Sulfuric Acid Manufacture

Analysis, Control and Optimization

Matt King, Michael Moats, William G. Davenport

• Publisher: Elsevier
• Year: 2013
• eBook ISBN: 9780080982267
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

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

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)....

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• 1. Overview
• 2. Production and consumption
• 3. Sulfur burning
• 4. Metallurgical offgas cooling and cleaning
• 5. Regeneration of spent sulfuric acid
• 6. Dehydrating air and gases with strong sulfuric acid
• 7. Catalytic oxidation of SO2 to SO3
• 8. SO2 oxidation catalyst and catalyst beds
• 9. Production of H2SO4(ℓ) from SO3(g)
• Break
• 10. Oxidation of SO2 to SO3—Equilibrium curves
• 11. SO2 oxidation heatup paths
• 12. Maximum SO2 oxidation: Heatup path-equilibrium curve intercepts
• 13. Cooling first catalyst bed exit gas
• 14. Second catalyst bed heatup path
• 15. Maximum SO2 oxidation in a second catalyst bed
• 16. Third catalyst bed SO2 oxidation
• 17. SO3 and CO2 in feed gas
• 18. Three catalyst bed acid plant
• 19. After-H2SO4-making SO2 oxidation
• 20. Optimum double contact acidmaking
• 21. Enthalpies and enthalpy transfers
• 22. Control of gas temperature by bypassing
• 23. H2SO4 making
• 24. Acid temperature control and heat recovery
• 25. Making sulfuric acid from wet feed gas
• 26. Wet sulfuric acid process fundamentals
• 27. SO3 gas recycle for high SO2 concentration gas treatment
• 28. Sulfur from tail gas removal processes
• 29. Minimizing sulfur emissions
• 30. Materials of construction
• 31. Costs of sulfuric acid production
• Appendix A. Sulfuric acid properties
• Appendix B. Derivation of equilibrium equation (10.12)
• Appendix C. Free energy equations for equilibrium curve calculations
• Appendix D. Preparation of Fig. 10.2’s equilibrium curve
• Appendix E. Proof that volume%=mol% (for ideal gases)
• Appendix F. Effect of CO2 and Ar on equilibrium equations (none)
• Appendix G. Enthalpy equations for heatup path calculations
• Appendix H. Matrix solving using Tables 11.2 and 14.2 as examples
• Appendix I. Enthalpy equations in heatup path matrix cells
• Appendix J. Heatup path-equilibrium curve: Intercept calculations
• Appendix K. Second catalyst bed heatup path calculations
• Appendix L. Equilibrium equation for multicatalyst bed SO2 oxidation
• Appendix M. Second catalyst bed intercept calculations
• 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
• 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
• 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
• Appendix V. Moist air in H2SO4 making calculations
• Appendix W. Calculation of H2SO4 making tower mass flows
• Appendix X. Equilibrium equations for SO2, O2, H2O(g), N2 feed gas
• Appendix Y. Cooled first catalyst bed exit gas recycle calculations
• Answers to numerical problems
• Index