Reactant gas

Macroscopic properties often influence tlie perfoniiance of solid catalysts, which are used in reactors tliat may simply be tubes packed witli catalyst in tlie fonii of particles—chosen because gases or liquids flow tlirough a bed of tliem (usually continuously) witli little resistance (little pressure drop). Catalysts in tlie fonii of honeycombs (monolitlis) are used in automobile exliaust systems so tliat a stream of reactant gases flows witli little resistance tlirough tlie channels and heat from tlie exotlieniiic reactions (e.g., CO oxidation to CO,) is rapidly removed. 

Vapor—vapor reactions (14,16,17) are responsible for the majority of ceramic powders produced by vapor-phase synthesis. This process iavolves heating two or more vapor species which react to form the desired product powder. Reactant gases can be heated ia a resistance furnace, ia a glow discharge plasma at reduced pressure, or by a laser beam. Titania [13463-67-7] Ti02, siUca, siUcon carbide, and siUcon nitride, Si N, are among some of the technologically important ceramic powders produced by vapor—vapor reactions. 

The rate of formation of the products of steam reforming is extremely slow in the homogeneous reaction, and the reaction only proceeds at a useful rate in the presence of the catalyst. This indicates that adsorption of the reactant gases on the catalyst surface is a necessary step in the initiation of a useful reaction. The nature of the adsorption processes can be described by the equations ... 

Reactant gases enter the reactor by forced flow. 

As shown schematically in Fig. 2.4a, the reactant gases are introduced in the upstream side, then flow down the reactor tube, and exhaust downstream through the vacuum pump. 

In the case of laminar flow, the velocity of the gas at the deposition surface (the inner wall of the tube) is zero. The boundary is that region in which the flow velocity changes from zero at the wall to essentially that of the bulk gas away from the wall. This boundary layer starts at the inlet of the tube and increases in thickness until the flow becomes stabilized as shown in Fig. 2.4b. The reactant gases flowing above the boundary layer have to diffuse through this layer to reach the deposition surface as is shown in Fig. 2.3. 

In the case of control by surface reaction kinetics, the rate is dependent on the amount of reactant gases available. As an example, one can visualize a CVD system where the temperature and the pressure are low. This means that the reaction occurs slowly because of the low temperature and there is a surplus of reactants at the surface since, because of the low pressure, the boundary layer is thin, the diffusion coefficients are large, and the reactants reach the deposition surface with ease as shown in Fig. 2.8a. 

Thermal gradient Reactant gases enter cold side. Deposit on hot zone... 

Thermal gradient/ forced flow Reactant gases flow from cold to hot surface... 

A SiC buffer layer was grown on a silicon wafer at 1150-1300°C from one to 45 minutes using C3Hg and H2 as reactant gases. The thickness of the film increased gradually by diffusion of Si into the deposit until a thickness controlled by temperature and silicon etching was reached. 

Another example of the CVD of TiN is deposition by laser activation using a CO2 laser with N2H2 and TiC reactant gases. Deposition temperatures are not mentioned but presumed to be low. The composition of the film is substoichiometric (N/Ti[Pg.285]

A vertical CVD reactor (cf. Figure lb) consists of an axlsymmetrlc enclosure with the deposition surface perpendicular to the Incoming gas stream. The reactant gases are typically Introduced at the top and fiow down towards the heated susceptor. Thus, the least dense gas Is closest to the growth Interface which destabilizes the fiow. The result Is recirculation cells which Introduce not only film thickness and composition variations but also broaden Junctions between layers. This Is particularly of... 

In solid electrolyte fuel cells, the challenge is to engineer a large number of catalyst sites into the interface that are electrically and ionically connected to the electrode and the electrolyte, respectively, and that is efficiently exposed to the reactant gases. In most successful solid electrolyte fuel cells, a high-performance interface requires the use of an electrode which, in the zone near the catalyst, has mixed conductivity (i.e. it conducts both electrons and ions). Otherwise, some part of the electrolyte has to be contained in the pores of electrode [1]. 

The whole set-up for partial oxidation comprises a micro mixer for safe handling of explosive mixtures downstream (flame-arrestor effect), a micro heat exchanger for pre-heating reactant gases, the pressure vessel with the monolith reactor, a double-pipe heat exchanger for product gas cooling and a pneumatic pressure control valve to allow operation at elevated pressure [3]. 

The usual working temperature of fuel cells with Nafion-type membranes is 80 to 90°C. Under these conditions, moisture must be supplied to keep the membranes wet, which usually is attained by passing the reactant gases through water that is somewhat warmer (by 5 to 10°C) than the cell s working temperature, thus saturating them with water vapor. 

Figure 26. Effect of trimethylamine (TMA) addition to the reactant gases on propylene epoxidation [88].

The second characteristic pertains to the fact that a very small amount of a catalyst may be able to maintain a high reaction rate over a long time. Although in practice some of the catalysts are extremely efficient for certain specific reactions, it is observed that in the majority of the cases of homogeneous catalysis, the enhancement in the reaction rate is proportional to the concentration of the catalyst used. In heterogeneous catalytic reactions involving reactant gases and solid surfaces, the total surface area of the solid may also affect the reaction rate. 

Experiments were carried out in a U-type quartz reactor. The sample (0.025-0.2 g) was held between plugs of quartz wool and the temperature was monitored through a WET 4000 or Eurotherm 2408 temperature controllers. Reactant gases were fed from mass flow controllers (Brooks 5850TR). 

SR greater than 1 refers to fuel lean conditions, while SR less than 1 refers to fuel rich conditions. To convert from fuel rich to fuel lean experimentally, a portion of the helium in the reactant gases was replaced with an equal volume of 02 using a 4-way switching valve. The pressures of the switching valve outlets were balanced such that only the reactant concentration is changed while keeping the flow rates and the pressure constant. 

---