Chaudhari and Ramachandran

The thrust of this chapter is on reactions and reactors involving a gas phase, a liquid phase, and a solid phase which can be either a catalyst (but not a phase-transfer catalyst) or a reactant, with greater emphasis on the former. The book by Ramachandran and Chaudhari (1983) on three-phase catalytic reactions is particularly valuable. Other books and reviews include those of Shah (1979), Chaudhari and Ramachandran (1980), Villermaux (1981), Shah et al. (1982), Hofmann (1983), Crine and L Homme (1983), Doraiswamy and Sharma (1984), Tarmy et al. (1984), Shah and Deck wer (1985), Chaudhari and Shah (1986), Kohler (1986), Chaudhari et al. (1986), Hanika and Stanek (1986), Joshi et al. (1988), Concordia (1990), Mills et al. (1992), Beenackers and Van Swaaij (1993), and Mills and Chaudhari (1997). Doraiswamy and Sharma (1984) also present a discussion of gas-liquid-solid noncatalytic reactions in which the solid is a reactant. 

Figure 17.3 Overall eflectiveness factor plots for LHHW kinetics (Chaudhari and Ramachandran, 1980). = (I + kA(/l) )/ vk. A in figure = [A]. ...

Chaudhari and Ramachandran (8-9) give a detailed analysis of slurry reactors including all mass transfer resistances and different kinds of kinetic expressions but they do not account for catalyst settling, dispersion and conversion induced volume change in the gas phase. 

Chaudhari and Ramachandran [7] derived expressions for R.a for other kinetic laws, as well. A selection is given in Table 2. 

In many industrial applications BCR are operated in a semi-batch manner. Only the gas is in continuous flow while the liquid is batch. As shown by Chaudhari and Ramachandran [7], the reaction time needed for a desired conversion of B can be calculated easily if the assumption underlying models <11>, <12>, and <13> are fulfilled. The variation of B follows from... 

For conditions where this criterion and Equations (69) are satisfied, Chaudhari and Ramachandran recently published design equations taking the internal particle diffusion effects into account [l58]. They applied this model to the hydrogenation of DNT, the ethynylation of formaldehyde and the oxidation of cyclohexene. Fig. 28 shows the influence of pressure on the conversion rate. The decrease in ("R ) below a critical pressure indeed could be predicted by this model within 23%. A complication not accounted for... 

Note that in the previous two cases, [/42]b was assumed to be either zero or equal to its interfacial concentration (solubility) [A2]. The situation gets quite complicated mathematically when [/42]b has a finite value between zero and the solubility, that is, 0 < [/42)b < To write the absorption equations for this case, knowledge of the concentration profiles in the films is necessary. Ramachandran and Sharma (1971) assumed that the profiles are linear, based on which an approximate solution was obtained. The assumption of a linear profile is only a first-order approximation, because the simultaneous occurrence of diffusion and reaction can lead to a nonlinear profile in the film. This problem was addressed by Sada et al. (1976) and Chaudhari and Doraiswamy (1974) who assumed certain nonlinear profiles and obtained analytical solutions that gave results comparable to those from numerical solution. 

A hst of 74 GLS reacdions with hterature references has been compiled by Shah Gas-Liquid-Solid Reactions, McGraw-HiU, 1979), classified into groups where the solid is a reactant, or a catalyst, or inert. A hst of 75 reactions made by Ramachandran and Chaudhari (Three-Phase Chemical Reactors, Gordon and Breach, 1983) identifies reactor types, catalysts, temperature, and pressure. They classify the processes according to hydrogenation of fatty oils, hydrodesulfurization, Fischer-Tropsch reactions, and miscellaneous hydrogenations and oxidations. 

Ramachandran, P. A. and Chaudhari, R. V., Three Phase Catalytic Reactors, Gordon and Breach Science Publishers. 

Ramachandran, RA. and R.V. Chaudhari, Three Phase Catalystic Reactors. 1983 Gordon and Breach. 

Ramachandran, P A, and R V Chaudhari, Three Phase Chemical ReactorsA Gordon Breach, 1983... 

If the process is continuous and in the complete mixed-flow mode, for both the gas and slurry phases, the equations derived for agitated sluny reactors are valid (see Section 3.5.1) (Ramachandran and Chaudhari, 1980) by simply applying the appropriate mass transfer coefficients. Note that in sluiiy-agitated reactors, the material balances are based on the volume of the bubble-free liquid. Furthermore, in reactions of the form aA(g) + B(l) — products, if gas phase concentration of A is constant, the same treatment holds for the plug flow of the gas phase. 

For maximum utilization of the solid phase in a bubble column, it is essential that all particles be suspended in the reactor (Ramachandran and Chaudhari, 1984). This means that the gas velocity should be sufficiently high to enable suspension of all solids in the liquid. In slurry bubble column reactors, two suspension states exist ... 

The gas velocities that are required for homogeneous suspension are far greater than those required for complete suspension. The gas velocity required to achieve complete suspension can be obtained from the correlation of Roy et al. (Ramachandran and Chaudhari, 1984) ... 

For the heterogeneous flow regime, the Akita-Yoshida correlation derived for bubble column reactors is proposed (Akita and Yoshida, 1973 Ramachandran and Chaudhari, 1984 Behkish, 2004 Koide, 1996) ... 

The liquid-phase dispersion coefficient can be estimated using the Deckwer et al. (1974) correlation (Ramachandran and Chaudhari, 1980) ... 

The above correlations have been suggested for gas-hquid systems. In the presence of a solid, in three-phase systems the energy of dissipation can be approximately 25-50% higher depending on the loading and density of particles (Ramachandran and Chaudhari, 1984). 

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