Modulation of liquid holdup along a trickle bed reactor with periodic operation

Temporal variations of the liquid holdup in a mini-pilot scale trickle bed reactor cold-mockup, induced by an ON-OFF liquid flow modulation strategy of operation, are explored at different axial positions. The reactor is packed with porous beads of γ-Al₂O₃ and the liquid holdup is approximately estimated with a conductimetric technique, using probes that mimic the packing. The effects of the liquid and gas superficial velocities, the bed depth and the cycling parameters, cycle period and split, on the liquid holdup modulation are examined for a wide range of conditions. For slow and intermediate cycle periods, the liquid holdup time dependence observed during the dry period is represented by an exponential function. The characteristic value of the decay is correlated to the examined variables. The correlation allows reconstruction of the liquid holdup time dependence along the column.     

General criteria to analyze the role of mass transfer and hydrodynamics in trickle-bed reactors

General simple criteria to predict and analyze the influence on trickle-bed reactors behavior of three phenomena, namely, plug flow deviations, external wetting efficiency and external mass transfer resistance, are developed. Criteria predictions are compared with results arising from a comprehensive mathematical model widely employed to describe trickle-bed reactors behavior and with other particular criteria reported in the literature. Present criteria applications to different sets of experimental data exemplify their practical utility for various actual situations and verify their validity. Besides, they constitute simple tools to diagnose detrimental on trickle-bed reactors operation.     

Features of the motion of gel particles in a three-phase bubble column under foaming and non-foaming conditions

Features of the motion of gel particles in a three-phase bubble column with non-foaming and foaming gas–liquid systems,determined by using experiments of radioactive particle tracking(RPT),have been compared.The tracer used is a gel particle which resembles typical immobilized biocatalyst.The tracer trajectory is analyzed to extract relevant information for design purposes.The solid velocity field,turbulence parameters,dispersion coefficients,mixing times and flow transitions are determined and compared.The presence of foam significantly affects many quantified parameters,especially within the heterogeneous flow regime.The hydrodynamic stresses are reduced in the presence of foam,especially close to the disengagement.The dispersion coefficients also decrease,and the solid mixing time is only slightly affected by the presence of foam.Gas holdup,inferred both from RPT experiments and from gamma ray scanning,is higher for foaming systems and leads to a shift in the transition gas velocity towards higher values.     

Liquid back-mixing in packed-bubble column reactors: a state-of-the-art correlation

The extent of liquid back-mixing in gas–liquid concurrent upflow packed-bubble column reactors is quantified in terms of an axial dispersion coefficient or its corresponding dimensionless Péclet number. Effects of reactor operating conditions on the axial dispersion coefficient are not properly accounted for by the available literature correlations, wherein most often the Péclet number is expressed solely in terms of the gas and liquid Reynolds numbers or superficial velocities. Based on the broadest experimental databank (1322 measurements, 11 liquids, four gases, 28 packing materials, 14 columns diameters, Newtonian, non-Newtonian, aqueous, organic, coalescing and non-coalescing liquids, high pressure, bubble and pulsing flow regime conditions), a state-of-the-art liquid axial dispersion coefficient correlation is obtained by combining neural network modeling and dimensional analysis. Thorough qualitative and quantitative analyses of the constructed databank demonstrate the robustness of the proposed correlation to restore the variety of trend variations of liquid Péclet numbers reported in the literature.     

Ethanol oxidation in a trickle-bed reactor using a hydrophobic catalyst: Effect of dilution with hydrophilic particles

Ethanol oxidation at moderate temperature and atmospheric pressure is carried out in a trickle-bed reactor using a hydrophobic catalyst and pure oxygen as the oxidizing agent. The influence of diluting the hydrophobic catalyst with different proportions of the hydrophilic inert support is particularly studied by performing comparative experiments at different operating conditions. Reactor performance is not significantly modified when the hydrophobic catalyst is diluted with the hydrophilic support in a 50% mass proportion. For a larger dilution, 75% (w/w), significant differences in ethanol conversions are found for certain conditions, presumably due to the influence of the liquid holdup, gas–liquid interfacial areas and wetting efficiency characterizing a bed with a major proportion of hydrophilic particles.     

Visual remapping by vector subtraction: analysis of multiplicative gain field models

Saccadic eye movements remain spatially accurate even when the target becomes invisible and the initial eye position is perturbed. The brain accomplishes this in part by remapping the remembered target location in retinal coordinates. The computation that underlies this visual remapping is approximated by vector subtraction: the original saccade vector is updated by subtracting the vector corresponding to the intervening eye movement. The neural mechanism by which vector subtraction is implemented is not fully understood. Here, we investigate vector subtraction within a framework in which eye position and retinal target position signals interact multiplicatively (gain field). When the eyes move, they induce a spatial modulation of the firing rates across a retinotopic map of neurons. The updated saccade metric can be read from the shift of the peak of the population activity across the map. This model uses a quasi-linear (half-rectified) dependence on the eye position and requires the slope of the eye position input to be negatively proportional to the preferred retinal position of each neuron. We derive analytically this constraint and study its range of validity. We discuss how this mechanism relates to experimental results reported in the frontal eye fields of macaque monkeys.     

Trickle bed reactors: Effect of liquid flow modulation on catalytic activity

The effect of slow ON-OFF liquid flow modulation on the oxidation of aqueous solutions of ethanol using a 0.5% Pd/Al₂O₃ commercial egg-shell catalyst was investigated in a laboratory trickle bed reactor (TBR). In this mode of operation, the catalyst was cyclically exposed to oxidative and reductive environments.The study was carried out under different gas and liquid flow rates, cycle periods and splits. Cycling results have been compared with the steady-state experiments performed at the corresponding average liquid flow rate. Significant improvements over the continuous operation were obtained when the catalyst was exposed to a short surplus of oxygen (to minimize deactivation by overoxidation in the kinetic regime) after a longer time of working in the mass transfer limited regime. According to the results presented here, it is recommended to work with high liquid flow rates and moderate gas flow rates to ensure complete wetting of the catalyst during the ON cycle and to minimize the overoxidation process during the OFF cycle.     

Bed Expansion and Particle Classification in Liquid Fluidized Beds with Structured Internals

The inclusion of a structured packing as internal in a liquid‐solid fluidized bed allows expansion of the liquid velocity operation range before elutriation, promoting the liquid solid contact and mixing. The bed expansion of liquid‐solid fluidized beds provided with structured packing as internals is examined, for solids denser than the liquid phase and within a wide range of operating conditions. A correlation to estimate the bed expansion in liquid‐solid fluidized beds using structured packing as internals is developed. In addition, the feasibility of employing structured packing as internals for favoring classification of different density particles is demonstrated by analyzing the mass elutriated from the column at different liquid velocities for single particles or binary mixtures.     

Yield Optimization in a Cycled Trickle‐Bed Reactor: Ethanol Catalytic Oxidation as a Case Study

The effect of slow ON‐OFF liquid flow modulation on the yield of consecutive reactions is investigated for oxidation of aqueous ethanol solutions using a 0.5 % Pd/Al₂O₃ commercial catalyst in a laboratory trickle‐bed reactor. Experiments with modulated liquid flow rate (MLFR) were performed under the same hydrodynamic conditions (degree of wetting, liquid holdup) as experiments with constant liquid flow rate (CLFR). Thus, the impact of the duration of wet and dry cycles as well as the period can be independently investigated. Depending on cycling conditions, acetaldehyde or acetic acid production is favored with MLFR compared to CLFR. Results suggest both the opportunity and challenge of finding a way to tune the cycling parameters for producing the most appropriate product.