reviews - American Chemical Society

Biotechnol. Prog. 1995, 11, 601-607

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REVIEWS Studies of High Solidity Ratio Hydrofoil Impellers for Aerated Bioreactors. 1. Review Caroline M. McFarlane* and Alvin W. Nienow The BBSRC Centre for Biochemical Engineering, School of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.

This first part of a four-part series of papers presents a review of the literature on high solidity ratio axial flow hydrofoil impellers. Interest in the application of highflow, low power number, hydrofoil impellers, such as the Prochem Maxflo T and the Lightnin A315,has developed over recent years and has largely been stimulated by reports of improved fermentation performance relative to the Rushton disc turbine. Initially, the review briefly examines the characteristics of the radial flow Rushton turbine, as traditionally used in fermenters, with particular reference to its weaknesses. It also discusses the use of pitched blade turbines (mixed flow impellers) for air dispersion since these show some characteristics similar to those of the axial flow hydrofoils. The consideration of these two impeller types provides the background to explain the advantages and potential problems associated with high solidity ratio, axial flow, downward pumping hydrofoils. These aspects are discussed in relation to existing literature which is still relatively scarce. Finally, the advantages of retrofitting are introduced. Parts 2-4 report on a recent, detailed, fluid dynamic study of the Prochem Maxflo T and Lightnin A315 hydrofoil impellers which extends our knowledge of these important impeller types.

Contents 1. Introduction 2. Review A. Characteristics and Limitations of the Radial Flow Rushton Disc Turbine in Aerated Bioreactors B. Downward Pumping Axial Flow Impellers C. Hydrofoil Impellers (i) Mixing and Ma88 Tranefer Studies (ii) Design Considerations 3. Conclusions

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1. Introduction During the 1980s the search to improve process performance and to reduce energy costs in mechanically agitated reactors led to the development of a series of novel “high flow efficiency”hydrofoil impellers. Some of the designs were optimized for blending and solid suspension operations (Oldshue, 1989; Salzman et al., 1988). However, because of their narrow blade design, their performance in gassed systems was found to be limited (Eliezer, 1987; Oldshue, 1989; Ibrahim, 1992). Therefore, hydrofoil impellers, such as the F’rochem Maxflo T and the Lightnin A315 (Figure 11, with high solidity ratios (defined as the ratio of the total blade area to the total projected, or swept, area of the impeller) were developed specifically for gas-liquid applications. These designs were claimed to combine pumping efficiency with a high

gas handling capability and have been receiving considerable attention as alternatives to the traditional Rushton disc turbine in fermentations. Substantial increases in oxygen transfer efficiencies and yields in viscous, mycelial fermentations have been reported when retrofitting existing vessels with either of these designs to replace the same number of Rushton disc turbines (Buckland et al., 1988a,b; Gbewonyo et al., 1986, 1987; Oldshue et al., 1988). Although mixing studies have been conducted in order to assess the manufacturers claims and to identify the physical processes responsible for the reported improvements, these have been limited primarily to air-water systems. No comparisons have yet been made between the Merent commercial hydrofoil impeller designs and, in general, information in the open literature on their gas-liquid mixing characteristics remains relatively scarce. This paper forms the first part of a series of four papers, the overall objective of which is to describe in detail the gas-liquid mixing characteristics of the &hem Maxflo T and the Lightnin A315 hydrofoil impellers and to compare their performance with that of the more extensively studied Rushton disc turbine and pitched blade turbine (mixed flow impeller). This w i l l be achieved by combining a review of the relevant literature especially that on high solidity ratio hydrofoil impellers, presented in part 1,with previously unpublished results in parts 2-4. The characteristics of the traditional Rushton turbine and downward pumping pitched blade turbine are also described in part 1in order to provide a basis for comparison with hydrofoil impellers. Part 2 deals with the equipment used in the study and presents the hydrodynamic and power characteristics of the

8756-7938/95/3011-0601$09.00/00 1995 American Chemical Society and American Institute of Chemical Engineers

Biotechnol. Prog., 1995, Vol. 11, No. 6

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Caroline M. McFarlane is a lecturer in biochemical engineering in the School of Chemical Engineering at The University of Birmingham, U.K. She graduated with a BSc in biochemistry in 1984 and an MSc in process biotechnology in 1985. She subsequently gained a Ph.D. in chemical engineering from The University of Birmingham and was awarded an ESSO Engineering Teaching Fellowship in 1993. Her research interests fall in the generic fields of mixing and interfacial phenomena with application to bioprocesses. Her current research includes scale-down modeling of mixing phenomena in bioreactors, the characterization and application of high-efficiency hydrofoil impellers, studies on the influence of liquid properties on bubble coalescence and mass transfer in stirred reactors, and the impact of hydrodynamics on flocculation and floc stability. >

Alvin W. Nienow is a professor of biochemical engineering and Director of the BBSRC Centre for Biochemical Engineering a t The University of Birmingham, U.K. He received his BSc (Eng) in chemical engineering (first class honours) a t University College London and later his Ph.D. and higher doctorate, DSc (Eng), from the University of London. He is a Fellow of the Royal Academy of Engineering and also a Fellow of the Institution of Chemical Engineers and their representative on the European Federation of Biotechnology Working Party on Bioreactor Performance. He is on the editorial board of Trends in Biotechnology and Chemical Engineering Communications and is an International Advisor to the Canadian Journal of Chemical Engineering. His main research is concerned with the many physical aspects of mixing, especially related to bioreactors with application to Xanthan and mycelial fermentations and animal cell culture; he also has interests in fluidized bed processes, especially fluidized bed coating and drying.

Prochem and Lightnin hydrofoilsin an air-water system. Part 3 examines the influence of liquid properties on hydrofoil impeller behavior and reports the findings for a coalescence inhibited system (sodium sulphate solution) and a viscous, non-Newtonian fluid (sodium carboxymethylcellulose solution). Comparisons are made with the results in water presented in part 2. Part 4 compares the performance of the two hydrofoils, the Rushton turbine and the mixed flow impeller, in the different model fluid systems and presents gas hold-up data for all of the fluids studied.

2. Review A. Characteristicsand Limitations of the Radial Flow Rushton Disc Turbine in Aerated Bioreactors. The agitator or agitators in aerated, stirred bioreactors are required to perform a range of functions in order to promote satisfactory rates for the necessary transport processes. These functions typically include (i) the breakup of sparged air, thus increasing the surface area available for mass transfer, (ii)the dispersion of bubbles to allow local oxygen demands to be met, (iii) the bulk mixing of gas, solid, and liquid phases to provide a uniform volumetric dispersion and to eliminate dead zones in the reactor, and (iv) the provision of sufficient liquid velocity at the heat transfer surfaces to achieve the desired heat transfer coefficient. One of these processes will usually be the most demanding on mixing; which one will depend on the rheological properties of the broth, which generally change with time, the scale of operation, the oxygen demand of the culture, and its sensitivity to damage. The gas dispersion performance of various agitator types in relation to fermenter operation in low- and high-viscosity broths has been recently reviewed by Nienow (1990a). Traditionally, the fermentation industries have adopted radial flow disc turbines, typically Rushton disc turbines (D T/3),since these are considered to be good for gas dispersion. Because of their industrial importance, these impellers have been widely studied in gas-liquid and gas-liquid-solid systems (Chapman et al., 1983;Nienow et al., 198513, 1977). However, disc turbines have a number of disadvantages. Thus, for example, they exhibit relatively high unaerated power numbers and a significant fall in the power drawn on aeration, this being particularly pronounced in viscous fluids (Nienow, 1990b). Since kLa, the volumetric oxygen mass transfer coefficient, is dependent on aeration rate and power input per unit volume (van’t Riet, 1979), this fall in power implies a reduced mass transfer potential. Because of the complex relationship between power, speed, and air flow rate, the accurate prediction of power is still not possible. In addition, disc turbines generate a nonuniform distribution of energy dissipation rates in the vessel with much of the energy dissipated in the impeller region (Laufhutte and Mersmann, 1985). High local levels of shear stress are generated in this region which may be deleterious to sensitive microorganisms. A significant proportion of the overall oxygen transfer into the liquid phase also occurs in the vicinity of the sparged turbine impeller (Wilhelm et al., 1966). Away from this region the strength of the flow field, along with oxygen availability, decreases. Radial flow turbines also tend to give poor top-to-bottommixing in multiple agitator geometries, as typically found in fermenters, and in highly viscous shear thinning fluids, compartmentalization is particularly severe (Kuboi and Nienow, 1986). As the scale of operation is increased, bulk mixing is likely to become the most demanding agitator function. Mixing times inevitably increase (Einsele, 1978),giving rise to spatial variations in, for example, dissolved oxygen and nutrient concentrations, pH, and temperature (Vardar, 1983; Oosterhuis and Kossen, 1984). Thus, microorganisms will be subjected to fluctuating environmental conditions. These problems are accentuated in viscous, non-Newtonian fermentations as a result of a more rapid fall in fluid velocity away from the impeller and the formation of stagnant regions (Metzner and Taylor, 1960; Wichterle and Wein, 1981). Such conditions may result in lower yields, inferior process control and unreliable scale-up. To improve liquid pumping capacity and gas handling capability, large diameter impellers can be used (Nienow,

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D

D

a)

b)

I

Figure 1. Hydrofoil impeller geometry: (a) five-blade Prochem Maxflo T; (b) four-blade Lightnin A315.

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-

1990a,b). However, direct replacement of D T/3 disc T/2) is difficult turbines with larger ones (e.g., D because of the very different speed-torque characteristics of the two and retrofitting in this way may not be permissible, given the limited capacity of many existing motors and drive trains. The alternative is to use axial flow agitators which provide a high flow for a given power input (Weetman and Oldshue, 1988). B. Downward Pumping Axial Flow Impellers. In recent years interest has arisen in the application of axial flow impellers in systems where the functions of gas dispersion, solid suspension, and blending have to be performed simultaneously. Axial flow impellers can be conveniently classified according to the design of their blades into two types: the pitched blade turbines, which posses flat inclined blades, and the more modern hydrofoils, which are distinguished by their profiled, angled blades. High solidity ratio hydrofoils, such as the Prochem Maxflo T and the Lightnin A315, have relatively large blade areas and are designed by their manufacturers to meet the needs of processes which demand high levels of gas dispersion, bulk blending, and mass transfer (as in viscous fermentations). Both classes of impeller can be operated so as to pump liquid in either a downward or upward direction, the former being the more usual mode of operation. The distinction between different impellers on the basis of their flow patterns must be made with care since, for a given blade design, the flow pattern generated may vary with the Reynolds number (Weetman, 1991). Pitched blade turbines are commonly referred to as mixed flow impellers since they do not produce truly axial flow even under fully developed turbulent flow conditions (Re > lo4). Instead, the resultant flow has a relatively strong radial component of velocity, with a discharge angle of 45-60’ to the horizontal (Ranade and Joshi, 19891, and upflow may occur around the vertical axis below the impeller (Jaworski et al., 1991; Ranade and Joshi, 1989). In comparison, it has been shown that hydrofoil impellers induce a “stronger” axial flow (Jaworski and Nienow, 1994). For the A315, higher relative pumping efficiencies have been reported (Bakker and van den m e r , 1990; Weetman and Oldshue, 1988)which have been attributed to a lower tangential velocity in the impeller outflow (Bakker, 1992). The axial velocity profiles of the two impeller types were found to be very similar with low velocities near the shaft and peaks at a distance from the axis of approximately 0.40 (Bakker, 1992). Over the time course of certain fermentations, e.g., mycelial and Xanthan gum, there may be significant increases in the apparent viscosity, shear thinning tendency, and yield stress of the broth. With increasing viscosity, t o give Reynolds numbers of the order of 5000, or less, the flow generated by hydrofoil impellers becomes

progressively more radial (Jaworski and Nienow, 1994). This increased radial discharge may be expected to significantly affect mixing and gas dispersion capabilities. The presence of a yield stress gives rise to cavern formation, comprising of fluid movement close to the impeller (the cavern) with essentially stagnant fluid outside it (Galindo and Nienow, 1992; Solomon et al., 1981; Wichterle and Wein, 1981). This type of flow structure is exhibited by all impeller types. Ideally a sufficiently high impeller speed should be used to ensure movement throughout the fluid. Large impeller to tank diameter ratios can achieve this with a lower power demand than small ones (Solomon et al., 1981). One of the potential advantages offered by axial flow impellers and in particular hydrofoils is their low unaerated power number which can be exploited in retrofitting operations. Table 1 shows typical values of unaerated power numbers (Po) in the turbulent flow regime. In comparison with the Rushton disc turbine, a larger diameter hydrofoil impeller can be driven with the same power at the same speed and torque and this should be advantageous with respect to bulk mixing (Nienow, 1990a,b). In principle, retrofitting in this way should lead to improved mixing through a reduction in mixing time (Ruszkowski, 1994). In practice there may be restrictions on retrofitting existing vessels with large diameter impellers, for example, because of the additional weight of the impeller or the presence of vessel internals, such as cooling coils. The hydrodynamic and power characteristics of mixed flow impellers in gas-liquid and gas-liquid-solid systems have been extensively reported (Bujalski et al., 1988; Chapman et al., 1983; Frijlink et al., 1984; Nienow et al., 1985a, 1983;Warmoeskerken et al., 1984). At high gas rates, downward pumping mixed flow impellers, especially those with a small DIT and projected blade area, tend to flood easily and therefore have a limited gas handling ability (Chapman et al., 1983; Nienow et al., 1983). Rather less has been published on the hydrofoil impellers although, in some respects, the two classes of downward pumping impeller do exhibit similarities in hydrodynamic behavior. Two regimes have been identified by which gas from a sparger can reach a downward pumping impeller (Warmoeskerken et al., 1984). At low gas flow rates and/or high impeller speeds, gas from the sparger is pumped downward and only reaches the impeller via recirculated liquid (indirect loading (Figure 2a)). Conversely, at high gas flow rates and/or low impeller speeds, the liquid flow is not sufficiently strong to deflect the rising gas and gas reaches the impeller directly (direct loading (Figure 2b)). The transition from indirect to direct loading has been reported to coincide with the formation of large gas-filled cavities, resulting in a sharp fall in power drawn (Bakker


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Table 1. Retrofitting for Equal Speed, Power, and Torque (Nienow, 1990a) impeller type Rushton Prochem Lightnin disc turbine Maxflo T A315 -1.0 -0.75 Po -5.0 no. of impellers, n 1 1 2 1 2 Da 0.332' 0.462' 0.402' 0.482' 0.422' a

Re

Assuming: (i) P = PopN3D5, M = P/(27cN);(ii) Po = const, lo4; (iii) (nPOD5)~ushton = (nP0D')~ydrofoil.

>

and van den Akker, 1990,1994a; Chapman et al., 1983; McFarlane, 1989; Nienow et al., 1983; Warmoeskerken et al., 1984), and in three-phase systems a severe reduction in solid suspension capabilities (Chapman et al., 1983; Frijlink et al., 1990; Nienow et al., 1985a). Oscillations in flow and in some cases large fluctuations in torque and power have been reported over the direct-indirect loading transition (Balmer et al., 1987; Bujalski et al., 1988; Chapman et al., 1983; McFarlane, 1989; Nienow et al., 1985a, 1983)for downward pumping impellers, and several attempts have been made to explain this unstable behavior. Bujalski et al. (1988) indicated that certain hydrodynamic instabilities arise as a result of the natural opposition of the rising gasliquid plume from the sparger and the downward pumping of liquid. Nienow et al. (1983) found that the torque instabilities were greatest when using a point sparger and a four-bladed, rather than six-bladed, mixed flow impeller with a small DIT ratio ( 5 V3) and explained the difference in the magnitude of the fluctuations partly with reference to the work of Tatterson et al. (1980) on the unaerated flows of pitched blade turbines. Nienow et al. (1983) suggested that for large-diameter impellers the well-formed vortex systems, which develop at the rear of the blades, fill with gas and these have a constant presence. The relatively small torque and flow oscillations result from periodic vortex shedding. With small impellers, the pulsing of high-speed jets, which tend to dominate the flow in the unaerated system, would have an exaggerated effect in a gassed system, resulting in much larger fluctuations in torque. Bakker and van den Akker (1994a) indicated that torque fluctuations could be caused by oscillations in the strength of two types of interacting vortex systems (one horizontal and one vertical) generated in the vessel and also between the Merent flow patterns. At high viscositys, especially in viscoelastic broths, the torque fluctuations may become even larger (Galindo and Nienow, 1992). Vessel vibrations have also been reported for the Lightnin A315 (Nienow et al., 1993)and Prochem impellers (Buckland et al., 1988b; Nienow et al., 1994) in large-scale equipment, especially with high-viscosity broths. Even under unaerated conditions, the amplitude of the vibrations has been found to be much higher than in the same vessel equipped with Rushton disc turbines. Although both torque fluctuations and vessel vibrations are associated with unsteady hydrodynamic conditions, they do not necessarily occur simultaneously. While these mechanical instabilities require serious consideration, they can be overcome by suitable structural design (Kirpekar et al., 1992). C. Hydrofoil Impellers. (i) Mixing and Mass Transfer Studies. Much of the recent interest in hydrofoil impellers has arisen following reports of substantial increases in oxygen mass transfer efficiency, typically of the order of 30-40%, and process yields in large-scale, viscous, non-Newtonian fermentations when replacing standard (D z TI31 disc turbines with large-

diameter Prochem Maxflo T or Lightnin A315 impellers (Buckland et al., 1988a,b; Gbewonyo et al., 1987, 1986; Oldshue et al., 1988). These improvements have been attributed to enhanced bulk mixing and have been explained in terms of an increase in the size of the wellmixed, low-viscosity,high-kLa region around the impeller relative to the poorly-mixed, high-viscosity, low-kLa region in the remainder of the vessel. The improved performance could be exploited either to reduce power costs for the same productivity or to increase productivity at the same cost. Buckland and his co-workers performed mycelial fermentations of up to 19 000 L in capacity. However, there are problems in obtaining accurate oxygen transfer measurements at such scales and in viscous, non-Newtonian fluids. In particular, the assumption of well-mixed gas and liquid phases breaks down and the accurate measurement of dissolved oxygen concentrations is very difficult. It has also been suggested that hydrofoils induce a more uniform distribution of energy dissipation rates within the reactor (Gbewonyo et al., 1987) which would significantly affect gas dispersion, bubble coalescence, and breakup phenomena. At present such claims are difficult to assess with any certainty due to the lack of direct evidence. Certainly, spatial distributions of relevant quantities (local turbulence, gas holdup, bubble size, and mass transfer rate) are far from homogeneous for both radial and axial flow impellers (Bakker and van den Akker, 1994b). However, more data on local quantities are required to firmly establish the differences between impeller types. It has been suggested that the improvements in oxygen transfer performance observed with hydrofoils may only apply to viscous fermentations where transport is thought to be limited by bulk mixing capability (Gbewonyo et al., 1986). Investigations conducted in air-water systems variously show similar or enhanced bulk average kLa values for the hydrofoils relative to the Rushton turbine. Using agitators of the same diameter (D = 0.411, Bakker and van den Akker (1994a) found that the mass transfer efficiencies and gas holdups of a single A315 and Rushton disc turbine were comparable and higher than those of the mixed flow impeller. The poor performance of the mixed flow impeller was attributed to its relatively low pumping efficiency under gassed conditions. Balmer et al. (1987) also reported that the mass transfer efficiency of the Prochem (D = 0.4511was similar to that of a largediameter Rushton disc turbine (D= TI2). On the other hand, Oldshue et al. (1988) found that mass transfer coefficients were 30% higher, on average, when using dual A315's (DIT = 0.51 and 0.43) as compared with dual Rushton disc turbines (DIT = 0.23) at a given power input, torque, and gas rate. It is worth noting that previous studies on a range of mixed flow and disc turbine agitators have shown that kLa and gas holdup are largely independent of impeller type at a given gassed power input and gas rate in the same low-viscosity fluid (Bujalski et al., 1988, 1990; Chapman et al., 1983; van't Riet, 1979; Warmoeskerken et al., 1984). Recent results confirm this broad trend for the Rushton turbine and Prochem Maxflo T hydrofoil in water, electrolyte, and low-concentration polypropylene glycol antifoam solutions (Martin et al., 1994). In highly coalescence inhibiting solutions, i.e., at high antifoam concentrations, the maximum holdup generated by the Prochem is much greater than that for the Rushton turbine due the presence of an unstable "foam" at the liquid surface (Machon et al., 1991; Martin et al., 1994). For both impeller types, increases in holdup in antifoam solutions are not matched by kLa, which instead is similar to that in water.


c

-t; r

605

0 0

Figure 2. Gas loading regimes of downward pumping impellers: (a) indirect loading; (b) direct loading. Mixing time measurements, using a decolorization technique, also suggest that the mixing efficiencies (mixing time per unaerated or aerated specific power input) of a D = TI2 Prochem and an equivalent diameter Rushton are comparable in the turbulent flow regime (Hass and Nienow, 19891,as suggested by the equations of Cooke et al. (1988)and Ruszkowski (1994). Therefore, from the analysis presented in Table 1 and by Ruszkowski (1994),it is likely that an improvement in bulk mixing would result from a typical retrofitting operation, in which a standard (Dm Tl3) disc turbine is replaced by a large-diameter hydrofoil, although this still requires direct confirmation. When multiple Lightnin A315 hydrofoils are compared with multiple Rushton turbines, mixing times are significantly reduced (Otomo et al., 1995). Although hydrofoils have been claimed to give good top-to-bottom flow throughout the reactor (Oldshue, 1989),compartmentalization is still evident (Manikowski et al., 1994). However, in comparison with the Rushton turbine, the number of compartments per impeller stage is reduced from two to one and, as a result, overall bulk blending is improved. (ii) Design Considerations. Modern hydrofoil impellers are designed for efficient pumping. As such they rely on the generation of appropriate pressure distributions on the suction and pressure surfaces of the blades to produce a positive lift, in the same way as an aircraft wing. In the case of a hydrofoil impeller, in the usual mode of operation, this is manifest as a downward pumping force. The profiled blade also reduces the drag forces associated with the motion of the blades and therefore the energy requirements for a mixing process. Impeller efficiency is directly related to blade characteristics, e.g., blade angle and camber. For the blade to act efficiently the flow over it should in principle be highly ordered. Boundary layer separation, vortex, and gasfilled cavity formation at the rear of the blade can result in changes in the pressure distribution and may produce significant changes in the direction of the outflow from the impeller and, therefore, bulk circulation, power drawn, and energy dissipation. A variety of hydrofoil impellers are commercially available and their design is dependent on the application. Narrow-blade hydrofoil designs, such as the Lightnin A310 and A6000,have been optimized with respect

to impeller pumping efficiency and solid suspension. Unfortunately, these have low gas handling capabilities, tending to flood easily at relatively low gas velocities (Eliezer, 1987; Oldshue, 1989; Salzman et al., 1988; Ibrahim, 1992). Oldshue (1989)has indicated the importance of a high solidity ratio for gas-liquid and highviscosity (Re < 103-104)applications. The solidity ratio (SR) of an impeller is closely related to the projected blade area and is given as,

An impeller with a high solidity ratio can operate at a higher pressure loading and stall occurs less easily. In stall, separation of the boundary layer from the suction (rear) surface of the blade causes the flow to degenerate to a disorderly recirculation zone and most of the lift and hence pumping capacity is lost. Both the Prochem Maxflo T and Lightnin A315 have solidity ratios of -0.9, as compared with -0.4 and 0.6 for a four- and six-blade, 45"mixed flow impeller, respectively. The advantage of a high solidity ratio in aerated systems has also been confirmed by the improved gas handling capability of the six-blade, relative to the four-blade mixed flow impeller (Chapman et al., 1983;Nienow et al., 1983). Although similar in appearance, several specific design features can be identified for each impeller (Figure 1). The blades of the A315 are mounted on a small boss, the diameter of which is approximately 20% of the total impeller diameter. In contrast, the Prochem has a large hollow hub, the hub to impeller diameter ratio being approximately 40%. The manufacturers of both impellers claim that their respective designs should prevent gas bypassing around the vertical axis, where there is likely to be a region of low downward liquid velocity or even a region of upflow. For the A315,the manufacturers also indicate that the more open four-blade design should assist in the development of flow between the blades and prevent the formation of a continuous sheath of gas around the impeller.

3. Conclusions Similarities exist in the hydrodynamic behavior of downward pumping axial (hydrofoil and mixed flow)


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impellers. Both impeller types exhibit flow and torque instabilities which are undesirable for mechanical reasons. Low solidity ratio axial flow impellers are generally considered unsuitable for aerated bioreactor operation as they tend to flood easily. Retrofitting of Rushton turbines with low power number impellers enables larger DIT ratios to be used, while matching speed, torque, and power. The use of large diameter, high solidity ratio hydrofoils should improve air handling capacity as well as bulk mixing which is likely to be a demanding function a t large scales and in viscous fermentation broths. In multiple impeller systems, the very strong compartmentalization, which is associated with radial flow impellers, is reduced and mixing times are much shorter. In general, information in the open literature on the gas-liquid mixing characteristics of high solidity ratio hydrofoil impellers remains relatively scarce. Because of the industrial interest in these impellers, there is a requirement for an improved understanding of their gas dispersion mechanisms, an independent assessment of their performance and comparisons with the more traditional impeller types, such as the Rushton disc turbine and mixed flow impeller. These aspects are considered in detail in parts 2-4 of this series of papers.

U

A315

Ab D e

H kL kLa

M n N P PMD Po Re T V

Notation bubble surface area per unit volume, m2 m-3 refers to the Lightnin A315 hydrofoil impeller horizontally projected blade area, m2 impeller diameter, m hub or boss diameter, m liquid height, m oxygen mass transfer coefficient, m 8-1 volumetric oxygen mass transfer coefficient, s-l torque, Nm number of blades, or number of impellers impeller rotational speed, rpm or rps power drawn by the impeller, W refers to the Prochem Maxflo T hydrofoil impeller power number = PIpWD5 impeller Reynolds number = pND21p vessel diameter, m volume, m3

Greek Symbols dynamic viscosity, N s m-2 density, kg m-3

P P

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