Process Tomography: Principles, Techniques and Applications

Abstract

Process tomography is the general name given to a range of techniques in which data are recorded using remote sensors and then manipulated to provide measurements of concentration distribution and flow phenomena within process equipment such as pipes and reactors. In principle, this technology can be used to identify, for example, the distribution of mixing zones in stirred reactors, and the position of interfaces within systems such as multiphase flows and complex mixtures during separation processes. Potential applications of process tomography fall into three broad categories. First, on-line, real-time sensing offers exciting developments in on-line process control. Second, visualization of concentration distributions as a function of time and flow phenomena within process vessels provides new insights into the operation of process equipment. Third, these visualizations yield data against which to validate the output of, for example, computational fluid dynamics simulations. This book is written in four sections: Principles of Process Tomography (Part I), Sensing Techniques (Part II), Data Processing Techniques (Part III) and Process Applications (Part IV). The articles are written assuming the reader has no particular specialization in the basic discipline of process tomography; the principles of the techniques are therefore discussed and other more detailed publications in each area are referenced. Part I introduces the fundamental principles of process tomography in the context of the three broad sensor types which are based on transmission, diffraction and electrical measurements. Examples of typical process applications and the sensing methods that can be used to probe these systems are briefly described. The computational techniques that are used in process tomography are also introduced. Part II provides a more detailed description of sensing methods; the section is introduced with an overview of the relevant strengths of different tomographic methods in various process applications, with a particular focus on dual-modality and multi-modality tomographic imaging systems. Subsequent chapters address each of the major tomographic methods including impedance sensors for application to dielectric and conducting systems, mutual inductance tomography, electrodynamic sensors, ultrasonic sensors, microwave sensors, optical tomography and emission tomography. Part III addresses data processing techniques in process tomography and includes chapters both on hardware and software requirements necessary for fast data capture and real-time image reconstruction and display. The principles upon which the image reconstruction algorithms required for each tomographic method are based are also described. The use of computer modelling in the design and optimization of process tomography sensors and systems, and in the interpretation of the sensor output data is discussed. The important topic of error analysis in the processing of tomography data is also addressed. The book concludes with chapters on specific process applications. The examples included are the measurement of bed porosity distribution in particulate flows using photon transmission tomography; powder mixing studied by positron-emission tomography; pneumatic conveying and control using capacitance imaging; capacitance imaging of fluidized beds; mixing processes in fluids studied by light absorption and interferometric techniques; resistivity imaging of mixing, transport and separation processes and, finally, optical tomography as applied to the visualization of temperature distributions in flames. Clearly, this book covers all the major aspects of development and application of process tomography. If there was a weakness it would be that the limitations of the various techniques and the theoretical limit to their application are not always obvious. For example, how does resolution vary throughout an image during typical process applications? How soon will we see the tomographic techniques described yielding real-time flow measurements? Undoubtedly, the answers to these questions are contained within this book but an overview of limitations and the expected short-term developments in process tomography in general would have been useful. However, this is a minor criticism and the text is essential reading for any researcher or industrial process engineer wishing to learn about the state-of-art in the field of process tomography. To those already working in the field it provides a useful collection of papers representing current practice in hardware, data analysis and process applications.