Bubble column reactors flows

This gas-liquid modeling approach has been used performing dynamic simulations of two-phase bubble column reactor flows operating at low gas holdups [201, 202, 19]. A major limitation revealed in these simulations is that there is some difficulties in conserving mass for the dispersed phases, so this concept is not recommended for the purpose of simulating chemically reactive flows. 

In a recent study Jakobsen et al [66] examined the capabilities and limitations of a dynamic 2D axi-symmetric two-fluid model for simulating cylindrical bubble column reactor flows. In their in-house code all the relevant force terms consisting of the steady drag, bulk lift, added mass, turbulence dispersion and wall lift were considered. Sensitivity studies disregarding one of the secondary forces like lift, added mass and turbulent dispersion at the time in otherwise... 

In a recent study Jakobsen et al. [71] examined the capabilities and limitations of a dynamic 2D axi-symmetric two-fluid model for simulating cylindrical bubble column reactor flows. In their in-house code all the relevant force terms consisting of the steady drag, bulk lift, added mass, turbulence dispersion and wall lift were considered. Sensitivity studies disregarding one of the secondary forces like lift, added mass and turbulent dispersion at the time in otherwise equivalent simulations were performed. Additional simulations were run with three different turbulence closures for the liquid phase, and no shear stress terms for the gas phase. A standard k — e model [95] was used to examine the effect of shear induced turbulence, case (a). In an alternative case (b), both shear- and bubble induced turbulence were accounted for by linearly superposing the turbulent viscosities obtained from the A — e model and the model of Sato and Sekoguchi [138]. A third approach, case (c), is similar to case (b) in that both shear and bubble induce turbulence contributions are considered. However, in this model formulation, case (c), the bubble induced turbulence contribution was included through an extra source term in the turbulence model equations [64, 67, 71]. The relevant theory is summarized in Sect. 8.4.4. 

Bubble columns in series have been used to establish the same effective mix of plug-flow and back-mixing behavior required for Hquid-phase oxidation of cyclohexane, as obtained with staged reactors in series. WeU-mixed behavior has been established with both Hquid and air recycle. The choice of one bubble column reactor was motivated by the need to minimize sticky by-products that accumulated on the walls (93). Here, high air rate also increased conversion by eliminating reaction water from the reactor, thus illustrating that the choice of a reactor system need not always be based on compromise, and solutions to production and maintenance problems are complementary. Unlike the Hquid in most bubble columns, Hquid in this reactor was intentionally weU mixed. 

This study investigates the hydrodynamic behaviour of an aimular bubble column reactor with continuous liquid and gas flow using an Eulerian-Eulerian computational fluid dynamics approach. The residence time distribution is completed using a numerical scalar technique which compares favourably to the corresponding experimental data. It is shown that liquid mixing performance and residence time are strong functions of flowrate and direction. 

Many industrial processes which employ bubble column reactors (BCRs) operate on a continuous liquid flow basis. As a result these BCR s are a substantially more complicated than stationary flow systems. The design and operation of these systems is largely proprietary and there is, indeed a strong reliance upon scale up strategies [1]. With the implementation of Computational Fluid Dynamics (CFD), the associated complex flow phenomena may be anal)rzed to obtain a more comprehensive basis for reactor analysis and optimization. This study has examined the hydrodynamic characteristics of an annular 2-phase (liquid-gas) bubble column reactor operating co-and coimter-current (with respect to the gas flow) continuous modes. 

An investigation into the applicability of numerical residence time distribution was carried out on a pilot-scale annular bubble column reactor. Validation of the results was determined experimentally with a good degree of correlation. The liquid phase showed to be heavily dependent on the liquid flow, as expected, but also with the direction of travel. Significantly larger man residence times were observed in the cocurrent flow mode, with the counter-current mode exhibiting more chaimeling within the system, which appears to be contributed to by the gas phase. 

In a properly operated bubble-column reactor, the liquid phase can be considered to be perfectly mixed, i.e. concentrations in the liquid are the same everywhere and correspond to those in the effluent. The gas is supposed to flow like a piston, i.e. the reactor is a plug-flow reactor with respect to the gas. These two assumptions are not entirely true, but within a certain flow regime they are not far from the reality. 

Runaway criteria developed for plug-flow tubular reactors, which are mathematically isomorphic with batch reactors with a constant coolant temperature, are also included in the tables. They can be considered conservative criteria for batch reactors, which can be operated safer due to manipulation of the coolant temperature. Balakotaiah et al. (1995) showed that in practice safe and runaway regions overlap for the three types of reactors for homogeneous reactions (1) batch reactor (BR), and, equivalently, plug-flow reactor (PFR), (2) CSTR, and (3) continuously operated bubble column reactor (BCR). 

Toseland, B. A., Brown, D. M., Zou, B. S., and Dudukovi..M., Flow Patterns in a Slurry-Bubble-Column Reactor Conditions, Trans. Inst. Chem. Engrs., 73 297 (1995)... 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

In a bubble-column reactor for a gas-liquid reaction, Figure 24.1(e), gas enters the bottom of the vessel, is dispersed as bubbles, and flows upward, countercurrent to the flow of liquid. We assume the gas bubbles are in PF and the liquid is in BMF, although nonideal flow models (Chapter 19) may be used as required. The fluids are not mechanically agitated. The design of the reactor for a specified performance requires, among other things, determination of the height and diameter. 

Repeat Example 24-2 for the xylene (B) oxidation reaction carried out in an agitated tank reactor (instead of a bubble-column reactor). Use the data given in Example 24-2 as required, but assume the diameter D is unknown. Additional data are the power input without any gas flow is 8.5 kW the impeller rotates at 2.5 Hz the height and diameter of the tank are the same (h = D) the impeller diameter is DI3, and the impeller contains 6 blades assume ubr = 1.25usg. In addition to the vessel dimensions for the conversion specified (/B = 0.16), determine the power input to the agitator (P,). 

Example 12-2 An aqueous solution contains 10 ppm by weight of an organic contaminant af molecular weight 120, which must be removed by air oxidation in a lo-cm-diameter bubble column reactor at 25°C. The liquid flows downward in the tube at an average velocity af 1 cm/sec. The air at 1 atm is admitted at 0.1 liter/sec and is injected as bubbles 1 mm diameter, which rise at 2 cm/sec. Assume no coalescence or breakup and that both gas and liquid are in plug flow. The reaction in the Hquid phase has the stoichiometry A + 2O2 products with a rate C. ... 

Slurry Bubble Column Reactors (SBCR) This reactor is tubular (Figure 3.12). The liquid is agitated by means of dispersed gas bubbles. Gas bubbles provide the momentum to suspend the catalyst particles. The gas phase flows upward through the reactor at a constant rate. This reactor could be of continuous type or of semibatch type. This type is used only in catalysis. 

In the common case, in slurry bubble column reactors, the catalyst phase remains in the reactor while the liquid phase could remain in the reactor with a continuous flow of gas (semibatch operation). Both gas and liquid could be in plug flow or could be well mixed. 

Flow regimes in slurry bubble column reactors... 

The hydrodynamics of bubble columns and slurry bubble column reactors depend strongly on the flow regime (Figure 3.27). There are three flow patterns that prevail in these reactors (Wallis, 1969 Shall et al., 1982) ... 

For the heterogeneous flow regime, the Akita-Yoshida correlation derived for bubble column reactors is proposed (Akita and Yoshida, 1973 Ramachandran and Chaudhari, 1984 Behkish, 2004 Koide, 1996) ... 

Well-Mixed Bubble Column Reactors Gas-Liquid Flow Patterns and Mass Transfer... 

Although the most realistic model for a bubble column reactor is that of dispersed plug-flow in both phases, this is also the most complicated model in view of the uncertainty of some of the quantities involved, such a degree of complication may not be warranted. Because the residence time of the liquid phase in the column... 

Svendsen, H. F., et al., Local flow structures in internal-loop and bubble column reactors, Chem. Eng. Sci., 47, 3297, 1992. 

Slurry Bubble Column Reactors As in the case of gas-liquid slurry agitated reactors, bubble column reactors may also be used when solids are present. Most issues associated with multiphase bubble columns are analogous to the gas-liquid bubble columns. In addition, the gas flow and/or the liquid flow have to be sufficient to maintain the solid phase suspended. In the case of a bubble column fermenter, the sparged oxygen is partly used to grow biomass that serves as the catalyst in the system. Many bubble columns operate in semibatch mode with gas sparged continuously and liquid and catalyst in batch mode. 

As part of the work undertaken by APCI under contract to the DOE, to develop a slurry phase Fischer-Tropsch process to produce selectively transportation fuels, a study of the hydrodynamics of three phase bubble column reactors was begun using cold flow modelling techniques (l ). Part of this study includes the measurement of solid concentration profiles over a range of independent column operating values. 

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