Observed surface residence time

Unfortunately, intraparticle readsorption cannot be corrected using this method. If intraparticle readsorption is significant, it can be detected by adding unlabelled product to the feed stream, to compete for readsorption sites with the labelled product formed during reaction. The observed surface residence time of product will approach the true surface residence time at higher concentrations of product added. 

A comparison of the surface residence times, x, reveals no significant difference for the experiments. From this observation, it can be concluded that the variation in methane 5deld is due to a difference in the amount of active sites. The intrinsic site activity for the methanation reaction remains unchanged after the water treatment, and it is also unaffected by the presence of the rhenium promotor. It is proposed that rhenium supports the reduction of cobalt, but the water pretreatment leads to a reoxidation of the active metal, particularly the fraction of the cobalt metal that is reduced only when rhenium is present. 

Kim et studied the fast initial deactivation observed during the first 45 min TOS at 150°C. The SSITKA results, after correction for readsorption, showed an increase in the surface residence time of iso-C4 intermediates and a corresponding decrease in site activity, as well as a... 

Product readsorption at reactive sites can lead to substantial contributions to the transient response, lowering the measured activity and reaction rate. Product readsorption at nonreactive sites will also inflate the measurement of surfece intermediates leading to the observed product and overestimate the mean surface residence time. Effects of product readsorption can be addressed by decreasing the bed length or increasing the space velocity. 

At still higher temperatures, when sufficient oxygen is present, combustion and "hot" flames are observed the principal products are carbon oxides and water. Key variables that determine the reaction characteristics are fuel-to-oxidant ratio, pressure, reactor configuration and residence time, and the nature of the surface exposed to the reaction 2one. The chemistry of hot flames, which occur in the high temperature region, has been extensively discussed (60-62) (see Col ustion science and technology). 

Baskaran and Santschi (1993) examined " Th from six shallow Texas estuaries. They found dissolved residence times ranged from 0.08 to 4.9 days and the total residence time ranged from 0.9 and 7.8 days. They found the Th dissolved and total water column residence times were much shorter in the summer. This was attributed to the more energetic particle resuspension rates during the summer sampling. They also observed an inverse relation between distribution coefficients and particle concentrations, implying that kinetic factors control Th distribution. Baskaran et al. (1993) and Baskaran and Santschi (2002) showed that the residence time of colloidal and particulate " Th residence time in the coastal waters are considerably lower (1.4 days) than those in the surface waters in the shelf and open ocean (9.1 days) of the Western Arctic Ocean (Baskaran et al. 2003). Based on the mass concentrations of colloidal and particulate matter, it was concluded that only a small portion of the colloidal " Th actively participates in Arctic Th cycling (Baskaran et al. 2003). 

Backflushing of injected wastes can also be a good way to observe waste/reservoir geochemical interactions. Injected wastes are allowed to backflow (if formation pressure is above the elevation of the wellhead) or are pumped to the surface. Backflowed wastes are sampled periodically (and reinjected when the test is completed) the last sample taken will have had the longest residence time in the injection zone. Keely165 and Keely and Wolf166 describe this technique for characterizing... 

The desorption of arsenate previously sorbed onto Fe- or Al-oxides or onto an Andisol containing 42% of allophanic materials (Vacca et al. 2002) by phosphate has been demonstrated to be affected by time of reaction, residence time of arsenate onto the surfaces and the pH of the system (Pigna et al. 2006 Pigna et al. 2007, unpublished data). Figure 9 shows the desorption of arsenate at pH 6.0 (phosphate/arsenate molar ratio of 4) when phosphate was added onto the soil (Andisol) sample 1, 5 or 15 days after arsenate (surface coverage of arsenate about 60%). After 60 days of reaction, 55% of arsenate was desorbed by phosphate when the residence time of arsenate onto the surfaces of the Andisol was 1 day, but 35 and 20% of arsenate was desorbed by phosphate with increase in the residence time up to 5 and 15 days. Further, it was also observed that by keeping the... 

Recently, such a temperature oscillation was also observed by Zhang et al (27,28) with nickel foils. Furthermore, Basile et al (29) used IR thermography to monitor the surface temperature of the nickel foil during the methane partial oxidation reaction by following its changes with the residence time and reactant concentration. Their results demonstrate that the surface temperature profile was strongly dependent on the catalyst composition and the tendency of nickel to be oxidized. Simulations of the kinetics (30) indicated that the effective thermal conductivity of the catalyst bed influences the hot-spot temperature. 

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