Mass Transfer Complicated by a Surface Chemical Reaction

Mass Transfer Complicated by a Surface Chemical Reaction 

Boundary condition on a surface. If a surface (heterogeneous) chemical reaction with finite rate occurs on the interface, the (3.1.4) must be replaced by the more complicated boundary condition [133,270] 

The specific form of the function Fs = FS(C) is determined by the kinetics of the surface chemical reaction. The function Fs must satisfy the condition Fs(0) = 0, whose physical meaning is obvious if the reagent is absent, there is no reaction. For reactions of rate order n, in (3.1.5) one must set [270] 

We point out that in a majority of cases the form of the function FS(C) does not describe the actual kinetics of catalytic chemical transformations but only determines the effective rate of chemical reaction. 

Mass Transfer in Films, Tubes, and Boundary Layers 

---

In the last two chapters, relatively simple linear problems of mass and heat transfer were discussed. However, no processes of mass transfer complicated by surface (heterogeneous) or volume (homogeneous) chemical reactions with finite rates have been considered so far. Moreover, it was assumed that the basic parameters of the fluid are temperature- and concentration-independent. This assumption permitted the hydrodynamic part of the problem to be solved first and then the linear thermal or diffusion problem to be considered for a known velocity field. 

A to products by considering mass transfer across the external surface of the catalyst. In the presence of multiple chemical reactions, where each iRy depends only on Ca, stoichiometry is not required. Furthermore, the thermal energy balance is not required when = 0 for each chemical reaction. In the presence of multiple chemical reactions where thermal energy effects must be considered becanse each AH j is not insignificant, methodologies beyond those discussed in this chapter must be employed to generate temperature and molar density profiles within catalytic pellets (see Aris, 1975, Chap. 5). In the absence of any complications associated with 0, one manipulates the steady-state mass transfer equation for reactant A with pseudo-homogeneous one-dimensional diffusion and multiple chemical reactions under isothermal conditions (see equation 27-14) ... 

The reaction is carried out over a silver gauze or low surface supported catalyst at 600—700°C, indicating a very fast chemical reaction. This implies that determination of the intrinsic reaction rate in laboratory reactors is complicated by the interference of heat and mass transfer limitations. To avoid this problem, studies have been made at much lower temperatures, which in turn run the risk of being non-representative. 

A gas-solid reaction usually involves heat and mass transfer processes and chemical kinetics. One important factor which complicates the analysis of these processes is the variations in the pore structure of the solid during the reaction. Increase or decrease of porosity during the reaction and variations in pore sizes would effect the diffusion resistance and also change the active surface area. These facts indicate that the real mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure during the reaction. 

Most reactions on surfaces are complicated by variations in mass transfer and adsorption equilibrium [70], It is precisely these complexities, however, that afford an additional means of control in electrochemical or photoelectrochemical transformations. Not only does the surface assemble a nonstatistical distribution of reagents compared with the solution composition, but it also generally influences both the rates and course of chemical reactions [71-73]. These effects are particularly evident with photoactivated surfaces the intrinsic lifetimes of both excited states and photogenerated transients and the rates of bimolecular diffusion are particularly sensitive to the special environment afforded by a solid surface. Consequently, the understanding of surface effects is very important for applications that depend on chemical selectivity in photoelectrochemical transformation. 

A trickle bed reactor (TBR) consists of a fixed bed of catalyst particles, where liquid and gas phases flow cocurrently downward through the bed. Although its wide application in chemical and petrochemical industry it is one of the most complicated type of reactor in its design and scale-up. Essencially, the overall rate can be controlled by one or a combination of the following processes mass transfer between interphases, intraparticle diffusion, adsorption and surface reaction. The hydrodynamics, solid-liquid contacting efficiency and axial mixing can also affect the performance of TBR. 

Step), and leave the reaction area into bulk solution (second mass transfer). The mass transfer step, as well as the electrochemical one, are always present in any electrochemical transformation. Importantly, the electrochemical step is always accompanied by transfer of a charged particle through the interface. That is why this step is called the transfer step or the discharge-ionization step. Other complications are also possible. They are related to the formation of a new phase on the electrode (surface diffusion of adatoms, recombination of adatoms, formation of crystals or gas bubbles, etc.). The transfer step may be accompanied by different chemical reactions, both in bulk and on the electrode surface. A set of all the possible transformations is called the electrode process. Electrochemical kinetics works with the general description of electrode processes over time. While related to chemical kinetics, electrochemical kinetics has several important features. They are specific to the certain processes, in particular - the discharge-ionization step. Determination of a possible step order and the slowest (rate-determining) step is crucial for the description of the specific electrode process. 

Furthermore, the high reactivity of sulfur compounds poses a host of difficulties during chemical analysis. Irreversible losses, elimination, and oxidation reactions catalyzed by heated metal surfaces easily take place during the sampling and transfer of sulfur compounds. Even oxidants in ambient air are known to oxidize analytes sampled cryogenically or on solid adsorbents. In addition, the dryers necessary for the cryogenic and adsorptive sampling of low molecular mass compounds can cause severe losses of sulfur compounds. Last but not least, the absorptive, adsorptive, and photooxidative behavior of sulfur compounds complicate their analysis. 

---