Observed surface residence time product

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. 

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). 

Chiche et a/.[56] have studied the oligomerization of butene over a series of zeolite (HBeta and HZSM-5), amorphous silica alumina and mesoporous MTS-type aluminosilicates with different pores. The authors found that MTS catalyst converts selectively butenes into a mixture of branched dimers at 423 K and 1.5-2 MPa. Under the same reaction conditions, acid zeolites and amorphous silica alumina are practically inactive due to rapid deactivation caused by the accumulation of hydrocarbon residue on the catalyst surface blocking pores and active sites. The catalytic behaviour observed for the MTS catalyst was attributed to the low density of sites on their surface along with the absence of diffusional limitations due to an open porosity. This would result in a low concentration of reactive species on the surface with short residence times, and favour deprotonation and desorption of the octyl cations, thus preventing secondary reaction of the olefinic products. 

Initial runs using Cu-containing Y zeolites yielded remarkable conversion levels (T = 523-623 K residence time = 1-6 s concentration c = 23 or 37 mg/1, respectively). During these runs, however, we observed deactivation of the catalyst and deposition of crystalline by-products at the exit of the fixed bed reactor. These deposits were identified as congeners of polychlorinated benzenes by means of HRGC/MS. The phenomenon of transchlorination is also known in connection with the thermal decomposition of 1,2-dichlorobenzene [6,7], As we confirmed for catalytic experiments with unchlorinated aromatics carried out in our laboratory [8], no oxygenated products were released from the catalyst surface even in the case of chlorobenzene... 

Employing a silicon micro reactor [channel dimensions = 500 or 1000 pm (width) x 250 pm (depth)], wall-coated with the acidic zeolite titanium silicate-1 (TS-1, Si Ti ratio = 17) (83) (3 pm), Gavrilidis and co-workers [52] demonstrated a facile method for the epoxidation of 1-pentene (84) (Scheme 6.23). Using H202 (85) (0.18 M, 30wt%) as the oxidant and 84 (0.90 M) in MeOH, the effect of reactant residence time on the formation of epoxypentane (86) was evaluated at room temperature. The authors observed increased productivity within the 500 pm reaction channel compared with the 1000 pm channel, a feature that is attributed to an increase in the surface-to-volume ratio and thus a higher effective catalyst loading. 

Observations from operating BWR plants suggest that on the surfaces wetted by high-temperature reactor water, one has to expect a deposition mechanism which is similar to that on the surfaces of a PWR primary circuit. The activation products released from the activated in-core materials, as well as from the fuel rod deposits, as dissolved ions are incorporated into the oxide layers on the austenitic out-of-core surfaces directly from the reactor water. The activated crud which is resuspended from the fuel rod surfaces is also partly deposited on the out-of-core surfaces here, colloid chemistry processes may participate in the deposition process. These corrosion products often show a higher cobalt content than the non-acti-vated corrosion products that are brought in with the feedwater. During the residence time of the particulate corrosion products on the out-of-core surfaces, this excess cobalt content is reduced therefore, the activated crud can be considered as an additional source of ionic cobalt (Alder et al., 1992). 

The particular reactivity of bare Si02 for the production of HCHO is a matter of debate and has not yet been completely rationalized. Parmaliana et al. [113] pointed out that the performance of the silica surface in CH4 partial oxidation is controlled by the preparation method. For several commercial Si02 samples, the following reactivity trend has been established, based on the preparation method precipitation > sol-gel > pyrolysis. The activity of such silicas has been correlated with the density of surface sites stabilized under steady-state conditions acting as O2 activation centers [114], and the reaction rate was the same for all the silicas when expressed as TOF (turnover frequency). Klier and coworkers [115] reported the activity data for the partial oxidation of CH4 by O2 to form HCHO and C2 hydrocarbons over fumed Cabosil and silica gel at temperatures ranging from 903 to 1953 K under ambient pressure. They observed that short residence times enhanced HCHO (and C2 hydrocarbon) selectivity, suggesting that HCHO did not originate from methyl radicals, but rather from methoxy complexes formed upon direct chemisorption. 

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