Comprehensive information is crucial in process industries. When processing plants operate at optimal range, they increase overall productivity and reduce maintenance costs. Instrumentation assists operators in predicting process problems and planning cost-effective maintenance. Process measurements can be categorized into point-like and volume measurements. Point-like measurements give process information from a single point in a fixed location, such as temperature, salinity and pH. These measurements often are sufficient for efficient process monitoring and control. However, when operators are working with homogeneities and/or multiple material phases, point-like measurements cannot identify spatial variations. How a certain material is distributed over a certain volume sometimes becomes more important to effective process monitoring than information from a single point. Process tomography is a general term for volume measurement techniques intended for cross-sectional or 3-D imaging of material properties and distributions. Tomographic measurements enable noninvasive process monitoring, which can provide valuable information for process optimization and control. The techniques of process monography apply to various industrial positions such as pipes, vessels and reactors.
The imaging technique provides valuable data on concentration and material distribution for process industries.
09/21/2015
Figure 2. Illustration of the iterative solution of ECT imaging problem. Actual permittivity distribution and measured data are shown in the leftmost panel.
These constraints can be qualitative or quantitative depending on how much is known about the permittivity distribution. Different scaling conditions may require different constraints. ECT imaging begins with an appropriate guess for the permittivity distribution and by gradually proceeding toward model data and measurements (see Figure 2).
ECT is typically used for imaging electrically insulating materials. However, ECT can also be used in cases where the conductivity is not negligible. In such situations, material permittivity is a complex-valued quantity. Solving the real and imaginary parts of the permittivity distribution is essential to the image reconstruction. For this purpose, the measurement system must measure the electrical charges on the electrodes and the phase shift between the charges and excitation voltage.
Image 1. Paraffin wax formed on an ECT pipe sensor wall. The thickness of the paraffin layer is about 2 to 3 mm covering four of the eight electrodes.