Bubble column reactor solid suspension

For maximum utilization of the solid phase in a bubble column, it is essential that all particles be suspended in the reactor (Ramachandran and Chaudhari, 1984). This means that the gas velocity should be sufficiently high to enable suspension of all solids in the liquid. In slurry bubble column reactors, two suspension states exist ... 

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. 

A Slurry Bubble Column Reactor (SBCR) is a gas-liquid-solid reactor in which the finely divided solid catalyst is suspended in the liquid by the rising gas bubbles. SBCR offers many advantages over fixed-bed type reactors such as 1) improved heat transfer and mass transfer 2) isothermal temperature profile is maintained and 3) relatively low capital and operating cost. Fischer-Tropsch Synthesis (FTS) takes place in a SBCR where the synthesis gas is converted on catalysts suspended as fine particles in a liquid. The synthesis gas flows in a bubble phase through the catalyst/wax suspension. The volatile products are removed with unconverted gases, and the liquid products are separated firom the suspension. A gas distributor located in the bottom of the reactor produces the bubbles in the reactor. 

Bubble columns in which gas is bubbled through suspensions of solid particles in liquids are known as slurry bubble columns . These are widely used as reactors for a variety of chemical reactions, and also as bioreactors with suspensions of microbial cells or particles of immobilized enzymes. 

Heat transfer in bubble column slurry reactors was studied by Kolbel and coworkers (75-77) and Deckwer et al. (13). The addition of solids increases the wall-to--suspension heat transfer coefficient. However, this increase is only due to changes in the physico-chemical properties and represents no independent contribution of the particles. Therefore, the heat transfer model, i.e. eqn. (17), developed by Deckwer (<53) for two-phase BCR also applies to slurry reactors as was proved for particle sizes up to 120 yum. This confirms that solids and liquid in the slurry can be regarded as a pseudo-homogeneous phase provided the gas velocity is large enough to provide for complete fluidization of the particles. 

In general, the gas holdups and kLa for suspensions in bubbling gas-liquid reactors decrease substantially with increasing concentrations of solid particles, possibly because the coalescence of bubbles is promoted by presence of particles, which in turn results in a larger bubble size and hence a smaller gas-liquid interfacial area. Various empirical correlations have been proposed for the kLa and gas holdup in slurry bubble columns. Equation 7.46 [24], which is dimensionless and based on data for suspensions with four bubble columns, 10-30 cm in diameter, over a range of particle concentrations from 0 to 200 kg m 3 and particle diameter of 50-200 pm, can be used to predict the ratio r of the ordinary kLo values in bubble columns. This can, in turn, be predicted for example by Equation 7.41, to the kLa values with suspensions. 

Solid particle size in stirred tank reactos, loop reactors and bubble columns is usually in the range of 100 - 200 and solid concentration is rather low whereas in multiphase fluidized beds the solid concentration as well as solid particle size is larger. The density difference of liquid and solid usually is large, except in bioreactors, where one is faced also with agglomeration problems and nonnewtonian behavior of the suspension. 

Suspension reactors are frequently operated continuously as so-called bubble column or slurry reactors (Figure 4.10.12c). The liquid represents the continuous phase in which the gas and the solid catalyst are distributed. The particle size is much smaller than in other three-phase reactors (<0.1 mm). The solid concentration is merely 1%, to keep the energy required for suspension low. However, the small particles complicate the situation as the separation of the solid catalyst is essential. Batchwise operated suspension reactors (not shown in Figure 4.10.12) are mostly mechanically stirred to keep the particles in suspension. 

Hydrodynamics of slurry reactors includes the study of 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 number of slurry reactor types in use (see fig. 1) and the fact that most correlations available are at least partially of an empirical nature. We will therefore restrict ourselves to sparged slurry columns and slurries in stirred vessels. A second problem is the difference with three phase fluidization. To avoid too much overlap we will only consider those cases where superficial liquid velocities are so low that its contribution to suspension of the particles is relatively unimportant. 

Since the reactor or synthesis vessel used in this instrument is a batch-type reactor unlike a column-type reactor used for small-scale DNA synthesis where the entire amount of solid support is suspended into reaction solution to give efficient coupling, it is very essential to have proper fluidization of the large amount of supports. Fluidization in this approach is effected by bubbling dry gas (argon) through the bottom of a reactor, resulting in complete suspension of the support. 

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