৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

Heavy petroleum fractions have very high boiling points. If we tried to distill them at atmospheric pressure, the required temperatures would be extremely high.
​At such high temperatures, thermal cracking occurs, breaking large hydrocarbon molecules into smaller ones, which is undesirable in vacuum distillation.
​By reducing the pressure (creating a vacuum), the boiling points of the hydrocarbons decrease, allowing them to vaporize at lower temperatures.
​This prevents thermal decomposition while still achieving effective separation.
The other options are incorrect because:
ক) Reducing pressure actually lowers boiling points, not increases them.
গ) Solubility of gases is not the main concern in vacuum distillation.
ঘ) Reflux ratio is a design parameter and not directly related to vacuum operation.

Atmospheric distillation columns operate at or near atmospheric pressure, handling crude oil fractions that boil at relatively lower temperatures.

Vacuum distillation columns operate at reduced pressure (vacuum), allowing high-boiling fractions (like residual oils) to distill at lower temperatures, preventing thermal cracking.

The main distinction between these two types of columns is therefore the operating pressure and temperature ranges, not the packing, tray number, or steam use.

Other options:
ক) Both types can use trays or packing; it’s not the main differentiator.
খ) Tray number varies based on design and desired separation, not strictly lower in vacuum columns.
ঘ) Steam injection may be used in either column for stripping or other purposes—it is not exclusive.

In vacuum distillation, the column operates at reduced pressure, which lowers the boiling points of hydrocarbon fractions.

As a result, the overhead vapors condense at a much lower temperature compared to atmospheric distillation, preventing thermal cracking of heavy fractions.

The other options are incorrect because:
ক) Density does not determine condenser temperature.
খ) Vacuum does not increase viscosity in a way that affects condensation temperature.
গ) Reflux ratio affects separation efficiency but not the absolute temperature of the condenser.

Heavy oil fractions contain large, high-boiling hydrocarbons. Even under vacuum, some thermal cracking occurs, which produces carbonaceous deposits (coke) on column trays, internals, and furnace tubes.

This coking can reduce heat transfer efficiency, block flow, and require periodic cleaning.

The other options are less relevant:
ক) Foaming is more common in light hydrocarbon or contaminated feeds, not in heavy vacuum fractions.
গ) Azeotrope formation is not an issue in simple hydrocarbon vacuum distillation.
ঘ) Pumping light fractions is straightforward; the main problem is handling heavy, viscous residues.

In vacuum distillation, steam injection is used to lower the partial pressure of hydrocarbons in the column.

According to Raoult’s law, reducing the partial pressure allows hydrocarbons to vaporize at lower temperatures, preventing thermal cracking of heavy fractions.

This also helps improve separation efficiency by stripping light components from heavier residues.

Other options are incorrect:
ক) Steam does not increase pressure; it effectively reduces hydrocarbon partial pressure.
খ) Steam lowers, not raises, boiling points.
ঘ) Condensation of heavy fractions occurs in the reboiler or bottom section naturally; steam is not for condensing.

Thermal cracking is a non-catalytic process where heavy hydrocarbons are broken into lighter products using high temperatures.

The product distribution—amounts of gasoline, diesel, or light gases—depends primarily on:
       1.Temperature: Higher temperatures favor production of lighter gases like ethylene and propylene.
       2.Residence time: Longer residence times increase the extent of cracking, producing more light products but also more coke.

The other options are incorrect because:

      ক) Catalyst type is irrelevant in thermal cracking (it’s critical in catalytic cracking).
     গ) Hydrogen partial pressure affects hydrocracking, not thermal cracking.
     ঘ) Feedstock sulfur content influences product quality but not the fundamental cracking reactions

Catalytic cracking is a refining process that uses a solid acid catalyst (typically zeolites) at moderate temperatures (~500°C) to break heavy hydrocarbon molecules into lighter products.

Its main goal is to maximize gasoline and light olefin production from heavy gas oils or residues.

Other options are incorrect:

         ক) While some aromatics are formed, the primary goal is gasoline production, not just aromatics.
         খ) Extremely high temperatures without a catalyst describe thermal cracking, not catalytic cracking.
         ঘ) Methane is not a significant product of catalytic cracking; ethylene is mainly produced in thermal cracking of lighter fractions.

Hydrocracking is a catalytic process that combines cracking and hydrogenation.

It operates at high pressures (typically 80–200 bar) and uses hydrogen to saturate olefins and aromatics, producing high-quality, stable middle distillates like diesel and kerosene.

Key differences from catalytic cracking:
         Catalytic cracking operates at moderate pressure and does not use hydrogen, so it produces more olefins and gasoline.
         Hydrocracking minimizes coke formation and sulfur content.

Other options are incorrect:

          ক) Hydrocracking does require a catalyst (usually bifunctional: acidic + hydrogenation sites).
          খ) Hydrocracking operates at high pressure and hydrogenation reduces aromatics.
         গ) Hydrocracking can process heavy gas oils and residues, not only naphtha.

Naphtha reforming is a catalytic process aimed at improving gasoline quality, not quantity.

It converts naphthenes to aromatics and straight-chain alkanes to branched alkanes, which increases the octane number of the reformate.

Hydrogen is also produced as a by-product, which is used in other refinery units.

Other options are incorrect:

          ক) Reforming is not cracking; it does not primarily break molecules to produce more gasoline.
          গ) Conversion of methane to ethylene occurs in steam cracking, not reforming.
          ঘ) Sulfur removal is done via hydrodesulfurization, not naphtha reforming.

Thermal cracking (or steam cracking) uses very high temperatures (700–900°C) to break large hydrocarbon molecules into smaller ones.

It is particularly effective at producing light olefins such as ethylene and propylene, which are key feedstocks for the petrochemical industry.

Other options are incorrect:

         খ) Catalytic cracking: Produces mainly gasoline and some light olefins, but not as much ethylene/propylene as thermal cracking.
         গ) Hydrocracking: Produces middle distillates and saturated hydrocarbons; hydrogenation minimizes olefin content.
         ঘ) Reforming: Increases octane number by forming aromatics and branched alkanes, but does not produce significant light olefins.

Hydrocracking combines catalytic cracking and hydrogenation under high pressure.

It can process heavier feedstocks (gas oils, vacuum residues) and produce high-quality middle distillates like diesel and kerosene with low sulfur, aromatics, and olefins.

The hydrogenation step saturates olefins and aromatics, improving stability and product quality.

Other options are incorrect:

         ক) Hydrocracking actually produces fewer olefins due to hydrogenation.
         খ) Hydrogen consumption is higher in hydrocracking, not lower.
         ঘ) Hydrocracking operates under high pressure, not without pressure.

Naphtha reforming requires a bifunctional catalyst that can facilitate dehydrogenation, isomerization, and cyclization reactions.

Platinum on alumina (Pt/Al₂O₃) is widely used because it provides excellent activity for:
               1.  Dehydrogenation of naphthenes to aromatics
               2.Isomerization of paraffins to branched alkanes

Hydrogen is also produced as a valuable by-product, which can be used in hydroprocessing units.

Other options are incorrect:

           ক) Zeolite-based catalyst: Mainly used in fluid catalytic cracking (FCC).

           গ) Iron oxide: Used in some dehydrogenation or water-gas shift reactions, not reforming.

           ঘ) Nickel-molybdenum: Common in hydrodesulfurization (HDS), not naphtha reforming.

In catalytic cracking (FCC), coke forms on the catalyst surface as a by-product of hydrocarbon cracking.

Continuous regeneration of the catalyst by burning off coke with air restores its activity and prevents deactivation.

This is a core feature of fluid catalytic cracking units, allowing long-term operation.

Other options are incorrect:

            ক) Feed temperature is controlled, but coke formation is inevitable; regeneration is the main solution.
            খ) Increasing hydrogen pressure is relevant in hydrocracking, not FCC.
            ঘ) Vacuum distillation is a crude separation process and does not affect catalyst coke in FCC.

Hydrocracking uses hydrogen under high pressure along with a bifunctional catalyst to:
                       1.Crack heavy, aromatic-rich feedstocks.
                       2.Saturate olefins and aromatics.
                       3.Reduce sulfur and nitrogen compounds.
This makes hydrocracking ideal for sulfur- and aromatic-rich feeds, producing clean, high-quality middle distillates like diesel and kerosene.

Other options are incorrect:

           ক) Light naphtha can be produced in catalytic cracking; hydrocracking is typically for heavier fractions.

           খ) Hydrocracking produces fewer olefins because hydrogenation saturates double bonds.

          গ) Hydrocracking is more complex than catalytic cracking due to high pressure and hydrogen handling.

Catalytic reforming of naphtha primarily aims to increase octane number of gasoline.

The key reactions include:
       1.Dehydrogenation of naphthenes → aromatics (main reaction).
       2.Isomerization of paraffins to branched alkanes.
       3.Cyclization of paraffins to naphthenes.

Hydrogen is also produced as a by-product, which is used in hydroprocessing.

Other options are incorrect:
          খ) Cracking of heavy residues occurs in catalytic or thermal cracking, not reforming.
         গ) Hydrodesulfurization is a separate process for sulfur removal.
          ঘ) Steam reforming of methane is used for hydrogen production, not naphtha reforming.

Naphtha is the primary feedstock for the steam cracking process, which produces ethylene, propylene, and other light olefins.

These olefins are the building blocks for plastics (like polyethylene, polypropylene) and synthetic fibers (like polyester, nylon).

Other options are incorrect:
​
           ক) Kerosene: Used mainly as aviation fuel and heating fuel.
           গ) Fuel oil: Used for power generation and industrial heating.
           ঘ) Lubricating oil: Used for machinery lubrication, not as a petrochemical feedstock.

Kerosene is the primary fraction used for aviation fuel (Jet-A, Jet-A1, JP-8) because:
              1.It has a high energy density, providing efficient fuel for long flights.
              2.It has a low freezing point, ensuring reliable flow at high altitudes where temperatures are very low.

Other fractions are less suitable:

           ক) Diesel: Too viscous and has a higher freezing point.
           খ) Gasoline: Too volatile and low flash point for aviation use.
           ঘ) Residual fuel oil: Very heavy, used in marine boilers and industrial furnaces.

Lubricating oils are refined petroleum fractions designed to:
                1.Reduce friction between moving parts.
                2.Minimize wear and tear.
                3.Dissipate heat and prevent corrosion in engines and machinery.

Other options are incorrect:
            খ) Lubricating oil is not primarily a fuel for power generation.
            গ) Fuel for stoves is typically kerosene or LPG, not lubricating oil.
​            ঘ) Lubricating oil is not a feedstock for gasoline production; it serves as a functional lubricant.

Residual fuel oils are the heaviest fractions from petroleum refining, with high viscosity and high boiling points.

They are mainly used as fuel in marine engines, ships, and industrial boilers because they provide high energy content and are cost-effective for large-scale combustion.

Other options are incorrect:
           ক) Residual oils are not typically used as petrochemical feedstock.
           গ) Their viscosity and low volatility make them unsuitable for automobile engines.
           ঘ) Jet engines require lighter, low-freezing-point fuels like kerosene, not residual oils.

Gasoline blending additives are primarily used to:
              1.Increase octane number to prevent engine knocking.
              2.Improve combustion efficiency, ensuring smoother engine performance and lower emissions.

Other options are incorrect:

           ক) Viscosity and freezing point are not primary concerns for gasoline; they are more relevant for diesel or jet fuels.

           খ) Lubricity and flash point are secondary properties; flash point is controlled, but lubricity is less critical in gasoline.

           গ) Sulfur content and density are more relevant to regulatory compliance, not performance improvement via additives.

Naphtha is the primary feedstock for steam cracking, the process that produces light olefins such as ethylene and propylene, which are essential for the petrochemical industry.

Other fractions are less suitable:

          ক) Heavy fuel: Too heavy, mainly used for boilers and marine fuel.
          খ) Lubricating oil: Used for machinery lubrication, not cracking.
          ঘ) Diesel: Heavier fraction; mainly used for fuel, not for olefin production.

LPG (mainly propane and butane) is a light, easily liquefied hydrocarbon gas.

Its most common application is as a clean, convenient fuel for domestic cooking and heating.

Other options are incorrect:

          ক) LPG is not a raw material for kerosene production.
          গ) Jet engines use kerosene-based fuels, not LPG.
          ঘ) While LPG can be used industrially, it is not exclusive to industrial boilers.

Fluid Catalytic Cracking (FCC) units use zeolite-based catalysts because they provide:

         1.High activity for cracking heavy gas oils.
         2.High selectivity towards gasoline and light olefins.
         3.Good resistance to deactivation by coke.

Other catalysts are used in different refinery processes:

          খ) Nickel-molybdenum on alumina: Hydrodesulfurization.
          গ) Alumina-silica catalyst: Less active; used in some older FCC units.
          ঘ) Platinum on alumina: Used in naphtha reforming, not FCC.

Current Standard: As of 2025, Bangladesh's diesel fuel sulfur content is generally capped at 500 ppm (0.05%), aligning with Euro II specifications.

Historical Context: In 2016, Bangladesh ceased importing diesel with sulfur content exceeding 500 ppm, adopting Euro II standards. 

Recent Developments: To address supply challenges, the Bangladesh Petroleum Corporation (BPC) has been permitted to import diesel with sulfur content up to 500 ppm, blending it with lower-sulfur diesel to meet overall emission standards.

Alkylation is a refinery process that combines light olefins (like propylene and butylene) with isobutane to produce high-octane branched paraffins, which are blended into gasoline.

This increases the octane number of the gasoline pool, improving engine performance and preventing knocking.

Other options are incorrect:
           ক) Delayed coking: Converts heavy residues into lighter hydrocarbons and coke; not primarily for octane improvement.
           খ) Hydrodesulfurization: Removes sulfur from fuels; does not significantly increase octane.
           গ) Solvent deasphalting: Separates heavy asphaltic components from crude residues; unrelated to octane enhancement.

Vacuum residues are the heaviest fractions from vacuum distillation, with very high boiling points and viscosity.

The delayed coker thermally cracks these heavy residues into:
         1.Lighter, transportable hydrocarbons (like naphtha, diesel, and gas oils).
         2.Solid petroleum coke as a by-product.

Other Options:
          ক) Hydrocracker: Also upgrades heavy fractions but is more suited for producing middle distillates under hydrogenation.
​
          গ) Naphtha reformer and  ঘ) Catalytic reformer: Operate on lighter naphtha fractions to improve octane; cannot process heavy residues.

High TAN (Total Acid Number) crude contains organic acids that can corrode refinery equipment and cause operational problems.

The desalting unit is the first step in crude processing and is essential to:
        1.Remove salts, water, and acids from the crude.
        2.Prevent corrosion in heaters, distillation columns, and pipelines.

Other units are not designed for acid removal:

         ক) FCC: Cracks heavy fractions to gasoline and light products.

         খ) Hydrocracker: Upgrades heavy fractions under hydrogen pressure.

         ঘ) Alkylation: Produces high-octane gasoline components; unrelated to acid removal.

Hydrogen is a critical feedstock for hydroprocessing units such as hydrocrackers and hydrodesulfurization units.

In Bangladeshi refineries, hydrogen is primarily produced via steam methane reforming (SMR):
     
​           1.Methane (CH₄) reacts with steam at high temperature (700–900°C) over a catalyst.
           2.This produces hydrogen (H₂) and carbon monoxide (CO).
           3.CO is further converted to CO₂ and more H₂ via the water–gas shift reaction.

Other options are less relevant:
           ক) Electrolysis of water: Very energy-intensive and not commonly used in refineries.
           গ) Partial oxidation of crude: Not standard for hydrogen production in Bangladeshi refineries.
           ঘ) Coal gasification: Not used in Bangladesh’s petroleum refineries.

Refinery utility systems—like steam, electricity, and cooling water—are critical to run various units efficiently.

In Bangladesh, even if crude supply and storage are adequate, insufficient or limited utility capacity can restrict throughput and prevent expansion of refinery operations.

Other options are less limiting:

          ক) Lack of crude import infrastructure: Some ports exist, so this is manageable.
         খ) Excess storage for products: Not a bottleneck; more storage can be added if needed.
         গ) Surplus skilled workforce: Workforce is usually available; training is ongoing.

In a vacuum distillation column, the operating pressure is lowered to reduce the boiling points of heavy fractions, allowing their separation without reaching temperatures that cause thermal cracking.

The vapor–liquid equilibrium (VLE) of the heavy fractions determines:
           1.At what pressure the fractions will vaporize at safe temperatures.
           2.How efficiently the vacuum column can separate the heavy components without thermal degradation.

Other options are less critical:
           ক) Boiling point distribution at atmospheric pressure helps in initial column design but doesn’t directly dictate vacuum pressure.
           খ) Temperature difference is a design consideration but VLE is the controlling factor for safe operation.
           ঘ) Heat transfer coefficient affects column efficiency but not the maximum allowable pressure.