Mixing Technology Modern Impeller Development To Optimize Mixing Processes

Author / Editor: Andreas Enz M. Sc., Dr.-Ing. Marc Stadtaus and Nikolaus Schmidt-Rose M. Sc.* / Charles Butcher

Solid deposits often have a negative impact on process performance. Learn how Ekato solved these problems by developing an optimised impeller geometry.

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The Endsfoil produces a single-compartment recirculation with little secondary flow.
The Endsfoil produces a single-compartment recirculation with little secondary flow.
(Source: Ekato)

Engineering solutions often start with a challenge. One of these are solid deposits occurring frequently in crystallization applications. Besides that, a variety of other processes are affected by the phenomenon of solid deposits at the vessel bottom and wall. These deposits often have a negative impact on the process performance as e. g. the heat transfer but can also agglomerate into larger lumps at the wall above the liquid level, from where they can break off and damage the impeller or other internals.

This can be counteracted by using adapted, robust impeller blade geometries, which can suspend and break up larger agglomerates without damage, and at the same time by generating high flow velocities near the wall to fulfill the initial mixing tasks.

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According to the feedback given to Ekato, above described problems occur in a wide variety of processes. For example, deposits in the bottom area often lead to problems and failures in the crystallization of aluminum hydroxide. In the production of terephthalic acid, the basic monomer of PET, the formation of larger lumps can result in bending and distortion of impeller blades and agitator shafts. This was the motivation for Ekato to develop an optimized impeller geometry.

The aim was to engineer a geometry, which, on the one hand, should be robust enough to handle larger agglomerates without damage of the equipment and, on the other hand, ensures a good suspension performance and overall homogeneity with high flow velocities near the vessel wall.

As a starting point, suspension tests were carried out with various types of solid particles using commonly applied suspension impellers on a test scale of 50 liters. Various glass bead fractions with diameters from 75 to 150 µm and quartz sand with grain sizes from 15 to 30 µm were used. The use of a transparent glass vessel allowed comparing the suspension behavior for the different impellers.

Based on the observations, 3D geometries were designed using state-of-the-art CAD methods and then produced using additive manufacturing or 3D printing technology. The knowledge and experience gained using well-established impellers were also considered and discussed in a team of experts during this development step.

In the next step, the suspension behavior of these new prototypes was compared to the standard impeller types. The suspension performance was assessed at different filling levels, while of course also adjusting for constant solids concentration.

In additional test series the influence of evolving gases, as they are generated e.g. during flash crystallization, on the suspension performance was also considered.

However, not only the mixing performance played an important role, but also the economic efficiency and the complexity of manufacturing must be considered in an impeller development.

For this reason some geometry variants were not pursued further since they would have required complex welding work.

After the first validation stage in the 50 liter scale, the most promising prototypes were produced to be tested in the 1 m3 scale. Ekato offers the possibility to perform tests in a wide range of vessel sizes, whether in laboratory scale from 3 liters to 100 liters or in pilot plant scale from 1 m3 up to 30 m3 and 100 m3. Tests in the 1 m3 scale reconfirmed the good suspension results achieved with the developed prototype.

Due to the tendency of flash crystallizers to form large agglomerates above the liquid level, additional blade force measurements were performed in the 1 m3 scale to determine the peak loads resulting from dropping agglomerates. At the same time, the disintegration of the agglomerates was to be simulated. That means, agglomerate model particles were needed that would, on the one hand, induce representative peak loads, and, on the other hand, decompose over time in the agitated vessel. To form these substitutes, special plastic flakes were frozen in water and the resulting ice blocks were then used to simulate the effect of agglomerate chunks. The favored prototype, the Endsfoil, showed no difficulties in suspending the solids even with these large chunks inside the vessel. For the scale-up into production scale and an examination of the new impeller’s stability, the tests in the 1 m3 vessel were supplemented using modern numerical simulations.

Numerical simulations

The use of computers for engineering purposes has developed rapidly over the last two decades and has now become an indispensable tool for the mechanical design of agitated reactors, internal components, and impellers. Therefore, the use of numerical simulations, for example computational fluid dynamics (CFD) and finite element analysis (FEA), has become mandatory.

The general advantages of numerical simulations include the following:

  • Reduced development time for faster time to market;
  • Application specific dimensioning of vessels, agitators, and their components for savings in time, energy and money;
  • Improvement of plant safety;
  • Increased reliability and operational safety; better planning of shutdown and maintenance intervals;
  • Prevention of potential safety and environmental risks at an early stage.

FEA allows the identification of local stress peaks and a prediction of deflections of structural components. Potential weak points are evaluated quickly and, if necessary, a mechanical redesign can be done instantly. Furthermore, the stochastic changes of turbulent flow fields induce cyclic loads which can accelerate fatigue failure. Also, harmonic and vortex detachment excitation can cause resonance phenomena. As a result, a reliable design must consider not only mechanical (hydraulic) loads but also the vibration behavior (natural frequencies) which is done by a modal analysis.

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The most important fields of applications of FEA in mixing technology can be summarized as follows:

  • Structural-mechanic simulations (fatigue & strength, deflections);
  • Modal analysis with fluid-structure-interaction to assess the risk of resonances;
  • Thermal and thermo-mechanical simulations (transient and quasi-static);
  • Multi-Physics, e.g. coupling of CFD and FEA.

After the impeller blade geometry of the Endsfoil was optimized and finalized, structural requirements needed to be considered. To achieve this, finite element simulations were conducted using results of the blade force measurements. Additionally, an iterative optimization routine was applied to further improve the mechanical design. It turned out that a beam design is advantageous compared to a blade-to-hub arrangement. In this way the blade thickness could be reduced by more than 50 percent. Regarding stresses, the critical locations are the weldments of the beam with the hub as well as with the impeller blade. Besides the static and the fatigue load case, the special load case already mentioned needed to be considered. Again, the additional loads measured earlier were used to simulate the mechanical impact of large chunks on the impeller, which makes it possible to assess their effect on the blades. Finally, a modal analysis was carried out to prevent the occurrence of resonance effects due to harmonic excitation. So far, investigations of the Endsfoil’s performance have been done in two lab scales, but it is anyhow important to assess its performance in production scale. Simulation is the ideal tool for that, since most common crystallization vessels are too large for practical testing. CFD allows visualizing the resulting flow to get more insight into what happens inside a vessel at any scale. With Ekato’s simulation capabilities, it is possible to compare the performance of the newly developed Endsfoil impeller with those of already-established impellers.

The first step to the CFD was modelling the geometries for the Endsfoil and for two standard impeller systems. Each of them was put into the same typical vessel – elliptical head, 4 baffles, 7 meters in diameter. The three resulting fluid geometries were then split into a computational mesh each. Special attention was paid to mesh them all in the same way, since this is important for the subsequent comparisons. After the meshes were generated, setting appropriate boundary conditions was required for each of them — most are identical, but each impeller got its own rotational speed to adjust power input.One of the main purposes of the Endsfoil in described applications is the suspension of particles, therefore the introduction of particles into the simulation suggested itself. These particles are typically small, so different simplifications can be applied: Particles are modelled to be point-masses, and to not affect the flow around them (one-way coupling). They are also assumed to be inert — in the sense that they do not react or grow, as well as in the sense that they are not massless. This means that they need to be accelerated by the flow instead of always following the fluid movement perfectly.

After the simulations were completed, the next step was to generate videos for the particle movement for each of the impellers, as well as for the velocities on vertical cuts through the vessel. This gra­phical representation of the flow allows a comparison of the suspension and mixing performance of the Endsfoil to those of the established impellers. It became obvious that the Endsfoil produces a different flow pattern compared to both other impeller types, which results from its position very close to the bottom head of the vessel. From the analyzing of the particle movement, it became clear that the suspension performance of the Endsfoil will be at least as good as those of the established impellers. With the more robust mechanical design, this signifies a substantial overall improvement.

Overall, an optimized impeller design could be developed to achieve good suspension behavior while maintaining a robust design. As a result, production downtimes can be reduced due to a significantly longer service life. From a process and mechanical point of view, the combination of laboratory tests and the use of modern numerical methods provides the optimum conditions for the development of new geometries.

* * A. Enz is Process Engineer, Research & Development, Dr.-Ing. M. Stadtaus is Project Leader Engineering, Mechanical Engineering & Simulations and N. Schmidt-Rose is Project Engineer CFD, Research & Development, all at Ekato, Schopfheim/Germany.