Slurry column

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Slurry column (bubbles rise through L containing fine suspended solids). 

Slurry column oxidation. Instead of using a trickle bed reactor for ethanol oxidation (see previous problem), let us consider using a slurry reactor. For this type of unit... 

In the third section an extensive writing on two types of slurry catalytic reactors is proposed Bubble Slurry Column Reactors (BSCR) and Mechanically Stirred Slurry Reactors (MSSR). All the variables relevant in the design and for the scale-up and the scale-down of slurry catalytic reactors are discussed particularly from the point of view of hydrodynamics and mass transfer. Two examples of application are included at the end of the section. 

Bubble slurry column reactors (BSCR) and mechanically stirred slurry reactors (MSSR) are particular types of slurry catalytic reactors (Fig. 5.3-1), where the fine particles of solid catalyst are suspended in the liquid phase by a gas dispersed in the form of bubbles or by the agitator. The mixing of the slurry phase (solid and liquid) is also due to the gas flow. BSCR may be operated in batch or continuous modes. In contrast, MSSR are operated batchwise with gas recirculation. 

Design of bubble slurry column reactors (BSCR)... 

Figure 5.3-7. Scheme of the flow regimes in bubble slurry column [6]. 

Most laboratory experiments demonstrating the utility of EO transport of organic compounds were conducted with kaolinite as the model clay-rich soil medium. Shapiro et al. (1989) used EO to transport phenol in kaolinite. Bruell et al. (1992) have shown that TCE can be transported down a slurry column by electroosmotic fluid flow, and more recently, Ho et al. (1995) demonstrated electroosmotic movement of p-nitrophenol in kaolinite. Kaolinite is a pure clay mineral, which has a very low cation exchange capacity and is generally a minor component of the silicate clay mineral fraction present in most natural soils. It is not, therefore, representative of most natural soil types, particularly those which are common in the midwestem United States. The clay content can impact the optimization and effectiveness of electroosmosis in field-scale applications, as has recently been discussed by Chen et al. (1999). 

Hydrodynamics of slurry reactors include the minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large variety in reactor types (sec Fig. I) and the fact that most correlations are of an empirical nature. We will therefore focus on sparged slurry columns and slurries in stirred vessels. 

Plate columns may give higher values for eg than found in bubble slurry columns due to smaller bubbles... 

Figure 5. Axial dispersion in the liquid phase of a slurry column. Comparison of data of Kara et al [70] with correlations.

Provided the particle settling velocities vt are known, this equation allows the calculation of )e,s Usually, experiments at non-zero liquid rates are used to evaluate t , and )e,s separately. A similar concentration profile might occur in practice if slurry column reactors are operated close to the conditions given by the minimum suspension criterium. In this case, reactor calculations should take the solids concentration profiles into account. A recommended correlation for the solids dispersion coefficient for small particles is given by Kato et al. [15] ... 

For k a data in draft tube slurry columns, the paper by Goto et al. [77] probably is the best entry. Zahradnik et al. [9] published some data for a slurry tray column. 

Figure 8. Pressure effect on the volumetric mass transfer coefficient in a sparged slurry column (from Dewes et al. [51]).

Cybulski et al. [39] have studied the performance of a commercial-scale monolith reactor for liquid-phase methanol synthesis by computer simulations. The authors developed a mathematical model of the monolith reactor and investigated the influence of several design parameters for the actual process. Optimal process conditions were derived for the three-phase methanol synthesis. The optimum catalyst thickness for the monolith was found to be of the same order as the particle size for negligible intraparticle diffusion (50-75 p.m). Recirculation of the solvent with decompression was shown to result in higher CO conversion. It was concluded that the performance of a monolith reactor is fully commensurable with slurry columns, autoclaves, and trickle-bed reactors. 

During the liquid phase FT program completed at the demonstration scale slurry column at the La Porte Alternative Fuels Development unit it was found that the system was essentially isothermal, with a temperature difference smaller than 2°C along the height of the reactor. ExxonMobil has reported that their AGC-21 FT reactor can be operated with an essentially flat temperature profile. All these publications confirmed the earlier results observed at the Rheinpreussen-Koppers demonstration scale plant the temperature gradient in the (slurry FT) reactor never amounted to more than 1°C . 

Heat transfer coefficients in two-phase and three-phase (i.e. slurry) column reactors were recently reviewed by Deckwer (82) and Deckwer et al.(83). The available data can be excellently described on the basis of a theoretical model which gives (82)... 

Kressin and Waterbury used a "slurry-column" technique for the rapid separation of Pu from other ions. The 7 M HNOg solution of the ions is slurried with about half of the resin to adsorb the bulk of the Pu before placing in a column containing the other half of the resin. The solution can then be run through the column at a more rapid rate without Pu breakthrough, because most of the Pu is already adsorbed. These authors used a low cross-linked resin (Dowex 1 X 2) to speed the kinetics of the adsorption reaction. They report greater than 99.9% recovery of the Pu by this technique when mixed with substantial quantities of over 40 elements. The Pu is desorbed with an HCl-HF mixture, again to speed the elution. 

In WAO with solid catalysts, three-phase reactors are used trickle bed, bubble slurry column, and bubble fixed-bed (monolith) or three-phase fluidized-bed reactors. When the catalyst is present in the liquid phase (homogeneous) or absent, two-phase reaetors such as bubble columns, jet-agitated reactors, and mechanically stirred reactor vessels are used. The limitations and advantages of these reactors for the application to WAO are listed in Table 10.7. 

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