Bubble size in a forced circulation loop reactor

BACKGROUND: The bubble size distribution in gas‐liquid reactors influences gas holdup, residence time distribution, and gas‐liquid interfacial area for mass transfer. This work reports on the effects of independently varied gas and liquid flow rates on steady‐state bubble size distributions in a new design of forced circulation loop reactor operated with an air–water system. The reactor consisted of a cylindrical vessel (～26 L nominal volume, gas‐free aspect ratio ≈ 6, downcomer‐to‐riser cross‐sectional area ratio of 0.493) with a concentric draft tube and an annular riser zone. Both gas and liquid were in forced flow through a sparger that had been designed for minimizing the bubble size. RESULTS: Photographically measured bubble size distributions in the riser zone could be approximated as normal distributions for the combinations of gas and liquid flow rates used. This contrasted with other kinds of size distributions (e.g. bimodal, Gaussian) that have been reported for other types of gas‐liquid reactors. Most of the bubbles were in the 3 to 5 mm diameter range. At any fixed low value of aeration rate (≤1.8 × 10^?4 m³s^?1), increase in the liquid flow rate caused earlier detachment of bubbles from the sparger holes to reduce the Sauter mean bubble size in the riser region. CONCLUSION: Unlike in conventional bubble columns where bimodal and Gaussian bubble size distributions have been reported, a normal bubble size distribution is attained in forced circulation loop reactors with an air–water system over the entire range of operation. Copyright © 2007 Society of Chemical Industry     

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.     

The effect of operating and design parameter on bubble column performance: The LOPROX case study

It is known that the performances of multi-phase reactors depend on the operating parameters (the temperature and the pressure of the system), the phase properties, and the design parameters (the aspect ratio (AR), the bubble column diameter, and the gas sparger design). Hence, the precise design and the correct operation of multi-phase reactors depends on the understanding and prediction of the fluid dynamics parameters. This paper contributes to the existing discussion on the effect of operating and design parameter on multi-phase reactors and, in particular, it considers an industrial process (e.g., the LOPROX (low pressure oxidation) case study, which is typical example of two-phase bubble columns). Based on a previously-validated set of correlations, the influence of operating and design parameter on system performances is studied and critically analyzed. First, we studied the effects of the design parameter on the liquid–gas interfacial area, by keeping constant the fluid physical–chemical properties as well as the operating conditions; subsequently, we discussed for a fixed system design, the influence of the liquid phase properties and the operating pressure. In conclusion, this paper is intended to provide guidelines for the design and scale-up of multi-phase reactors.     

ESTIMATION OF EFFECTIVE SHEAR RATE FOR AERATED NON-NEWTONIAN LIQUIDS IN AIRLIFT BIOREACTOR”“

Effective shear rate is one of the indispensable parameters for the design of aerobic fermentors using a viscous non-Newtonian system. The estimation of effective shear rate in airlift loop bioreactors has been investigated with liquid circulation velocity. An empirical correlation of effective shear rate in airlift loop reactors is proposed.

γ_eπ= 3.26-3.51 ; 10²U_G + 1.48 10⁴U²_G

It is found that the effective shear rate is lower in airlift reactors than in bubble columns. This equation can be used for the cultivation of cells sensitive to shear stress.     

Mixing Characteristics and Bubble Behavior in an Airlift Internal Loop Reactor with Low Aspect Ratio

The present study summarizes the results of macro-and micro-mixing characteristics in an airlift inter-nal loop reactor with low aspect ratio （H/D≤5） using the electrolytic tracer response technique and the method of parallel competing reactions respectively. The micro-mixing has never been investigated in airlift loop reactors. The dual-tip electrical conductivity probe technique is used for measurement of local bubble behavior in the reactor. The effects of several operating parameters and geometric variables are investigated. It is found that the increase in su-perficial gas velocity corresponds to the increase in energy input, liquid circulation velocity and shear rate, decreas-ing the macro-mixing time and segregation index. Moreover, it is shown that top clearance and draft diameter affect flow resistance. However, the bubble redistribution with a screen mesh on the perforated plate distributor for macro-mixing is insignificant. The top region with a high energy dissipation rate is a suitable location for feeding reactants. The analysis of present experimental data provides a valuable insight into the interaction between gas and liquid phases for mixing and improves the understanding of intrinsic roles of hydrodynamics upon the reactor de-sign and operating parameter selection.     

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

The volumetric gas‐liquid mass transfer coefficient, k_Lα, 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, k_Lα 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 holdup and pressure drop for air-water flow through plate bubble columns

Gas holdup and pressure drop measurements were made in two multistage bubble columns. Plates made from 6 mesh/in screen (α = 0.64) and those of Karr design (α = 0.53) were tested. An improved separated-flow model was developed for correlating the two-phase friction losses over the ranges: 0 ≤ V_G ≤ 8.22 cm/s and 0 ≤ V_L ≤ 6.12 cm/s. This model was also sound for the vibrating disk column introduced by Tojo et al. (1974). Empirical correlations are presented for the gas holdup and the pressure drop due to liquid circulation within these two columns.     

Pressure Drop and Mass Transfer Study in Static Mixers with Gas Continuous Phase

The aim of this work was to characterize both the influence of the gas and the liquid flow rates on the pressure drop generated by static mixers type Sülzer, and on the mass transfer performances of these gas‐liquid contactors. The originality of this work rests on the use of the static mixers with a gas continuous phase. Several configurations were studied: vertical upward flow, vertical downward flow, and horizontal positions, with one to five mixing elements. It was concluded that the pressure drop is mainly generated by the gas phase, whatever the configuration chosen. Moreover, the volumetric mass transfer coefficients k_La found were lower than those obtained with the same static mixers used in classic conditions (that is with a liquid continuous phase), but greater than values obtained with classic reactors like bubble columns or packed columns. The efficiency of these gas‐liquid reactors was found high, as well as the energy dissipated, unfortunately.     

A study of the pressure balance around the loop of a circulating fluidized bed

Pressure measurements around the loop of a circulating fluidized bed with 152 mm ID riser and L-valve fecuer were analysed to determine the effect of operating parameters (superficial gas velocity in the range 2.2 - 4.0 m/s, solids circulation flux in the range 5 - 50 kg/m² · s and solids inventory, in the range 80 - 180 kg) on the components of the pressure balance. The riser pressure drop, and hence, riser solids holdup were not affected by changes in the inventory of solids in the system, provided riser superficial gas velocity and solid circulation flux were held constant. The mean suspension concentration in the riser was found to be directly proportional to the ratio of solids flux to superficial gas velocity (G / U) in the riser.     

Enhancement in gas holdup in bubble columns through use of vibrating internals

Including internals in bubble columns is known to enhance the gas holdup. In this paper, a method to achieve this objective substantially has been proposed via the use of vibrating helical spring internals. Experimental observations on effect of vibrating internals such as vibrating helical springs on gas holdup in bubble columns are presented. Effects of superficial gas velocity, H/D ratio (height of the static liquid to column diameter ratio), volume fraction of helical springs, and thickness of the helical spring wires on hydrodynamics parameters are studied. Increase in gas holdup up to 135% is observed by using vibrating helical spring internals in bubble columns compared to bubble columns without internals. This method offers a simple, cost‐effective, and easy way to enhance gas holdup even at high gas fluxes. It has been reported that this enhancement stems from the fact that the vibrating springs breakup the gas into fine bubbles, which effectively reduces their rise velocity and enhances their average residence time in the liquid column.     

INVISCID LIQUID CIRCULATION IN BUBBLE COLUMNS

For circulation in axi-symmetric (cylindrical) bubble columns, the recently developed mathematical model^25,26 has been used along with the criterion of minimum circulation strength to determine the height of each circulation cell in a tall column. This is then used to derive a theoretical expression, first of its kind, for gas hold-up inside a bubble column. The predictions of this equation as well as the equation derived here for axial liquid velocity at column axis have been compared with available data and the comparison is found to be excellent for both the variables. An explicit relation is derived for the average liquid circulation velocity. The model is also used to derive an expression for liquid axial dispersion coefficient which compares almost exactly with Deckwer et al.'s⁴ correlation.

For circulation in two-dimensional bubble columns a new mathematical model is developed. The predictions of bubble envelope shape and bubble envelope area compare well with published data. The predictions of number of circulation cells in the horizontal direction also compare well with published data.     

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.     

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.     

Studies of spouted beds with small outlet-inlet cross-section ratios

An experimental study of the hydrodynamics, structure, mixing and interphase heat transfer of spouted beds of fine dispersed materials in conic-cylindrical columns with the small outlet-to-inlet cross-section ratio D/D₀ ≤ 3 has been made by different methods. The effect of design and operation parameters on the shape, sizes and stability of a central core, the solid phase concentration variation in the core and annulus, the circulation rate, entrainment and heat transfer in the columns with D = 100 mm have been explored. Empirical correlations have been suggested for calculating pressure drops across such beds under the developed spouting conditions and for effective gas-to-solid phase heat transfer coefficients.     

Liquid dispersion,gas holdup and frictional pressure drop in a packed bubble column at elevated pressures

The gas holdup, frictional pressure drop and liquid dispersion have been investigated in a packed bubble column at elevated pressures for the air–water system. The bubble column, which had an internal diameter of 0.15 m and which was packed with 15 mm plastic Pall rings was operated in the semibatch mode. The operating pressures ranged from 0.1 to 0.66 MPa. It was found that increasing the pressure increases both the gas holdup and the dispersion coefficient. In contradiction to the results obtained from packed bubble columns fed with a continuous net flow of liquid, a maximum point of the frictional pressure drop was observed at the transition point between bubble and pulse flow region.     

Gas holdup and bubble dynamics in a three-phase internal loop reactor with external slurry circulation

As modified three-phase fluidized reactors, loop reactors have been widely used in the area of chemical and energy processes. An external slurry circulation is introduced into a traditional internal loop reactor to improve the transfer between gas and slurry phases. Gas holdup and bubble dynamics are investigated by using the double-sensor conductivity probe technique in the present work. The results show that gas holdup inside the draft tube is greatly affected by the geometrical configuration and is much higher than that in the corresponding section of the annular region. Local, section-averaged, and overall gas holdups increase with increasing superficial gas velocity, while the effects of solid loading and external slurry circulation velocity are less significant than that of superficial gas velocity. Both local bubble size and bubble rise velocity vary significantly in different regions.     

Gas Holdup in Circulating Bubble Columns with Pseudoplastic Liquids

The contributions of pressure drop due to wall frictional losses to the total gas holdup of two‐phase viscous non‐Newtonian systems were experimentally investigated using a 150 dm³ circulating bubble column. The column had a downcomer‐to‐riser cross‐sectional area ratio of 0.54 and a dispersion height of 2.5 m. Aqueous solutions of xanthan gum and carboxymethyl cellulose were used to simulate a wide range of rheological properties. The average wall shear stress was estimated from Al‐Masry's (1999) correlation for the average wall shear rate in external loop airlift reactors. Pressure drop due to wall shear stress was found significantly contributed by 10–70 % to the total gas holdup. This contribution has always been ignored in the data presented in the literature due to the absence of reliable and simple correlations for the average shear rate and shear stress. Corrections to gas holdup were found necessary for non‐Newtonian solutions with concentrations of ≥ 0.5 wt/wt.‐%.     

Numerical Simulation of Dispersed Gas/Liquid Flows in Bubble Columns at High Phase Fractions using OpenFOAM®. Part II – Numerical Simulations and Results

The design of industrial gas/liquid reactors such as bubble columns requires detailed information with respect to the flow structure and characteristics of two‐ or multiphase systems in the reactor. The contribution is focused on the evaluation of the simulation results obtained by model selection. The results are further compared with those reported in literature. The simulation has been performed with the CFD software OpenFOAM®. The main focus of the numerical simulation was set on capturing the characteristic process and design parameters of bubble columns.     

Modellbildung für Mehrphasenströmungen in Reaktoren

Modelling multiphase flow in reactors. This paper is intended to enhance the knowledge of two phase reactors. Their performance depends not only on reaction kinetics but essentially on fluid-dynamically fixed quantities such as volume fractions, dispersion coefficients in the fluid and in the gas phase, as well as the volumetric mass transfer coefficient. These are basically dependent on the slip velocity of the phases and on the internal circulation speed of the fluid. The circulation either results from the fluid passing through the vessel (as in a bubble column) or it has to be effected separately in order to avoid separation. Suspension of solids or aeration of liquids in loop and stirred reactors can serve as examples. Dimensionless groups that characterize the main material and geometric properties can be derived using the slip velocity of the largest stable bubble and a certain circulation parameter. The power of this method will be shown by the characterization of bubble columns and gas-liquid mixers. Simple laws comparable with those applying to bubble columns emerge when approaching the flooding point of a disc turbine in a tank. Power numbers, gas volume fractions, and volumetric mass transfer coefficients can be bundled over the whole spectrum of the non-flooded states, when their characteristic quantities are divided by the quantities relating to the flooding point.     

Computational flow modelling and design of bubble column reactors

The present design practice of the bubble column reactors is still closer to an art than science because of the complexity of the fluid mechanics. In view of this, there have been continuous attempts to understand the complex three-dimensional turbulent two-phase flow. The present paper reviews the modelling efforts on the flow patterns published in the last 30 years with relatively more focus on the last 10 years. Over this period, there have been sustained efforts to improve our understanding of the governing equations of the change (equations of continuity and motion) for two-phase flows. Both Eulerian and Lagrangian approaches have been extensively used. The development has been mainly on three fronts: (i) formulation of interfacial forces (ii) closure problem for the eddy viscosity and (iii) modelling of the correlations arising out of Reynolds averaging procedure.As regards to interface force terms, the published literature has been critically analysed. The present status of our understanding of the drag force, virtual mass force and lift force has been presented. The physical significance of the various formulations has been brought out. The mechanism of the energy transfer from gas to liquid phase has been explained. The developments in closure problem have been most dramatic. The progress of the past 30 years has been reviewed with a focus on the past 10 years. The published literature has been critically analysed and chronology of development has been presented. The effort has been concentrated on cylindrical bubble columns where results on flow pattern could be extended to the design. The studies on transient flow pattern in two-dimensional columns have not been covered because the subject is still under development and the results cannot be extended to the design objective. The closure problem is intimately linked with the physics of turbulence.An attempt has been made to develop a complete correspondence between an operation of real column and the model simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. The effects of the superficial gas velocity, column diameter and bubble slip velocity on the flow pattern have been examined. Extensive comparison has been presented between the predicted and the experimental velocity profiles.For the design of the bubble columns, the knowledge of various design parameters (such as pressure drop, rate of mixing, residence time distribution of both the phases, heat and mass transfer coefficients) is needed. For the estimation of these parameters, the prevailing procedures are largely empirical. The fundamental basis for the estimations is possible through the understanding of the detailed macro- and micro-flow patterns. This basic direction has been the subject of several publications, particularly during the last 5 years. All these studies have been critically analysed in the present review paper. A coherent and holistic approach has been presented on the modelling of fluid mechanics and design of bubble column reactors.Recommendations have been made for the future research in this area.