Catalytic ducts

Pick s law stipulates that a negative sign is required to calculate the molecular flux of reactants in a positive coordinate direction. Term 2 requires the kinetic rate law for surface-catalyzed chemical reactions. It is not necessary to re-express the rate law pseudo-volumelrically. In other words, the product of and Papparent is not appropriate here or in equation (23-1). When term 2 is written as a first-order or psendo-first-order rate law, the balance between diffusion and chemical reaction at the catalytic surface is recognized canonically as a radiation boundary condition. This is illustrated below for a catalytic duct with rectangular cross section ... 

Comparison with Exact Results. It is not unreasonable to suspect that truncation errors in the numerical approximation of first and second derivatives might accumulate in the computational scheme used to integrate the mass transfer equation. One check for accuracy involves a comparison between numerical results and exact analytical solutions. Of course, only a limited number of analytical solutions are available. For example, the following solutions have been obtained analytically for catalytic duct reactors ... 

Fig. 5. Catalytic system designs (11) of (a) basic VOC catalytic converter containing a preheater section, a reactor housing the catalyst, and essential controls, ducting, instmmentation, and other elements (b) a heat exchanger using the cleaned air exiting the reactor to raise the temperature of the incoming process exhaust and (c) extracting additional heat from the exit gases by a secondary heat exchanger.

Point Combustible Gas Detectors (IR) are used to indicate the presence of gas at a particular location (e.g., in a congested area of the planter in small ducts.) IR technology has proven to be more reliable than catalytic bead detectors. The point detector functions in the same manner as the open path detector, by comparing absorbed and reference frequencies of IR light. The main difference between these and open path type is that the path length of the point type is short (3 inches) and is kept within the confines of the instrument. 

A Chloro-Cat catalytic oxidizer was used with SVE at a Superfund Site in Deer Park, New York, to treat VOCs and SVOCs in soils. The total treatment cost for this application was 450,420. The costs associated with instrumentation were greater than anticipated due to corrosion of process duct work. Unit costs for this application were estimated to be 360/yd of soil treated (D13943P, p.192 D22776T, p.2). 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

The boundary conditions for equation (29) are that Y is bounded, that Y — Yq at z = 0, and that 7 = 0 at x = 0, x — a, y = 0, and y = b (that is, at the walls of the duct). This last condition follows directly from an analysis in the spirit of Section B.4 in the steady state, as the specific rate constant for the surface reaction approaches infinity, the concentrations of the reactant on the surface and in the gas phase adjacent to the surface both approach zero, and steps 1 and 5 of Section B.4 (the diffusion transport steps) become rate controlling. For recombination processes, the limit of an infinite surface rate constant is usually referred to as the case of a perfectly catalytic wall. Systems with finite surface reaction rates have been studied in [19] and [20], for example. ... 

Cybulski and Moulijn [27] proposed an experimental method for simultaneous determination of kinetic parameters and mass transfer coefficients in washcoated square channels. The model parameters are estimated by nonlinear regression, where the objective function is calculated by numerical solution of balance equations. However, the method is applicable only if the structure of the mathematical model has been identified (e.g., based on literature data) and the model parameters to be estimated are not too numerous. Otherwise the estimates might have a limited physical meaning. The method was tested for the catalytic oxidation of CO. The estimate of effective diffusivity falls into the range that is typical for the washcoat material (y-alumina) and reacting species. The Sherwood number estimated was in between those theoretically predicted for square and circular ducts, and this clearly indicates the influence of rounding the comers on the external mass transfer. 

The conversion of fuel nitrogen to NO, presents a problem when biomass or waste with a high nitrogen content is used as a fuel. This is a common problem for both conventional boiler plants and IGCC plants, In boilers, NO, reduction can be achieved by primary measures, eg, fuel staging or by selective non catalytic NO, reduction (SNCR) with ammonia injection in the boiler, or selective catalytic NO, reduction (SCR) with ammonia injected in the flue gas duct after the boiler. 

Catalytic effect of transition metals on the smooth conjugate addition reaction was next tested [38]. Alkynylation at the / position was followed by capture of the aluminum enolate in the presence of Ni(acac)2 (Scheme 6.18). Yields of 1,4-ad-ducts were moderate, however, indicating the methods were still unsatisfactory for enolate generation. 

Desimoni and coworkers reported on the enantioselective ene cyclization using a stoichiometric or catalytic amount of a bisoxazoline-Mg(0104)2 complex (Scheme 21) [ 61 ]. In all cases, although a hetero -Diels - Alder pro duct was formed, an ene product was obtained predominantly in trans,syn fashion as the major kinetic product. The minor kinetic ene products, the trans,anti and cis,anti products, were readily isomerized at the 03 atom by silica gel to give the thermodynamically stable products, the trans,syn and cis,syn isomers, respectively. The... 

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