Gas-Liquid Mass Transfer with Reaction

When the reaction rate is comparable to that of the mass transfer throngh the diffnsion film, interactions mnst be taken into acconnt. The interactions can be delineated as five regimes, as shown in Fignre 11-27. These are identified by the 

REGIME CONDITIONS IMPORTANT VARIABLES CONCENTRATION PROFILES  

V Instantaneous reaction Reaction at Interface . Controlled by transfer of B to Interface from bulk. J a kLa R 9Cal Rate a a a kL Independent of Cal Independent of k Independent of El P al/ j Cab 

For regimes III and IV the reaction effectively enhances the mass transfer rate, and an enhanced effective valne of kL is often used, defined as kp 

Note that this is now a fnnction of the reaction rate, not the hydrodynamics. If heat of reaction is significant, this expression must be modified to allow for the effects of local temperature on gas solubility and reaction rate (Mann and Moyes, 1977). 

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DP E F f f. Ha He AG Degree of polymerization Activation energy, enhancement factor for gas-liquid mass transfer with reaction, electrochemical cell potential Faraday constant, F statistic Efficiency of initiation in polymerization Ca/CaQ or na/nao, fraction of A remaining unconverted Hatta number Henry constant for absorption of gas in liquid Free energy change kj/kgmol Btu/lb-mol... 

Dk DP E Knudsen diffusivity Degree of polymerization Activation energy, enhancement factor for gas-liquid mass transfer with reaction. mVs ft"/s... 

Figure 11-27 Regimes of gas-liquid mass transfer with reaction. (From Middleton, 1997 reproduced by permission of Butterworth-Heinemann.)...

Fluid-fluid systems are widely used in chemical, petroleum, pharmaceutical, hydrometaflurgical, and food industries. Commercially important examples of gas-liquid mass transfer with or without reaction include gas purification, oxidation, halogenations, hydrogenation, and hydroformylation to name but a few. Important liquid-liquid reactions include nitration, phase transfer catalysis (PTC), cyclization, emulsion polymerization, homogenous catalyst screening, enzymatic reactions, extraction, precipitation, crystallization, and cell separation. 

Example 11.8 With highly reactive absorbents, the mass transfer resistance in the gas phase can be controlling. Determine the number of trays needed to reduce the CO2 concentration in a methane stream from 5% to 100 ppm (by volume), assuming the liquid mass transfer and reaction steps are fast. A 0.9-m diameter column is to be operated at 8 atm and 50°C with a gas feed rate of 0.2m /s. The trays are bubble caps operated with a 0.1-m liquid level. Literature correlations suggest = 0.002 m/s and A, = 20m per square meter of tray area. 

Chapter 24. In this chapter, we are concerned with the kinetics of these reactions, and hence with reaction models, which may have to include gas-liquid mass transfer as well as chemical reaction. 

Hydrogen transfer from the gas phase to the liquid phase becomes rate limiting with very fast hydrogenations (or with insufficient agitation). The observed reaction rate is then equal to the rate of gas-liquid mass transfer of hydrogen and becomes first order in hydrogen and independent of substrate concentration. The activation energy decreases to that of a diffusion process. 

The heart of the pilot plant study normally involves varying the speed over two or three steps with a given impeller diameter. The analysis is done on a chart, shown in Fig. 36. The process result is plotted on a log-log curve as a function of the power applied by the impeller. This, of course, implies that a quantitative process result is available, such as a process yield, a mass transfer absorption rate, or some other type of quantitative measure. The slope of the line reveals much information about likely controlling factors. A relatively high slope (0.5-0.8) is most likely caused by a controlling gas-liquid mass transfer step. A slope of 0, is usually caused by a chemical reaction, and a further increase of power is not reflected in the process improvement. Point A indicates where blend time has been satisfied, and further reductions of blend time do not improve the process performance. Intermediate slopes on the order of 0.1-0.4, do not indicate exactly which mechanism is the major one. Possibilities are shear rate factors, blend time requirements, or other types of possibilities. 

E will be different from 1 only if R4 is small relative to / 2, resulting in a bulk concentration of c — 0 and in a real parallel mechanism of the enhancement. The advantage of the concept of the enhancement factor as defined by eq 33 is the separation of the influence of hydrodynamic effects on gas-liquid mass transfer (incorporated in Al) and of the effects induced by the presence of a solid surface (incorporated in E ), indeed in a similar way as is common in mass transfer with homogeneous reactions. The above analysis shows that an adequate description of mass transfer with chemical reaction in slurry reactors needs reliable data on ... 

Estimation of gas-liquid mass-transfer rates also requires the knowledge of solubilities of absorbing and/or desorbing species and their variations with temperature (i.e., knowledge of heats of solution). In some reactions, such as hydrocracking, significant evaporation of the liquid occurs. The heat balance in a hydrocracker would thus require an estimation of the heat of vaporization of the oil as a function of temperature and pressure. The data for the solubility, heat of solution, and heat of vaporization for a given reaction system should be obtained experimentally if not available in the literature. 

The separation performance of the reaction section is key to realizing the full potential of catalytic distillation as described above. Efficient gas-liquid mass transfer ensures immediate removal of the products from the reaction zone, effectively shifting the equilibrium such that reactants arc completely converted. Poor separation, on the other hand, will result in the reaction s proceeding little beyond the equilibrium value associated with a fixed bed operating under the same conditions. The remaining reactants will then have to be separated from the products downstream. The potential of catalytic distillation to avoid difficulties in separation due to azeotropes with the reactants is then lost. (It should be kept in mind, however, that while this is one of the major attractions of catalytic distillation, the process cannot help avoid all azeotropes that may form.)... 

Although the penetration theory better describes the gas-liquid mass transfer than the film theory, its advantage is only significant with physical absorption. For gas absorption with fast chemical reactions in the liquid phase, the entire mass transfer process is gas-phase-controlled rendering the penetration theory inapplicable.f ... 

Several of the factors of Figure 3 controlling the activity and selectivity of the biphasic selective hydrogenation of ,/ -unsaturated aldehydes to allylic alcohols, for instance, 3-methyl-2-butenaldehyde to 3-methyl-2-buten-l-ol (Eq. 11) with rutheni-um-sulfonated phosphine catalysts were investigated [11], such as the effect of agitation speed and the influence of aldehyde, ligand, and metal concentrations. Under optimized reaction conditions, where gas-liquid mass transfer was not rate-determining, the kinetic equation (Eq. 12) was found to apply. A zero-order dependence with respect to the concentration of the ,/i-unsaturated aldehyde was found. 

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