Three-phase bubble column reactor

The Effect of Gas and Liquid Velocities and Solid Size on Solid Suspension in a Three-Phase Bubble Column Reactor... 

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. 

Three phase bubble column reactors are of interest for Fischer-Tropsch synthesis because of their following characteristics ... 

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. 

Schliiter S, Steiff P, Weinspach M. Heat transfer in two- and three-phase bubble column reactors with internals. Chem. Eng. Process. 1995 34 157-172. 

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

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. 

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. 

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. 

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)]. 

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.

Three phase, upflow, bubble column reactors are used in the process industry because of their simplicity and good liquid mixing characteristics, without the need for mechanical agitation. 

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. 

Three phase fluidized bed reactor/slurry reactor FIGURE I l.l Types of bubble column reactors (from Lee and Tsui, 1999). 

Krishna, R., van Eaten, J.M. and Ursenau, M.I. (2000a), Three-phase Eulerian simulations of bubble column reactors operating in the churn-turbulent regime a scale-up strategy, Chem. Eng. Sci., 55, 3275-3286. 

In order to reduce attrition, recirculation of the particles can be achieved by the gas bubbles (gas lift) instead of by mechanical agitation (bubble column, see Fig. 8.2). External liquid recirculation offers the possibility of improving the gas-liquid mass transfer by special liquid ejector types. In the case of gas and liquid upflow a three-phase fluidized bed reactor is produced. Here particles of a few millimeters are needed to retain them in the reactor and allow separation from the gas/liquid phase. 

The fundamental principles of the gas-to-liquid mass transfer were concisely presented. The basic mass transfer mechanisms described in the three major mass transfer models the film theory, the penetration theory, and the surface renewal theory are of help in explaining the mass transport process between the gas phase and the liquid phase. Using these theories, the controlling factors of the mass transfer process can be identified and manipulated to improve the performance of the unit operations utilizing the gas-to-liquid mass transfer process. The relevant unit operations, namely gas absorption column, three-phase fluidized bed reactor, airlift reactor, liquid-gas bubble reactor, and trickled bed reactor were reviewed in this entry. 

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