Sulfuric acid manufacture analysis, control and optimization

5 Regeneration of spent sulfuric acid 475.5 Optimum decomposition furnace operating conditions 535.6 Preparation of offgas for SO2oxidation and H2SO4making 54 8.2 Maximum and minimum cat

Sulfuric Acid Manufacture

Sulfuric Acid Manufacture

Analysis, Control, and Optimization

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5 Regeneration of spent sulfuric acid 47

5.5 Optimum decomposition furnace operating conditions 535.6 Preparation of offgas for SO2oxidation and H2SO4making 54

8.2 Maximum and minimum catalyst operating temperatures 95

9.7 Gas temperatures 116

11.15 Significance of heatup path position and slope 148

12.5 Inadequate % SO2oxidized in first catalyst bed 15412.6 Effect of feed gas SO2strength on intercept 154

12.8 Minor influence—O2strength in feed gas 155

12.10 Catalyst degradation, SO2strength, and feed gas temperature 157

12.12 Exit gas composition
intercept gas composition 159

15.1 Second catalyst bed equilibrium curve equation 177

17 SO3and CO2in feed gas 189

18.11 Major effect—Catalyst bed input gas temperatures 207

19.6 Heatup path-equilibrium curve intercept calculation 216

20.1 Total % SO2oxidized after all catalyst beds 230

22.4 Heat transfer requirement for 480 K economizer output gas 245

22.7 Bypassing for 460, 470, and 480 K economizer output gas 24722.8 Bypassing for 470 K economizer output gas while input gas

23.4 H2O(g) input from moist acid plant input gas 253

23.6 Calculation of mass water in and mass acid out 255

24.5 Effect of input gas SO3concentration on output acid temperature 273

25.5 Preparing the oxidized gas for H2SO4(ℓ) condensation 288

26.1 Wet gas sulfuric acid process SO2oxidation 29526.2 Injection of nanoparticles into cooled process gas 299

26.5 Condenser acid composition up the glass tube 307

27.4 Effect of recycle gas temperature on recycle requirement 31527.5 Effect of gas recycle on first catalyst SO2oxidation efficiency 31727.6 Effect of first catalyst exit gas recycle on overall acid plant

28.4 Industrial acid plant tail gas treatment methods 328

28.5 Technology selection 337

29.1 Industrial catalytic SO2þ 0.5O2! SO3oxidation 341

Appendix B Derivation of equilibrium equation (10.12) 369Appendix C Free energy equations for equilibrium curve

Appendix D Preparation of Fig 10.2’s equilibrium curve 383Appendix E Proof that volume% = mol% (for ideal gases) 387Appendix F Effect of CO2and Ar on equilibrium equations (none) 389Appendix G Enthalpy equations for heatup path calculations 393Appendix H Matrix solving using Tables 11.2 and 14.2 as examples 399Appendix I Enthalpy equations in heatup path matrix cells 401Appendix J Heatup path-equilibrium curve: Intercept calculations 405Appendix K Second catalyst bed heatup path calculations 413Appendix L Equilibrium equation for multicatalyst bed SO2

Appendix M Second catalyst bed intercept calculations 421Appendix N Third catalyst bed heatup path worksheet 427

Appendix P Effect of SO3in Fig 10.1’s feed gas on equilibrium

Appendix R CO2- and SO3-in-feed-gas intercept worksheet 441Appendix S Three-catalyst-bed “converter” calculations 443Appendix T Worksheet for calculating after-intermediate-H2SO4-

making heatup path-equilibrium curve intercepts 451

Appendix U After-H2SO4-making SO2oxidation with SO3

Appendix V Moist air in H2SO4making calculations 459Appendix W Calculation of H2SO4making tower mass flows 461Appendix X Equilibrium equations for SO2, O2, H2O(g), N2

Appendix Y Cooled first catalyst bed exit gas recycle calculations 475

The second is the addition of seven new chapters:

Chapter25 Making Sulfuric Acid from Wet Feed Gas

Chapter26 Wet Sulfuric Acid Process Fundamentals

Chapter27 SO3Gas Recycle for High SO2Concentration Gas TreatmentChapter28 Sulfur from Tail Gas Removal Processes

Chapter29 Minimizing Sulfur Emissions

Chapter30 Materials of Construction

Chapter31 Costs of Sulfuric Acid Production

We add one new unit to this edition—parts per million SO2by volume, where SO2can

be any gas It is defined as

ppmv¼ Nm3of SO2per total Nm3of gas

 1  106

where Nm3may be (i) measured or (ii) calculated from measured gas masses by therelationship:

22:4Nm3contains 1 kg mol of ideal gas:

Once again we have received exceptional help from our industrial colleagues, who sokindly showed us around their plants and answered all our questions We have con-tinued to visit acid plants during preparation of this edition—we thank our hosts mostprofusely

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

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

Matthew J KingPerth, Western AustraliaWilliam G DavenportTucson, ArizonaMichael S MoatsRolla, Missouri

1 Overview

Sulfuric acid is a dense clear liquid It is used for making fertilizers, leaching metallicores, refining petroleum, and manufacturing a myriad of chemicals and materials World-wide, about 200 million tonnes of sulfuric acid is consumed per year (Apodaca, 2012).The raw material for sulfuric acid is SO2gas 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

plant SO2feed is always mixed with other gases

Sulfuric acid is almost always made from these gases by:

(a) catalytically reacting their SO2and O2to form SO3(g)

(b) reacting (a)’s product SO3with 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 CO2is gas in the acid plant Each isreferenced in this book, for example, as O2not O2(g) The standard state for H2O, S,and H2SO4is gas or liquid in the acid plant Each is referenced accordingly

1.1 Catalytic oxidation of SO2 to SO3

O2doesnot oxidize SO2to SO3without a catalyst All industrial SO2oxidation is done

by sending SO2bearing gas down through “beds” of catalyst (Fig 1.2) The reaction is:

At normal operating temperature, 400-630C, SO

2oxidation catalyst consists of a moltenfilm of V, K, Na, Cs pyrosulfate salt on a solid porous SiO2substrate The molten filmrapidly absorbs SO2and O2and rapidly produces and desorbs SO3(Chapters 7and8)

Sulfuric Acid Manufacture http://dx.doi.org/10.1016/B978-0-08-098220-5.00001-0

© 2013 Elsevier Ltd All rights reserved.

1.1.2 Feed gas drying

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

(a) accidental formation of H2SO4by the reaction of H2O(g) with the SO3product of alytic SO2oxidation

of the stack The catalytic converter is 16.5 m diameter

Table 1.1 Typical compositions (volume%) of acid plant feed gases entering SO2oxidation

“converters,” 2013 The gases may also contain small amounts of CO2and SO3.Gas Sulfur burning

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

H2SO4(ℓ) is not made by reacting SO3(g) with pure H2O(ℓ) This is because

would be hot H SO vapor—which is difficult and expensive to condense

Figure 1.2 Catalyst pieces in a catalytic SO2oxidation “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 at3000 Nm3/min Catalyst pieces are10 mm in diameterand length Copyright 2013 MECS, Inc All rights reserved Used by permission of MECS, Inc

The small amount of H2O(ℓ) and the massive amount of H2SO4(ℓ) in

extent of the reaction The large amount of H2SO4(ℓ) warms only 25C, while it

absorbs Eq.(1.2)’s heat of reaction

1.3 Industrial flowsheet

(a) the three sources of SO2for acid manufacture (metallurgical, sulfur burning, and spentacid decomposition gas)

(b) acid manufacture from SO2by Reactions(1.1)and(1.2)

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

“downcomer” pipes are shown The acid flows through slots in the downcomers down across thebed (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 is7 m diameter Its packed bed is 4 m deep About

25 m3of acid descends per minute, while 3000 Nm3of gas ascends per minute

metallurgical offgas

Dust removal by electrostatic

precipitation and aqueous

decomposition Spent acid

H 2 SO 4 strengthened acid to dilution, recycle, and market

The sulfur is made into SO2acid plant feed by

(a) melting the sulfur

(b) spraying it into a hot furnace

(c) burning the droplets with dried air

The reaction is:

explains the two-step oxidation shown inFig 1.4:

(a) burning of sulfur to SO2

then:

(b) catalytic oxidation of SO2to SO3, 400-630C

The product of sulfur burning is hot, dry SO2, O2, N2gas After cooling to400C, it

is ready for catalytic SO2oxidation and subsequent H2SO4(ℓ) making

1.5 Metallurgical offgas

SO2in smelting and roasting gas accounts for about 30% of sulfuric acid production

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 SO2oxidation

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 contains1 mg of dust per dry Nm3of gas It is ready fordrying, heating, catalytic SO2oxidation, and H2SO4(ℓ) making

1.6 Spent acid regeneration

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

man-(a) spraying the acid into a hot (1050C) furnace—where the acid decomposes to SO

2,

O2, and H2O(g)

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

(c) catalytically oxidizing the offgas’s SO to SO

(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 petroleumrefining and polymer manufacture

1.7 Sulfuric acid product

Most industrial acid plants have three flows of sulfuric acid—one gas-dehydrationflow and two H2SO4(ℓ)-making flows These flows are connected through automaticcontrol 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 marketdemand The main product in cold climates is 94% H2SO4(ℓ) because of its low(35C) freezing point (Gable et al., 1950) A small amount of oleum (H

2SO4(ℓ) withdissolved SO3) is also produced (King and Forzatti, 2009)

Sulfuric acid is mainly shipped in stainless steel trucks, steel rail tank cars, anddouble-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-resistantmaterials

(b) improved SO2þ0.5O2!SO3catalyst, specifically V,Cs, K, Na, S, O, SiO2catalystwith 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 H2SO4and energy recovery

1.9 Alternative processes

1.9.1 Wet gas sulfuric acid

An alternative to the conventional acidmaking described above is theWet gas SulfuricAcid (WSA;Laursen and Jensen, 2007) process This process:

(a) catalytically oxidizes the SO2in H2O(g), SO2, O2, N2gas

and:

(b) condenses strong (98 mass% H2SO4(ℓ)2 mass% H2O(ℓ)) sulfuric acid directly fromthis oxidized gas

It is described inChapters 25and26.

In 2013, it is mainly used for removing SO2from 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 activatedcarbon catalytic reactor where SO2reacts with O2and H2O(ℓ) at 30-80C to produce

catalyst which produces weak sulphuric acid The cleaned gas is discharged tothe atmosphere

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

1.10 Summary

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

Sulfuric acid is made from dry SO2, O2, N2gas 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 SO2in the gas is made into sulfuric acid by

(a) catalytically oxidizing it to SO3(Chapters 7and8)

(b) reacting this SO3with the H2O(ℓ) in 98.5 mass% H2SO4(ℓ), 1.5 mass% H2O(ℓ) sulfuricacid (Chapter 9)

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

King, M.J., Forzatti, R.J., 2009 Sulphur based by-products from the non-ferrous metals industry.In: Liu, J., Peacey, J., Barati, M., Kashani-Nejad, S., Davis, B (Eds.), Pyrometallurgy ofNickel and Cobalt 2009: Proceedings of the 48th Conference of Metallurgists of CIM.CIM METSOC, Montreal, pp 137–149.

Kruger, B., 2004 Recovery of SO2 from low strength off-gases In: International PlatinumConference ‘Platinum Surges Ahead’ The Southern African Institute of Mining andMetallurgy, Johannesburg, pp 59–61

Laursen, J.K., Jensen, F.E., 2007 WSA—meeting industry demands Sulfur 312, 80–85.Louie, D.K., 2008 Handbook of sulphuric acid manufacturing, second ed DKL EngineeringInc., Richmond Hill, Ontario, Canada

Viergutz, M.D., 2009 Heat recovery system update In: Proceedings of the Sulphur and SulphuricAcid Conference The Southern African Institute of Mining and Metallurgy, Johannesburg

2 Production and consumption

Sulfuric acid was first produced around the tenth century AD (Al Hassan and Hill,

1986) It was made by decomposing natural hydrated sulfate minerals and condensingthe resulting gas Example reactions are:

CuSO4
5H2O sð Þ ƒƒƒ!heat

SO3þ 5H2O gð Þ ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!acidmaking and condensation H2SO4ð Þ þ 4Hl 2Oð Þl (2.2)

The process was carried out in a ceramic retort (inside a furnace) and in a “bird-beak”condenser (outside the furnace) Acid composition was adjusted by adding orevaporating water

The earliest uses of sulfuric and other mineral acids were as solvents for:(a) separating gold and silver

(b) decorative etching of metals, e.g., Damascus Steel

(Killick, D., University of Arizona, personal communication, 2005)

Thermal decomposition of sulfates was still being used in the nineteenth century—

to make 90þ% H2SO4sulfuric acid The process entailed (Kiefer, 2001):

(a) making Fe2(SO4)3(s) by oxidizing pyrite FeS2(s) with air

(b) thermally decomposing the Fe2(SO4)3(s) in a retort to make SO3(g) and Fe2O3(s), i.e.:

Industrial sulfuric acid production began in the eighteenth century with theburning of sulfur in the presence of natural niter (KNO3) and steam This developed

Sulfuric Acid Manufacture http://dx.doi.org/10.1016/B978-0-08-098220-5.00002-2

© 2013 Elsevier Ltd All rights reserved.

into the lead chamber and tower processes—which used nitrogen oxides to form

an aqueous catalyst for SO2oxidation The overall acidmaking reaction with this alyst was:

cat-SO2þ 0:5O2þ H2Oð Þ ƒƒƒƒƒƒƒƒƒƒƒ!l in aqueous solutionNOHSO

4 catalyst H2SO4ð Þl (2.5)

The lead chamber and tower processes were used in the twentieth century tunately, their H2SO4strength was limited to below about 70 mass% H2SO4 Above70% H2SO4, the product acid contained stable nitrosyl hydrogen sulfate which made itunsuitable for many purposes

Unfor-The twentieth century saw the nitrogen oxide processes gradually but tely replaced by the catalytic SO2 oxidation/SO3—sulfuric acid contact process

concen-trations and was first described in a patent by Phillips in 1831 (Sander et al., 1984).Platinum was the dominant catalyst until the 1930s V, K, Na, Cs, S, O, SiO2catalyst

World production of sulfuric acid since 1950 is shown inFig 2.1 Sources of SO2for this production are given inTable 2.1

Calculated from total world sulfur production assuming that 90%

of this production is made into H2SO4 (Buckingham, Ober and Apodaca, 2010); (Gustin, 2011)

Figure 2.1 World sulfuric acid production, 1970-2011, in megatonnes of contained H2SO4

(Buckingham et al., 2010; Gustin, 2011) The increase in production over the years is notable It

is mainly due to the increased use of phosphate and sulfate fertilizers, virtually all of which aremade with sulfuric acid

2.1 Uses

Sulfuric acid is mostly used for making phosphate fertilizers (Table 2.2) The mostcommon process is:

(a) production of phosphoric acid by reacting phosphate rock with sulfuric acid, i.e.:

Caphosphate rock3ðPO4Þ2ð Þs þ3H2SO4ð Þ þ 6Hl 2Oð Þ ! 2Hl phosphoric acid3PO4ð Þl þ 3 CaSO½ gypsum4
2H2O sð Þ

(2.6)followed by:

(b) reaction of the phosphoric acid with ammonia to make monoammonium phosphate(NH4H2PO4(s)) and diammonium phosphate ((NH4)2H2PO4(s)) These are oftenreferred to as MAP and DAP (King and Forzatti, 2009)

Table 2.2 World uses of sulfuric acid by percentage, 2009 (Chemsystems, 2009)

Table 2.1 Sources of sulfur and SO2for producing sulfuric acid Virtually, all the sulfur and SO2

production is involuntary, i.e., it is the by-product of other processes.Source: Interpreted

fromGustin (2011)andApodaca (2012)

Elemental sulfur from natural gas purification

and petroleum refining (Chapter 3)

60

SO2from smelting and roasting nonferrous

minerals and pyrites (Chapter 4)

30

SO2from decomposing spent petroleum/polymer

sulfuric acid catalyst (Chapter 5)

10

Sulfuric acid is also used extensively for making other fertilizers (e.g., sulfates) andchemicals of all sorts.

2.2 Acid plant locations

Sulfuric acid plants are located throughout the industrialized world (Fig 2.2) Mostare located near their product acid’s point of use, i.e., near fertilizer plants, copperore leach plants, and petroleum refineries This is because elemental sulfur is cheaper

to transport than sulfuric acid Examples of long-distance sulfur shipment are fromnatural gas purification plants in Alberta, Canada, to acid plants near phosphate ores

in Florida and Australia

Smelter acid, on the other hand, must be made from the by-product SO2at thesmelter and transported to its point of use An example of this is the production of acid

at the Cu-Ni smelters in Sudbury, Canada, and rail transport of the product acid tofertilizer plants in Florida Another example of this is the production of acid at

Cu smelters in Sonora, Mexico, and Arizona, United States, and rail and truck port of product acid to numerous copper ore leaching operations in Arizona andNew Mexico, United States, and Sonora, Mexico

trans-Production of pure sulfuric acid from contaminated “spent” sulfuric acid catalyst isalmost always done near the source of the spent acid—to minimize forward and returnshipping distance

2.3 Price

of the graph are:

(a) a stable price from 2003 to 2007

(b) a sharp price spike in 2008

(c) a price collapse in 2009

(d) a steady price increase to about $150 per tonne at the end of 2012

The 2008 price spike was caused by:

(a) a sharp increase in China’s demand for metals and chemicals, all of which require furic acid for their manufacture

sul-(b) inability of the world’s sulfuric acid producers to rapidly increase their acid productionrate, i.e., their inability to meet this increased demand

The 2009 catastrophic price fall was the reverse of the above China’s demandfor metals and chemicals decreased briefly, but sharply, leading to a sharp decrease

in acid demand and a glut of sulfuric acid at the world’s Cu, Zn, and Ni sulfidesmelters In some cases, the acid had to be given away in order to avoid shutting down

a smelter

North America including Mexico 18.1%

Oceana including Australia 2.2%

South and Central America 6.1%

Western Europe 6.4%

Africa 10.3%

Asia including China 44.5%

Eastern Europe including Russia 9.9%

2.4 Summary

Worldwide, about 200 million tonnes of sulfuric acid are produced per year Sixty cent comes from burning elemental sulfur The remainder comes from the SO2insmelter, roaster, and spent acid regeneration furnace offgases

per-By far, the largest use of sulfuric acid is for making phosphate fertilizers, e.g.,ammonium phosphate Other large uses are for making other fertilizers and chemicals

of all sorts

Sulfuric acid price averaged about $50 per tonne from 2003 to 2007, spiked to over

$400 in 2008, and settled at$150 per tonne in 2012

Figure 2.3 Spot price for sulfuric acid at U.S Gulf of Mexico ports (Boyd, 2011) Shipping toother ports is paid for by the buyer The peak price during 2008 is notable It was caused mainly

by increased Chinese demand (tonnes per year) and an inflexible supply (tonnes per year)

King, M.J., Forzatti, R.J., 2009 Sulphur based by-products from the non-ferrous metals try In: Liu, J., Peacey, J., Barati, M., Kashani-Nejad, S., Davis, B (Eds.), Pyrometallurgy

indus-of Nickel and Cobalt 2009: Proceedings indus-of the 48th Conference indus-of Metallurgists indus-of CIM,Sudbury, Ontario, Canada METSOC, Montreal, Quebec, pp 137–149

Sander, U.H.F., Fischer, H., Rothe, U., Kola, R., More, A.I., 1984 Sulphur, Sulphur Dioxide,Sulphuric Acid British Sulphur Corporation Ltd., London

3 Sulfur burning

Sixty percent of sulfuric acid is made from elemental sulfur The elemental sulfur is:(a) received molten or melted with pressurized steam-heated pipes (sulfur melting point

120C)

(b) atomized to small droplets in a hot (1150C) furnace

(c) burnt in the furnace with excess dry air to form hot SO2, O2, N2gas

Sulfuric acid is then made from this gas by:

(d) cooling the gas in a boiler

(e) catalytically reacting its SO2and O2to form SO3

(f) contacting step (e)’s product gas with strong sulfuric acid to make H2SO4(ℓ) by thereaction

SO3ð Þ þ Hg 2Oð Þl

in acid

! H2SO4ð Þl

in strengthened acid

Steps (b)-(f) are continuous

Figure 3.0 View of spinning cup sulfur burner from inside sulfur burning furnace—burningcapacity 870 tonnes of molten sulfur per day The thermocouple at top and central blue sulfur-rich flame are notable Photograph courtesy of Outotec OYJ

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© 2013 Elsevier Ltd All rights reserved.

This chapter describes steps (a)-(d) (Fig 3.1) Steps (e) and (f) are described in

3.1 Objectives

The objectives of this chapter are to describe:

(a) the physical and chemical properties of elemental sulfur

(b) transportation of elemental sulfur to the sulfur burning plant

(c) preparation of elemental sulfur for combustion

(d) sulfur burners and sulfur burning furnaces

(e) control of sulfur burning offgas composition, temperature, and volume

3.2 Sulfur

Virtually, all of the elemental sulfur used for making sulfuric acid is a by-product ofnatural gas and petroleum refining It contains 99.9þ% S Its main impurity is carbonfrom natural gas or petroleum

Its melting point is 115-120C, depending on its crystal structure It is easily

melted with pressurized steam pipes

3.2.1 Viscosity

Molten sulfur viscosity is described inFig 3.2 Its key features are (i) a viscosity imum at 160C and (ii) a 10,000-fold viscosity increase just above 160C.

min-Sulfur burners are fed with 140C molten sulfur, near the viscosity minimum, but

safely below the steep viscosity increase Sulfur temperature is maintained by

Molten sulfur (140 °C) delivered

molten or delivered solid and

steam-melted on site

12 volume% SO2, 9 volume% O2,

79 volume% N2gas (420 °C) to catalytic

SO2 oxidation and H2SO4 making

Sulfur burning furnace Heat recoveryboiler

Steam

Clean, dry air, 120 °C, 1.4 bar

1150 °C

Boiler feed water

Figure 3.1 Sulfur burning flowsheet—molten sulfur to clean dry 420C SO

2, O2, N2gas Thefurnace is supplied with excess air to provide O2needed for subsequent catalytic oxidation of

SO2to SO3 The air also provides N2which keeps the furnace from overheating.Table 3.1givesindustrial sulfur burning data

circulating 150C steam through sulfur storage tank steam pipes just ahead of sulfur

burning Below ground or insulated above ground, storage tanks are used

Sulfur’s huge increase in viscosity just above 160C is due to a transition

from S8 ring molecules to long interwoven S chain molecules (Tobolosky and

3.3 Molten sulfur delivery

Elemental sulfur is produced molten It is also burnt molten

Where possible, therefore, sulfur is transported molten from sulfurmaking to sulfurburning It is mainly shipped in double-walled, steam-heatable railway tank cars,barges, and ships This gives easy handling at both ends of the journey Even if thesulfur solidifies during the journey, it is easily melted out with 150C steam jackets

and pipes to give a clean, atomizable raw material Short-distance deliveries are times made in single-walled tanker trucks, occasionally with provision for carryingsolid cargo (e.g., fertilizer granules) back to the starting point (Louie, 2008).Sulfur that is shipped molten is ready for burning Sulfur that is shipped as solid-ified pellets or flakes picks up dirt during shipping and storage This sulfur is meltedand filtered (Durco, 2012) before being burnt

some-Sulfur is shipped solid when there are several intermediate unloading-loading stepsduring its journey, e.g., train-ship-train An example of this is shipment of solid sulfurfrom interior Canada to interior Australia

3.3.1 Sulfur pumps and pipes

Molten sulfur has a viscosity (
0.01 kg/(m s), 140C,Fig 3.2) about 10 times that of

water (
0.001 kg/(m s), 20C) Its density is
1.8 tonnes/m3 It is easily moved insteam-jacketed steel pipes (Jondle and Hornbaker, 2004) Steam-heated pumps much

0.001 0.010 0.100 1.000 10.000

like that inFig 9.2are used Molten sulfur is an excellent lubricant at 140C Sulfur

pump impellers need no additional lubrication

3.4 Sulfur atomizers and sulfur burning furnaces

Sulfur burning consists of:

(a) atomizing molten sulfur and spraying the tiny droplets into a hot furnace

(b) blowing clean, dry 120C air into the furnace.

The tiny droplets and warm air give:

(c) rapid vaporization of sulfur in the hot furnace

(d) rapid and complete oxidation of the sulfur vapor by O2in the air

Representative reactions are:

The combined exothermic heat of reaction (DH

25C) for Reactions(3.1)and(3.2)isapproximately300 MJ/kg mol of S(ℓ)

3.4.1 Sulfur atomizers

Molten sulfur spraying is done with:

(a) a stationary spray nozzle at the end of a horizontal lance (Fig 3.3)

(b) a spinning cup sulfur atomizer (Fig 3.0) (Outotec, 2008)

In both cases, molten sulfur is pumped into the atomizers by steam-jacketed pumps.The stationary spray nozzle has the advantage of simplicity and no moving parts

sul-fur pressure (1 versus 10 bar), smaller droplets, easier flow rate adjustment, and ashorter furnace

3.4.2 Dried air supply

Air for sulfur burning is filtered through fabric and dried It is heated by the heat ofcompression in the main acid plant blower then blown into the sulfur burning furnace

It is blown in behind the sulfur spray to maximize droplet-air contact

The drying is done by contacting the air with strong sulfuric acid (Chapter 6) Thisremoves H2O(g) down to
0.05 g/Nm3

of air Drying to this level prevents accidental

H2SO4(ℓ) formation and corrosion after catalytic SO3production

3.4.3 Main blower

The dried air is blown into the sulfur burning furnace by the acid plant’s main blower

The blower is a steam or electricity-driven centrifugal blower (Louie, 2004) Itblows air into the sulfur burning furnace and the furnace’s offgas through the remain-der of the acid plant A pressure of 0.3-0.5 bar is required.

Two thousand tonnes of H2SO4per day sulfur burning acid plant typically requires

a 4000 to 4500 kW main blower

3.4.4 Furnace

Sulfur burning furnaces are 2-cm-thick cylindrical steel shells lined internally with

30-50 cm (total) of fire brick and insulating brick (Fig 3.3) Air and atomized molten sulfurenters at one end and hot SO2, O2, N2gas departs at the other end into a boiler (Fig 3.4).Some furnaces are provided with internal baffles The baffles create a tortuous path forthe sulfur and air, promoting complete sulfur combustion Complete sulfur combustion

is essential to prevent elemental sulfur condensation in downstream equipment.Industrial sulfur burning furnace operating details are listed inTable 3.1