Three-Phase Gas-Liquid-Solid Systems

Fractional gas holdup is an indication of the effective interfacial area in any gas-liquid system. Detailed discussion on fractional gas holdup in stirred reactors is available in Section 7A.5. Analogous to other parameters, the gas holdup is also a function of the operating parameters (superficial gas velocity, type of impeller and its size/position in the reactor, etc.) and system properties (liquid-phase viscosity, surface tension, solid density and loading, presence of surfactant, etc.). As discussed in Section 7A.5, YawaUcar et al. (2002a) have been able to obtain a unique correlation for the gas holdup using the concept of relative dispersion N/N  

This correlation was found to give a better fit of data covering a broad range of system properties, impeller types/size, and geometric configurations. Both Equations 7B.14 and 7B.15 require information on In the case of stirred bioreactors for both suspension and microcarrier-anchored culture, it is desirable to use low shear/ low mechanical impact severity impellers (Collignon et al. 2010 Bustamante et al. 2013 Fig. 7B.15). For these new impellers, correlations for are not available but 

Yawalkar et al. (2002b) used the concept of relative dispersion to unify all gas-liquid mass transfer data available in the literature for stirred reactors. This matter has been discussed in great detail in Section 7A.5. The correlation proposed was 

It is evident that the above correlation also requires knowledge of Therefore, studies for establishing reliable correlations for for the new impellers deserve immediate attention. 

STIRRED TANK REACTORS EOR CELL CULTURE TECHNOLOGY 

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The volumetric mass transfer coefficient is also determined for three-phase (gas-liquid-solid) systems using both physical and chemical methods described above. A summary of these studies is given in Table XXXII. 

Govindarao10 also postulated generalized nonisothermal (constant reactor wall temperature) models for batch as well as cocurrent- and countercurrent-flow three-phase gas-liquid-solid systems carrying out a first-order reaction. 

Typically, this additional biological step is carried out in a pachuca, a cone-bottomed column familiar to the mining industry. Ground or milled ore, mixed with the aqueous bacterial solution, is introduced into the top of the column, and air is injected at the base. The injected air serves a number of functions it maintains the solid in suspension, it mixes the solid with liquid— giving a three-phase gas/liquid/ solid system—and it provides the oxygen and carbon dioxide required by the bacteria. The bacteria also require a feed of nitrogen and phosphorous, which can be added to the colunm if they are not indigenous to the ore. 

Precombustion of sulfur from coal by selective oxidation is an important step in coal cleaning, without the need for a postcombustion cleanup step to remove sulfur oxides. This process, known as oxydesulfurization of coal, is a three-phase gas-liquid-solid system. As an example, a sparged reactor wiU be designed to oxydesulfurize 100 tons/day of coal in an aqueous slurry of coal with air. 

It is evident from Figure 7A.4 that the power number attains a constant value in the turbulent region. For the case of two- (gas-liquid) and three-phase (gas-liquid-solid) systems, additional parameters that represent the gas- and solid-phase holdups have an important bearing on the power required. Hence, these are also included. The effects of introduction of gas and solid phases on the power number are discussed... 

Three-phase gas-liquid-solid system with single impeller (Pangarkar et al. 2002) ... 

Critical speed for just suspension of solid in three-phase (gas-liquid-solid) system (rev/s)... 

B.9.1.6 Mixing Time in Three-Phase (Gas-Liquid-Solid) System There are no reported experimental data on gas-liquid-solid systems that are relevant to animal cell culture using microcarrier beads as support for the cells. As mentioned earlier in the case of gas-liquid and solid-liquid systems, because of (i) the low density difference (Ap 30-50 kg/m ) and (ii) low aeration rates, there is insignificant impact of introduction of the solid and gas phases. Further, if an upflow impeller is used, the power drop due to aeration is less than 10% at low aeration rates (Fig. 7A.6). Therefore, Equation 7B.11 can be used in this case also. 

For comprehensive treatments of gas-solid fluidization, see Hetsroni [1], Yates [2], Davidson et al. [3], Geldart [4], Pell [5], Kunii and Levenspiel [6], Grace et al. [7], and Yang [8]. While fluidization can be applied to liquid-solid and three-phase (gas-liquid-solid) systems, the great majority of applications, especially in terms of catalytic processes, are for gas-solid systems, and this chapter is limited to this case. Readers interested in three-phase catalytic fluidized-bed reactors should consult Fan [9]. For a detailed review of spouted bed catalytic... 

In this Chapter, a heterogeneous system is one in which the reactants are present in at least two phases. The discussion will concentrate on two such conditions, two-phase gas/liquid systems and three-phase gas/liquid/solid systems. Chemists tend to favor homogeneous conditions, with the reactants all in one phase, because they provide more controlled and reproducible conditions. However, heterogeneous conditions are often preferred in industrial processes because of the ease of separating the catalyst from the products. In many mechanistic studies, heterogeneity adds a complicating feature to be avoided, but there are times when this cannot be done, or when it happens unexpectedly. 

Three-phase (gas-liquid-solid) systems such as gaseous slurry reactions in stirred vessels are common in the chemical industry. They present special mixing challenges. The presence of gas tends to disturb the liquid flow patterns established... 

One goal of our experimental program with the bench-scale unit was to develop the necessary correlations for use in the ultimate design of large commercial plants. Because of the complexity inherent in the three-phase gas-liquid-solid reaction systems, many models can be postulated. In order to provide a background for the final selection of the reaction model, we shall first review briefly the three-phase system. 

Heterogeneously catalyzed hydrogenation is a three-phase gas-liquid-solid reaction. Hydrogen from the gas phase dissolves in the liquid phase and reacts with the substrate on the external and internal surfaces of the solid catalyst Mass transfer can influence the observed reaction rate, depending on the rate of the surface reaction [15]. Three mass transfer resistances may be present in this system (Fig. 42.1) ... 

FIGURE 7A.10 Variation of power number with impeller speed for two-phase (gas-liquid) and three-phase (gas-liquid-solid) stirred reactors. Two phase (solid-liquid) system. A, fillet formation B, disappearance of fillets C, off-bottom suspension of solids D, recirculation of mixture. Three phase (gas-liquid-sohd). A, no dispersion of gas solid settled on bottom B, gas dispersed beginning of solid suspension C, gas dispersed off-bottom suspension of solids D, recirculation of mixture. (Reproduced from Rewatkar et al. 1991 with permission from American Chemical Society. 1991, American Chemical Society.)... 

Difference in the critical speed for just suspension of solid in three-phase (gas-liquid-solid) and two-phase (solid-liquid) system (rev/s) Difference in the minimum speed for just suspension of the solid in three-phase system in the presence and absence of helical coil (rev/s) Power input (W, kW)... 

Radial heat transport may usually be represented by a pseudo-homogeneous model with two parameters (Specchia and Baldi [81], Specchia et al. [82], Specchia and Sicardi [83]). The catalytic bed is assumed to be a pseudo-homogeneous system characterized by an effective thermal conductivity k - and by a heat transfer resistance located at the wall of the reactor. The corresponding coefficient is h. In any point in the reactor the three phases (gas, liquid, solid) are supposed to be at the same temperature. 

Hydroprocessing reactors generally are three-phase (gas-liquid-solid) reaction systems. The gas phase is composed majorly of hydrogen, gaseous reaction products, and partially vaporized hydrocarbons the hydrocarbon feed is the liquid phase, whereas the catalyst bed is the solid phase. The only exception is naphtha HDT, which exhibits just two phases (gas-solid) as a result of the complete vaporization of the hydrocarbon. The coexistence of these three phases puts hydroprocessing FBRs... 

Numerous studies have been made of the hydrodynamics and other aspects of the behavior of gas-liquid-solid systems, in particular of trickle beds, and including absorption and extraction in packed beds. A selection of correlations of these parameters is presented in problem P8.03.02. They tell something of what is going on in three-phase reactors. 

In Fig. 1.2, phase transformations are pnt into their context of physical processes used for separation of mixtures of chemical compounds. However, the figure has been drawn asymmetrically in that two Uqnids (I and II) are indicated. Most people are familiar with several organic Uqnids, Uke kerosene, ether, benzene, etc., that are only partially miscible with water. This lack of miscibility allows an equilibrium between two liquids that are separated from each other by a common phase boundary. Thus the conventional physical system of three phases (gas, liquid, and solid, counting all solid phases as one), which ordinarily are available to all chemists, is expanded to four phases when two immiscible liquids are involved. This can be of great advantage, as will be seen when reading this book. 

The Wellman-Lord Process is not, in itself, a conversion method, but rather a solution phase technique for concentrating a dilute SO2 effluent stream to provide a suitably rich feed for Claus redox conversion. When coupled with the Claus Process, it constitutes an overall desulphurisation system which involves all three phases gas, liquid solution, and solid crystalline. 

It is convenient to begin with a simple one-component system having three phases - gas, liquid and solid. At the primitive level these phases differ by a density n, i.e., the number of particles per unit volume. If we fix now for simplicity the pressure in this system, in the thermodynamical equilibrium,... 

The various volumetric mass-transfer coefficients are defined in a manner similar to that discussed for gas-liquid and fluid-solid mass transfer in previous sections. There are a large number of correlations obtained from different gas-liquid-solid systems. For more details see Shah (Gas-Liquid-Solid Reactor Design, McGraw-Hill, 1979), Ramachandran and Chaudhari (Three-Phase Catalytic Reactors, Gordon and Breach, 1983), and Shah and Sharma [Gas-Liquid-Solid Reactors in Carberry and Varma (eds.), Chemical Reaction and Reactor Engineering, Marcel Dekker, 1987],... 

Young>s Equation A fundamental relationship giving the balance of forces at a point of three-phase contact. For a gas—liquid—solid system Young s equation is 7SL + 7 cos 6 = 7so, where 7SL, 7LG, and 7SG are interfacial tensions between solid—liquid, liquid—gas, and solid—gas,... 

In this chapter, different types of microstructured devices for three-phase reactions are described. The characterization of mass transfer for gas-liquid-solid systems is presented. Finally, hterature examples of both gas-liquid-solid and gas-liquid-liquid reactions are briefed. 

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