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Stiffness in combustion models is quite different from that in more conventional kinetic descriptions. The steady-state approximation, so useful in other contexts, cannot be applied in the usual ad hoc manner but can be applied with attention to its origins in singular perturbation analysis. The magnitude and time scaling on the equations representing the isothermal kinetics of hydrogen combustion reveal distinct regions of the transient to which this approximation may naturally and successfully be made to reduce computation time. The steady-state approximation is found to apply to some free radicals, but not all, and these can change between regions. An analytical solution to an important section of the transient is found and illustrates dramatically the power of the approach. The analysis may be applied a priori and leads to a stagewise efficient numerical solution.

There is an equivalence between stiff and singularly perturbed systems of ordinary differential equations. This feature is exploited in this paper by numerically employing recent singular perturbation methods to attack troublesome boundary layer stage of the solution in which some variables have very short response times. The numerical method affords a means of essentially determining the thickness of this boundary layer. The algorithm is capable of high stability and accuracy for the commonly occurring stiff system, whether or not it is in singularly perturbed form. Application to a singularly perturbed reaction system and a highly stiff reactor system not in singularly perturbed form demonstrate the effectiveness and utility of this approach.

Simulation studies are presented of single- and two-component adsorption in periodic, countercurrent, multicolumn systems. It is demonstrated that long adsorption profiles are likely to occur in the adsorption of organic contaminants from wastewater onto activated carbon, thus favoring the use of multicolumn rather than single-column systems. For two-component adsorption in a multicolumn system, it is shown that more switching periods are needed than in the single-column case before the profiles stabilize, that the stabilized profiles can differ substantially from those found in a single-column system, and that their shape is strongly influenced by the relative concentrations of the components in the feed.

Utilisation de l'approximation de la perturbation singuliere, pour estimer les parametres des equations differentielles difficiles a calculer. Application de l'algorithme a l'estimation des constantes de vitesse d'une reaction a catalyseur enzymatique

Highly selective transfer of hydrogen sulfide (H2S) from much larger quantities of carbon dioxide (CO2) has been accomplished by absorption and reaction into fine sprays of buffered carbonate solution. Selectivity after the intensely agitated atomization zone is dramatically improved once the droplets have formed and stagnate internally. H2S transfer in this latter zone is virtually independent of the presence of CO2, even at CO2:H2S ratios of 100; CO2 transfer, however, is adversely affected by the presence of H2S. The gas film resistance for mass transfer is effectively eliminated with the 50 μm mean droplet diameter. The data indicate approximately constant hydrodynamics throughout the formed droplet zone and allow good theoretical prediction of enhancement factors for this complex system. Thus the design of more efficient equipment is possible if the gas-liquid contacting method is tailored to take advantage of differences in the transfer and reaction rate properties of gases to be separated.