Finals Notes

ChE 410: Chemical Engineering Calculations 2

Lecture notes

OXIDATION OF SULFUR AND ITS COMPOUNDS

   







A. BURNING OF RAW SULFUR Raw sulfur is a combination of pure sulfur and INERT materials, which are unburned during combustion and separates into the cinder. Unburned elemental sulfur may be lost in the cinder resulting in incomplete gasification of the raw S charged. Gases from the burner consist of SO2, O2, N2, SO3 and water. Orsat analysis of the burner gas doesn’t include SO3, since it is soluble in water. Reactions: o Main Reaction: S + O2 SO2 3 o Side Reaction: S + ⁄2O2 SO3 Calculations of Theoretical Oxygen: o Theoretical O2 (S SO2) = total S atom o Theoretical O2 (S SO3) = total S atom * 3⁄2 o Theoretical oxygen is based on conversion to SO2 unless otherwise specified The determination of excess O2 and % excess O2 should identify whether the calculations are based on the conversion of sulfur to SO2 or SO3. o

% 𝐸𝑥𝑐𝑒𝑠𝑠 𝑂2 (𝑆 𝑡𝑜 𝑆𝑂2 ) =

𝑂2 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑓𝑟𝑜𝑚 𝑎𝑖𝑟−𝑡ℎ𝑒𝑜 𝑂2 (𝑆 𝑡𝑜 𝑆𝑂2 ) ∗ 𝑡ℎ𝑒𝑜 𝑂2 (𝑆 𝑡𝑜 𝑆𝑂2 )

100%

o

% 𝐸𝑥𝑐𝑒𝑠𝑠 𝑂2 (𝑆 𝑡𝑜 𝑆𝑂3 ) =

𝑂2 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑓𝑟𝑜𝑚 𝑎𝑖𝑟−𝑡ℎ𝑒𝑜 𝑂2 (𝑆 𝑡𝑜 𝑆𝑂3 ) ∗ 𝑡ℎ𝑒𝑜 𝑂2 (𝑆 𝑡𝑜 𝑆𝑂3 )

100%

Streams:

Burner Gas (ORSAT): SO2 COMPLETE: SO2 O2 O2 N2 N2 SO3 H2O

Raw Sulfur o Pure S o Inerts Air

  





Cinder: Inerts May contain unburned S

B. ROASTING OF IRON PYRITES Iron pyrites refer to the sulfide ore most commonly burned for SO2 manufacture. It consists primarily of Iron Sulfide (FeS2), small amounts of metallic sulfides and appreciable amounts of totally incombustible materials, called GANGUE. Reactions: o Main Reaction: 4FeS2 + 11 O2 8 SO2 + 2Fe2O3 o Side Reaction: 4FeS2 + 15 O2 8 SO3 + 2Fe2O3 Calculations of Theoretical Oxygen: o Theoretical O2 (FeS2 SO2) = total moles FeS2 * 11⁄4 o Theoretical O2 (FeS2 SO3) = total moles FeS2 * 15⁄4 During burning, the gangue and the iron oxide goes to the cinder. Unburned FeS2 may also be present in the cinder. 1

ChE 410: Chemical Engineering Calculations 2   

Lecture notes

Any formation of SO2 in the cinder is very small and may be neglected. SO3 ma be absorbed in the cinder by iron oxide, and unburned FeS2 Streams: Burner Gas (ORSAT): SO2 COMPLETE: SO2 O2 O2 N2 N2 Iron pyrites SO3 o FeS2 H2O o Gangue Cinder: Gangue Fe2O3 Air May contain unburned FeS2 and absorbed SO3

Problems on burning of raw sulfur and iron pyrites: 1. Calculations based on Raw Sulfur Analysis Raw sulfur analyzing 95% S and 5% inerts is burned with 65% excess air (S→SO2). Air is supplied at 30°C, 740 mmHg with 80% RH. The analysis of the cinder shows 10% S and 90% inerts. 90% of the S gasified burns to SO2, the rest to SO3. Calculate: a. % excess air (S → SO3) c. m3 air/ kg raw S b. Complete analysis of the burner gas 2. Calculations based on Burner Gas Analysis The burner gas from a sulfur burner analyzes 9.2% SO2, 7.13% O2 and 83.67% N2. The raw sulfur charged contains 82% pure sulfur and analysis of the cinder shows 20% unburned sulfur. Calculate: a. % excess air (S → SO2) c. m3 of saturated air (28°C, 750 mmHg)/kg raw S b. % excess air (S → SO3) d. m3 of burner gas (300°C, 730 mmHg)/ kg raw S 3. Calculations based on Burner Gas Analysis The gases from a sulfur burner have the following analysis: 9.86% SO2, 8.54% O2, 81.60% N2. After a passage of the gases through a catalytic converter, the analysis is 0.605% SO2, 4.50% O2, and 94.9% N2. What percentage of the SO2 entering the converter has been oxidized to SO3? 4. Calculations based on Pyrite Analysis Pyrites fines containing 85% FeS2 and 15% gangue are charged to a burner. An analysis of the cinder shows 11.11% FeS2, 66.63% Fe2O3, 2.66% SO3 and 19.6% gangue. Air is supplied 17.33% in excess (FeS2 → SO3) at 25°C, 740 mmHg and 80% RH. If 8% of the SO3 formed is absorbed in the cinder. Calculate: a. % excess air (FeS → SO2) c. Orsat analysis of the burner gas b. % of the FeS2 charged lost in the cinder d. m3 of the burner gas at 350°C and 750 mmHg / kg pyrite 5. The cinder from the combustion of iron pyrites containing 85% FeS2 and 15% gangue carries 1% S as FeS2. How many pounds of FeS2 are lost in the cinder per 100 lb of pyrites fired? 6. The cinder from the combustion of iron pyrites containing 85% FeS2 and 15% gangue carries 1% S as SO3. How many pounds of FeS2 are lost in the cinder per 100 lb of pyrites fired? 7. Dry pyrites fines containing 82% FeS2 and 18% gangue are burned in a Herreshoff burner. The cinder produced contains 3.06% SO3 and no unburned FeS2. Orsat analysis of the burner gas showed 8.16% SO2, 8.46% O2 and 83.38% N2. Calculate: a. % of the FeS2 charged converted to SO2 b. % excess air (FeS2 → SO2 c. complete analysis of the burner gas 2

ChE 410: Chemical Engineering Calculations 2

Lecture notes

8. In the burning of pyrite containing 92% FeS2 and 8% gangue, 12% of the FeS2 charged is lost in the cinder. A partial analysis of the cinder also shows 5.31% SO3. The Orsat analysis of the burner gases shows 6.75% SO2, 6.88% O2 and 86.38% N2. Air is supplied at 23C, 743 mmHg and 88% RH. Calculate: a. % excess air (FeS2 → SO2) c. m3 air/ kg pyrite b. % excess air (FeS2 → SO2 d. m3 of burner gas (250C, 750 mmHg)





C. PRODUCTION OF SULFURIC ACID AND OLEUM Sulfuric acid (oil of vitriol) is one of the most important of all manufactured chemicals. Not only is it one of the most common reagents in the laboratory, but enormous quantities of it are used in many of the industries, especially in the refining of petroleum, the manufacture of nitroglycerin, sodium carbonate, and fertilizers. Sulfuric acid has been historically produced by two methods: the Contract Process and the Chamber Process. Both of these involve the formulation of SO2 in a burner using either raw sulfur or pyrite: followed by conversion of the SO2 to SO3; and adsorption of SO3 in water to give sulfuric acid and oleum. 1. CONTACT PROCESS: a. BURNER  Raw Sulfur, Iron pyrites (Sometimes referred to as Iron ore), or mixed pure sulfur and ore may be charged into the burner for oxidation.  Primary air as source of oxygen b. CONVERTER  The contact process involves the catalytic oxidation of SO2 to SO3 using vanadium pentoxide or platinum dispersed in asbestos or silica gel as catalyst under appropriate conditions in an equipment known as converter. The reaction is: SO2 + ½ O2 SO3  Two or more converters may be in series and extra air (called secondary air) may be supplied.  Factors to favor forward reaction: 1. Maintain temperature at 425C 2. Increase the concentration of SO2 and O2 3. Remove some SO3 by scrubbing c. GAS ABSORBERS The SO3 from the gases in the converters is absorbed by countercurrent passage of the gases (upward) and absorbing liquid (downward) in ceramic packed towers. Sulfuric acid is formed by the reaction: SO2 + H2O H2SO4  This absorption cannot be satisfactorily accomplished by water alone, because the vapor pressure of water is sufficiently high to cause the formation of an acid mist that hinders absorption. Thus it is customary to feed sulfuric acid solution.  98% acid has been found to be the most efficient absorbing agent to produce fuming sulfuric acid, otherwise known as oleum. Water is added to the product to give grades of lower concentrations.  To determine if further oxidation of SO2 to SO3 takes place in the absorber, the waste gases are compared with the converter gas. If O2 in the waste gas is less than O2 in the converter gas, oxidation took place

3

ChE 410: Chemical Engineering Calculations 2 



Lecture notes

For computational purposes, if the absorbing agent is dilute acid, it will be assumed that concentrated sulfuric acid is formed; if the absorbing agent is concentrated acid, it will be assumed that oleum is formed. A 20% oleum product means that it contains 20% SO3 and 80% H2SO4. CONTACT PROCESS Flow Diagram Waste Gas Absorbing Medium Absorber Gas Sulfuric Acid Solution (maybe dilute or a concentrated acid) GAS ABSORBER

Raw Sulfur, BURNER Pyrite Or Mixed Pure Sulfur & Ore

Burner Gas (SO2, SO3, O2, N2)

CATALYTIC CONVERTER

Converter Gas (SO2, SO3, O2, N2)

Concentrated Acid or Oleum

Cinder Primary Air

  

Secondary Air

2. CHAMBER PROCESS The method of manufacture exclusively employed until recent years, and still in very extensive use Much more complicated than the contact process. The conversion of water, sulfur dioxide, and oxygen into sulfuric acid is accomplished by the catalytic action of oxides of nitrogen. The reactions are brought about in large lead-lined chambers, into which oxides of nitrogen, sulfur dioxide, steam, and air are introduced in suitable proportions.

Problems on Sulfuric Acid Production: 1. Two hundred and fifty pounds per hour of 98% H2SO4 enters an absorption tower of a contact sulfuric acid plant. If 20% oleum is produced per hour, how many pounds of SO3 are absorbed? 2. Dry pyrites is burned with dry air in a plant for the manufacture of sulfuric acid by the contact process. The cinder contains 2% w S (present as SO3). Seventy thousand cubic feet of burner gas (measured at 730 mmHg and 200⁰F) is produced per hour. An analysis of the burner gas shows 20% SO2 and 7% O2. The burner gas is passed through an absorber, where all the SO3 is removed, and is then conducted to a contact catalyst chamber. Fifty-nine and half pounds of 70% H2SO4 is used per hour for the absorption of the SO3 in the burner gas. Ninety percent H2SO4 is formed in the absorber. No SO3 is formed in the absorber. Calculate the percentage of FeS2 in the dry pyrites. 3. A sulfur burner, burning sulfur of 98% purity with dry air, discharged gas at 1300°F containing 16% SO2, 5% O2, and 79% N2. Both the sulfur and the air supplied are at 80°F. The heat of combustion of sulfur to SO2 is 127690 4

ChE 410: Chemical Engineering Calculations 2

Lecture notes

BTU/lbmole at 25°C. The burner gas is mixed with more dry air (secondary air) and passed through a converter. Analysis of the converter gas shows 4.2% SO2, 7.5% O2 and 88.3% N2. An absorber subsequently removes 95% of the SO3 in the converter gas by absorbing it in 97% H2SO4 to produce 100% H2SO4 solution. No SO3 is formed in the absorber. a. Calculate the volume of secondary air supplied at 80°F and 735 mmHg per 100 lbs impure sulfur entering. b. How many pounds of 97% H2SO4 solution must be supplied per mole of SO3-free converter gas?

   

 

D. BISULFITE LIQUOR PRODUCTION In bisulfite liquir production, the gases from the sulfur burner are passed through a cooler, an entrainment separator and then into an absorption tower. In the tower, it is made to come into contact with milk of lime or slaked lime (lime mixed with water), to form the bisulfite. Lime is a mixture of CaO, MgO and inerts. The following reactions take place: 1. SLAKER: CaO + H20 Ca(OH)2 MgO + H2O Mg(OH)2 2. ABSORPTION TOWER: a. Main Reactions: Ca(OH)2 + 2SO2 Ca(HSO3)2 Mg(OH)2 + 2SO2 Mg(HSO3)2 b. Side Reactions: Ca(OH)2 + SO3 CaSO4 + H2O Mg(OH)2 + SO3 MgSO4 + H2O H2O + SO3 H2SO4 SO2 + ½ O2 SO3 Bisulfite liquor is composed of the mixtures of Ca and Mg bisulfite, Ca and Mg sulfates, H2SO4, inerts and water. Analysis of bisulfite liquor is usually reported in terms of %SO2, both “free” and present as bisulfites.

Problems on Bisulfite Liquor Production: 1. The burning of raw S consisting of 95%S and 5% inerts produces a gas whose orsat analysis shows 11.40% SO2, 7.76% O2, and 80.84% N2. Ten percent of the total sulfur charged is lost in the cinder, The burner gases are cooled and absorbed in milk of lime obtained by slaking lime consisting of 58% CaO, 32% MgO and 10% inerts with water. The bisulfite liquor formed contains 12% SO2, of which 2% is free and the rest present as bisulfites. Orsat analysis of the waste gas shows that it contains 7.39% O2 and 92.61% N2. Calculate: a. kg bisulfite liquor/ kg raw sulfur c. kg of water used for slaking/ kg raw sulfur b. kg lime consumed/ kg raw sulfur 2. The roasting of pyrites analyzing 85% FeS2 and 15% gangue utilizes 40% excess air (FeS2 to SO2) supplied at the rate of 358m3/hr at 23 degC, 743 mmHg and 88% RH. A partial analysis of the cinder showed 25.92% FeS2 and 17.83% gangue. Only 65% of the FeS2 gasified is converted to So2, and the rest to SO3. The burner gases are cooled and charged to a converter together with slaked lime containing 75% CaO, 25% MgO. If 850 kg/hr of bisulfite liquor are produced with no oxidation of SO2 to SO3 taking place. Assume all SO2 converted to bisulfite. Calculate: a. kg/hr of lime b. kg/hr of water for slaking c. complete analysis of the burner gas 5

ChE 410: Chemical Engineering Calculations 2

Lecture notes

PRODUCTION OF LIME   

     

Lime can be formed from the calcination of limestone, which is a mixture of CaCO3, MgCO3 and inerts. Calcination takes place in a kiln where heat used for calcining comes from the combustion of a fuel. Reactions: CaCO3 CaO + CO2 MgCO3 MgO + CO2 The CaO, MgO and inerts combine to form the lime. In some cases, calcination may not be complete so the underburned lime may contain small amounts of CaCO3 and MgCO3 which may be reported in terms of its CO2 content. The kiln gases formed after combustion and calcination contain the products of combustion, CO2 from the calcination of lime, and possible water vapor from air and if wet limestone is used. CO2 and H20 from calcination MUST BE SEPARATED from the rest of the products of combustion, when determining the fuel consumption and lime production. Fuel ratio = amount of lime formed per amount of fuel Calculation analysis depend on whether the fuel contains negligible or considerable N2. o If the N2 in the fuel is negligible, the fuel is related with the kiln gas using theo O2 as the tie substance. o If N2 in the fuel is considerable, the tie substance is the sum of the N2 from the fuel and N2 for theoretical O2.  N2 form fuel + N2 for theo O2 = N2 from fuel + [ theo O2 * (79/21)]

Problems on Lime Production: 1. (Negligible N2 in the fuel). The burning of limestone containing 65% CaCO3, 25% MgCO3 and 10% inerts, using gas mixture made up of 75% ethane ans 25% propane produces a burner gas containing 22.07% CO2, 0.9% CO, 3.02% O2 and 74% N2. Calculate: a. fuel ratio by weight b. % excess air 2. (Considerable N2 in the fuel). A calcination plant manufacturing 10 tonnes lime/day consisting of 83% CaO, 5% CaCO3 and 12% inerts. The fuel used is coal gas analyzing 5.9% CO, 53.2% H2, 29.6% CH4, 4.1% CO2, 0.7%O2 and 6.5%N2 entering at 25degC, 740 mm Hg with 80% RH. Orsat analysis of the kiln gas shows 10.63% CO2, 0.66% CO, 0.66% H2, 6.75% O2 and 81.3% N2. Calculate: a. kg of limestone charged/day b. m3 of coal gas/day c. % excess O2. 3. A plant is burning limestone which analyzes 52% CaO, 41% CO2 and 7% inerts using coal that analyzes 90%C and 10% ash. The lime product obtained contains 3% CO2 and 1% unburned C. The top gas shows 22% CO2, 1% CO and 87% N2. Calculate: a. Kg limestone.kg coal b. b. Fuel ratio

6