Ion Iliuta

Pressure drop through structured packings: Breakdown into the contributing mechanisms by CFD modeling

Determination of dry pressure drops is often the preliminary diagnostic tool for characterizing structured packing-containing columns. One conventional approach that ushered in this area evolves around the use of Ergun expressions along with mandatory experimental pressure drops for the fitting of some empirical constants characterizing a given packing. This method is strictly representational, and incapable of predicting the impact on bed pressure drop of changes in packing geometry, e.g., corrugation angle, channel size, or packing topography. In this work, a combined mesoscale—microscale predictive approach was developed to apprehend the aerodynamic macroscale phenomena in structured packings. The proposed method consists in identifying recurrent mesoscale patterns (the representative elementary units, REU) wherein the constitutive microscale dissipation mechanisms occur. The dissipative phenomena that were identified to be important are: the elbow loss and jet splitting at the packed bed entrance, the elbow loss at the column wall, the elbow loss at the jump from one layer to another, and the collisional losses at the criss-crossing junctions. Each mechanism was simulated over a wide Reynoids range spanning the pure creeping flow to the fully developed turbulent flow using three-dimensional computational fluid dynamics (CFD). Postulating additiveness of dissipation, the overall pressure drop was reconstructed. The approach was validated using experimental dry pressure drop data for five packing types (Flexipac, Gempak, Mellapak, Sulzer BX and Montz-Pak) having different channel sizes, corrugation angles, and surface topography. Our goal was to advocate CFD as a quicker and cheaper means for design and optimization, in terms of energy dissipation, of new structured packing shapes.

Gas}liquid interfacial mass transfer in trickle-bed reactors: state-of-the-art correlations

The state-of-the-art of the gas-liquid mass transfer characteristics in trickle-bed reactors was summarized and its quantification methods were reevaluated based on a wide-ranging data base of some 3200 measurements. A set of three unified whole-flow-regime dimensionless correlations for volumetric liquid- and gas-side mass transfer coefficients, and gas–liquid interfacial area, each of which spanned four-order-of-magnitude intervals, were derived. The correlations involved combination of artificial neural networks and dimensional analysis. The dimensionless interfacial area, ShL and ShG were expressed as a function of the most pertinent dimensionless groups: ReL, ReG, WeL, WeG, ScL, ScG, StL, XG, MoL, FrL, Eom, Sb.

Hydrodynamics and mass transfer in trickle-bed reactors: an overview

Abstract The fluid dynamic and the gas–liquid mass transfer characteristics of trickle-bed reactors were revisited and their quantification methods reevaluated based on extensive experimental historic flow databases (22,000 experiments) set up from the open literature published over the last 50 years. The state-of-the-art of trickle-bed fluid dynamics was summarized and a set of unified and updated estimation methods relying on neural network, dimensional analysis and phenomenological hybrid approaches were discussed.

Tailoring the pressure drop of structured packings through CFD simulations

A computational fluid dynamic methodology is proposed to breakdown into elementary dissipation mechanisms the overall single-phase gas flow bed pressure drop in towers containing corrugated sheet structured packings. The goal behind was to allow piecewise geometry optimization of such packings in terms of capacity enlargement and efficiency enhancement. The dissipations sorted in order of decreasing importance were the collision losses by jet streams at criss-crossing junctions within corrugated channels, elbow loss by form drag at interlayer transition, elbow loss by jets striking wall and subsequent flow redirection to upper channels, and elbow loss in bed entrance. Replacement of sharp bends at the interlayer junctions by progressive direction change was beneficial for the reduction of the dissipations at the wall and the interlayer junction thus stretching capacity of the structured packing. However, this improvement was not spectacular because the most energy-intensive component (criss-crossing) remained unaffected by such modifications. Computational fluid dynamics is foreseen to be a successful, rapid and economic tool to theoretically explore new geometries coping with this limitation.

Residence time, mass transfer and back-mixing of the liquid in trickle flow reactors containing porous particles

Abstract A residence time distribution (RTD) model to describe the liquid trickle flow in a trickle-bed reactor packed with porous particles and operated both under partially and fully wetted conditions was proposed based on a simple representation of the liquid flow structure. The model views the external liquid stream as divided into a dynamic zone where an axially dispersed plug flow pattern prevails, and an external stagnant (or static) zone contiguous to both the dynamic zone and the partially wetted porous particles. The model incorporates mass transfer between (i) external dynamic and stagnant zones, (ii) dynamic zone and nearby partially wetted porous particles, (iii) stagnant zone and adjacent partially wetted particles, and (iv) finally intraparticle diffusion. The model parameters were derived from liquid residence time distribution tests with various air/Newtonian and air/non-Newtonian systems. Analysis of the dynamic tracer impulse–response data of the liquid revealed the significance of the mass transfer resistance between static liquid and adjacent wetted particles, intraparticle diffusion resistance, as well as partial wetting. By properly accounting for intraparticle diffusion, peculiarly high liquid axial dispersion coefficients were obtained for low liquid velocities and high carboxymethyl-cellulose (CMC) concentrations. Finally, the deficiency of lumping static liquid–solid mass transfer, internal diffusion, and partial wetting in the Peclet number and the number of transfer units was discussed.

Allothermal steam gasification of biomass in cyclic multi-compartment bubbling fluidized-bed gasifier/combustor - new reactor concept.

A new reactor concept of allothermal cyclic multi-compartment fluidized bed steam biomass gasification is proposed and analyzed numerically. The concept combines space and time delocalization to approach an ideal allothermal gasifier. Thermochemical conversion of biomass in periodic time and space sequences of steam biomass gasification and char/biomass combustion is simulated in which the exothermic combustion compartments provide heat into an array of interspersed endothermic steam gasification compartments. This should enhance unit heat integration and thermal efficiency and procure N(2)-free biosyngas with recourse neither to oxygen addition in steam gasification nor contact between flue and syngas. The dynamic, one-dimensional, multi-component, non-isothermal model developed for this concept accounts for detailed solid and gas flow dynamics whereupon gasification/combustion reaction kinetics, thermal effects and freeboard-zone reactions were tied. Simulations suggest that allothermal operation could be achieved with switch periods in the range of a minute supporting practical feasibility for portable small-scale gasification units.

New mechanistic film model for pressure drop and liquid holdup in trickle flow reactors

Abstract This paper, building on a series of previous articles on the slit models, provides a new mechanistic film model for the description of trickle-bed reactor hydrodynamic parameters (two-phase pressure drop, total external liquid holdup) in the low interaction regime. An important feature of the model is a more physical assumption of continuity in the velocity and shear stress profiles at the gas–liquid interface. The model also introduces a concept borrowed from the modeling of falling-film reactors, in which the degree of interaction between fluids is described by incorporating a phenomenological gas–liquid interaction factor ψ gl . This interaction factor has been extracted by solving an inverse formulation of the film model over a comprehensive database consisting of ca. 5000 pressure gradient and liquid holdup measurements. It was thereafter correlated through artificial neural networks and dimensional analysis to the input characteristics of the trickle-bed reactor. Including the resulting correlation [ψ gl =ψ gl (Re gl ,We l ,Fr l ,St l ,Ga l ,Ga g ,X l ,S b )] in the film model yielded mean absolute relative errors of 18.1% for the total pressure drop and 18.5% for the liquid holdup. Finally, parametric simulations of the behavior of ψ gl as a function of the various trickle-bed operating variables confirmed the physical coherence of the proposed interfacial interaction factor.

Hydrodynamic characteristics of two-phase flow through fixed beds: Air/Newtonian and non-Newtonian liquids

An experimental investigation was carried out to determine the effects of gas and liquid how rates and flow consistency index on the liquid-phase axial dispersion coefficient, pressure drop and the liquid holdup for two-phase downflow and upflow in a fixed bed. Water and non-Newtonian liquids were employed as liquid phase. The liquid-phase flow in a fixed bed was examined using the piston-diffusion-exchange (PDE) model. The time-domain analysis of tracer response data was used for the how model parameter estimation. Copyright (C) 1996 Published by Elsevier Science Ltd

Catalytic wet air oxidation with a deactivating catalyst analysis of fixed and sparged three-phase reactors

Abstract A comparative analysis is made for a trickle-bed reactor, a packed-bubble column, a three-phase fluidized bed and a slurry-bubble column with an active and moderately deactivating catalyst for the wet oxidation at high pressure and temperature of organic-containing aqueous wastes. Compared to other mature industrial sectors where multiphase reactors are prevalent, the design of three-phase catalytic reactors for wet air oxidation processes is still at an emerging stage. This paper discusses, from a multiphase reactor engineering perspective, the design of such contactors by setting an exhaustive modeling framework of catalytic wet oxidation in which the molecular, particle and reactor scales are integrated. The simulation results indicate that when wet oxidation is liquid-reactant limited, packed-bubble columns outperform trickle beds, whereas slurry-bubble columns are the most vulnerable to “coke” deactivation.

Residence time distribution of the liquid in gas-liquid cocurrent upflow fixed-bed reactors

The authors present an experimental investigation of the residence time distribution (RTD) of the liquid in a gas-liquid upflow fixed-bed reactor with porous and nonporous particles and air/Newtonian or non-Newtonian systems. The piston-dispersion-exchange model with Danckwerts boundary conditions was used to describe the liquid flow. In the case of porous particles, the dynamic evolution of the tracer concentration in the particles was described in terms of diffusion phenomena. An imperfect pulse method was used to estimate the model parameters directly from the experimentally nonideal input and output response.