- Email: [email protected]
Third catalyst bed SO2 oxidation
16 Third catalyst bed SO
2
oxidation
Simplest industrial sulfuric acidmaking consists of: SO2 oxidation in three catalyst beds with gas cooling between beds
then: H2SO4 making by contact of cooled third catalyst bed exit gas with strong sulfuric acid (Fig. 16.1).
The SO2 oxidation reaction is: Catalyst
! SO3 SO2 þ 0:5O2 700-900 K |fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} in catalyst bed input gas
The H2SO4 making reaction is SO3
350-380 K þ H2 OðlÞ ! H2 SO4 ðlÞ
in cooled third catalyst
in strong
in strengthened
bed exit gas
sulfuric acid
sulfuric acid
This chapter describes cooling of second catalyst bed exit gas and oxidation of the cooled gas’ SO2 in a third catalyst bed. Its objectives are to: (a) prepare a 2-3 cooldown path (b) prepare a third catalyst bed heatup path (c) calculate a third bed heatup path-equilibrium curve intercept.
Specifications for the calculations are shown in Fig. 16.2. Calculation results are summarized in Figs. 16.3 and 16.4.
16.1
2-3 Cooldown path
This chapter’s 2-3 cooldown path is prepared like Fig. 13.3’s 1-2 cooldown path. It is a horizontal line at: (a) 94.2% SO2 oxidized (second catalyst bed intercept % SO2 oxidized)
between: (b) 773.2 K (second catalyst bed intercept temperature) Sulfuric Acid Manufacture. http://dx.doi.org/10.1016/B978-0-08-098220-5.00017-4 © 2013 Elsevier Ltd. All rights reserved.
184
Sulfuric Acid Manufacture 690 K SO2, O2, N2 gas
First catalyst bed SO2 + 0.5O2 → SO3 + heat ~890 K, ~70% SO2 oxidized Impervious plate
Gas cooling
Heat
Gas cooling
Heat
700 K Second catalyst bed SO2 + 0.5O2 → SO3 + heat ~770 K, ~95% SO2 oxidized Impervious plate 710 K Third catalyst bed SO2 + 0.5O2 → SO3 + heat ~720 K, SO3 bearing gas to cooling & H2SO4 making ~98% SO2 oxidized
Figure 16.1 Schematic of single contact, third catalyst bed sulfuric acid plant. It is a single contact plant because it has only one H2SO4 making step. Note gas cooling between catalyst beds. It permits further SO2 oxidation in the next catalyst bed.
and: (c) 710 K (specified third catalyst bed input gas temperature).
It is shown in Figs. 16.3 and 16.4.
16.2
Heatup path
This chapter’s third catalyst bed heatup path is calculated much like Chapter 14’s second catalyst bed heatup path. Differences are: (a) the input gas temperature is 710 K rather than 700 K (b) the input SO3, SO2, O2 (kg mol) are different (Fig. 16.2).
The latter are represented by the following equations: kg mol SO3 in ¼ 0:0942
(16.1)
kg mol SO2 in ¼ 0:0058
(16.2)
kg mol O2 in ¼ 0:0629 kg mol N2 in ¼ 0:7900
(16.3) (16.4)
where the numerical values are from Section 15.2.2.
1 kg mol first catalyst bed feed gas 10 volume% SO2 (0.1 kg mol) 11 volume% O2 79 volume% N2 690 K
First catalyst bed SO2 + 0.5O2 → SO3 1.2 bar pressure
893.3 K Gas cooling Cooled first catalyst bed exit gas ≡ cooled first catalyst bed intercept gas 0.0692 kg mol SO3 (Section 12.2) 0.0308 kg mol SO2 0.0754 kg mol O2 0.7900 kg mol N2 700 K
Second catalyst bed SO2 + 0.5O2 → SO3 1.2 bar pressure
773.2 K Gas cooling Cooled second catalyst bed exit gas ≡ cooled second catalyst bed intercept gas 0.0942 kg mol SO3 (Section 15.2.2) 0.0058 kg mol SO2 0.0629 kg mol O2 0.7900 kg mol N2 710 K
Third catalyst bed SO2 + 0.5O2 → SO3 1.2 bar pressure 715 K Level L SO3, SO2, O2, N2 gas
Figure 16.2 Specifications for (i) 2-3 cooldown and (ii) third catalyst bed heatup path and intercept calculations. The first and second catalyst bed exit gas quantities are equivalent to: (a) 69.2% SO2 oxidized after the first catalyst bed (b) 94.2% SO2 oxidized after the second catalyst bed (Fig. 15.1).
186
Sulfuric Acid Manufacture
100
721.1 K 773.2 K 710 K
Φ, % SO2 oxidized
80
Second catalyst bed heatup path 893.3 K 700 K
60
40
20
0 600
First catalyst bed feed gas 10 volume% SO2 First catalyst bed heatup path 11 volume% O2 79 volume% N2 1.2 bar pressure, all beds 690 K 700 800 900 1000 Gas temperature (K)
Figure 16.3 Third catalyst bed SO2 oxidation with gas cooling between beds. The equilibrium curve is the same for all beds (Section 15.1.1) because: (a) no component gas (e.g., SO3) is selectively removed from Fig. 16.2 gas stream (Appendix L) (b) no air is added to Fig. 16.2 gas stream (c) all beds are at the same pressure.
100 Third heatup
98.0% SO2 oxidized
2-3 cooldown
94.2% SO2 oxidized
Φ, % SO2 oxidized
90
Second heatup
80
70 1-2 cooldown
69.2% SO2 oxidized
First heatup 60
600
700
800 Gas temperature (K)
900
1000
Figure 16.4 Blowup of top portion of Fig. 16.3. Overall SO2 oxidation efficiency increases with each bed but the gain diminishes.
Appendix N shows a third catalyst bed heatup path matrix with these equations. It also shows several heatup path points. Figures 16.3 and 16.4 show the entire heatup path.
Third catalyst bed SO2 oxidation
16.3
187
Heatup path-equilibrium curve intercept
Appendix O describes a third catalyst bed intercept calculation—with Fig. 16.2 specifications. The third bed intercept with these specifications is: 721.1 K 98.0% SO2 oxidized.
Its gas quantities are: 0.098 kg mol 0.002 kg mol 0.061 kg mol 0.790 kg mol
SO3 SO2 O2 N2
per kg mol of first catalyst bed feed gas. These quantities go forward to the next acidmaking step—almost always to H2SO4 making, but occasionally to a fourth catalyst bed.
16.4
Graphical representation
Figures 16.3 and 16.4 describe 3-bed SO2 oxidation with Fig. 16.2’s specifications. They indicate that the percentages of SO2 oxidized in each bed are: 69.2% in the first catalyst bed 25.0% in the second catalyst bed 3.8% in the third catalyst bed
for a total of 98.0%.
16.5
Summary
Second and third catalyst bed heatup path and intercept calculations are very similar. Their differences are: (a) different SO3, SO2, O2, and N2 input quantities (b) different gas input temperatures.
Third catalyst bed SO2 oxidation efficiency is about 98%. Beds 1, 2, and 3 contribute 69%, 25%, and 4%. Chapter 17 examines the effects of SO3 and CO2 in feed gas on these catalytic oxidation efficiencies.
16.6
Problems
16.1 Prepare a graph like Fig. 16.3 for (a) 12 volume% SO2, 13.2 volume% O2, 74.8 volume% N2 first catalyst bed feed gas
188
Sulfuric Acid Manufacture
(b) the following gas input temperatures first catalyst bed: 675 K second catalyst bed: 685 K third catalyst bed: 695 K. (c) 1.2 bar gas pressure in all beds (d) attainment of intercept % SO2 oxidized in all beds.
Hints (a) Your answer to Problem 15.1 contains most of the required graph. You only need to prepare (i) a 2-3 cooldown path (ii) a third catalyst bed heatup path (iii) a third catalyst bed intercept point (which you can join to the heatup path). (b) The third catalyst bed’s input gas quantities are those in your Problem 15.2 answer. 16.2 Tabulate the intercept kg mol of SO3, SO2, O2, and N2 equivalent to Problem 16.1’s third catalyst bed intercept point (all per kg mol of first catalyst bed feed gas).
2
oxidation
Simplest industrial sulfuric acidmaking consists of: SO2 oxidation in three catalyst beds with gas cooling between beds
then: H2SO4 making by contact of cooled third catalyst bed exit gas with strong sulfuric acid (Fig. 16.1).
The SO2 oxidation reaction is: Catalyst
! SO3 SO2 þ 0:5O2 700-900 K |fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} in catalyst bed input gas
The H2SO4 making reaction is SO3
350-380 K þ H2 OðlÞ ! H2 SO4 ðlÞ
in cooled third catalyst
in strong
in strengthened
bed exit gas
sulfuric acid
sulfuric acid
This chapter describes cooling of second catalyst bed exit gas and oxidation of the cooled gas’ SO2 in a third catalyst bed. Its objectives are to: (a) prepare a 2-3 cooldown path (b) prepare a third catalyst bed heatup path (c) calculate a third bed heatup path-equilibrium curve intercept.
Specifications for the calculations are shown in Fig. 16.2. Calculation results are summarized in Figs. 16.3 and 16.4.
16.1
2-3 Cooldown path
This chapter’s 2-3 cooldown path is prepared like Fig. 13.3’s 1-2 cooldown path. It is a horizontal line at: (a) 94.2% SO2 oxidized (second catalyst bed intercept % SO2 oxidized)
between: (b) 773.2 K (second catalyst bed intercept temperature) Sulfuric Acid Manufacture. http://dx.doi.org/10.1016/B978-0-08-098220-5.00017-4 © 2013 Elsevier Ltd. All rights reserved.
184
Sulfuric Acid Manufacture 690 K SO2, O2, N2 gas
First catalyst bed SO2 + 0.5O2 → SO3 + heat ~890 K, ~70% SO2 oxidized Impervious plate
Gas cooling
Heat
Gas cooling
Heat
700 K Second catalyst bed SO2 + 0.5O2 → SO3 + heat ~770 K, ~95% SO2 oxidized Impervious plate 710 K Third catalyst bed SO2 + 0.5O2 → SO3 + heat ~720 K, SO3 bearing gas to cooling & H2SO4 making ~98% SO2 oxidized
Figure 16.1 Schematic of single contact, third catalyst bed sulfuric acid plant. It is a single contact plant because it has only one H2SO4 making step. Note gas cooling between catalyst beds. It permits further SO2 oxidation in the next catalyst bed.
and: (c) 710 K (specified third catalyst bed input gas temperature).
It is shown in Figs. 16.3 and 16.4.
16.2
Heatup path
This chapter’s third catalyst bed heatup path is calculated much like Chapter 14’s second catalyst bed heatup path. Differences are: (a) the input gas temperature is 710 K rather than 700 K (b) the input SO3, SO2, O2 (kg mol) are different (Fig. 16.2).
The latter are represented by the following equations: kg mol SO3 in ¼ 0:0942
(16.1)
kg mol SO2 in ¼ 0:0058
(16.2)
kg mol O2 in ¼ 0:0629 kg mol N2 in ¼ 0:7900
(16.3) (16.4)
where the numerical values are from Section 15.2.2.
1 kg mol first catalyst bed feed gas 10 volume% SO2 (0.1 kg mol) 11 volume% O2 79 volume% N2 690 K
First catalyst bed SO2 + 0.5O2 → SO3 1.2 bar pressure
893.3 K Gas cooling Cooled first catalyst bed exit gas ≡ cooled first catalyst bed intercept gas 0.0692 kg mol SO3 (Section 12.2) 0.0308 kg mol SO2 0.0754 kg mol O2 0.7900 kg mol N2 700 K
Second catalyst bed SO2 + 0.5O2 → SO3 1.2 bar pressure
773.2 K Gas cooling Cooled second catalyst bed exit gas ≡ cooled second catalyst bed intercept gas 0.0942 kg mol SO3 (Section 15.2.2) 0.0058 kg mol SO2 0.0629 kg mol O2 0.7900 kg mol N2 710 K
Third catalyst bed SO2 + 0.5O2 → SO3 1.2 bar pressure 715 K Level L SO3, SO2, O2, N2 gas
Figure 16.2 Specifications for (i) 2-3 cooldown and (ii) third catalyst bed heatup path and intercept calculations. The first and second catalyst bed exit gas quantities are equivalent to: (a) 69.2% SO2 oxidized after the first catalyst bed (b) 94.2% SO2 oxidized after the second catalyst bed (Fig. 15.1).
186
Sulfuric Acid Manufacture
100
721.1 K 773.2 K 710 K
Φ, % SO2 oxidized
80
Second catalyst bed heatup path 893.3 K 700 K
60
40
20
0 600
First catalyst bed feed gas 10 volume% SO2 First catalyst bed heatup path 11 volume% O2 79 volume% N2 1.2 bar pressure, all beds 690 K 700 800 900 1000 Gas temperature (K)
Figure 16.3 Third catalyst bed SO2 oxidation with gas cooling between beds. The equilibrium curve is the same for all beds (Section 15.1.1) because: (a) no component gas (e.g., SO3) is selectively removed from Fig. 16.2 gas stream (Appendix L) (b) no air is added to Fig. 16.2 gas stream (c) all beds are at the same pressure.
100 Third heatup
98.0% SO2 oxidized
2-3 cooldown
94.2% SO2 oxidized
Φ, % SO2 oxidized
90
Second heatup
80
70 1-2 cooldown
69.2% SO2 oxidized
First heatup 60
600
700
800 Gas temperature (K)
900
1000
Figure 16.4 Blowup of top portion of Fig. 16.3. Overall SO2 oxidation efficiency increases with each bed but the gain diminishes.
Appendix N shows a third catalyst bed heatup path matrix with these equations. It also shows several heatup path points. Figures 16.3 and 16.4 show the entire heatup path.
Third catalyst bed SO2 oxidation
16.3
187
Heatup path-equilibrium curve intercept
Appendix O describes a third catalyst bed intercept calculation—with Fig. 16.2 specifications. The third bed intercept with these specifications is: 721.1 K 98.0% SO2 oxidized.
Its gas quantities are: 0.098 kg mol 0.002 kg mol 0.061 kg mol 0.790 kg mol
SO3 SO2 O2 N2
per kg mol of first catalyst bed feed gas. These quantities go forward to the next acidmaking step—almost always to H2SO4 making, but occasionally to a fourth catalyst bed.
16.4
Graphical representation
Figures 16.3 and 16.4 describe 3-bed SO2 oxidation with Fig. 16.2’s specifications. They indicate that the percentages of SO2 oxidized in each bed are: 69.2% in the first catalyst bed 25.0% in the second catalyst bed 3.8% in the third catalyst bed
for a total of 98.0%.
16.5
Summary
Second and third catalyst bed heatup path and intercept calculations are very similar. Their differences are: (a) different SO3, SO2, O2, and N2 input quantities (b) different gas input temperatures.
Third catalyst bed SO2 oxidation efficiency is about 98%. Beds 1, 2, and 3 contribute 69%, 25%, and 4%. Chapter 17 examines the effects of SO3 and CO2 in feed gas on these catalytic oxidation efficiencies.
16.6
Problems
16.1 Prepare a graph like Fig. 16.3 for (a) 12 volume% SO2, 13.2 volume% O2, 74.8 volume% N2 first catalyst bed feed gas
188
Sulfuric Acid Manufacture
(b) the following gas input temperatures first catalyst bed: 675 K second catalyst bed: 685 K third catalyst bed: 695 K. (c) 1.2 bar gas pressure in all beds (d) attainment of intercept % SO2 oxidized in all beds.
Hints (a) Your answer to Problem 15.1 contains most of the required graph. You only need to prepare (i) a 2-3 cooldown path (ii) a third catalyst bed heatup path (iii) a third catalyst bed intercept point (which you can join to the heatup path). (b) The third catalyst bed’s input gas quantities are those in your Problem 15.2 answer. 16.2 Tabulate the intercept kg mol of SO3, SO2, O2, and N2 equivalent to Problem 16.1’s third catalyst bed intercept point (all per kg mol of first catalyst bed feed gas).