor. For each liquid velocity, gas velocities are varied and corresponding maldistribution factors are calculated. The results are reported in view of the varying operating conditions.

Effect of surfactant additives on co-current gas–liquid downflow

The influence of a surfactant additive on the interfacial structure and on the transition from the smooth to the wavy stratified flow regime in inclined pipes is investigated for various gas–liquid flow rates. The effect of a surfactant on the gas–liquid stratified downflow patterns is studied using a dilute aqueous solution of a non-ionic surfactant ( Tween ® 80 ) , which is found to have a dramatic effect on two-phase flow characteristics. A conventional solution having a surface tension similar to that of the surfactant solution is also employed and this verifies that the atypical behavior of the surfactant solution is ascribed to its special chemical structure and not to its low surface tension. Liquid layer thickness time records are acquired using a parallel-wire conductance technique from which the statistics of the layer thickness, as well as wave celerities, are calculated. Laser doppler anemometry (LDA) is employed to investigate the flow structure in the thin liquid layer both with and without interfacial shear induced by a co-current gas flow. A differential pressure transducer is used for pressure drop measurements in the liquid phase. The interpretation of the results verifies that the “ delayed” appearance of the interfacial waves should be associated with the turbulence retardation within the liquid layer and it facilitates the clarification of the mechanisms in which the surfactant interacts with the turbulent structures and causes significant drag reduction.

Evaluation of pseudohomogeneous models for heat transfer in packed beds with gas flow and gas–liquid cocurrent downflow and upflow

Several pseudohomogeneous models are used by researchers in the study of heat transfer in packed beds. In this work, five of the most used pseudohomogeneous models (to one, two and three parameters) are analyzed, for gas and gas–liquid flow configurations. The models were evaluated concerning the following aspects: (a) the fitting between calculated and measured temperatures, (b) the values of thermal parameters, (c) their confidence intervals, (d) the quality of the estimation of the thermal parameters by analysis of their Box biases, and (e) the nonlinear dependence of the calculated temperatures on the thermal parameters (using the curvature measures of Bates and Watts). It was observed, particularly in gas–liquid flow, that the fittings between calculated and measured temperature profiles are better for models in which a wall heat transfer coefficient is incorporated to consider the convective resistance at the bed wall. It was also noted that the values of the thermal parameters fitted from the pseudohomogeneous models may be very different at identical operational conditions. The effective axial thermal conductivity may be neglected in the modeling because its estimation does not affect the residual functions. Besides, the estimation of k a is tricky because it depends on the initial guess and also because the parameter is extremely sensitive to changes in the operational conditions. The confidence intervals for the parameters depend on the model and are also affected by the experimental conditions. The estimation of the parameters was adequate for the k r – h W and k r – k a models and the curvatures measures were satisfactory only for models in which h W was not incorporated.

Liquid holdup and pressure drop in the gas–liquid cocurrent downflow packed-bed reactor under elevated pressures

An experimental investigation of the residence time distribution, liquid holdup, and pressure drop in a gas–liquid downflow packed bed reactor with porous particles operated under elevated pressures is presented. The effects of the two-phase flow rates and reactor pressures on the external liquid holdup and pressure drop are discussed. A mechanistic model, which accounts for the interaction between the gas and liquid phases by incorporating the shear and velocity slip factors between phases, is employed to predict the external liquid holdup and pressure drop for the experimentally covered flow regime. The involved parameters, such as shear and velocity slip factors and Ergun single-phase flow bed constants, are calculated from the correlations developed via neural network regression. The model's predictions and the experimental observations at elevated pressure are compared.

Total Liquid Saturation in Gas−Liquid Cocurrent Downflow and Upflow through Packed Beds and Analysis of Correlations for Predicting the Total Liquid Saturation

In this work, the effects of the gas and liquid flow rates, as well as of the size and shape of particles, on the total liquid saturation in gas-liquid cocurrent downflow and upflow through packed beds are investigated. Experimental values of the liquid saturation are obtained using air and water as the percolating fluids. The results indicate that the air and water flow rates affect the liquid saturation under downflow and upflow conditions. Concerning the particle characteristics, it was observed that the particle shape affects the values of β t for downflow and upflow conditions, whereas the influence of particle size is more significant in the downflow configuration. The experimental values are compared to the predictions of some correlations presented in the literature, and new correlations are proposed for predicting β t in downflow and upflow configurations.

Bubble flow in trickle beds: investigations using resistive sensors

The local hydrodynamics of co-current gas–liquid down-flow through porous media are investigated in a quasi two-dimensional regular arrangement by means of a network of resistive sensors. The investigations are focused on a liquid continuous flow regime, the dispersed bubble flow, where the gas is divided into elongated bubbles. Due to the variation of the local flow channel orientation and the local void fraction, the average bubble velocity strongly depends on the local geometry. The flow is more coherent in vertical constrictions, compared to all other types of sites, this is probably due to bubble stagnation in the flow channel enlargements. At a given liquid superficial velocities and for sufficiently high superficial gas velocities, the average bubble size is independent of the gas flow rate; it is of the order of magnitude of the volume of the enlargements of the porous medium. The maximum bubble size is about three times its average size, corresponding thus to the coalescence of three average sized bubbles.

Multiphase catalytic reactors: a perspective on current knowledge and future trends

ABSTRACT Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas–liquid downflow, trickle-beds with countercurrent gas–liquid flow, and packed-bubble columns where gas and liquid are contacted in cocurrent upflow. The advantages of cyclic operation are also outlined. This is followed by a discussion on conventional reactors with mobile catalysts, such as slurry bubble columns, ebullated beds, and agitated reactors. Several unconventional reactor types are reviewed also, such as monoliths for two-phase flow processing, membrane reactors, reactors with circulating solids, rotating packed beds, catalytic distillation, and moving-bed chromatographic reactors. Numerous references are cited throughout the review, and the state-of-the-art is also summarized. Measurements and experimental characterization methods for multiphase systems as well as the role of computational fluid dynamics are not covered in a comprehensive manner due to other recent reviews in these areas. While it is evident that numerous studies have been conducted to elucidate the behavior of multiphase reactors, a key conclusion is that the current level of understanding can be improved further by the increased use of fundamentals.

Evaluation of heat transfer parameters in packed beds with cocurrent downflow of liquid and gas

The results of an experimental investigation on heat transfer from a packed bed with cocurrent gas–liquid downflow to the wall are presented and analyzed in this contribution. The measurements cover the range of operating variables corresponding to the so-called trickle regime in beds presenting aspect ratios (tube to particle diameter ratio) from 4.67 to 34.26. Water and air were employed as model fluids. The heat transfer process was first analyzed by means of a two-dimensional pseudohomogeneous plug-flow model with two parameters, the effective radial thermal conductivity ( k er ) and the wall heat transfer coefficient ( h w ). k er is well correlated with liquid and gas Reynolds numbers and particle diameter, except for the lowest experimental aspect ratio (4.67). Instead, a meaningful correlation of h w stands only for aspect ratios larger than 15. These results are analyzed and the evidence points out to sustain the hypothesis that the model fails at low aspect ratios because an apparent contact resistance (1/ h w ) can no longer accommodate the effects of significant fluid bypassing and finite size of the near-wall region. The experimental set of data were also used to develop a correlation for the overall heat transfer coefficient ( h T ), which can be employed satisfactorily to predict heat transfer rates in the whole range of variables here investigated.

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).