৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

Gypsum (CaSO₄·2H₂O): Contains sulfur in sulfate form (SO₄²⁻), not elemental sulfur. It’s used for plaster and cement but not a major source of sulfur.

Pyrite (FeS₂): This is also called fool’s gold and is the most important natural mineral source of elemental sulfur. When roasted in air, it produces SO₂, which is then converted to sulfuric acid or elemental sulfur.

Barite (BaSO₄): Also contains sulfur in sulfate form, used mainly as drilling mud and in paints, not for sulfur extraction.

Dolomite (CaMg(CO₃)₂): Contains no sulfur at all, just calcium, magnesium, and carbonate.

Frasch Process Principle:
It is used to extract elemental sulfur from underground deposits.
Superheated water (≈170°C) is injected into the deposit to melt the sulfur (melting point ≈115°C).
Compressed air is then pumped in to force the molten sulfur to the surface in liquid form.
This method works because sulfur is insoluble in water but can be melted and lifted using air pressure.
​
​ক) Electrolysis of sulfur compounds → Not used in sulfur extraction; electrolysis is more common for metals like Al or Na.

গ) Roasting of sulfide ores in presence of oxygen → This is used for converting sulfide ores to SO₂ for sulfuric acid, not for extracting pure sulfur directly.

ঘ) Dissolution of sulfur in organic solvents → Not practical on an industrial scale for sulfur mining.

Thiophenes:
These are heterocyclic sulfur compounds commonly found in crude oil and petroleum fractions.
They are very stable and are the main source of sulfur in petroleum refining.
During refining, sulfur in thiophenes must be removed to meet fuel standards (done by hydrodesulfurization).

Sulfates (SO₄²⁻):
Rare in crude oil; more common in minerals like gypsum and barite, not petroleum.

Sulfites (SO₃²⁻):
Not naturally present in crude oil; these are mostly industrial additives or intermediates.

Sulfur dioxide (SO₂):
This is not present in crude oil; it is a by-product of sulfur combustion, not a native petroleum component.

The Claus process is the main industrial method for recovering elemental sulfur from hydrogen sulfide (H₂S), which is abundant in natural gas and refinery gases.
​The process has two stages:
​
1. ​Thermal Stage (Partial Combustion):
 2H₂S+3O₂→2SO₂+2H₂O
​Here, some H₂S is oxidized to SO₂.
​
2. ​Catalytic Stage (Main Reaction):
2H₂S+SO₂→3S+2H₂O

This is the primary Claus reaction, producing elemental sulfur.

Salt Domes:
These are large underground formations of evaporite salts (mainly NaCl) created by ancient seas.
Elemental sulfur often accumulates in the cap rock of salt domes due to the reduction of sulfate minerals by bacteria.
Historically, salt domes in the U.S. Gulf Coast region were the primary source of sulfur for the Frasch process.

Coal seams:
Contain sulfur mainly as pyritic sulfur (FeS₂) and organic sulfur, not as elemental sulfur.

Igneous rocks:
May contain sulfide minerals, but not large elemental sulfur deposits.

Limestone beds:
Mostly calcium carbonate; can contain sulfates but not elemental sulfur in significant amounts.

Current global trend:
Today, most industrial sulfur comes from desulfurization of fossil fuels (petroleum refining and natural gas sweetening).
This is mainly due to strict environmental regulations that require ultra-low sulfur fuels.
The recovered sulfur is often converted into sulfuric acid for fertilizers, chemicals, and industrial applications.

ক) Native sulfur from volcanic deposits:
Historically important, but now a minor source due to environmental and economic factors.

খ) Pyrite roasting:
Was a major source in the past for sulfuric acid manufacture, but declined after fossil fuel desulfurization became widespread.

ঘ) Sulfur mining from salt domes:
The Frasch process used this method, but it’s now almost obsolete because by-product recovery is cheaper and cleaner.

When pyrite is heated in the presence of oxygen, the main reaction is:
 4FeS₂+11O₂⟶2Fe₂O₃+8SO₂

The primary gaseous product is sulfur dioxide (SO₂), which is later converted to SO₃ for sulfuric acid manufacture (Contact Process).
​
​SO₃: Produced later by catalytic oxidation of SO₂ in the Contact Process, not in the roasting step.

H₂S: Occurs in natural gas, not in pyrite roasting.

S₂: Elemental sulfur vapors can form in reducing conditions, but under roasting (oxidizing), SO₂ dominates.

In the Frasch process, three concentric pipes are used:
Superheated water (≈170 °C) is sent down the outer pipe to melt the sulfur (melting point ≈115 °C).
Compressed air is injected through the inner pipe to:
     Agitate and reduce the viscosity of molten sulfur (making it flow easily).
    Create an air-lift effect to push the molten sulfur to the surface.

This avoids the need for mechanical pumping at great depths.

ক) To oxidize sulfur → No oxidation happens here; sulfur remains elemental.

খ) To maintain pressure and prevent water loss → Pressure is secondary; the main reason is lifting and viscosity reduction.

ঘ) To cool the sulfur → Opposite of what we need; we want sulfur to stay molten, not cool.

Sulfur recovery processes aim to capture sulfur from fuels and ores to prevent its emission as SO₂ into the atmosphere.
SO₂, when released, reacts with water vapor and oxygen to form H₂SO₄ (sulfuric acid), leading to acid rain.
Acid rain damages ecosystems, corrodes infrastructure, and acidifies water bodies.


ক) Ozone layer depletion: Linked to chlorofluorocarbons (CFCs), not sulfur.

গ) Global warming: Primarily due to CO₂, CH₄, N₂O; SO₂ actually has a cooling effect but causes other issues.

ঘ) Soil salinization: Related to irrigation practices, not sulfur emissions.

Desulfovibrio:
These are sulfate-reducing bacteria.
They convert sulfates (SO₄²⁻) into hydrogen sulfide (H₂S) under anaerobic conditions, playing a key role in the biological sulfur cycle.
H₂S produced can later react with metals or oxidize to form elemental sulfur, contributing to natural sulfur recovery.

ক) Nitrosomonas → Involved in nitrification, converting NH₄⁺ → NO₂⁻; not sulfur.

খ) Rhizobium → Fixes nitrogen in legumes; unrelated to sulfur cycling.

গ) Azotobacter → Free-living nitrogen-fixing bacteria; does not participate in sulfur transformations.

Contact Process Overview:
Used to manufacture sulfuric acid (H₂SO₄) from SO₂.

​Key step: 
​Oxidation of SO₂ to SO₃:
2SO₂+O₂→2SO₃     (In presence of Catalyst)
​
​Why V₂O₅ (Vanadium(V) oxide)?
1. Cost-effective: Platinum (Pt) is much more expensive.
2. Durability: V₂O₅ is less easily poisoned by trace impurities in SO₂ gas (e.g., arsenic or dust).
3. Suitable activity: Works efficiently at ~450°C and moderate pressure.

Why not Pt?
Platinum is highly active but very expensive and susceptible to poisoning, making it impractical for industrial-scale sulfuric acid production.

Reaction in Contact Process:
 2SO₂(g)+O₂(g)⇌2SO3(g)        ΔH=−197 kJ/mol (exothermic)
​ 
​Key Points:
Exothermic reaction: According to Le Chatelier’s principle, lower temperatures favor higher equilibrium conversion to SO₃.
Reaction rate: At low temperatures, the kinetic rate is very slow.
Industrial compromise: Around 450°C is chosen because:
              The reaction is fast enough (good kinetics).
              The equilibrium conversion is still reasonably high
​
​ক) Higher temperature increases the yield → Wrong; higher temperature reduces equilibrium yield because the reaction is exothermic.
খ) Higher temperature favors oxidation → Oxidation is exothermic, so higher temperature actually disfavors product formation.
গ) At lower temperatures, SO₂ decomposes → SO₂ is stable; decomposition is not an issue.

SO₃ behavior with water:
1. ​When sulfur trioxide (SO₃) is directly added to water, it reacts very exothermically:
SO₃ + H₂O → H₂SO₄

2. This reaction releases a lot of heat, causing the formation of fine droplets or mist of concentrated sulfuric acid, which is highly corrosive and difficult to handle.
​
​Industrial solution:
Instead of direct absorption in water, SO₃ is absorbed in concentrated H₂SO₄ to form oleum (H₂S₂O₇):
SO₃ + H₂SO₄ → H₂S₂O₇

Oleum can then be diluted safely with water to produce concentrated sulfuric acid.

Pyrite roasting reaction:
                      4FeS₂+11O₂⟶2Fe₂O₃+8SO₂

​Role of excess air:
Ensures complete oxidation of pyrite (FeS₂) to iron(III) oxide (Fe₂O₃) and sulfur dioxide (SO₂).
Without sufficient oxygen, partial oxidation could occur, forming lower oxides or leaving unreacted FeS₂, which reduces efficiency.
​
​ক) To reduce the temperature of roasting → Air addition slightly affects temperature but that’s not the main reason.

খ) To avoid formation of Fe₂O₃ → Actually, Fe₂O₃ is the desired solid product.

গ) To ensure complete oxidation of SO₂ to SO₃ in the converter → SO₂ → SO₃ conversion happens later in the Contact Process, not during roasting.

Contact Process Key Steps:

SO₂ production: Roasting of pyrite (FeS₂) or burning sulfur.

Purification: Cooling and drying of SO₂ gas.

Catalytic oxidation:
2SO₂+O₂→2SO₃           (In presence of V₂O₅)
​
​Exothermic reaction over V₂O₅ catalyst at ~450°C.
This step is slow compared to other steps and thus rate-controlling.

Absorption: SO₃ is absorbed in concentrated H₂SO₄ to form oleum.

Jamuna Fertilizer Company Limited (JFCL), located in Tarakandi, Jamalpur, Bangladesh, is the country's largest producer of granular urea fertilizer. Established in 1991, JFCL has a designed capacity of producing 1,700 metric tons of urea per day, though production has been affected by factors such as gas supply issues and technical challenges. Despite these challenges, JFCL remains a significant player in the urea fertilizer sector in Bangladesh. 

TSP Complex Limited, located in Patenga, Chattogram, is the only fertilizer plant in Bangladesh dedicated to producing Triple Superphosphate (TSP). Established in 1974 under the Bangladesh Chemical Industries Corporation (BCIC), it operates two production units with a combined annual capacity of 240,000 metric tons . The facility produces TSP by reacting rock phosphate with phosphoric acid, resulting in a high-analysis phosphorus fertilizer essential for agriculture.

Fertilizer production in Bangladesh:
Most nitrogen-based fertilizers, especially urea, rely heavily on natural gas as a feedstock for ammonia synthesis.
Shortages or interruptions in gas supply directly reduce production capacity, causing fertilizer scarcity.

Impact on industry:
Plants like Jamuna Fertilizer Company Limited (JFCL) and Ghorashal Fertilizer Factory frequently face production halts due to limited gas availability.
This makes natural gas supply the key limiting factor for nitrogen fertilizer output in Bangladesh.

Government Policy:
The Bangladesh government subsidizes fertilizers, especially nitrogen-based fertilizers like urea, to make them affordable for farmers.
This ensures food security and supports agricultural productivity, as fertilizers are essential for high crop yields.

Mechanism of subsidy:
The government often covers the difference between production/import cost and the sale price to farmers.
This helps prevent price spikes due to fluctuations in natural gas prices or international fertilizer markets.

Ghorashal Polash Fertilizer PLC has a production capacity of  2800 MTPD (9,24,000 MTPY). This Factory is fully environment friendly. It will be the first urea fertilizer factory in Bangladesh where the environmental pollutant Carbon-Di-Oxide (CO2) will be captured from the primary reformer flue gas and the production of urea fertilizer will be increased (about 10%) by using the captured CO2. As a result, there is no carbon emission for preventing Climate Change.
Two Steam Turbine Generator having electricity production capacity of 32 MW each has been installed in this Factory.

Source: Ghorashal Polash Fertilizer PLC​

Haber-Bosch Catalyst: Fe with promoters (K₂O, Al₂O₃)

Iron (Fe): Main catalyst because it is cheap, durable, and effective.

Promoters (K₂O, Al₂O₃): Improve activity, stability, and lifespan of the catalyst.

Purpose: Helps speed up ammonia production by making nitrogen easier to react with hydrogen.

Why not others: Pt, Ni, or Cu-Zn are either too expensive, easily poisoned, or meant for other reactions.

Shift Converter (Water-Gas Shift Reaction):
In ammonia production, syngas from steam reforming contains CO and H₂.
CO is toxic to the ammonia synthesis catalyst (Fe), so it must be reduced.

The shift converter reacts CO with water vapor to form CO₂ and more H₂:
1. This increases hydrogen concentration, which is essential for ammonia synthesis.
2. CO₂ is later removed in the CO₂ scrubber before the synthesis loop.

Avoiding Biuret Formation in Urea Manufacturing

Problem:
Biuret  (NH₂CONHCONH₂) forms when urea molecules react with each other, especially under high temperatures or long residence times.
Biuret is harmful to plants, so its formation must be minimized.

Essential Conditions:

Moderate Temperature:
Typically around 130–170°C in the urea reactor.
Prevents excessive decomposition or condensation of urea.

Low Residence Time:
Urea should not stay too long in the reactor.
Reduces the chance of urea molecules reacting to form biuret.

Additional Notes:
Proper pressure control (typically 150–250 bar in high-pressure reactors) ensures efficient conversion of ammonia and CO₂ to urea.
Maintaining excess ammonia helps shift equilibrium toward urea formation, but alone it doesn’t prevent biuret.

Stepwise Reaction in Urea Synthesis:

Formation of Ammonium Carbamate:
NH₃ reacts with CO₂ under high pressure to form ammonium carbamate:
2NH₃+CO₂→NH₂COONH₄

​This is the primary intermediate in the industrial urea process.

Dehydration to Urea:
Ammonium carbamate dehydrates under heat to form urea (NH₂CONH₂) and water:
NH₂COONH₄→NH₂CONH₂+H₂O

Purpose of the acid in TSP production:
Phosphate rock primarily contains tricalcium phosphate (Ca₃(PO₄)₂), which is insoluble in water.
Plants cannot absorb it directly.
Sulfuric acid reacts with phosphate rock to produce monocalcium phosphate (Ca(H₂PO₄)₂), which is water-soluble and suitable as fertilizer.

Reaction and By-product:
 ​Ca_3​(PO₄​)_2​+2H₂​SO₄​→Ca(H₂​PO_4​)_2​+2CaSO_4​
​
​Ca(H₂PO₄)₂ → TSP (Triple Superphosphate)
CaSO₄ → gypsum (by-product)


খ) Nitric acid: Would produce nitrate fertilizers, not TSP.
গ) Phosphoric acid: Used in DAP (Diammonium Phosphate), not TSP.
ঘ) Hydrochloric acid: Not economical and produces soluble calcium chloride, not TSP.

Ammonia Plant Effluent:
During ammonia production (Haber-Bosch process), excess ammonia may dissolve in process water or get discharged in wastewater.
This results in high ammoniacal nitrogen (NH₃–N), which is toxic to aquatic life and can cause eutrophication if released untreated.

Environmental Impact:
Ammoniacal nitrogen in water increases pH, reduces dissolved oxygen, and harms aquatic organisms.
It is a major concern for regulatory compliance and often requires neutralization, stripping, or biological treatment before discharge.

Why ammonia is emitted:
During prilling or granulation, molten urea is cooled and formed into solid granules or prills.
Some unreacted ammonia or ammonia trapped in urea melt can volatilize into the atmosphere during this cooling and solidification step.

Environmental concern:
Ammonia emission contributes to air pollution, odor issues, and can lead to ammonium deposition in nearby soils and water bodies.
Plants often use scrubbers or condensers to recover ammonia and reduce emissions.
​
​ক) Carbamate decomposition section → Ammonia reacts with carbamate to form urea; emission is minimal.
খ) Ammonia refrigeration system → Closed-loop system; negligible atmospheric release.
ঘ) High-pressure synthesis loop → System is sealed; ammonia is under high pressure, so emission is very low.

TSP Production:
Triple Superphosphate (TSP) is made by reacting phosphate rock with sulfuric acid.
Phosphate rock contains impurities, including fluorides (CaF₂, MgF₂, etc.) and dust particles.

Pollutants Generated:

1. Fluoride compounds:
Released as HF or particulate fluorides during acid attack on rock.
Can cause air pollution, corrosion, and health hazards.

2. Dust:
Fine particulate matter is emitted from grinding, handling, and reaction vessels.
Contributes to local air quality problems.

Environmental Control:
TSP plants use scrubbers, electrostatic precipitators, and dust collectors to reduce emissions.

Ammonia production relies heavily on steam reforming of natural gas (CH₄) to produce hydrogen.
This reaction inherently produces CO₂ as a by-product:
          Even with efficient reforming, CO₂ is unavoidable because carbon in methane ends up partly as CO₂.

Why CO₂ Reduction is Important:
CO₂ is a greenhouse gas, contributing to global warming.
Ammonia plants are significant point sources of CO₂ in industrial regions.

Effective Process Change:
Integrating Carbon Capture Systems:
      Captures CO₂ from reformer flue gas or shift converter gas before it is emitted.
     Techniques include amine scrubbing, physical absorption, or membrane separation.
     Captured CO₂ can be stored underground (CCS) or used in other chemical processes (CCU).

Why Other Options Are Less Effective:

Heavier hydrocarbon feedstock → Produces more CO₂ per unit hydrogen; counterproductive.
Replacing primary reforming with partial oxidation → Still generates CO₂; not a significant reduction.
Increasing steam-to-carbon ratio → Helps avoid CO formation but increases energy demand and indirectly CO₂.

Fertilizer Plant Effluent:
Wastewater from plants producing nitrogen (urea, ammonium sulfate) and phosphate (TSP, DAP) fertilizers often contains high levels of nitrogen (NH₄⁺, NO₃⁻) and phosphorus (PO₄³⁻).

Primary Environmental Impact – Eutrophication:
When these nutrients enter rivers, lakes, or ponds, they stimulate excessive growth of algae and aquatic plants.

This leads to:
      1. Oxygen depletion in water (hypoxia)
      2. Death of fish and other aquatic organisms
     3. Disruption of aquatic ecosystems

Other Environmental Concerns:
High ammoniacal nitrogen can also be toxic, but the major ecological effect is eutrophication.

As of 2024, China is the world's largest producer of elemental sulfur, with an estimated production of 19 million metric tons, accounting for approximately 22% of global production.

Other major sulfur-producing countries include:

United States: Approximately 8.2 million metric tons.

Canada: Approximately 5 million metric tons.

Russia: Approximately 7.5 million metric tons. 

In petroleum refining, sulfur is mainly removed from fuels to meet environmental regulations. The Hydrodesulfurization (HDS) unit is specifically designed to:
1.Treat sulfur-containing hydrocarbons in crude oil fractions (like diesel and gasoline).
2.Convert organosulfur compounds into hydrogen sulfide (H₂S).
3.Recover elemental sulfur from H₂S in a Claus process, making it a major by-product of petroleum refining.

Other Options:
খ) Fluid catalytic cracking (FCC) – Primarily produces lighter hydrocarbons; it does not generate significant sulfur by-products.
গ) Alkylation unit – Produces high-octane gasoline components; minimal sulfur is involved.
ঘ) Isomerization unit – Mainly rearranges hydrocarbon structure; not a sulfur source.

In volcanic regions, sulfur is commonly found as elemental sulfur:
  1.It is often deposited around fumaroles, hot springs, and volcanic vents.
  2.Formed directly from the cooling and deposition of sulfur gases like sulfur dioxide (SO₂) and hydrogen sulfide (H₂S).

Other options:
ক) Sulfides mixed with halides – Found in some mineral deposits, but not typical in volcanic zones.
খ) Sulfate ores – Such as gypsum, more common in sedimentary environments.
ঘ) Sulfonate compounds – Synthetic or industrial, not naturally occurring.

The Claus process is the most widely used method to recover elemental sulfur from sour natural gas and refinery gas streams containing hydrogen sulfide (H₂S):
1.H₂S is first partially combusted to produce sulfur dioxide (SO₂).
2.SO₂ then reacts with the remaining H₂S over a catalyst to produce elemental sulfur:
    2H₂S+SO₂→3S+2H₂O
This process can recover up to 95–98% of sulfur from gas streams.

Other Options:
ক) Mond process – Used for purifying nickel, not sulfur.
খ) Contact process – Used for producing sulfuric acid, not elemental sulfur from gas.
ঘ) Frasch process – Used to extract native sulfur from underground deposits, not from gas streams.

The Frasch process is used to extract elemental sulfur from underground deposits. Its principle involves:
1.Injecting superheated water (around 160–170 °C) into the sulfur deposit to melt the sulfur.
2.Compressed air or hot water is then used to lift the molten sulfur to the surface through a pipe.

Other options:
ক) Pressure reduction at the surface – Not the main principle.
গ) Density difference between sulfur and brine – Helps in separation but is not the core principle.
ঘ) Solubility of sulfur in water – Sulfur is insoluble in water, so this does not apply.

1.Thiobacillus is a genus of chemolithotrophic bacteria that oxidizes sulfide compounds (like H₂S, FeS₂) into sulfate (SO₄²⁻).
2.This process occurs naturally in soils, sediments, and acidic mine drainage, contributing to biogeochemical sulfur cycling.

Other options:
খ) Nitrosomonas – Oxidizes ammonia to nitrite, involved in nitrogen cycle.
গ) Pseudomonas – Versatile heterotrophs, not specific to sulfide oxidation.
ঘ) Azotobacter – Nitrogen-fixing bacteria in soil, unrelated to sulfur oxidation.

In the Contact Process for producing sulfuric acid:
1.Sulfur dioxide (SO₂) is oxidized to sulfur trioxide (SO₃) using a vanadium(V) oxide (V₂O₅) catalyst.
2.The oxidation is exothermic, and higher temperatures favor reaction rate but reduce equilibrium conversion.
3.To maximize SO₂ conversion, the catalyst bed is arranged in multiple stages with inter-stage cooling:
       1. Each stage gradually converts SO₂ to SO₃.
       2. Cooling between stages shifts equilibrium towards more SO₃ formation.

Other options:
ক) To increase reaction temperature – Actually, the bed is cooled between stages to optimize equilibrium.
গ) To absorb SO₃ in each stage – Absorption happens in the absorption tower, not in the catalyst bed.
ঘ) To regenerate the catalyst continuously – Catalyst is not regenerated stage-wise; it is stable over long periods.

In the Contact Process, the oxidation of SO₂ to SO₃ is exothermic:
  2SO₂+O₂↔2SO₃(ΔH<0)
1.High temperatures increase reaction rate but reduce equilibrium conversion.
2.Low temperatures favor equilibrium but slow down the reaction.
3.The interpass heat exchanger between catalyst beds:
      1. Removes excess heat from the reaction.
      2. Maintains the temperature at an optimal range for high conversion in subsequent stages.

Other options:
ক) To cool SO₂ before catalytic oxidation – Cooling occurs, but the main goal is controlling reaction temperature, not simply cooling SO₂.
খ) To condense SO₃ into oleum – This is done in the absorption tower, not the heat exchanger.
গ) To recycle unreacted SO₂ – Recycling is done separately, not via interpass heat exchangers.

When sulfur trioxide (SO₃) is dissolved in concentrated sulfuric acid (H₂SO₄), it forms oleum (also called fuming sulfuric acid or pyrosulfuric acid):
 SO₃+H₂SO₄→H₂S₂O₇
Oleum is an important intermediate in the Contact Process for producing concentrated sulfuric acid.

Other options:
খ) Sulfurous acid (H₂SO₃) – Formed from SO₂ + H₂O, not SO₃.
গ) Thiosulfuric acid (H₂S₂O₃) – Rare and unstable, not formed here.
ঘ) Persulfuric acid (H₂SO₅) – Formed by special oxidation processes, not by dissolving SO₃ in H₂SO₄.

1.BCIC is the government organization responsible for managing and controlling most large-scale fertilizer and chemical plants in Bangladesh.
2.It oversees production, operation, and distribution of major fertilizers, including urea, ammonia, and phosphate-based fertilizers.

Other options:
ক) BPDB (Bangladesh Power Development Board) – Manages electricity generation and distribution, not fertilizers.
গ) Petrobangla – Handles oil, gas, and petroleum sectors, though it supplies raw gas to fertilizer plants.
ঘ) BADC (Bangladesh Agricultural Development Corporation) – Primarily handles fertilizer distribution and agricultural services, not plant management.

1.Ashuganj Fertilizer & Chemical Co. Ltd. is a major urea and ammonia fertilizer plant in Bangladesh.
2.It uses natural gas as the primary feedstock for ammonia synthesis, which is then converted into urea.

Other options:
ক) Lafarge Surma Plant – Cement production, not fertilizer.
খ) Karnafuli Paper Mills – Paper industry, not fertilizer.
গ) Chhatak Cement Factory – Cement production, not fertilizer.

The first urea fertilizer company established in what is now Bangladesh was the Natural Gas Fertilizer Factory (NGFF) in Fenchuganj, Sylhet, which began operations in December 1960. It was a groundbreaking facility for the region at the time, though it was later permanently closed upon the start-up of the Shahjalal Fertilizer Company Ltd. (SFCL) in the same location.

The Haber-Bosch process is used for ammonia synthesis.
In the high-pressure section (typically 150–250 bar) and high temperature (400–500 °C), nitrogen (N₂) reacts with hydrogen (H₂) over an iron-based catalyst:
N₂+3H₂↔2NH₃

This is a reversible, exothermic reaction, and high pressure favors ammonia formation according to Le Chatelier’s principle.

Other options:
ক) NH₃ + CO₂ → NH₂COONH₄ – Formation of ammonium carbamate in urea production, not Haber-Bosch.
গ) 2NH₃ + CO₂ → (NH₂)₂CO + H₂O – Urea synthesis reaction, occurs after ammonia is produced.
ঘ) 2H₂ + O₂ → 2H₂O – Combustion of hydrogen, unrelated to ammonia synthesis.

In the Haber-Bosch process for ammonia synthesis:
Pressure: 150–300 atm (15–30 MPa)
Temperature: ~400–500 °C
Catalyst: Iron-based (Fe with promoters like K₂O, Al₂O₃)
High pressure is needed because the reaction:
N₂+3H₂↔2NH₃  (Δn=−2)
decreases the number of gas molecules, so high pressure favors ammonia formation by Le Chatelier's principle.

Other options:
ক) 1–5 atm – Way too low for significant conversion.
খ) 20–30 atm – Used in some modern low-pressure processes, but not typical for conventional Haber-Bosch.
ঘ) 500–700 atm – Too high; early experiments used such pressures, but industry optimized it to 150–300 atm.

In the conventional urea manufacturing process (Bosch–Meiser process), compressed ammonia (NH₃) and carbon dioxide (CO₂) are fed into an autoclave (a high-pressure reactor).

Inside the autoclave:

1. Formation of ammonium carbamate occurs under high pressure (140–200 bar) and temperature (170–200 °C).
2NH₃+CO₂  →  NH₂COONH₄

2. Dehydration of ammonium carbamate gives urea.
NH₂COONH₄  →  (NH₂)CO+H2O

Thus, the autoclave is the main high-pressure equipment where the carbamate formation and its partial decomposition to urea take place.

Triple Superphosphate (TSP) is produced by reacting phosphate rock (Ca₃(PO₄)₂) with concentrated phosphoric acid (H₃PO₄):
Ca₃(PO₄)₂ + 4H₃PO₄ → 3Ca(H₂PO₄)₂
Single Superphosphate (SSP) is made by reacting phosphate rock with sulfuric acid, which is the main difference.

Other options:
ক) Nitric acid → used in nitrophosphate production.
ঘ) Hydrochloric acid → not used in large-scale phosphate fertilizer production.

1.TSP (Triple Superphosphate) contains about 44–48% P₂O₅, mostly in the form of monocalcium phosphate [Ca(H₂PO₄)₂], which is highly water-soluble.
2.SSP (Single Superphosphate) contains about 16–20% P₂O₅ and includes gypsum (CaSO₄·2H₂O), making it less concentrated.

Other options:
ক) Higher nitrogen content → Neither TSP nor SSP contains nitrogen.
গ) Presence of sulfur → SSP contains sulfur (from gypsum), not TSP.
ঘ) Lower cost per unit of P₂O₅ → TSP is actually more expensive than SSP per unit weight but cheaper per unit of P₂O₅.

1.SO₂ (Sulfur dioxide) is the primary gas emitted from sulfuric acid plants and, when released into the atmosphere, oxidizes to SO₃ and then reacts with water to form H₂SO₄ (sulfuric acid), which causes acid rain.
​
2.SO₃ also contributes but is usually absorbed in the plant to make sulfuric acid or oleum, so its atmospheric release is minimal.
​
3.NO₂ is related to nitric acid plants and vehicle emissions, not sulfuric acid plants.
​
4.CO₂ does not significantly cause acid rain (it causes mild natural acidity in rain but not industrial acid rain).

1.Wet scrubbers are commonly used in sulfuric acid plants to remove SO₂ and other acidic gases from flue gases by absorption in a liquid (often water or an alkaline solution).

2.Cyclone separators are for large particulate matter, not gases like SO₂.

3.Bag filters capture dust but are not effective for gas-phase pollutants.

4.Electrostatic precipitators (ESPs) also target particulates, not acidic gases.

During prilling of urea, molten urea is sprayed into a prilling tower to form small spherical prills.

If the retention time of molten urea is longer than necessary and temperature is kept higher than just above the melting point, then condensation of two urea molecules occurs, producing biuret.
Biuret is an undesirable impurity in urea fertilizer because it reduces its suitability for agricultural use (toxic to plants above ~1–2%).