New technique for simultaneous measurement of the local solid and gas holdup by using optical fiber probes in the slurry bubble column
Introduction
Bubble column reactors are widely used in the various processes in the chemical and petrochemical industries. Based on the process situation and the reaction type, solid particles can be added to the system as a catalyst and this consequently affects on the main characteristics such as the hydrodynamic, heat and mass transfer and the conversion rate of the bubble column. The slurry bubble column reactors (SBCR), as one of the most important types of reactors, are considered to produce liquid fuels from low-demand material in the last decades.
For instance, gasoline and kerosene can be produced from heavy oil by using the hydrogenation process at high pressure and high temperature. Also, these fuels may be produced in the well-known Fischer-Tropsch process by converting syngas to the required products [1], [2]. To do these processes, slurry bubble column reactors are known as an efficient approach [3]. There are lots of studies that focused on the performance of the SBCRs, but most of them are done on the laboratory or pilot scale reactors. So, the importance of a reliable scale-up scheme is undeniable in these processes. In fact, the success in commercialization of the processes extensively depends on the quality of the scale-up [4]. It is clear that a good scale-up is substantially related to the hydrodynamics and the reaction kinetics of the process. Therefore, wide investigations have been done to recognize and enhance the performance of the slurry bubble columns [5], [6], [7]. Due to the complexity of the hydrodynamics of the bubble columns, the scale-up of SBCRs steel needs more precise investigations. Researchers have tried to develop a precise and reliable method to scale-up the bubble column from pilot scale to industrial scale and various procedures are proposed, the review of these works shows that the majority of them are based on the global hydrodynamic parameters of the columns [8]. These kinds of approaches do not consider all phenomena which affect the performance of the column. Consequently, after scale-up, the performance of the SBCR will not correspond to the pilot or lab scale. The local phase distribution can be used to promote the precision of the design and scale-up of the SBCRs. Thus, it is required to measure the local solid, liquid and gas hold up in the slurry systems. Although there are various studies and measurement techniques which used to identify the local gas and liquid holdup inside two-phase bubble columns [9], limited studies focused on the solid phase distribution in the column. However, for the solid-liquid two-phase reactors and mixers, some efforts were done to define the solid concentration distribution.
Few studies have been done to investigate the local solid holdup in three-phase bubble columns [10], [11], [12], [13], [14], [15]. Dziallas et al. [10] used a sensing probe that combines the differential pressure measurement (DPM) with the time domain reflectometry (TDR) or the electrical conductivity measurement (ECM) to estimate the local gas and solid holdup in a bubble column. They showed that the solid and gas holdups are rotationally symmetric. Also, they claimed that the distribution of the solid holdup is independent of the superficial gas velocity if the perforated plate was used as the gas distributor. Warsito et al. [11] and Soong et al. [12] utilized an ultrasonic technique to measure the gas and solid holdup in the slurry bubble column. Moreover, Hidaka et al. [13] used a special bubble column to determine the average solid holdup in each axial position by closing some shutter during the process and calculate the amount of solid in each section. In addition, Jin et al. [14] utilized the electrical resistance tomography (ERT) coupled with the differential pressure transducer to estimate the local solid and gas distribution in the slurry bubble column. They showed that the solid holdup increases with the increase of the superficial gas velocity and then reaches a constant value in the heterogeneous flow regime. Besides, the same method was used by the Razzak et al. [15] to measure the phase holdup in a three-phase fluidized bed reactor. Meanwhile, some researchers tried to measure solid holdup in the SBCs, but their methods were not sufficiently accurate or suitable. For instance, the radial distribution of the solid particles in the SBC cannot be obtained by the Ultrasonic technique. Besides, the ERT method measures the solid holdup indirectly and also is not usable for all kind of materials.
On the other hand, many researchers have tried to measure gas holdup, bubble size, bubble rise velocity in bubble column and slurry bubble column reactors by optical fiber probes (OFP). They have used mono, dual and four tip probes to measure the gas phase characteristics [16], [17], [18], [19].
Manjrekar et al. [16] applied a four-tip OFP to measure the gas holdup and bubble characteristics in a slurry bubble column reactor. They investigated the effect of the solid particle on the hydrodynamics of the gas phase. They used 60-µm aluminum oxide particles and found that the local gas holdup and bubble frequency decrease because of the presence of the solid particles. Gheni et al. [17] studied the effect of L/D on the gas holdup and bubble characteristics in a slurry bubble column by using a 4-tip optical fiber probe. They showed that the gas holdup has a significant increment along the axial direction of the column. Esmaili et al. [18] investigated the influence of liquid rheology on the gas phase characteristics by performing a mono-tip OFP. Effect of viscosity and elasticity on the local bubble size, bubble velocity, and gas holdup were shown by them. Also, a dual tip OFP was utilized by Wang et al. [19] to evaluate the local gas and oil distribution in a water-oil-air flow inside a pipe. In fact, they proved that the probe receives various intensities of light based on the refractive index of oil, water and gas and the output voltage will consequently be different.
In the present study, we tried to measure the local solid and gas phase distribution simultaneously by using a newly developed technique. To do so, an in-house made optical fiber probes are used. The OFP can measure the local phase distribution based on the difference in refractive index of the slurry phase and the gas phase. The results prove that it is possible to measure gas and solid holdup by OFP simultaneously.
Section snippets
Bubble column reactor and the materials
In this study, a plexiglass column with 270 cm height and 29.2 cm diameter was used as the slurry bubble column. A perforated plate with 94 × 1 mm diameter holes (density of 1400 holes/ m2) enters the air, in the form of bubbles, to the column. Two precise rotameters were used to measure and adjust the air flow rate. The superficial gas velocity could be changed from low velocities up to 22 cm/s. So, the column can operate in both homogeneous and heterogeneous flow regime. The slurry phase is
Results and discussion
By using the available measurement techniques which discussed previously, the solid and gas holdups were measured under various experimental conditions. The data were collected for two different solid loadings within various superficial gas velocities. The flow pattern in the bubble column is turbulent and, a non-stationary behavior may be observed. To ensure the accuracy of the obtained results, all experiments were repeated for three times. Moreover, the data of each test were recorded for
Conclusion
In this study, a new method is presented for the simultaneous measuring of solid and gas holdup in the slurry bubble column. Optical fiber probes are calibrated to have the possibility of measuring the solid concentration in addition to the gas holdup measurement. These calibrated OFPs are utilized in various axial positions to measure the local gas holdup and solid concentration. The obtained results showed that the solid concentration in the axial position of the column is not uniform despite
Acknowledgments
The authors are grateful to the TOTAL American Services, Inc. and the Natural Science and Engineering Research Council, Canada (NSERC) for financial support of the project. We also would like to appreciate Mr. El Mahdi Lakhdissi for his valuable assistance in the apparatus preparation.
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