Improved gas'liquid contacting in co-current flow

A new method of increasing interfacial area for gas-liquid contacting in co-current flow using screen packings has been evaluated in a 51/2-in. I. D. column using CO₂ chemisortpion in sodium hydroxide solution and CO² physical desorption from water. The study investigated the effect of gas velocity (0.3-2.7-ft/sec.), liquid velocity (0.018-0.1-ft/sec), and of column height (0.67-7.8-ft.) on the interfacial area, a, and the physical desorption liquid phase mass transfer coefficient, K_L. The two types of screen packing tested produced interfacial areas of 2 to 4 times that generated in an unpacked column. K_L showed no effect of gas velocity but increased with liquid velocity, a and K_L decreased with increasing column height. Generally, photographic evaluation gave unrealistically high values of a.     

Mass transfer and mixing characteristics in an airlift‐driven fibrous‐bed bioreactor

An internal loop airlift‐driven fibrous bed bioreactor (ILALFBB) was designed and developed with a high degree of flexibility to handle genetically engineered and fragile shear‐sensitive cells. The mixing and oxygen mass transfer characteristics have been investigated. A cotton fibre was set in the downcomer of the ILALB to represent the fibrous packed bed and the outcome results were compared with those of the polyurethane foam (PUF) packed and unpacked ILALB systems. The effects of fibre, packing height, bed top and bottom clearances, spacing between adjacent fibre surfaces, and superficial gas velocity were investigated. The liquid phase mixing output variables included the liquid circulation velocity (U_Lc), circulation time (t_Lc), mixing time (t_Lm), Bodenstein number (Bo_L), and axial dispersion coefficient (E_L), whereas the mass transfer out variable was the K_La. Bo_L and E_L in the riser and downcomer regions of all packed systems increased with increasing in packing height, packing top clearance, and superficial gas velocity, except the overall Bo_L was independent of gas velocity at low gas velocities. The Bo_L was found highest in the riser of the large cotton and small PUF packed system with large spacing and the E_L in the downcomer of PUF packed systems with smaller spacing between fibre surfaces. Increased amounts of packing in the ILALB, whether in the form of cotton or PUF decreased the U_Lc in the bioreactor because of the increased frictional resistance and tortuosity. The reduction in U_Lc was significant for large packing with smaller spacing between fibre surfaces and increased bottom clearances of the cotton packed system. High circulation times (t_Lc) and shorter mixing times (t_Lm) were achieved using small PUF packing with large top clearance. Relatively high K_La values were obtained using large packing with large top clearances and spacing between fibre surfaces. The boost in K_La was associated with increased gas holdup and/or interfacial area, due to bubble breakage by the shearing action of the fibrous‐bed. Empirical correlation proposed for E_L, Bo_L, and K_La gave a good fit of the experimental data.     

EFFECT OF STATIC LIQUID HEIGHT ON GAS-LIQUID MASS TRANSFER IN A DRAFT-TUBE BUBBLE COLUMN AND THREE-PHASE FLUIDIZED BED

Experiments are performed under batch-liquid operating conditions to investigate the effect of static liquid height on the gas-liquid mass transfer coefficient (K_La) in a draft-tube bubble column (DTBC) and a draft-tube three-phase fluidized bed (DTFB). In addition, the effects of column diameter, gas-distributor, and draft-tube diameter are studied. The results indicate that for a given system with a porous plate gas-distributor at low superficial gas velocities (<70 m/hr), increasing static liquid height decreases K_La. At high gas velocities, K_La is independent of the static liquid height. For systems with a perforated gas-distributor, there is no effect of static liquid height on K_La. The formation of small dispersed bubbles at low gas velocities in the porous plate distributor system accounts for the considerably high K_La values and the observed effect of liquid height. On the other hand, the formation of large spherical-cap bubbles and the bubble coalescence at high gas velocities reduce the performance of the porous plate distributor system to that of the perforated one.     

Bubble column research in Japan

Many experimental studies on the bubble column have been reported by Japanese researchers since around 1960. They include studies of bubble behaviour, bubble size distribution, transition from the homogeneous bubbly flow regime to the heterogeneous liquid circulation regime, liquid velocity distribution, longitudinal liquid mixing, hydrodynamic modelling, the gas holdup, and the volumetric coefficient of gas-liquid mass tranfer k_La. Studies covered various modified bubble columns, such as the airlift reactor with an external or internal loop, the packed bubble column, and others. Performance of three-phase bubble columns, which deal with suspensions or emulsions, and their use as bioreactors or chemical reactors were also studied.     

Oxygen transfer in viscous non-Newtonian liquids having yield stress in bubble columns

The oxygen transfer and hydrodynamics in viscous media having a yield stress in bubble columns operated under the slug flow regime were investigated to design an optimum bubble column fermentor for culture media having a yield stress.The gas holdup of escapable bubbles was well estimated by the equation of Nicklin et al. (Trans. Inst. Chem. Eng. 40 (1962) 61), which was modified for the viscous liquid having a yield stress by Terasaka and Tsuge (Chem. Eng. Sci. 58 (2003) 513). The volumetric oxygen transfer coefficient k_La increased with increasing superficial gas velocity and decreasing column diameter under the present conditions. To predict k_La in the non-Newtonian liquids having a yield stress under the operation in slug flow regime, the proposed correlation equation estimated relatively well the experimental k_La.To increase oxygen transfer rate, two types of novel bubble columns were compared with the standard bubble column. The partitioned bubble column presented the better performance than those of the other ones.     

Design and characterization of a multistage,mechanically stirred column absorber

A multistage, mechanically stirred column absorber has been designed and built with a modular construction, based on preliminary experiments with a test column. The column has been characterized as a gas-liquid contactor by its gas holdup, gas and liquid axial dispersion, mixing times, oxygen transfer coefficients and power consumptions, determined as a function of gas velocity, liquid velocity and impeller speed for one and two impellers per stage.Gassed power was correlated with ungassed power, gas rate and impeller speed. The gas phase axial mixing was essentially plug flow and the liquid phase axial mixing varied between 5 and 12 equivalent stages.Oxygen transfer coefficients were correlated with power consumptions and aeration rates by the equation K_La γ (P/V)^a(υ_sg)^b. The oxygen transfer coefficients with single stiffer stages were 25% above those for the double stirrer stages for equal power consumption and gas rates. Except for the low aeration and high power consumption extremes, the column showed superior oxygen transfer performance. in comparison to tubular loop and tank fermenters.     

Hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer

The hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer were investigated. The contactor consisted of two concentric columns of 0.11 and 0.2 m i.d., with the annulus acting as the downcomer. The packing used in this investigation was standard 16 mm stainless steel Pall rings. The superficial gas and liquid velocities, V_G and V_L, were varied from 0.01 to 0.09 and 1 × 10^?3 to 8.8 × 10^?3 m s^?1 respectively. Two flow patterns, namely the bubble and pulse flows were observed in the packed bubble column with downcomer, as shown by a flow map. The liquid circulation velocity in both the contactors was observed to be constant throughout the ranges of V_G and V_L covered in this work. The effect of liquid viscosity (0.8 to 9.5 mPa ? s) and surface tension (45 to 72 mN m^?1) on the flow pattern, liquid circulation, gas hold-up and pressure drop was investigated. The pressure drop characteristics across the two contactors have been compared with those across a bubble column. Values of the effective interfacial area, a, and the volumetric mass transfer coefficient, k_L a, were measured by using chemical methods. Values of a as high as 180 and 700 m^?1 and k_L a as high as 0.075 and 0.22 s^?1, in the bubble and packed bubble columns with downcomer, respectively, were obtained. The values of true liquid-side mass transfer coefficient, k_L, were found to be independent of V_G and were of the order of 5.5 × 10^?4 and 3.5 × 10^?4 m s^?1, respectively, in the two contactors.     

Mass transfer in packed columns: Co-current (downflow) operation: 1 in. And 1.5 in. Metal pall rings and ceramic intalox saddles: Multifilament gauze packings in 20 cm and 38 cm i.d. columns

The theory of gas absorption accompanied by fast pseudo-mth order reaction was used to obtain values of effective interfacial area, a, in 20 and 38 cm i.d. packed columns which were operated co-currently (downflow). Values of a were obtained for 1 in. and 1.5 in. metal Pall rings; 1 in. stainless steel Pall rings, having length (height) to diameter ratio of 1.0, 0.75, and 0.5; 1 in and 1.5 in. ceramic Italox saddles; and stainless steel multifilament wire gauze type packing over a wide range of gas and liquid superficial velocities. The gas superficial velocity was varied from 30 to 255 cm/sec in the 20 cm i.d. column and 14 to 73 cm/sec in the 38 cm i.d. column. The liquid superficial velocity was varied from 0.2 to 3 cm/sec in the 20 cm i.d. column and 0.2 to 1 cm/sec in the 38 cm i.d. column. Different flow regimes, namely, trickle flow (film flow), pulse flow and transition from pulse to disperse flow, were covered. The values of a were found to be in the range of 0.6 to 2 cm²/cm³ for the trickle flow (film flow) regime, and 2.4–5 cm²/cm³ for the pulse flow regime. In the case of multifilament wire gauze packing (MFWGP) remarkably high values of a up to 16 cm²/cm³ were obtained in the pulse to disperse flow regime.     

Model development,parameter identification,and simulation of SCP production processes in air lift tower reactors with external loop—II: Extended culture modelling and quasi steady state parameter identification

Yeast was cultivated in extended culture in a bench-scale 275 cm high air lift tower reactor 15 cm dia. with an external loop. Longitudinal dissolved oxygen concentration profiles, substrate and cell mass concentrations in the medium, O₂ and CO₂ concentrations in the gas phase, as well as gas flow rates and liquid recirculation rates were measured. A distributed parameter model was used to describe the cultivation process variation along the column, cell mass, substrate and oxygen balances in the medium, O₂ and CO₂ balances in the gas phase, variation of the volumetric mass transfer coefficient along the column due to bubble coalescence, as well as double substrate Monod kinetics. Based on simulation runs it was assumed that under non limited and oxygen transfer limited growth conditions, the cell mass and substrate concentrations are uniform in the reactor. The simulation was carried out by a hybrid computer. The unknown model parameters (volumetric mass transfer coefficient at the gas entrance, k_La^E, and coalescence factor K_ST) and two kinetic parameter R_Omax and K_O were identified by means of experimental results with quasi steady state simulation methods.     

Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD

Computational Fluid Dynamics (CFD) is used to compare the hydrodynamics and mass transfer of an internal airlift reactor with that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulation velocities are significantly higher in the airlift configuration than in bubble columns, leading to significantly lower gas holdups. Within the riser of the airlift, the gas and liquid phases are virtually in plug flow, whereas in bubble columns the gas and liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k_La, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k_La are practically identical.     

Axial dispersion in a rectangular bubble column

This paper presents the results of an experimental study on the gas holdup and the liquid phase axial dispersion coefficient in a narrow packed and unpacked rectangular bubble column. In both cases the gas and liquid flow rates were varied and the data were obtained by employing standard tracer technique. The gas holdup and the axial dispersion coefficient for both the packed and unpacked columns were found to be dependent on the gas and liquid flow rates. For given gas and liquid velocities and a given packing size in the case of the packed column, the rectangular column gave significantly higher dispersion coefficients than a cylindrical column of the equivalent cross sectional area. This result agrees very well with the one predicted by the velocity distribution model. The correlations for the Peclet number, the axial dispersion coefficient, and the fluid holdup for both the unpacked and packed bubble columns are presented.     

Mixing and mass transfer in tall bubble columns

In cocurrent bubble columns (15 and 20 cm diameter, 440 and 723 cm high) with different gas distributors measurements were carried out with tap water and solutions of salts and molasses. A stationary and a transient method were applied to determine the dispersion coefficients. Absorption and desorption of oxygen was studied by measuring the concentration profiles of oxygen in the liquid phase. Liquid phase mass transfer rates k_La were obtained adjusting the experimental profiles with the predictions of the axial dispersed plug flow model. Owing to the different gas spargers the k_La values of both columns differ by a factor of about two. Correlations are proposed for the k_La data of the various liquid phases which only depend on the gas velocity.     

Gas Holdup and Volumetric Mass Transfer Coefficient in a Slurry Bubble Column

The gas holdup, ?, and volumetric mass transfer coefficient, k_La, were measured in a 0.051 m diameter glass column with ethanol as the liquid phase and cobalt catalyst as the solid phase in concentrations of 1.0 and 3.8 vol.‐%. The superficial gas velocity U was varied in the range from 0 to 0.11 m/s, spanning both the homogeneous and heterogeneous flow regimes. Experimental results show that increasing catalyst concentration decreases the gas holdup to a significant extent. The volumetric mass transfer coefficient, k_La, closely follows the trend in gas holdup. Above a superficial gas velocity of 0.04 m/s the value of k_La/? was found to be practically independent of slurry concentration and the gas velocity U; the value of this parameter is found to be about 0.45 s^–1. Our studies provide a simple method for the estimation of k_La in industrial‐size bubble column slurry reactors.     

Mass Transfer Characteristics of Syngas Components in Slurry System at Industrial Conditions

The gas‐liquid mass transfer behavior of syngas components, H₂ and CO, has been studied in a three‐phase bubble column reactor at industrial conditions. The influences of the main operating conditions, such as temperature, pressure, superficial gas velocity and solid concentration, have been studied systematically. The volumetric liquid‐side mass transfer coefficient k_La is obtained by measuring the dissolution rate of H₂ and CO. The gas holdup and the bubble size distribution in the reactor are measured by an optical fiber technique, the specific gas‐liquid interfacial area aand the liquid‐side mass transfer coefficient k_L are calculated based on the experimental measurements. Empirical correlations are proposed to predict k_L and a values for H₂ and CO in liquid paraffin/solid particles slurry bubble column reactors.     

Using image analysis in the study of multiphase gas absorption

For the air-water-calcium alginate beads system, the effect of the presence of solids on the mass transfer characteristics in a bubble column was experimentally studied.Volumetric liquid side mass transfer coefficient, k_La, specific interfacial area, a, and hence liquid side mass transfer coefficient, k_L, were determined under different solid concentrations (0, 5, and 10 vol%), superficial gas velocities (up to 0.27 cm/s) and solid sizes (1.2 and 2.1 mm diameter). The bubble characteristics, namely the interfacial area, were obtained using an image analysis technique.This technique proved to be a suitable and practical method to characterize mass transfer phenomena in bubble columns for the range of operating conditions used. The solids affect negatively k_La, decreasing both a and k_L, the effect being more pronounced for the smaller particles. For these particles the variation of k_La is due to the variation of its two components, while for larger particles k_La variation is due, essentially, to changes in k_L as no significant differences in a were observed.     

Mass transfer in bubble column for industrial conditions—effects of organic medium, gas and liquid flow rates and column design

Most of available gas-liquid mass transfer data in bubble column have been obtained in aqueous media and in liquid batch conditions, contrary to industrial chemical reactor conditions. This work provides new data more relevant for industrial conditions, including comparison of water and organic media, effects of large liquid and gas velocities, perforated plates and sparger hole diameter.The usual dynamic O₂ methods for mass transfer investigation were not convenient in this work (cyclohexane, liquid circulation). Steady-state mass transfer of CO₂ in an absorption-desorption loop has been quantified by IR spectrometry. Using a simple RTD characterization, mass transfer efficiency and k_La have been calculated in a wide range of experimental conditions.Due to large column height and gas velocity, mass transfer efficiency is high, ranging between 40% and 90%. k_La values stand between 0.015 and and depend mainly on superficial gas velocity. No significant effects of column design and media have been shown. At last, using both global and local hydrodynamics data, mass transfer connection with hydrodynamics has been investigated through k_La/ε_G and k_La/a.     

填料特性对鼓泡填料萃取塔性能影响

填料的结构与表面性能对鼓泡填料萃取塔性能有直接影响。利用空气 煤油 (苯甲酸 ) 水体系 ,测定了未装填料和分别装填板波填料、丝网填料、压延孔环填料的鼓泡萃取塔水力学性能和传质性能。实验表明 ,对未装填料和装有填料的萃取塔 ,气相搅拌都可以显著提高液液两相的接触与传质性能 ;液泛速度随表观气速的增大而下降 ;流道设计合理的规整填料传质性能明显高于散装填料 ;表面光滑的填料分散相滞存率低 ,因而液泛速度较高 ;填料的作用有利于降低轴向返混 ,明显提高萃取塔传质性能。     

Effect of surfactants on liquid-side mass transfer coefficients

In the present paper, the effect of liquid properties (surfactants) on bubble generation phenomenon, interfacial area and liquid-side mass transfer coefficient was investigated. The measurements of surface tension (static and dynamic methods), of critical micelle concentration (CMC) and of characteristic adsorption parameters such as the surface coverage ratio at equilibrium (s_e) were performed to understand the effects of surfactants on the mass transfer efficiency. Tap water and aqueous solutions with surfactants (cationic and anionic) were used as liquid phases and an elastic membrane with a single orifice as gas sparger. The bubbles were generated into a small-scale bubble column. The local liquid-side mass transfer coefficient (k_L) was obtained from the volumetric mass transfer coefficient (k_La) and the interfacial area (a) was deduced from the bubble diameter (D_B), the bubble frequency (f_B) and the terminal bubble rising velocity (U_B). Only the dynamic bubble regime was considered in this work (Re_OR=150-1000 and We=0.002-4).This study has clearly shown that the presence of surfactants affects the bubble generation phenomenon and thus the interfacial area (a) and the different mass transfer parameters, such as the volumetric mass transfer coefficient (k_La) and the liquid-side mass transfer coefficient (k_L). Whatever the operating conditions, the new k_La determination method has provided good accuracy without assuming that the liquid phase is perfectly mixed as in the classical method. The surface coverage ratio (s_e) proves to be crucial for predicting the changes of k_L in aqueous solutions with surfactants.     

Interfacial Area and Liquid‐Side Volumetric Mass Transfer Coefficient in a Downflow Bubble Column

An experimental investigation was made to measure interfacial area, a, and liquid‐side volumetric mass transfer coefficient, k_La, in a downflow bubble column by chemical methods viz., absorbing CO₂ in aqueous sodium hydroxide and sodium carbonate/bicarbonate buffer solution respectively. The effect of gas and liquid flowrate and nozzle sizes on a and k_La were investigated. The experimental data obtained in the present system were analyzed and correlations were developed to predict a and k_La in terms of superficial gas velocity. The variation of a and k_La with specific power input were shown in graphical plot and compared with other gas‐liquid systems.     

Counter-current absorption using wire gauze packings

The dynamic liquid hold-up, ?_LD, effective interfacial area, a, and the liquid side mass transfer coefficient k_La were determined for 0.1 m and 0.2 m multifilament wire gauze packings, 0.0125 m double walled wire gauze partition rings and 0.025 m wire gauze saddle packings in columns operated countercurrently. The theory of gas absorption accompanied by fast pseudo mth order reaction was used to determine the effective interfacial area. The values of liquid side mass transfer coefficient for the multifilament wire gauze packings were obtained by absorbing lean carbon dioxide in a buffer solution of sodium carbonate and sodium bicarbonate. K_La values for the other packings were obtained by absorbing pure carbon dioxide in tap water. The values of a and k_La for multifilament wire gauze packings were found to be two to four times higher as compared to the conventional ring or saddle packings. Further, the superficial liquid velocity was found to have marginal effect on a. The double walled wire gauze partition rings offered a values which were 1.5–2.0 times higher than that offered by 0.016 m s.s. Pall rings at low values of superficial liquid velocity (<3 × 10^{?3 m/s}.