Gas-liquid Mass Transfer Coefficient Research Articles

Simulation of ethylene polymerization in continuous slurry reactors

A large volume of polyethylene is manufactured in continuous slurry reactors at an industrial scale. A complete reactor model of the polyethylene slurry reactors consists of the kinetic model, intraparticle diffusion model and the mass balance equations for individual species. In this paper, a modified polymeric multigrain model (PMGM) is used to describe the intraparticle monomer transport, rate of polymerization, particle growth and polymer yield in an ideal continuous stirred tank reactor (CSTR) using a single-site, non-deactivating catalyst. The effect of parameters such as the original catalyst size, ethylene partial pressure and the gas–liquid mass transfer coefficient on the radial monomer concentration profile, the rate of polymerization, polymer yield, reactor safety factor and the polydispersity index of the macroparticle is discussed.

A Review on Gas-Liquid Mass Transfer Coefficients in Packed-Bed Columns

This review provides a thorough analysis of the most famous mass transfer models for random and structured packed-bed columns used in absorption/stripping and distillation processes, providing a detailed description of the equations to calculate the mass transfer parameters, i.e., gas-side coefficient per unit surface ky [kmol·m−2·s−1], liquid-side coefficient per unit surface kx [kmol·m−2·s−1], interfacial packing area ae [m2·m−3], which constitute the ingredients to assess the mass transfer rate of packed-bed columns. The models have been reported in the original form provided by the authors together with the geometric and model fitting parameters published in several papers to allow their adaptation to packings different from those covered in the original papers. Although the work is focused on a collection of carefully described and ready-to-use equations, we have tried to underline the criticalities behind these models, which mostly rely on the assessment of fluid-dynamics parameters such as liquid film thickness, liquid hold-up and interfacial area, or the real liquid paths or any mal-distributions flow. To this end, the paper reviewed novel experimental and simulation approaches aimed to better describe the gas-liquid multiphase flow dynamics in packed-bed column, e.g., by using optical technologies (tomography) or CFD simulations. While the results of these studies may not be easily extended to full-scale columns, the improved estimation of the main fluid-dynamic parameters will provide a more accurate modelling correlation of liquid-gas mass transfer phenomena in packed columns.

Selecting carrier material for efficient biomethanation of industrial biogas-CO2 in a trickle-bed reactor

Biogas upgrading via biomethanation of renewable H2 and the CO2 in biogas poses a potential key technology in the decarbonized energy system. This study uses a methanogenic trickle-bed reactor for upgrading raw biogas from an industrial anaerobic digester to natural gas-grade biomethane through ex situ biomethanation. The carrier material comprises a critical design parameter for trickle-bed reactors by supporting the catalytic methanogenic biofilm and thereby the interfacial area for H2 mass transfer. The present study applied an abiotic mass transfer-based screening of six different carrier types to identify a candidate material that can facilitate efficient H2 mass transfer in a biomethanation trickle-bed reactor. The screening included gas-liquid mass transfer experiments in an air-water system in addition to characterizations of the carriers’ surface area, dynamic liquid hold-up volumes, and external porosities. The abiotic experiments showed that the gas-liquid mass transfer coefficient is highly influenced by material type and cannot be predicted solely from differences in the carriers’ surface area. The abiotic characterizations led to the selection of crushed clay particles, whose application in biomethanation-based upgrading of raw biogas from an industrial anaerobic digester was examined. The clay particles facilitated an apparent volumetric mass transfer coefficient for H2 of 10 min−1, enabling CH4 production rates up to 6.1 L · L−1 · d−1 and more than 99 % H2 conversion over 30 days of continuous operation, where after accumulation of metabolic water in the packed bed impaired H2 mass transfer. Apart from the adverse effect of water accumulation, there was no indication that the maximum H2 mass transfer rate was reached, demonstrating the selected carrier material’s industrial potential for biomethanation-based biogas upgrading.

Encapsulation of red sorghum extract rich in proanthocyanidins: Process formulation and mechanistic model of foam-mat drying at various temperature

Foam-mat drying (FD) was applied to encapsulate red sorghum extract (RSE) rich in proanthocyanidins. A sufficient amount of whey protein isolate (WPI) to build the foam that supported the drying acceleration and a mechanistic model for process quantification were required to design FD precisely. This research investigated the WPI percentage necessary for creating the best foam-microstructure to accelerate the drying considering the foam properties profiles during whipping and built a mechanistic model to quantitatively describe the drying phenomena illustrating the moisture distribution in foam. The foam obtained by the best formula (10% of WPI, 15 min of whipping) was then dried at 40, 50, and 60 ℃ of temperature. A mechanistic model was developed considering intraparticle diffusion, liquid-gas mass transfer, and liquid-gas equilibrium, involving effective-diffusivities (Deff) and gas-liquid mass transfer coefficient (K) as parameters. The parameters were solved numerically, illustrating drying phenomena at several temperatures. It was observed that the model proposed works well, and the value of Deff and K was temperature-dependent, with the activation energy of 5.904 kJ/mol and 13.857 kJ/mol, respectively. The encapsulation reached the yield (wb) of 16.24 - 18.29%, the encapsulation productivity of 84.17 - 96.01%, and proanthocyanidin content in the product of 9.113–9.227 mg/g.

Investigation of growth limitation by CO2 mass transfer and inorganic carbon source for the microalga Chlorella vulgaris in a dedicated photobioreactor

An experiment was set up using a well-controlled photobioreactor to characterize the CO2 transfer and assimilation by photosynthetic microorganisms. This lab-scale system enabled accurate monitoring of the carbon sources in liquid and gas phases for in-depth characterization of the CO2 physical transfer and biological consumption of dissolved carbon.This paper presents a validation of the set-up by assessing the carbon mass balance in various absorption/desorption experiments. This is represented by the Carbon Recovery Percentage, which was found to be equal to 1 ± 0.1 confirming the accuracy of the time monitoring for the various carbon fluxes. These carbon fluxes were modeled to determine on-line the gas-liquid mass transfer coefficient (kLa). This enabled investigation of some aspects of CO2 dissolution in water, such as the effect of the culture medium composition and its pH value. Finally, carbon assimilation and growth limitation by the carbon source were studied for the microalga Chlorella vulgaris.

Gas-liquid and gas-liquid-solid mass transfer model for Taylor flow in micro (milli) channels: A theoretical approach and experimental proof

New theoretical approach to model Gas-Liquid and Gas-Liquid-Solid mass transfer for two-phase Taylor flows in milli and microchannels is proposed. The main idea of the proposed mathematical model is to take into account not only diffusion but the convection inside of liquid slugs caused by circulation within Taylor vortices and the convective transport from the film surrounding the bubble to the slug induced by the film movement. For that aim a three-layer model elaborated earlier by Abiev (CEJ, 2013) is used to decompose the liquid slug onto three interacting layers. Diffusion within the film around bubbles and the transit film belonging to the slug, as well as diffusion from the bubble caps was included in the model as well. Experimental data of Bercic & Pintar (CES, 1997), Butler et al. (CHERD, 2016; IJMF, 2018) and Haase et al. (TFCE, 2020) used for proof have shown quite better coincidence with the proposed model than the other available models. Special tests without convective term have demonstrated much less accuracy with the experimental data confirming the huge role of convection in the overall gas-liquid and gas-liquid-solid transport. The correlation between frequency of circulations within Taylor vortices and overall gas-liquid mass transfer coefficient found recently (Abiev et al., CEJ, 2019) was confirmed by use of Bercic & Pintar (CES, 1997) data. In the frame of the moving bubble the velocity of the transit film equals to the absolute value of two-phase velocity. That is why the convective term significantly influences the total mass transfer in the film and overall gas-liquid transport as well.

Enhancing the gas–liquid mass transfer during microbial electrosynthesis by the variation of CO2 flow rate

Carbon dioxide (one of the main greenhouse gases) can be used as a raw material in microbial electrosynthesis system (MES) for the production of valuable organic compounds. However, one of the major drawbacks associated with the MES is the mass transfer limitation of CO2 in the aqueous phase. In order to overcome this limitation, several operational strategies such as the increase of CO2 flow rate have been proposed. Therefore, the present paper assessed an H-type MES (H-Cell) carried out under CO2 pure gas supplied at 5, 10 and 20 mL min−1, and a gas diffusion electrode (GDE) MES (VITO-Cell) under 5 and 20 mL min−1. In both the MESs, the increase of the CO2 flow rate led to the improvement of inorganic carbon concentration, reaching until 1068 ± 115 mg L−1 in VITO-Cell. Consequently, in H-Cell the maximum acetate production rate increased from 45 to 270 mg L−1 d−1 when the CO2 flow varied from 5 to 20 mL min−1. In VITO-Cell the maximum acetate production rate reached 85 mg L−1 d−1 at 5 mL min−1 CO2 flow rate due to a better gas-liquid mass transfer coefficient of CO2 provided by the GDE.

Effect of solid particles on the volumetric gas liquid mass transfer coefficient in slurry bubble column reactors

The effect of solid particles on the volumetric gas liquid mass transfer coefficient klal in slurry bubble column reactors was investigated in the present work. klal was measured for an air-water-glass bead system using the dynamic oxygen absorption technique. Three solid concentrations and two particle diameters were used. Solid particles had a negligible effect on klal due to two opposite effects. First, a fraction of the particles tends to be located in the bulk liquid, altering its viscosity. In the heterogeneous regime, increasing the solid concentration enhances bubble coalescence, which led to an increase in size and, as a result, a decrease in the gas-liquid interfacial area al. Second, another fraction of particles moves to the bubble surface due to the collision phenomenon and tends to accumulate in the liquid film, resulting in local turbulence and an increase in the liquid-side mass transfer coefficient kl. The hydrodynamic effect mechanism was the governing mechanism of the effect of solid particles on gas-liquid mass transfer within the range of the investigated operating conditions.

Behavior of CO-water mass transfer coefficient in membrane sparger-integrated bubble column for synthesis gas fermentation

The gas–liquid mass transfer coefficient (kLa) of O2 was investigated in a bubble column reactor (BCR) using a sintered gas filter (SF), ceramic membrane module (CMM), and hollow fiber membrane module (HFM), which have different ranges of gas supply areas. kLa was enhanced by increasing flow rate in all of the spargers. Different responses when changing the gas supply area were obtained depending on the sparger type. Average values of kLa that were 52 and 258% higher were obtained using a CMM-integrated BCR compared to SFs and HFMs. CO-water kLa was investigated using CMMs for application to gas fermentation. The CO-water kLa ranged from 28.3 to 113.7/h under the experimental conditions. Based on the experimental data from CO and O2, a model to predict kLa was constructed for CMM-integrated BCRs. A dimensionless number indicating a gas supply area of the sparger was newly defined and included in the developed model.

Toward Sustained Product Formation in the Liquid-Phase Hydrogenation of Mandelonitrile over a Pd/C Catalyst.

The liquid-phase hydrogenation of the aromatic cyanohydrin mandelonitrile (MN, C6H5CH(OH)CN) over a carbon-supported Pd catalyst to produce the primary amine phenethylamine (PEA, C6H5CH2CH2NH2) is investigated with respect to the transition from operation in single-batch mode to repeat-batch mode. While a single-batch reaction returns a complete mass balance, product analysis alongside mass balance measurements for a six-addition repeat-batch procedure shows an attenuation in the rate of product formation and an incomplete mass balance from the fourth addition onward. This scenario potentially hinders possible commercial operation of the phenethylamine synthesis process, so it is investigated further. With reference to a previously reported reaction scheme, the prospects of sustained catalytic performance are examined in terms of acid concentration, stirrer agitation rate, catalyst mass, and hydrogen availability. Gas–liquid mass transfer coefficient measurements indicate efficient gas → liquid transfer kinetics within the experimental constraints of the Henry’s law limitation on hydrogen solubility in the process solvent (methanol). Deviations from the optimized product selectivity are attributed to mass transport constraints, specifically the H2(solv) → 2H(ads) transition, which is ultimately restrained by the availability of H2(solv). Finally, in an attempt to better understand the deactivation pathways, inelastic neutron scattering measurements on a comparable industrial-grade catalyst operated in an analogous reaction in fed-batch mode indicate the presence of an oligomeric overlayer postreaction. This overlayer is thought to be formed via oligomerization of hydroxyimine or imine species via specific pathways that are identified within a postulated global reaction scheme.

Mass transfer in a single-use angled-shaft aerated stirred bioreactor applicable for animal cell culture

The single-use stirred bioreactors are increasingly recognized as a viable alternative in animal cell culture due to the higher production capacity, increased flexibility, prevention of cross contamination, reduction of the cleaning cost, and shortened downtime. In this paper, the effects of the impeller speed, the volumetric gas flow rate, and the impeller type on the volumetric gas-liquid mass transfer coefficient in a single-use unbaffled angled-shaft bioreactor applicable for the animal cell culture were analyzed and compared with those attained for the baffled vertical-shaft bioreactors. The volumetric gas-liquid mass transfer coefficient (KLa) was experimentally determined by the simplified dynamic pressure method. The lowest P/V values as well as the highest KLa values were achieved for an unbaffled angled-shaft bioreactor. It was found that the Rushton impeller was the most efficient impeller in term of mass transfer for this bioreactor. An empirical correlation for the mass transfer coefficient was also developed.

Kinetics of hot metal desulfurization by bottom-blowing magnesium vapor

To solve the technical problems of hot metal desulfurization by injecting magnesium particulate, a new idea of hot metal desulfurization by bottom-blowing magnesium vapor was put forward. The reaction mechanism of hot metal desulfurization with magnesium vapor injection was analyzed, and the kinetic model of the desulfurization rate during the process of hot metal desulfurization with magnesium vapor injection was established. The dimensionless equation of the gas–liquid mass transfer coefficient under the injection conditions was obtained by the dimensional analysis method. And the theoretical calculation results were in good agreement with the experimental measurements. The results show that the diameter of the bubbles and the viscosity of the melt significantly affect the desulfurization rate of hot metal injected with magnesium vapor. When the temperature is 1573 K and the gas flow rate is 3 L/min, the desulfurization rate can reach 79% and the utilization rate of magnesium can reach 83%.

Production of ethanol fuel via syngas fermentation: Optimization of economic performance and energy efficiency

In this work, a model was developed to predict the performance of a bubble column reactor for syngas fermentation and the subsequent recovery of anhydrous ethanol. The model was embedded in an optimization framework which employs surrogate models (artificial neural networks) and multi-objective genetic algorithm to optimize different process conditions and design variables with objectives related to investment, minimum selling price, energy efficiency and bioreactor productivity. The results indicate the optimal trade-offs between these objectives while providing a range of solutions such that, if desired, a single solution can be picked, depending on the priority conferred to different process targets. The Pareto-optimal values of the decision variables were discussed for different case studies with and without the recovery unit. It was shown that enhancing the gas-liquid mass transfer coefficient is a key strategy toward sustainability improvement.

A colourimetric method for the measuring of the mass transfer kinetics of carbon dioxide in aqueous media

The measurement of the gas-liquid mass transfer coefficient of carbon dioxide in aqueous media is relevant in the field of phototrophic microorganism culture. Here, we present an approach that allowed the handling of a system composed of seven chemical reactions, in which the transfer process of CO2 was one of them, pursuing the objective of quantifying the rate of dissolution of carbon dioxide in a designed liquid medium. A simple methodology, involving the use of an acid-base indicator, acting as a tracer, was successfully employed. It was established, that starting from a condition distant from its unique equilibrium, through the alteration of the initial pH value of a designed arbitrary state by adding onto it exact amounts of a strong base, the incorporation of CO2, from an air current in contact with the perturbed liquid, occurred in order to restore a new balance. For each experience, achieved under the same operational conditions, it was possible to calculate nearly equal values of kinetic constants which holds information about the behaviour of the system. Collectively the results presented here suggest that this methodology could be adapted to several different systems, enlightening easier ways of incorporating the transfer kinetics of this major carbon source on the existing growth kinetic models of phototrophic microorganisms. At the same time, it highlights the fact that probably the rate-controlling step it is not the transfer reaction of CO2 by itself, as the process of hydration of CO2 is.

Mechanistic Description of Convective Gas-Liquid Mass Transfer in Biotrickling Filters Using CFD Modeling.

The gas-liquid mass transfer coefficient is a key parameter to the design and operation of biotrickling filters that governs the transport rate of contaminants and oxygen from the gas phase to the liquid phase, where pollutant biodegradation occurs. Mass transfer coefficients are typically estimated via experimental procedures to produce empirical correlations, which are only valid for the bioreactor configuration and range of operational conditions under investigation. In this work, a new method for the estimation of the gas-liquid mass transfer coefficient in biotrickling filters is presented. This novel methodology couples a realistic description of the packing media (polyurethane foam without a biofilm) obtained using microtomography with computational fluid dynamics. The two-dimensional analysis reported in this study allowed capturing the mechanisms of the complex processes involved in the creeping porous air and water flow in the presence of capillary effects in biotrickling filters. Model predictions matched the experimental mass transfer coefficients (±30%) under a wide range of operational conditions.

Characterization of power input and its impact on mass transfer in a rocking disposable bioreactor

The impact of specific power input on gas-liquid mass transfer and mixing has not been extensively reported for rocking disposable bioreactors. An electrical method was applied for measuring the specific power input into a disposable rocking (Wave) bioreactor at benchtop scales. The peak power input was shown to be suitable for characterizing the impact of operational parameters including rocking frequency, rocking angle and liquid volume. The average power inputs ranged from 66.5 W/m3 to 680.1 W/m3 which were comparable to those for stirred tank and orbitally-shaken disposable bioreactors. The gas-liquid oxygen mass transfer coefficient was shown to correlate with the peak power input in a power law model, confirming that the mass transfer capacity rapidly increased with power input, especially at 600 W/m3 and greater. The correlation between mixing time and power input indicated that power input at higher than 400 W/m3 was sufficient to induce relatively rapid mixing.

Modelling gas-liquid mass transfer in wastewater treatment: when current knowledge needs to encounter engineering practice and vice versa.

Gas-liquid mass transfer in wastewater treatment processes has received considerable attention over the last decades from both academia and industry. Indeed, improvements in modelling gas-liquid mass transfer can bring huge benefits in terms of reaction rates, plant energy expenditure, acid-base equilibria and greenhouse gas emissions. Despite these efforts, there is still no universally valid correlation between the design and operating parameters of a wastewater treatment plant and the gas-liquid mass transfer coefficients. That is why the current practice for oxygen mass transfer modelling is to apply overly simplified models, which come with multiple assumptions that are not valid for most applications. To deal with these complexities, correction factors were introduced over time. The most uncertain of them is the α-factor. To build fundamental gas-liquid mass transfer knowledge more advanced modelling paradigms have been applied more recently. Yet these come with a high level of complexity making them impractical for rapid process design and optimisation in an industrial setting. However, the knowledge gained from these more advanced models can help in improving the way the α-factor and thus gas-liquid mass transfer coefficient should be applied. That is why the presented work aims at clarifying the current state-of-the-art in gas-liquid mass transfer modelling of oxygen and other gases, but also to direct academic research efforts towards the needs of the industrial practitioners.

Mass transfer estimation for bubble column scale up

The productivity of a bubble column reactor (BCR) critically depends upon gas-liquid mass transfer coefficient, kLa. The prediction of kLa as function of design and operating conditions is central to BCR scale up. A large number of researchers have successfully characterized kLa experimentally in terms of superficial gas velocity, Usg using the power law relation kLa=αUsgβ, with α,β as fit parameters. We probe the applicability of such correlations to the design of a scaled up BCR, which differs from laboratory BCR in two important aspects: (i) the scale of operation, which can be O(102-103) times larger, and (ii) the type of sparger used. To this end, experiments were performed with air and water in a pilot scale (DC=1.6 m diameter) BCR using a coarse bubble sparger. We found that the existing correlations do, indeed, describe kLa over a wide range of BCR sizes, suggesting that these correlations are fairly scale insensitive. However, the correlations provide no means to capture the role of sparger explicitly. We cast our experimental kLa values in terms of a mass transfer efficiency and independently recover the power law relation with β=1. We suggest that the role of sparger design can be incorporated in the definition of α through the well-documented sparger efficiency factors. The α and β estimates thus obtained are in good agreement with the literature.

Determination of reactive mass transfer coefficients for CO2 absorption predictions

ABSTRACTAbsorption of carbon dioxide (CO2) by solvents is an important process in many practical applications such as capture of greenhouse gases from flue gas, gas processing in the chemical and petroleum industries, capture of radioactive isotopes in the nuclear cycle, etc. High pH alkaline solutions were used in this research project to capture CO2. The chemical reaction between CO2 and the hydroxyl ion is known to significantly increase the absorption rate compared to the same process without chemical reaction. The goal of this work is to study the influence of the chemical reaction on the absorption rate. For this purpose, the gas-liquid mass transfer coefficient was measured under reactive and nonreactive conditions. Reactive mass transfer coefficient values were higher than similar ones without chemical reaction. Under the operating conditions used in this work, the mass transfer process was found to be controlled by the liquid phase resistance.

Hydrodynamics, mixing and mass transfer in a pilot-scale bubble column with dense internals

Bubble column reactors with exothermic reactions are often equipped with dense tube bundle heat exchangers. While there is some knowledge about the impact of such internals on hydrodynamics and mass transfer for narrow columns, its role in pilot-scale columns is less clear. In this paper we report on a study of hydrodynamics and mass transfer in a BCR of 4.2 m height and 0.392 m diameter. We investigated different tube arrangements with triangular and square pitch and tube diameters of 32 × 10−3 m and 45 × 10−3 m at the same cross-sectional coverage (∼25%). The column was operated at homogeneous and heterogeneous flow conditions. A customized three-layer wire-mesh sensor was utilized to visualize gas phase dynamics and liquid mixing characteristics in the column’s cross-section. We found that sub-channel size is the most crucial geometric design parameter. Tracer mixing experiments revealed that internals reduce the mixing time due to the induction of large-scale liquid circulation. Mass transfer was studied with the oxygen stripping method. Here we found, that the effect of the internals on the gas-liquid mass transfer is almost negligible. Eventually, correlations for gas holdup, axial liquid dispersion and the volumetric gas-liquid mass transfer coefficient are given, which take the internals’ geometry into account.