Considerations for Formulation of Products in Stirred Tanks

When formulating complex products, stirred tanks are traditionally used because of inherent advantages in semi-batch operations.  While there is interest in moving from batch to continuous processes in the context of process intensification [1], a semi-batch stirred tank will remain the best option for complex formulations. For instance, a single stirred tank can be designed for a variety of products because the recipes (ingredients, hold times, temperature ramps, etc.) are easily varied within the system design limits.  Some products may not need the full capabilities of other products, but there is an advantageous flexibility.

Although stirred tanks have benefits, there are many considerations to be made.  Based on this author’s personal experience, here is a list of design questions when creating a new formulation:

1. Does the viscosity/rheology vary significantly during the course of a run?

• An example is an initial monomer solution that polymerizes.
• Multiple types of impellers may be required due to transition in flow regimes.
• In the photo above, the (reacting) formulation is viscoelastic and climbs the mixer shaft via the Weissenberg effect. It’s not a show-stopper, but it is best to consider the possibility and consequences before implementation.

2. Are multiple phases expected during any stage of the recipe?  For example,

• Steam stripping of residual solvent at end of batch.
• Solids addition and incorporation, possibly with dissolution. Once incorporated, the agitation system must maintain suspension and prevent accumulation at the bottom or top of the vessel.
• Immiscible liquid addition for emulsification.

3. Are there reactions during the batch that need to achieve yield and selectivity targets?

• The reaction overall rate and selectivity may be sensitive to methods of addition of a reactant to the bulk, for instance in design and location of a dip tube in the vessel.
  • See my recent blog for comments on mixing-sensitive chemistries Mixing with Reaction; Practical Considerations

4. Does the level vary considerably during the recipe?

• The level may change from an initial heel in the dish of the vessel to 100% level along the straight side.
• The average density may also change substantially, so a level measurement based on static head will not be correct without corrections during the batch.
• Multiple impellers are likely required as the level increases.
• Level affects whether dip tubes, instruments, and other internals are submerged when they are needed.

5. Is the product, or an intermediate, shear-sensitive or subject to instabilities?

• For example, a latex intermediate will irreversibly flocculate if an impeller generates too much local shear. The is due to stabilizing surfactants on the oil/water surface being excessively disturbed during polymerization.
• If so, use low-shear impellers such as low area hydrofoils.

6. Are there critical heat transfer requirements?

• The time for heat-up and cool-down needs to be carefully evaluated during the tank design to ensure the total time to manufacture is economically viable.
• Are jacketed walls sufficient, accounting for level change? The wall heat transfer area per volume changes with level, particularly if the vessel base is jacketed.
• Is a wall (proximity) impeller needed to refresh the wall surface?
• Are internal coils required, and if so, do they affect anything else such as mixing and cleaning time?
• A pump-around loop with a heat exchanger for heating and cooling is often a preferred option for enhancement of heat transfer. A loop also offers the capability of injecting and mixing gas or liquid components with a static mixer after the injection point. A soluble, fast-dissolving gas may fully absorb in the recycle liquid before entering the stirred vessel.

7. Are there additional steps with solids that can be especially problematic?
   • For solids that are difficult to wet (e.g., flour into water), a rotor stator can be installed inside the stirred tank (off-center to the main shaft), or installed into a pump-around loop.
   • In some cases, the liquid surface can be made to vortex to “suction in” solids fed through the vessel headspace. This depends on the flow regime (turbulent is best) and liquid level relative to the vessel impellor(s).
   • Some solids will clump and form large particles containing a dry powder interior. These will eventually break apart after many passes through the impeller zones, but may require an extra “wait period” before proceeding.
   • For reacting solids, rapid incorporation into solution may be mandatory. A layer of floating and reacting solids may lead to quality and safety problems.  Note that solids that are heavier than the liquid medium will float if they are difficult to wet; the wettability thermodynamics dominate, especially if particles are small (specific surface area is high).
   • Evaluate if a pre-slurry can be made (or provided from the raw material supplier) for easier injection to the formulation.
8. Can the product be drained in a timely fashion?
   • If the product is sufficiently viscous, or a yield stress fluid, or viscoelastic, the emptying of the vessel may take excessively long. Padding the vessel with pressurized gas won’t help to a great extent. The only solution to this is pre-design of the vessel, such as a wide exit nozzle, minimization of tank internals, and/or a conical base with a wall impeller such as a helical ribbon.
   • If the product is a fast-settling slurry and needs to be partitioned into multiple totes or other product vessels, it is necessary to keep the stirring going during the draining step. A so-called tickler impeller such as the KT-3 [2] can be used to ensure the slurry stays uniform during draining, and as well leaves no solid heel in the vessel.
9. What are the requirements for clean-out between runs?

• If the product can be sufficiently drained, there may not be a need for cleaning if the same product is being made in the next batch. However, consider the impact of product in piping dead legs that can excessively contaminate the next batch.
• If surface cleaning is required, a spray ball, through which a cleaning solvent is added, is a convenient and cheap option. See my previous blog Sprays in the Process Industries.
• Cleaning of a pump-around loop may require that a solvent be recirculated to fully clean heat transfer and static mixer surfaces.

In this author’s experience, there are also certain rules of thumb to avoid problems.

• Do not add a supersack of solids “all at once” and expect it to melt sufficiently before agitation is started. Without agitation, melting will be slow. Eventually, the bottom impeller will be bent up by prematurely starting it. (This is a “when”, not an “if”.)
• If you are adding material through the headspace (including a pump-around loop return), and an impeller is not submerged, the material will quite possibly splash and sling off the impeller. In one case encountered by this author, the added material was relatively viscous and, upon splashing, stuck to the vessel walls and mixer shaft, forming considerable fouling.
  • A good practice to introduce liquids (besides using a submerged dip pipe) is to install a pipe bend (i.e. 90 to downward 135 degrees) to flow towards the wall with a tangential component. This will cause the liquid to flow down the wall as a film.
• Floating solids that contact a hot wall can form a dried-out rind that will eventually need to be hydroblasted from the wall. Worse yet, a large chunk may dislodge and plug the discharge from the vessel bottom. Designing for rapid solids incorporation is advised.
• Avoid a floating liquid phase, especially if it can react at the interface. Without mixing, it may form a floating solid layer that can’t be removed from the exit nozzle. A vessel entry is likely required.
• Do not expect an open pipe to be an effective gas sparger. Use a multi-holed approach to make smaller, jetted bubbles.
• If adding solids and you want to use a site glass to look at the interior of a stirred vessel during addition, consider that a very stable dust cloud can form that will obscure any view of the vessel contents. (Dust clouds have obvious safety ramifications as well.)

Fortunately for the designer, there are tools available to address concerns as they arise (see for instance [3] for solids suspension, [4] for drawdown of flowing liquid phase, and [5] for scaling liquid drop size based on power/mass.) Oftentimes these tools are based on empirical correlations.  Computational Fluid Dynamics (CFD) can certainly be helpful for some steps of the process. Keep in mind that solids incorporation into a liquid surface, for example, is inherently three-phase (including the gas contained in the vortex), three-dimensional, and transient. This would require significant CFD effort, if it becomes practical. An analogous physical experiment can be done in an afternoon.

If designing from scratch, as opposed to retrofitting an existing vessel, there may be prefabricated systems available that address your needs. Companies such as Ekato [6], Ross [7], and SPXFlow®[8] make market development and production scale vessels, and may be able to prepare a test formulation of your product in one of their facilities before a capital investment is made.

Feel free to contact Becht for further discussions on this topic or other topics related to mixing, whether stirred tank, jetted tank, inline, or options.

Further Reading

1. Paul, E.L., Atiemo-Obeng, V.A., Kresta, S.M., eds., Handbook of Industrial Mixing: Science and Practice, John Wiley & Sons, 2003.
2. Kresta, S.M., Etchells, III, A.W., Dickey, D.S., Atiemo-Obeng, V.A., eds., Advances in Industrial Mixing: A Companion to the Handbook of Industrial Mixing, John Wiley & Sons, 2016.

References

1. RAPID Manufacturing Institute for Process Intensification (aiche.org)
2. https://www.spxflow.com/lightnin/products/kt-3-low-level-mixing-impeller/
3. Zwietering, T. N., “Suspending of Solid Particles in Liquids by Agitators, Chemical Engineering Science, Vol. 8, pp. 244-253, 1958.
4. Armenante, P.M., Huang, Y.-T., Tong, L., “Determination of the Minimum Agitation Speed to Attain the Just Dispersed State in Solid-Liquid and Liquid-Liquid Reactors Provided with Multiple Impellers,” Chemical Engineering Science, Vol. 47, No. 9-11, pp. 2865-2870, 1992.
5. Davies, J.T., “A Physical Interpretation of Drop Sizes in Homogenizers and Agitated Tanks, Including the Dispersion of Viscous Oils,” Chemical Engineering Science, 42, pp. 1671-1676, 1987.
6. https://www.ekato.com/products/process-plants-and-units/
7. https://www.mixers.com/products/multi-shaft-mixers/
8. https://www.spxflow.com/industries/chemical-processing/