Gas composition, change

Example 11.4 The assumption in Example 11.3 that the gas composition changes instantly is suspicious. Remedy this defect in the analysis. 

An analysis by Cairns and Liebhafsky (3) for a H2/air fuel cell shows that a change in the gas composition that produces a 60 mV change in the reversible cell potential near room temperature corresponds to a 300 mV change at 1200°C (2192°F). Thus, gas composition changes are more significant in high temperature fuel cells. 

Hydrate kinetics studies of methane mixed hydrate in sll hydrate are limited. This is mostly due to complication when working with mixture where the gas composition changes over time due to preferential guest dissolution in water and preferential encapsulation in the... 

Figure 28 shows the result of the kinetic in-cell investigation. For three anodic overpotentials there are plotted the obtained current densities versus gas composition, which simulates different degrees of hydrogen conversion. The gas composition changes from pure hydrogen (left) to H20/C02 = 1/1 mixtures. As predicted by Eq. (30), not only for complete conversion but also for vanishing conversion the current approaches zero as neither vapor nor carbon dioxide is present. 

If additional points are required, gas composition changed and Steps 3-6 repeated. 

Equation (5.41) assumes that the gas is of a uniform composition throughout the reactor at all times. If the gas composition changes with the time or position within the reactor, a different equation must be used. To account for the effect of particle size distribution in addition to the residence time distribution is difficult because different size particles can remain in the reactor for different periods of time. To account for these effects completely a population balance must be performed, where the conversion is an internal variable (see Chapter 3). This type of treatment is beyond the scope of this chapter. A simplified method of accounting for the effects of a particle size distribution, mQt), on the mean conversion, is by... 

Headspace gas composition changes dynamically due to several factors. Gas exchange with the enviromnent occurs over the packaging film if the partial pressure of a gas differs on the two sides of the film. The amount of gas that permeates the film can be calculated from equation 9.1 (Robertson, 1993) ... 

The next question is whether this reaction proceeds in a retort. Since the gas compositions change with location in the retort, this question is not answered easily. If we use a typical offgas composition with steam dilution of 40% steam and 5% hydrogen, we find, using the expression for K4, that PH2S must be less than 500 ppm. This indicates that the reaction would be severely limited thermodynamically. At 727°C, AG for Reaction (4) is 11.71 kcal/mol, which results in K4 =... 

By raising the temperature from 550° to 650°C and simultaneously reducing the pressure by 2.5 atm, the role of the monomolecular decomposition reaction should become more important, thus increasing the methane and propylene content and decreasing the ethylene content the gas composition changes only slightly if we take into account the error in defining the components. 

Moreover, we need to compare the permeability change with a constant Cj and with the gas composition changing from pure CH4 to COj The changing Cf is defined by following Eq. (6) ... 

The above picture of the performance of the fnmace is idealised and is comphcated by non-nniformity across the shaft section, giving rise to significant gas compositional changes from the walls to the centre of the shaft with higher oxidation potential at the walls. Conditions at each tuyere... 

The carbon conversion in zone 2 is dominated by pyrolysis, with 60% of the carbon in the reed being converted into volatile products. In the subsequent zone 3, reed pyrolysis and char gasification lead to a carbon conversion of 70%. The conversion increases further to 93% at a reactor length of 2.4 m. From this point on, no char is gasified, and only the gas composition changes by the water-gas shift reaction. 

---