X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-06-22
2003-06-10
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000, C324S686000, C324S691000
Reexamination Certificate
active
06577700
ABSTRACT:
TECHNICAL FIELD OF INVENTION
This invention relates to electrical capacitance tomography. Specifically, the invention relates to a new image reconstruction technique for imaging two- and three-phase flows using electrical capacitance tomography (ECT).
BACKGROUND OF INVENTION
Applications of process tomography as a robust non-invasive tool for direct analysis of the internal characteristics of process plants in order to improve the design and operation of the equipment have increased in number in recent years (Williams and Beck, 1995, Beck and Williams, 1996). Process tomography involves utilization of tomographic imaging methods to manipulate data from remote sensors to obtain precise quantitative information from inaccessible locations. The need for tomography in industrial applications is analogous to the medical need for body scanners. A tomographic technique involves two basic tasks: (1) the acquisition of measurement signals from sensors located on the periphery of an object, such as a process vessel or column; and (2) the process of abstracting the measurement signals, which reveals information on the nature and distribution of components within the sensing zone, to form a cross-sectional image of the object. The task of generating image from the measurement signals is also known as tomographic reconstruction. The basic components of a tomographic instrument are, therefore, embodied in a sensor system, a signal/data acquisition system and a computer system for measurement control, image reconstruction and display.
Successful implementation of a tomographic technique lies on the selection of a sensor system deployed for the specified application and the tomographic image reconstruction algorithm suited to the sensor selected. A variety of sensing methods can be employed based on measurements of transmission, diffraction or electrical phenomena using radiation, acoustic or electrical sensors (Beck, 1995). For the purpose of multiphase flow imaging in process industries, one of the most critical things may be the speed of the technique to capture the real time data of the turbulent fluctuation in the flow field. In this regard, electrical capacitance tomography (ECT) is considered to be the most powerful tool among other available tomographic techniques because of its high-speed capability, low construction cost, high safety and suitability for small or large vessels. Currently commercially available electrical capacitance tomography systems could capture up to 100 image frames per second, compared to corresponding electrical resistance tomography (ERT) of 2 image frames per second, or X-ray computer tomography (CT) of only one image every 2 seconds.
ECT is gaining acceptance as a laboratory and industrial tool to analyze multiphase systems. Early efforts with ECT have been concerned with the imaging of two-phase stratified flows in industrial pipelines, especially oil-gas and oil-water flows from offshore oil wells (Yang et al., 1996), manufacturing processes involving gas-solid systems such as pneumatic conveyers (Ostrowski et al., 1997, Dyakowski et al., 1999) and gas-solid fluidizations (Dyakowski et al., 1997, Halow and Nicoletti, 1992, Wang et al., 1995), and trickle bed reactors for measuring water content (Reinecke and Mewes, 1997, 1998). However, application of the technique to more complex multiphase flow systems such as bubbly flows in gas-liquid as well as gas-liquid-solid systems which are widely used in chemical processes (Fan, 1989) is very limited. ECT has prospective uses for applications to gas-liquid and gas-liquid-solid systems in real chemical processes, since they are mostly using organic liquids (Fan et al., 1999), which are non-conductive, rather than water which is a widely used model liquid in laboratories. The implementation of the technique would considerably further research in these fields since the information provided by the rapid online imaging method offers a means to address long-standing problems in the modeling, optimization and control of these processes.
However, the most critical problems that still challenge the application of the technique to such systems may be the relatively low spatial resolution and the accuracy of the reconstructed image using existing techniques. Most work currently done by researchers is focused on efforts to address these problems. While leaving the problem on the spatial resolution to other researchers working on sensor hardware, this work deals with a reconstruction technique aimed at improving the accuracy of the reconstructed image. The selection of image reconstruction technique suited to the sensor is highly important, because it determines the quality of the image that gives the information required to analyze the flow system. In the case of capacitance tomography, unfortunately, the reconstruction problem is non-linear, so that commonly used and commercially available and well-developed reconstruction techniques for linear tomographies based on electromagnetic radiation as widely used in the medical field are not directly applicable to the non-linear problem. Although many studies have been reported on the development of image reconstruction techniques for capacitance tomography, the reconstructed results using the techniques reported so far are still more qualitative rather than quantitative. For application to relatively complex multiphase system such as bubbly flows, high accuracy of the reconstruction results is especially necessary. There is an urgent need for development of an accurate reconstruction technique for capacitance tomography to meet the requirement of real chemical process applications.
Multi-modal Tomographic Techniques for Three-phase Flow Imaging
Tomographic technique for two-phase flow imaging is referred to as single modal tomography, where the requisite sensed signal contains only one parameter in the object space, (e.g., energy absorption (electromagnetic radiations), permittivity (capacitance sensor) or conductivity (impedance sensor) (Nooralahiyan and Hoyle, 1997)). In a three-phase system, the requisite sensed signal contains two or more parameters that require a multi-sensing method. The tomographic technique for imaging a three-phase flow system is referred to as multi-modal tomography. Most of the tomographic techniques developed so far are for single-modal systems, which are not readily applicable to three-phase systems. There are three strategies to perform three-phase imaging using a tomographic technique: (1) by combination of two different single-modal sensing systems, (2) using an inherently multi-modal sensing system, and (3) by means of a single-modal sensing system having a reconstruction technique capable of differentiating between three phases in the object space (Warsito et al., 1999).
An example of the first approach is the use of electrical capacitance tomography combined with Gamma-ray tomography for imaging a multi-component composition of gas, oil and water in a pipeline (Johansen et al., 1996). Water has a permittivity approximately 40 times to that of oil, or 80 times to that of gas, so that it can be easily differentiated from gas and oil using the electrical capacitance tomography. The gas distribution is then determined using Gamma-tomography which is responsive to the large density difference between the gas and the liquids. George et al. (2000) used electrical resistance tomography (ERT) combined with Gamma densitometry tomography (GDT) to measure the gas and solid concentration (holdup) profiles in three-phase bubble column. They employed dry air as the gas phase, water with sodium nitrate as the liquid phase, and both polystyrene and glass-beads for the solid phase. Polystyrene is an electrical insulator like air but has a gamma attenuation coefficient similar to water, so that ERT is influenced by both the solid and gas phases while GDT is primarily sensitive to the gas alone. In the reconstruction, the gas volume fraction profile from GDT is subtracted from the insulating phase profile determined by ERT to get first-approximations of volume
Fan Liang-Shih
Warsito W.
Bruce David V.
Standley & Gilcrest LLP
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