Heatup path

It does not, however, show where a feed gas s equilibrium point lies on its equilibrium curve. That is the task of this chapter and Chapter 12. 

This chapter discusses catalytic SO2 oxidation in terms of heatup paths. 

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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 value becomes apparent when the equilibrium curves of this chapter are combined with the approach-to-equilibrium heatup paths of Chapters 11 and 12. 

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.

Our heatup path calculations assume that there is no transfer of heat between gas and reactor walls or between gas and catalyst. The result is, therefore, an adiabatic heatup path. 

The following example problem shows how a heatup path point is determined. The problem is ... 

The next step in calculating a heatup path point is to develop steady state molar S, O and N balances for Fig. 11.3 s feed and level L gases. 

The heatup path for the Section 11.5 feed gas is prepared by re-doing the above calculation for many different levels and temperatures in the catalyst bed, Fig. 11.4. Only cells G8 to J8 in Table 11.2 are changed. 

An efficient method of calculating heatup path points is to put enthalpy equations directly into cells D8 - J8 of Table 11.2. This is detailed in Appendix I. 

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. 

Table 11.3. Heatup path points for 10 volume% S02, 11 volume% 02, 79 volume% N2, 690 K feed gas. The values are represented graphically in Fig. 11.1.

The effect of feed gas S02 strength on heatup path is determined by inserting different values of ... 

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.

The effect of feed gas temperature on heatup path is determined by inserting new enthalpy values into Eqn. (11.7). With 660 K feed gas (for example), enthalpy Eqn. (11.7) becomes ... 

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 ... 

Prepare a heatup path for 12 volume% S02, 13.2 volume% 02, 74.8 volume% N2, 675 K feed gas - as described in Appendix I. Plot the path with Excel s Chart Wizard function. 

For an intercept calculation to be valid, its heatup path and equilibrium curve must be for the same feed gas. Each intercept calculation must, therefore, specify a feed gas... 

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. 

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