Liquid reactors, gas

Gas-liquid reactions are used in several industrial processes. In the synthesis of chemical compounds, gas-liquid reactions are used in, for example, the oxidation of hydrocarbons. For a synthesis reaction, it is typical that one organic compound is transformed into another organic compoimd in the presence of a homogeneous catalyst. Typical reactions are, for example, chlorination of aromatic compounds in the production of chlorinated hydrocarbons, chlorination of carboxylic acids (mainly acetic acid), and oxidation of toluene and xylene in the production of benzoic acid and phthalic acid. In the production of hydrogen peroxide (H2O2), an oxidation process can also be used, namely oxidation of anthraquinole to anthraquinone. 

An important area where gas-liquid reactors are used is the cleaning of industrial gases. A low-concentration gas component is absorbed with the aid of a chemical reaction in the liquid phase. Such an absorption can be purely physical in nature, but when aided by a 

FIGURE 7.1 Phases in a gas-liquid reactor according to the film model. 

For a review of noncatalytic or homogeneously catalyzed gas-liquid reactions, see Table 7.1 [2-4]. 

Two reactor types dominate in the synthesis of chemicals in the case of gas-liquid reactions the tank reactor and the bubble column. Both types can be operated in a continuous 

At first glance gas-liquid reactors might appear to be easier to analyze than slurry reactors since they both involve gas and liquid phases, but the solid phase is not present in the former. On the other hand, the fluid mechanics and transport behavior have been investigated in more detail in gas-liquid systems than in gas-liquid-solid systems, so it is possible to include a little more detail in analysis if desired. The analysis and design equations can also be applied to liquid-liquid systems, as described below. 

In Chapter 7 we discussed the basics of the theory concerned with the influence of diffusion on gas-liquid reactions via the Hatta theory for flrst-order irreversible reactions, the case for rapid second-order reactions, and the generalization of the second-order theory by Van Krevelen and Hofitjzer. Those results were presented in terms of classical two-film theory, employing an enhancement factor to account for reaction effects on diffusion via a simple multiple of the mass-transfer coefficient in the absence of reaction. By and large this approach will be continued here however, alternative and more descriptive mass transfer theories such as the penetration model of Higbie and the surface-renewal theory of Danckwerts merit some attention as was done in Chapter 7. 

Gas-liquid reactions are most often conducted in stirred-tank systems with flow of both gas and liquid through the reactors, or in bubble eolumns, or in packed columns—with countercurrent flow typical in the last two. For the most part the analysis given is independent of the specific configuration of the reaetor (bubbles are still with us and still important in design), but correlations for transport eoeffieients may vary with the individual reactor and type of operation. 

If we return to the Hatta picture of reaction and diffusion, recall that reaction and diffusion occur only in the film. Reaction also occurs in the bulk liquid phase, of course, but there the concentration of reactants as a function of position is determined by the nature of mixing in that phase. Let us reformulate the problem so that the fraction of liquid phase occupied by the film, a, is defined explieitly. If L is film thickness and interfacial area, then 

We may also define a phase utilization factor as the ratio of observed rate to the intrinsic rate kBA V/Sf) 

Conventional mechanically agitated gas-liquid reactors, wherein gas and liquid make contact in batch, semibatch, or continuous mode, are widely used in processes involving chlorination, sulfonation, hydrogenation, selective absorptions in amine solutions, etc. (Doraiswamy and Sharma, 1984). These reactors are popular for laboratory studies because of their simplicity in construction and low cost. As a rule of thumb with noncorrosive liquids, the mechanically agitated reactor is most economical when the overall reaction rate is five times greater than the mass transfer rate in a bubble column. If a 

Liquid-phase processes such as oxidation, hydrogenation, sulfonation, nitration, halogenation, hydrohalogenation, alkylation, sulfonation, polycondensation, polymerization, etc. Examples oxidation of acetaldehyde to acetic acid 

Examples absorption of S03 in dilute sulfuric acid absorption of N02 in dilute nitric acid 

Examples manufacture of H2S04, BaC03, BaCl2, adipic acid, phosphates, etc. 

Various processes in petroleum refining and recovery of nuclear materials. 

If kiAi is known with good accuracy, it may be possible to back out the intrinsic kinetics using the methods of Section 7.1. Knowing the intrinsic kinetics may enable a scaleup where kiAj(af — ai) is dilferent in the large and small units. However, it is better to adjust conditions in the pilot reactor so that they are identical to those expected in the larger reactor. Good pilot plants have this versatility. The new conditions may give suboptimal performance in the pilot unit but achievable performance in the full-scale reactor. 

Does increased agitator speed improve performance in the pilot plant If so, there is a potential scaleup problem. Installing a variable-speed drive with a somewhat over-sized motor can provide some scaleup insurance, the cost of which is apt to be minor compared with the cost of failure. 

Example 11.18 Consider a gas-sparged CSTR with reaction occurring only in the liquid phase. Suppose a pilot-scale reactor gives a satisfactory product. Propose a scaleup to a larger vessel. 

Solution Ideally, the scaleup will maintain the same inlet concentrations for the two phases, the same relative flow rates and holdups for the two phases, and the same ratio of gas transferred to liquid throughput. It is also necessary to maintain a constant residence time in the liquid phase. It is simple to set the flow rates  

We would also like the following to be true, but their achievement is less direct than for the flow rates. 

SOLUTION Ideally, the scaleup will maintain the same inlet concentrations for the two 

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Figure 8.12 Gas-liquid reactor model (Yagi, 19H6) where...

Figure 8.33 Schematic development of an industrial gas-liquid reactor (Wachi and Jones, 1994)...

The interfacial area AtV usually excludes the contact area between the vapor space and the liquid at the top of the reactor. The justification for this is that most gas-liquid reactors have gas bubbles as a dispersed phase. This gives a much larger interfacial area than the nominal contact area at the top of the reactor. There are exceptions—e.g., polyester reactors where by-product water is removed only through the nominal interface at the top of the reactor— but these are old and inefficient designs. This nominal area scales as while the contact area with a dispersed phase can scale as S. 

TABLE 11.4 Typical Flow and Mixing Regimes for Gas-Liquid Reactors... 

Static mixing catalysts Operation Monolithic reactors Microreactors Heat exchange reactors Supersonic gas/liquid reactor Jet-impingement reactor Rotating packed-bed reactor... 

Table 5.2 Comparison of reaction conditions and results for hydrogenation of methyl (Z)-a-acetamidocinnamate in mini batch, micro liquid/liquid and micro gas/liquid reactors [70. ...

A survey of the mathematical models for typical chemical reactors and reactions shows that several hydrodynamic and transfer coefficients (model parameters) must be known to simulate reactor behaviour. These model parameters are listed in Table 5.4-6 (see also Table 5.4-1 in Section 5.4.1). Regions of interfacial surface area for various gas-liquid reactors are shown in Fig. 5.4-15. Many correlations for transfer coefficients have been published in the literature (see the list of books and review papers at the beginning of this section). The coefficients can be evaluated from those correlations within an average accuracy of about 25%. This is usually sufficient for modelling of chemical reactors. Mathematical models of reactors arc often more sensitive to kinetic parameters. Experimental methods and procedures for parameters estimation are discussed in the subsequent section. 

Mass and heat balance equations for typical gas-liquid reactors in heterogeneous systems at steady state... 

Requirements regarding laboratory liquid-liquid reactors are very similar to those for gas-liquid reactors. To interpret laboratory data properly, knowledge of the interfacial area, mass-transfer coefficients, effect of contaminants on mass-transport processes, ionic characteristics of the system, etc. is needed. Commonly used liquid-liquid reactors have been discussed by Doraiswamy and Sharma (1984). 

Gas-liquid reactors. Gas-liquid reactors are quite common. Gas-phase components will normally have a small molar mass. Consider the interface between a gas and a liquid that is assumed to have a flow pattern giving a stagnant film in the liquid and the gas on each side of the interface, as illustrated in Figure 7.2. The bulk of the gas and the liquid are assumed to have a uniform concentration. It will be assumed here that Reactant A must transfer from the gas to the liquid for the reaction to occur. There is diffusional resistance in the gas film and the liquid film. 

Figure 7.4c shows an in-line static mixer. Dispersion is usually promoted by repeatedly changing the direction of flow locally within the mixing device as the liquids are pumped through. This will give a good approximation to plug-flow in both phases in cocurrent flow. As with gas-liquid reactors, static mixers are particularly suitable when a short residence time is required. 

Lo, S., Application of population balance to CFD modelling of gas-liquid reactors . Conference on Trends in Numerical and Physical Modelling for Industrial Multiphase Flows , Cargese, Corse 27-29 September (2000). 

The design of gas-liquid reactors requires the consideration of four sets of basic data ... 

In this section, we have examined how the coupling between mass transfer and the chemical reaction defines the concentration profile of the limiting reagent (i.e., hydrogen), and how the mass or molar flow between the gas and the liquid phase can be computed. In the next section, the estimation of the overall rate of reaction (i.e., the reactor productivity) will be reviewed for different gas-liquid reactors. 

Mechanically stirred gas-liquid reactor performances are affected by the degree of mixing, apparatus geometry, stirring power, flow rate, discharge and feed locations for the gas and liquid. For a correct design, the following requirements must be satisfied ... 

The scale-up of mechanically stirred gas-liquid reactors mainly involves reactor size and stirrer size, and is generally based on homothetic designs from pilot tests. The similitude in the scale-up means that the following parameters are - or at least should be - kept constant ... 

Regarding this new edition first of all I should say that in spirit it follows the earlier ones, and I try to keep things simple. In fact, I have removed material from here and there that I felt more properly belonged in advanced books. But I have added a number of new topics—biochemical systems, reactors with fluidized solids, gas/liquid reactors, and more on nonideal flow. The reason for this is my feeling that students should at least be introduced to these subjects so that they will have an idea of how to approach problems in these important areas. 

Double-impeller combinations Bouaifi et al. (2001) derived the following correlations for stirred gas-liquid reactors with various combinations of double impellers. The impellers used were the lightning axial flow impeller (A-310), the four 45° pitched blade turbine pumping down (PBTD) and the Rushton disk turbine (RDT). Furthermore, the tank was a dish-bottom cylindrical tank equipped with four baffles, while the gas was introduced by a ring sprager. The gas-flow rate ranged from 0.54 to 2.62 L/s, whereas the rotational speed was from 1.66 to 11.67 s. The gas holdup is... 

The correlations for as discussed above are for homogeneous liquids. Bubbling gas-liquid reactors are sometimes used for suspensions, and bioreactors of this type must often handle suspensions of microorganisms, cells, or immobilized cells or enzymes. Occasionally, suspensions of nonbiological particles, to which organisms are attached, are handled. Consequently, it is often necessary to predict how the values for suspensions will be affected by the system properties and operating conditions. In fermentation with a hydrocarbon substrate, the substrate is usually dispersed as droplets in an aqueous culture medium. Details of... 

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