Sulfuric Acid Manufacture: Analysis, Control and Optimization

By Matt King, Michael Moats and William G. Davenport

By some measure the most widely produced chemical in the world today, sulfuric acid has an extraordinary range of modern uses, including phosphate fertilizer production, explosives, glue, wood preservative and lead-acid batteries. An exceptionally corrosive and dangerous acid, production of sulfuric acid requires stringent adherence to environmental regulatory guidance within cost-efficient standards of production.

This work provides an experience-based review of how sulfuric acid plants work, how they should be designed and how they should be operated for maximum sulfur capture and minimum environmental impact. Using a combination of practical experience and deep physical analysis, Davenport and King review sulfur manufacturing in the contemporary world where regulatory guidance is becoming ever tighter (and where new processes are being required to meet them), and where water consumption and energy considerations are being brought to bear on sulfuric acid plant operations. This 2e will examine in particular newly developed acid-making processes and new methods of minimizing unwanted sulfur emissions.

The target readers are recently graduated science and engineering students who are entering the chemical industry and experienced professionals within chemical plant design companies, chemical plant production companies, sulfuric acid recycling companies and sulfuric acid users. They will use the book to design, control, optimize and operate sulfuric acid plants around the world.

• Unique mathematical analysis of sulfuric acid manufacturing processes, providing a sound basis for optimizing sulfuric acid manufacturing processes
• Analysis of recently developed sulfuric acid manufacturing techniques suggests advantages and disadvantages of the new processes from the energy and environmental points of view
• Analysis of tail gas sulfur capture processes indicates the best way to combine sulfuric acid making and tailgas sulfur-capture processes from the energy and environmental points of view
• Draws on industrial connections of the authors through years of hands-on experience in sulfuric acid manufacture

• Language: English
• Publisher: Elsevier Science
• Release date: May 11, 2013
• ISBN: 9780080982267

Book preview

Sulfuric Acid Manufacture

Analysis, Control, and Optimization

By

Matthew J. King

Perth, Western Australia

William G. Davenport

Tucson, Arizona

Michael S. Moats

Rolla, Missouri

Table of Contents

Cover image

Title page

Copyright

Preface

1. Overview

1.1 Catalytic oxidation of SO2 to SO3

1.2 H2SO4 production

1.3 Industrial flowsheet

1.4 Sulfur burning

1.5 Metallurgical offgas

1.6 Spent acid regeneration

1.7 Sulfuric acid product

1.8 Recent developments

1.9 Alternative processes

1.10 Summary

References

Suggested reading

2. Production and consumption

2.1 Uses

2.2 Acid plant locations

2.3 Price

2.4 Summary

References

Suggested reading

3. Sulfur burning

3.1 Objectives

3.2 Sulfur

3.3 Molten sulfur delivery

3.4 Sulfur atomizers and sulfur burning furnaces

3.5 Product gas

3.6 Heat recovery boiler

3.7 Summary

References

Suggested reading

4. Metallurgical offgas cooling and cleaning

4.1 Initial and final SO2 concentrations

4.2 Initial and final dust concentrations

4.3 Offgas cooling and heat recovery

4.4 Electrostatic collection of dust

4.5 Water scrubbing (Tables 4.5 and 4.6)

4.6 H2O(g) removal from scrubber exit gas (Tables 4.5 and 4.6)

4.7 Summary

References

Suggested reading

5. Regeneration of spent sulfuric acid

5.1 Spent acid compositions

5.2 Spent acid handling

5.3 Decomposition

5.4 Decomposition furnace product

5.5 Optimum decomposition furnace operating conditions

5.6 Preparation of offgas for SO2 oxidation and H2SO4 making

5.7 Summary

References

Suggested Reading

6. Dehydrating air and gases with strong sulfuric acid

6.1 Chapter objectives

6.2 Dehydration with strong sulfuric acid

6.3 Dehydration reaction mechanism

6.4 Residence times

6.5 Recent advances

6.6 Summary

References

7. Catalytic oxidation of SO2 to SO3

7.1 Objectives

7.2 Industrial SO2 oxidation

7.3 Catalyst necessity

7.4 SO2 oxidation heatup path (Chapter 11)

7.5 Industrial multicatalyst bed SO2 oxidation (Tables 7.2–7.7)

7.6 Industrial operation (Table 7.2)

7.7 Recent advances

7.8 Summary

References

8. SO2 oxidation catalyst and catalyst beds

8.1 Catalytic reactions

8.2 Maximum and minimum catalyst operating temperatures

8.3 Composition and manufacture

8.4 Choice of size and shape

8.5 Catalyst bed thickness and diameter

8.6 Gas residence times

8.7 Catalyst bed temperatures

8.8 Catalyst bed maintenance

8.9 Summary

References

Suggested reading

9. Production of H2SO4(ℓ) from SO3(g)

9.1 Objectives

9.2 Sulfuric acid rather than water

9.3 Absorption reaction mechanism

9.4 Industrial H2SO4 making (Tables 9.3–9.8)

9.5 Choice of input and output acid compositions

9.6 Acid temperature

9.7 Gas temperatures

9.8 Operation and control

9.9 Double contact H2SO4 making (Tables 19.3 and 23.2)

9.10 Intermediate versus final H2SO4 making

9.11 Summary

References

Suggested reading

Break

10. Oxidation of SO2 to SO3—Equilibrium curves

10.1 Catalytic oxidation

10.2 Equilibrium equation

10.3 KE as a function of temperature

10.4 KE in terms of % SO2oxidized

10.5 Equilibrium % SO2 oxidized as a function of temperature

10.6 Discussion

10.7 Summary

10.8 Problems

Reference

11. SO2 oxidation heatup paths

11.1 Heatup paths

11.2 Objectives

11.3 Preparing a heatup path—The first point

11.4 Assumptions

11.5 A specific example

11.6 Calculation strategy

11.7 Input SO2, O2, and N2 quantities

11.8 Sulfur, oxygen, and nitrogen molar balances

11.9 Enthalpy balance

11.10 Calculating level L quantities

11.11 Matrix calculation

11.12 Preparing a heatup path

11.13 Feed gas SO2 strength effect

11.14 Feed gas temperature effect

11.15 Significance of heatup path position and slope

11.16 Summary

11.17 Problems

12. Maximum SO2 oxidation: Heatup path-equilibrium curve intercepts

12.1 Initial specifications

12.2 % SO2 oxidized-temperature points near an intercept

12.3 Discussion

12.4 Effect of feed gas temperature on intercept

12.5 Inadequate % SO2 oxidized in first catalyst bed

12.6 Effect of feed gas SO2 strength on intercept

12.7 Minor influence—Equilibrium gas pressure

12.8 Minor influence—O2 strength in feed gas

12.9 Minor influence—CO2 in feed gas

12.10 Catalyst degradation, SO2 strength, and feed gas temperature

12.11 Maximum feed gas SO2 strength

12.12 Exit gas composition ≡ intercept gas composition

12.13 Summary

12.14 Problems

13. Cooling first catalyst bed exit gas

13.1 First catalyst bed summary

13.2 Cooldown path

13.3 Gas composition below equilibrium curve

13.4 Summary

13.5 Problem

Hints

14. Second catalyst bed heatup path

14.1 Objectives

14.2 % SO2 oxidized redefined

14.3 Second catalyst bed heatup path

14.4 A specific heatup path question

14.5 Second catalyst bed input gas quantities

14.6 S, O, and N molar balances

14.7 Enthalpy balance

14.8 Calculating 760 K (level L) quantities

14.9 Matrix calculation and result

14.10 Preparing a heatup path

14.11 Discussion

14.12 Summary

14.13 Problem

15. Maximum SO2 oxidation in a second catalyst bed

15.1 Second catalyst bed equilibrium curve equation

15.2 Second catalyst bed intercept calculation

15.3 Two bed SO2 oxidation efficiency

15.4 Summary

15.5 Problems

Hints

16. Third catalyst bed SO2 oxidation

16.1 2-3 Cooldown path

16.2 Heatup path

16.3 Heatup path-equilibrium curve intercept

16.4 Graphical representation

16.5 Summary

16.6 Problems

17. SO3 and CO2 in feed gas

17.1 SO3

17.2 SO3 effects

17.3 CO2

17.4 CO2 effects

17.5 Summary

17.6 Problems

18. Three catalyst bed acid plant

18.1 Calculation specifications

18.2 Example calculation

18.3 Calculation results

18.4 Three catalyst bed graphs

18.5 Minor effect—SO3 in feed gas

18.6 Minor effect—CO2 in feed gas

18.7 Minor effect—Bed pressure

18.8 Minor effect—SO2 strength in feed gas

18.9 Minor effect—O2 strength in feed gas

18.10 Summary of minor effects

18.11 Major effect—Catalyst bed input gas temperatures

18.12 Discussion of book’s assumptions

18.13 Summary

Reference

19. After-H2SO4-making SO2 oxidation

19.1 Double contact advantage

19.2 Objectives

19.3 After-H2SO4-making calculations

19.4 Equilibrium curve calculation

19.5 Heatup path calculation

19.6 Heatup path-equilibrium curve intercept calculation

19.7 Overall SO2 oxidation efficiency

19.8 Double/single contact comparison

19.9 Summary

19.10 Problems

Reference

20. Optimum double contact acidmaking

20.1 Total % SO2 oxidized after all catalyst beds

20.2 Four catalyst beds

20.3 Improved efficiency with five catalyst beds

20.4 Input gas temperature effect

20.5 Best bed for Cs catalyst

20.6 Triple contact acid plant

20.7 Summary

Reference

21. Enthalpies and enthalpy transfers

21.1 Input and output gas enthalpies

21.2 H2SO4 making input gas enthalpy

21.3 Heat transfers

21.4 Heat transfer rate

21.5 Summary

21.6 Problems

22. Control of gas temperature by bypassing

22.1 Bypassing principle

22.2 Objective

22.3 Gas to economizer heat transfer

22.4 Heat transfer requirement for 480 K economizer output gas

22.5 Changing heat transfer by bypassing

22.6 460 K Economizer output gas

22.7 Bypassing for 460, 470, and 480 K economizer output gas

22.8 Bypassing for 470 K economizer output gas while input gas temperature is varying

22.9 Industrial bypassing

22.10 Summary

22.11 Problems

23. H2SO4 making

23.1 Objectives

23.2 Mass balances

23.3 SO3 input mass

23.4 H2O(g) input from moist acid plant input gas

23.5 Water for product acid

23.6 Calculation of mass water in and mass acid out

23.7 Interpretations

23.8 Summary

23.9 Problem

24. Acid temperature control and heat recovery

24.1 Objectives

24.2 Calculation of output acid temperature

24.3 Effect of input acid temperature

24.4 Effect of input gas temperature

24.5 Effect of input gas SO3 concentration on output acid temperature

24.6 Adjusting output acid temperature

24.7 Acid cooling

24.8 Target acid temperatures

24.9 Recovery of acid heat as steam

24.10 Steam production principles

24.11 Double-packed bed absorption tower

24.12 Steam injection

24.13 Sensible heat recovery efficiency

24.14 Materials of construction

24.15 Summary

24.16 Problems

References

25. Making sulfuric acid from wet feed gas

25.1 Chapter objectives

25.2 WSA feed Gas

25.3 WSA flowsheet

25.4 Catalyst bed reactions

25.5 Preparing the oxidized gas for H2SO4(ℓ) condensation

25.6 H2SO4(ℓ) condenser

25.7 Product acid composition

25.8 Comparison with conventional acidmaking

25.9 Appraisal

25.10 Alternatives

25.11 Summary

References

Suggested reading

26. Wet sulfuric acid process fundamentals

26.1 Wet gas sulfuric acid process SO2 oxidation

26.2 Injection of nanoparticles into cooled process gas

26.3 Sulfuric acid condensation

26.4 Condenser temperature choices

26.5 Condenser acid composition up the glass tube

26.6 Condenser re-evaporation of H2O(ℓ)

26.7 Condenser acid production rate

26.8 Condenser appraisal

26.9 Summary

References

Suggested reading

27. SO3 gas recycle for high SO2 concentration gas treatment

27.1 Objectives

27.2 Calculations

27.3 Effect of recycle extent

27.4 Effect of recycle gas temperature on recycle requirement

27.5 Effect of gas recycle on first catalyst SO2 oxidation efficiency

27.6 Effect of first catalyst exit gas recycle on overall acid plant performance

27.7 Recycle equipment requirements

27.8 Appraisal

27.9 Industrial SO3 gas recycle

27.10 Alternatives to gas recycle

27.11 Summary

References

28. Sulfur from tail gas removal processes

28.1 Objectives

28.2 Environmental standards

28.3 Acid plant tail gas characteristics

28.4 Industrial acid plant tail gas treatment methods

28.5 Technology selection (after Hay et al., 2003)

28.6 Capital and operating costs

28.7 Summary

References

29. Minimizing sulfur emissions

29.1 Industrial catalytic SO2+0.5O2→SO3 oxidation

29.2 Methods to lower sulfur emissions

29.3 Summary

References

Suggested reading

30. Materials of construction

30.1 Chapter objectives

30.2 Corrosion rate factors for sulfuric acid plant equipment

30.3 Sulfuric acid plant materials of construction

30.4 Summary

References

31. Costs of sulfuric acid production

31.1 Investment costs

31.2 Production costs

31.3 Summary

References

Appendix A. Sulfuric acid properties

A.1 Sulfuric acid specific gravity at constant temperature

A.2 Specific gravity of sulfuric acid at elevated temperatures

A.3 Sulfuric acid freezing points

A.4 Oleum specific gravity

A.5 Electrical conductivity of sulfuric acid

A.6 Absolute viscosity of sulfuric acid

Appendix B. Derivation of equilibrium equation (10.12)

B.1 Modified equilibrium equation

B.2 Mole fractions defined

B.3 Feed and oxidized gas molar quantities

B.4 Mole fractions in oxidized gas

B.5 Equation applicability

B.6 Equilibrium equation

B.7 Equilibrium constant and molar quantities

and ΦE

Appendix C. Free energy equations for equilibrium curve calculations

C.1 Production of SO3(g) from SO2(g) and O2(g)

C.2 Production of H2SO4(g) from SO3(g) and H2O(g)

Appendix D. Preparation of Fig. 10.2’s equilibrium curve

D.1 Integer temperature calculations

D.2 Second and third catalyst bed equilibrium curves

Appendix E. Proof that volume%=mol% (for ideal gases)

E.1 Definitions

E.2 Characterization of partial volumes

E.3 Equality of volume% and mol%

Appendix F. Effect of CO2 and Ar on equilibrium equations (none)

F.1 CO2

F.2 Ar

F.3 Conclusions

Appendix G. Enthalpy equations for heatup path calculations

G.1 An example—Enthalpy of SO3(g) at 600 K

G.2 Preparation of equations

References

Appendix H. Matrix solving using Tables 11.2 and 14.2 as examples

Appendix I. Enthalpy equations in heatup path matrix cells

I.1 Example results

Appendix J. Heatup path-equilibrium curve: Intercept calculations

J.1 Calculation strategy

J.2 Worksheet

J.3 Intercept worksheet preparation instructions

J.4 Goal Seek instructions

J.5 Another example

Appendix K. Second catalyst bed heatup path calculations

Appendix L. Equilibrium equation for multicatalyst bed SO2 oxidation

L.1 Proof

L.2 Inapplicability

Appendix M. Second catalyst bed intercept calculations

M.1 Calculation strategy

M.2 Specifications (Fig. 14.2)

M.3 Worksheet

M.4 Goal Seek instructions

Appendix N. Third catalyst bed heatup path worksheet

Appendix O. Third catalyst bed intercept worksheet

Appendix P. Effect of SO3 in Fig. 10.1’s feed gas on equilibrium equations

P.1 Molar balances

P.2 Total kg mol of oxidized gas

P.3 Mole fractions in oxidized gas

P.4 New equilibrium equation

P.5 % SO2 oxidized in equilibrium equation

P.6 Equilibrium % SO2 oxidized as a function of temperature

Appendix Q. SO3-in-feed-gas intercept worksheet

Appendix R. CO2- and SO3-in-feed-gas intercept worksheet

Appendix S. Three-catalyst-bed converter calculations

S.1 First catalyst bed calculations (cells A1 through M47)

S.2 Second catalyst bed calculations (cells AA1 through AM47)

S.3 Third catalyst bed calculations (cells BA1 through BM47)

Appendix T. Worksheet for calculating after-intermediate-H2SO4-making heatup path-equilibrium curve intercepts

Appendix U. After-H2SO4-making SO2 oxidation with SO3 and CO2 in input gas

U.1 Equilibrium equation with SO3 in after-H2SO4-making input gas

U.2 H2SO4 making input gas quantity specification

U.3 H2SO4 making exit gas quantity calculation

U.4 Calculation of H2SO4 making exit gas volume percents

U.5 Worksheet construction and operation

U.6 Calculation of % SO2 oxidized after all catalyst beds

Appendix V. Moist air in H2SO4 making calculations

V.1 Calculation

Appendix W. Calculation of H2SO4 making tower mass flows

W.1 Input and output gas specifications

W.2 Input SO3(g) equation

W.3 Input and output acid composition equations

W.4 Total mass balance equation

W.5 Sulfur balance equation

W.6 Solving for flows

W.7 Effect of output acid mass% H2SO4 on input and output acid flows

Appendix X. Equilibrium equations for SO2, O2, H2O(g), N2 feed gas

X.1 Equilibrium equations

X.2 Modified equilibrium equations

X.3 Mole fractions defined

X.4 Feed and oxidized gas molar quantities

X.5 Preparing Eqs. (X.1) and (X.2) from Eqs. (X.19) and (X.21)

Appendix Y. Cooled first catalyst bed exit gas recycle calculations

Y.1 Exit gas temperature without recycle

Y.2 Recycle calculation setup

Y.3 Recycle matrix (Table Y.2)

Y.3.2 Result

Y.4 Recalculation to steady state

Y.5 Different feed and recycle temperatures

Y.6 Third catalyst bed exit gas recycle calculations

Answers to numerical problems

Chapter 10

Chapter 11

Chapter 12

Chapter 13

Chapter 14

Chapter 15

Chapter 16

Chapter 17

Chapter 19

Chapter 21

Chapter 22

Chapter 23

Chapter 24

Index

Copyright

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Preface

We have made several additions and changes to this second edition of Sulfuric Acid Manufacture.

The first change is the addition of a third author, Dr. Michael S. Moats, Associate Professor of Metallurgical Engineering at the Missouri University of Science and Technology. We welcome Michael to our team.

The second is the addition of seven new chapters:

We add one new unit to this edition—parts per million SO2 by volume, where SO2 can be any gas. It is defined as

where Nm³ may be (i) measured or (ii) calculated from measured gas masses by the relationship:

Once again we have received exceptional help from our industrial colleagues, who so kindly showed us around their plants and answered all our questions. We have continued to visit acid plants during preparation of this edition—we thank our hosts most profusely.

One of the authors would specifically like to thank his son George Davenport and his nephew Andrew Davenport for their help with (i) wet sulfuric acid and (ii) cooled catalyst bed exit gas recycle calculations.

In our first edition preface, we expressed the hope that our book would bring us as much joy as Professor Dr. von Igelfeld’s masterpiece Portuguese Irregular Verbs had brought him. Indeed it has! We hope now that this second edition will continue to bring us this same good fortune.

Matthew J. King

Perth, Western Australia

William G. Davenport

Tucson, Arizona

Michael S. Moats

Rolla, Missouri

1

Overview

Sulfuric acid is a dense clear liquid. It is used for making fertilizers, leaching metallic ores, refining petroleum, and manufacturing a myriad of chemicals and materials. Worldwide, about 200 million tonnes of sulfuric acid is consumed per year (Apodaca, 2012).

The raw material for sulfuric acid is SO2 gas. It is obtained by:

(a) burning elemental sulfur with air

(b) smelting and roasting metal sulfide minerals

(c) decomposing contaminated (spent) sulfuric acid catalyst.

Elemental sulfur is far and away the largest source.

Table 1.1 describes three typical sulfuric acid plant feed gases. It shows that acid plant SO2 feed is always mixed with other gases.

Table 1.1

Typical compositions (volume%) of acid plant feed gases entering SO2 oxidation converters, 2013. The gases may also contain small amounts of CO2 and SO3.

Sulfuric acid is almost always made from these gases by:

(a) catalytically reacting their SO2 and O2 to form SO3(g)

(b) reacting (a)’s product SO3 with the H2O(ℓ) in 98.5 mass% H2SO4(ℓ), 1.5 mass% H2O(ℓ) sulfuric acid.

Industrially, both processes are carried out rapidly and continuously (Fig. 1.1).

Figure 1.1 Modern 4100 tonnes/day sulfur burning sulfuric acid plant, courtesy PCS Phosphate Company, Inc. (2012). The main components are the catalytic SO2 oxidation converter (tall, right), twin H2SO4(ℓ) making (absorption) towers (middle, right of stack) and a sulfur burning furnace (middle, bottom). The air dehydration (drying) tower is left of the stack. The catalytic converter is 16.5 m diameter.

The standard state for SO2, SO3, O2, N2, and CO2 is gas in the acid plant. Each is referenced in this book, for example, as O2 not O2(g). The standard state for H2O, S, and H2SO4 is gas or liquid in the acid plant. Each is referenced accordingly.

1.1 Catalytic oxidation of SO2 to SO3

O2 does not oxidize SO2 to SO3 without a catalyst. All industrial SO2 oxidation is done by sending SO2 bearing gas down through beds of catalyst (Fig. 1.2). The reaction is:

(1.1)

It is strongly exothermic (ΔH°25 °C = − 100 MJ/kg mol of SO3). Its heat of reaction provides considerable energy for operating the acid plant.

Figure 1.2 Catalyst pieces in a catalytic SO2 oxidation converter. Converters are typically ~ 20 m high and 12 m diameter. They typically contain four, 0.5- to 1-m-thick catalyst beds. SO2-bearing gas descends the bed at ~ 3000 Nm³/min. Catalyst pieces are ~ 10 mm in diameter and length. ©MECS, Inc. All rights reserved. Used by permission of MECS, Inc.

1.1.1 Catalyst

At normal operating temperature, 400-630 °C, SO2 oxidation catalyst consists of a molten film of V, K, Na, Cs pyrosulfate salt on a solid porous SiO2 substrate. The molten film rapidly absorbs SO2 and O2 and rapidly produces and desorbs SO3 (Chapters 7 and 8).

1.1.2 Feed gas drying

Equation (1.1) indicates that catalytic oxidation feed gas is almost always dry.¹ This dryness avoids:

(a) accidental formation of H2SO4 by the reaction of H2O(g) with the SO3 product of catalytic SO2 oxidation

(b) condensation of the H2SO4(ℓ) in cool flues and heat exchangers

(c) corrosion.

The H2O(g) is removed by cooling/condensation (Chapter 4) and by dehydration with H2SO4(ℓ) (Chapter 6).

1.2 H2SO4 production

Catalytic oxidation’s SO3 product is made into H2SO4(ℓ) by contacting catalytic oxidation’s exit gas with strong sulfuric acid (Fig. 1.3). The reaction is:

(1.2)

Figure 1.3 Top of H2SO4 making (absorption) tower, courtesy MECS (www.mecsglobal.com). The tower is packed with ceramic saddles. 98.5 mass% H2SO4(ℓ), 1.5 mass% H2O(ℓ) sulfuric acid is distributed uniformly across this packed bed. Distributor headers and downcomer pipes are shown. The acid flows through slots in the downcomers down across the bed (see buried downcomers at the right of the photograph). It descends around the saddles, while SO3-rich gas ascends, giving excellent gas-liquid contact. The result is efficient H2SO4(ℓ) production by Reaction (1.2). A tower is ~ 7 m diameter. Its packed bed is ~ 4 m deep. About 25 m³ of acid descends per minute, while 3000 Nm³ of gas ascends per minute.

Reaction (1.2) produces strengthened sulfuric acid because it consumes H2O(ℓ) and makes H2SO4(ℓ).

H2SO4(ℓ) is not made by reacting SO3(g) with pure H2O(ℓ). This is because Reaction (1.2) is so exothermic that the product of the SO3 + H2O(ℓ) → H2SO4 reaction would be hot H2SO4 vapor—which is difficult and expensive to condense.

The small amount of H2O(ℓ) and the massive amount of H2SO4(ℓ) in Reaction (1.2)’s input acid avoid this problem. The small amount of H2O(ℓ) limits the extent of the reaction. The large amount of H2SO4(ℓ) warms only 25 °C, while it absorbs Eq. (1.2)’s heat of reaction.

1.3 Industrial flowsheet

Figure 1.4 is a sulfuric acid manufacture flowsheet. It shows:

(a) the three sources of SO2 for acid manufacture (metallurgical, sulfur burning, and spent acid decomposition gas)

(b) acid manufacture from SO2 by Reactions (1.1) and (1.2).

Figure 1.4 Double contact sulfuric acid manufacture flowsheet. The three main SO2 sources are at the top. Sulfur burning is by far the biggest source. The acid product leaves from two H2SO4(ℓ) making towers at the bottom. Barren tail gas leaves the final H2SO4(ℓ) making tower, right arrow.

(b) is the same for all three sources of SO2. The next three sections describe (a)’s three SO2 sources.

1.4 Sulfur burning

About 60% of sulfuric acid is made from elemental sulfur (Chapter 3). Virtually, all the sulfur is obtained as a byproduct from refining natural gas and petroleum.

The sulfur is made into SO2 acid plant feed by

(a) melting the sulfur

(b) spraying it into a hot furnace

(c) burning the droplets with dried air.

The reaction is:

(1.3)

Very little SO3 forms at the 1150 °C flame temperature of this reaction (Fig. 7.4). This explains the two-step oxidation shown in Fig. 1.4:

(a) burning of sulfur to SO2

then:

(b) catalytic oxidation of SO2 to SO3, 400-630 °C.

The product of sulfur burning is hot, dry SO2, O2, N2 gas. After cooling to ~ 400 °C, it is ready for catalytic SO2 oxidation and subsequent H2SO4(ℓ) making.

1.5 Metallurgical offgas

SO2 in smelting and roasting gas accounts for about 30% of sulfuric acid production (Chapter 4). The SO2 is ready for sulfuric acid manufacture, but the gas is dusty. If left in the gas, the dust would plug the downstream catalyst layers and block gas flow.

It must be removed before the gas goes to catalytic SO2 oxidation.

It is removed by combinations of:

(a) settling in heat recovery boilers

(b) electrostatic precipitation

(c) scrubbing with water (which also removes impurity vapors).

After treatment, the gas contains ~ 1 mg of dust per dry Nm³ of gas. It is ready for drying, heating, catalytic SO2 oxidation, and H2SO4(ℓ) making.

1.6 Spent acid regeneration

A major use of sulfuric acid is as catalyst for petroleum refining and polymer manufacture (Chapter 5). The acid becomes contaminated with water, hydrocarbons, and other compounds during this use. It is regenerated by:

(a) spraying the acid into a hot (~ 1050 °C) furnace—where the acid decomposes to SO2, O2, and H2O(g)

(b) cleaning, drying, and heating the furnace offgas

(c) catalytically oxidizing the offgas’s SO2 to SO3

(d) making the resulting SO3 into new H2SO4(ℓ) by contact with strong sulfuric acid (Fig. 1.4).

About 10% of sulfuric acid is made this way. Virtually, all is reused for petroleum refining and polymer manufacture.

1.7 Sulfuric acid product

Most industrial acid plants have three flows of sulfuric acid—one gas-dehydration flow and two H2SO4(ℓ)-making flows. These flows are connected through automatic control valves to:

(a) maintain proper flows and H2SO4(ℓ) concentrations in the three acid circuits

(b) draw off newly made acid.

Water is added where necessary to give prescribed acid strengths.

Sulfuric acid is sold in grades of 93-99 mass% H2SO4(ℓ) according to market demand. The main product in cold climates is ~ 94% H2SO4(ℓ) because of its low (− 35 °C) freezing point (Gable et al., 1950). A small amount of oleum (H2SO4(ℓ) with dissolved SO3) is also produced (King and Forzatti, 2009).

Sulfuric acid is mainly shipped in stainless steel trucks, steel rail tank cars, and double-hulled steel barges and ships (Louie, 2008). Great care is taken to avoid spillage.

1.8 Recent developments

The three main recent developments in sulfuric acidmaking have been:

(a) improved materials of construction (Chapter 30), specifically more corrosion-resistant materials

(b) improved SO2 + 0.5 O2 → SO3 catalyst, specifically V, Cs, K, Na, S, O, SiO2 catalyst with low activation temperatures (Christensen and Polk, 2011; Felthouse et al., 2011)

(c) improved techniques for recovering the heat from Reactions (1.1)–(1.3) (Viergutz, 2009).

All of these improve H2SO4 and energy recovery.

1.9 Alternative processes

1.9.1 Wet gas sulfuric acid

An alternative to the conventional acidmaking described above is the Wet gas Sulfuric Acid (WSA; Laursen and Jensen, 2007) process. This process:

(a) catalytically oxidizes the SO2 in H2O(g), SO2, O2, N2 gas

and:

(b) condenses strong (~ 98 mass% H2SO4(ℓ) − 2 mass% H2O(ℓ)) sulfuric acid directly from this oxidized gas.

It is described in Chapters 25 and 26.

In 2013, it is mainly used for removing SO2 from moist, dilute (~ 3 volume% SO2) waste gases (Chapter 25). It accounts for ~ 3% of world sulfuric acid production.

1.9.2 Sulfacid®

About 20 Sulfacid® installations worldwide produce weak sulfuric acid (10-20% H2SO4) from very low concentration gases (< 1.0 volume% SO2) using an activated carbon catalytic reactor where SO2 reacts with O2 and H2O(ℓ) at 30-80 °C to produce H2SO4 (Kruger, 2004). The acid is intermittently washed with water from the catalyst which produces weak sulphuric acid. The cleaned gas is discharged to the atmosphere.

The sulfuric acid is often used for other on-site processes (e.g., titanium dioxide production) or sold.

1.10 Summary

About 200 million tonnes of sulfuric acid are produced/consumed per year. The acid is used for making fertilizer, leaching metal ores, refining petroleum and for manufacturing a myriad of products.

Sulfuric acid is made from dry SO2, O2, N2 gas. The gas comes from:

(a) burning molten elemental sulfur with dry air (Chapter 3)

(b) smelting and roasting metal sulfide minerals (Chapter 4)

(c) decomposing contaminated (spent) sulfuric acid catalyst (Chapter 5).

Sulfur burning is far and away the largest source.

The SO2 in the gas is made into sulfuric acid by

(a) catalytically oxidizing it to SO3 (Chapters 7 and 8)

(b) reacting this SO3 with the H2O(ℓ) in 98.5 mass% H2SO4(ℓ), 1.5 mass% H2O(ℓ) sulfuric acid (Chapter 9).

References

1. Apodaca LE. Sulfur Mineral Commodity Summary. Washington, DC: United States Geological Survey; 2012.

2. Christensen K, Polk P. SO2 emission reduction by Topsøe’s new VK-701 LEAP5™ catalyst. Sulfuric Acid Today. 2011;17(1):23–24.

3. Felthouse TR, DiGiovanni MP, Horne JR, Richardson SA. Improving sulfuric acid plant performance with MECS’ new GEAR catalysts. Sulfuric Acid Today. 2011;17(2):16–18.

4. Gable CM, Betz HF, Maron SH. Phase equilibria of the system sulfur trioxide-water. J Am Chem Soc. 1950;72:1445–1448.

5. King MJ, Forzatti