Relative Power Demand Research Articles

Gas–liquid mixing performance of a non‐Newtonian fluid in a multiple‐impeller agitated tank

AbstractBACKGROUNDThis target is to decide power input and gas hold‐up for gas–liquid mixing aqueous CMC solutions. The analysis considers the impact by varying the impeller speed and gas flow rates in a stirred tank bioreactor of three kinds of multiple impellers. The effects of the impeller type, rheology, and operating conditions were investigated on power drawn, relative power demand (RPD), gas holdup, and volumetric mass transfer coefficient.RESULTCompared to the Rushton turbine (6RT) and 4‐pitch blade (4PBT) impeller, the propeller (3PP) impeller presented a minimal event of the gassing on the RPD. 4PBT impeller has shown a higher gas hold‐up compared to the Rushton turbine and propeller impellers. The aerated agitated tank was established a dimensionless correlation for the RPD as a function of flow number and web number. Besides, the gas–liquid agitated system was also introduced a dimensionless correlation to compute the overall gas hold‐up as a function of specific power consumption. For the maximum dispersion mixing intensity, the impeller structure with low RPD looks to be more adequate. Further, maximizing gas holdup in the structures with high RPD is advantageous. The effects of impeller speed, gas superficial velocity, and rheology on the volumetric mass transfer coefficient were examined.CONCLUSIONThe volumetric mass transfer coefficient increased with an increase in impeller speed, gas superficial velocity, and power consumption per unit volume and decreased as rheology increased. The averaged kLa for each multiple‐impeller was correlated well with the specific gassed power consumption and gas superficial velocity. © 2024 Society of Chemical Industry (SCI).

Experimental and Numerical Investigation of the Characteristics of Novel Disc Turbines in Aerated Stirred Tanks

In this work, the gas dispersion and transport characteristics of three different disc turbines, namely, parabolic disc turbine (PDT), novel swept-back parabolic disc turbine (SPDT), and staggered fan-shaped parabolic disc turbine (SFPDT), are characterized by experiments and two-fluid model/RANS simulations. The physical explanation of the superior power characteristics of SFPDT and SPDT is elaborated using the pressure distribution and drag force coefficient on a blade surface. Gas–liquid performance parameters such as critical dispersion speed, relative power demand (RPD), and volumetric mass transfer coefficient are assessed under different conditions. The results show that SFPDT and SPDT can produce smaller flow separation regions and trailing vortices, resulting in smaller gas cavities under aeration conditions. SFPDT has the lowest critical dispersion speed under a high flow rate, while SPDT consumes the least power. Both SFPDT and SPDT exert roughly 7% higher gassed power input than PDT under aeration conditions and have higher efficiency on gas–liquid mass transfer than PDT. Moreover, empirical correlations from experimental data for SFPDT and SPDT are presented to provide a reference to the design of industrial stirred tanks.

Investigating the power consumption for the intensification of gas dispersion in a dual coaxial mixer containing yield-pseudoplastic fluids

Coaxial mixers with double central impellers and an anchor have demonstrated promising results in the intensification of gas dispersion in complex fluids. While previous studies have evaluated power consumption for gas dispersion in pseudoplastic fluids, no investigation has been conducted on aerated dual coaxial mixers containing pseudoplastic fluids with yield stress. This study aimed to evaluate the power consumption of the aerated double coaxial mixer under various operating conditions, including anchor and central impeller speed, rotational mode, pumping direction, and aeration rate. Experimental power consumption measurements were performed for both gassed and ungassed systems. Our findings showed that the anchor power contributed substantially less to overall power consumption than the central impellers. The counter-rotational mode used more power than the co-rotational mode due to the opposing flow created by the rotation of the central impellers and anchor impeller. The co-rotational mode had a higher relative power demand (RPD) than the counter-rotational mode, with the upward pumping mode of the central impellers yielding the highest RPD values. The modified Jamshidzadeh's correlation successfully predicted the impact of the anchor speed, central impeller speed, and aeration rate on power consumption for the investigated double coaxial mixers containing the xanthan gum solution.

Study on hydrodynamics characteristics in a gas-liquid stirred tank with a self-similarity impeller based on CFD-PBM coupled model

BackgroundIn general, radial impeller has been the most widely employed for the gas-liquid mixing process in the stirred tank. However, gas cavities are easy to form behind the impeller blades, which results in the waste of power consumption, the limited gas handling capacity of impeller and the poor dispersion of gas. Based on self-similarity property of nonlinear theory, a type of self-similarity impeller was employed to strengthen gas-liquid dispersion process in this work. MethodsThe hydrodynamics of gas-liquid dispersion process in a stirred tank with Rushton turbine and self-similarity impeller were comparatively investigated by computational fluid dynamics (CFD) simulation combined with a population balance model (PBM). The gas holdup distribution, gas cavities, relative power demand (RPD), bubble size distribution and turbulent intensity analysis in the gas-liquid dispersion process were predicted. Significant findingsResults showed that self-similarity impeller could destroy the gas cavity structure, reduce the negative pressure zone and gas accumulation, enhance the relative power demand (RPD) and improve the uniformity of gas holdup distribution compared with Rushton turbine, and the gas-liquid mixing efficiency can be further improved with the increase of the self-similar iteration number of self-similarity impeller. The proper increase of impeller speed was beneficial to overcome gas flooding and improve the gas-liquid two phase dispersion uniformity. The increase of gas flow rate obviously increased the gas holdup in liquid phase. Meanwhile, self-similarity impeller could increase the uniformity of bubble size compared with Rushton turbine, and increasingly so with the increase of self-similar iteration number of self-similarity impeller. The uniformity of bubble size was improved with the increase of impeller speed and was decreased with the increase of gas flow rate. In addition, self-similarity impeller could significantly enhance the turbulent intensity and turbulent kinetic energy dissipation rate by the jet flows through the concave-convex edges, which was beneficial to the gas dispersion process.

Gas–liquid mixing in the stirred tank equipped with semi-circular tube baffles

Abstract Despite extensive contributions have been made over the past several decades, there are still ongoing challenges in improving gas dispersion performance in stirred tanks. To that end, a stirred tank equipped with four semi-circular (SC) tube baffles was designed. Hydrodynamics in the stirred tank before and after aeration were studied. The turbulent fluid flow was simulated using the standard k-ε turbulence model. The gas–liquid two-phase flow was simulated by the Eulerian–Eulerian multiphase model and the k-ε dispersed turbulence model. The impeller rotation was modeled with the multiple reference frame (MRF) approach. Firstly, the grid independence test was made. By comparing the distributions of gas holdup in the flat-plate (FP) baffled stirred tank with literature results, the reliability of the numerical model and simulation method was verified. Subsequently, the flow field, gas holdup and power consumption of the SC and FP configurations were studied, respectively. Results show that the former can increase the fluid velocities and promote the gas holdup dispersion. Besides, it is energy-saving and has a higher relative power demand (RPD). The findings obtained here lay a preliminary foundation for the potential application of the SC configuration in the process industry.

Investigation of power consumption, torque fluctuation, and local gas hold-up in coaxial mixers containing a shear-thinning fluid: Experimental and numerical approaches

The performance of a coaxial bioreactor in the intensification of gas dispersion in shear-thinning fluids in terms of the power consumption of the central and anchor impellers, torque fluctuation for the anchor impeller, global and local gas hold-up, and cavity size was assessed. Electrical resistance tomography and computational fluid dynamics methods were used to evaluate the effects of the central impeller type, impeller speed, pumping direction, and rotating mode on the efficacy of the aerated coaxial mixers. Unlike the coaxial mixer equipped with a radial-flow impeller, for the coaxial mixer furnished with an axial-flow impeller, the rotating mode had a pronounced effect on the relative power demand. Surprisingly, it was observed that increasing the central impeller speed increased the contribution of the anchor impeller to the total power consumption of the co-rotating coaxial mixer furnished with a down-pumping impeller. Furthermore, by increasing the central impeller speed the torque fluctuation of the coaxial mixer was diminished due to the uniform distribution of the energy dissipation rate. Unexpectedly, for a coaxial mixer furnished with a down-pumping impeller in both rotating modes and an up-pumping impeller in the counter-rotating mode, the gas hold-up decreased by a further increase in the central impeller speed.

Parametric study of gas−liquid two-phase flow field in horizontal stirred tank

Based on Fluent software, the gas−liquid two-phase flow in the horizontal stirred tank was simulated with SST k−ω turbulence model, Eulerian−Eulerian two-fluid model, and multi-reference flame method. The mixing process in the tank was calculated by tracer method. The results show that increasing the rotating speed or gas flow is conducive to a more uniform distribution of the gas phase and accelerates the mixing of the liquid phase. When the rotating speed exceeds 93 r/min, the relative power demand remains basically constant. The change in the inclination angle of the upper impeller has minimal effect on the gas phase distribution. When the inclination angle is 50°, the relative power demand reaches the maximum. An appropriate increase in the impeller distance from the bottom improves the gas holdup and gas phase distribution but increases the liquid phase mixing time.

Chaotic mixing and mass transfer characteristics of fractal impellers in gas-liquid stirred tank

Gas-liquid mixing characteristics in fractal impeller stirred tank were experimentally investigated by measuring multi-scale entropy (MSE), relative power demand (RPD), local gas holdup, bubble size, and mass transfer coefficient (KLa). Results showed that fractal impeller could enhance 21.69% of MSE and 11.94% of RPD on the basis of pitched-blade impeller. Fractal impeller can enhance the local gas holdup, decrease the bubble size, reduce the dispersion coefficient (σ) and improve the gas-liquid dispersion performance. Meanwhile, the regression based on d32− for pitched-blade impeller and fractal impeller were obtained. In addition, fractal impeller can also increase 11.07% of volume mass transfer coefficient (KLa) compared with pitched-blade impeller in the gas-liquid mixing process. The results can provide theoretical guidance for the optimal design of gas-liquid stirred tank.

Gas dispersion and solid suspension in a three-phase stirred tank with triple impellers

The hydrodynamics is still not fully understood in the three-phase stirred tank equipped with multi-impeller due to the intensive interaction between phases. In this work, the solid critical suspension speed ( N JSG ), relative power demand (RPD) and overall gas holdup ( ε G ) were measured in an air–water–glass beads stirred tank equipped with multi-impeller, which consists of a parabolic blade disk turbine below two down-pumping hydrofoils. Results show that either the N JSG or the specific power consumption increases when increasing the volumetric solid concentration or superficial gas velocity. RPD changes less than 10% when solid volumetric concentration ranges from 0 to 15%. ε G decreases with the increase of solid concentration, and increases with the increase of both superficial gas velocity and the total specific power consumption. The quantitative correlations of N JSG , RPD and ε G were regressed as the function of superficial gas velocity, specific power consumption, Froude number and gas flow number, in order to provide the reference in the design of such three-phase stirred tank with similar multi-impellers. • Just-suspension speed, RPD and gas holdup with triple impeller were measured. • Superficial gas velocity significantly affects the suspension speed. • Overall gas holdup is evidently affected by the solid.

Design of Impeller Blades for Intensification of Gas-Liquid Dispersion Process in a Stirred Tank

Abstract Gas-liquid dispersion characteristics were experimentally investigated by measuring largest Lyapunov exponent (LLE), relative power demand (RPD), and local gas holdup in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers. Results showed that punched rigid-flexible impeller could enhance the value of LLE, namely, the chaotic mixing degree of gas-liquid system compared with rigid impeller and rigid-flexible impeller. RPD for punched rigid-flexible impeller was higher than that for rigid impeller and rigid-flexible impeller. The local gas holdup of punched rigid-flexible impeller system was higher than those of rigid impeller system and rigid-flexible impeller system at the same Pg,m. In addition, a long flexible connection piece length could improve the chaotic mixing degree, RPD, and local gas holdup. The aperture diameter of 8 mm and free area ratio of 12 % of punched rigid-flexible impeller were particularly suitable for the gas-liquid dispersion process in this work.

Gas-liquid mixing performance, power consumption, and local void fraction distribution in stirred tank reactors with a rigid-flexile impeller

In this work, the mixing performance of gas–liquid was characterized by the largest Lyapunov exponent (LLE), relative power demand (RPD), power number NP and local void fraction in a rigid-flexible impeller reactor equipped with a tube sparger. The effects of impeller types, gas flow rates, agitation speed, flexible piece lengths, axial spacing distances lh and angles θ were investigated. Meanwhile, global variable analysis (GVA) based on the mixing index (MI) was proposed to characterize the mixing process. The results show that the increases in flexible piece length, agitation speed, and gas flow rate could enhance the mixing degree. Meanwhile the reductions of axial spacing, and angle were also able to improve the mixing performance of gas–liquid two phases to a certain extent. These findings were in conformity with the results of investigation into MI. The results of this study could provide theoretical guidance for the optimal design and engineering scale-up of STRs.

Experimental study on gas–liquid dispersion and mass transfer in shear-thinning system with coaxial mixer

The effects of impeller type, stirring power, gas flow rate, and liquid concentration on the gas–liquid mixing in a shear-thinning system with a coaxial mixer were investigated by experiment, and the overall gas holdup, relative power demand, and volumetric mass transfer coefficient under different conditions were compared. The results show that, the increasing stirring power or gas flow rate is beneficial in promoting the overall gas holdup and volumetric mass transfer coefficient, while the increasing system viscosity weakens the mass transfer in a shearing–thinning system. Among the three turbines, the six curved-blade disc turbine (BDT-6) exhibits the best gas pumping capacity; the six 45° pitched-blade disc turbine (PBDT-6) has the highest volumetric mass transfer coefficient at the same unit volume power.

Improving oxygen transfer efficiency by developing a novel energy-saving impeller

Radial flow impeller is the energy-intensive and fundamental component in reactors for the gas–liquid transfer process. A newly designed fan-shaped turbine (FT) assembly with annular-sector-shaped concave blades was characterized and compared with the Rushton turbine (RT) and Bakker turbine (BT). A new surface equation was established to design the blade of the FT impeller. Under turbulence conditions, the FT impeller showed a lower power number and higher relative power demand (RPD) compared with RT and BT impellers. The power number of the FT impeller was 1.7, lower by 26% than that of BT impeller. The RPD of the FT impeller was nearly 0.95 at a high impeller speed. The critical dispersion speed, gas holdup, and volumetric oxygen transfer coefficient of the FT impeller were close to those of BT impeller. Moreover, the oxygen transfer efficiency of the FT impeller was remarkably higher by 35%–66% and 23%–34% than that of RT and BT impellers, respectively. The FT impeller showed competence in a broad operation range, strong robustness, energy-saving feature, and efficient mass transfer characteristics.

Power consumption and mass transfer in a gas-liquid-solid stirred tank reactor with various triple-impeller combinations

Five triple-impeller combinations were used to study the effects of impeller combination on gassed power consumption and volumetric mass transfer coefficient kLa in a baffled gas-liquid-solid stirred tank reactor with air, water, and 12vol% of glass beads. The bottom impeller was chosen from two radial flow type impellers of half-elliptical-blade disk turbine (HEDT) and parabolic-blade disk turbine (PDT), and the upper two impellers were chosen from four axial flow type impellers with different blade widths and pumping modes, forming five different triple-impeller combinations: HEDT+2WHU, HEDT+2WHD, PDT+2WHD, PDT+2CBYW, and PDT+2CBYN. The experimental results show that the addition of solid particles has little effect on the relative power demand (RPD), but significant effect on kLa. kLa in a three-phase system is smaller than that in a two-phase system, and HEDT+2WHU with two up-pumping upper impellers shows the best mass transfer performance in these five triple-impeller combinations at various superficial gas velocities uG, much different from what HEDT+2WHU behaves in two-phase systems. The bottom impeller mainly influences the absolute power consumption, but the two upper impellers play a more important role on the change of RPD than the bottom impeller. The impeller combinations with a higher power number and larger projection cross-sectional area can lead to a higher kLa. The effect of operating conditions (e.g., power consumption and superficial gas velocity), the bottom impeller, the blade width and pumping mode of the two upper impellers, and solid particles were investigated and discussed in detail. To better explain the difference of mass transfer performance between impeller combinations HEDT+2WHD/U, the Eulerian-Eulerian formulation of the k-ε turbulence model with the population balance model (PBM) was used to simulate the flow field in gas-liquid systems.

Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations

The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT+2CBYN, PDT+2CBYW, PDT+2WHD, HEDT+2WHD and HEDT+2WHU, respectively. The results show that the relative power demand of HEDT+2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT+2WHD shows the best mass transfer performance and HEDT+2WHU shows the worst mass transfer performance under all operating conditions. At high uG and a given power input, the impeller combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design.

Influence of impeller diameter on overall gas dispersion properties in a sparged multi-impeller stirred tank

The impeller configuration with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT+2CBY, was used in this study. The effect of the impeller diameter D, ranging from 0.30T to 0.40T (T as the tank diameter), on gas dispersion in a stirred tank of 0.48m diameter was investigated by experimental and CFD simulation methods. Power consumption and total gas holdup were measured for the same impeller configuration PDT+2CBY with four different D/T. Results show that with D/T increases from 0.30 to 0.40, the relative power demand (RPD) in a gas–liquid system decreases slightly. At low superficial gas velocity VS of 0.0078m·s−1, the gas holdup increases evidently with the increase of D/T. However, at high superficial gas velocity, the system with D/T=0.33 gets a good balance between the gas recirculation and liquid shearing rate, which resulted in the highest gas holdup among four different D/T. CFD simulation based on the two-fluid model along with the Population Balance Model (PBM) was used to investigate the effect of impeller diameter on the gas dispersion. The power consumption and total gas holdup predicted by CFD simulation were in reasonable agreement with the experimental data.

Influence of the Top Impeller Diameter on the Gas Dispersion in a Sparged Multi-impeller Stirred Tank

The impeller configuration with a hollow half-elliptical blade dispersing turbine below two up-pumping wide-blade hydrofoils, identified as HEDT+2WHU and recommended in previous work, was used in this study. The effect of the top impeller diameter, ranging from 0.33T to 0.50T, on gas–liquid flow in a stirred tank of 0.48 m diameter was investigated by experimental and CFD methods. Power consumption, total gas holdup, and local void fraction were measured for the impeller configurations with different top impeller diameters. Results show that while the ratio of top impeller diameter to tank diameter (Dtop/T) increases from 0.33 to 0.50, the relative power demand (RPD) in a gas–liquid system decreases slightly. The increase of total gas holdup with rising gas flow rate becomes less evident as Dtop/T increases from 0.33 to 0.50. Local void fractions at the measurement points above the height of 0.8T increase significantly with the increase of top impeller diameter. When Dtop/T = 0.50, there is an extremely l...

Temperature Effects on Gas Dispersion and Solid Suspension in a Three-Phase Stirred Reactor

Temperature effects on gas dispersion and solid suspension have been investigated in a fully baffled, dished-base stirred tank of 0.48 m diameter holding 0.145 m3 of liquid stirred by a triple-impeller combination. The impeller combination consisted of a half-elliptical disk turbine below two up-pumping wide-blade hydrofoils (WHU). This configuration is efficient for both gas dispersion and solid suspension. Power consumption, gas holdup, and the critical off-bottom just-suspension agitation speed have been measured at solid concentrations up to 21 vol % at six different temperatures ranging from 24 to 95 °C in increments of about 14 °C. The results confirm significant effects of temperature on the hydrodynamic characteristics. The relative power demand increases somewhat at increased temperature, although this effect is less when more solids are present. Gas holdup decreases significantly at higher temperatures, again an effect that is reduced at higher solid concentrations. The critical impeller speed f...

Solid Suspension in a Boiling Stirred Tank with Radial Flow Turbines

This paper considers conditions in reactors in which solids must be maintained in suspension in a boiling liquid. The situation is analogous to, but differs from, the more commonly studied conditions when the liquid is well-below its boiling point in a gas-liquid-solid reactor. Impeller performance in both cases is usually controlled by the development of gas or vapor cavities behind the impeller blades. Power consumption, critical off-bottom suspension speed, and vapor holdup measurements are reported for impellers working in a three-phase model system of 0.1 mm glass beads at concentrations of 3, 6, 9, 15, and 21 vol %, suspended in deionized boiling water. The vessel was electrically heated, dished based, and of 0.48 m diameter. It was agitated by single 0.24 m diameter impellers. The three radial-flow disc turbines studied were a flat-blade Rushton turbine, RT-6, a hollow half-pipe blade turbine, Chemineer CD-6, and a hollow semi-elliptical blade turbine, HEDT. The modem CD-6 and HEDT impellers are highly efficient in handling high gas-phase loading and in maintaining high levels of power input throughout a range of impeller tip speeds. For all three impellers, the relative power demand (RPD, i.e., the ratio between the gassed and ungassed power at a given impeller speed) was only slightly affected by solids, even up to 21 vol %. In contrast to the effect of varying the gas sparge rate in a cold aerated system, the boil-off rate had only a limited effect on the RPD, the minimum impeller speed needed to maintain suspension, or the retained void fraction. When boiling, the void fraction was, as might have been expected, much lower than in cold systems at similar power input and off-gas rates. Correlations of vapor holdup and RPD for the three radial flow turbines are suggested as a reference for future industrial applications in which knowledge of both the power requirements and the actual liquid fraction retained in the reactor may be of commercial importance. The HEDT impeller is recommended for both vapor dispersion and solid suspension in ungassed boiling three-phase systems.

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

ABSTRACT Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WHU) and down-pumping (WHD) modes. Glass beads of 50 ∼ 150 μm diameter and density 2500 kg · m−3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = PG/PU) than those with WHD or WHU as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WHU has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.