Volumetric Gas-liquid Mass Transfer Coefficient Research Articles

Gas–liquid mass transfer in three‐phase inverse fluidized bed reactor with Newtonian and non‐Newtonian fluids

AbstractLiquid‐phase volumetric mass transfer coefficients, kLa were determined in three‐phase inverse fluidized beds of low‐density polyethylene (LDPE) and polypropylene (PP) spheres fluidized by a countercurrent flow of air and Newtonian (water and glycerol solutions) or non‐Newtonian liquids [carboxy methyl cellulose (CMC) solutions]. The effects of liquid and gas velocities, particle size, solid loading and addition of organic additives (glycerol and CMC) on the volumetric mass transfer coefficient, kLa were determined. The superficial liquid velocity had a weak effect on the mass transfer whereas the gas flow rate affected the mass transfer positively. kLa increased with increase in particle diameter and decreased with increase in initial bed height (solid loading). kLa decreased as the concentration of glycerol (viscosity) and CMC increased. Empirical correlations are presented to predict the gas–liquid volumetric mass transfer coefficient in terms of operating variables. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

Pt-catalyzed ozonation of aqueous phenol solution using high-gravity rotating packed bed

In this study, a high-gravity rotating packed bed (HGRPB or HG) was used as a catalytic ozonation (Cat-OZ) reactor to decompose phenol. The operation of HGRPB system was carried out in a semi-batch apparatus which combines two major parts, namely the rotating packed bed (RPB) and photo-reactor (PR). The high rotating speed of RPB can give a high volumetric gas–liquid mass transfer coefficient with one or two orders of magnitude higher than those in the conventional packed beds. The platinum-containing catalyst (Dash 220N, Pt/γ-Al 2O 3) and activated alumina (γ-Al 2O 3) were packed in the RPB respectively to adsorb molecular ozone and the target pollutant of phenol on the surface to catalyze the oxidation of phenol. An ultra violet (UV) lamp (applicable wavelength λ = 200–280 nm) was installed in the PR to enhance the self-decomposition of molecular ozone in water to form high reactive radical species. Different combinations of advanced oxidation processes (AOPs) with the HGRPB for the degradation of phenol were tested. These included high-gravity OZ (HG-OZ), HG catalytic OZ (HG-Cat-OZ), HG photolysis OZ (HG-UV-OZ) and HG-Cat-OZ with UV (HG-Cat-UV-OZ). The decomposition efficiency of total organic compound ( η TOC) of HG-UV-OZ with power of UV ( P UV) of 16 W is 54% at applied dosage of ozone per volume sample m A, in = 1200 mg L −1 (reaction time t = 20 min), while that of HG-OZ without the UV irradiation is 24%. After 80 min oxidation ( m A, in = 4800 mg L −1), the η TOC of HG-UV-OZ is as high as 94% compared to 82% of HG-OZ process. The values of η TOC for HG-Cat-OZ process with m S = 42 g are 56% and 87% at m A, in = 1200 and 4800 mg L −1, respectively. By increasing the catalyst mass to 77 g, the η TOC for the HG-Cat-OZ process reaches 71% and 90% at m A, in = 1200 and 4800 mg L −1, respectively. The introduction of Pt/γ-Al 2O 3 as well as UV irradiation in the HG-OZ process can enhance the η TOC of phenol significantly, while γ-Al 2O 3 exhibits no significant effect on η TOC. For the HG-Cat-UV-OZ process with m S = 42 g, the values of η TOC are 60% and 94% at m A, in = 1200 and 4800 mg L −1, respectively. Note that the decomposition of TOC via HG-UV-OZ is already vigorous. Thus, the enhancing effect of catalyst on η TOC is minor.

Influence of suspended solid particles on gas-liquid mass transfer coefficient in a system stirred by double impellers

AbstractThe aim of the research work was to investigate the effect of the presence and concentration of solid particles on the gas-liquid volumetric mass transfer coefficient in a mechanically stirred gas-solid-liquid system. Experimental studies were conducted in a tall vessel of the diameter of 0.288 m, equipped with two designs of double stirrers. Three high-speed stirrers were used: A 315, Smith turbine, and Rushton turbine. The following operating parameters were changed: gas flow rate, stirrer speed, and solid concentration. The volumetric mass transfer coefficient was determined using the dynamic gassing-out method. In the range of the measurements conducted, this coefficient was strongly affected by both the presence and the concentration of particles in the system. Generally, a low concentration of particles in the system, equal to 0.5 mass %, caused an increase of the volumetric mass transfer coefficient values for both stirrer configurations compared to a system without solids whilst more particles (2.5 mass %) caused a decrease of this coefficient. It could be supposed that an increase of slurry viscosity affected the decrease of the volumetric mass transfer coefficient at higher solid concentration. An empirical correlation was proposed for volumetric mass transfer coefficient prediction. Its parameters were fitted using experimental data.

Carbon Monoxide Mass Transfer for Syngas Fermentation in a Stirred Tank Reactor with Dual Impeller Configurations

This study compares the power demand and gas-liquid volumetric mass transfer coefficient, kLa, in a stirred tank reactor (STR) (T = 0.211 m) using different impeller designs and schemes in a carbon monoxide-water system, which is applicable to synthesis gas (syngas) fermentation. Eleven different impeller schemes were tested over a range of operating conditions typically associated with the "after large cavity" region (ALC) of a Rushton-type turbine (D/T = 0.35). It is found that the dual Rushton-type impeller scheme exhibits the highest volumetric mass transfer rates for all operating conditions; however, it also displays the lowest mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) for all conditions due to its high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller show improved mass transfer rates without dramatic increases in power draw. At high gas flow rates, dual impeller schemes with a lower concave impeller have kLa values similar to those of the Rushton-type dual impeller schemes but show improved mass transfer performance. It is believed that the mass transfer performance can be further enhanced for the bottom concave impeller schemes by operating at conditions beyond the ALC region defined for Rushton-type impellers because the concave impeller can handle higher gas flow rates prior to flooding.

Oxygen transfer and hydrodynamics in three-phase inverse fluidized beds

Experiments were performed at ambient temperature and pressure in a 152 mm inner diameter column with air, tap water or 0.5% wt. aqueous ethanol solution, and polypropylene particles. An increase in liquid velocity and solids loading, and the presence of a surfactant reduces the gas velocity required to reach full bed expansion, which is delimited by the gas sparger. With an increase in gas velocity, solids holdups remain constant after full bed expansion, liquid holdups increase to a maximum and then decrease and gas holdups continuously increase. The addition of ethanol greatly increases the gas holdups leading to significant reductions in liquid holdups. The volumetric gas–liquid mass transfer coefficient, k L a , increases with increasing gas velocity but does not change significantly with liquid velocity. There are complex interaction effects between solids loading and surfactants as the values of k L a in the aqueous ethanol solution were greater than those in water when particles were present and smaller without particles. k L a data in water were found to be proportional to gas holdup whereas for the ethanol solution this proportionality constant first decreased with increasing gas velocity to eventually stabilize at a value smaller than for water.

Gas–liquid mass transfer in bubble column reactor: Analytical solution and experimental confirmation

Gas–liquid mass transfer behavior in a bubble column is investigated. The used reactor is of 6 m height and has an internal diameter of 0.15 m. The classical ‘gassing out’ dynamic method is used to obtain the dissolved oxygen (DO) concentration profiles. An analytical description of the dissolved oxygen concentration evolution with time following the switch in the inlet gas is presented and allows the evaluation of the gas–liquid volumetric mass transfer coefficient for the different superficial gas velocities used. The comparison between the k L a values determined from this exact solution and those obtained in the same reactor from a more complete and sophisticated model, numerically solved, shows a good agreement.

Mass transfer characteristics of a novel three-phase airlift contactor with a semipermeable membrane

The effects of hydrocarbon contaminants and solid loading on gas–liquid volumetric mass transfer coefficient, K L a , were studied in a three-phase 25 l concentric cylindrical airlift contactor with a semipermeable membrane. The porous membrane could retain a large volume of immobilized solid particles and enhanced mass transfer and mixing in the airlift contactor. A specific form of the continuity equation was combined with gas holdup values in order to set up an expression for the mass transfer coefficient calculation. Water with different hydrocarbon concentrations of 0–200 mg/l p -xylene and 0–25 mg/l naphthalene were used as liquid media, under varying solid loadings and superficial gas velocities. The results indicated that deviation of mass transfer coefficient could go either way when hydrocarbons were dissolved in water: the most significant reduction in K L a occurred at the concentration of 37 (mg/l) for p -xylene after which K L a recovered and increased slightly. A less severe depression of K L a was observed for naphthalene. Total solid loading in the airlift contactor was limited to the maximum amount of solid entrapped within the porous textile membrane. Higher superficial gas velocity resulted in the increase of the effect of solid loading on K L a .

Gas–liquid mass transfer in an up-flow cocurrent packed-bed biofilm reactor

Gas–liquid mass transfer was investigated in an up-flow cocurrent packed-bed biofilm reactor. In aerobic processes gas–liquid mass transfer can be considered as a key operational parameter as well as in reactor scale-up. The present paper investigates the influence of the liquid phase mixing in the determination of the volumetric gas–liquid mass transfer coefficient ( k L a) coefficient. Residence time distribution (RTD) experiments were performed in the reactor to determine the flow pattern of the liquid phase and to model mathematically the liquid phase mixing. The mathematical model derived from RTD experiments was used to evaluate the influence of the liquid mixing on the experimental estimation of the ( k L a) in this reactor type. The methods used to estimate the k L a coefficient were: (i) dynamic gassing-out, (ii) sulphite method, and (iii) in-process estimation through biological conversion obtained in the reactor. The use of standard chemical engineering correlations to determine the k L a in this type of bioreactors is assessed. Experimental and modelling results show how relevant can be to take into consideration the liquid phase mixing in the calculations of the most-used methods for the estimation of k L a coefficient. k L a coefficient was found to be strongly heterogeneous along the reactor vertical axis. The value of the k L a coefficient for the packed-bed section ranged 0.01–0.12 s −1. A preliminary correlation was established for up-flow cocurrent packed-bed biofilm reactors as a function of gas superficial velocity.

GAS-LIQUID MASS TRANSFER CHARACTERISTICS OF LARGE-SCALE IMPELLERS: EMPIRICAL CORRELATIONS OF GAS HOLDUPS AND VOLUMETRIC MASS TRANSFER COEFFICIENTS IN STIRRED TANKS

ABSTRACT Gas dispersion with large-scale impellers consisting of modified large paddle impellers in stirred tanks, with rather large ratios of both impeller diameter and impeller height to tank diameter, was experimentally examined in transition and turbulent mixing ranges. Gas holdups and volumetric gas-liquid mass transfer coefficients with large-scale impellers, i.e., Maxblend and Fullzone impellers, were measured in 0.31 and 0.6 m I.D. stirred tanks, and the gas dispersion performance of large-scale impellers was compared with that of double conventional small-scale high-speed impeller systems, i.e., double four-flat blade disk turbine impellers and double four-flat paddle impellers. The gas holdups of the large-scale impellers were comparable with those of the small-scale impeller systems at a given rotational speed. The volumetric gas-liquid mass transfer coefficients for large-scale impellers were also similar to those of the small-scale impeller systems. It was found that the large-scale impellers are not more energy efficient than the small-scale impellers in obtaining good gas dispersion. Empirical correlations for gas holdups and volumetric gas-liquid mass transfer coefficients were developed. They fit the experimental data in transition and turbulent mixing ranges reasonably well, with correlation factors greater than 0.84.

Correlating Gas-Liquid Mass Transfer in a Stirred-Tank Reactor

A correlation is developed for the volumetric gas–liquid mass transfer coefficient (kLa) in stirred-tank reactors (STRs) using results from the literature. The volumetric mass transfer coefficient is correlated on the basis of the relative dispersion parameter (N/NCD) for similar impeller hydrodynamics and operating regimes. Using a bench-top STR of diameter T= 0.211 m, gas–liquid mass transfer data are also obtained and found to follow the proposed correlation when the appropriate hydrodynamic conditions are satisfied. A STR scale-up technique from bench-scale (T= 0.211m and D/T= 0.35) to industrial-scale (up to T= 2.7 m) is proposed using a normalized hydrodynamic flow regime map and shown to be useful in understanding the range of operational conditions for the successful scale-up of STRs.

Effect of n-dodecane on Crypthecodinium cohnii fermentations and DHA production

The potential use of n-dodecane as an oxygen vector for enhancement of Crypthecodinium cohnii growth and docosahexaenoic acid (DHA) production was studied. The volumetric fraction of oxygen vector influenced the gas-liquid volumetric mass transfer coefficient k (L) a positively. The k (L) a increased almost linearly with the increase of volumetric fraction of n-dodecane up to 1%. The stirring rate showed a higher influence on the k (L) a than the aeration rate. The effects of this hydrocarbon on C. cohnii growth and DHA production were then investigated. A control batch fermentation without n-dodecane addition (CF) and a batch fermentation where n-dodecane 1% (v/v) was added (DF) were carried out simultaneously under the same experimental conditions. It was found that, before 86.7 h of fermentation, the biomass concentration, the specific growth rate, the DHA, and total fatty acids (TFA) production were higher in the CF. After this fermentation time, the biomass concentration, the DHA and TFA production were higher in the DF. The highest DHA content of biomass (6.14%), DHA percentage of TFA (51%), and DHA production volumetric rate r (DHA) (9.75 mg l(-1 )h(-1)) were obtained at the end of the fermentation with n-dodecane (135.2 h). The dissolved oxygen tension (DOT) was always higher in the DF, indicating a better oxygen transfer due to the oxygen vector presence. However, since the other C. cohnii unsaturated fatty acids percentages did not increase with the oxygen availability increase due to the n-dodecane presence, a desaturase oxygen-dependent mechanism involved in the C. cohnii DHA biosynthesis was not considered to explain the DHA production increase. A selective extraction through the n-dodecane was suggested.

Liquid mixing and gas–liquid mass transfer in a three-phase inverse turbulent bed reactor

In this research work, hydrodynamic characteristics and gas–liquid mass transfer in a laboratory scale inverse turbulent bed reactor were studied. In order to characterize internal flow in the reactor, the residence time distribution (RTD) was obtained by the stimulus-response technique using potassium chloride as a tracer. Different solid hold-up (0–0.37) and air superficial velocity (2.7–6.5 mm s−1) values were assayed in RTD experiments. The parameters that characterize the RTD curve, mean residence time and variance were independent of the solid hold-up, thus the solid particle concentration did not influence liquid mixing in the reactor. The hydrodynamic of the inverse turbulent bed was well represented by a model that considers the reactor as two-mixed tank of different volumes in series. The value of the volumetric gas–liquid mass transfer coefficient (kLa) was independent of the solid hold-up. This result enhances a previously suggested hypothesis, which considers that the solid and liquid form a pseudo-fluid in the inverse turbulent bed reactor.

A comparative evaluation of oxygen mass transfer and broth viscosity using Cephalosporin-C production as a case strategy

The production of Cephalosporin-C (CPC) a secondary metabolite, using a mold Acremonium chrysogenum was studied in a lab scale Internal loop air lift reactor. Cephalosporin-C production process is a highly aerobic fermentation process. Volumetric gas–liquid mass transfer coefficient (kLa) and viscosity (η) were evaluated, during the growth and production phases of the microbial physiology. An attempt has been made to correlate the broth viscosity, η and volumetric oxygen transfer coefficient, kLa during the Cephalosporin-C production in an air lift reactor. The impact of biomass concentration and mycelial morphology on broth viscosity has been also evaluated. The broth exhibits a typical non-Newtonian fermentation broth. Rheology parameters like consistency index and fluidity index are also studied.

Characteristics of three-phase internal loop airlift bioreactors with complete gas recirculation for non-Newtonian fluids

Hydrodynamic and gas-liquid mass transfer characteristics, such as liquid velocity, gas holdup, solid holdup and gas-liquid volumetric mass transfer coefficient, in the riser and downcomer of the gas-liquid-solid three-phase internal loop airlift bioreactors with complete gas recirculation for non-Newtonian fluids, were investigated. A mathematical model for the description of flow behavior and gas-liquid mass transfer of these bioreactors was developed. The predicted results of this model agreed well with the experimental data.

Comparison of mass transfer efficiency in horizontal rotating packed beds and rotating biological contactors

AbstractIn this study, the mass transfer efficiencies of a novel horizontal rotating packed (h‐RPB) bed and the conventional disc‐type rotating biological contactor (RBC) were studied at four speeds and seven submergences. Pall rings of two different sizes (25, 38 mm), superintalox saddles and a wiremesh spiral bundle were used as packings in the h‐RPB. Volumetric gas–liquid mass transfer coefficients were determined by unsteady state absorption of atmospheric oxygen in de‐aerated water. Power consumption per unit liquid volume has been found for all geometries tested. The oxygen transfer efficiency values for the h‐RPB were found to be 2–5 kg kWh−1 and for the disc RBC were found to be 1–2 kg kWh−1. The performance of the h‐RPB was also compared with other gas–liquid contactors such as surface aerators. The study proves that the h‐RPB is a energy efficient alternative to conventional contactors. Copyright © 2005 Society of Chemical Industry

Influence of alcohol addition on gas hold-up, liquid circulation velocity and mass transfer coefficient in a split-rectangular airlift bioreactor

In this work, the effect of alcohol addition on gas hold-up, liquid circulation velocity and gas–liquid volumetric mass transfer coefficient was studied in a split-rectangular airlift bioreactor using air-water as a system to which propanol was added in concentrations ranging from 0.01 to 0.1% (v/v). In order to compare the effect of the propanol with other alcohols, methanol and butanol were also used. The experimental results showed that, at high superficial gas velocities, the addition of small amounts of alcohols decreases the difference between the gas hold-up in the riser and the downcomer, which is due to smaller bubble diameter than with the tap water system. This behaviour is enhanced by alcohols with long carbon chain lengths, which has been explained on the basis of surface tension effects. As a result, the addition of alcohol solutions results in a significant decrease in the liquid circulation velocity, which is mainly due to the decrease in the circulation driving force, but also in a strong decrease in the volumetric gas–liquid mass transfer coefficient for superficial gas velocities higher than 0.033 m/s. This surprising result has been explained by a strong decrease in K L values due to the presence of alcohol and by oxygen depletion due to the increasing amount of small recycled bubbles at high superficial gas velocity.

Hydrodynamic and Mass Transfer of Gas-Liquid-Solid Three-Phase Internal Loop Airlift Reactors with Nanometer Solid Particles

Hydrodynamic and mass transfer characteristics such as liquid velocity, gas holdup and gas-liquid volumetric mass transfer coefficient in the riser and downcomer of the gas-liquid-solid three-phase internal loop airlift reactors with nanometer solid particles were investigated. A mathematical model for the description of flow behavior and gas-liquid mass transfer of those reactors was also developed. The predicted results of this model agreed well with the experimental data.

Experimental Study of Oxygen Mass Transfer Coefficient in Bubble Column with High Temperature and High Pressure

AbstractThe volumetric gas‐liquid mass transfer coefficient, kLα, for oxygen was studied by using the dynamic method in slurry bubble column reactors with high temperature and high pressure. The effects of temperature, pressure, superficial gas velocity and solids concentration on the mass transfer coefficient are systemically discussed. Experimental results show that the gas‐liquid mass transfer coefficient increases with the increase in pressure, temperature, and superficial gas velocity, and decreases with the increase in solids concentration. Moreover, kLα values in a large bubble column are slightly higher than those in a small one at certain operating conditions. According to the analysis of experimental data, an empirical correlation is obtained to calculate the values of the oxygen volumetric mass transfer coefficient for a water‐quartz sand system in two bubble columns with different diameter at high temperature and high pressure.

LOCAL HYDRODYNAMICS IN AN EXTERNAL LOOP AIRLIFT SLURRY REACTOR WITH AND WITHOUT A RESISTANCE-REGULATING ELEMENT

The hydrodynamic behavior of an external loop airlift slurry reactor (ALSR) with and without a resistance-regulating element was studied with a fiber optic probe and ultrasound Doppler velocimetry. The influences of the superficial gas velocity and solid holdup on the global gas holdup and radial profiles of the suspension circulation velocity in the downer and of gas holdup, bubble size, and bubble rise velocity in the riser were studied. Local measurements allow a better understanding of the flow behavior in the reactor and can be used for CFD modeling and validation. Experimental results show that the resistance-regulating element increases the gas holdup and decreases the suspension circulation velocity, indicating that an optimum design of the flow resistance is needed to obtain the maximum gas-liquid volumetric mass transfer coefficient for a specific superficial gas velocity. A high superficial gas velocity and low solid holdup are favorable for increased uniformity of the radial profile of the gas holdup and bubble rise velocity. Hydrodynamic models that predict the gas holdup and suspension circulation velocity were developed for an ALSR with and without a resistance-regulating element. Good agreement was obtained between measured and predicted values.

Prediction of the gas–liquid volumetric mass transfer coefficients in surface-aeration and gas-inducing reactors using neural networks

Almost all available literature correlations to predict the volumetric gas–liquid mass transfer coefficient, k L a in agitated reactors are systems- or operating conditions-dependent. In this study, two back-propagation neural networks (BPNNs), one dimensional and one dimensionless were developed to correlate k L a for numerous gas–liquid systems in both surface-aeration reactors (SAR) and gas-inducing reactors (GIR) operating under wide ranges of industrial conditions. A total of 4435 experimental data points obtained from more than 10 publications for 50 gas–liquid systems were used to train, validate the dimensional and dimensionless BPNNs, which were able to correlate all k L a values with R 2 of 90.5 and 88.6%, respectively. The dimensional BPNN was used to predict the effect of various operating parameters on k L a in a number of important industrial processes. The predictions showed that increasing liquid viscosity decreased k L a values in the SAR, while k L a values in the GIR increased and then decreased with increasing liquid viscosity, following the gas holdup behavior. Increasing liquid density decreased k L a in both reactor types. Increasing liquid surface tension increased k L a values in the SAR, whereas in the GIR, k L a decreased due to the increase of bubble size. Increasing gas diffusivity or gas partial pressure or mixing speed, increased k L a in both reactor types. k L a values in the GIR were always higher than those in the SAR and increasing D Imp./ D T and H F/ H L increased k L a in both reactor types.