Throughout industry components are mixed in stirred vessels or pipes. The challenge is to determine exactly how well the ingredients are being mixed.

Factors such as the composition and nature of the ingredients, the position, speed and design of the stirrer and the structure of the vessel all contribute to mixing effectiveness.

Any reduction in mixing effectiveness:

- may increase the mixing time with consequential reduction in capacity

- increase the energy consumption

- reduce the yield and quality of the downstream processes.

The challenge is to improve understanding and detailed knowledge of the mixing and hence improve performance and availability. Often the structure is complex and the nature of the ingredients and the operating conditions make visual inspection either impossible or hazardous.

Solid-liquid mixing is common in the process industries and a recurring issue is the suspension of denser solids in a fluid.

Electrical tomography can identify the axial solids distribution in a stirred tank and the just suspension speed. In these cases ITS uses its unique linear probe as shown below:

This is inserted into the mixing vessel from the top and connected to an Industrial Tomography Systems p2+ instrument designed and established to measure and display ERT data. The linear ERT Probe produces a conductivity map of an orthogonal slide perpendicular to the probe.

Additional tomography probes can be used to determine mixing at different points in a vessel or coupled together to scan particular regions.

This information can be reduced to show the axial conductivity profile through the height of the vessel. Again, concentration of the dispersed phase, in this case solids, can be determined from the conductivity data by applying Maxwell’s equation. The axial solid concentration profiles for a range of agitation rates are shown in this figure.

This plots the % solids in the mixture as a function of tank height and allows quantification of the degree of mixing throughout the vessel.

They agree qualitatively well with the CFD results.

Key benefits include:

It has been widely demonstrated that ERT technology can be applied to a range of mixing processes. The benefits to the customers are:

  • At the R&D stage of mixing development, improved process understanding and validation of CFD models to aid process design and scale-up.
  • Through measurement improved control and product quality in industrial reactors.
  • Higher yields through improved mixing with shorter mixing time and less energy consumption.

Register to access our "Mixing" case studies available from the Download section on the right.

Register to access our "Mixing" case studies available from the Download section on the right.

Publications:

Hui, L.K., Bennington, C.P.J. and Dumont, G.A. (2009) Cavern formation in pulp suspensions using sideentering axial-flow impellers, Chemical Engineering Science, Vol. 64, pp 509-519

Williams, RA, Jia, X, West, RM and Roberts, KJ (1998) On Line Measurement of Solids Distributors in Stirred Tanks and Crystallizers Using Electrical Computed Tomography, International Conference on Mixing and Crystallisation, 22nd–25th April 1998, Tioman Island, Malaysia

For more information about this paper, please contact ITS.

In the Press:

  •  The Chemical Engineer Feb 2009 - Seeing is believing

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