Mass transfer homogeneous reactions

Homogeneous reactions occur within one phase, here taken to be fluid. Included are reactions in which a reactant is supplied from another phase by mass transfer. Multistep reactions consist of a combination of elementary steps. No distinction is made between complex reactions (with trace-level intermediates) and multiple reactions (with intermediates at higher than trace concentrations). 

Behavior of this general type is also occasionally found in systems in which a reactant must be supplied from another phase by mass transfer, e.g., in heterogeneous catalysis or homogeneous hydrogenation, air oxidation, etc. Usually, the activation energy of the reaction is fairly high, that of mass transfer only a few kilojoule per mole. Rate control may then shift from reaction at low temperature to mass transfer or a combination of mass transfer and reaction at high temperature. 

A number of fine reviews have appeared recently which address in part the problems mentioned and the models employed. Rietema (R12) discusses segregation in liquid-liquid dispersion and its effect on chemical reactions. Resnick and Gal-Or (RIO) considered mass transfer and reactions in gas-liquid dispersions. Shah t al. (S16) reviewed droplet mixing phenomena as they applied to growth processes in two liquid-phase fermentations. Patterson (P5) presents a review of simulating turbulent field mixers and reactors in which homogeneous reactions are occurring. In Sections VI, D-F the use of these models to predict conversion and selectivity for reactions which occur in dispersions is discussed. 

In industrial practice, the most convenient way of accounting for mass-transfer effects is to view the penetrable catalyst particle as a pseudo-homogeneous phase. Obstruction of mass transfer by the solid material in the particle then is reflected by an "effective" intraparticle mass-transfer or diffusion coefficient that is appropriately lower than in the contacting fluid. If this approach is taken, two fundamentally different mass-transfer situations appear Mass transfer to and from the particle across an adherent boundary layer is affected by the reaction only in that the latter sets the boundary condition at the particle. Here, mass transfer and reaction are sequential and occur in different parts of the system, and the slower of the two is the bottleneck and dictates the overall rate and its temperature dependence. Within the particle, however, mass transfer and reaction occur simultaneously and in the same volume element. Here, the reaction introduces a source-or-sink term into the basic differential material balance. If the reaction is slow, it alone controls the overall rate and its temperature dependence. If mass transfer is slow, both reaction and mass transfer affect the rate, and the apparent reaction order and activation energy are the arithmetic means of those of reaction and mass transfer. 

Another way of combining the terms for mass transfer and reaction in series is to use N, the number of reaction units, and N, the number of mass transfer units. The group kpi,L/ug is equivalent to kt for a homogeneous reaction and is called the number of reaction units. The group KL/ug is the number of mass transfer units and is equivalent to the NTU in mass transfer operations such as gas absorption. Using Eq. (9.19), these terms can be combined to give N, the overall number of units for mass transfer and reaction ... 

To demonstrate one such difference, Figure 2 shows the observed rate for a heterogeneous catalytic oxidation reaction as a function of temperature. At a low temperature, the reaction lakes place on the catalyst surface. Although the overall reaction rate can be controlled by surface kinetics or mass transfer, the reaction still occurs on the catalyst surface. As the temperature is raised, as may occur in a flame, the bulk gas temperature is high enough that the homogeneous gas phase reaction and catalytic reaction occur simultaneously. Finally, the homogeneous reaction dominates. 

Third, the restriction associated with the mass action law was rmtil now used without consideration of kinetics as it was applied only to sufficiently fast reactions. However, in the interaction between water and rock participate reactions of various kinetics. Whereas the relaxation of homogeneous processes in water completes in hours or minutes, it is between water and rock, especially silicate ones, may last for years and decades. Simultaneously, in conditions of lowered temperature hypogene minerals only dissolve, hypergene ones dissolve or form, and the ground water composition continuously maintains thermodynamical equilibrium. In order to account for such kinetic variety of the mass transfer processes, they are tentatively subdivided into two groups reactions of irreversible mass transfer and reactions of instantaneous relaxation. 

According to this equation, the concentration of any component C. is a function of not only time but also of the water saturation state by individual minerals which continuously changes due to the mass transfer and reactions of homogeneous relaxation. 

Distillation with reaction, where the normal process is coupled with a liquid phase reaction, is also interesting and esterifications of certain alcohols with acids are typical industrial applications. These include, among others the homogeneously catalyzed butyl acetate process and the production of the plasticizer di-octyl-phthalate from phthalic anhydride and 2-ethyl-hexanol. However, the subject which involves both product formation and separation aspects has not usually been treated in the literature relating specifically to "mass transfer with reaction". 

Moreover, gas-liquid reactors represent the simplest departure from reactors which involve purely homogeneous reactions, but nevertheless the interactions of the processes of simultaneous mass transfer and reaction in gas-liquid reactors have presented and continue to present a formidable challenge to the applied science of chemical reaction engineering. This is particularly true of the vigorous intense reactions which are involved in the production of many major chemical intermediates. For example, in many liquid phase hydrocarbon oxidation processes, partial oxidation competes with complete combustion to produce a wide range of oxidised hydrocarbons, where the kinetics may be summarised as... 

In a gas—sohd CFB with heterogeneous reactions and mass transfer, in Hne with the structural characteristics of the SFM model (Hong et al, 2012), as shown in Fig. 12, the mass transfer and reaction in any local space can be divided into components of the dense cluster (denoted by subscript c), the dilute broth (denoted by subscript f), and in-between (denoted by subscript i), respectively. And these terms can be represented by Ri (1 = gc, gf, gi, sc, sf, si). Both the dense and dilute phases are assumed homogenous and continuous inside, and the dense phase is fiarther assumed suspended uniformly in the dilute phase in forms of clusters of particles. Then the mass transfer terms can be described with Ranz-Marshall-hke relations for uniform suspension of particles (Haider and Basu, 1988). In particular, the mesoscale interaction over the cluster will be treated as is for a big particle with hydrodynamic equivalent diameter of d. Due to dynamic nature of clusters, there are mass exchanges between the dilute and dense phases with rate ofTk (k = g, s), pointing outward from the dilute to the dense phase. 

Before we proceed further, we need to differentiate a homogeneous and a heterogeneous reaction system. We will do so based on the relative rates of mass transfer and reaction kinetics. A fast reaction can only take place at the interface therefore, the rate expression is expressed in the boundary condition. On the contrary, a slow reaction at rates comparable to that of diffusion resides mathematically in the continuity equation. In vector form, we express the continuity equations as follows ... 

Unlike homogeneous reactions, even in the laboratory, overall reaction time can have an effect on yield that is caused by the effect of mixing on mass transfer rate if there are parallel reactions in the continuous phase or in the films between phases. With slower mass transfer, these reactions have longer to generate by-products—often decomposition products—so that time of reaction is important on all scales. Determination of this possibility must be included in the experimental plan. Increased amounts of S and other by-products for longer overall reaction times would indicate this sensitivity to mass transfer rate. 

Mass-Transfer Coefficients with Chemical Reaction. Chemical reaction can occur ia any of the five regions shown ia Figure 3, ie, the bulk of each phase, the film ia each phase adjacent to the iaterface, and at the iaterface itself. Irreversible homogeneous reaction between the consolute component C and a reactant D ia phase B can be described as... 

Fluid mixing is a unit operation carried out to homogenize fluids in terms of concentration of components, physical properties, and temperature, and create dispersions of mutually insoluble phases. It is frequently encountered in the process industry using various physical operations and mass-transfer/reaction systems (Table 1). These industries include petroleum (qv), chemical, food, pharmaceutical, paper (qv), and mining. The fundamental mechanism of this most common industrial operation involves physical movement of material between various parts of the whole mass (see Supplement). This is achieved by transmitting mechanical energy to force the fluid motion. 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

Consider a closed PVT system, either homogeneous or heterogeneous, of uniform T and P, which is in thermal and mechanical equilibrium with its surroundings, but which is not initially at internal equilibrium with respect to mass transfer or with respect to chemical reaction. Changes occurring in the system are then irreversible, and must necessarily bring the system closer to an equihbrium state. The first and second laws written for the entire system are... 

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

For the first assumption, the value of Kw for the shift appears to be too high. It must be this high because it is necessary to make C02 appear while both C02 and CO are being consumed rapidly by methanation. The data may be tested to see if the indicated rate appears unreasonable from the standpoint of mass transfer to the gross catalyst surface. Regardless of the rate of diffusion in catalyst pores or the surface reaction rate, it is unlikely that the reaction can proceed more rapidly than material can reach the gross pill surface unless the reaction is a homogeneous one that is catalyzed by free radicals strewn from the catalyst into the gas stream. 

Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

---