GB2312174A - Agitator impeller having hydrofoil blades with a swept back trailing edge - Google Patents

Abstract

An agitator impeller 10 has four radially extending hydrofoil blades 14, each of which has a swept back trailing edge 18. The trailing edge 18 of each blade 14 may define an arc. Each trailing edge 18 may initially extend radially outwards at an obtuse angle to a tangential line passing through the blade edge. Each blade 14 may have a radially extending leading edge 16. The solidity ratio of the agitator impeller 10 is preferably 100%. The blades 14 may be pitched at angle of around 30 degrees and tilted at an angle of around 15 degrees.

Description

AGITATOR This invention relates to an agitator for use in mixing processes and gas dispersion.

In many industries it is necessary for materials to be mixed and blended on a large scale. Typically this is achieved by placing the materials in a cylindrical vessel provided with one or more agitator impellers driven to rotate about the vessel axis. One of the most common impellers currently in use is the Rushton turbine which comprises a flat disc with a series of radially extending rectangular flat plates spaced circumferentially around the disc edge. Rushton turbines are characterised by a relatively high power number (P,), that is a relatively high torque is required to drive the turbine for a given power input to the material being mixed. This tends to increase the initial costs of agitating or mixing systems provided with Rushton turbines as drive motors, drive shafts and gearboxes must be of relatively heavy construction to accommodate these high torques.

Many materials are mixed or processed in the presence of gas, and it is often desired to disperse gas in a material. The Rushton turbine is not well suited to accommodating changes in gas flow rate; with an increase in gas flow rate the power input to the material from the turbine drops, which leads to loss of mass transfer potential; that is the circulation rate of material within the mixing vessel drops. This problem may be reduced by increasing the number of blades present, however this increases the ungassed power number, for example from P0 5 with a six blade turbine to P0 x 8 or 9 with a nine or twelve blade turbine.

With increasing gas flow an impeller may eventually become "flooded", when the bulk flow pattern associated with the particular agitator (radial in the case of a Rushton turbine) is lost to be replaced by an upward gasliquid plume in the centre of the vessel and annular downward liquid flow. The problems of flooding and the fall in power input with gassing have been overcome to a limited extent by the development of hydrofoil agitators with large solidity ratios, which also tend to have low power numbers. However, the flow characteristics of these agitators still change with gassing, from predominantly axial flow to predominantly radial flow, which may result in vertical zoning within a mixing vessel with multiple agitators. Further, the power draw of the agitators still tends to drop with increased gassing, often accompanied by significant torque fluctuations, which may lead to excessive vessel vibrations and accelerate deterioration of the agitator system.

It is among the objects of embodiments of the present invention to provide an agitator configuration which obviates or mitigates these disadvantages.

According to the present invention there is provided an agitator impeller comprising a plurality of radially extending hydrofoil blades, each blade having a swept back trailing edge.

It has been found that, in use, the provision of this form of trailing edge is capable of providing an agitator which demonstrates a relatively constant power number and power draw with increased gassing, and is not subject to the level of torque fluctuations experienced with conventional agitators. Without wishing to be bound by theory, it is believed that the swept back trailing edges of the blades eliminate or reduce the creation of low pressure regions at the blade tips which may arise with conventional agitator configurations, and in the presence of gassing result in large ventilated cavities being created. These cavities have been found to disrupt the ungassed flow pattern and lead to the undesirable torque fluctuations and decreases in power number as described above. Accordingly, the agitator of the present invention eliminates or reduces flow instabilities and may reduce top-to-bottom zoning in multiple impeller configurations.

The provision of hydrofoil blades tends to result in agitators having relatively low power numbers, such that a relatively low torque is required to drive an agitator when used with a large impeller-to-tank diameter (D\T) ratio.

This relatively large diameter ratio also enables greater volumes of gas to be handled at similar energy dissipation rates when compared to conventional agitators.

Preferably, the trailing edge of each blade defines an arc. Most preferably, each trailing edge initially extends radially outwardly at an obtuse angle to a tangential line passing through the blade edge root.

Preferably also, each blade defines a leading edge which extends substantially perpendicularly to a tangential line passing through the blade edge root. Most preferably, each leading edge is slightly convex.

Preferably also, the agitator has a relatively high solidity ratio, to further minimise the possibility of gas flooding. The solidity ratio is preferably greater than 70%, most preferably in excess of 85%, and in the preferred embodiment is 100%.

Preferably also, the blades are pitched, typically at an angle of 150 and 450, and most preferably at 300.

Preferably also, the blades are tilted, typically at an angle of between 0 and 300 degrees, and most preferably at 150 to the axis of blade rotation.

Preferably also, the blades are curved. Most preferably the blades follow a constant radius of curvature about a radially extending axis.

Preferably also, the agitator has four blades.

The agitator may be configured for pumping upwards or for pumping downwards.

This and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a view from below of an agitator impeller in accordance with a preferred embodiment of the present invention; Figure 2 is a side view of the agitator impeller of Figure 1, illustrating a side view of one impeller blade and sectional views of two other blades; Figure 3 is a plan view of one of the blades of the agitator impeller of Figure 1 (on same sheet as Figure 1); Figure 4 is a side view of an agitator impeller in accordance with a further embodiment of the present invention; and Figure 5 illustrates the relative power consumption of three impellers for different levels of gassing.

Reference is first made to Figures 1, 2 and 3 of the drawings, which illustrate an agitator impeller 10 in accordance with a preferred embodiment of the present invention. The illustrated impeller 10 is intended to be used to provide upward flow of material in a vessel. This form of impeller is used, for example, for gas dispersion in fermenters.

As may be seen from Figure 2, the impeller comprises a cylindrical hub 12 to which four radially extending hydrofoil blades are mounted. Each blade features a substantially radially extending leading edge 16 and a swept back trailing edge 18. It will be noted from Figure 1 that the leading and trailing edges 16, 18 of adjacent blades 14 overlap, to provide the impeller 10 with a 100% solidity ratio.

From Figure 2 it will be noted that each blade 14 is curved and has a pitch of 300. Further, in this embodiment, the blades 14 are tilted at 150 to the hub 12.

The illustrated impeller 10 is formed of stainless steel, the blades 14 being cut from 0.32 cm (") thick sheet which is then rolled to produce the desired degree of curvature. The blades 14 are then attached to the hub 12.

The dimensions and configuration of the impeller 10 will now be described in detail, but solely by way of example. The hub 12 is 5.72 cm (21W") in diameter and defines a 3.81 cm ( ") diameter bore. The hub 12 is 10.32 cm (4 1\16") long and is provided with two 900 spaced radially extending holes for use in fixing the impeller to a driving shaft. The blades 14 are sized to provide an impeller diameter of 26.99 cm (10A). The leading edge 16 of each blade is slightly convex, with a radius of curvature of 26.99 cm (10 5/8"). The leading tip of each blade has an 1.27 cm (M") radius. Each trailing edge 18 has a radius of curvature of 5.40 cm (21/a") , and the trailing tip has a 0.64 cm (l,4íí ) radius. Each blade is curved with an 26.99 cm (10%") radius about a radially extending axis spaced 400 from the blade root at the leading edge and 700 from the blade root at the trailing edge.

Reference is now made to Figure 4 of the drawings which illustrates a side view of an agitator impeller 30 in accordance with a further embodiment of the present invention. The impeller 30 is substantially similar to the impeller 10 described above, but is intended for use in downward pumping, the typical mode for material mixing.

Agitator impellers similar to those described above were tested in a fully baffled, flat bottomed, cylindrical, Perspex (RTM) tank of 0.61 m diameter. The liquid was water of a height equal to the tank diameter and of volume 0.178 m3. Air was supplied to the tank below the impeller through a ring sparger of a diameter of about 1.9 D (D = impeller diameter). The power dissipated by the impeller was measured accurately using shaft mounted strain gauges and cavity observations were made through a derotational prism, as described by Kuboi, R et al in "A Multipurpose Stirred Tank Facility for Flow Visualisation and Dual Impeller Power Measurement", Chem.Eng.Comm., 22, 29 - 39, (1983). Mixing time was measured by a decolorisation technique as described by Cronin, D.G. et al in "An Experimental Study of the Mixing in a Proto-Fermenter Agitated by Dual Rushton Turbines", Food and Bioproducts Processing, (Trans.I.Chem.E., Part C), 72, 34 - 40 (1994).

UNGASSED POWER CHARACTERISTICS The power number for the up pumping impeller was 0.85.

Hence, compared to a Rushton turbine, the impeller may be used at a large impeller-to-tank diameter ratio without increasing the torque requirements. This facilitates retro-fitting of impellers to systems previously provided with Rushton turbines.

GAS - LIQUID MIXING STUDIES In the downward pumping mode, the interaction of the gas flow from the sparger with the pumping action of the impeller still led to unstable flow patterns. This resulted in torque fluctuations, though with the impeller of the present invention these were less than with other down pumping agitators tested and the unaerated axial flow pattern remained substantially unchanged. In the upward pumping mode these fluctuations were eliminated and, even at low impeller speeds, the impeller was capable of dispersing gas, that is flooding took place at only extremely low speeds.

Reference is now made to Figure 5 of the drawings illustrates the characteristic power curves of the impeller of the present invention (APV-B2\PU) pumping up, a Lightnin' Mixers Inc A315 impeller (PO 0.83) with hydrofoil shaped blades pumping down, and a six bladed Rushton disc turbine (BDT). These gassed power characteristics were each obtained at a speed corresponding to an unaerated power input of 1 kW m-3, a typical value for industrial operation. The gas flow rate was varied from 0 to 2.5 vvm which is also a typical range. The relative power consumption, Pg\PO (power when gassed\power when unaerated), remained close to unity for the impeller of the present invention but was reduced considerably with the six bladed Rushton disc turbine, where cavities formed readily upon gassing. The reduction in relative power consumption with the A315 was between the impeller of the present invention and the Rushton turbine, but the torque fluctuations were large over a wide range of gas velocities as indicated by the upper and lower lines for that impeller in Figure 5.

The above experiments indicate that the agitator impellers in accordance with the preferred embodiments of the present invention allow the provision of an agitator vessel configuration giving improved performance over conventional configurations, and in particular provides: (a) an upward pumping agitator which eliminates flow instabilities and reduces top to bottom zoning in multiple impeller configurations; (b) a hydrofoil impeller that eliminates or reduces low pressure regions and the formation of ventilated cavities behind the blades; and (c) i) a low power number, high pumping capacity agitated so that a large D\T ratio can be used, suitable for good bulk blending especially with multiple impellers; ii) a minimal fall in power under aerated conditions; and iii) a high gas handling capacity.

It will be clear to those of skill in the art that the above-described embodiments are merely exemplary of the present invention, and that various modifications and improvements may be made thereto without departing from the scope of the invention.

Claims (15)

CLAIMS:

1. An agitator impeller comprising a plurality of radially extending hydrofoil blades, each blade having a swept back trailing edge.

2. The agitator of claim 1, wherein the trailing edge of each blade defines an arc.

3. The agitator of claim 2, wherein each blade trailing edge initially extends radially outwardly at an obtuse angle to a tangential line passing through the blade edge root.

4. The agitator of claim 1, 2 or 3, wherein each blade defines a radially extending leading edge.

5. The agitator of claim 4, wherein each blade leading edge is slightly convex.

6. The agitator of any of the preceding claims wherein the agitator has a solidity ratio of greater than 70%.

7. The agitator of any of the preceding claims wherein the agitator has a solidity ratio of greater than 85%.

8. The agitator of any of the preceding claims wherein the agitator has a solidity ratio of 100%.

9. The agitator of any of the preceding claims wherein the blades are pitched.

10. The agitator of any of the preceding claims wherein the blades are pitched at an angle of between 150 and 450.

11. The agitator of any of the preceding claims wherein the blades are pitched at an angle of around 300.

12. The agitator of any of the preceding claims wherein the blades are tilted.

13. The agitator of any of the preceding claims wherein the blades are tilted at an angle between 0 and 300.

14. The agitator of any of the preceding claims wherein the blades are tilted at an angle of around 150.

15. The agitator substantially as described herein and as illustrated in Figures 1, 2 and 3 or Figure 4 of the accompanying drawings.

15. -The agitator of any of the preceding claims wherein the blades are curved.

16. The agitator of claim 15 wherein the blades follow a substantially constant radius of curvature about a radially extending axis.

17. The agitator of any of the preceding claims wherein the agitator has four blades.

18. The agitator of any of the preceding claims wherein the agitator is configured for pumping upwards.

19. The agitator of any of claims 1 to 17 wherein the agitator is configured for pumping downwards.