Gaseous pressure, reactants

In the case of a three-phase electrocatalytic system, such as those of fuel cells, the cylindrical pore model is not applicable since it does not consider the problem of the partial pressure of the reactant in the gas phase. In this case, an equilibrium between the gaseous pressure inside the pore (which tends to force the electrolyte out of it) and the capillary forces of the electrolyte (which tend to flood the electrolyte away from the pore) must occur. This is known as the stable meniscus condition ... 

CVD gaseous reactants (precursors) delivered to a heated substrate in a flow reactor undergo tliennal reaction to deposit solid films at atmospheric or reduced pressure, and volatile side products are pumped away. CVD is used for conductors, insulators and dielectrics, elemental semiconductors and compound semiconductors and is a workliorse in tire silicon microelectronics industry. 

The differential material balances contain a large number of physical parameters describing the structure of the porous medium, the physical properties of the gaseous mixture diffusing through it, the kinetics of the chemical reaction and the composition and pressure of the reactant mixture outside the pellet. In such circumstances it Is always valuable to assemble the physical parameters into a smaller number of Independent dimensionless groups, and this Is best done by writing the balance equations themselves in dimensionless form. The relevant equations are (11.20), (11.21), (11.22), (11.23), (11.16) and the expression (11.27) for the effectiveness factor. 

Urea is produced from liquid NH and gaseous CO2 at high, pressure and temperature both reactants are obtained from an ammonia-synthesis plant. The latter is a by-product stream, vented from the CO2 removal section of the ammonia-synthesis plant. The two feed components are deUvered to the high pressure urea reactor, usually at a mol ratio >2.5 1. Depending on the feed mol ratio, more or less carbamate is converted to urea and water per pass through the reactor. 

The stmcture of residual char particles after devolatilization depends on the nature of the coal and the pyrolysis conditions such as heating rate, peak temperature, soak time at the peak temperature, gaseous environment, and the pressure of the system (72). The oxidation rate of the chat is primarily influenced by the physical and chemical nature of the chat, the rate of diffusion and the nature of the reactant and product gases, and the temperature and pressure of the operating system. The physical and chemical characteristics that influence the rate of oxidation ate chemical stmctural variations, such as the... 

Vapor-phase fugacity coefficients are needed not only in high-pressure phase equilibria, but are also of interest in high-pressure chemical equilibria (D6, K7, S4). The equilibrium yield of a chemical reaction can sometimes be strongly influenced by vapor-phase nonideality, especially if reactants and products have small concentrations due to the presence in excess of a suitably chosen nonreactive gaseous solvent (S4). 

Gases. The reactants (including diluent, extender, and carrier gases) must be transported and metered in a controlled manner into the reactor. In the case of gaseous reactants, this does not present any particular problem and is accomplished by means of pressure controllers, gauges, flowmeters, and mass-flow controllers. 

The low-pressure chemical vapor deposition of silicon nitride on silicon involves two gaseous reactants dichlorosilane and ammonia. The following reactions are believed to be important under typical conditions of P = 1 torr and T= 1000 1200 K ... 

Many liquid phase or heterogeneous solid—liquid or gas—liquid reactions result in gaseous products or byproducts. These products may be toxic (refer to Table 4.1) or flammable (refer to Table 5.1), or result in overpressurization of any sealed container or vessel. Unless pressure relief is provided, relatively small volumes of reactants — the presence of which may not be expected — may generate sufficient gas pressure to rupture a container. The causes of pressure build-up may be ... 

Shaker tube reactors are commonly used for the evaluation of catalysts at elevated pressure. The liquid reactant and powdered catalyst are introduced into a metal or glass ampoule, which is sealed and pressurized to a predetermined level with the gaseous reactant. The ampoule is immersed into a thermostatted liquid and maintained at this temperature for a certain period of time while shaking. Then the reactor is opened and the reaction mixture analysed. Ampoules of ca. 10-100 cm are typically used. The usefulness of data obtained using such reactors for process scale-up is nearly zero due to poor agitation and unknown hydrodynamics in the ampoule. These reactors are, however, very useful for fast screening of catalysts. 

A frequent theme in high-pressure STM is the possibility of artefact structures developing due to low levels of impurities present in the gaseous reactants it is an aspect that has been discussed13 for the Pt (llO)-CO system. 

The pressure of gaseous reactants. In general, the higher the pressure of gaseous reactants, the faster the reaction. This factor is merely a corollary of factor 4, since the higher pressure is in effect a higher concentration. 

Physical methods involve the measurement of a physical property of the system as a whole while the reaction proceeds. The measurements are usually made in the reaction vessel so that the necessity for sampling with the possibility of attendant errors is eliminated. With physical methods it is usually possible to obtain an essentially continuous record of the values of the property being measured. This can then be transformed into a continuous record of reactant and product concentrations. It is usually easier to accumulate much more data on a given reaction system with such methods than is possible with chemical methods. There are certain limitations on physical methods, however. There must be substantial differences in the contributions of the reactants and products to the value of the particular physical property used as a measure of the reaction progress. Thus one would not use pressure measurements to follow the course of a gaseous reaction that does not... 

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