Mixing device

Abstract

A mixing method, a controller and a mixing device for mixing components in a mixing vessel are provided. The mixing method includes providing a mixing impeller in the mixing vessel; accelerating the mixing impeller from an inactive state to a rotating state in which the mixing impeller rotates at a first desired speed in a first rotation direction; rotating the mixing impeller at the first desired speed for a first time t.sub.steady,1 in the first rotation direction; changing the rotation direction of the mixing impeller, so that the mixing impeller rotates in a second rotation direction at a second desired speed; and rotating the mixing impeller at the second desired speed for a second time t.sub.steady,2.

Claims

1. A mixing device for mixing components, comprising: a mixing vessel being adapted to accommodate the components to be mixed; a rotatable shaft with at least one mixing impeller mounted thereon, the rotatable shaft and the at least one mixing impeller being arranged inside of the mixing vessel and being configured to mix the components when being rotated; a drive unit for driving the rotatable shaft and the at least one mixing impeller mounted thereon; at least one sensor provided at the rotatable shaft or at the at least one mixing impeller for detecting a decrease in torque; and a controller configured to change a rotational speed or a rotational direction of the rotatable shaft when the at least one sensor detects a specified decrease in torque that is indicative of a vortex in the components being mixed.

2. The mixing device of claim 1, wherein the controller is configured to: accelerate the mixing impeller from an inactive state to a rotating state in which the mixing impeller rotates at a first desired speed in a first rotation direction; rotate the mixing impeller at the first desired speed for a first time t.sub.steady,1 in the first rotation direction; change the rotation direction of the mixing impeller, so that the mixing impeller rotates in a second rotation direction at a second desired speed; and rotate the mixing impeller at the second desired speed for a second time t.sub.steady,2.

3. The mixing device of claim 2, wherein the mixing vessel is a single-use container.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a top plan view of a mixing impeller having straight blades.

(2) FIG. 2 is a graph indicating the speed of the mixing impeller in view of the time when applying the mixing method according to the first embodiment of the invention.

(3) FIG. 3 is a graph further graph of the torque of the mixing impeller in view of the time indicating various fluctuations in the torque.

(4) FIG. 4 is a top plan view of a mixing impeller having back-swept blades.

DETAILED DESCRIPTION

(5) According to a first embodiment of the invention, a mixing impeller 1 is provided (see FIG. 1) and may be arranged in a mixing vessel. The mixing vessel may be a rigid or flexible container in which various fluids, like solid, liquid and/or gaseous products, are mixed by the mixing impeller 1. The mixing impeller 1 is controllable by a control system so that the mixing impeller 1 is rotatable in a first rotation direction and in a second rotation direction that is opposite the first rotation direction. Exemplary, the first rotation direction may be a clockwise direction CW and the second rotation direction may be a counterclockwise direction (CCW), or vice versa. Preferably, the mixing impeller 1 has equivalent behaviors in both rotation directions, like e.g. a Rushton or straight blade turbine. FIG. 1 shows a Rushton turbine. The mixing impeller 1 may be a radial flow impeller having a circular basis 3 from which at least one blade 5 radially extends. FIG. 1 shows the specific case of six blades 5 arranged evenly along the circular basis 3. A rotational axis of the mixing impeller 1 extends through the center 7 of the circular basis 3 and the blades 5 extend vertically along the rotational axis.

(6) The above described mixing impeller 1 is applied for a mixing method according to the first embodiment of the invention, by which swirling tangential flow in the components to be mixed is prevented.

(7) FIG. 2 shows the mixing method by means of a graph. The graph indicates the speed of rotation N of the mixing impeller 1 in view of the time.

(8) Initially, the mixing impeller 1 is accelerated from an inactive state, in which the speed of rotation N is 0, to a rotating state. The rotating state starts as soon as the mixing impeller 1 is rotating. In the step of accelerating the mixing impeller 1, the mixing impeller 1 is accelerated from the speed of rotation N of 0 to the first desired speed 10. As shown in FIG. 2, the first desired speed 10 may be the maximum speed of the mixing impeller 1. The mixing impeller 1 rotates in a first rotation direction, which is clockwise in FIG. 2. Alternatively, the first rotation direction may be counterclockwise. The time within which the mixing impeller 1 is accelerated from the speed of rotation N of 0 to the first desired speed 10 (ramp time t.sub.ramp) may be determined in the control system. Usually the ramp time t.sub.ramp depends on the design limitations of the mixing impeller 1, a rotation shaft to which the mixing impeller 1 is connected, and/or the motor that drives the mixing impeller 1 and the rotation shaft. Preferably, the motor is equipped with a variable frequency drive capable of accelerating and decelerating the motor at a specified ramp speed.

(9) The mixing impeller is rotated at a constant rotation speed N for a time t.sub.steady,1. after reaching the first desired speed 10. Preferably, the duration of time t.sub.steady,1. is as long as possible, but should be limited to the point of time when swirling flow is detected in the components to be mixed. This time usually depends on the geometry of the mixing vessel, the geometry of the mixing impeller 1, and the properties of the components to be mixed.

(10) The speed of rotation N of the mixing impeller 1 is reduced from the first desired speed 10 to the speed of rotation N of 0 when swirling flow appears. Afterwards the mixing impeller 1 is accelerated again, but now to a second desired speed 20 in a second rotation direction. The second rotation direction in FIG. 2 is counterclockwise. In other words, the rotation direction of the mixing impeller 1 is alternated, preferably as soon as swirling flow is detected in the components to be mixed.

(11) The ramp time t.sub.ramp, within which the mixing impeller 1 has alternated its rotation direction and has achieved the second desired speed 20, preferably is kept short, but it should not be so short that harmful transients are created when switching rotation directions.

(12) At the second desired speed 20, the mixing impeller 1 is rotated constantly for the time t.sub.steady,2. The second desired speed 20 is maintained for the time t.sub.steady,2 as long as possible, but should be limited to the point of time when swirling flow is detected in the components to be mixed. If swirling flow appears, the rotation direction is alternated again, i.e. from the second rotation direction toward the first rotation direction. Again, the ramp time t.sub.ramp, within which the mixing impeller 1 has alternated its rotation direction and has achieved the first desired speed 10, is kept short, but should not be so short that harmful transients are created when switching rotation directions. Preferably, the time t.sub.ramp is identical whenever the rotation direction is alternated. It is, however, also possible that the time t.sub.ramp differs in the different cycles of changing the rotation direction

(13) The time t.sub.steady,1 and t.sub.steady,2 may be identical or different.

(14) The point of time when the mixing impeller 1 alternates its rotation direction or, in other words, the duration of t.sub.steady,1 and t.sub.steady,2 may be determined in the control system, so that the control system induces the alternation of the rotation direction. The determination may be carried out by various methods.

(15) Option 1:

(16) According to Option 1, a desired duration of time t.sub.steady may be determined and stored in the control system. Accordingly, as soon as the time t.sub.steady expires, the control system would induce a change of the rotation direction.

(17) The determined duration of time t.sub.steady may be based on the knowledge about properties of the fluids to be mixed, the liquid level in the mixing vessel and/or the effects of shape of the mixing vessel on the fluid flow. Based on this knowledge the typical time may be determined after which usually a swirling flow is detected in the components to be mixed.

(18) Option 2:

(19) When a swirling motion is fully developed and the components to be mixed start to rotate as a body, the torque required to turn the mixing impeller drops. The control system may detect this drop as Option 2 and induce afterwards an alternation of the rotation direction. The amount of the drop after which an alternation of the rotation direction is induced may be determined in the control system. One or more sensors may be provided at the rotation shaft or the mixing impeller for detecting the drop.

(20) Option 3:

(21) As Option 3 fluctuations regarding the torque required to rotate the mixing impeller may be detected.

(22) When air is ingested through a central vortex into the mixing vessel, the blades of the mixing impeller experience sudden fluctuations in torque since one or more blades may have air on one side and liquid on the other side. One or more sensors may be provided e.g. at the rotation shaft that applies the torque to rotate the mixing impeller for detecting the fluctuations in torque. The strength and/or the length of such fluctuations may be determined in the control system so that the control system may induce an alternation of the rotation direction of the mixing impeller when such fluctuations are detected.

(23) FIG. 3 graphically shows such fluctuations in the torque of the mixing impeller 1 in view of the time.

(24) At first the torque of the mixing impeller 1 is substantially constant. However, as soon as a swirling flow appears in the components to be mixed, a gradual decline in the torque appears (see time interval a) as explained with respect to Option 2. If air is ingested through a central vortex, sudden fluctuations in the torque appear as explained above (see time intervals b).

(25) The second and third Options may be complemented by the determination of minimum and maximum time durations of t.sub.steady stored in the control system. Thereby incorrect sensor measurements or process errors could be compensated.

(26) The undesired swirling flow can be prevented and the mixing quality can be enhanced by means of the periodic alternations of the rotation direction of the mixing impeller 1.

(27) The first embodiment describes that a swirling flow may be suppressed by alternating the rotation direction of the mixing impeller as soon as a swirling flow is detected. However, it is also possible any one of the following actions are carried out when detecting a swirling flow: reducing the speed at which the mixing impeller rotates to a preset speed, fully stopping the rotation movement of the mixing impeller or continuously reducing the speed until a vortex in the fluid to be mixed is no longer detected. As soon as swirling flow and/or a vortex in the fluids to be mixed is no longer detected, the mixing impeller can again rotate at its original speed. Any of the above described detection methods could be used for starting any one of the previously described alternative actions.

(28) Alternatively or additionally, an alert may be sent to the operator when detecting a swirling flow.

(29) According to a second embodiment of a mixing method of the invention, a mixing impeller 100 is provided and has a circular base 102. As shown in FIG. 4 a rotation axis of the mixing impeller 100 extends through a center 104 of the circular base 102. At least one blade 106 radially extends from the circular base 102 and has mixing surfaces 108 that extend vertically along the rotation axis. In particular, the at least one blade 106 has two opposite mixing surfaces 108.

(30) The at least one blade 106 is arranged with respect to the circular base 102 in a back-swept manner so that an angel between a first mixing surface 108a and the circular base 102 is smaller than 90 degrees, and an angle between a second mixing surface 108b and the circular base 102 is larger than 90 degrees. In other words, the at least one blade 106 is back-swept with respect to a first rotation direction FD. A synonym for back-swept is backward-leaning. Preferably, as shown in FIG. 3, the at least one blade 106 is curved.

(31) When rotating the mixing impeller 100 in the first rotation direction FD, which is the clockwise direction in FIG. 4, a gentle mixing is achieved, since the curved blade 106 reduces the torque required to turn the mixing impeller 100 in comparison to a mixing impeller having straight blades and the retreating blade 106 reduces the shear stress applied to the fluids to be mixed. When rotated in a second rotation direction SD (counterclockwise direction in FIG. 3), which is opposite to the first rotation direction FD, a chaotic mixing is achieved, since more torque is required to turn the mixing impeller 100 at a given rotation speed. This results in a higher power draw for the mixing impeller 100 and again results in a lower blend time. When rotating the mixing impeller 100 in the second rotation direction SD, the back-swept blade 106 could be also considered as a forward-leaning blade 106.

(32) A gentle mixing method is beneficial for mixing liquid-liquid homogenization of an aqueous solution containing sensitive molecules, like e.g. therapeutic proteins, because proteins are sensitive to shear and to interfacial forces. In contrast, a chaotic mixing method is beneficial when the mixing includes the dissolution of powder in an aqueous solution which does not contain sensitive molecules. Any concentrations gradients could be disrupted and the powder suspended could be maintained.

(33) Accordingly, by rotating the above described mixing impeller 100 in two different rotation directions two different ways of mixing can be achieved so that the same mixing impeller 100 can be used for different applications.