Bubble column reactor
[1] A bubble column reactor is a very simple device consisting of a vertical vessel filled with water with a gas distributor at the inlet.
[3] In recent years, Computational Fluid Dynamics (CFD) has become a very popular tool to design and optimize bubble column reactors.
[1] The gas is supplied in the form of bubbles to either a liquid phase or a liquid-solid suspension.
In this case, the solid particle size (typically a catalyst) ranges from 5 to 100 μm.
[4] The liquid flow rate may be fed co-currently or counter-currently to the rising bubbles, or it may be zero.
[1] Bubble columns offer a significant number of advantages: excellent heat and mass transfer between the phases, low operating and maintenance costs due to the absence of moving parts, solids can be handled without any erosion or plugging problems, slow reactions can be carried out due to the high liquid residence time (this is the case for gas-liquid reactions with a Hatta number Ha <0.3), reasonable control of temperature when strongly exothermic reactions take place.
The reactor may be equipped with internals, baffles, or sieve plates, to overcome the back-mixing problem with an inevitable modification in the fluid dynamics.
[2] n-paraffins, cyclohexane aromatic hydrocarbons propionic acid Due to the increasing importance of bubble column reactors in most industrial sectors, the study of their hydrodynamics acquired significant relevance in recent years.
The design of bubble columns depends on the quantification of three main phenomena: (1) mixing characteristics, (2) heat and mass transfer properties, (3) chemical kinetics in case of reactants systems.
The fluid dynamics properties in bubble columns depend on the interaction between the gas and liquid phases, which are related to the prevailing flow regime.
[9] Although the superficial velocity concept is based on a simple one-dimensional flow assumption, it can be used to characterize and determine the hydrodynamics in bubble columns since an increase in its value can determine a flow regime transition.
Where: The gas holdup provides information about the mean residence time of bubbles inside the column.
Combined with bubble dimensions ( a fundamental local flow property), it determines the interfacial area for the heat and mass transfer rate between the phases.
The flow regime can vary significantly depending on several factors, including gas and liquid flow rates, geometric aspects of the column (column diameter, column height, sparger type, sparger holes diameter, and eventually, the size of the solid particles) and physical properties of the phases.
The former is characterized by a mono-dispersed bubble size distribution, the latter by a poly-dispersed one, according to the change in sign of the lift force.
The latter is characterized by a central core of gas surrounded by a thin liquid film.
When dealing with industrial applications, larger-diameter bubble columns are typically employed so that the slug flow regime is not usually observed due to the so-called Rayleight-Taylor instabilities.
[14][15] The quantification of these instabilities at the reactor-scale is obtained by comparing the dimensionless bubble diameter,
For example, at ambient temperature and pressure and considering air and water as working fluids, a bubble column is classified as a large-diameter if it has a hydraulic diameter greater than 0.15 m.[3] Due to the very high gas velocity, the annular flow regime is not usually observed in industrial bubble columns.
The recent increase in interest in Computational Fluid Dynamics (CFD) spurred substantial research efforts in determining numerical models that can obtain reasonably accurate predictions with limited computational time, thus overcoming the limitations of traditional empirical methods.
[16] The Eulerian-Lagrangian model couples the Eulerian description of the continuous phase with a Lagrangian scheme for tracking the individual particulates.
[8] Considering an isothermal flow without mass transfer, the Unsteady Reynolds Average Navier-Stokes equations (URANS) are:[8]
The drag force has a dominant role and can be considered as the most important contribution in bubbly flows.
[18] It reflects the resistance opposing bubble motion relative to the surrounding fluid.
Depending on the regime under investigation, different approaches can be used to model the dispersed gas phase.
This approximation is suitable to simulate the homogeneous flow regime, where the interactions between the bubbles are negligible.
In addition, this approach calls for the knowledge of the bubbles diameter since it is an input parameter for the simulations.
[8] However, in industrial practice, large-scale bubble columns are typically employed, equipped with gas distributors characterized by large openings, so a heterogeneous flow regime is commonly observed.
A Population Balance Model consists of a transport equation derived from the Boltzmann statistical transport equation, and it describes the particles entering or leaving a control volume via several mechanisms.
The right and side term of the Population Balance Equation is the source/sink term due to bubbles coalescence, breakup, phase change, pressure change, mass transfer, and chemical reactions.
