Three-phase fluidized bed reactors

Chapter 22 G/L Reactions on Solid Catalyst Trickle Beds, Slurry Reactors, Three Phase Fluidized Beds /500... 

Trickle bed reactors Slurry reactors Three-phase fluidized beds No Little Little Modeling on basis of unit cell approach + development of correspondence rules for macroscopic system behavior Modeling of the effect of the solids phase on interfacial transport phenomena Modeling of the effect of the solids phase on interfacial transport phenomena -I- development of refined models for particle-particle and particle-wall interaction... 

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. 

Stirred slurry reactor Slurry bubble column reactor Three-phase fluidized bed reactor... 

Three-phase fluidized bed reactors are used for the treatment of heavy petroleum fractions at 350 to 600°C (662 to 1,112°F) and 200 atm (2,940 psi). A biological treatment process (Dorr-Oliver Hy-Flo) employs a vertical column filled with sand on which bacderial growth takes place while waste liquid and air are charged. A large interfacial area for reaction is provided, about 33 cmVcm (84 inVirr), so that an 85 to 90 percent BOD removal in 15 min is claimed compared with 6 to 8 h in conventional units. 

Hydrodynamics, Heat and Mass Transfer in Inverse and Circulating Three-Phase Fluidized-Bed Reactors for WasteWater Treatment... 

Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The three-phase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gas-liquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. 

To provide the pr equisite knowledge for designing the three-phase fluidized-bed reactors with new modes, the hydrodynamics such as phase holdup, mixing and bubble properties and heat and mass transfer characteristics in the reactors have to be determined. Thus, in this study, the hydrodynamics and heat and mass transfer characteristics in the inverse and circulating three-phase fluidized-bed reactors for wastewater treatment in the present and previous studies have been summarized. Correlations for the hydrod3aiamics as well as mass and heat transfer coefficients are proposed. The areas wherein future research should be undertaken to improve... 

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. 

New applications and novel reactor configurations or operational modes for three-phase systems are continually being reported. These include the operation of a three-phase fluidized bed in a circulatory mode (Liang et al., 1995), similar to the commonly applied gas-solid circulating fluidized bed the application of a three-phase fluidized bed electrode that can be used as a fuel cell (Tanaka et al., 1990) magnetically stabilized three-phase fluidized beds centrifugal three-phase reactors and airlift reactors. 

There are a wide variety of three-phase fluidized bioreactor designs possible. The conventional reactor, shown in Fig. 9, is fluidized by both gas and liquid entering at the bottom of the reactor and leaving at the top and is the most common type of three-phase fluidized bed bioreactor. This reactor may be configured to operate with little axial liquid mixing or in a well-mixed mode by adding a recycle stream. The airlift reactor or draft tube fluidized bed reactor, Fig. 10, is also frequently used. In this reactor, gas is injected at the bottom of a draft tube placed in the center of the... 

Understanding the effect of reactor diameter on the volumetric mass transfer coefficient is critical to successful scale up. In studies of a three-phase fluidized bed bioreactor using soft polyurethane particles, Karamanev et al. (1992) found that for a classical fluidized bed bioreactor, kxa could either increase or decrease with a change in reactor diameter, depending on solids holdup, but for a draft tube fluidized bed bioreactor, kxa always increased with increased reactor diameter. 

An example of the application of a unified model to the design of a three-phase, fluidized bed bioreactor is the scale down, scale up procedure. A model of the full scale reactor is developed, then is used to design... 

If substrate inhibition exists, a well-mixed bioreactor is desirable. Mixing in three-phase fluidized bed bioreactors can be increased by adding an external recycle loop, by inserting a draft tube in the reactor, or by decreasing the height to diameter ratio. 

Catalytic coal liquefaction processes do not specifically use hydrogen donor solvents although coal is introduced into the liquefaction reactor as a slurry in a recycle liquid stream. Catalyst is used as a powder or as granules such as pellets or extrudates. If powdered catalyst is used, it is mixed with the coal/liquid stream entering the reactor. Pelleted catalyst can be used in fixed bed reactors if precautions are taken to avoid plugging with solids or in fluidized bed reactors. In the latter case, the reacting system is actually a three phase fluidized bed, that is, catalyst particles and coal solids, as well as liquid, are fluidized by gas. 

Reactors with moving solid phase Three-phase fluidized-bed (ebullated-bed) reactor Catalyst particles are fluidized by an upward liquid flow, whereas the gas phase rises in a dispersed bubble regime. A typical application of this reactor is the hydrogenation of residues. 

Leading characteristics of five main kinds of reactors are described following. Stirred tanks, fixed beds, slurries, and three-phase fluidized beds are used. Catalyst particle sizes are a compromise between pressure drop, ease of separation from the fluids, and ease of fluidization. For particles above about 0.04 mm dia, diffusion of liquid into the pores and, consequently, accessibility of the internal surface of the catalyst have a minor effect on the overall conversion rate, so that catalysts with small specific surfaces, of the order of 1 m2/g, are adequate with liquid systems. Except in trickle beds the gas phase is the discontinuous one. Except in some operations of bubble towers, the catalyst remains in the vessel, although minor amounts of catalyst entrainment may occur. 

In slurry reactors, an attempt is made to realize intensive and intimate contact between a gas-phase component, usually to be dissolved in the liquid phase, a liquid-phase component and a finely dispersed solid. In this respect, slurry reactors are related to packed-bed reactors with the various gas/liquid flow regimes that can be realized (such as trickle flow, pulsed flow and dispersed bubble flow). Also, there is much similarity with three-phase fluidized beds. 

The main difference is the particle size. In three-phase fluidized beds these are so large that a net upward liquid flow is necessary to keep the solids in suspension, whereas in slurry reactors the turbulence of the liquid is sufficient to keep the solids suspended particle sizes in slurry reactors are usually below 200 ftm. Particularly for fast reactions where intraparticlc dif-... 

Typical properties of slurry reactors, and of packed bed co-current downflow trickle flow reactors, are summarized in Table 1. Most properties indicated for slurry reactors also hold for three-phase fluidized beds. These properties can be advantageous or disadvantageous, depending on the application ... 

The holdups can play an important role in the reactor performance. For example, in a pilot-scale trickle-bed reactor, the liquid holdup can play an important role in changing the nature of the apparent kinetics of the reaction. When homogeneous and catalytic reactions occur simultaneously, the liquid holdup plays an important role in determining the relative rates of homogeneous and catalytic reactions. In a three-phase fluidized-bed reactor, the holdup of the solid phase plays an important role in the reaction rate, particularly when the solid phase is a reactant. The gas holdup, of course, always plays an important role in reactor performance when a gaseous reactant takes part in the reaction. 

Little is known about the fluid wall heat transfer in the case of gas -liquid flow in a fixed-bed reactor. Some research on this subject, however, has been carried out for the specific case of cocurrent downflow over a fixed-bed reactor. This is summarized in Chap. 6. Some work on the slurry-wall heat-transfer rate for a three-phase fluidized bed has also been reported. The heat-transfer rate is characterized by the convective heat-transfer coefficient between the slurry and the reactor wall. Some correlations for the heat-transfer coefficient in a three-phase slurry reactor are discussed in Chap. 9. 

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