Cross-over effect

Heating stage. The temperature is brought to the maximum (e.g. 3300 K over around 5 s with graphite tubes) so as to remove any sample residue from the evaporation device and to minimize memory and cross-over effects. 

The electro-chemical reactions are assumed to take place in the fluid phase. The explicit introduction of the occupation numbers (0,-, S,- 0,- = 1) as variables is a natural description of the possible lack of catalytic sites and methanol cross over effects. Due to the use of expensive catalysts, electrochemical saturation effects may be reached by fuel cells in some... 

Suresh NS, Jayanti S (2009) Modelling of cross-over effects in direct ethanol fuel cells (DEFCs), AIChE proceedings, paper 161991... 

Many papers have appeared in which QES results have been reported, and some of these have been mentioned earlier, when the collapse and cross-over effects were discussed.Others are concerned with polymer characterization per se and are not discussed here (see Chapter 12). Here we examine a few areas of controversy, or illustrate novel methods that may prove useful in the future. 

The stereochemical isomerization implies a competition between ring closure and bond rotation in the biradical reactive intermediate. As with tetramethylene, a competition is supported by studies of appropriately substituted diazenes which, on thermolysis, lose N2 and presumably produce the biradical (Eq. 11.84). However, in an early indication that things are not as simple as with tetramethylene, the diazene experiments show a "cross-over" effect. The cis diazene preferably produces trans cyclopropane, and vice versa. 

Intermediate temperature polarization curves for two different PEM single cells equipped with the Nafion and Aquivion membranes under slightly pressurized conditions are reported in Fig. 2.23. Polarizations are carried out at 130 °C (3 bar abs. 100 % RH). A higher OCV is registered for the Aquivion-based MEA at this temperature than for the Nafion-based MEA. This behavior could be attributed principally to an increase of the hydrogen cross-over effect in... 

For the saturation of aromatics in a hydrotreating or hydrocracking unit, equilibrium effects, which favor formation of aromatics, start to overcome kinetic effects above a certain temperature. This causes a temperature-dependent aromatics cross-over effect, which explains the degradation of important middle distillate product properties—including kerosene smoke point and diesel cetane number—at high process temperatures near the end of catalyst cycles. The cross-over temperature is affected by feed quality and hydrogen partial pressure, so it can differ from unit to unit. 

The y-axis shows the mole fraction of total aromatics remaining in product. For this case study, the feed rate, feed type, and catalyst activity were kept constant, which means that deactivation effects were not included. With less-active end-of-run catalysts, the rates of forward (saturation) reactions are inhibited, so the aromatics cross-over effect is amplified. 

G. Weill and J. des Cloizeaux. Dynamics of polymers in dilute solutions an explanation of anomalous indices by cross-over effects. J. Phys.(Paris), 40 (1970), 99-106. 

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