Process Tomography

Tomography comes from the Greek words tomo (slice) and graph (picture). As defined in one encyclopaedia, it is the obtaining of plane section images, which show a slice through an object.

Process tomography evolved in the late 1980′s to meet a widespread need for the direct analysis of the internal characteristic of process plants in order to improve the design and operation of equipment. The measuring instruments for such applications must use robust non-invasive sensors which, if required, can operate in aggressive and fast moving fluids and multiphase mixtures. Process tomography involves using tomographic imaging methods to manipulate data from remote sensors in order to obtain precise quantitative data from inaccessible locations. The need for tomography in process engineering is analogous to the medical need for body scanners, which has been met by the development of computer-aided tomography.

Process tomography is a developing measurement technology. It is gaining attention from industry because of some recent successes and the increased work into various fundamental aspects of the technology. This is demonstrated by the first international workshop on Process Tomography held in 1992 and in three subsequent years which reported new sensing techniques, new algorithms and began to address the fundamental challenges such as spatial resolution, speed of acquisition, quality of information and other factors that are essential to make the technology acceptable to industrial practice.

The specific characteristic of tomographic measurement is its proven ability to interrogate the dynamic state of a process condition within a unit operation such as a mixing vessel or conveyor without interfering with the process itself. This is achieved using non-invasive sensors along a cross-sectional boundary of the process equipment. The tomographic measurement data is manipulated using algorithms for image reconstruction, profile analysis and computation of numerical quantities such as flow rates, concentration, size and phase distribution.

For instance process tomography will improve the operation and design of processes handling multi-component mixtures by enabling boundaries between different components in a process to be imaged in real-time using non-intrusive sensors. Information about the flow regime, vector velocity, component size distributions and concentrations in process vessels and pipelines may be determined from the images.

The basic tomographic system consists of an array of sensors around the pipe or vessel to be imaged. The sensors output signals dependent on the position of the component boundaries within their sensing zones. The sensor signals are transferred to a computer which is used to reconstruct a tomographic image of the cross section being observed by the sensors.

The need for process imaging is analogous to that of medical imaging. In the process industry, information describing material distribution and validating internal modes of the process are necessary for the optimum design and operation of process equipment. Hence, there is need for the process engineer to be able to visualise the inside of the mixing vessel or reactor, thus relevant measurement techniques are necessary. However, complex experimental approaches are not economically viable for many process design and operation needs. For these latter cases, the process tomography approach using simple non-invasive sensors has much to offer.
Fig. 1: Process tomography system using capacitance electrodes.

There are three basic components in a typical process tomography system :
  1. The sensors
  2. The data acquisition system
  3. The image reconstruction system and display.
Sensor technology is the most essential and critical part of any measurement system. The sensors in a tomography system are usually placed in a circular array around the circumference of the process vessel. This is to provide multiple projections of the object or event being measured. The main advantage derived from such a configuration is multi-component spatial distributions can be obtained through image reconstruction techniques.

Several process tomography systems are as follows:
  1. Positron Emission Tomography (PET)
  2. X-rays
  3. Nuclear Magnetic Resonance (NMR)
  4. Ultrasonic Tomography (UT)
  5. Electrical Impedance Tomography (EIT)
  6. Electrical Capacitance Tomography (ECT)
  7. Optical Sensors (Visible and Infra-red)
  8. Magnetic Induction Tomography (MIT)
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