Hydrodynamics of bubble column reactors operating with non-Newtonian liquids

摘要

Processes based on the contact between gas and liquid/slurry phases are commercially used for performing a variety of chemical reactions. Although different types of reactors are used for this purpose, bubble column reactors have received more attention during the past decade since they offer some unique advantages, such as ease of operation, high rates of heat and mass transfer, and lower maintenance costs due to the absence of moving parts. The design and scale-up of a bubble column reactor require a complete understanding of its complex hydrodynamics, which is influenced by the physical properties of the phases, the operating variables, and the design parameters. Current design procedures for bubble columns involve several steps of pilot-plant experimentation using equipment of different scales, which is expensive and time consuming.;In recent years, the liquid and/or slurry phases which are processed in bubble columns in many applications are viscous and normally demonstrate non-Newtonian behaviors during the process operation. Hydroconversion of heavy oil and petroleum residues, wastewater treatment, processing of fermentation broths, polymer composite processing, and slurry-phase synthesis are some of those processes in which viscous and non-Newtonian liquids are often encountered in bubble column reactors. On the other hand, in bubble columns operating with non-Newtonian liquids, the viscosity changes upon the flow conditions, and also a variety of non-Newtonian liquids possess elastic properties that can affect and alter bubble behavior to a great extent. Although there has been an increasing application of non-Newtonian fluids in bubble column reactors, our present understanding of the effects of non-Newtonian properties on different hydrodynamic aspects of bubble columns is far from complete. Only few studies are reported on the effect of liquid phase rheological properties in bubble columns so that the influence of liquid elasticity on the hydrodynamic parameters such as gas holdup and bubble properties has never been studied distinctly, and the models and concepts currently available on this subject are insufficient for chemical practice. To gain adequate insight into the performance of bubble columns operating with non-Newtonian liquids, the effects of all rheological properties of the liquid phase need to be investigated rather that the effect of a single parameter like viscosity. This thesis is, therefore, dedicated to investigating the hydrodynamics of bubble columns operating with non-Newtonian liquids having different rheological properties.;The operation principle and basic hydrodynamic aspects of the bubble column reactors, as well as non-Newtonian liquids and their rheological properties, are briefly discussed in the first two chapters. The first objective of this work is to understand the effect of the rheological properties of liquid on different hydrodynamic aspects of a bubble column reactor including gas holdup and its radial and axial distributions, bubble size and its axial distribution, standard deviation, power spectral density and average frequency of pressure signals. In this regard, the effect of liquid phase rheology on the hydrodynamics of a pilot-scale bubble column reactor is extensively investigated by strategically selecting various types of liquids. The selected liquids include water as a reference and low-viscosity liquid, an aqueous glucose solution as a highly viscous Newtonian and inelastic liquid, a Boger fluid which has a constant viscosity identical to the glucose solution but it is slightly elastic, and finally two non-Newtonian (shear-thinning) and elastic Carboxymethyl cellulose (CMC) and Xanthan gum solutions. Gas holdup and its radial and axial variations, the operating flow regime transition and bubble size are evaluated by means of two in-house made optical fiber probes and several pressure transducers. Different time-domain and frequency-domain analyses are applied to the pressure fluctuation signals in order to better understand the effect of liquid phase rheology on the gas holdup and bubble size. The simultaneous viscous and elastic effects of non-Newtonian liquids are studied by proposing a new approach based on the dynamic moduli of viscoelastic solutions. It was found that the viscosity of liquid is more favorable for bubble coalescence; however, the elasticity can hinder bubble coalescence as it can demonstrate a solid-like behavior at the interface of two bubbles. The presence of elasticity in the liquid was shown to reduce the average bubble chord length and increase the overall gas holdup. The results obtained in this part of the work are essential for achieving the second objective, which is aimed at studying the local hydrodynamic parameters such as local bubble frequency and bubble rise velocity and developing new correlations to estimate bubble size and gas holdup in bubble column reactors operating with non-Newtonian liquids. Therefore, in the second part of this work, local bubble properties such as bubble frequency, bubble chord length, and bubble rise velocity, as well as their radial and axial distributions, are evaluated by installing two optical fiber probes at various locations within a bubble column reactor operating with different non-Newtonian liquids. It was observed that the radial profiles of bubble frequency, bubble chord length and bubble rise velocity are relatively flat at low superficial gas velocities, while they become parabolic as the superficial gas velocity increases. Moreover, by applying the dimensional analysis, two new correlations are developed to predict the bubble size and gas holdup in bubble columns operating with non-Newtonian liquids. The two correlations are developed by taking into consideration the ratio between the dynamic moduli of viscoelastic solutions and are capable of accurately predicting both bubble size and gas holdup.;Moreover, a variety of commercial processes such as Fischer-Tropsch synthesis, Methanol synthesis, Partial oxidation of ethylene, Residuum hydrotreating, and Hydroformylation are carried out in bubble columns at elevated pressures. The operating pressure is found to have a significant effect on the hydrodynamic characteristics of bubble columns such as bubble properties and gas holdup. For instance, an increase in the operating pressure normally results in the formation of smaller bubbles at the gas distributor and this is mainly due to the higher gas density at elevated pressure. Although investigating the pressure effects in the bubble columns has been the subject of some research, there is still a strong need toward more studies on the influence of operating pressure on different hydrodynamic aspects, and, accordingly, on the performance of bubble column reactors. Therefore, the last objective of this work is devoted to investigating the effect of operating pressure on the hydrodynamics of bubble column reactors in presence of non-Newtonian liquids. For this purpose, a high-pressure/high-temperature multiphase reactors unit including a bubble column reactor with an inner diameter of 0.152 m and a total height of 4.8 m has been designed and constructed to perform experiments at elevated pressures. The multiphase reactors unit was equipped with different equipment, including air compressors, high-pressure gas storage cylinders, gas heating elements, liquid supply tank, liquid centrifugal pump, gas-liquid separators, PLC control unit, etc. This experimental unit is introduced in more detail in Chapter 5. Various hydrodynamic characteristics of bubble column reactors, such as the total gas holdup and its axial distribution, operating flow regime transition point, pressure fluctuation and its standard deviation have been studied by means of pressure signal measurements with several differential and dynamic pressure traducers. The superficial gas velocity varied from 1 to 35 (cm s -1), covering both homogeneous and heterogeneous flow regimes. Operating pressure also changed from 0.1 to 1 (MPa) during the experiments. The total gas holdup was found to increase with both operating pressure and the elasticity of liquid phase, and the effect of pressure was shown to be more pronounced at lower operating pressures. The operating pressure was shown to shift the flow regime transition point to higher superficial gas velocities. A new correlation was also derived for predicting the gas holdup in bubble column reactors operating at elevated pressure. As a conclusion, both the rheology of the liquid phase and operating pressure are shown to have important effects on the hydrodynamics of bubble column reactors. Moreover, the scientific findings of the present work may have significant implications for the more accurate design, operation and scale-up of commercial bubble column reactors, where highly viscous and non-Newtonian liquids and high pressures are often applied.