GAS-LIQUID MIXING TECHNOLOGY
New developments that affect productivity

Multi-phase processes involving either Gas-Liquid or Gas-Liquid-Solids are amongst the more complicated and demanding of mixing applications. The critical design criterion for mixer selection in these processes is usually gas dispersion. However, the overall process result frequently requires the mixer to satisfy other mixing objectives, for which the chosen impeller may not be totally suitable. Typical mixing objectives may include absorption, mass transfer, blending, solids suspension, thermal homogeneity, and heat transfer, along with gas dispersion.

For over 40 years the Rushton turbine, with its good gas handling and mass transfer characteristics, has been the traditional impeller of choice for gas-liquid systems. However, recent research suggests that the use of modern hydrofoil impellers can have an impact on the productivity of these systems. It is possible to achieve significant improvements in both productivity and quality by identifying the various mixing objectives required by a process and optimising mixer performance to meet these objectives.

TRADITIONAL TECHNOLOGY
Developed in the 1950’s as a general purpose mixing impeller, the Rushton turbine, is rarely the ideal impeller for achieving all the mixing objectives required for optimum process results. Typical characteristics that have ham-pered its performance include staged blending, poor solids suspension and a comparatively high power demand when compared to axial flow impellers. The Rushton turbine also exhibits a significant decrease in power draw upon aeration. This implies a reduced mass transfer potential because the mass transfer coefficient is dependent on both aeration rate and power per unit volume.
Over the years various hybrids of the Rushton turbine have evolved in an attempt to overcome its inadequacies. One such example is the Concave Disc Turbine. The improved streamlining over the back of the blade, reduces turbulence, and results in improved power characteristics for this hybrid. However, despite improvements to power characteristics, the mass transfer and flow characteristics of the Concave Disc Turbine are essentially the same as those of the Rushton turbine, at equal power input. Furthermore, it still exhibits the inadequacies of staged mixing, long blend times, poor solids suspension, and poor heat transfer like all radial flow impellers.

IMPROVED TECHNOLOGY
In the early 1980’s it was recognised that whilst the bottom impeller was responsible for gas dispersion and increasing the surface area of the gas-liquid interface by reducing bubble size, the upper impellers did nothing to further decrease bubble size. Their function was primarily to maintain the dispersion and provide circulation in the upper part of the vessel.
The use of high efficiency axial flow impellers in place of the upper radial impellers significantly improved the flow patterns and eliminated the staging effects, which had hampered the performance of multiple radial flow impellers. As a result significant improvements in blend times, solids suspension capability, and temperature uniformity were achieved. Additionally, axial flow impellers with their relatively lower power numbers compared to radial flow impellers, provide the opportunity for significant reductions in energy costs.
However, despite the considerable potential of the mixed flow approach its use was restricted by the limited gas handling capabilities of the available axial flow impellers.
In the late 1980’s this deficiency was overcome, and productivity further improved, with the introduction of high solidity, hydrofoil impellers. Speci-fically designed to provide high efficiency operation under gassed condi-tions, the large, profiled and angled blades of these impellers produce strong axial flow patterns while provi-ding superior gas handling capabilities. When high efficiency axial flow impel-lers cannot effectively handle the gas, high solidity hydrofoil impellers can offer an effective alternate.

NEW TECHNOLOGY
Many gas-liquid processes, including the majority of pharmaceutical fermen-tations, are coalescing systems, that is, the gas bubbles have a tendency to converge and recombine to form larger bubbles. This tendency for gas bubbles to coalesce increases as shear levels are reduced due to increasing viscous drag. Furthermore, pressure differentials set up around an operating impeller, cause gas bubbles to migrate to areas of low pressure where they coalesce to form larger bubbles. These become caught behind the impeller blades where they eventually cause the impeller to stall. This phenomenon is particularly pro-nounced in Rushton turbines but it is also a problem with down pumping axial flow impellers.
To date, all the axial flow technology applied to gas-liquid systems, has been founded on the concept of down pumping. This requires that the impellers pump against the rising gas, with the intention of driving it to the tank bottom and increasing the time it is in contact with the liquid. However, the most recent research carried out by Lightnin Mixers into gas-liquid sys-tems, suggests that using hydrofoil impellers in an up-pumping mode can not only eliminate the opportunity for gas bubbles to coalesce, but also provide dramatic enhancements in productivity and mechanical reliability. Based on the Lightnin A340, a high solidity hydrofoil impeller optimised for up-pumping in gassed systems, this new development in gas-liquid mixing technology has demonstrated unique advantages in gas handling compared to the conventional Rushton turbine and down pumping high solidity impellers. In up pumping mode, the A340 impeller enhances the natural direction of the gas flow, and since it is not working against the gas stream, it will not flood. Additionally, by encouraging gas bubbles to circulate in the direction they are naturally prone to move, the up-pumping A340 does not provide a surface for bubbles to coalesce. The result of this is that the A340 does not exhibit a significant difference between gassed and ungassed horse-power. This suggests an improved mass transfer potential at equal un-gassed agitator power, and better utilisation of motor power without the need for expensive variable frequency drives, two speed motors, or additional controls. In full scale commercial installations, significant improvements in productivity have been observed. These include a 20 to 40% increase in fermenter yield, reaction times reduced from 12 hours to 3 hours, temperature uniformity of ± 0.2ºC, a doubling of catalyst life, a four fold decrease in blend time and a 2 fold increase of fluid velocities over heat transfer surfaces resulting in improved heat transfer. Other benefits observed include improved nutrient utilisation, improved product uniformity and purity.
Along with productivity improvements, improved mechanical reliability has also been observed. Since the up-pumping A340 is pushing against a free surface, that is, the liquid surface as opposed to the tank bottom, this effectively dampens vibration and reduces torque fluctuations resulting in decreased wear on seals, stuffing boxes, steady bearings and dampened stress on tank structures.

SUMMARY
The evidence so far, suggests that most, if not all, gas-liquid processes can benefit from up pumping technology. How-ever, there are a number of applications where up pumping technology would appear to be particularly beneficial. These include coalescing systems, applications near or at the boiling point, or where additional gas is being gene-rated, high gas rate applications where traditional gas handling impellers will be flooded, and cone bottomed tanks.
As companies continue to come under price pressure with the inevitable squeeze on operating margins, the potential for significant improvements in productivity offered by using up-pumping hydrofoil impellers is one means by which manufacturers can improve their competitiveness and profitability.