Gaseous phase reactants

Three-phase reactor systems are ideally suited for methanol production because of the ability to provide intimate contact between the gaseous phase reactants and the solid phase catalysts and to remove the large amounts of heat created by the high heats of reaction. In the three-phase system, an inert liquid phase circulates between the reactor and an external... 

Selective deposition in PEVD is clearly indicated in Figure 13. As discussed previously, this is because of the unique feature of the electrochemical reaction for deposition. As schematically shown in Figure 18, the only area to meet the requirement for nucleation is the three phase boundary (point O) of Na -P"-alumina, Pt and gaseous phase, where Na, e and gaseous phase reactants (CO + O ) are all... 

The use of a fluidized-bed reactor is possible only when the reactants are essentiaUy in the gaseous phase. Eluidized-beds are not suitable for middle distiUate synthesis, where a heavy wax is formed. Eor gasoline synthesis processes like the MobU MTG process and the Synthol process, such reactors are especiaUy suitable when frequent or continuous regeneration of the catalyst is required. Slurry reactors and ebuUiating-bed reactors comprising a three-phase system with very fine catalyst are, in principle, suitable for middle distiUate and wax synthesis, but have not been appHed on a commercial scale. 

The flux of flie adsorbed species to die catalyst from flie gaseous phase affects die catalytic activity because die fractional coverage by die reactants on die surface of die catalyst, which is determined by die heat of adsorption, also determines die amount of uncovered surface and hence die reactive area of die catalyst. Strong adsorption of a reactant usually leads to high coverage, accompanied by a low mobility of die adsorbed species on die surface, which... 

Example 11.16 Model the consumption of a solid when the gas-phase reactant is available at constant concentration and the reaction products are gaseous. 

Figure 5.30 Conversion of CO2 as a function of reactant concentration and flow direction of the gaseous phase [5].

New NH3/NH4+ buffer When 0.142 mol per liter of HC1 is added to the original buffer presented in (a), it reacts with the base component of the buffer, NH3, to form more of the acid component, NH4+ (the conjugate acid of NH3). Since HC1 is in the gaseous phase, there is no total volume change. A new buffer solution is created with a slightly more acidic pH. In this type of problem, always perform the acid-base limiting reactant problem first, then the equilibrium calculation. 

Gasification reactions of solids The reactions of solids with gas-phase reactants to form gaseous products are generally described in the same manner as are surface-catalyzed reactions. The reaction of carbon with water vapor is an example ... 

External diffusion of reactants. This step depends on the fluid dynamic characteristics of the system. Reactants must first diffuse from the bulk gaseous phase to the outer surface of the carrier through a stagnant thin film of gas. Molecular diffusion rates in the bulk have the activation energy E1 = 2 to 4 kcal/mol and they vary with Tm. 

A kinetic study requires the determination of the concentration (in mol dnr3) of at least one of the reactant or product as a function of time. In case of gaseous phase, in place of concentration, the partial pressure is determined. The method of analysis employed must be faster than the rate of reaction. The conventional methods of analysis can be applied to the reactions which have a half-life of at least a few minutes. The measurement of some physical property which is proportional to the concentration/partial pressure can also be taken for determination of the rate. In many cases of reactions in solution, it is necessary to take out aliquots from the reaction mixture at suitable intervals of time, arrest the reaction in aliquots by means of suitable means and then analyse the sample. Some conventional physical methods used to study the kinetics of slow reactions are described as follows. 

For both reactions to occur, a three-phase boundary is required where the reactant gas, protons, and electrons react at the catalyst surface. The CLs should be able to facilitate transport of protons, electrons, and gases to the catalytic sites. Under normal PEM fuel cell operating conditions (<80°C), the reactants are gaseous phase H2 and O2 (from air), and the product is water, primarily in the liquid phase. Water removal is a key factor affecting catalyst... 

Numerous variables influence the yields and selectivity of surface reactions, for example (i) nature and loading of the metal salt or organometallic precursor adsorbed on the inorganic oxide (ii) nature of the inorganic oxide (iii) physical and chemical properties of the surface as such or after addition of some reactants (e.g., alkali or acids) (iv) nature and composition of the gaseous phase (v) temperature, pressure and reaction time [6]. 

The gaseous phase is more random than the aqueous and solid phases. Thus, the tendency toward maximum randomness is in favor of the reactant in reaction II, and that of the products in reaction III. 

The gaseous limiting reactant has a very low solubility. This means that the moles lost by the dissolution of gas in the liquid are very small, and thus the moles that disappear are mainly due to reaction. In this case, the expansion can be evaluated as described in two-phase systems. 

The first and the second law of thermodynamics allow the description of a reversible fuel cell, whereas in particular the second law of thermodynamics governs the reversibility of the transport processes. The fuel and the air are separated within the fuel cell as non-mixed gases consisting of the different components. The assumption of a reversible operating fuel cell presupposes that the chemical potentials of the fluids at the anode and the cathode are converted into electrical potentials at each specific gas composition. This implies that no diffusion occurs in the gaseous phases. The reactants deliver the total enthalpy J2 ni Hi to the fuel cell and the total enthalpy J2 ni Hj leaves the cell (Figure 2.1). 

The hydrodesulfurization of low boiling (naphtha) feedstocks is usually a gas-phase reaction and may employ the catalyst in fixed beds and (with all of the reactants in the gaseous phase) only minimal diffusion problems are encountered within the catalyst pore system. It is, however, important that the feedstock be completely volatile before entering the reactor as there may be the possibility of pressure variations (leading to less satisfactory results) if some of the feedstock enters the reactor in the liquid phase and is vaporized within the reactor. 

Usually, the typology of batch reactors also includes the semi-batch gas-liquid reactors, in which a gaseous phase is fed continuously in order to provide one of the reactants. A typical example is given by the reactors used both in different oxidative industrial processes and in the active sludge processes for the treatment of wastewater. It is possible to distinguish between the bubble columns (Fig. 7.1(c)), in which the gas rises undisturbed in the liquid phase, and the bubble stirred reactor, in which a mechanical mixer is added. Finally, the slurry reactors can be considered, in which the liquid phase contains a finely dispersed solid phase as well, which can act as a reactant or as a heterogeneous catalyst these reactors assume in general the features of Fig. 7.1(d). 

A more complex behavior is expected when multiphase reacting systems are examined. As an example, consider the gas-liquid reactor sketched in Fig. 7.1(c), which behaves as a batch reactor with respect to the liquid phase and as a continuous reactor with respect to the gaseous phase. A reactant is transferred from the gaseous to the liquid phase, where it reacts with a substrate. 

In our attempts to determine the possible role of free radical lipid peroxidation in smoke induced injury, the levels of lipid peroxidation products - thiobarbituric acid reactants, mainly malondialdehyde - wereQmeasured in lung homogenates with or without prior incubation at 37 C for one hour, contrary to our expectation, the levels of thiobarbituric acid reactants were found to be decreased, rather than increased, in the lungs of cigarette-smoke-exposed rats (Table III). Such a depression effect, however, was observed only when animals were exposed to whole smoke, and not to the gaseous phase of smoke. 

Physical Vapour Deposition (PVD) is another coating technique. The reactants (precursors) are solids, which are forced in a gaseous state. This can be done by simple heating, but mostly, this procedure involves ion - bombing in order to create a plasma. The gaseous phase deposits on the solid substrate at relatively low temperatures. 

A simultaneous countercurrent movement of solid and gaseous phases makes it possible to enhance the efficiency of an equilibrium limited reaction with only one product (Fig. 4(a)) [34]. A positive effect can be obtained for the reaction A B if the catalyst has a higher adsorption capacity for B than for A. In this case, the product B will be collected mainly in the upper part of the reactor, while some fraction of the reactant A will move down with the catalyst. Better performance is achieved when the reactants are fed at some side port of the column inert carrier gas comes to the bottom and the component B is stripped off the catalyst leaving the column (Fig 4(a)). The technique was verified experimentally for the hydrogenation of 1,3,5-trimethylbenzene to 1,3,5-trimethylcyclohexane over a supported platinum catalyst [34]. High purity product can be extracted after the catalytic reactor, and overequilibrium conversion can be obtained at certain operating conditions. 

The kinetics of catalytic reactions on nonuniform surfaces have been discussed by Roginskii (330,331) certain general features of his discussion will be presented here. The rate of a complex multistage heterogeneous catalytic reaction is controlled by the rate of the slowest step. The slowest step may be the adsorption of the reactants, the chemical reactions on the surface, desorption of the products or diffusion of reactants or products through the gaseous phase near the surface of the catalyst. 

The Collision Theory of Bimolecular Gaseous Reactions. This is the earliest theory of reaction rates. Since reaction between two species takes place only when they are in contact, it is reasonable to suppose that the reactant species must collide before they react. Since our knowledge of molecular collisions is more complete for the gaseous phase than for the liquid phase (in the latter case we speak of encounters rather than collisions), we will restrict our discussion to bimolecular reactions in the gaseous phase. 

---