Three-phase slurry reactors bubble columns

No systematic research has been done yet on the influence of pressure and gas density on the holdup in three-phase slurry reactors. The only data [45-51] for three-phase bubble columns under pressure suggest rather high values of eq under pressure. Qualitatively, this is in line with the effect of a high density as predicted by Wilkinson [44], The effect might decrease with increased solids concentration [52]. Clearly, additional research is necessary here. 

Three-phase reactions comprise gas-liquid-solid and gas-liquid-liquid reactions. Gas-liquid reactions using solid catalysts represent a very important class of reactions. Conventionally, they are carried out in slurry reactors, (bubble columns, stirred tanks), fluidized beds, fixed bed reactors (trickle beds with cocurrent downflow or cocurrent upflow, segmented bed, and countercurrent gas-liquid arrangements) and structured (catalytic wall) reactors. 

Figure 6.1 Schematic diagrams of industrial three-phase slurry reactors, (a) Slurry bubble column, (b) Three-phase fluidized bed. (c) Three-phase agitated vessel...

Table 6.4 shows the macroscopic attributes that may be achieved in these three-phase slurry reactors from a mechanical and configurational standpoint. For instance, in stirred slurry reactors, the action of the stirrer causes chopping of the bubbles emanating from the distributor, and hence the steady-state bubble size achieved in the reactor is largely determined by the breakage caused by the stirrer action. In slurry bubble columns and three-phase fluidized beds, however, fine bubbles emanate from the distributor, and as they rise, coalescence dominates and the bubbles increase in size (causing reduction in interfacial area... 

Let us consider the mass balance of two kinds of three-phase reactors bubble columns and tube reactors with a plug flow for the gas and the liquid phases, and stirred tank reactors with complete backmixing. Modeling concepts can be implemented in most existing reactors backmixing is typical for slurry reactors, bubble columns, and stirred tank reactors, whereas plug flow models describe the conditions in a trickle bed reactor. The interface between the gas and the liquid is supposed to be surroimded by gas and liquid films. Around the catalyst particles, there also exists a liquid film. In gas and liquid films, physical diffusion, but no chemical reactions, is assumed to take place. A volume element is illustrated in Figure 6.15. 

In connection with the engineering content of the book, a large number of reactors is analyzed two- and three-phase (slurry) agitated reactors (batch and continuous flow), two-and three-phase fixed beds (fixed beds, trickle beds, and packed bubble beds), three-phase (slurry) bubble columns, and two-phase fluidized beds. All these reactors are applicable to catalysis two-phase fixed and fluidized beds and agitated tank reactors concern adsorption and ion exchange as well. 

Additional information on hydrodynamics of bubble columns and slurry bubble columns can be obtained from Deckwer (Bubble Column Reactors, Wiley, 1992), Nigam and Schumpe (Three-Phase Sparged Reactors, Gordon and Breach, 1996), Ramachandran and Chaudhari (Three-Phase Catalytic Reactors, Gordon and Breach, 1983), and Gianetto and Silveston (Multiphase Chemical Reactors, Hemisphere, 1986). Computational fluid mechanics approaches have also been recently used to estimate mixing and mass-transfer parameters [e.g., see Gupta et al., Chem. Eng. Sci. 56(3) 1117-1125 (2001)]. 

The general difficulties in design and scale-up of bubble column reactors concern reaction specific data, such as solubilities and kinetic parameters as well as hydrodynamic properties. The paper critically reviews correlations and new results which are applicable in estimation of hydrodynamic parameters of two-phase and three-phase (slurry) bubble column reactors. 

Gas-liquid-solids reactors Stirred slurry reactors, three-phase fluidized bed reactors (bubble column slurry reactors), packed bubble column reactors, trickle bed reactors, loop reactors. 

A related reactor is that for coal liquefaction, which can be carried out in a three-phase slurry bubble column (see Fig. 5). Hydrogen can be supplied at the bottom of a column of downcoming product—oil. The solid coal reactant is blended with the product or carrier oil and fed at the top. The generic process depicted in Fig. 5 is a generalization of the liquefaction reactor in the Exxon Donor Solvent Process. As the gas flow rate increases, the bubbles change from uniformly small to chaotic. In the H-coal process, both the gas and a coal-oil slurry are fed from the bottom in an ebullating-bed reactor. Catalyst solids are fed from the top. This reactor operates as an expanded... 

Gamwo, I. K. Gidaspow, D. Computational and Experimental Modeling of Three-phase Slurry Bubble Column Reactor, Annual Report to DOE Illinois Institute of Technology, 1999. 

The three phase slurry-bubble column-reactors used in the DCL process have been modeled by using an axial dispersion model. 

FIGURE 3.2 (Cont d) (b) Slurry reactor or bubble column with a draft tube cum heat exchanger, (c) Airlift (external downcomer type) slurry or three-phase sparged reactor. (Reprinted from de Deugd et al. (2003) with kind permission from Springer Science + Business Media. 2003.)... 

The concept of three-phase sparged reactor or bubble column with a draft mbe (BCDT) can be advantageously applied in this oxidation process. Section 10.9 presents a detailed discussion of various aspects of BCDT. The BCDT (Fig. 3.2b) is a simple variadon of the conventional slurry or three-phase sparged reactor. The major conclusions that can be drawn with respect to the present application are as follows (i) the overall gas holdup in a BCDT is approximately the same as that in a conventional bubble column. Further, the gas holdup is independent of sohd loading, (ii) There is a well-directed Uquid circulation—upward in the draft tube (riser) and downward in the... 

Aspect Micro slurry reactor Jet loop reactor Stirred slurry reactor Slurry bubble column reactor Three-phase fluidized reactor Packed bubble column reactor Trickle bed reactor... 

Three-phase slurry bubble columns, in which the fine catalyst particles are suspended in the liquid and a gas is sparged into the vessel. The gas rises due to the buoyancy difference between the gas and the liquid-solid slurry, driving a circulation. These reactors usually exhibit vigorous mixing pattern driven by cross-sectional variation in the gas volume fraction. Typical schematic of a three-phase slurry bubble column is shown in Figure 6.1a. Particle sizes are typically less than 500 pm. 

Matos EM, Guirardello R, Mori M, Nunhez JR. Modeling and simulation of a pseudo-three-phase slurry bubble column reactor applied to the process of petroleum hydrodesulfurization. CompuL Chem. Eng. 2009 33 1115-1122. 

Al-Dahhan MH, Fan L-S, Dudukovic MP, Toseland B. Advanced Diagnostic Techniques for Three-phase Slurry Bubble Column Reactors (SBCR). Final Technical Report, DE-FG-26-99FT40594 August 2003. Washington University in St. Louis, St. Louis, MO. 

The development of three-phase reactor technologies in the 1970 s saw renewed interest in the synthetic fuel area due to the energy crisis of 1973. Several processes were developed for direct coal liquefaction using both slurry bubble column reactors (Exxon Donor Solvent process and Solvent Refined Coal process) and three-phase fluidized bed reactors (H-Coal process). These processes were again shelved in the early 1980 s due to the low price of petroleum crudes. 

Note that three-phase bubble columns and agitated tank reactors are sometimes referred to as slurry reactors. 

The alkylated anthraquinone process accounts for over 95% of the world production of H202, mainly because the it operates under mild conditions and direct contact of 02 and H2 is avoided. In this process, 2-alkylanthraquinone (the alkyl group is typically an ethyl, terf-butyl or amyl group) is dissolved in a mixture of a non-polar solvent (C9-Cn alkylbenzene) and a polar solvent [Trioctyl phosphate (TOP), or tetrabutyl urea (TBU) or diisobutyl carbinol (DIBC)] and then hydrogenated over a precious metal (Pd or Ni) catalyst in a three-phase reactor (trickle bed or slurry bubble column) under mild reaction conditions (<5bar, <80 °C) to generate 2-alkylanthrahydroquinone [1-3, 5], The latter is then auto-oxidized with air in a... 

Gas-liquid bubble columns and gas-liquid-solid slurry bubble columns are widely used in the chemical and petrochemical industries for processes such as methanol synthesis, coal liquefaction, Fischer-Tropsch synthesis and separation methods such as solvent extraction and particle/gas flotation. The hydrodynamic behavior of gas-liquid bubble columns and gas-liquid-solid slurry bubble columns are of great importance for the design and scale-up of reactors. Although the hydrodynamics of the bubble and slurry bubble columns has been a subject of intensive research through experiments and computations, the flow structure quantification of complex multi-phase flows are still not well understood, especially in the three-dimensional region. In bubble and slurry bubble columns, the presence of gas bubbles plays an important role to induce appreciable liquid/solids mixing as well as mass transfer. The flows within these systems are divided into two... 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

In the design of upflow, three phase bubble column reactors, it is important that the catalyst remains well distributed throughout the bed, or reactor space time yields will suffer. The solid concentration profiles of 2.5, 50 and 100 ym silica and iron oxide particles in water and organic solutions were measured in a 12.7 cm ID bubble column to determine what conditions gave satisfactory solids suspension. These results were compared against the theoretical mean solid settling velocity and the sedimentation diffusion models. Discrepancies between the data and models are discussed. The implications for the design of the reactors for the slurry phase Fischer-Tropsch synthesis are reviewed. 

An important aspect of the design of three phase bubble columns is the variation of catalyst distribution along the reactor height, and its effect on reactor performance. Many factors influence the degree of catalyst distribution, including gas velocity, liquid velocity, solid particle size, phase densities, slurry viscosity, and, to a lesser extent, column diameter, solid shape and chemical affinity between the solid and liquid phases. 

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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