Two Phase Gas-Liquid Reactions

This type of reaction is relatively simple for laboratory investigation. A number of inexpensive model contactors are available for these investigations. These include stirred cell and laminar jet apparatus (Danckwerts 1970). The gradient-less contactor 

)/ is the stoichiometric factor. We consider the general case where the orders of the reactions are p and q with respect to the gaseous and liquid-phase reactant, respectively. For the sake of simplicity, the film model is chosen for the discussion that follows. 

This situation can be explained through the concept of characteristic times for the two steps of diffusion and reaction (Astarita 1967). We define the characteristic diffusion time and reaction time, r, respectively, for the diffusion across the liquid film and the reaction between the dissolved gaseous species and the liquid-phase reactant The reciprocals of these characteristic limes signify the respective rate coefficients. Thus, 

Regime Conditions to be satisfied Rate expression Comment 

B Ha 1 kj a depends on hydrodynamics and is always first order with respect to [A ] but independent of [S ] 

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A batch reactor is an agitated vessel in which the reactants are precharged and which is then emptied after the reaction is completed. More frequently for exothermic reactions, only part of the reactants are charged initially, and the remaining reactants and catalysts are fed on a controlled basis this is called a semi-batch operation. For highly exothermic reactions and for two-phase (gas-liquid) reactions, loop reactors with resultant smaller volumes can be used. 

Wood, J, (2007) Two phase gas-liquid reaction studies in a circular capillary. Chem. Eng. Sci, 62, 5397-5401,... 

As mentioned earlier many two-phase gas-liquid flows are highly dynamic and this property requires the use of dynamic simulation methods. A well-known example of such a type of flow is encountered in bubble columns where recirculating flow structures are present which are not stationary but which continuously change their size and location in the column. Due to their frequent application in chemical reaction engineering and their relatively simple geometry, CFD analyses of bubble columns have received significant attention and are discussed in more detail here. 

We apply the concepts discussed above to design a CSTR that operates at 55 °C for the chlorination of benzene in the hquid phase. It is necessary to account for all three chlorination reactions. Chlorine gas is bubbled through the liquid mixture in the CSTR and it must diffuse across the gas-liquid interface before any of the reactions can occur. For this particular problem, it is reasonable to assume that chlorine is present as a solubilized liquid-phase component, and its molar density in the inlet liquid stream is given as a fraction e of the inlet molar density of pure liquid benzene. In a subsequent example discussed in Chapter 24, a two-phase gas-liquid CSTR analysis is presented which accounts for the realistic fact that benzene enters the reactor in an undiluted liquid stream, and chlorine is actually bubbled through as a gas. It is sufficient to consider that the fraction e = 0.25 remains constant for all simulations. In the first chlorination step, benzene reacts irreversibly with dissolved chlorine to produce monochlorobenzene and hydrogen chloride ... 

Design a two-phase gas-liquid CSTR that operates at 55°C to accomplish the liquid-phase chlorination of benzene. Benzene enters as a liquid, possibly diluted by an inert solvent, and chlorine gas is bubbled through the liquid mixture. It is only necessary to consider the first chlorination reaction because the kinetic rate constant for the second reaction is a factor of 8 smaller than the kinetic rate constant for the first reaction at 55°C. Furthermore, the kinetic rate constant for the third reaction is a factor of 243 smaller than the kinetic rate constant for the first reaction at 55°C. The extents of reaction for the second and third chlorination steps ( 2 and 3) are much smaller than the value of for any simulation (i.e., see Section 1-2.2). Chlorine gas must diffuse across the gas-liquid interface before the reaction can occur. The total gas-phase volume within the CSTR depends directly on the inlet flow rate ratio of gaseous chlorine to hquid benzene, and the impeller speed-gas sparger combination produces gas bubbles that are 2 mm in diameter. Hence, interphase mass transfer must be considered via mass transfer coefficients. The chemical reaction occurs predominantly in the liquid phase. In this respect, it is necessary to introduce a chemical reaction enhancement factor to correct liquid-phase mass transfer coefficients, as given by equation (13-18). This is accomplished via the dimensionless correlation for one-dimensional diffusion and pseudo-first-order irreversible chemical reaction ... 

Design a two-phase gas-liquid CSTR for the chlorination of benzene at 55°C by calculating the total volume that corresponds to an operating point where r/X = 500 on the horizontal axis of the CSTR performance curve in Figure 24-1. The time constant for convective mass transfer in the liquid phase is r. The time constant for second-order irreversible chemical reaction in the liquid phase is If the liquid benzene feed stream is diluted with an inert, then 7 increases. The liquid-phase volumetric flow rate is 5 gal/min. The inlet molar flow rate ratio of chlorine gas to liquid benzene... 

Similar to the liquid-liquid system, the volume-surface diameter of dispersed phase particles in a liquid-gas flow is determined by the initial size of the bubbles at the gas input points [81-83]. An increase of the liquid-gas flow rate leads to an increase of the shear deformation influence of the dispersed phase particles and therefore, to a decrease in the diameter of the gas bubbles in the input area of the device. Finally, it leads to the formation of a finely dispersed system in a device with an increase of the liquid-gas flow rate (Figure 2.23). Fast chemical reactions in two-phase gas-liquid systems usually occur in a gas-phase excess, so it is reasonable to analyse the influence of the gas content in a flow on the change of phase contact surface. 

An important consequence of this cathodic reaction is the fact that the I EG becomes filled with a two-phase, gas-liquid... 

In heterogeneous non-catalytic reactors, the reaction medium is a two-phase medium. The two-phase reaction medium is composed of a gas phase and a solid phase for the reaction between a gaseous reactant and a solid reactant, whereas for the reaction between a gaseous reactant and a liquid reactant the reaction phase is a two-phase gas-liquid medium. The design of heterogeneous non-catalytic reactors for gas-solid reactions and gas-liquid reactions are discussed in this section. 

Ideally, the model reaction is the same as the reaction under investigation. The difficulty here is to establish the conditions under which R2 Ri, R3 and Ri. Alternatively, also one of the many well known gas-liquid model reactions may be applied for the purpose of measuring kj a but then the problem is to carry out that model reaction in a slurry with properties as close as possible to the actual slurry under investigation both with respect to the liquid and the solids. Probably most popular nowadays are transient techniques, often using physical absorption only [62,63] but also with reaction (see e.g. [64,65]). In fact, there exists quite a variety of methods to measure kj a and we refer to the methods of Laurent et al. [66] and Sharma et al. [67,68] which concentrate on gas-liquid systems but are equally well applicable to gas-slurry systems. As long as the presence of the solids does not affect k and a, the existing relations for kj a in two phase gas-liquid reactors may be applied to predict k a for slurry reactors too. For reviews summarizing the available information for gas liquid reactors see [69-79]. 

The hydroformylation of olefins is a type of CO insertion reaction that is one of the most important industrial applications of homogeneous catalysis with transition metal complexes (208,209). Conventional industrial processes (e.g., the Oxo process) typically use either cobalt- or rhodium-based catalysts and conduct the reaction in two-phase gas-liquid reactors. Efficient transfer of the reactants from the gas phase into the liquid phase is of primary importance to minimize inherent mass transfer limitations (208). Reactor design thus focuses on optimizing this mass transfer rate by maximizing the interfacial area between phases. An SCE process eliminates this transport restriction since the hydrogen... 

Chapter 11 treats reactors where mass and component balances are needed for at least two phases and where there is interphase mass transfer. Most examples have two fluid phases, typically gas-liquid. Reaction is usually confined to one phase, although the general formulation allows reaction in any phase. A third phase, when present, is usually solid and usually catalytic. The solid phase may be either mobile or stationary. Some example systems are shown in Table 11.1. 

Liquid-liquid reactors. Examples of liquid-liquid reactions are the nitration and sulfonation of organic liquids. Much of the discussion for gas-liquid reactions also applies to liquid-liquid reactions. In liquid-liquid reactions, mass needs to be transferred between two immiscible liquids for the reaction to take place. However, rather than gas-and liquid-film resistance as shown in Figure 7.2, there are two liquid-film resistances. The reaction may occur in one phase or both phases simultaneously. Generally, the solubility relationships are such that the extent of the reactions in one of the phases is so small that it can be neglected. 

In this chapter, we consider multiphase (noncatalytic) systems in which substances in different phases react. This is a vast field, since the systems may involve two or three (or more) phases gas, liquid, and solid. We restrict our attention here to the case of two-phase systems to illustrate how the various types of possible rate processes (reaction, diffusion, and mass and heat transfer) are taken into account in a reaction model, although for the most part we treat isothermal situations. 

In this chapter, we consider process design aspects of reactors for multiphase reactions in which each phase is a fluid. These include gas-liquid and liquid-liquid reactions, although we focus primarily on the former. We draw on the results in Section 9.2, which treats the kinetics of gas-liquid reactions based on the two-film model. More detailed descriptions are given in the books by Danckwerts (1970), by Kastanek et al. (1993), and by Froment and Bischoff (1990, Chapter 14). 

We first present further examples of the types of reactions involved in two main classifications, and then a preliminary discussion of various types of reactors used. Following an examination of some factors affecting the choice of reactor, we develop design equations for some reactor types, and illustrate their use with examples. The chapter concludes with a brief introduction to trickle-bed reactors for three-phase gas-liquid-solid (catalyst) reactions. 

To incorporate mixing by the dispersed plug flow mechanism into the model for the bubble column, we can make use of the equations developed in Chapter 2 for dispersed plug flow accompanied by a first-order chemical reaction. In the case of the very fast gas-liquid reaction, the reactant A is transferred and thus removed from the gas phase at a rate which is proportional to the concentration of A in the gas, i.e. as in a homogeneous first-order reaction. Applied to the two-phase bubble column for steady-state conditions, equation 2.38 becomes ... 

Danckwerts, P. V. (1970). Gas-Liquid Reactions. McGraw-Hill, New York. Culick, F. E. C. (1964). Boltzmann equation applied to a problem of two phase flow. Physics of Fluids, 7 1894-1904. 

Heterogeneous reactions involve two or more phases. Examples are gas-liquid reactions, solid catalyst-gas phase reactions and products, and reactions between two immiscible liquids. Catalytic reactions as illustrated in Chapter 1 involve a component or species that participates in various elementary reaction steps, but does not appear in the overall reaction. In heterogeneous systems, mass is transferred across the phase. 

Within the German public funded project p.PRO.CHEM, a concept for a continuously operated modularly assembled flexible pilot plants for highly exothermic two-phase liquid-liquid or gas-liquid reactions will be developed and validated [48]. The plant features process intensifying microprocess technologies. A goal of the project is the demonstration of the technical and economic feasibility of the plant concept on pilot scale with selected model processes. 

Experimental measurements of absorption fluxes and colour development for the gas-liquid reaction between sulphur trioxide and dodecylbenzene have been carried out in a stirred cell absorber. A model with two parallel reaction paths representing sulphonation and discolouration has been applied to analyse the exothermic absorption accompanying conversions up to 70%. The results show that the two reactions have similar activation energies and that temperature increases greater than 100°C occur at the interface during absorption. The absorption enhancement factor exhibits a maximum value as liquid phase conversion proceeds. 

Two-phase vapor-liquid flow of the type that can affect relief system design occurs as a result of vaporization and gas generation during a runaway reaction. Boiling can take place throughout the entire volume of liquid, not just at the surface. Trapped bubbles, retareded by viscosity and the nature of the fluid, reduce the effective density of the fluid and cause the liquid surface to be raised. When it reaches the height of the relief device, two-phase flow results. 

In the third type of gas-liquid-solid reaction, only two of the three phases take part into the reaction, the third phase being an inert phase. This type of reaction can be further subdivided into three catagories. Some reactions are strictly gas liquid reactions, but they are often carried out in packed-bed reactors operating under countercurrent-flow conditions. Here, the solid imparts momentum transfer and allows better gas-liquid contact and gas-liquid interfacial mass... 

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