Sulfuric acid manufacture analysis, control and optimization

Sulfuric Acid Manufacture


Intentionally left as blank


Sulfuric Acid Manufacture
Analysis, Control, and Optimization

By

Matthew J. King
Perth, Western Australia

William G. Davenport
Tucson, Arizona

Michael S. Moats
Rolla, Missouri

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD
PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO


Elsevier
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Second edition
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ISBN: 978-0-08-098220-5

7 6 5 4

3 2 1


Contents

Preface

xv

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

2

Production and consumption
2.1
Uses
2.2
Acid plant locations
2.3
Price
2.4
Summary

11
13
14
14
16

3

Sulfur
3.1
3.2
3.3
3.4
3.5
3.6
3.7

19
20

20
21
22
23
28
29

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
4.6
H2O(g) removal from scrubber exit gas
4.7
Summary

burning
Objectives
Sulfur
Molten sulfur delivery
Sulfur atomizers and sulfur burning furnaces

Product gas
Heat recovery boiler
Summary

1
1
3
4
4
6
6
7
7
7
8

31
31
33
34
35
37
43
44


vi

Contents

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

47
47
51
51
52
53
54
56

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

59
59
61
64
65
70
70

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
7.5
Industrial multicatalyst bed SO2 oxidation
7.6
Industrial operation
7.7
Recent advances
7.8
Summary

73
73
73
75
84
84
87
89
89

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

91
91
95
95
96
97
98
99
100
100

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
9.5
Choice of input and output acid compositions
9.6
Acid temperature

103
103
104
105
107
115
116


Contents

9.7
9.8
9.9
9.10
9.11

vii

Gas temperatures

Operation and control
Double contact H2SO4 making
Intermediate versus final H2SO4 making
Summary

116
116
118
120
120

Break

123

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 % SO2 oxidized
10.5
Equilibrium % SO2 oxidized as a function of temperature
10.6
Discussion

10.7
Summary
10.8
Problems

125
125
127
128
129
129
132
132
132

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

135
135
135
136
136
136
137
138
139
140
142
143
143
145
147

148
149
150

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

151
151
151
153
153
154
154
154

viii

Contents

12.8
12.9
12.10
12.11
12.12
12.13
12.14

Minor influence—O2 strength in feed gas
Minor influence—CO2 in feed gas
Catalyst degradation, SO2 strength, and feed gas temperature
Maximum feed gas SO2 strength
Exit gas compositionintercept gas composition
Summary
Problems

155
155
157
158
159
160
160

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

161
161
161
164
164
164

14

Second
14.1
14.2
14.3
14.4
14.5
14.6

14.7
14.8
14.9
14.10
14.11
14.12
14.13

catalyst bed heatup path
Objectives
% SO2 oxidized redefined
Second catalyst bed heatup path
A specific heatup path question
Second catalyst bed input gas quantities
S, O, and N molar balances
Enthalpy balance
Calculating 760 K (level L) quantities
Matrix calculation and result
Preparing a heatup path
Discussion
Summary
Problem

167
167
167
168
170
170
171

171
172
173
173
173
175
176

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

177
177
178
180
181
181

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

183
183
184
187
187
187
187


Contents

ix

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

189
189
193
193
197
197
198

18

Three
18.1
18.2
18.3
18.4
18.5

18.6
18.7
18.8
18.9
18.10
18.11
18.12
18.13

199
199
199
199
201
202
202
204
204
206
206
207
208
209

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

211
211
213
213
215
216
216
217
221
222
227

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

229
230
230
231
231
232
233
234

21

Enthalpies and enthalpy transfers
21.1

Input and output gas enthalpies
21.2
H2SO4 making input gas enthalpy

235
235
238

catalyst bed acid plant
Calculation specifications
Example calculation
Calculation results
Three catalyst bed graphs
Minor effect—SO3 in feed gas
Minor effect—CO2 in feed gas
Minor effect—Bed pressure
Minor effect—SO2 strength in feed gas
Minor effect—O2 strength in feed gas
Summary of minor effects
Major effect—Catalyst bed input gas temperatures
Discussion of book’s assumptions
Summary


x

Contents

21.3
21.4

21.5
21.6
22

Heat transfers
Heat transfer rate
Summary
Problems

239
240
241
241

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

243
243
243
245
245
246
247
247

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

251
252
252
253
253
255
255
258
261
262

24

Acid temperature control and heat recovery
267
24.1
Objectives
267
24.2
Calculation of output acid temperature
267
24.3

Effect of input acid temperature
272
24.4
Effect of input gas temperature
273
24.5
Effect of input gas SO3 concentration on output acid temperature 273
24.6
Adjusting output acid temperature
274
24.7
Acid cooling
275
24.8
Target acid temperatures
276
24.9
Recovery of acid heat as steam
276
24.10 Steam production principles
278
24.11 Double-packed bed absorption tower
278
24.12 Steam injection
279
24.13 Sensible heat recovery efficiency
279
24.14 Materials of construction
280

248
249
249
250


Contents

24.15
24.16

xi

Summary
Problems

280
280

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

283
283
284
285
287
288
289
291
291
292
292
293

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

295
295
299
302
305
307
307
308
309
310

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

313
313
313
314
315
317

Sulfur
28.1
28.2
28.3
28.4

325
325
325
326
328

28

from tail gas removal processes
Objectives
Environmental standards
Acid plant tail gas characteristics
Industrial acid plant tail gas treatment methods

318
319
319
319
321
323


xii

Contents

28.5
28.6
28.7

Technology selection
Capital and operating costs
Summary

337
338
338

29

Minimizing sulfur emissions
29.1
Industrial catalytic SO2 þ 0.5O2 ! SO3 oxidation
29.2
Methods to lower sulfur emissions
29.3
Summary

341
341
343
347

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

349
349
349
351
356

31

Costs of sulfuric acid production
31.1
Investment costs
31.2
Production costs
31.3
Summary

357
357

360
362

Appendix A
Appendix B
Appendix C

363
369

Sulfuric acid properties
Derivation of equilibrium equation (10.12)
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-H2SO4making heatup path-equilibrium curve intercepts

379
383
387
389
393
399
401
405
413
417
421
427
429
431
439
441
443
451


Contents

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

xiii

Appendix U

453
459
461
465
475
481
493


Intentionally left as blank


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:
Chapter
Chapter
Chapter
Chapter
Chapter
Chapter
Chapter

25
26
27
28
29
30
31

Making Sulfuric Acid from Wet Feed Gas
Wet Sulfuric Acid Process Fundamentals
SO3 Gas Recycle for High SO2 Concentration Gas Treatment
Sulfur from Tail Gas Removal Processes
Minimizing Sulfur Emissions
Materials of Construction
Costs of Sulfuric Acid Production

We add one new unit to this edition—parts per million SO2 by volume, where SO2 can
be any gas. It is defined as
À
Á
ppmv ¼ Nm3 of SO2 per total Nm3 of gas à 1  106

where Nm3 may be (i) measured or (ii) calculated from measured gas masses by the
relationship:
22:4 Nm3 contains 1 kg mol of ideal gas:
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.


xvi

Preface

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.
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).
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:
400-630  C
þ 0:5O2 ƒƒƒƒƒ!
SO3
SO2
in dry SO2 , O2 , N2 gas
in feed gas catalyst in SO3 , SO2 , O2 , N2 exit gas

(1.1)

It is strongly exothermic (DH 25 C ¼ À 100 MJ/kg mol of SO3). Its heat of reaction
provides considerable energy for operating the acid plant.

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).
Sulfuric Acid Manufacture. />© 2013 Elsevier Ltd. All rights reserved.


2

Sulfuric Acid Manufacture

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.
Gas

Sulfur burning
furnace

Sulfide mineral smelters
and roasters

Spent acid decomposition
furnace

SO2

12

10

9

O2

9

11

11

N2

79

79

76

Absorption
Converter
Drying

Furnace

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.

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
1

A small amount of sulfuric acid is made by wet catalysis. This is discussed in Section 1.9 and Chapters 25
and 26.


Overview

3

Catalyst

Catalyst

~20 m
Catalyst

Catalyst

~0.5-1 m

~12 m

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. Copyright 2013 MECS, Inc. All rights reserved. Used by permission of MECS, Inc.

(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:
80-110  C
þ
H 2 O ðl Þ
H2 SO4 ðlÞ
,

SO3
ƒƒƒƒƒƒƒ!
in SO3 SO2 , O2 , N2 gas
in strengthened sulfuric acid
in 98:5% H2 SO4 ðlÞ, 1:5% H2 OðlÞ sulfuric acid

DH  25  C % À130 MJ=kg mol of SO3

(1.2)

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 þ H2O(ℓ) ! H2SO4 reaction
would be hot H2SO4 vapor—which is difficult and expensive to condense.


4

Sulfuric Acid Manufacture

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.

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).

(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.


Spent sulfuric acid

Elemental sulfur
Dried air

Gas cooling

Spent acid

Sulfur burning furnace
S + O2 → SO2

Dilution with low
SO2 in-plant gas

Overview

10-75% SO2 dusty
metallurgical offgas

decomposition
8-14% SO2, 2% O2, remainder
CO2, H 2O(g), and N2

Dust removal by electrostatic
precipitation and aqueous
scrubbing

12% SO2, 9% O2, 79% N 2
Gas cooling and aqueous
scrubbing

Gas cooling
Gas drying and heating

Gas drying and heating
~420 °C

Air

Air
Catalyst layers
Gas cooling
SO2 + 0.5O2 → SO3

Gas cooling

Gas cooling
SO3 rich, SO3, SO2, O2, N 2 gas

SO3 free, SO2, O2, N 2 gas

SO2 (trace), O2, N 2 gas
to environment

Gas heating

98.5% H 2SO4 acid

98.5% H 2SO4 acid
SO3(g) + H 2O in acid → H 2SO4(l)
Packed bed

SO2 +0.5O2 → SO3

SO3(g) + H 2O in acid → H 2SO4(l)

Gas cooling

Packed bed

SO2 (trace), SO3, O2, N 2 gas

H 2SO4 strengthened acid to
dilution, recycle, and market

H 2SO4 strengthened acid to
dilution, recycle, and market

5

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.


6

Sulfuric Acid Manufacture

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:
1150  C

SðlÞ þ O2 ƒƒƒƒ!

in dry air

SO2
,
in SO2 , O2 , N2 gas

DH  25  C % À300 MJ=kg mol of SðlÞ:
(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


Overview

7

(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
(À35  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 þ 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
1.9.1

Alternative processes
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) À 2 mass% H2O(ℓ)) sulfuric acid directly from
this oxidized gas.


8

Sulfuric Acid Manufacture

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
Apodaca, L.E., 2012. Sulfur Mineral Commodity Summary. United States Geological Survey,
Washington, DC.
Christensen, K., Polk, P., 2011. SO2 emission reduction by Topse’s new VK-701 LEAP5™
catalyst. Sulfuric Acid Today 17 (1), 23–24.
Felthouse, T.R., DiGiovanni, M.P., Horne, J.R., Richardson, S.A., 2011. Improving sulfuric
acid plant performance with MECS’ new GEAR catalysts. Sulfuric Acid Today 17 (2),
16–18.

Gable, C.M., Betz, H.F., Maron, S.H., 1950. Phase equilibria of the system sulfur trioxide-water.
J. Am. Chem. Soc. 72, 1445–1448.