Volumetric Gas-liquid Mass Transfer Research Articles

Analysis of petroleum biodesulfurization in an airlift bioreactor using response surface methodology

For the first time, growing cells of Gordonia alkanivorans RIPI90A were used for biodesulfurization (BDS) of diesel. This process was carried out in an internal airlift bioreactor. BDS parameters (oil/water phase ratio and initial sulfur concentration) were optimized in flasks using response surface methodology. Predicted results were found to be in good agreement with experimental results. Initial sulfur concentration had a remarkable effect on BDS process. Maximum removal of sulfur (21mg/l) can be achieved at oil/water phase ratio of 25% (v/v) and initial sulfur concentration of 28mg/l. Moreover, effect of superficial gas velocity (Ug) and working volume (v) on volumetric gas liquid mass transfer coefficient was studied in an airlift bioreactor for BDS of diesel. The best results were achieved at Ug and v of 2.5l/min and 6.6l, respectively. Subsequently, BDS of diesel was investigated in an airlift bioreactor under optimized conditions. Sulfur reduction after 30h was 14mg/l.

Hydrodynamics and mass transfer studies on a plate microreactor

Prototype of structured microreactor plates with 300 μm deep flow channels was developed. Two plates with different structural elements, square and triangular, were used in the study. Flow behavior was investigated and the results showed excellent mixing behavior of gas and liquid phases. Hydrodynamic parameters and volumetric gas–liquid mass transfer coefficients were determined at various flow conditions. Gas holdup and the gas–liquid interfacial area were determined by an optical method using image processing software. Volumetric mass transfer coefficients were measured using nitrogen to strip oxygen from deionized water. The experimental results indicate a high mass transfer rate. Gas holdup and the total gas–liquid interfacial area were in the range of 20–45% and 2600–5600 m 2 m −3, respectively. Volumetric mass transfer coefficients were as high as 1.1 s −1. An empirical correlation for the mass transfer coefficient was derived.

Effect of rotor–stator distance and rotor radius on the rate of gas–liquid mass transfer in a rotor–stator spinning disc reactor

Abstract This paper describes the effect of rotor radius, rotor–stator distance, liquid flow rate and rotational disc speed on the rate of gas–liquid mass transfer in a rotor–stator spinning disc reactor. A rotor radius of 0.135 m is studied with rotor–stator distances of 1, 2 and 5 mm, at rotational disc speeds up to 209 rad s −1, and compared with a rotor radius of 0.066 m. At rotational disc speeds lower than 70 rad s −1, elongated gas bubbles are formed, that are larger than the rotor–stator distance. At rotational disc speeds above 100 rad s −1, spherical gas bubbles are formed that are smaller than the rotor–stator distance. The volumetric gas–liquid mass transfer coefficient increases with increasing rotational disc speed and decreases with increasing liquid flow rate. This decrease is larger than predicted by the Wallis drift flux model because of the complex two-phase flow pattern. The rate of gas–liquid mass transfer per unit of reactor volume increases with decreasing rotor–stator distance. The maximum observed volumetric mass transfer coefficient in case of the 0.135 m rotor is a factor 3 higher than in case of the 0.066 m rotor, while the rate of energy dissipation is a factor 15 higher.

The effect of the physical properties of the liquid phase on the gas-liquid mass transfer coefficient in two- and three-phase agitated systems

AbstractThe results of studies concerning two- and three-phase systems in an agitated vessel are presented. The aim of our research was to investigate the effect of the physical properties of the liquid phase on the value of the volumetric gas-liquid mass transfer coefficient in mechanically agitated gas-liquid and gas-solid-liquid systems. Our experimental studies were conducted in a vessel with an internal diameter of 0.288 m. The flat bottom vessel, equipped with four baffles, was filled with liquid up to a height equal to the inner diameter. The liquid volume was 0.02 m3. Three high-speed impellers of a diameter equal to 0.33 of the vessel diameter were used: Rushton turbine (RT), Smith turbine (CD 6), or A 315 impeller. The measurements were carried out in coalescing and non-coalescing systems. Distilled water and aqueous solutions of an electrolyte (sodium chloride) of two different concentrations were used as the liquid phase. The gas phase was air. In the three-phase system, particles of sea sand were used as solid phase. The measurements were conducted at five different gas-flow rates and three particle loadings. Volumetric gas-liquid mass transfer coefficients were measured using the dynamic method. The presence and concentration of an electrolyte strongly affected the value of the gas-liquid mass transfer coefficient in both two- and three-phase systems. For all agitators used, significantly higher k l a coefficient values were obtained in the 0.4 kmol m−3 and 0.8 kmol m−3 aqueous NaCl solutions compared with the data for a coalescing system (with distilled water as the liquid phase). The k l a coefficient did not exhibit a linear relationship with the electrolyte concentration. An increase in the sodium chloride concentration from 0.4 kmol m−3 to 0.8 kmol m−3 caused a considerable decrease in the volumetric mass transfer coefficient in both the two-phase and three-phase systems. It was concluded that the mass transfer processes improved at a certain concentration of ions; however, above this concentration no further increase in k l a could be achieved.

Bubble shape, gas flow and gas–liquid mass transfer in pulp fibre suspensions

AbstractGas–liquid mass transfer in pulp fibre suspensions in a batch‐operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical‐capped/dimpled‐elliptical bubbles. At relatively low mass concentrations (Cm = 2–3% for the softwood and Cm ≅ 7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas–liquid mass transfer rate, kLa, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in kLa occurred between Cm = 2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed.

Relation between pristinamycins production by Streptomyces pristinaespiralis, power dissipation and volumetric gas–liquid mass transfer coefficient, kLa

During bioreactor cultures, microorganisms are submitted to non-optimal conditions such as nutritional and hydrodynamic stresses which may lead to modifications of the physiological cell response; this is especially true for filamentous microorganisms like Streptomycetes also subjected to significant morphological changes. In the present work, growth and production of pristinamycins by Streptomyces pristinaespiralis in shaking flasks have been related to power dissipation. The filamentous bacteria were grown in different flask conditions with various total and working volumes and at two agitation rates, to test the influence of power dissipation and gas–liquid mass transfer coefficient on growth and antibiotics production. As a first step, computational fluid dynamics–volume of fluid (CFD–VOF) calculations were shown to be able to predict power dissipations for the various operating conditions in Newtonian flow conditions. Then, in non-Newtonian flow conditions (biomass concentration superior to 14 g L −1), the rheological model of Sisko was implemented in CFD simulations for the calculation of the fluid viscosity and then of power dissipation. Whereas microbial growth was correlated to k L a, the antibiotics production onset was linked to the volume mean power dissipation. Once a minimal cell concentration of 15 g L −1 was reached, the concentration of antibiotics was correlated to power dissipation with an optimal range of production, between 5.5 and 8.5 kW m −3. Higher power dissipation entailed a drop in production which could be explained by hydrodynamic cell damages.

Gas–liquid mass transfer in a rotor–stator spinning disc reactor

This paper describes a new multiphase reactor, the rotor–stator spinning disc reactor, which shows high rates of gas–liquid mass transfer in comparison to conventional multiphase reactors. The volumetric gas–liquid mass transfer coefficient k GL a GL in the rotor–stator spinning disc reactor increases with increasing rotational disc speed, due to the higher surface renewal rate caused by the increasing turbulence, and with increasing gas flow rate. Measured k GL a GL values are as high as 0.43 m L 3 m R - 3 s - 1 at 7.3 × 10 - 6 m 3 s - 1 gas flow and a rotational disc speed of 179 rad s - 1 , and are expected to increase even further at increasing rotational disc speed. This is twice as high as for conventional reactors as bubble columns, in spite of the low gas holdup of 0.021 m G 3 m R - 3 with only one gas inlet. The volumetric mass transfer per unit volume of gas, k GL a GL / ɛ G , of 20.5 m L 3 m G - 3 s - 1 is 40 times higher than 0.5 m L 3 m G - 3 s - 1 for a bubble column.

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.

Investigation of gas properties, design, and operational parameters on hydrodynamic characteristics, mass transfer, and biomass production from natural gas in an external airlift loop bioreactor

An external airlift loop bioreactor (EALB) was used for production of biomass from natural gas. The effect of riser to downcomer cross sectional area ratio ( A r / A d ), volume of gas–liquid separator, superficial gas velocity ( U sgr ), and physical properties of gases and their mixtures [ υ g ( μ/ ρ) and D g ] were investigated on mixing time, gas hold-up, and volumetric gas liquid mass transfer coefficients ( k La ). It was found that A r / A d has remarkable effects on gas hold-up and k La due to its influence on mixing time. Kinematic viscosity ( υ g ) showed its significant role on mixing time, gas hold-up and k La when different gases used (mixing time changes directly whereas gas hold-up and k La change indirectly). Moreover, it was found that diffusion coefficient of gas in water ( D g ) has remarkable effect on k La . The volumetric mass transfer coefficients for methane and its mixtures with oxygen (three different mixtures) were determined at different geometrical and operational factors. In average, the rate of oxygen utilization is approximately 1.8 times higher than that of methane. A gas mixture of 25 vol% methane and 75 vol% oxygen was the best gas mixture for biomass production in the EALB. The correlations developed for predicting the mixing time, gas hold-up, and k La in terms of U sgr , A r / A d , volume of gas–liquid separator, and gas phase properties have been found to be encouraging.

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 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.

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.

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.

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.

Gas‐Liquid Mass Transfer in Pulp Retention Towers

AbstractThe volumetric gas‐liquid mass transfer rate, kLa, was measured under batch conditions in a 0.28 m diameter laboratory‐scale retention column. Tests on water, and on unbleached kraft (UBK) pulp suspensions (mass fractions, Cm from 0.013 to 0.09) were made with air or nitrogen sparged through the column at superficial gas velocities between 0.0015 to 0.05 m/s. kLa varied with suspension mass concentration and superficial gas velocity, initially decreasing with increasing mass concentration, reaching a minimum between Cm = 0.03 and 0.06, and then increasing. The minimum in kLa coincided with a change in hydrodynamics within the column, from bubble column behaviour below Cm = 0.03 to porous solid behaviour above Cm = 0.06.

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.