Riser Gas Holdup Research Articles

Characterization of packed-bed in the downcomer of a concentric internal-loop airlift bioreactor

A packed-bed basket containing corn cob powder was used for the adaptation of a concentric-tube airlift reactor for enzymatic reactions. The bed was inserted in the downcomer region of the bioreactor generating a decrease in liquid downflow. To characterize the liquid velocity through the packed-bed and calculate the residence time, the intrinsic permeability was determined using Darcy’s law, resulting in 1.275 × 10−9 m2 (40-mesh particle size), and the pressure difference was calculated using the riser gas holdup, reaching values up to 56.6% for water at 3.5 vvm. Thus, a superficial liquid velocity model was obtained to describe the liquid velocity at different airflow rates and bed lengths, covering a wide range of support masses commonly used in enzymatic reactions. The model also allows the variation of viscosity, caused by different reaction temperatures and the use of larger bed sizes for scaling. Using the dye tracer method, it was possible to visualize the flow through the bed and corroborate the residence times obtained by the model for a wide range of airflows. Therefore, it describes the use of a basket filled with potential enzyme support, contributing to the applicability of pneumatic systems as enzymatic bioreactors, particularly internal-loop airlift reactors.

Influence of the geometry on the riser gas holdup in an external-loop airlift reactor: empirical and neural network modelling

Experimental studies on the effect of geometrical characteristics (restriction orifice free area and sparger configuration) on the riser gas holdup of an external-loop airlift reactor have been conducted in this paper. The impact of the addition of non-coalescing media, such as n-butanol, was also investigated. In general, the decrease in the restriction orifice free area led to a substantial increase (up to 90%) in the riser gas holdup. The addition of n-butanol effected gas holdup to a lesser extent, as it increased the gas holdup in the range of 4-15%, depending on the restriction orifice free area. On the other hand, the impact of the sparger type was observed only at lower gas velocities, for which the sinter plate increased the gas holdup up to 20% in comparison to the single orifice. The paper also presents an empirical correlation for the estimation of the riser gas holdup with an average relative error of 7%. Finally, a neural network model with an average relative error of 2.4% was proposed.

CFD simulation of the hydrodynamics in an industrial scale cyclohexane oxidation airlift loop reactor

The two-fluid model was applied to simulate the hydrodynamics based on the process conditions in a cyclohexane oxidation airlift loop reactor (ALR). A lab-scale ALR was first investigated to validate the simulation methods and the simulated gas holdup and liquid velocity agree well with the experimental data. Following the validation, computational fluid dynamics (CFD) was then extended to study the industrial scale cyclohexane oxidation ALR. The CFD results indicate that the key hydrodynamic parameters for the circulation flow are the liquid circulation velocity and liquid circulation flux. The increasing liquid circulation flux in the downcomer enhances the downcomer gas holdup, while the increasing liquid circulation velocity in the riser reduces the riser gas holdup. The factor influencing the gas phase entrainment in the draft-tube is the liquid circulation flux rather than the liquid circulation velocity. With a constant reactor diameter, the gas holdup and liquid circulation flux increases with the increasing cross-sectional area ratio of downcomer to riser. The draft-tube with a horn-mouth can efficiently enhances the gas–liquid separation at the reactor top. Within a certain range, the axial position height of the draft-tube can influence the circulation driving force and resistance loss in the reactor. The results of this study are significant for guiding production, the optimization and scale-up study of this kind of reactors.

Enhanced hydrodynamics in a novel external-loop airlift reactor with self-agitated impellers

In the present work, self-agitated impellers, as a novel type of internals, have been proposed for improving hydrodynamic and mass transfer characteristics of external-loop airlift reactors. The influence of inserted self-agitated impellers, in the riser section, in various liquid phases and with different sparger types, on main hydrodynamic parameters, was studied. The results show that the insertion of impellers led to significant bubble breakage and decrease in mean bubble size, particularly in pseudoplastic liquid. Obtained riser gas holdup values were up to 47% higher, in comparison to the configuration without impellers. Higher improvements were obtained with single orifice, given that this is the least effective gas distributor. Even though impellers represent an obstacle to the flow, relatively low reduction (about 10%) in downcomer liquid velocity was observed, for all investigated cases. Having this in mind, the benefit of inserting self-agitated impellers for improving the performance of external-loop airlift reactors was apparent.

Hydrodynamics of a self-agitated draft tube airlift reactor

The main hydrodynamic characteristics of a novel-constructed, self-agitated draft tube airlift reactor (DT-ALR) were investigated. Ten impellers, driven only by the means of gas throughput and induced liquid circulation, were inserted in the draft tube. The insertion of impellers caused bubble breakup and reduction of both mean bubble size and coalescence, even under the conditions of high gas throughputs. Although the impellers induced energy losses, the resistance to the flow was relatively lower due to their rotation, unlike the internals used in other research reported in the literature. In comparison to the conventional configuration of a DT-ALR, it was found that the presence of impellers led to significant changes in hydrodynamics: riser gas holdup and mixing time increased, while overall gas holdup and liquid velocity in the downcomer decreased.

Dynamic and local gas holdup studies in external loop recirculating airlift reactor with two rolls of fiberglass packing using electrical resistance tomography

AbstractBACKGROUND: Airlift bioreactors have been used extensively in biotechnology industries in recent years in a variety of arrangements and applications. The insertion of packing inside the bioreactors has the potential to provide high productivity within a compact size through utilizing immobilized species.RESULTS: A novel recirculating external loop airlift bioreactor that has two rolls of fiberglass packing and a gas distributor in between was designed and built. Electrical resistance tomography (ERT) images showed that the gas holdup increased after installing the packing and the gas distributor. Gas holdup in the riser increased with decreasing static liquid height in the bioreactor. This decreased the liquid superficial velocity, which contributed to a higher gas holdup in the bioreactor. Results also showed that riser gas holdup varied slightly with different sparger configurations. Higher gas holdup increases the oxygen mass transfer rate by increasing the residence time and interfacial mass transfer area.CONCLUSION: ERT results showed that fiberglass packing with an installed gas distributor in bioreactors can achieve higher gas holdup at higher superficial gas velocity. This can contribute to improved conversion in bioreactors with packing through utilizing higher biomass concentrations and higher oxygen concentration. © 2012 Society of Chemical Industry

Effect of Contaminants on the Gas Holdup and Mixing in Internal Airlift Reactors Equipped with Microbubble Generator

The impact of contaminants on the gas holdup and mixing characteristics encountered in internal airlift reactors was investigated using a 200 L pilot scale unit equipped with a two-phase transonic sparger capable of generating microbubbles. Small dosages of a cationic surfactant (0–50 ppm of sodium dodecyl sulfonate (SDS)) were used to simulate the coalescence-retarding effect encountered in most industrial streams and resulted in the formation of bubbles that varied in size between 280 and 1,900 μm. Gas holdups as high as 0.14 were achieved in the riser under homogeneous flow regime when slowly coalescent systems were aerated at the relatively low superficial velocity of 0.02 ms−1, whereas liquid circulation velocities as high as 1.3 ms−1were achieved in conjunction with rapidly coalescent systems at the same superficial velocity. This excellent hydrodynamic performance represents a 5-fold improvement in the riser gas holdup and up to 8-fold enhancement in the liquid circulation velocity and is expected to yield good mixing and mass transfer performance at low energy dissipation rates.

CFD analysis of two‐phase turbulent flow in internal airlift reactors

AbstractThe hydrodynamic performance of three internal airlift reactor configurations was studied by the Eulerian–Eulerian k–ε model for a two‐phase turbulent flow. Comparative evaluation of different drag and lift force coefficient models in terms of liquid velocity in the riser and downcomer and gas holdup in the riser was highlighted. Drag correlations as a function of Eötvös number performed better results in comparison to the drag expressions related to Reynolds number. However, the drag correlation as a function of both Reynolds and Eötvös numbers fitted well with experimental results for the riser gas holdup and downcomer liquid velocity in configurations I and II. Positive lift coefficients increase the liquid velocity and decrease the riser gas holdup, while opposite results were obtained for negative values. By studying the effects of bubble size and their shape, the smaller bubbles provide a lower liquid velocity and a gas holdup. The effects of bubble‐induced turbulence and other non‐drag closure models such as turbulent dispersion and added mass forces were analysed. The gas velocity and gas holdup distributions, liquid velocity in the riser and downcomer, vectors of velocity magnitude and streamlines for liquid phase, the dynamics of gas holdup distribution and turbulent viscosity at different superficial gas velocities for different reactor configurations were computed. The effects of various geometrical parameters such as the draft tube clearance and the ratio of the riser to the downcomer cross‐sectional area on liquid velocities in the riser and the downcomer, the gas velocity and the gas holdup were explored. © 2011 Canadian Society for Chemical Engineering

Prediction of Riser Gas Holdup in Three- Phase External Loop Air Lift Fluidized Bed Reactor

Hydrodynamics is an important issue for design and development of three phase external-loop airlift fluidized bed. This paper deals on the experimental investigations on the effect of superficial gas and liquid velocities and properties of solids on the riser gas holdup of a three phase external-loop airlift fluidized bed reactor and it was characterized using Newtonian and non Newtonian systems. Water, 65% and 85% of glycerol and n-butanol were used as Newtonian liquids and different concentration of Carboxyl Methyl Cellulose (CMC) i.e., 0.2%, 0.5% and 1% were used as non Newtonian liquids. Spherical glass beads, bearl saddles and rasching rings of different sizes were used as solid phase. The phase flow rates and properties of solid particle had significant effect on the hydrodynamic characteristics of the external-loop airlift fluidized bed reactor such as riser gas holdup. Unified correlations have been developed to estimate the riser gas holdup as a function of superficial phase velocities, properties of solid particle and physical properties of both Newtonian and Non-Newtonian liquid systems. The predicting ability of the correlations were tested with the experimental data and found to be good fit with an absolute average relative error (AARD) of ± 9.7 % for riser gas holdup.

Hydrodynamic Considerations in an External Loop Airlift Reactor with a Modified Downcomer

Gas holdup and superficial liquid velocity in the downcomer and riser are studied for an external loop airlift reactor with a downcomer-to-riser area ratio of 1:16. Two downcomer configurations are investigated over a range of superficial gas velocities (0.5 ≤ UG ≤ 20 cm/s) using three aeration plate open area ratios (A = 0.62, 0.99, and 2.22%). These results are compared to a bubble column operated with similar operating conditions. Gas holdup in both the riser and downcomer are found to increase with increasing superficial gas velocity. Results show that riser gas holdup varies slightly with downcomer configuration, while a considerable variation is observed for downcomer gas holdup. The superficial liquid velocity varies considerably for the two downcomer configurations and is a function of superficial gas velocity and flow conditions in the downcomer. Observed variations are independent of aeration plate open area ratio.

EFFECTS OF AERATED LIQUID LEVEL ABOVE DOWNCOMER ON HYDRODYNAMIC CHARACTERISTICS OF AN EXTERNAL-LOOP AIRLIFT REACTOR

Effects of the aerated liquid level above the downcomer on riser superficial liquid velocity, gas flow rates, and gas holdups in individual sections were investigated in an external-loop airlift reactor. The aerated liquid level is an important operating parameter, and the valve connected to the extension tube could adjust it. As the aerated liquid level was increased up to 0.158 m, which was slightly larger than the diameter of the riser, both the liquid circulation velocity and the gas flow rate in the extension tube increased, whereas the riser gas holdup, the downcomer gas holdup, and the gas flow rate in other individual sections except the extension tube decreased. A combination of the gas-liquid separation ability and hydraulic resistance of the head region and the surface aeration could explain the impact of the aerated liquid level on the hydrodynamic characteristics. However, the effect was negligible when the aerated liquid level was higher than 0.158 m.

Analysis of Hydrodynamics of External Loop Circulating Bubble Columns with Open Channel Gas Separators Using Neural Networks

Gas holdups and liquid circulation velocities in two external loop circulating bubble columns of the open channel gas separators using air–water and air–glycerol systems were extensively reported by Al-Masry (1999, 2004). The effects of changing the volume of the liquid in the gas–liquid separators on the columns hydrodynamics were analysed numerically using neural network with four inputs and three outputs. The inputs were superficial gas velocity UGR, volume ratio TVR, liquid viscosity μL and scale-up factor AD/AR, while the outputs were liquid circulation velocity ULR, riser gas holdup ɛGR and downcomer gas holdup ɛGD. The network was trained on 60% of the data, and then used to predict 40% of the data that have never been seen by the network. The training was successfully accomplished and results obtained with average normalized square error

Three‐phase airlift internal loop reactor: correlations for predicting the main fluid dynamic parameters

AbstractThree‐phase airlift loop reactors have many industrial applications. Examples include applications in the chemical, metallurgical, biochemical and minerals‐processing industries; fluid dynamics in such systems is a critical factor affecting efficiency. The experimental work was carried out at a pilot‐plant scale in a tank with a cylindrical shape and conical bottom (height 1.25 m; diameter 0.42 m), water and air were used as liquid and gas phases, and for different solid phases, different loads of glass spheres (diameters: 0.25 and 1 mm; density 2.6 g cm−3), and polystyrene cylinders (diameter and length 3 mm; density 1.0 g cm−3) were introduced. Air was injected through the bottom of the tank by means of 12 nozzles (diameter 1 mm each). An internal draft tube riser was tested on different configurations as its diameter was varied (44, 82, 125 and 240 mm) as was its height (1050 and 630 mm). Corresponding liquid velocities in the adjacent annular downcomer were determined by a thermal tracer technique, solid holdups were determined by conductivity methods, riser overall gas holdups were deduced from the liquid level, and riser gas holdup from manometer readings. Several adjusted correlations were considered in a method to predict the main fluid dynamic parameters (solids and gas holdups, and superficial liquid velocities). Copyright © 2005 Society of Chemical Industry

Mass Transfer Studies in Stirred AirLift Reactor

In the present work, water and three phase compositions of Solka-Floc, a cellulose fiber for simulating the biomass in bacteria, yeast, and fungal fermentation were studied in a 1.4 m3 stirred airlift reactor. The fractional dispersed phase holdup and the overall volumetric mass transfer coefficients were measured. The dispersed phase riser gas holdup and overall volumetric mass transfer coefficients both increased with increasing riser superficial dispersed phase velocity (0.02–0.1 ms−1) and agitator speed in the range of 0–5 rs−1. An increase in the Solka-Floc concentration (1–3% w/v) was found to reduce ϵGR and K L a L . Empirical correlations have been developed for fractional dispersed phase gas holdup and overall volumetric mass transfer coefficients.

Influence of Additives and Geometric Design on Hydrodynamic Characteristics of an Internal Loop Airlift Reactor

The influence of organic additives such as benzoic acid, iso-amyl alcohol and propanol on hydrodynamic characteristics of an internal loop airlift reactor was investigated. The effects of the draft tube to column diameter ratio, DE/D, and superficial gas velocity, Usg on the gas holdup in various sections of the reactor (riser, downcomer, overall) and liquid circulation velocity were reported. The gas holdup and the superficial liquid velocity in the riser and downcomer increased with an increase in the superficial gas velocity, Usg. In the present investigation, the DE/D ratio varied from 0.34 to 0.76 and the overall gas holdup and downcomer gas holdup increased with an increase in the DE/D ratio and the optimum was found to be 0.49. The riser gas holdup decreased with an increase in the DE/D ratio. The maximum volumetric liquid circulation rate was obtained with a DE/D ratio of 0.49 to 0.59. The addition of additives influenced the bubble coalescence reduction and increased the gas holdup. The addition of iso-amyl alcohol resulted in the maximum gas holdup. The superficial liquid velocity in the riser and downcomer decreased with the addition of additives. Correlations for the prediction of gas holdups in the different sections of the reactor and liquid circulation velocity were proposed.

Role of Perforated Plates in the Airlift Contactor.

This work investigated the effects of perforated plates on the performance of an airlift contactor. The result showed that the rate of gas-liquid mass transfer in the airlift contactor with perforated plates was significantly greater than those found in a conventional airlift contactor and in a bubble column. This was because the perforated plates induced resistance to fluid flows which lengthened the residence time of gas bubbles in the riser. This in turn gave a greater local gas holdup and also a higher rate of mass transfer between gas and liquid. On the other hand, perforated plates reduced the liquid velocity in the system which was found to negatively influence the gas-liquid mass transfer rate. However, in most cases, the effect of the increase in riser gas holdup on the gas-liquid mass transfer rate overwhelmed that from the low liquid velocity, and the resulting overall volumetric mass transfer coefficient was found to increase in the system with perforated plates. In this work, it was reported that there existed the optimal hole size and the optimal number of holes which gave the highest gas-liquid mass transfer rate. The number of perforated plates in the ALC was also examined and the investigation revealed that there was a maximum number of plates that could be inserted in the ALC above which no further enhancement of overall volumetric mass transfer rate was obtained.

Performance of airlift contactors with baffles

This work investigated the effect of baffles on hydrodynamics and gas–liquid mass transfer in an internal-loop airlift contactor (ALC). It was found that the introduction of baffles reduced liquid circulating velocity in the system. The riser gas holdup in the baffled contactor was found to be higher than the unbaffled, but due to the lower liquid circulation, the use of baffles resulted in a decreased downcomer gas holdup. The overall gas holdup and the rate of gas–liquid mass transfer were not significantly influenced by the presence of the baffles.

Stochastic modelling of axial dispersion in external-loop airlift bioreactors

The paper presents a model of the motion of a particle subjected to several transport processes in connection with mixing in two phase flow. A residence time distribution technique coupled with a one-dimensional dispersion model was used to obtain the axial dispersion coefficient in the liquid phase, Dax. The proposed model of Dax for an external-loop airlift bioreactor is based on the stochastic analysis of the two-phase flow in a cocurrent bubble column and modified for the specific flow in the airlift reactor. The model takes into account the riser gas superficial velocity, the riser liquid superficial velocity, the Sauter bubble diameter, the riser gas hold-up, the downcomer-to-riser cross sectional area ratio. The proposed model can be applied with an average error of ±20.

Hydrodynamics of a three-phase external-loop airlift bioreactor

The effect of the distributing plate orifice diameter, airflow rate, solids loading and solids density on the hydrodynamic characteristics — gas holdup, circulation time and liquid velocity — of a three-phase external-loop airlift reactor was characterized. It was observed that the gas distributor has a small effect on riser gas holdup, circulation time and downcomer liquid velocity. On the contrary, the airflow rate, solids loading and solids density significantly affect the hydrodynamic characteristics of the external-loop airlift reactor. A previously described model was used to estimate simultaneously both the riser gas holdup and the downcomer linear liquid velocity. The model simulated with high-accuracy experimental data obtained with three different distributing plate orifice diameters, two solids density and solids loading up to 30% (v/v).

Hydrodynamics and Mass-Transfer Studies in a Pilot-Plant Airlift Reactor: Non-Newtonian Systems

Experimental studies on the effects of surface active agents on the hydrodynamics and masstransfer performance of a 700 dm 3 pilot-plant external loop airlift reactor have been conducted. The effects of the addition of a silicone polymer antifoam agent on foaming caused by the viscous non-Newtonian solutions of xanthan gum and carboxymethyl cellulose were examined. In general, the addition of the antifoam led to significant reductions in all design parameters, i.e., gas holdup, liquid circulation velocities, and volumetric mass-transfer coefficients relative to the values determined for the air-water system. Presented in this paper are new correlations for riser gas holdup (r) and volumetric mass-transfer coefficient (kla) with effective viscosity ( eff) based on shear rate determined in this reactor. Also, it was found and/or confirmed that the geometric parameter Ad/Ar and Vsg had the most effect on the nonadjustable parameters such as riser and downcomer gas holdup, superficial liquid velocity, and specific interfacial area. The disengagement section design has a profound effect on the rate of foaming.