Heatup paths oxidation

Fig. 7.5 also shows that S02 oxidation and gas heat up end when the heatup path reaches Fig. 7.4 s equilibrium % S02 oxidized curve. 

Fig. 7.5. Heatup path for gas descending the Fig. 7.1 catalyst bed. It begins at the feed gas s input temperature and 0% SO2 oxidized. Its temperature rises as S02 oxidizes. Maximum attainable S02 oxidation is predicted by the heatup path-equilibrium curve intercept, 69% oxidized at 893 K in this case. This low % SO> oxidized confirms that efficient SO-> oxidation cannot be obtained in a single catalyst bed. Multiple catalyst beds with gas cooling between must be used.

Fig. 7.8. Heatup paths, intercepts and cooldown paths for Fig. 7.6 converter. They are described in Section 7.5. Final % SO2 oxidation after Fig. 7.6 s three catalyst beds is -98%.

This chapter discusses catalytic S02 oxidation in terms of heatup paths. 

Fig. 11.1. Heatup path for S02, 02, N2 gas descending a catalyst bed. The S02 and 02 in feed gas react to form S03, Eqn. (1.1). The gas is heated by the exothermic heat of reaction. The result is a path with increasing % S02 oxidized and increasing gas temperature. Notice how the feed gas s heatup path approaches its Chapter 10 equilibrium curve.

Table 11.3 summarizes % S02 oxidized vs gas temperature as calculated by the above described method. The points are equivalent to the heatup path in Fig. 11.1. As expected, high gas temperatures are equivalent to extensive SO2+I/2O2 —> S03 oxidation and vice versa. 

Fig. 11.5 summarizes the effects of S02 feed gas strength on heatup paths. It shows that each % SO2 oxidized gives a larger temperature increase with ... 

Fig. 11.5. Heatup paths for 7, 10 and 13 volume% S02 (volume% 02/volume% S02 =1.1) feed gas. Per % S02 oxidized, strong S02 gas heats up more than weak S02 gas.

A new heatup path is then calculated as described in Section 11.11. The result is a path nearly parallel to the 690 K path 30 K cooler at all % S02 oxidized values, Fig. 11.6. 

Fig. 11.7. Heatup paths and equilibrium curve for 10 volume% SO2, 11 volume% 02, 79 volume% N2 feed gas. Notably, the 660 K heatup path will reach the equilibrium curve at a higher % S02 oxidized value than the 690 K heatup path. 660 K is about the lowest feed gas temperature that will keep V, alkali metal, S, 0, Si02 catalyst active and S02 oxidation rapid, Table 8.1.

S02 oxidizes and gas temperature increases as S02, 02, N2 gas descends through active V, alkali metal, S, O, Si02 catalyst. This behavior is shown by the heatup paths of this chapter. 

Chapters 12 onwards combine these heatup paths with Chapter 10 s % SO2 oxidized equilibrium curves to show how ... 

Table 12.1 shows heatup path and equilibrium curve % S02 oxidized-temperature points near a heatup path-equilibrium curve intercept. They are for ... 

Temperature, K Heatup path % S02 oxidized, Equilibrium % S02 oxidized, E... 

This indicates that at 893 K and below, S02 + /202 —> S03 oxidation can proceed further up the heatup path towards equilibrium, Fig. 12.1. 

At 894 K and above, however, heatup path % S02 oxidized is greater than equilibrium % S02 oxidized. This is, of course, impossible because equilibrium % S02 oxidized cannot be exceeded up a heatup path. 

Fig. 12.1. Plot of Table 12.1 heatup path points and equilibrium curve, expanded from Fig. 11.7. Below the equilibrium curve, S02 is being oxidized, gas temperature is increasing and equilibrium is being approached up the heatup path. Maximum (equilibrium) oxidation is attained where the heatup path meets the equilibrium curve.

Fig. 12.3. Heatup paths, equilibrium curves and intercepts for 7, 10, and 13 volume% S02 feed gas. Volume% 02/volume% S02 ratio =1.1. Intercept temperature increases with increasing S02 strength. Intercept % S02 oxidized decreases with increasing S02 strength. The intercepts have been calculated as described in Appendix J.

These pressure differences have no effect on heatup paths, Fig. 12.4 - and little effect on equilibrium curves and intercepts. Intercept temperature and % S02 oxidized both increase slightly with increasing pressure. 

Fig. 12.6. Effect of CO2 on intercept temperature and % S02 oxidized. Heatup path slope increases slightly with increasing CO2 in gas - because C02 heat capacity is greater than N2 heat capacity, Appendix G. This decreases intercept temperature and increases % S02 oxidized. C02 calculations are described in Chapter 17.

Catalyst bed S02 + /202 — S03 oxidation is represented by heatup paths and equilibrium curves. Maximum S02 oxidation occurs where a feed gas s heatup path intercepts its equilibrium curve. 

Now determine the %S02 oxidized - temperature point at which the Problem 11.4 heatup path intercepts the Problem 10.4 equilibrium curve. 

Fig. 13.2. 1st catalyst bed heatup path, equilibrium curve and intercept point, from Fig. 12.1. The 1st catalyst bed s exit gas is its intercept gas, Section 12.12. It is cooled and fed to a 2nd catalyst bed for more S02 oxidation.

Table 14.3. Heatup path points for Fig. 14.2 s 2nd catalyst bed. The points are shown graphically in Fig. 14.3. They have been calculated using matrix Table 14.2 with enthalpy equations in cells H15-K15, Appendix K. An increase in gas temperature from 700 K to 760 K in the 2nd catalyst bed is seen to be equivalent to an increase in % SO oxidized from 69.2% to 89.7%.

Fig. 14.3. Fig. 13.3 with the addition of Table 14.3 s heatup path (upper left). The heatup path starts at 1st catalyst bed intercept % S02 oxidized and 2nd catalyst bed gas input temperature. 

Table 17.3. Is1 catalyst bed heatup path matrix with S03 and C02 in feed gas". Cells D15 to H15 contain -H°9QK values. Cells 115 to M15 contain H°2QK values. All are calculated with Appendix G s enthalpy equations. 820 K part way down the catalyst bed is shown to be equivalent to oxidation of 46.7% of the feed gas s S02. 

Fig. 17.1. Effect of C02-in-feed-gas on 1st catalyst bed heatup path and heatup path-equilibrium curve intercept. C02 increases heatup path slope and slightly increases intercept (equilibrium) % SO2 oxidized. Section 17.4. Appendix Table R.l describes the 10 volume% C02 intercept calculation.

Fig. 18.3. Heatup paths and intercepts for 0 and 10 volume% C02 1st catalyst bed feed gas. C02 heatup paths are steeper than non C02 heatup paths because C02 heat capacity > N2 heat capacity, Appendix G. The steeper paths give higher intercept % S02 oxidized values in each catalyst bed.

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