From Sulfur to Sulfuric Acid

The Contact Process

www.mecsglobal.com/.../difficultapplications.asp

History

� Process of burning sulfur with saltpeter was first described in the 15th

century

� 1746 – lead chamber process� 1831 – contact process

- use of catalyst- absorption of SO3 into 98.5 % to 99 % H2SO4

universe-review.ca/F13-

atom.htm

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e/Multimedia.js...

Process

S (l) O2 (g) SO2 (g) Η = − 298.3 kJ, 25 0C

SO2 (g) O2 (g) SO3 (g) Η = − 98.3 kJ, 25 0C1/2

SO3 (g) H2SO4 (l) H2S2O7 (l) H = - 130.4 kJ, 25 0C

“Because sulphur is exothermic and burns in air, heat is generated and most sulphuric acid plants generate steam as a by-product.”

www.engineeringnews.co.za/article/new-sulphur...

• large plants can produce 1.3 tons in excess per metric ton of acid

• in some countries, energy from sulfuric acid plants is used in homes and offices saving ~ 35 700 tons of fuel oil per day!

S (l) O2 (g) SO2 (g) Η = − 298.3 kJ, 25 0C

SO2 (g) O2 (g) SO3 (g) Η = − 98.3 kJ, 25 0C1/2

SO3 (g) H2SO4 (l) H2S2O7 (l) H = - 130.4 kJ, 25 0C

How can more SO3 (g) be produced?

Kp =PSO3

PSO2 PO2

12

x

0.18451000

397400

48.1500

9.53600

0.09801100

0.384900

0.915800

2.63700

KpTemp, 0C

But, rate of reaction increases with increase in temperature → balance between rate of reaction and optimum conversion

Use of a catalyst:� earlier catalysts included iron oxide and platinum

� replaced with V2O5

(vanadium pentoxide) – can last up to 20 years

� is used in the shape of hollow cylinders →increased surface area together with good flow of gas through the catalyst

www.britannica.com/EBchecked/topic-art/572815...

How else can we optimize the conversion to SO3?

SO2 (g) O2 (g) SO3 (g) 1/2Four-pass Converter

S (l) O2 (g) SO2 (g) Η = − 298.3 kJ, 25 0C

SO2 (g) O2 (g) SO3 (g) Η = − 98.3 kJ, 25 0C1/2

SO3 (g) H2O (l) H2SO4 (l) H = - 130.4 kJ, 25 0C

SO3 (g) H2O (l) H2SO4 (aq) X

highly corrosive mist of sulfuric acid fumes

SO3 (g) H2SO4 (l) H2S2O7 (l)

� 98.5 – 99 % sulfuric acid is a good absorber of SO3 (g)

� lined with acid proof bricks with ceramic packing with conc acid flowing over it allows for SO3 (g) to have close contact with the conc acid

� concentrated acid is withdrawn and portion going back into tower is diluted with dilute H2SO4 or water

H2S2O7 (l) H2O (l)

H2SO4 (l)

2 H2SO4 (l)

H2SO4 (aq)H2O

Absorbing tower

Steel

Typical flow chart for a sulfur-burning single-absorption contact sulfuric acid plant. Monsanto Enviro-Chem from Shreve’s Chemical Process Industries

Remove CO2, N2, moisture & other

impurities

Furnace similar to that used for burning fuel oils

Removes heat from sulfur burner gas →steam

Cooling gases

between passing through catalyst –why?

“Heart” of the contact process. Discuss in terms of the equilibrium reaction, rate of reaction, rate of conversion and increased SO3 formation?

� Absorption of SO3 into H2SO4

� RecirculatedH2SO4 is cooled

What is a furnace?

www.petrocoke.com/pics1.html

www.oup.co.uk/.../pictures/industry/furnace/

http://en.wikipedia.org/wiki/File:Furnace2-en.svg

Schematic diagram of an industrial process furnace

� air blower directs air towards burner that is powered by fuel

� heat transferred to tubes in radiant section → flow of liquid through the tubes is heated to desired temperature

� combustion gases vented via the stack but their heat is recovered in the convection section first

http://www.freepatentsonl

ine.com/2622007.pdfMelt vessel where the sulfur is melted

Combustion chamber where the

molten sulfur is partially burned and vaporized

Secondary combustion chamber

where vaporized sulfur is burned –650 – 900 oC

Steam heating

coil

O2 gas introduced to molten sulfur

What is a heat exchanger?

�A device that allows for the exchange of heat from a fluid on one side of a barrier to fluid on the other side of the barrier without the two liquids coming into contact with each other.

�Give examples of heat exchangers. http://www.thefreedictionary.com/_/vie

wer.aspx?path=hm&name=A4heatex

� Rate of heat transfer is dependent on:

- temperature difference between hot and cold fluids/surfaces

- surface area available for heat transfer

� the geometry and physical properties of a system will also influence the heat transfer system, for eg., if fluids are involved then the flowrate is important

� heat exchange or transfer is used in industry to

- heat or cool reactions- keep raw materials at an optimal

temperature (eg., keep solid fats in a liquid state)

- conserve energy (eg., re-use of steam condensate)

� most commonly used between liquids but may get transfer between gas and liquid or between 2 gases.

Jacketed vessel External coil

� lower surface area → smaller contact area

� not generally used for temperature control in reactors

� used in low duty heating requirements, for eg., maintaining the temperature in raw material tanks or heating pipe lines

Types of Heat Transfer Equipment

Internal helical coil Internal baffle type coil

� Used in reactors even if contains viscous material.

� High surface area for heat transfer

� Problem? Makes cleaning of reactor difficult and interferes with mixers in reactors

solution is pumped to outside the vessel where heat transfer occurs

External Heat Exchangers

external heat exchanger

inner tube

outer annulus

Double pipe heat exchanger

Hot and cold liquids move in a counter current direction

Most efficient operation is achieved when inlet and outlet temperatures are as close together as possible

� larger surface area → many tubes inside a shell or cavity

Single pass tubular heat exchanger

divides the header from the rest of the apparatus & prevents the fluid from entering the shell

directs the flow of liquid counter current

www.absoluteastronomy.com/topics/Shell_and_tu...

Provide labels for this diagram

Plate heat exchanger

� is a series of metal plates

� 2 fluids pass through the metal plates and are in contact with each other through the plates but do not mix

� plates are sealed with high temperature rubber gaskets for easy cleaning or repair

� usually used where there is a lot of fouling and the system needs a lot of cleaning

Removes heat from sulfur burner gas →steam

Furnace similar to that used for burning fuel oils

“Heart” of the contact process. Discuss in terms of the equilibrium reaction, rate of reaction, rate of conversion and increased SO3 formation?

Remove CO2, N2, moisture & other

impurities

Cooling gases

between passing through catalyst –why?

� Absorption of SO3 into H2SO4

� RecirculatedH2SO4 is cooled

Is Continuous or Batch Processing Better?

� the aim is to produce the maximum amount of product

� the types of solutions dealt with on the plant determine which process is better

� costs of labour, amount of product, research and instrumentation also play a role in determining which process is used.

� Continuous process:

- runs continuously

- fairly automated → minimize labour

- have much less material in process therefore lower chance of ruining large quantities of feedstock

- more uniform operating conditions→requires good quality instrumentation for concise control

- suitable for processes that are liquids or gases. Why?

- entire plant cannot be shut down for maintenance → shut down small sections of the plant at a time to carry out repairs and maintenance

- Disadvantage – very expensive to build

� Batch process:

- Relatively smaller quantities of feedstock are processed in individual reactors→ produce smaller quantities of product

- Precise quantities for each batch can be weighed out accurately but temperature control can be a problem

- Easier to control the reaction in a small reactor

- Maintenance is easier

- Disadvantage – very labour intensive

� Contact process:

- Gaseous feedstock → produce liquid product

- Large demand for end product

What type of process does the Contact Process use?

Environmental Awareness

�What are the main pollutants in a sulfuric

acid process?

� Identify, on the process chart, where these pollutants would be present.

�Why are these regarded as pollutants?

�Describe in detail the methods that can be used to reduce these pollutants

Why are these regarded as pollutants?

- SO2 is a throat and nose irritant and causes breathing problems if high concentrations are inhaled.

- Damage to vegetation and animals

- SO3 combines with water vapour

in the atmosphere to form sulfuric acid mist.

�Use of higher stacks:

- Tall stacks helpto disperse the SO2 gas higher into the atmosphere away from ground life

- Climate conditions and topography of the surrounding must be considered during emission of gases and building of stacks

http://sine.ni.com/cms/images/casestudies/stacks_01.jpg

� Use of scrubbers:- Scrub with � water (very effective in

reducing 50 – 75 % of SO2)

� soda ash (produce sodium bisulfite as by-product)

� lime slurries � salt solutions (slightly

alkaline, heated conc salt soln)

http://www.thermalenergy.com.au/images/GasCleaning/LandfillGas.jpg

�Removal of acid mist- The smaller the mist particles the greater

the visibility which results in plumes from stacks

- Removal of acid mist is carried out by:� Packed bed separators

containing sand, coke or fibers of glass or metal which cause the particles to coalesce

� Wire mesh mist eliminators cause particles to coalesce when high velocity gas pass through them

http://upload.wikimedia.org/wikipedia/commons/thum

b/3/31/Packedtowerex.gif/250px-Packedtowerex.gif

� Porous ceramic filter tubes made up of aluminium are effective in collecting acid mist that passes through it from the top downwards

� Electrostatic precipitators

� Sonic agglomerators use sound waves to cause small particles to stick together to form a large particle which is then removed by cyclones

What is Sulfuric Acid Used for?

�~ 70 % used to make superphosphatefertilizers

� Processing of metal ores

�Manufacture of detergents

�Manufacture of paper

�Manufacture of rayon and other polymers

�Manufacture of paints and

pigments

�Electrolyte in heavy duty batteries

� Laboratory reagent

Economic Role of Sulfuric Acid

� sulfuric plays an important role in a countries industrial economy

WW1 (1917-1919)

Stock market crash (1929)

WW2

(1938)

Post war (1940-1965) – greatest economic growth

http://www.pafko.com/history/h_s_acid.html

• Most produced chemical in the US

• About 40 million tons produced per year (US - 1990)

• Cost of sulfuric acid

is affected by supply &

demand which is

affected by the

demand for the chemicals

it is used to make

http://www.purchasing.com/article/talkback/227813-

Sulfuric_acid_prices_continue_to_decline_as_demand_dries_up.php#52758

“We may judge with great accuracy the commercial prosperity of a country from the amount of sulfuric acid it consumes”

1840 Liebig