Two-Phase Solid-Liquid Systems

STIRRED TANK REACTORS FOR CELL CULTURE TECHNOLOGY 

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Caution must be exercised in the addition of the glacial acetic acid in order to avoid frothing of the hot solution. The frothing becomes most vigorous as the 6-aminouracil begins to precipitate from the solution. The heating and subsequent neutralization assure cycli-zation of the initially formed cyanoacetylurea to 6-aminouracil. The nitrosation is carried out on the two-phase (solid-liquid) system. 

Expression is the separation of liquid from a two-phase solid-liquid system by compression of the system under conditions that permit the liquid to escape, while the solid is retained between the compressing surfaces. Expression serves the same purposes as filtration but is distinguished from the latter in that the pressure is applied by movement of the retaining walls instead of pumping the material into a fixed space. Expression is usually employed to separate systems that are not easily pumped. It is also used instead of filtration when a more thorough removal of liquid from the cake is desired. The usual equipment for expression is a hydraulic press. Most of the common vegetable oils are produced by expression, In the expl in-... 

Expression is the separation of a liquid from a two-phase solid-liquid system by compression, due to movanent of the retaining wall rather than the pumping of the solid-liquid system into a fixed chamber as in filtration. In filtration, the original mixture is sufficiently fluid to be pumpable in expression, the material may appear either entirely sani-solid or slurry. Based on an expression theory [6,7], the time for expression is also proportional to the square of cake thickness. 

The variety of impellers available can further be divided into two categories based on whether they aeate a predominantly shear field or bulk movement. The axial flow propeller, the hydrofoils, and the mixed flow impellers (when D/r< 0.4) develop bulk axial patterns. The downflow type in the mixed flow class develops a mean flow directed toward the base of the vessel and is therefore useful for solid suspension in two-phase (solid-liquid) systems. But the same are less efficient in three-phase... 

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.)... 

TABLE 7A.3 Summary of Important Literature Studies on Solid Suspension in Two-phase (solid-liquid) Systems... 

Two-phase (solid-liquid) system with single impeller (Jadhav and Pangarkar 1991) ... 

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)... 

The synthesis of glycidic esters in a two-phase solid-liquid system has been reported yields of between 32 and 92 % are obtained for eleven examples. 2,3-Dibromoalkyl esters react with excess sodium azide on heating to give 2-azido-2-alkenoic esters in 71 to 87 % yield over five examples. The chiral formamido-ester (71) reacts with oxomethylenebis-(3 f+-imidazolium) bis(methanesulphon-ate) (72), a non-basic dehydrating agent, to give the a-isocyano-ester (73) in 80 % yield with only 1 % racemization, as shown by the re-hydration of the product (73) (Scheme 38). ... 

Viswanathan et al. (V6) measured gas holdup in fluidized beds of quartz particles of 0.649- and 0.928-mm mean diameter and glass beads of 4-mm diameter. The fluid media were air and water. Holdup measurements were also carried out for air-water systems free of solids in order to evaluate the influence of the solid particles. It was found that the gas holdup of a bed of 4-mm particles was higher than that of a solids-free system, whereas the gas holdup in a bed of 0.649- or 0.928-mm particles was lower than that of a solids-free system. An attempt was made to correlate the gas holdup data for gas-liquid fluidized beds using a mathematical model for two-phase gas-liquid systems proposed by Bankoff (B4). 

The adoption of a particular isolation procedure will depend to a large extent upon the physical and chemical properties of the product. Some guidelines for useful general approaches may however be given with regard to the physical state at ambient temperature of the crude mixture resulting from the reaction, i.e. whether it is a one-phase (either solid or liquid) or a two-phase (solid/liquid or liquid/liquid) system. 

A two-phase solid-liquid transfer catalytic system has been developed for aromatic aldehydes and for aliphatic aldehydes which are branched at the a-carbon.116 The aldehyde and the a-chloroester are stirred in dimethylformamide in the presence of solid potassium carbonate and triethylbenzylammonium chloride. The isolated yield of glycidic ester compares favourably with that of the procedure above. 

Solid—Liquid Tie-Lines. Tabulated in Table III are the weight percentages of the originally prepared complexes which were chosen as points within the two-phase, solid-liquid region in the potassium chloride-water-THF system. The weight percentages of the saturated solutions for each of the original complexes are presented also in this table. 

DYNAMICS OF DISTRIBUTION The natural aqueous system is a complex multiphase system which contains dissolved chemicals as well as suspended solids. The metals present in such a system are likely to distribute themselves between the various components of the solid phase and the liquid phase. Such a distribution may attain (a) a true equilibrium or (b) follow a steady state condition. If an element in a system has attained a true equilibrium, the ratio of element concentrations in two phases (solid/liquid), in principle, must remain unchanged at any given temperature. The mathematical relation of metal concentrations in these two phases is governed by the Nernst distribution law (41) commonly called the partition coefficient (1 ) and is defined as = s) /a(l) where a(s) is the activity of metal ions associated with the solid phase and a( ) is the activity of metal ions associated with the liquid phase (dissolved). This behavior of element is a direct consequence of the dynamics of ionic distribution in a multiphase system. For dilute solution, which generally obeys Raoult s law (41) activity (a) of a metal ion can be substituted by its concentration, (c) moles L l or moles Kg i. This ratio (Kd) serves as a comparison for relative affinity of metal ions for various components-exchangeable, carbonate, oxide, organic-of the solid phase. Chemical potential which is a function of several variables controls the numerical values of Kd (41). 

Most of the studies referred to in the previous discussion used two-phase (gas-liquid) systems. The considerations are substantially similar when the liquid jet ejector is to be used as a three-phase catalytic reactor, particularly because the catalyst loading commonly used in a venturi loop system is relatively low. In terms of mass transfer, besides gas-liquid mass transfer, solid-liquid mass transfer step assumes great importance. As discussed in Chapters 7A and 7B, factors relating to dispersion of the gas phase, suspension of solids, and concentration profile of the catalyst phase need to be addressed in the case of a three-phase reactor. 

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. 

At the still lower temperature at point d, the system point is within the two-phase solid-liquid area. The tie line through this point is line e-f. The compositions of the two phases are given by the values of zb at the ends of the tie line Xg = 0 for the solid and Xg = 0.50 for the liquid. From the general lever rule (Eq. 8.2.8 on page 209), the ratio of the amounts in these phases is... 

In contrast to three phase fluid beds where the relation between the three phase hold-ups can be rather complex, in slurry reactors with the much smaller particles and slip velocities the relation between and is often simple as it is fixed by the feed ratio of solids and liquid phases, or liquid and solids volumes are constant (in batch systems). The bubble hold- up is much more difficult to predict, first of all because of the different regimes that might prevail both in stirred vessels and slurry sparger columns. These regimes are shared with two phase gas liquid systems (see fig. 4). In a three phase sparger the regimes are (see Shah [b]) ... 

A one-component system (C = 1) has two independent state variabies (T and p). At the tripie point three phases (soiid, iiquid, vapour) coexist at equiiibrium, so P = 3. From the phase ruie f = 0, so that at the tripie point, T and p are fixed - neither is free but both are uniqueiy determined. If T is free but p depends on T (a sloping line on the phase diagram) then f = 1 and P = 2 that is, two phases, solid and liquid, say, co-exist at equilibrium. If both p and T are free (an area on the phase diagram) F = 2 and P = 1 only one phase exists at equilibrium (see Fig. A1.18). 

Strictly gas-phase CSTRs are rare. Two-phase, gas-liquid CSTRs are common and are treated in Chapter 11. Two-phase, gas-solid CSTRs are fairly common. When the solid is a catalyst, the use of pseudohomogeneous kinetics allows these two-phase systems to be treated as though only the fluid phase were present. All concentration measurements are made in the gas phase, and the rate expression is fitted to the gas-phase concentrations. This section outlines the method for fitting pseudo-homogeneous kinetics using measurements made in a CSTR. A more general treatment is given in Chapter 10. 

Phase transfer catalysis (PTC) refers to the transfer of ions or organic molecules between two liquid phases (usually water/organic) or a liquid and a solid phase using a catalyst as a transport shuttle. The most common system encountered is water/organic, hence the catalyst must have an appropriate hydrophilic/lipophilic balance to enable it to have compatibility with both phases. The most useful catalysts for these systems are quaternary ammonium salts. Commonly used catalysts for solid-liquid systems are crown ethers and poly glycol ethers. Starks (Figure 4.5) developed the mode of action of PTC in the 1970s. In its most simple... 

In earlier work, Bhaumik and Kumar (1995) have reported that the use of two liquid phases in the oxidation of hydrophobic organic substances with aqueous H2O2 using titanium silicate as the catalyst not only enhances the rate of oxidation but also improves selectivity for species like toluene, anisole, and benzyl alcohol. For a single liquid phase acetonitrile was u.sed a solvent. The solid-liquid system gives high ortho selectivity. Thus, in the case of anisole the ratios of o to p for. solid-liquid and solid-liquid-liquid system were 2.22 1 and 0.35 1, respectively. 

The term two-phase flow covers an extremely broad range of situations, and it is possible to address only a small portion of this spectrum in one book, let alone one chapter. Two-phase flow includes any combination of two of the three phases solid, liquid, and gas, i.e., solid-liquid, gas-liquid, solid-gas, or liquid-liquid. Also, if both phases are fluids (combinations of liquid and/or gas), either of the phases may be continuous and the other distributed (e.g., gas in liquid or liquid in gas). Furthermore, the mass ratio of the two phases may be fixed or variable throughout the system. Examples of the former are nonvolatile liquids with solids or noncondensable gases, whereas examples of the latter are flashing liquids, soluble solids in liquids, partly miscible liquids in liquids, etc. In addition, in pipe flows the two phases may be uniformly distributed over the cross section (i.e., homogeneous) or they may be separated, and the conditions under which these states prevail are different for horizontal flow than for vertical flow. 

The scope of coverage includes internal flows of Newtonian and non-Newtonian incompressible fluids, adiabatic and isothermal compressible flows (up to sonic or choking conditions), two-phase (gas-liquid, solid-liquid, and gas-solid) flows, external flows (e.g., drag), and flow in porous media. Applications include dimensional analysis and scale-up, piping systems with fittings for Newtonian and non-Newtonian fluids (for unknown driving force, unknown flow rate, unknown diameter, or most economical diameter), compressible pipe flows up to choked flow, flow measurement and control, pumps, compressors, fluid-particle separation methods (e.g.,... 

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