What is Electrical Process Tomography?

Electrical Tomography is a measurement technique for obtaining information about the contents of process vessels and pipelines. Multiple electrodes are arranged around the boundary of the vessel at fixed locations in such a way that they do not affect the flow or movement of materials.  Tomographic measurement techniques differ from point measurement techniques, because they sample a substantial proportion of the process volume rather than at a single point. Circular pipeline-based sensors measure an entire cross-sectional volume.

The technology can be used for liquid/liquid, solid/ liquid, gas/liquid, gas/solid/liquid systems. The spatial resolution of the imaging method and the sensitivity of the method depend specifically on the electrical properties of the system being measured and upon the dimensions of the process.

Typically, a sensor consists of 16 electrodes and for research applications up to 8 x 16 electrodes may be arranged within a process vessel.

The technique can be used with a wide range of research and development applications demonstrated, including:

• interrogation of mixing processes
• investigating a solid-liquid filtration process
• monitoring the performance of a hydrocyclone
• measurement and control of bubble columns
• measurement of multiphase flow

One of the main application areas of electrical tomography is the measurement of multiphase flow regimes.

Flow processes may involve a variety of phases or components in the gas, liquid or solid phase and are complex in their nature. Electrical tomography techniques provide the capability for flow visualisation, regardless of material opacity, to enhance the understanding of such complex flow processes.

Measuring Solid-Liquid Flow Using Electrical Resistance Tomography

CSIRO (Melbourne, Australia) required a measurement technique to measure the particle and fluid properties in solid-liquid flows for both research and industrial application. Electrical Resistance Tomography (ERT) met the criteria due to the robust and simple nature of the equipment and the absence of any radioactive, cryogens or dangerous components means the equipment can be readily employed at mine sites both above and below ground. Measurements were performed on a 100 mm diameter flow loop with closely graded 2 mm silica sand suspended in clear shear thinning polymer suspensions. These ‘model' suspensions mimic the behaviour of bimodal suspensions of particles containing a large fraction of fine rheologically active particles that would form a non-Newtonian carrier in which would be suspended the coarser fractions such as those found on mining co-disposal lines.

Figure 1 shows a comparison between a photograph of the actual pipe flow and ERT derived concentration maps using on-line single step Linear Back Projection (LBP) algorithm and off-line iterative Sensitivity Conjugate Gradients (SCG) algorithm. The LBP algorithm has the advantage that images are produced on-line at rates of multiple images per second. However, it can be seen that there is some blurring of the solid-liquid interface. The SCG algorithm is in substantial agreement with actual pipe flow conditions.

By: Michael Dunne, Guest Author, Industrial Tomography Systems PLC