Batch Mixing

Overview
Solutions
Key Benefits
References

Batch Mixing -Single phase mixing / blending

Batch mixing is widely used in the Pharmaceutical, Fine Chemicals, mineral processing and FMCG (such as food and household products) industries. The principle is that two or more components are added to a vessel which is agitated or rotated in a manner designed to ensure to fully mix the ingredients. In blending there is no chemical reaction, in reactive processes, mixing may be part of a unit process such as crystallization.

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

As most substances have different electrical properties, electrical tomography is an ideal tool to monitor mixing as it can determine the spatial distribution of different components and so provide a measure of homogeneity.

Process Challenges

While at first sight a simple and low cost, means of mixing the challenge is to ensure when complete mixing throughout a batch is achieved. Depending on the nature of the components material may “stick” to the mixer, walls or any irregularities in the surface. Also, where the two components have different densitities, they can settle or separate when agitation completes. This will 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. This process is also complicated by the underlying rheology of the components, an area where process ultrasound spectroscopy can be of benefit.

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. Where point-based measurement techniques or sampling are used to determine mixing, difficulties can arise through where and how data is acquired as well as delays caused by at-line or lab based measurements.

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. As noted above, on line rheological measurements can also be of assistance in such cases.

Where sheer a sheer-thinning substrate is used (that is one whose viscosity reduces when it is put under a force, such as mixing impeller) mixing caverns can form. The size of the mixing cavern limits the extent of the mixing region and can lead to significant processing challenges.

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. In addition, when mixing caverns are formed, even long mixing periods will not lead to complete 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.

Process tomography can measure many regions in a vessel simultaneously. The p2+ instrument can measure up to 2,500 points in a vessel several times per second using simple peripheral or probe based sensors that do not alter mixing conditions. 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 (Photo).

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.

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.

This can deliver solutions to many mixing problems. For each sensor array, tomography data is provided in an array of 200 (for probe-based sensors) or 314 (for circular sensors). Multiple probes – up to 8 arrays for a p2+ instrument - 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.

Applications:

Liquid-Liquid/Solid-Liquid/Gas-Liquid
Lab/Pilot/Production
Batch/Semi-Batch
Move from batch to continuous in-line mixing using driven or static mixers

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

validation of CFD (computational fluid design) and other process models
process design
optimisation of process conditions

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

baffle design and 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:

progress and end-point detection of mixing
mixing quality
In addition concentration changes can also be monitored

Key benefits include:

development and evaluation 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

"FMCG company used ERT to optimise the toothpaste manufacturing process showing some mixing steps could be reduced in time by as much as 90%"

Publications:

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

Rodgers, T.L. and Kowalski, A. (2009) An electrical resistance tomography method for determining mixing in batch addition with a level change, Chemical Engineering Research and Design

Stanley, SJ, Mann, R and Primrose, K (2002) Tomographic imaging of fluid mixing in three dimensions for single-feed semi-batch operation of a stirred vessel, Trans IChemE, Volume 80, Part A, 903-909

Holden, PJ, Wang, M, Mann, R, Dickin, FJ and Edwards, RB (1999) On Detecting Mixing Pathologies Inside a Stirred Vessel using Electrical Resistance Tomography, Trans IChemE, Vol. 77, Part A, November 1999,pp 709-712

Holden, PJ, Wang, M, Mann, R, Dickin, FJ, Edwards, RB (1998) Imaging Stirred-Vessel Macromixing Using Electrical Resistance Tomography, American Institution of Chemical Engineers Journal, Vol. 44, No. 4, pp 780-790

For more information about this paper, please contact ITS.

In the Press:

The Chemical Engineer Feb 2009 - Seeing is believing

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