Double Helical Ribbon Impeller Research Articles

Mixing effects on decentralized high-solid digester for horticultural waste: Startup, operation and sensitive microorganisms

This work studied the use of a horizontal high-solid digester for the decentralized anaerobic treatment of horticultural waste (fallen leaves), where the effect of intermittent mixing by a modified double helical ribbon impeller was investigated. Before experimental verification, the flow pattern and theoretical mixing time were first characterized by CFD simulation. Subsequently, three mixing time intervals (i.e., 3 min/3 hr; 18 min/3 hr; 108 min/3 hr) and one control setup (i.e., without mixing) were compared for their performance during start-up and semi-continuous operation. It was found that minimal mixing was necessary for an efficient digester’s start-up but increased mixing intensity for semi-continuous operation. The results were further interpreted by correlating the digester performance and microbial communities. Those microorganisms sensitive to increased mixing intensity were highlighted and analysed.

Energy efficiency of different impellers in stirred tank reactors

The flow energy efficiency of different impellers for stirring rheologically complex fluids (yield stress fluids) in cylindrical tanks has been investigated in this paper. Four impellers have been used: a Maxblend, an anchor, a gate and a double helical ribbon impeller. Our investigations were achieved via numerical simulations with the help of a CFD (computational fluid dynamics) computer program CFX 13.0. It was found from the predicted results that the Maxblend impeller gives the best performance. The effects of some design parameters on the flow energy efficiency and the power consumption have been also studied; it concerns the Maxblend impeller: its grid size and its paddle curvature.

Hydrodynamics in a vessel stirred by simple and double helical ribbon impellers

Abstract The present paper treats the agitation of shear thinning fluids in a cylindrical unbaffled vessel with a flat-bottom. Two kinds of impellers have been used: simple and double helical ribbons. This work is achieved with the help of CFD package (CFX-12.0), which is based on the finite volume method to solve the continuity and momentum equations. The effect of impeller rotational speed, fluid rheology, impeller size, impeller clearance from the tank bottom on the flow fields and power consumption have been investigated. Finally, a comparison between the two impellers is made. It was found that the best performance is obtained with double helical ribbon impeller placed at the middle of the tank, with moderate blade size and operating at Re g > 20. Our results have been compared with those of other literatures and a satisfactory agreement is observed.

Hydrodynamics Characterization of an Ellipse Gate Impeller by Experimental and Numerical Studies

AbstractHydrodynamics characteristics like flow pattern, shear rate distribution, power consumption, axial pumping capacity, mixing time, and mixing efficiency of an ellipse gate (EG) impeller were investigated by experimental and numerical methods. The numerical simulation results were validated by experimental data of power consumption and mixing time. Results indicate that the axial pumping number of the EG impeller is larger than that of any other reported large‐scale impeller under laminar regime, and that the shear rate formed by this impeller is less sensitive to Reynolds numbers. In‐depth analysis reveals the different function of each part of the EG impeller under different flow regimes. This impeller provides an almost similar mixing efficiency like the double‐helical ribbon impeller under laminar regime, but much higher mixing efficiency both under transitional and turbulent flow regimes.

Effect of impeller type on continuous‐flow mixing of non‐Newtonian fluids in stirred vessels through dynamic tests

AbstractMixing of non‐Newtonian fluids with axial and radial flow impellers is prone to a significant extent of nonideal flows (e.g., dead zones and channelling) within the stirred reactors. To enhance the performance of the continuous‐flow mixing of pseudoplastic fluids with yield stress, close‐clearance impellers were utilised in this study. We explored the effects of various parameters such as the type of close‐clearance impeller (i.e., the double helical ribbon (DHR) and anchor impellers), impeller speed (25–500 rpm), impeller pumping direction, fluid rheology (0.5–1.5% xanthan gum solution), fluid flow rate (3.20–14.17 L min−1) and the locations of outlet (configurations: top inlet–top outlet, top inlet–bottom outlet) on the dynamic performance of the mixing vessel. The performance of the DHR impeller was then compared to the performance of various types of impellers such as axial‐flow (Lightnin A320) and radial‐flow (Scaba 6SRGT) impellers. The dynamic tests showed that the DHR impeller was the most efficient impeller for reducing the extent of nonideal flows in the continuous‐flow mixer among the impellers employed in this study. In addition, the mixing quality was further improved by optimising the power input, increasing the mean residence time, decreasing the fluid yield stress, using the up‐pumping impeller mode and using the top inlet–bottom outlet configuration. © 2011 Canadian Society for Chemical Engineering

Calculation of Metzner Constant for Double Helical Ribbon Impeller by Computational Fluid Dynamic Method

Using the multiple reference frames (MRF) impeller method, the three-dimensional non-Newtonian flow field generated by a double helical ribbon (DHR) impeller has been simulated. The velocity field calculated by the numerical simulation was similar to the previous studies and the power constant agreed well with the experimental data. Three computational fluid dynamic (CFD) methods, labeled I, II and III, were used to compute the Metzner constant k s. The results showed that the calculated value from the slop method (method I) was consistent with the experimental data. Method II, which took the maximal circumference-average shear rate around the impeller as the effective shear rate to compute k s, also showed good agreement with the experiment. However, both methods suffer from the complexity of calculation procedures. A new method (method III) was devised in this paper to use the area-weighted average viscosity around the impeller as the effective viscosity for calculating k s. Method III showed both good accuracy and ease of use.

Power and Mixing Time Study Involving a Maxblend ® Impeller with Viscous Newtonian and Non-Newtonian Fluids

Physical investigations have been carried out to characterize the power consumption and the mixing time in a vessel equipped with a Maxblend ® impeller (Sumitomo Mechanical Equipment, Japan). The laminar, transition and turbulent flow regimes were investigated using viscous Newtonian and non-Newtonian shear-thinning fluids. For the determination of the mixing times, a discolouration method based on a fast acid-base reaction was used in conjunction with image analysis to generate mixing evolution curves. Three vessel sizes were used from which the influence of the scale on the mixing efficiency and the power consumption could be determined. With the geometrically similar mixers used here, it was clearly shown that power and efficiency do not depend on the scale, under certain conditions. The power consumption was found almost identical as that of an anchor but far lower than that of a double helical ribbon impeller. The power and shear constants, K p and K s, were found very sensitive to small variations in the bottom clearance. The K s value remained almost constant for a wide range of flow indices. The dimensionless mixing time, Nt m, decreased with the reciprocal of the Reynolds number in the laminar flow region. Based on data from the literature, a comparison with many different large impellers also indicated that the Maxblend technology makes better use of the mixing power in the upper part of the laminar regime and in the low turbulent regime.

Rheology and Heat Transfer During Mineral Residue Phosphatation in a Rheo-Reactor

The rheo-reactor consists of a cylindrical vessel equipped with a double helical ribbon impeller installed in a RS150 controlled stress rheometer. It was used to investigate the rheological behaviour and the heat transfer in aqueous mineral residue suspensions during the phosphatation process. The absolute viscosity was determined using a Couette analogy allowing the quantitative analysis of the torque-rotational speed. The results show that the suspensions exhibit a viscoplastic or Binghamian behavior. The experimental operating conditions for the phosphatation process were then selected in order to ensure a good macromixing, even in laminar regime (Re<30). Heat was generated as the combined result of the exothermic phosphatation process and the viscous dissipation inside the rheo-reactor. The heat produced needed to be efficiently removed to reduce its effect on the behaviour of the suspension. This was evaluated using the Nusselt number and the heat transfer coefficient h.

Experimental investigation of mass transfer in high viscosity media

An experimental study on mass transfer in high viscosity media was carried out in a sparged stirred vessel with a double helical ribbon impeller (HRI). The effects of stirring speed, superficial velocity of sweeping gas and media viscosity on volumetric mass transfer coefficient k L a were investigated under the conditions of viscosity from 10 to 300 Pas, superficial velocity of sweeping gas 0.00067 to 0.01 m/s and stirring speed 5 to 250 rpm. The results show that k j a increases with increasing stirring speed and superficial velocity of sweeping gas and decreases with increasing media viscosity in normal operating conditions. Too high stirring speed will deteriorate the flow of liquid thereby resulting in the passive effect on the mass transfer in the sparged stirred vessel dealing with very high viscosity media. For prediction of the mass transfer coefficient, an empirical correlation based on stirring speed, superficial velocity of sweeping gas and media viscosity has been proposed. It reproduced our experimental results with relative error within 20%.

Numerical investigation on dispersive mixing characteristics of MAXBLEND and double helical ribbons

The local and total dispersive mixing performance of large type impellers, a standard type of MAXBLEND and double helical ribbons impellers, were numerically analyzed using two indices, local dispersive mixing efficiency and NPD function. The results indicated that a standard type of MAXBLEND has a satisfactory local dispersive mixing performance, especially in the grid region where the local dispersive mixing efficiency is high to near 1. However, when the Re is somewhat low, the total dispersive mixing performance is not as satisfactory as that operated under a moderate Re number. The double helical ribbon impeller can not provide a promising local mixing performance, although it can induce a good total circulation throughout the stirring tank.

Computation Transport Phenomena in Chemical Engineering. Numetical Analysis of Bulk Thermal Styrene Polymerization in Stirred Vessel with Double Helical Ribbon Impeller.

Computation Transport Phenomena in Chemical Engineering. Numetical Analysis of Bulk Thermal Styrene Polymerization in Stirred Vessel with Double Helical Ribbon Impeller.

Development and Mixing Characteristics of Foldiny Anchor for Round-Bottomed Flask.

A folding anchor impeller was developed for a round-bottomed flask with a small neck, which is frequently used for research and development of new materials and technologies in the laboratories of industries and universities. The power consumption and mixing time were measured over a wide range of Reynolds number and compared with those obtained for two traditional types of impellers for relatively high viscous liquids in a round-bottomed flask and those calculated for a helical ribbon impeller set in a cylindrical vessel. It was found that the power correlation developed for paddle impellers in a spherical agitated vessel was also applicable for data obtained for the folding anchor impeller. The power consumption for the folding anchor impeller was much higher than those of two traditional impellers, but almost the same with that for a double helical ribbon impeller. On the other hand, the mixing time for the folding anchor impeller was less than one-fifth those obtained for two traditional impellers, and it was comparable to that calculated for a helical ribbon impeller according to the correlation. Consequently, the folding anchor impeller needs the lowest mean energy dissipation rate to attain the same mixing time among the impellers considered in this work.

Design parameters and performance of a surface baffled helical ribbon impeller bioreactor for the culture of shear sensitive cells

A novel bioreactor equipped with a double helical ribbon impeller (HRI) and three surface baffles has been shown to be well adapted to shear sensitive suspension cultures. Data on power dissipation, mixing time, circulation time, mass transfer parameters (kL, KLa) were obtained at the 3 and 11 L scale. Rational design and scale-up of this type of unit are discussed. Performance of the HRI for the high density cultivation of plant cells (27g dry weight/L) and insect cells (6×106 cells/mL) are presented.

Development of a helical‐ribbon impeller bioreactor for high‐density plant cell suspension culture

A double helical-ribbon impeller (HRI) bioreactor with a 11-L working volume was developed to grow high-density Catharanthus roseus cell suspensions. The rheological behavior of this suspension was found to be shear-thinning for concentrations higher than 12 to 15 g DW . L(-1). A granulated agar suspension of similar rheological properties was used as a model fluid for these suspensions. Mixing studies revealed that surface baffling and bottom profiling of the bioreactor and impeller speeds of 60 to 150 rpm ensured uniform mixing of suspensions. The HRI power requirement was found to increase significantly for agar suspensions higher than 13 g DW . L(-1), in conjunction with the effective viscosity increase. Oxygen transfer studies showed high apparent surface oxygen transfer coefficients (k(L)a approximately 4 to 45 h(-1)) from agar suspensions of 30 g DW . L(-1) to water and for mixing speeds ranging from 120 to 150 rpm. These high surface k(I)a values were ascribed to the flow pattern of this bioreactor configuration combined with surface bubble generation and entrainment in the liquid phase caused by the presence of the surface baffles. High-density C. roseus cell suspension cultures were successfully grown in this bioreactor without gas sparging. Up to 70% oxygen enrichment of the head space was required to ensure sufficient oxygen supply to the cultures so that dissolved oxygen concentration would remain above the critical level (> or =10% air saturation). The best mixing speed was 120 rpm. These cultures grew at the same rate ( approximately 0.4 d(-1)) and attained the same high biomass concentrations ( approximately 25 to 27 g DW . L(-1), 450 to 500 g filtered wet biomass . L(-1), and 92% to 100% settled wet biomass volume) as shake flask cultures. The scale-up potential of this bioreactor configuration is discussed.