Liquid-Gas-Solid Systems

The hydrodynamics in a gas-liquid or gas-liquid-solid system is characterized by the... 

Advances in multiphase reactors for fuel industry are discussed in this work. Downer reactors have some advantages over riser reactors, but suffer from some serious shortcomings. The coupled reactors can fully utilize the advantages of the riser and the downer. For fuel industry that involves gas-liquid-solid system, slurry bed reactors especially airlift reactors are preferred due to their performance of excellent heat control and ease of seale up. For high-pressure processes, the spherical reactor is promising due to its special characteristics. 

Table 5.4-3 summarizes the design equations and analytical relations between concentration, C/(, and batch time, t, or residence time, t, for a homogeneous reaction A —> products with simple reaction kinetics (Van Santen etal., 1999). Balance equations for multicomponent homogeneous systems for any reaction network and for gas-liquid and gas-liquid-solid systems are presented in Tables 5.4-7 and 5.4.8 at the end of Section 5.4.3. 

In catalytic gas-liquid-solid systems mass transfer is more complex. The catalyst particles are present in the liquid phase. The expression for the rate of mass transfer from the gas to the liquid is identical to that for systems without a solid catalyst (Eqn. 5.4-67). However, now also mass transfer from the liquid to the solid surface (external mass transfer) and inside the particle (internal mass transfer) have to be considered. 

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. 

ZANKER, A. In Separation Techniques Vol 2 Gas Liquid Solid Systems (McGraw-Hill, 1980), p. 178. Hydrocyclones dimensions and performance. 

It is difficult to evaluate quantitatively the importance of such heterogeneous reactions in the overall oxidation of S(IV). Their rates depend on the physical and chemical natures of the surfaces involved, including specific surface areas, the presence of defects and surface adsorbed water, etc., yet these are not well understood, especially for highly complex environmental gas-liquid-solid systems. For example, the rates of oxidation of S02 at 80% relative humidity on two different samples of fly ash obtained from two coal-fired power plants differed by more than an order of magnitude (Dlugi and Gusten, 1983). Even in laboratory systems the nature of relatively simple surfaces such as carbon depends on the history of the material. 

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

Katti SS. Gas-liquid-solid systems an industrial perspective. Trans IChemE 1995 73(part A) 595-607. 

As mentioned previously, axial flow impellers are typically used for solids suspension. It is also typical to use radial flow impellers for gas-liquid mass transfer. In combination gas-liquid-solid systems, it is more common to use radial flow impellers because the desired power level for mass transfer normally accomplishes solids suspension as well. The less effective flow pattern of the axial flow impeller is not often used in high-uptake-rate systems for industrial mass transfer problems. There is one exception, and that is in the aeration of waste. The uptake rate in biological oxidation systems is on the order of 30 ppm/hr, which is about to the rate that may be required in industrial processes. In waste treatment, surface aerators typically use axial flow impellers, and there are many types of draft tube aerators that use axial flow impellers in a draft tube. The gas rates are such that the axial flow characteristic of the impeller can drive the gas to whatever depth is required and provide a very effective type of mass transfer unit. 

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],... 

Loiseau et al. (1977) found that their data for nonfoaming systems agreed well with Eq. (3.3). Calderbank (1958), Hassan and Robinson (1977), and Luong and Volesky (1979) have also proposed correlations for power consumption in gas-liquid systems. Nagata (1975) suggested that power consumption for agitated slurries can be reasonably predicted from these correlations by the correction factor psi/pL, where psl is the density of the slurry. Power consumption for a gas-liquid-solid system has also been studied by Wiedmann et al. (1980). They examined the influence of gas velocity, solid loading, type of stirrer, and position of the stirrer blades on power consumption plots of power numbers vs. Reynolds numbers for propeller and turbine type impellers proposed by them are shown in Fig. 13. 

Wiedmann et al. (1980) have compared the mixing of nonaerated liquids, aerated liquids, and slurries in a turbulent flow. They found that the torque required for stirred, aerated liquids is lower than that for nonaerated stirred liquids because of the decrease in the density of the gas-liquid mixture. The concentration distribution of the particles in aerated suspension becomes more uniform with increasing impeller speed, whereby the torque is higher than that for aerated liquids but lower than that for nonaerated slurries. For gas-liquid-solid systems, very limited data on dispersion of solids and gas phase are available, and further studies are necessary with different designs and for systems with different physical properties. The available literature has been reviewed by Stiegel et al. (1978), Shah et al. (1982), and Shah and Sharma (1986). 

For gas-liquid-solid systems, studies on gas-liquid and liquid-solid mass transfer in basket reactors have been rather limited. For the rotating basket reactor, gas-liquid mass-transfer coefficient data are needed. Liquid-solid mass transfer has been studied by Teshima and Ohashi (1977), and their data are correlated by... 

The power required for a given stirrer type and associated vessel configuration depends on the speed of rotation N, the stirrer diameter du the density p, and the kinematic viscosity v of the medium. In vessels without baffles, the liquid vortex, and therefore the acceleration due to gravity, g, is immaterial, as long as no gas is entrained in the liquid. Thus, P = f(N, dt,p, v), and in the dimensionless form, Ne = /(Re), a relationship generally known as the power characteristics of the stirrer. Here, Ne = P/(pN3df) is the Newton or Power number, and Re s Ndf/v the Reynolds number. This relationship was described in Sections II and III for gas-liquid and gas-liquid-solid systems. 

For a conventional mechanically agitated biological reactor, the information provided for aqueous gas-liquid and gas-liquid-solid systems in Sections II, III, and VII is applicable here. For power consumption, the most noteworthy works are those by Hughmark (1980) (see Eqs. (6.15) and (6.16)) and Schiigerl (1981). For gas-liquid mass transfer, the relationship kLaL = (P/V, ug) is applicable for biological systems. The relationships (6.19) and (6.20) are also valuable, and their use is recommended. The most generalized relation for kLaL is provided by Eq. (6.18). The intrinsic gas-liquid mass transfer coefficient is best estimated by Eq. (6.23). For liquid-solid mass transfer, the use of the study by Calderbank and Moo-Young (1961) (Eqs. (6.24)-(6.26)) is recommended. For viscous fluids, Eq. (6.27) should be used. 

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. 

The gas-liquid interfacial area measured form the physical techniques is generally about 35% higher than the one measured by the chemical technique. The chemical technique is generally more accurate than the physical technique. The reaction systems described in Table XXXI are reliable for the gas-liquid system. For gas-liquid-solid systems, the method is not reliable even when solids are inert, because of possible adsorptions of gas and/or liquid on the solid surface. 

The physical methods for the measurement of kLaL in batch, semi-batch, and continuous systems described earlier are accurate. The main limitation for the semi-batch and continuous systems is the availability of the analytical technique for the measurement of the gas concentration in the liquid phase. For gas-liquid-solid systems, Eq. (9.41) can be used to measure both kt and kL simultaneously. The liquid-solid mass-transfer coefficient can also be measured using the method of Ruether and Puri (1973) or the physical methods outlined earlier. 

Figure 12. Interfacial area as a function of radial position for a gas-liquid-solid system.

Figure 4.2 Concentration profiles for mass transfer and reaction in series in a gas - liquid - solid system (film theory).

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. 

Adlington andThompson,1 and, more recently, by Ostergaard and Suchozebrski,105 Ostergaard and Fosbol,103 and Nishikawa et al.92 A summary of the gas-liquid-solid systems used by these investigators is given in Table 9-3. 

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. 

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