Improving the quality of light products and the increasing demand for heavy oil in compliance with strict environmental concerns have become a challenge for petrochemical refineries. Catalytic hydroprocessing is considered one of the most promising technologies for the conversion of heavy oils into high-value products, which is a well-known technology to remove undesirable components (sulfur, nitrogen, organometallic, etc.) from hydrocarbon feed streams and has been extensively practically utilized nowadays in refineries worldwide [1]. In hydrotreatment processes, the main issues are the catalyst life and performance limitation because the impurities can deposit on the catalysts resulting in rapid loss of its activity [2]. Under high temperatures and pressure, coking, poisoning, and sintering could cause serious agglomeration and maldistribution leading to unexpected unit shutdowns. Various designs of residue hydrotreating reactors have been described in the literature for treating heavy feedstocks [3]. There are some commercial designs such as Moving Bed of Catalyst (MBR), (U.S. Pat. No. 5,076,908), Fixed Bed Reactor (FBR) (U.S. Pat. No. 3879642 A), Ebullating Catalyst Bed (EBR), (U.S. Pat. Nos. 4,571,326 and 4,744,887) have been reported in the last few decades. The fixed bed systems deal with middle distillate feed, but they cannot deal with every residuum feed that is available. Heavy feeds with highly metallic contaminations (> 250 ppm) make the fixed bed catalytic hydrotreating system inefficient because the deposition of metals on the catalyst will result in a rapid loss of hydrogenation activity [4]. However, high-metal feeds are always the most economically attractive because of their relatively lower price. To deal with drastic change in heavy petroleum feed properties, moving bed technology has been developed [5]. In general, the upflow Moving Bed Reactor has a conical bottom part that allows replacement of the deactivated catalysts with fresh new catalysts. The removed catalysts can be reprocessed and reinjected later on mixed with fresh catalyst from the top of the reactor. In this case, the upward flow fluid with a slight bed expansion could avoid coking and plugging and reduce the pressure drop of the system to some extent [5,6]. The advantages of the upflow moving packed bed reactors are the increased utilization of catalysts characterized by a slight back mixing for both the catalyst and the feedstock; providing a better quality of products and higher efficiency of the process than that of an ebullated bed reactor [7,8]; periodic replacement of spent catalyst without shutdown. The moving bed configuration offers a relatively large catalyst migration time in comparison with the liquid mean residence time, so it can be considered as a pseudo two-phase (gas-liquid) upflow with a fixed bed [9]. Therefore, for the sake of the hydrodynamics study, the moving bed reactor can be investigated as a two-phase upflow packed bed reactor. The liquid holdup in upflow moving packed bed reactor is one of the important design and operating hydrodynamic parameters. Its measurement is essential to get a better understanding of the prediction of pressure drop, mass, and heat transfer mechanisms since the liquid serves as a transport of mass and heat to and from the catalyst particles. Moreover, the upflow packed bed reactor offers advantages for liquid-limited reaction [10]. For exothermic reactions, higher liquid holdup and well distribution ensure complete wetting efficiency and better temperature control thus contributing to the prevention of hot spot formation and thermal instabilities. A two-phase upflow packed bed reactor could be a satisfactory alternative to the classical trickle bed reactor for liquid-limited reactions because of increased gas and liquid interactions, thus increasing the effectiveness of contact, leading to better heat transfer and higher overall mass transfer coefficient [11]. For porous catalytic packed bed reactors, the total liquid holdup (${\epsilon }_{l,t}$) means the overall volume of the liquid phase divided by the reactor volume. The total amount of liquid consists of two parts, which are dynamic liquid (${\epsilon }_{l,d}$) and static liquid (${\epsilon }_{l,st}$). The static liquid includes the liquid inside the porous catalyst (internal static liquid, ${\epsilon }_{l,st\_int}$) and the stagnant liquid attached between the catalyst particles (external static liquid, ${\epsilon }_{l,st\_ext}$). The dynamic liquid refers to the freely flowing liquid under operating conditions. Experimental data on the overall liquid holdup or saturation can be obtained by various techniques, such as for example drainage [12], weighing [13], tracer methods [14], and electric capacitance tomography (ECT) [15]. The liquid holdup measurement techniques can be divided into integral, semi-integral, and local measurement methods [16]. Integral methods provide liquid holdup information over the entire volume of the packed bed; these methods include the draining, weighting, and tracer methods. Semi-integral measurement methods provide liquid holdup information over a section or a line-integral of the packed bed, which includes radiation methods (e.g., Gamma-Ray and X-Ray) that can be applied at many axial and radial positions to get line-averaged information. The local measurement methods provide local liquid information by inserting a sensor (e.g., electromagnetic radiation) or a probe (e.g., optical fiber). Probes can be used inside the packed bed at different positions or using time-averaged tomography (Gamma-ray Computed Tomography) or instantaneous tomography (X-Ray or Electrical Capacitance Resistive Tomography). However, liquid draining and tracer methods can only provide the average holdup for the whole packed column. They cannot offer any information on how the liquid is distributed in the packing bed [5]. In fact, the liquid holdup can vary significantly with spatial position, and this information is critical for a better understanding of flow hydrodynamics and mass transport in packed columns. In gas–liquid–solid systems where the catalytic bed is dense and opaque, it’s hard to implement instrumentation inside three-phase systems. While the noninvasive methods such as advanced radioactive measurement techniques eliminate the alteration during the measurements [17], these techniques can determine the flow distribution over the whole reactor section with good spatial resolution and are not too intrusive [18]. The radiation method is based on the attenuation of the radiation beam as it passes through an absorption medium, which can be obtained by Beer-Lambert’s equation. Non-invasive techniques have become the tools of choice for detailed flow structures within porous media, unlike the more traditional interfering probes inserted within flows [15]. Since the catalytic packed bed is opaque, some noninvasive visualization techniques such as Digital Particle Image Velocimetry (DPIV) and Laser Doppler Anemometry (LDA) cannot measure the liquid/gas holdup distribution over its cross-section. Compared with other radiation techniques, the gamma-ray technique is well developed and more versatile because gamma rays have strong energy to penetrate wide ranges of material and can be chosen depending on the test section used [19]. Therefore, gamma-ray approaches have played a major role and have become the tools of choice in the measurement technology for gas–liquid two-phase systems and gas–liquid–solid three-phase systems [20]. A summary of literature review on the hydrodynamics studies of upflow packed bed is given elsewhere[21,22]. However, to the best of our knowledge, there are no studies reported in the literature that have studied liquid holdup in two-phase upflow moving beds with a conical bottom. Attenuation of the gamma radiation is mostly due to the presence of the liquid and solid compared to gas in the flow. Thus, information on the two-phase (liquid – gas) distribution can be obtained for flow over a fixed bed of catalyst, as the attenuation due to the catalyst will be fixed and the variation of attenuation is due to the flowing liquid. Hence, the measurement technique using gamma-ray densitometry leads to the measurement of the line averaged liquid holdup [20,23]. Accordingly, in this work, a comprehensive method has been developed using gamma-ray densitometry to measure and investigate the diameter profile of the line average solid, total external liquid holdup (dynamic liquid plus external static liquid), and internal static liquid holdup inside porous catalyst. For the cold flow laboratory packed bed used in this study, the column was packed randomly with commercial spherical catalyst. The flow conditions were scaled down from the industrial operation conditions as typical lab-scale hydroprocessing units. Owing to the distinct advantages of GRD, we employed it for the first time to measure the line average liquid holdup in a co-current gas-liquid upflow moving packed bed reactor operated under flow rates matching the operating conditions.

In this section, the line average diameter distribution of the parameters like solids holdup and the internal liquid holdup inside the catalyst are presented. All these parameters are fixed for a packed bed unit irrespective of the operating conditions. The methods used to obtain these parameters have been discussed in the previous section of the principle of measurements.

The current study investigated the effect of varying superficial gas velocity at a constant superficial liquid velocity on the line average external liquid holdup in a co-current two phase upflow moving packed bed reactor using Gamma ray densitometry (GRD). The line average internal porosity of the catalyst particles, and line average external bed porosity have been measured also. The experiments were conducted in upflow moving packed bed operated with an air-water system. The moving packed bed was packed randomly with 3 mm extrudate porous particles. The liquid holdup was calculated based on the new methodology developed using Beer – lambert’s equation. It has been found that the liquid holdup decreased as the superficial gas velocity increased at all axial and radial locations. The rate of decrease in liquid holdup at all location except at walls is higher after 3.8 cm/sec and it is due to transition from bubbly to pulse flow regime at this superficial gas velocity. The same trend was observed for reported studies on upflow packed bed reactors. The external liquid holdup is higher for the bottom part than middle section of the packed bed for the same range of superficial gas velocities. At lower flow rate the liquid holdup distribution is quite uniform at both axial locations, but at higher flow rate the middle sections shows better liquid flow distributions. The result shows that gamma ray densitometry can indicate and measure the online liquid holdup, and it’s a reliable method for measuring the holdup inside packed beds with a thick wall. The comparison with available correlation on upflow packed bed showed similar trend but large absolute deviation. This necessitates further studies to develop predictable correlations for this kind of system.