Minor influence—Equilibrium gas pressure

Industrial catalyst bed gas pressure varies slightly between acid plants depending on altitude. It also tends to increase slightly over time as catalyst beds become clogged with dust and catalyst fragments. 

These pressure differences have no effect on heatup paths. Fig. 12.4 - and little effect on equilibrium curves and intercepts. Intercept temperature and % SO2 oxidized both increase slightly with increasing pressure. 

Industrial first catalyst bed feed gas typically contains O2 and SO2 in the ratio  

This is two to four times the SO2 + O.5O2 SO3 stoichiometric O2/SO2=0.5 requirement. It gives rapid oxidation. 

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Figure 18.2 shows the density profiles of n-butane in the zeolite and close to the surface at equilibrium. The local density of guest molecules in the zeolite strongly varies with position in the zeolite. In the work of Inzoli et al. (2008,2009), the external surface has two different surface structures one where the surface is cut between the sinusoidal channels and the other one across the sinusoidal channels. This shifts the density profile as seen in Fig. 18.2, and these represent two quite different surface structures. Inzoli et al. (2009) showed that for this system the details of the surface structure have a minor influence on the surface resistance to transport. In Fig. 18.3 we show a typical snapshot of a zeolite/gas interface from an equilibrium simulation at a pressure of 100 kPa, at 400 K. Clearly, n-butane molecules are adsorbed on the external surface of silicalite-1 and some molecules are adsorbed in the zeolite pores. The external surface in this case has a flat structure. 

If we now inject additional ammonia into the container, a new set of equilibrium concentrations will eventually be established, with higher concentrations in each phase than were initially obtained. In this manner, we can eventually obtain the complete relationship between the equilibrium concentrations in both phases. If the ammonia is designated as substance A, the equilibrium mole fractions in the gas (yA) and liquid ( A) give rise to an equilibrium-distribution curve as shown in Figure 3.1. This curve results irrespective of the amounts of air and water that we start with, and is influenced only by the temperature and pressure of the system. It is important to note that at equilibrium the concentrations in the two phases are not equal instead, the chemical potential of the ammonia is the same in both phases. It is this equality of chemical potentials which causes the net transfer of ammonia to stop. The curve of Figure 3.1 does not, of course, show all the equilibrium concentrations existing within the system. For example, water will partially vaporize into the gas phase, the components of the air will also dissolve to a small extent into the liquid, and equilibrium concentrations for these substances will also be established. For the moment, we need not consider these equilibria, since they are of minor importance. 

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