Contents I. Introduction
Accurate and real-time multiphase flow metering is required in several applications. For instance, in the biomedical field, measuring the flow rate of blood inside blood vessels is required in order to avoid unpredictable accidents which may occur from various cardiovascular diseases. In oil and gas plants, real-time flow metering is required to ensure good quality of substances flowing through the pipes, while preventing contaminants from propagating along the pipeline network. Several technologies and devices have been designed and implemented in the industries that have provided successful results. In particular, tomography systems have been developed where reconstruction of images of the multiphase flow are done based on the data that are collected at different sensing orientation. For example, electrical capacitance tomography (ECT), magnetic induction tomography (MIT), and electrical resistance tomography (ERT or EIT) systems generate images based on measurements of the capacitance values, change in magnetic fields, and change in resistance values, respectively [1], [25], [26]. Out of these technologies, ECT systems have been proven to be cost-effective and efficient in generating images in real time [2]. However, the quality of the images generated from the tomographic systems make it difficult to distinguish different types of contents inside the pipeline. Another issue with these techniques is the difficulty in capturing images of very small particles, or solid contaminants having mixture of iron oxide and iron sulfide particles found in oil and gas pipelines which are of micrometer size [20], [27]. Research was also performed on utilizing different technologies that have the ability to “see through” the walls of the pipe and map out the image of the contents flowing inside. One technology that is gaining popularity is the use of spectroscopy. Spectroscopy, by definition, is the study of the interaction between matter and electromagnetic radiation [3]. It refers to the measurement of radiation intensity as a function of wavelength and is often used to describe experimental spectroscopic methods. Properties such as absorbance, transmittance, and refraction are measured in order to determine how an object of mass responds to a set of electromagnetic waves. One research study was performed on a system that used near-infrared (NIR) spectroscopy technology for detecting contaminants in water [4]. An emitter source generated waves in the range of 700–2500 nm inside the pipe segment. A detector then receives this signal and measures its intensity depending on items that pass through the system. This technology has provided satisfactory results for solid contaminants; however, more research work needs to be done to prove its effectiveness for multiphase flow and detecting the individual phases in the pipeline. X-rays have also been tested in the past; however, the thickness and material of the pipeline makes it difficult for X-rays to penetrate through the wall of the pipe and create a clear image of the process fluid [5]. A technology that has been researched extensively in the spectroscopy field over the past decade is the use of terahertz (THz) waves. THz waves are electromagnetic waves with a band of frequency from 0.1 to 10 THz (i.e., 1 THz is equal to Hz) and wavelengths ranging from 3 to 0.03 mm (or 30 μm) [6], [7]. Like infrared and microwave waves, THz waves travel in a line of sight and are nonionizing, unlike X-rays which are ionizing. This property of THz waves has given it a rise in popularity over X-rays due to its ability to penetrate body tissues without damaging them [8] and have consequently proven to be effective in many medical and industrial applications. In the biomedical industry, THz imaging systems have been used to detect differences in water content and density of tissues in order to diagnose several cardiovascular diseases that are present within the patient [8]. In addition, research was also performed on the effectiveness of THz waves in detecting cancer with an imaging system that is safe, noninvasive, and painless [9]. THz imaging systems have also had successful implementations for security and surveillance applications. THz imaging devices effectively uncovered concealed weapons on a person remotely by penetrating the cloth fabrics and plastic containers [10]. THz waves have also been implemented successfully in industries where they help companies detect flaws in their products and inspect packaged goods to ensure that their products meet the requirements before shipping to the customer [11]. Previous research studies have also been performed using THz spectroscopy for analyzing fuel oils. In [21], a spectral analysis study was performed on different fuel oils such as gasoline, diesel, and lubricants. Results showed how different compositions such as sulfur content in gasoline and methyl methacrylate content in diesel were used to differentiate different types of gasolines and diesel products. However, the study only focused on single-phase item, which is fuels in this case, and did not focus on studying the effects of mixing other phases with the fuel oils in order to see how THz spectroscopy can differentiate them. In [22], a study was performed in which THz spectroscopy was used to measure the water concentration in crude oil samples. Results from this study indicated how well the THz system was able to measure the water content in oil-water mixtures containing 50.05–100% of water. The amplitude and absorbance of the THz pulse was used to characterize the water content. The results shown in this paper demonstrates the suitability of the THz technology to deal rather only with very low water content, as it has been found that the THz absorption is very high in the case of using mixed fluid of only few fraction of water content (i.e., 15% water content). Beyond this value, no signal could be detected by the THz spectrometer. This can be explained by the fact that THz waves get very much absorbed by even low volume of conductive medium such as water. In addition, compared to their work, extra tests were also performed in order to measure the contents of a solid contaminant that was mixed in the water–oil sample and in addition to assess the effect of temperature on THz absorption for these samples.
