Chemical hydrodynamics of a downward microbubble flow for intensification of gas‐fed bioreactors

Bioreactors are of interest for value‐upgrading of stranded or waste industrial gases. Reactor intensification requires development of low cost bioreactors with fast gas–liquid mass transfer rate. Here we assess published reactor technology in comparison with a novel downward bubble flow created by a micro‐jet array. Compared to known technology, the advanced design achieves higher volumetric gas transfer efficiency (k_La per power density) and can operate at higher k_La. We measure the effect of four reactor heights (height‐to‐diameter ratios of 12, 9, 6, and 3) on the gas transfer coefficient k_L, total interfacial area a, liquid residence time distribution, energy consumption, and turbulent hydrodynamics. Leading models for predicting k_L and a are appraised with experimental data. The results show k_L is governed by “entrance effects” due to Higbie penetration dominate at short distances below the micro‐jet array, while turbulence dominates at intermediate distances, and finally terminal rise velocity dominates at large distances. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1399–1411, 2018     

Hydrodynamics and mass transfer of gas–liquid flow in a falling film microreactor

In this article, flow pattern of liquid film and flooding phenomena of a falling film microreactor (FFMR) were investigated using high‐speed CCD camera. Three flow regimes were identified as “corner rivulet flow,” “falling film flow with dry patches,” and “complete falling film flow” when liquid flow rate increased gradually. Besides liquid film flow in microchannels, a flooding presented as the flow of liquid along the side wall of gas chamber in FFMR was found at high liquid flow rate. Moreover, the flooding could be initiated at lower flow rate with the reduction of the depth of the gas chamber. CO₂ absorption was then investigated under the complete falling flow regime in FFMR, where the effects of liquid viscosity and surface tension on mass transfer were demonstrated. The experimental results indicate that k_L is in the range of 5.83 to 13.4 × 10^?5 m s^?1 and an empirical correlation was proposed to predict k_L in FFMR. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Kinetics and mass transfer in hydroformylation—bulk or film reaction?

The kinetics of a gas–liquid reaction, alkene hydroformylation was studied in the presence of a homogeneous catalyst in a pressurised laboratory‐scale semibatch reactor. Hydroformylation of propene to isobutyraldehyde and n‐butyraldehyde was carried out at 70–115°C and 1–15 bar pressure in 2,2,4‐trimethyl‐1,3‐pentanediol monoisobutyrate solvent with rhodium catalyst using the ligands cyclohexyl diphenylphosphine. In order to evaluate the influence of mass transfer, experiments were made using varied stirring rate from 100 to 1000 rpm at 100°C and 10 MPa syngas pressure. Only at higher stirrings rates, the reaction took place in the kinetic regime. A reactor model was developed comprising both complex kinetics and liquid‐phase mass transfer. The model was based on the theory of reactive films. The model is able to predict under which circumstances the hydroformylation process is affected by liquid‐phase diffusion of the reactants. Experimental data and model simulations are presented for the hydroformylation of propene in the presence of a homogeneous rhodium catalyst.     

Flow distribution and mass transfer in a parallel microchannel contactor integrated with constructal distributors

Flow distribution and mass transfer characteristics during CO₂‐water flow through a parallel microchannel contactor integrated with two constructal distributors have been investigated numerically and experimentally. Each distributor comprises a dichotomic tree structure that feeds 16 microchannels with hydraulic diameters of 667 μm. It was found that constructal distributors could ensure a nearly uniform gas–liquid distribution at high gas flow rates where the ideal flow pattern was slug‐annular flow. Nevertheless, at small gas flow rates where the ideal flow pattern was slug flow, a significant flow maldistribution occurred primarily due to the lack of large pressure barrier inside each distributor, indicating that dynamic pressure fluctuation in parallel microchannels greatly disturbed an otherwise good flow distribution therein. It was further shown that the present parallel microchannel contactor could realize the desired mass transfer performance previously achieved in one single microchannel under relatively wide operational ranges due to the integration of constructal distributors. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

A multi‐scale theoretical model for gas–Liquid interface mass transfer based on the wide spectrum eddy contact concept

On the basis of the wide spectrum eddy contact concept and the isotropic turbulence theory, a multi‐scale theoretical model for the prediction of liquid‐side mass transfer coefficient in gas–liquid system was developed. The model was derived from an unsteady‐state convection and diffusion equation and considered the contributions of eddies with different sizes to the overall mass transfer coefficient. The proper contact time distribution at the surface is need to be determined to obtain satisfactory results with this model. Moreover, a simplified model was also proposed based on the assumption of steady‐state mass transfer mechanism for single eddy. The results predicted by this model showed a very good agreement with the available experimental data in a comparatively wide range of turbulence intensities. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Experimental study of O2 diffusion coefficient measurement at a planar gas–liquid interface by planar laser‐induced fluorescence with inhibition

A new method for determining the molecular diffusivity of oxygen in liquids is described. The technique was applied through a flat air–liquid interface in a Hele‐Shaw cell (5 × 5 × 0.2 cm³) and was based on planar laser‐induced fluorescence (PLIF) with inhibition. A ruthenium complex (C₇₂H₄₈N₈O₆Ru) was used as the fluorescent dye sensitive to oxygen. A mathematical analysis was developed to determine the molecular diffusivity of oxygen simply by localizing the gas diffusion front. The specificity of this mathematical analysis is that it does not require the properties of the fluids (such as the saturation concentration) to be considered, which is especially relevant for complex media that are sometimes difficult to characterize properly. This technique was applied to three different fluids (viscosities ranging from 1 to 2.4 mPa·s) corresponding to binary diffusion coefficients ranging from 9.5 × 10^?10 to 2 × 10^?9 m²/s. Experimental data were found with an uncertainty of about 5% and were in good agreement with the literature. Particle image velocimetry and numerical simulations were also carried out to determine the optimal gas flow rate (0.01 L/s) to reach purely diffusive transfer, and the corresponding hydrodynamic profiles of the two phases. © 2012 American Institute of Chemical Engineers AIChE J, 59: 325–333, 2013     

Liquid‐bridge flow in the channel of helical string and its application to gas–liquid contacting process

To solve the problems of the traditional packings, such as high pressure drop, mal‐distribution and short liquid residence time, a helical flow structured packings was proposed. Two different flow patterns, liquid‐bridge flow and liquid‐drop flow were identified when the width of the channel of the helical string was adjusted. Moreover, the characteristics of the helical liquid‐bridge flow including maximum liquid loading, mean thickness of liquid film, mean residence time and effective specific surface area, were examined. And the separation efficiency was studied by the lab‐scale distillation column. In comparison, the effective specific surface area of the helical flow type packings is almost as large as the traditional B1‐350Y structured packings, but with thinner liquid film, longer liquid residence time and finally higher separation efficiency. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3360–3368, 2018     

Mass transfer and deodorization efficiency in a countercurrent spray tower for low superficial gas velocities

The aim of this study was to characterize mass transfer and deodorization efficiency in a countercurrent spray tower for low superficial gas velocities. The influence of operating parameters (U_G = 0.005 to 0.025 m s^?1, U_L = 6.1 × 10^?5 to 2.4 × 10^?4 m s^?1) on the liquid retention (ε_L), the drop diameter (d_g), the interfacial area (a) and the overall liquid and gas phase mass transfer coefficients (K_La, K_Ga) were estimated. The spray efficiency of some malodorous compounds was also estimated. A negative influence of the superficial gas velocity was demonstrated, during the spraying of water or chemical neutralizing scrubbing solutions. There was also an increase with the liquid flow rate. Abatements obtained were very good with respect to ammonia (>90%), and acceptable for the other compounds.     

Influence of the gas—liquid separator design on hydrodynamic and mass transfer performance of split-channel airlift reactors

Three geometric configurations of gas–liquid separators were used in split-channel airlift reactors (0·1 m³ liquid volume; riser-to-downcomer cross-sectional area ratio = 0·7; aspect ratio = 3.6) to test the effect of geometry on hydrodynamic performance and oxygen transfer behaviour. For otherwise fixed conditions, the design of gas–liquid separators affected the induced liquid circulation rate, the depth of penetration of the bubble layer in the downcomer, the gas holdup in the downcomer, the mixing time and the overall volumetric gas–liquid oxygen transfer coefficient. The gas holdup in the riser was only marginally affected by the design of the separator. The impact of the various separator designs on hydrodynamic behaviour could be explained as emanating from a combination of the gas–liquid separating ability of the design and its hydraulic resistance.     

Helical liquid flow on a vertical cylinder and its application to gas absorption

An experimental examination of a novel device for enhancing the gas absorption into an aqueous absorbent flowing down the outer wall of a vertical cylinder was reported. This device utilizes flexible strings tightly wound around the cylinder, taking the form of a multiple helix. The absorbent flows along parallel channels partitioned by the strings, maintaining mutual contact with the surrounding gas for a longer time than it would when it flows down the same cylinder wall in the absence of such strings. Both flow‐observation experiments and absorption experiments using water as the absorbent flowing along a single helical channel and carbon dioxide as the gas to be absorbed were carried out. The effectiveness of the helical‐flow device for promoting the absorption was recognized at water flow rates high enough to induce an oscillatory flow mode accompanied by periodical liquid?gas interface deformation. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3109–3118, 2013     

Experimental determination of maximum effective hydrodynamic stress in multiphase flow using shear sensitive aggregates

Maximum effective hydrodynamic stress, , responsible for the breakup of aggregates with size comparable to Kolmogorov eddies, was experimentally determined in an aerated stirred tank. The proposed method is based on the measurement of the maximum stable aggregates size consisting of poly(methyl methacrylate) nanoparticles. The fractal aggregates were broken under various operating conditions in an aerated stirred tank and calibrated with known flow conditions using contracting nozzles to convert the measured aggregate sizes into hydrodynamic stress. It was found that can vary substantially among studied conditions and its magnitude depends on the controlling mechanism including gas jet during bubble formation, bubble rise, bubble burst at the gas–liquid interface or the turbulence generated by the impeller. The measured values are in good agreement with literature data which supports the applicability of this method to characterize the maximum effective hydrodynamic stress in complicated multiphase flow. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1735–1744, 2015     

Mass transfer at the confining wall of helically coiled circular tubes with gas–liquid flow and fluidized beds

Experiments were conducted to investigate the effect of various dynamic and geometric parameters on mass transfer coefficients in two-phase helically coiled flow systems. Computation of mass transfer coefficients was facilitated by the measurement of limiting current at the electrodes fixed flush with the inner surface of the tube wall. Two flow systems were chosen: a two-phase liquid solid fluidized bed and a two-phase gas–liquid up flow. An equimolar potassium ferrocyanide and potassium ferricyanide solution in the presence of sodium hydroxide was used as the liquid phase. In the fluidized bed, glass spheres and sand of different sizes were employed as fluidizing solids. In two-phase flow system nitrogen was employed as inert gas. The pressure drop in the presence of fluidizing solids in helical coils was found to increase with increase in the pitch of the coil and was maximum for straight tube. The mass transfer coefficients were found to increase with increase in liquid velocity. The mass transfer coefficients in case of gas–liquid flow were found to be independent of liquid velocity and the pitch of the coil, and were largely influenced by gas velocity only. The data were correlated using j_D factor, Helical number, Froude number and Stanton number.