Batch Mixing

Single phase mixing / blending

Applications:

Liquid-Liquid/Solid-Liquid/Gas-Liquid
Lab/Pilot/Production
Batch/Semi-Batch

Key Benefits Include:

development of new mixing techniques
determine mixing efficiency
improved mixing homogeneity and efficiency
determine end point of mixing (eg T90 and T95)
research / process monitoring / process development

Introduction

Batch mixing is widely used in the Pharmaceutical, Fine Chemicals and FMCG (such as food and household products) industries. The principle is that the components are added to a vessel which is agitated or rotated in a manner designed to ensure to fully mix the ingredients.

“In 1989, the cost of poor mixing was estimated at $1 to $10 billion in the US chemical industry alone”

Video Showing Experimental Results from ITS Software:

Right Click to zoom

The two experimental results shown are from a study for a chemical company who were designing a new manufacturing facility. Part of the manufacturing process involved an addition into stirred tank reactors. It was crucial that this addition was mixed quickly and effectively as high concentrations could lead to unwanted byproduct formation and affect downstream processing and product quality.

Process Challenges

While at first sight a simple and low cost, means of mixing the challenge is to ensure complete mixing throughout the batch. Depending on the nature of the components material may “stick” to the mixer, walls or any irregularities in the surface. In addition, where the two components have different densitities, they can settle or separate once agitation completes. This can lead to mixing quality problems or de-mixing.

Ensuring complete mixing - homogeneity or blend uniformity - is particularly difficult when a low concentration of an ingredient, say below 2%, is to be accurately mixed in a bulk carrier. This is often the situation in the Pharmaceutical industry where the active is placed in a carrier and the regulated target is a uniform distribution in every “pill” produced from the batch.

Even defining complete mixing can present a challenge. The length scale at which mixing is to be measured also has a significant impact on whether a system can be defined as well mixed.

Mixing problems can also arise when there are small changes to product recipies. These can produce changes to viscosties of media which in turn can change the time taken to reach a well-mixed state.

To compensate for known difficulties in mixing efficiency the tendency is to significantly extend the mixing time to be on the “safe side”. This has the disadvantage of lost production opportunity and increasing the energy used for mixing.

The solution to many mixing problems is either to sample or measure mixing on-line. Sampling creates its own challenges, including where to sample, loss of product, time lost whilst samples are being analysed.

The ability to accurately measure on-line the completeness of batch mixing is highly desirable. Often it is difficult as the sensor is outside the batch mixer and any form of “window” is prone to becoming blocked by the mixture. When samples are placed in the vessel, again challenges can arise on the positioning of the measurement point vs the fluid dynamics of the process material within the vessel.

Answers from Process Tomography

Process tomography can measure many regions in a vessel simultaneously. The p2+ can measure up to 2,500 points in a vessel several times per second. For smaller vessels, fewer planes and more rapid measurements (up to 40 times per second) are typical. This configuration makes use of 8 cross sectional measurement planes.

Where space is at a premium in a vessel, a tomography probe can be used which can be substituted for a dip pipe or baffle
If the concentration is the same in each region, then the materials are mixed, if there are significant variations, it's not
The key is using tomography sensors that can detect the different components being mixed

This can deliver solutions to many mixing problems. Tomography data is provided in an array of 200 (for probe-based sensors) or 314 (for circular sensors). Multiple probes can produce even more measurement points. A variety of statistical techniques can be applied to determine homogeneity / mixing quality over time and through a volume.

In addition, process tomography can be used to characterise mixing processes. This can be applied to:

CFD validation
process design
optimisation of process conditions

Thus tomography can also be used to determine the effect of different:

baffle arrangements
impeller designs
vessel shapes (such as flat-bottomed or dish-bottomed)
positions of feed pipes and nozzles

In most batch mixing processes, the mixer is general driven radially, producing effective mixing in its plane, but poor axial mixing. Industrial Tomography Systems has developed a tomography probe (operated with the p2+ instrument) which readily delivers axial mixing information in real time. This provides an effective tool to determining:

end-point detection of mixing
mixing quality

References

Mann, R, Dickin, FJ, Wang, M, Dyakowski, T, Williams, RA, Edwards, RB, Forrest, AE and Holden PJ (1997) Application of Electrical Resistance Tomography to Interrogate Mixing Processes at Plant Scale, Chemical Engineering Science, Vol. 52, No. 13, pp 2087-2097

For more information about this paper, please contact ITS.