Inline Mixing

Inline mixing/blending

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

In addition to tomography equipment to study mixing, ITS also provides process ultrasound spectroscopy instrumentation (u2s) which provides on-line rheological information.

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. It can also degrade product quality where fragile components, such as pieces of meat or fruit in food products, need to be mixed into a substrate such as yoghurt, juice or gravy.

Inline mixing/blending is widely used in many industries, such as pulp and paper, food, petrochemical. In addition, a number of traditional batch processes are also being transferred to continuous in-line production.  There are two main inline mixing techniques:

• static mixers

• driven mixers

These offer different trade-offs in terms of process efficiency, energy efficiency and relative pressure drop across the mixer.

In the case of static mixers, the critical factor is the number of elements / overall length of the mixer.  Although other factors can include injection point, flow rate and changing physical properties of ingredients.

As with batch mixing, the same issues of identifying mixing end-points is a critical factor. The solution to many mixing problems is either to sample or measure mixing on-line. 

Process tomography provides a useful tool to quantifying mixing characteristics.

The mixing index σ is calculated as follows where:

C1i = conductivity of pixel i in tomogram 1
C1 = average conductivity of tomogram 1
C2i = conductivity of pixel i in tomogram 2
C2 = average conductivity of tomogram 2
N = the number of pixels in a tomogram

For example,
No mixing C1i = C2i σ1 = σ2  σ = 0
Perfect mixing C2i = C2 σ2 = 0  σ = 100

This technique can be used to:

• optimise set up conditions

• adjust mixing rates for changes to raw materials

Applications:

• Liquid-Liquid/Solid-Liquid/Gas-Liquid
• Lab/Pilot/Production
• Continuous/Batch/Semi-Batch
• Research/Process monitoring/Process Development

Process tomography can also be used to continuously sample materials as they pass through a pipe.  This can provide an on-line measurement of product quality to validate that materials have been mixed to the appropriate standard.

The instrumentation required for inline mixing is typically the two plane p2+.  This unit is highly portable and can be taken from plant to plant for process optimisation.

In cases where organic materials are being mixed, the m3000 should be used.

In rapid mixing systems (such as jet mixing which are frequently used in the production of nano-particles), the z8000 can be used.  The z8000 can measure at more than 1,000 frames per second, resolving rapidly changing flow features.  This provides an opportunity to improve homogeneity of the reaction space and so improve the quality of the nano-particles produced.

As noted before, process ultrasound spectroscopy with the u2s can provide on-line rheolgical information which can supplement tomographic data on mixing processes.

Generally inline sensors are fitted for inline mixing processes.

Key Benefits Include:

• determine optimal mixing lengths / settings

• determine residence times

• improvement of yield / quality by responding to process or material variations

Inline mixing - standard deviation and conductivity graphs

"ERT pipeline sensors used for in-line mixer optimisation improving yields and product consistency"

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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

Stephenson, D., Cooke, M., Kowalski, A. and Yorke, T.A. (2007) Determining jet mixing characteristics using electrical resistance tomography, Flow Measurement and Instrumentation, Vol. 18, No. 5-6, pp 204-210

For more information about this paper, please contact ITS.

In the Press:

•  The Chemical Engineer Feb 2009 - Seeing is believing

If you would like to receive a copy of an article, please contact ITS.