Micro-engineered catalyst systems: ABB’s advancement in structured catalytic packings

ABB has advanced catalysis with micro-engineered catalyst (MEC) systems by providing a uniquely small particle size on a formable catalyst support through the integration of catalysis and reaction engineering. A mechanically strong catalytic web of micro-fibers has been engineered and shaped utilizing both computational fluid dynamics (CFD) and cold flow experiments to optimize flow characteristics. This article discusses techniques used for the development of novel catalytic structured packings for catalytic distillation applications. CFD models (verified through experiments performed on small-sized structures) were shown to be of great utility in screening new structure ideas. Results will illustrate achievement of both high gas–liquid contacting and bulk mixing at low pressure drop with the potential to provide enhanced catalyst utilization by taking advantage of the intrinsic MEC properties, particularly its high porosity and exposed geometric fiber and catalyst surface area. This was shown by the successful testing of one of these catalyzed structures in the selective hydrogenation of C4 acetylenes.     

Counter-current gas−liquid flow between vertical corrugated plates

The paper is devoted to a theoretical analysis of the counter-current gas−liquid film flow between vertical corrugated plates. We use the Navier−Stokes equations in their full statement to describe the liquid phase hydrodynamics. For the gas phase equations, we use the Benjamin−Miles approach where the wavy liquid/gas interface is a small disturbance for the turbulent gas and where we can linearize the gas phase governing equations. We consider both the steady state and the two-periodical traveling solutions of the counter-current gas/liquid flow between the corrugated plates. The changes in the liquid film hydrodynamics with the increase in gas superficial velocity are the main interest of the investigation. What is the flooding mechanism in the case of flow between the corrugated plates and does the gas superficial velocity for the flooding depend on the wall corrugation parameters?     

Three-component distillation using structured packings: Performance evaluation and model validation

This paper presents results of continuous feed and total reflux distillation experiments carried out with a common type and size structured packing using two- and three-component mixtures of common alcohols and water. With the binary mixture, the packing performed slightly better than with a three-component mixture. Surprisingly, the continuous operation performance appeared slightly better than in case of the total reflux operation. Delft model predictions of the overall mass transfer efficiency appeared to be conservative enough around the operating/design point load of the packing in question. The composition profiles measured with the three-component mixture were used to validate the rate-based (non-equilibrium) model developed at the Nagoya Institute of Technology, which appeared to be highly accurate, but also sensitive to the choice of the predictive method for the interfacial area.     

Influence of the viscosity on the liquid hold-up in trickle-bed reactors with structured packings

The influence of liquid viscosity on liquid hold-up in structured packings under co-current gas–liquid downward flow operation has been investigated for liquid viscosity from 1 to 20 cP. The liquid hold-up has been determined on a 400 mm internal diameter column by gamma tomographic cross-sectional measurements. An important influence of the viscosity on the liquid hold-up is observed. It is shown that, the widely used model supported by Bravo et al. [J.L. Bravo, J.A. Rocha, J.R. Fair, Hydrocarbon Process. January (1985) 91] assuming 1D fully established vertical liquid film flow does not agree with the experimental data. From experiments, the different assumptions used in the 1D model are discussed. On the basis of these results, a new correlation is proposed, which enables to calculate the hold-up from the viscosity, the liquid flow rate and the geometry of the packing. A comparison with data of literature is done.     

Modeling of dry pressure drop for fully developed gas flow in structured packing using CFD simulations

Dry pressure drop in columns equipped with structured packings is considered to involve two components: drag force due to the direction changes near the column walls and in the transition region between two packing layers rotated to each other by 90°, and friction force between the different gas flows inside the crossing triangular channels and with the packing solid walls. It is believed that in a packed bed with compact sheet density and large packing surface area (above 250 m²/m³), the major contribution of the pressure drop is generated by the friction component.In this paper, a model is proposed to determine the dry pressure drop friction component. The gas is assumed to establish a fully developed turbulent flow inside the structured packing channels. The structured packing geometry consists of a combination of periodic elements. It is shown that the reproduction of one periodic element aerodynamics leads to determine the gas distribution and pressure drop inside the packed bed. Therefore, modeling the dry pressure drop through one periodic element is a meaningful representation of the dry pressure drop over the packing.CFD simulations are carried out on periodic elements using different turbulence models: RNG k−ε, realizable k−ε, and SST k−ω. The best results that agree with the experimental data in the literature are obtained with the SST k−ω model. The CFD model proposed is used to study the impact of packing geometry variations on the dry pressure drop and to bring up a correlation for the pressure drop with respect to changes of packing geometry: channel height dimension, channel opening angle, and corrugation angle.     

Direct heat and mass transfer in structured packings

Direct heat transfer is an important method in the exchange of heat between two countercurrent process streams within a column. The process can be simulated using either the theoretical stage or the rate based concept. With both concepts, a reliable heat transfer coefficient is needed. Additionally, the rate of the heat transfer coefficient is influenced by the simultaneous mass transfer.

A number of application-dependent methods to estimate the heat transfer coefficient have been developed, mainly for random packings. It is the purpose of this paper to extend this work to structured packings.

A number of experiments with air/water have been performed in a column of 300 mm inner diameter with Mellapak 250.Y, 250.X and 125.X at ambient conditions. A second group of measurements were done using an oil/air system where only sensible heat was transferred.

Based on these experimental results a method was developed to predict the heat transfer coefficient for structured packings. The method is applied to examples of industrial importance, like a gas quench, a gas saturator and a pump-around zone in an atmospheric tower.     

Film flow down a fibre at moderate flow rates

We consider axisymmetric fluid flow down the exterior of a rigid straight vertical cylinder and model the emergence and propagation of the finite-amplitude waves that are created at the free surface. A system of evolution equations for the film thickness and volumetric flow rate, that was first derived by Trifonov [1992. Steady-state travelling waves on the surface of a viscous liquid film falling down on vertical wires and tubes. A.I.Ch.E. Journal 38, 821-834] has been re-formulated as an extension of the classical falling film problem. To inspire confidence in the predictions of this model, its linear stability characteristics versus those of the full Navier-Stokes equations are examined yielding very good agreement. Travelling wave solutions are determined and analysed in detail over a wide range of system parameters. The solutions resemble those associated with film for sufficiently small film thickness to fibre radius ratios, and beads when these ratios are relatively large. Transient computations are also performed for comparison with the travelling wave solutions and demonstrate the selection mechanism leading to the development of so-called ‘dominating’ waves for comparison with experimental observations.     

Investigation of the convective motion through a staggered herringbone micromixer at low Reynolds number flow

A computational study is presented of the complex flow through a staggered herringbone micromixer (SHM), which utilises sequences of asymmetrical herringbone grooves in cycles where a set of topologically similar grooves represent a half cycle. It was analysed using finite-element (method) based software to elucidate the fluid flow within the channel and characterise the effect of the grooves at moving fluid across the channel thus creating non-axial fluid movement. Three separate physical systems were modelled: a channel containing a single groove, a half cycle of infinite grooves and an infinite system with one groove per half cycle. A range of groove heights were investigated for the single groove for the Reynolds number range 0-15 to identify the mechanics through which fluid is transported across the channel by the grooves, the effect that inertial and viscous forces have on the process and to identify a groove height range for optimised cross channel fluid transfer. The flow field within the grooves at various heights was analysed and their relationship with non-axial flow within the bulk channel identified. The culminating effect of increasing grooves per half cycle on their ability to transport fluid across the channel is analysed by comparing the entrainment of fluid into and across the groove for both a single and infinite grooves. The maximum increase in fluid entrainment per groove for the addition of extra grooves to a cycle was found to be 14%. The helicity (or swirl) of the flow within the channel is found to be small for all three systems, while increased helicity within the flow was found to correspond to an increase in energy dissipation.     

Thin film flow over spinning discs: The effect of surface topography and flow rate modulation

We examine the effect of disc topography and time modulation of the liquid flow rate at the inlet on the dynamics of a thin film flowing over a spinning disc. We use a combination of boundary-layer theory and the Kármán-Polhausen approximation to derive coupled equations for the film thickness, and radial and azimuthal flow rates. Substrate patterning is taken into account in the limit of small-amplitude topography. Our numerical results indicate that the combined effects of flow rate modulation at the inlet and disc patterning can lead to a significant increase in interfacial waviness, which greatly exceeds that associated with the constant flow rate, smooth disc case.     

Reactive distillation: Non-ideal flow behaviour of the liquid phase in structured catalytic packings

Reactive distillation, the combination of chemical reaction and multistage distillation, is one of the most important industrial applications of the multifunctional reactor concept. The most promising column internals for reactive distillation are the so-called structured catalytic packings that combine favourable characteristics of traditional structured packings and heterogeneous catalysts. The non-ideal flow behaviour of the gas and the liquid phase is a fundamental aspect in multiphase reactor design since it has a strong influence on the reactor performance. In this study, liquid phase residence time distributions for the catalytic structured packing MULTIPAK^® were measured by means of conductivity measurements under different liquid and gas flow rates and evaluated with differential models.     

Prediction of effective area and liquid hold-up in structured packings by CFD

Interfacial effective area and liquid hold-up in structured packing geometries are investigated using the volume of fluid method. Three-dimensional numerical simulations of gas–liquid flow on inclined plane plate and in a structured packing are performed. The VOF method is used to capture the gas–liquid interface motion. After a first validation case on the wetting phenomena prediction on an inclined plane plate, the effective interfacial area, the liquid hold-up and the degree of wetting of packing are studied as function of liquid flow rate and wall surface characteristic (adherence contact angle). Results show that the liquid flow-rate and the contact angle play a significant role. It is found that the interfacial effective area and the degree of wetting of packing increase as the liquid flow rate increases and as the contact angle decreases. Moreover, under the influence of the contact angle, different liquid film shapes are observed. The simulations results are compared to experimental data available in literature. This work shows that the CFD is a powerful tool to investigate performance characteristics of structured packings. Moreover, this work shows how CFD can be used as an effective tool to provide information on fluid flow behavior and determination of interfacial area, liquid hold-up and minimum flow-rate to ensure complete wetting. These parameters could be further used in process simulation at larger scale for the development and the design of efficient packings.     

Effect of contamination on rise velocity of bubble swarms at moderate Reynolds numbers

In this work, hydrodynamics of contaminated bubble swarms is numerically investigated using the free surface cell model combined with the spherical stagnant cap model. The governing field equations are solved numerically to elucidate the effect of Reynolds number, gas holdup and degree of contamination on the hydrodynamic behavior of bubble swarms. New extensive results are reported over the range of conditions as follows: Reynolds number, Re: 1–200, bubble holdup, Φ: 0.1–0.5, and stagnant cap angle, α: 0–180°. Finally, the effects of these parameters on streamlines and vorticity contours, surface pressure and vorticity distributions and on drag coefficients are discussed in detail. Briefly, the drag coefficients decrease with the decreasing stagnant cap angle and/or the decreasing bubble hold up and/or the increasing Reynolds number; whereas the ratio of the pressure and friction drag coefficients exhibits mixed trends with respect to these parameters.     

The effect of surfactant on the flow of a thin liquid film over a spinning disc

We examine the flow of a thin liquid film over a spinning disc in the presence of dilute insoluble surfactant. We use the integral method to derive a coupled system of equations that govern the axisymmetric evolution of the film thickness, radial flow rate, angular momentum and surfactant surface concentration; a linear equation of state is used for closure. This system of equations is parameterized by a modified Weber number, a Marangoni parameter, M, and a surface Peclet number, Pe_s. Numerical solutions of these equations reveal that the presence of surfactant gives rise to Marangoni stresses which interfere with the mechanisms of wave growth. For M∼0.1, our results indicate that the formation of interfacial waves is suppressed.     

Mass transfer from a single fluid sphere to power-law liquids at moderate Reynolds numbers

The steady-state convective inter-phase mass transfer from a single Newtonian fluid sphere (free from surfactants) to a continuous phase with power-law viscosity has been studied at moderate Reynolds and Schmidt numbers under the conditions when the resistance to mass transfer in the dispersed phase is negligible. The species continuity equation, segregated from the momentum equations of both phases, has been numerically solved using a finite difference method. The effects of the Reynolds number (Re_o), power-law index (n_o), internal to external fluid characteristic viscosity ratio (k) and Schmidt number (Sc) on the local and average Sherwood number (Sh) have been analysed over the following ranges of conditions: 5?Re_o?200, 0.6?n_o?1.6, 0.1?k?50 and 1?Sc?1000. It has been observed that irrespective of the values of the Reynolds number and of the power-law index, as the value of k increases the average Sherwood number decreases for intermediate to large values of the Peclet number. As the value of the power-law index increases, the rate of mass transfer decreases for all values of the Reynolds number and the characteristic viscosity ratio thereby suggesting that shear-thinning behaviour facilitates mass transfer, whereas shear-thickening behaviour impedes it. Based on the present numerical results, a simple predictive correlation is proposed which can be used to estimate the rate of inter-phase mass transfer of a fluid sphere sedimenting in power-law liquids.     

Drag on a single fluid sphere translating in power-law liquids at moderate Reynolds numbers

A numerical investigation has been carried out to obtain the steady state drag coefficients and flow patterns of a single Newtonian fluid sphere sedimenting in power-law liquids. A finite difference method based simplified marker and cell (SMAC) algorithm has been implemented on a staggered grid arrangement to solve the continuity and momentum equations. For both phases, the convective terms have been discretized using the quadratic upstream interpolation for convective kinematics (QUICK) scheme, and diffusive and non-Newtonian terms with central differencing scheme. An exponential transformation has been applied in the radial direction for the continuous phase computational domain. In order to ensure the accuracy of the solver, extensive validation has been carried out by comparing the present results with the existing literature values for a few limiting cases. Further, in this study the effects of the Reynolds number (Re_o), internal to external fluid characteristic viscosity ratio (k) and power-law index (n_o) on the continuous phase flow field, pressure drag (C_dp), friction drag (C_df) and total drag (C_D) coefficients have been analyzed over the range of parameters: 5?Re_o?500, 0.1?k?50 and 0.6?n_o?1.6. Based on numerical results obtained in this work, a simple correlation has been proposed for the total drag coefficient, which can be used to predict the rate of sedimentation of a fluid sphere in power-law liquids.     

Air pollutant absorption by single moving droplets with drag force at moderate Reynolds numbers

Mass transfer phenomena of hydrogen chloride around single water droplets at moderate initial Reynolds numbers are investigated to simulate air pollutant absorption by droplets in wet scrubbers. Of particular interest is the uptake mechanism in the droplet under the impact of deceleration. An examination of the mass transfer inside the droplet, in view of the solute transport delay from the gas-liquid interface to the droplet interior, a maximum distribution in concentration difference between the droplet surface and the internal minimum concentration is exhibited. Meanwhile, gaseous scavenging behavior is apparently characterized by the droplet. Regarding the effect of the decelerating motion, the predictions reveal that the variation of the droplet velocity due to drag force is faster than that of the uptake process. Therefore, the absorption rates of the decelerating droplet are substantially decreased when compared with that of a droplet without drag force. As a whole, increasing initial Reynolds number causes faster decay in the droplet velocity which further reduces the mass transfer rate in the aqueous phase. This suggests that the larger the initial Reynolds number, the more significant the absorption rate of the droplet affected by the drag force.     

Investigation of laminar flow in a stirred vessel at low Reynolds numbers

Many mixing applications involve viscous fluids and laminar flows where the detailed as well as overall flow structures are important. In order to understand the fluid dynamic characteristics of low Re laminar flows in mixing vessels, the flow induced by a Rushton impeller for three Re namely, 1, 10 and 28, was studied both experimentally and computationally. It was found that for the highest Re, the flow exhibited the familiar outward pumping action associated with radial impellers under turbulent flow conditions. However, as the Re decreases, the net radial flow during one impeller revolution was reduced and for the lowest Re a reciprocating motion with negligible net pumping was observed. This behaviour has not been reported in the literature in the past and represents a highly undesirable flow pattern from the standpoint of effective mixing. The CFD results successfully reproduced this behaviour. In order to elucidate the physical mechanism responsible for the observed flow pattern, the forces acting on a fluid element in the radial direction were analysed. The analysis indicated that for the lowest Re, the material derivative of radial velocity near the blade tip is small thus a balance exists between pressure and viscous forces; the defining characteristic of creeping flow. The velocity and pressure forces are in phase because the velocity is driven by the pressure field generated by the rotation of the impeller. Based on these findings, a simplified analytic model of the flow was developed that gives a good qualitative as well as quantitative representation of the flow.     

Drag on ensembles of fluid spheres translating in a power-law liquid at moderate Reynolds numbers

This work elucidates the role of power-law rheology on the sedimentation velocity of an ensemble of mono-size spherical Newtonian droplets (free from surfactants) translating in a power-law continuous phase numerically by solving the momentum equations of both phases. A simple sphere-in-sphere cell model has been used to account for inter-drop interactions. In particular, in this study, the effects of the Reynolds number (Re_o), the internal to external fluid characteristic viscosity ratio (k), the volume fraction of the dispersed phase (Φ) and the power-law index of the continuous phase (n_o) on the external flow field, pressure drag (C_dp), friction drag (C_df) and total drag (C_d) coefficients have been analyzed over wide ranges of parameters as follows: 1 ≤ Re_o ≤ 200, 0.1 ≤ k ≤ 50, 0.2 ≤ Φ ≤ 0.6 and 0.6 ≤ n_o ≤ 1.6. Based on the extensive numerical results obtained in this work, a simple predictive correlation has been proposed for the total drag coefficient, which can be used to predict the rate of sedimentation of ensembles of Newtonian fluid spheres in power-law liquids in a new application.     

Modeling of unsteady flow around accelerating sphere at moderate reynolds numbers

The flow around an accelerating spherical particle of diameter ranging from 50 to 200 m?m is studied in the range of Reynolds number between 0.1 and 100. The flow around the sphere is assumed to be laminar and two-dimensional axisymmetric. The calculated drag coefficient is compared with the theoretical predictions of added mass term and Basset history term. Appropriate corrections for those two terms are proposed as function of the acceleration rate and the particle diameter.