৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

Mercury cell technology produces caustic soda (NaOH) and chlorine by forming a sodium–mercury amalgam. While it yields very high-purity NaOH, the major limitation is mercury pollution. Mercury is highly toxic and can contaminate water, soil, and air, creating severe environmental and health hazards. This makes its adoption increasingly restricted worldwide, including in Bangladesh.​ 
​
​Option খ (High chlorine production cost) is not the main limitation; mercury cells are economically viable in terms of product yield.
​Option গ (Lower NaOH purity) is incorrect because mercury cells actually produce the highest purity NaOH among all cell types. 
​Option ঘ (High electricity demand of diaphragm cells) is irrelevant to mercury cells; it pertains to a different technology.

Membrane cell technology is becoming the preferred choice in Bangladesh (and globally) because it offers higher energy efficiency and better current efficiency compared to diaphragm cells. Key points:
​
Membrane cells allow selective transport of Na⁺ ions while preventing Cl⁻ back-migration, which reduces energy loss.
They produce relatively pure NaOH (around 32–35%) without the need for further purification.
Operating costs are lower due to reduced electricity consumption, which is significant for energy-intensive chlor-alkali production.
​
​Other Options:
​
​ক) Ability to operate with impure brine – Membrane cells actually require highly purified brine, unlike diaphragm cells.
গ) Lower initial capital investment – Membrane cells have higher capital costs, though they save on operating costs.
ঘ) Capability of producing concentrated NaOH directly (50%) – Membrane cells produce moderately concentrated NaOH (~32–35%), not 50%.

In Bangladesh, the caustic soda–chlorine industry generates chlorine as a co-product. Efficient utilization of chlorine is essential to make the process economically viable. Major industrial sectors that consume chlorine and its derivatives include:

PVC (Polyvinyl Chloride) manufacturing – uses chlorine as a primary feedstock.
Pulp and paper industry – uses chlorine compounds for bleaching.
​
​Other Options:
​
​ক) Pharmaceutical and beverage industry – may use some chlorine derivatives (like disinfectants), but on a smaller scale.
খ) Fertilizer and cement – mainly linked with sodium hydroxide and other chemicals, not chlorine.
ঘ) Textile dyeing and sugar refining – use caustic soda more than chlorine.

In Bangladesh, the Environment Conservation Act, 1995 is the primary legislation that regulates:

Effluent discharge standards for industries, including chlor-alkali plants.
Environmental compliance, including air, water, and soil pollution control.
Issuance of environmental clearance and permits through the Department of Environment (DoE).

Other options:

ক) Factories Act, 1965 – mainly deals with worker safety and factory operations, not environmental standards.
খ) Industrial Policy, 2021 – provides broad guidance for industrial growth and incentives but does not directly regulate effluent discharge.
গ) Bangladesh Labor Code, 2006 – focuses on labor rights and workplace conditions, not environmental control.

Membrane cell chlor-alkali plants are highly sensitive to impurities in brine, especially divalent cations like calcium (Ca²⁺) and magnesium (Mg²⁺). These ions can:

Foul or block the membrane, reducing efficiency.
Cause scaling, leading to increased maintenance and potential shutdowns.
Reduce current efficiency and NaOH purity.

Other options:

ক) High pH of brine – Brine is naturally slightly alkaline; pH is not the critical factor for membrane operation.
গ) High dissolved oxygen content – Dissolved oxygen is less critical; the membrane is more sensitive to divalent ions.
ঘ) Low sodium concentration – Membrane cells actually require a sufficient Na⁺ concentration; low sodium would reduce production, but the critical limiting factor is hardness.

The chlor-alkali industry is highly energy-intensive, especially for electrolytic production of NaOH and chlorine. Key points:

Electricity is the major operating cost, often accounting for 40–60% of total production cost, depending on the technology (mercury, diaphragm, or membrane).
 Any fluctuation in electricity tariff directly affects the profitability of the plant.
 While power outages or process issues can occur, the primary sensitivity is economic, not necessarily technical.

Other options:

খ) Brine preparation consumes steam – Steam usage is much less significant than electricity.
গ) Power outages cause irreversible membrane damage – Membranes are robust; temporary shutdowns do not usually cause irreversible damage.
ঘ) Electricity determines chlorine liquefaction efficiency – Liquefaction is a minor part; the major cost driver is electrolysis itself.

Diaphragm cell chlor-alkali plants produce moderately concentrated NaOH (~12%), but unlike mercury or membrane cells, they also allow significant NaCl to remain in the effluent. This leads to:

High chloride content in wastewater, which can harm aquatic ecosystems if discharged untreated.
The need for proper effluent treatment to meet environmental regulations in Bangladesh.

Other options:

ক) Emission of mercury vapors – Relevant only to mercury cells.
গ) Production of dilute NaOH requiring evaporation – This is an operational/economic issue, not primarily an environmental challenge.
ঘ) Generation of brine sludge with heavy metals – Not typical for diaphragm cell processes; heavy metals are not a major concern here.

Mercury-cell chlor-alkali plants use mercury to form a sodium–mercury amalgam for NaOH production. Legacy environmental impacts arise mainly from:

Mercury spills, leaks, and improper disposal of mercury-containing waste.
Bioaccumulation of mercury in water bodies and soil, leading to long-term ecological and human health risks.

Other options:
খ) Vinyl chloride monomer in air emissions – Relevant to PVC production, not directly to mercury cells.
গ) Sulfate aerosols from chlorine drying units – Not a primary concern in mercury-cell processes.
ঘ) Dioxins in wastewater – Typically linked to incineration or bleaching, not mercury-cell chlor-alkali plants.

In membrane cell chlor-alkali technology, current density is the key factor determining:

Specific energy consumption: Higher current density increases ohmic losses and overpotentials, leading to higher electricity usage per ton of NaOH produced.

Indirect CO₂ emissions: Since most CO₂ emissions are associated with electricity generation, higher energy consumption directly increases the plant’s carbon footprint.

Other options:

ক) Brine pre-treatment efficiency – Important for membrane longevity and purity, but its effect on energy consumption is secondary.
খ) Temperature of the electrolyzer – Affects reaction kinetics slightly but is less significant than current density for energy use.
ঘ) Type of diaphragm separator used – Membrane cells do not use diaphragms; this is relevant only for diaphragm cells.

Even when mercury is not used, diaphragm cells have higher environmental burdens compared to membrane cells because:

They produce dilute NaOH (~10–12%), requiring large volumes of water and subsequent concentration steps.
The process generates chloride-rich spent brine, which can pollute water bodies if not properly treated.
This leads to higher water usage, effluent treatment needs, and potential environmental impact.

Other options:

ক) Greater electricity requirement due to lower voltage efficiency – Diaphragm cells are moderately energy-efficient, though slightly less than membrane cells, but the main environmental concern is water and effluent, not electricity.

খ) Higher concentration of residual chlorine in hydrogen stream – This is minor and easily managed with scrubbing.

গ) Larger carbon footprint due to anode coating materials – Not a significant contributor compared to water and brine issues.

The formation of chlorinated organic compounds (AOX, chlorinated hydrocarbons, etc.) in chlor-alkali effluent primarily occurs during electrolysis, specifically at the anode compartment, because:
1. Organic impurities present in the brine (even in trace amounts) react with chlorine generated at the anode.
2. This leads to formation of chlorinated by-products, which can end up in the effluent if not properly managed.

Other Options:

ক) Brine purification – Removes impurities, reducing formation of chlorinated organics rather than causing it.
খ) Chlorine drying and liquefaction – Mainly affects gaseous chlorine; organics in the effluent are minimally impacted.
ঘ) NaOH concentration by evaporation – Concentrates caustic soda but does not generate chlorinated organics.

Old mercury-cell chlor-alkali sites remain environmentally hazardous long after closure because:

1. Mercury released during operation binds with organic matter in soils and sediments.
2. These form stable organomercury compounds, which are highly toxic and bioaccumulative.
3. Such compounds can persist for decades, contaminating water bodies, soil, and food chains.

Other options:

ক) Persistent chloride scaling that traps toxic ions – Chloride scaling is mostly an operational issue, not a long-term contamination source.
খ) Continuous chlorine emission from soil desorption – Chlorine gas does not persist in soil over decades.
ঘ) Mercury volatilization at low temperatures – Mercury volatilization occurs at high temperatures; long-term risks are mainly due to bound organomercury, not ongoing volatilization.

Chlorine gas is heavier than air, which makes fugitive emissions particularly hazardous because:
    1. It accumulates in low-lying areas, such as pits, basements, or plant trenches.
    2. This poses serious inhalation risks to workers and nearby populations.
    3. Controlling and ventilating these emissions is challenging due to gravity-driven pooling.

Other options:
ক) High solubility of chlorine in brine making leaks harder to detect – Chlorine solubility is not the main issue; leaks are mostly gaseous.
খ) Low odor threshold of chlorine requiring strict monitoring – While true, this is a detection concern, not the main control challenge.
গ) Rapid photochemical reaction forming ozone in the atmosphere – Formation of ozone occurs outdoors but is not the primary challenge in plant fugitive emissions.

Under EU BAT (Best Available Techniques) for chlor-alkali plants:
Specific energy consumption is a critical key performance indicator (KPI) because:
    1. Chlor-alkali electrolysis is highly energy-intensive, and energy use directly impacts CO₂ emissions and overall environmental footprint.
    2. Optimizing energy efficiency reduces indirect environmental impacts from electricity generation.

Other options:
ক) Hydrogen purity percentage – Important for safety and downstream use, but not a primary environmental KPI.
গ) Chlorine liquefaction efficiency – Operational efficiency concern, less tied to environmental impact.
ঘ) Sodium hydroxide concentration in catholyte – Process parameter affecting production, but energy consumption is the main environmental performance metric.

An indirect environmental concern arises from activities outside the immediate chemical process but associated with its operation. In chlor-alkali plants:
1. Electrolysis consumes large amounts of electricity.
2. If electricity comes from fossil-fuel-based power plants, it results in CO₂ emissions, contributing to the plant’s indirect carbon footprint.

Other options represent direct environmental impacts:
ক) Discharge of chlorinated organics in wastewater – Direct emission from the process.
খ) Formation of dioxins during chlorine drying – Direct chemical by-product formation.
ঘ) Mercury vapor release from decomposer units – Direct pollutant release from mercury-cell operation.

Branching density in polyethylene is largely controlled by the polymerization mechanism:
Ziegler–Natta catalysis:
Produces linear polyethylene (HDPE/LLDPE) with very low branching.
Branching is controlled by the catalyst’s active site geometry and steric hindrance, which dictate how monomer inserts into the growing polymer chain.

High-pressure free radical polymerization:
Produces low-density polyethylene (LDPE) with high branching, due to random chain transfer and termination events.

Other options:
খ) Monomer purity and reactor temperature – Important for activity and molecular weight, but not the main determinant of branching.
গ) Polymerization pressure and initiator concentration – Critical in free-radical LDPE, but not for Ziegler–Natta linear polyethylene.
ঘ) Chain transfer agent concentration in the feed – Influences molecular weight more than branching density.

In HDPE production, slurry-phase polymerization is often preferred for certain applications because:
   1. Slurry reactors use a liquid hydrocarbon (e.g., hexane) as the medium, which dissipates heat effectively, minimizing hot spots in the reactor.
   2. They allow better control over particle size and morphology, which is critical for powder handling, extrusion, and molding processes.
   3. This control also helps achieve uniform polymer properties for applications like pipes, containers, and sheets.

Other options:
খ) Higher comonomer incorporation – Gas-phase reactors (like fluidized bed) are generally more flexible for comonomer incorporation in LLDPE.
গ) Minimizes hydrogen usage – Hydrogen is used for molecular weight control, but slurry vs. gas phase does not inherently minimize it.
ঘ) Higher melt flow index – Melt flow depends on molecular weight and comonomer content, not strictly the phase of polymerization.

Long-chain branching in LDPE introduces –CH₃ (methyl) groups along the polymer backbone, which are absent in linear HDPE.

​In FTIR spectroscopy:
1377 cm⁻¹ corresponds to –CH₃ bending vibrations.
An increase in intensity at 1377 cm⁻¹ indicates the presence of branches along the main chain, characteristic of LDPE.

Other options:
ক) 1462 cm⁻¹ (CH₂ bending) – Present in all polyethylene; does not distinguish branching.
খ) 720 cm⁻¹ (CH₂ rocking, crystalline) – More prominent in HDPE due to high crystallinity; not indicative of branching.
ঘ) Shift of carbonyl band (1715 → 1735 cm⁻¹) – Indicates oxidation or ester formation, not branching.

Isotactic polypropylene requires high stereoregularity, meaning all methyl groups are aligned on the same side of the polymer chain. Industrially, this is achieved using:
Ziegler–Natta catalysts:
   1. Typically TiCl₄ supported on MgCl₂ combined with an organoaluminum co-catalyst (like AlEt₃).
   2. These catalysts provide stereospecific active sites, producing isotactic polypropylene with high tacticity.

Other options:
ক) Free radical initiator with peroxides – Produces atactic polypropylene, highly irregular.
খ) Metallocene catalyst with aluminoxane – Can also produce isotactic polypropylene, but Ziegler–Natta is dominant in industrial-scale production due to cost-effectiveness.
গ) Chromium-based Phillips catalyst – Mainly used for HDPE production, not polypropylene

In gas-phase polypropylene polymerization, reactor sheeting occurs when molten or sticky polymer sticks to the reactor walls, causing fouling and operational issues. The main factor influencing this is:

​Temperature gradients inside the reactor:
   1. Hot spots or uneven temperature distribution can locally melt polymer particles, which then adhere to reactor walls.
   2. Proper temperature control minimizes sheeting and ensures smooth fluidization of polymer particles.

Other options:
খ) Partial pressure of propylene monomer – Affects polymerization rate but is less directly responsible for sheeting.
গ) Catalyst particle size distribution – Influences polymer particle size and fluidization, but temperature is more critical for stickiness.
ঘ) Hydrogen concentration – Controls molecular weight, not sheeting.

Impact copolymer polypropylene (ICP) is produced by incorporating ethylene–propylene rubber (EPR) particles into a polypropylene matrix. This modification:
1. Enhances toughness, especially at low temperatures, where homopolymer PP can become brittle.
2. The rubber phase absorbs impact energy, preventing crack propagation.

Other options:
ক) Improved transparency – Not the main feature; ICP may be slightly less transparent due to rubber inclusions.
খ) Higher melting point – ICP has a slightly lower or similar melting point, not higher, compared to homopolymer PP.
ঘ) Higher degree of crystallinity – ICP generally has lower overall crystallinity due to amorphous rubber domain.

In suspension polymerization of PVC, the monomer droplets are dispersed in water with the help of stabilizers (suspending agents). The particle size distribution is primarily controlled by:
 1. Stirring speed: Higher agitation breaks monomer droplets into smaller sizes, resulting in finer PVC particles.
 2. Stabilizer concentration: Adequate stabilizer prevents coalescence of droplets, maintaining uniform particle size.

Other options:
ক) Initiator solubility in monomer phase – Affects polymerization rate and molecular weight, not droplet size.
খ) Water-to-monomer ratio – Influences overall slurry volume but is secondary for particle size control.
গ) Reactor pressure – Minor effect; suspension polymerization is generally operated near atmospheric pressure.

PVC (polyvinyl chloride) is inherently more flame-retardant than polyethylene because of its chemical composition:
   1.Each PVC monomer contains a chlorine atom.
Upon heating, PVC releases HCl gas, which:
    1. Acts as a radical scavenger, inhibiting further combustion.
    2. Promotes char formation, creating a protective layer that slows down flame propagation.

Other options:
ক) High degree of crystallinity – Affects mechanical properties and thermal conductivity, but does not impart significant flame retardancy.
গ) Increased molecular weight – Can improve thermal stability slightly but is not the primary factor for flame resistance.
ঘ) Crosslinking during polymerization – PVC is usually thermoplastic, and crosslinking is not intrinsic; crosslinking is not the main reason for inherent flame retardancy.

Plasticized PVC requires flexibility, which is achieved by adding plasticizers like dioctyl phthalate (DOP):
    1.DOP intersperses between PVC chains, reducing intermolecular forces.
    2.This lowers the glass transition temperature (Tg), making PVC soft and flexible at room temperature.
    3.Its relatively low volatility ensures that plasticizer does not evaporate easily during processing or in end-use applications.

Other options:
খ) Chemically bonds with PVC chains – DOP is a physical plasticizer, not covalently bonded, so it can migrate over time.
গ) Enhances degree of polymerization – Plasticizers do not affect polymerization; they modify physical properties.
ঘ) Provides additional chlorine content – DOP does not contribute chlorine; flame retardancy is not its role.

Polymethyl acrylate (PMA) differs from polymethyl methacrylate (PMMA) in that:
    1.PMA has a lower glass transition temperature (Tg ~10 °C) compared to PMMA (Tg ~105 °C).
    2.This low Tg makes PMA softer and more flexible, which is desirable for coatings, films, and adhesives where some elasticity is needed.

Other options:
ক) Higher Tg and rigidity – This describes PMMA, not PMA.
গ) Higher refractive index and optical clarity – PMMA is superior in optical clarity, not PMA.
ঘ) Greater resistance to UV degradation – PMMA is more UV-resistant; PMA is more prone to degradation.

In free-radical polymerization of methyl acrylate (MA), the average molecular weight of polymethyl acrylate (PMA) is primarily influenced by:

Monomer-to-initiator ratio ([M]/[I]):
    1.Higher [M]/[I] → fewer radicals per monomer → longer chains → higher molecular weight.
    2.Lower [M]/[I] → more radicals per monomer → shorter chains → lower molecular weight.
Solvent polarity can affect radical stability and termination rates, indirectly influencing chain length and molecular weight.

Other options:
ক) Type of initiator and its decomposition rate – Important for polymerization rate, but molecular weight is more sensitive to [M]/[I].
খ) Presence of ester side groups – Influences polymer properties, but not the chain length in radical polymerization.
গ) Pressure and temperature – Temperature affects rate constants, but the dominant factor for molecular weight control is [M]/[I].

Polymethyl acrylate (PMA) has a nonpolar methyl ester side group, which limits its ability to form hydrogen bonds or strong polar interactions with many substrates.
1.The absence of hydroxyl (-OH) or carboxyl (-COOH) groups reduces surface adhesion, especially to polar surfaces like glass, metals, or treated plastics.
2.Other acrylic polymers with polar pendant groups (e.g., acrylic acid or hydroxyethyl acrylate) exhibit much better adhesion due to hydrogen bonding or polar interactions.

Other options:
খ) Excessive branching – PMA is generally linear; branching is not the primary adhesion issue.
গ) Low molecular polarity of the methyl group – Partly true, but the main factor is the absence of specific polar functional groups.
ঘ) High crystallinity – PMA is mostly amorphous; crystallinity does not limit adhesion here.

General-purpose polystyrene (GPPS) is valued for its optical clarity, rigidity, and low residual monomer content. Industrially, this is achieved using:
    1.Bulk polymerization: Styrene monomer is polymerized without solvents or surfactants, preventing haziness and contamination.
    2.Continuous stirred-tank reactors (CSTR) or batch reactors: Provide uniform heat transfer and controlled polymerization rates.
    3.Thermal initiators (e.g., benzoyl peroxide, AIBN) are used to initiate free-radical polymerization.

Other options:
ক) Bulk polymerization using suspension technique – Suspension is mainly used for PVC or polymers needing granular particles, not GPPS.
খ) Emulsion polymerization – Produces latex, not clear solid polystyrene; requires surfactants which can leave residues.
গ) Solution polymerization – Introduces solvents, which can reduce clarity and require removal.

In expanded polystyrene (EPS) production:
    1.n-Pentane or isopentane is added to polystyrene beads as a physical blowing agent.
    2.During pre-expansion, the volatile hydrocarbon vaporizes upon heating, creating gas-filled voids in the beads.
    3.This results in lightweight, porous EPS with good thermal insulation properties.

Other options:
ক) Improve thermal resistance and reduce polymerization time – Blowing agents do not directly influence polymerization kinetics.
গ) Increase molecular weight by chain transfer – n-Pentane is inert in polymerization; it does not act as a chain transfer agent.
ঘ) Reduce residual styrene monomer – Pentane does not remove styrene; degassing or vacuum treatments are used for that purpose.

Syndiotactic polystyrene (SPS) differs from general-purpose polystyrene (GPPS) primarily in its stereoregularity:
1. SPS has alternating placement of phenyl groups along the polymer backbone (syndiotactic configuration).
2. This regular arrangement allows the chains to pack efficiently, resulting in high crystallinity.
3. Higher crystallinity translates to:
      i. Increased heat resistance
      ii. Improved chemical resistance
      iii. Better dimensional stability compared to amorphous GPPS.

Other options:
ক) Cross-linked during polymerization – SPS is not inherently cross-linked; crystallinity is the key difference.
খ) Amorphous with higher molecular weight – SPS is semi-crystalline, unlike amorphous GPPS.
ঘ) Isotactic arrangement – Isotactic polystyrene exists but is less common; syndiotactic is alternating, not isotactic, and less brittle than GPPS due to crystallinity.

1. Membrane cell technology produces NaOH typically at 30–35% concentration because the membranes (usually made of perfluorinated polymers like Nafion) have limited current density tolerance. Running them at very high current densities or trying to push for >50% NaOH directly would damage the membrane, increase energy consumption, and cause ion leakage.
2. To obtain 50% or higher NaOH, an additional evaporation step is required, which is energy-intensive.

Other options: 
খ) High chloride content in brine – This affects membrane life and quality but doesn’t specifically cap NaOH concentration beyond 35%.
গ) Temperature sensitivity of the catholyte – Important for operational stability, but not the main limiting factor for concentration.
ঘ) Excess hydrogen evolution – Common to all electrolysis processes, but it’s managed by proper venting and doesn’t dictate NaOH concentration.

1.In the chlor-alkali process (membrane or diaphragm cells), the major products are chlorine gas (Cl₂) at the anode, hydrogen gas (H₂) at the cathode, and caustic soda (NaOH) in solution.
2.PVC (Polyvinyl Chloride) is made by polymerizing vinyl chloride monomer (VCM), which is produced from ethylene dichloride (EDC). EDC itself is synthesized from ethylene + chlorine gas.
C₂​H_4​+Cl₂ ​→ C₂​H₄​Cl₂​(EDC)
Then EDC is cracked to make VCM:
C_2​H₄​Cl_2​ → C₂​H_3​Cl(VCM) + HCl
Therefore, chlorine gas is the critical feedstock for PVC production.
 
Other options:
ক) Sodium hypochlorite → Used in bleaching/disinfection, not PVC.
খ) Hydrogen gas → Used in hydrogenation or as fuel, not for PVC.
ঘ) Hydrochloric acid → A by-product in VCM cracking, sometimes recycled, but not the primary chlor-alkali by-product used for PVC.

1. Membrane cell technology requires extremely high-purity brine (very low Ca²⁺, Mg²⁺, sulfate, and iron content), because impurities damage the ion-exchange membrane and reduce current efficiency.
2. In Bangladesh, raw salt is often solar salt from coastal regions. Its quality fluctuates seasonally due to:
Monsoon rains → higher moisture and contaminants.
Impurities like CaSO₄, MgCl₂, silts vary with season and harvesting method.

3. This makes consistent brine purification expensive and challenging, especially for membrane cells.
 
Other options:
খ) Lack of skilled operators → A concern, but not the main technical challenge for brine purity.
গ) High electricity consumption → Affects cost but not purity sourcing.
ঘ) Excess nitrogen in brine → Not a relevant issue for chlor-alkali brine.

1. Mercury cells (Castner-Kellner process) release mercury into the environment, leading to severe health hazards and environmental contamination.
2. Bangladesh adopted membrane cell technology to meet:
    Global environmental regulations (e.g., Minamata Convention).
    Local health and environmental safety standards.
3. Although membrane cells require high-purity brine and have a higher capital cost, they:
    Eliminate mercury discharge.
    Produce eco-friendly caustic soda.
    Reduce the risk of costly mercury remediation.
 
Other options:
ক) Membrane cells can tolerate high magnesium content → False; they are more sensitive to Mg²⁺ and Ca²⁺.
গ) Membrane cells operate at higher temperatures → Not a key reason for preference.
ঘ) Membrane cells generate more hydrogen per ton of NaOH → All chlor-alkali processes generate similar H₂ amounts per stoichiometry.

1.In membrane cell technology, the membrane selectively allows Na⁺ ions to pass from anode to cathode compartment, while blocking Cl⁻ and other impurities.
2.If brine pre-treatment is inefficient, impurities like Ca²⁺, Mg²⁺, Fe³⁺, and sulfate:
       Precipitate and foul the membrane.
       Cause chloride leakage into the caustic soda product.
       Reduce current efficiency and increase energy consumption.
3.Thus, brine purification is the single most critical factor determining NaOH purity.
 
Other options:
ক) Anode material → Affects chlorine evolution efficiency, not NaOH purity.
খ) Hydrogen removal rate → Important for safety and pressure balance, but not NaOH purity.
ঘ) Chlorine liquefaction temperature → Relevant for chlorine handling, not NaOH concentration or purity.

1. AOX (Adsorbable Organic Halides) refers to halogenated organic compounds that can adsorb on activated carbon.
2. In chlor-alkali plants, AOX mainly originates from:
Organic matter present as impurities in raw salt or brine (from seawater or solar salt).
When chlorine gas or hypochlorite reacts with these organics, it forms chlorinated organics like chloroform and chlorophenols.
3. These compounds end up in effluent streams, posing toxic and persistent environmental hazards.
 
Other options:
ক) Hydrogen evolution at cathode → Only produces H₂ gas, not AOX.
গ) Membrane degradation → Releases fluoride compounds, not significant AOX.
ঘ) Chlorine liquefaction process → Involves cooling/compression of chlorine, not AOX formation.

Regulatory Context (Bangladesh & International)
1. AOX limits in effluent are typically 0.5–1.0 mg/L in many jurisdictions.
2. Bangladesh DoE and global environmental standards (e.g., EU BAT for chlor-alkali) require AOX monitoring and reduction.

Control Measures
1. Enhanced brine purification (activated carbon treatment to remove organics).
2. Avoid excess hypochlorite formation in side streams.
3. End-of-pipe AOX treatment (activated carbon adsorption, advanced oxidation).

1. Environment Conservation Rules (ECR), 1997 under the Bangladesh Environment Conservation Act, 1995 provide:
    Industry categorization (Red, Orange, Green) – chlor-alkali plants fall under Red category due to hazardous chemicals (chlorine, caustic soda).
    Requirements for hazardous chemical storage, effluent treatment plants (ETPs), and environmental clearance.
    Mandates safety measures for storage and handling of chemicals like chlorine gas.
2. Chlor-alkali plants must obtain Environmental Clearance Certificates (ECC) and maintain hazardous material management plans as per ECR 1997.
 
Other options:
ক) Factories Act, 1965 → Deals with worker safety, but not specifically hazardous chemical storage regulations.
খ) Industrial Policy, 2021 → Policy for industrial development, not environmental compliance.
গ) Bangladesh Labor Code, 2006 → Addresses labor rights and workplace safety, but not detailed chemical storage norms.

1. Hydrogen gas is produced at the cathode in chlor-alkali plants along with NaOH.
2. It is highly flammable and forms explosive mixtures with air at concentrations of 4–75% by volume.
3. The main environmental and safety concern is fire and explosion hazard, especially in storage or venting systems if:
    Poor ventilation occurs.
    Leaks go undetected.
    There is an ignition source nearby.
 
Other options:
ক) Global warming potential → Hydrogen has no direct GWP (it does not trap heat like CO₂).
খ) Water contamination → Hydrogen does not dissolve in water to a harmful extent.
ঘ) Soil acidification → Not relevant; hydrogen does not acidify soil.

1.Membrane cell chlor-alkali plants are electrolytic processes, consuming large amounts of electricity (typically 2,500–3,500 kWh per ton of NaOH).
2.In Bangladesh, much of this electricity comes from fossil-fuel-based grids, which results in indirect CO₂ emissions (from power generation).
3.By switching to renewable electricity (solar, wind, or hydro), the plant significantly reduces its carbon footprint without altering the chemical process.
 
Other options:
ক) Operating at higher temperatures → Can slightly affect energy efficiency but has minimal impact on indirect CO₂ emissions.
গ) Increasing brine concentration → Helps with membrane efficiency, not CO₂ emissions.
ঘ) Using mercury-free electrodes → Environmental benefit is mainly mercury pollution reduction, not CO₂ emission reduction.

1. LLDPE (Linear Low-Density Polyethylene) is produced by copolymerizing ethylene with α-olefins (like butene, hexene) using Ziegler–Natta catalysts.
2. The active site stereochemistry of the catalyst determines:
    Comonomer incorporation rate
    Polymer chain distribution
    Molecular weight and branching
3. Proper stereochemistry allows controlled insertion of comonomer units, giving LLDPE its characteristic linear backbone with short branches.
 
Other options:
ক) Reaction pressure → Affects polymerization rate and molecular weight, but less influence on comonomer incorporation.
গ) Hydrogen concentration → Primarily used as a molecular weight regulator, not for controlling comonomer content.
ঘ) Solvent polarity → Minor effect in heterogeneous Ziegler–Natta systems; not a key factor.

1. HDPE (High-Density Polyethylene) is used for pipes because it has:
      High crystallinity → molecules pack closely, giving greater stiffness and strength.
      Low branching → results in denser material.
2. LDPE (Low-Density Polyethylene) has:
      High branching density → less crystalline, more flexible, lower tensile strength.
 
Other options:
ক) Lower tensile strength → False; HDPE has higher tensile strength than LDPE.
খ) Higher branching density → Characteristic of LDPE, not HDPE.
ঘ) Lower melt index → Melt index varies with processing; it’s not the primary distinguishing property for pipe applications.

1. Polypropylene (PP) can have different tacticities:
    Isotactic PP → all methyl groups on the same side of the polymer chain.
    Atactic PP → random placement of methyl groups.
2. Uniform stereoregularity in isotactic PP allows:
    Tight chain packing
    High crystallinity
    Greater stiffness, tensile strength, and thermal resistance
3. Atactic PP is amorphous, soft, and has poor mechanical properties.
 
Other options:
খ) Higher melt flow → Affects processability, not mechanical strength.
গ) Lower molecular weight → Reduces mechanical strength, opposite effect.
ঘ) Presence of comonomer units → Mostly relevant for copolymers; isotacticity is the key factor here.

1. In PVC suspension polymerization, stabilizers (suspending agents) such as polyvinyl alcohol (PVA) or cellulose derivatives are used to:
      Prevent coagulation of PVC particles
      Stabilize droplets during polymerization
2. Increasing stabilizer concentration:
      Increases the number of nucleation sites
      Leads to smaller PVC particles
      Produces a more uniform particle size distribution, which improves flowability and processing
 
Other options:
ক) Reduces polymer molecular weight → Molecular weight is mainly controlled by initiator concentration and reaction temperature, not stabilizer.
খ) Produces larger particles with broader distribution → Opposite effect of increasing stabilizer.
ঘ) Enhances chlorine content in polymer → Chlorine content depends on monomer composition, not stabilizer.

1. Polymethyl acrylate (PMA) has a lower glass transition temperature (Tg ~10°C) compared to polymethyl methacrylate (PMMA, Tg ~105°C).
2. Lower Tg means the polymer chains can move more easily at room temperature, giving:
    Better flexibility
    Improved adhesion to substrates
    Suitable for flexible coatings, adhesives, and elastomeric applications
3. PMMA, with its high Tg, is rigid and brittle, making it unsuitable for flexible coating applications.
 
Other options:
ক) Higher Tg for heat resistance → Opposite effect; higher Tg reduces flexibility.
গ) Higher UV resistance → PMMA is actually better for UV stability.
ঘ) Higher refractive index → Relevant for optical applications, not flexibility.

Bulk polymerization involves polymerizing pure styrene monomer without solvents or dispersants, which gives:
   High-purity polystyrene
   Minimal residual monomer (due to controlled reaction conditions)
   Excellent optical clarity (since no stabilizers or emulsifiers are used that could cause haziness)

Other methods:
খ) Emulsion polymerization → Uses surfactants; produces latex, requires coagulation; not optically clear.
গ) Suspension polymerization → Produces bead polystyrene; some haze due to stabilizers.
ঘ) Solution polymerization → Uses organic solvents; may leave residual solvent affecting clarity.

1.Long-chain branching (LCB) in polyethylene (especially in LDPE and LLDPE) affects polymer properties:
      LCB introduces entanglements between chains.
      This improves resistance to environmental stress cracking, which is crucial for films, containers, and pipes.
      It does not significantly reduce tensile strength; mechanical strength can remain high.
2.Other effects of LCB:
      Slightly reduces crystallinity rather than increasing it.
      Melt index can change, but the primary industrial effect is enhanced stress crack resistance.
 
Other options:
ক) Reduces tensile strength → Not the main effect; tensile strength may stay similar.
খ) Increases crystallinity → LCB tends to decrease crystallinity.
ঘ) Lowers melt index → Melt flow is affected but is secondary to stress cracking improvement.

1.Rigid PVC is naturally brittle, so to improve impact resistance, impact modifiers are added.
2.MBS (Methyl Methacrylate-Butadiene-Styrene) and similar elastomeric additives:
       Create rubbery domains in the PVC matrix
       Absorb and dissipate impact energy
       Enhance toughness without affecting rigidity significantly
 
Other options:
ক) Plasticizer → Increases flexibility, mainly used for flexible PVC, not rigid PVC impact improvement.
খ) Stabilizer → Protects PVC from heat and UV degradation, does not improve impact.
গ) Blowing agent → Creates foam, not used for improving impact resistance in solid PVC.

1. Polypropylene (PP) copolymers are made by copolymerizing propylene with ethylene.
2. Ethylene units act as soft segments in the PP matrix:
     Reduce brittleness at low temperatures
     Improve impact toughness
     Enhance flexibility and low-temperature performance 
3. As ethylene content increases: 
     Stiffness and crystallinity may slightly decrease
     Impact resistance, especially at low temperatures, increases
 
Other options:
ক) Decreases toughness → Opposite effect; ethylene improves toughness.
খ) Increases stiffness → Actually decreases stiffness slightly.
গ) Increases crystallinity → Ethylene disrupts PP crystallinity slightly.

1. EPS (Expanded Polystyrene) is produced by expanding polystyrene beads using a blowing agent.
2. Pentane (or other hydrocarbons) serves as a physical blowing agent:
    Evaporates when heated during pre-expansion
    Creates gas-filled cells, producing lightweight foam
    Determines foam density and insulation properties
 
Other options:
ক) Increase polymer molecular weight → Molecular weight is set during polymerization, not by blowing agent.
গ) Reduce residual styrene content → Achieved via post-expansion washing, not blowing agents.
ঘ) Improve thermal stability → Stabilizers, not blowing agents, serve this purpose.