৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

Industrial separation processes rely on thermodynamic disequilibrium. This means that components in a mixture have different chemical potentials (μ) in different phases (e.g., vapor vs liquid). The system moves toward equilibrium by transferring species from the high phase to the low  phase.

Chemical potential (μ) is the partial molar Gibbs free energy of a component.

Separation occurs because we create conditions (temperature, pressure, composition) that favor the selective transfer of components between phases.

Why Option খ is correct:

Whether it is distillation, absorption, extraction, or membrane separation, the fundamental driving force is the gradient in chemical potential (or equivalently fugacity or activity) between phases.

Why others are wrong:

Option ক (Gradient in density): Only important in mechanical separations (like sedimentation or centrifugation), not in thermodynamic phase equilibrium-driven processes.

Option গ (Variation in molecular diffusivity): Diffusivity affects the rate of mass transfer, but not the driving force.

Option ঘ (Difference in optical properties): This is unrelated to separation in industrial processes; color or refractive index may differ, but they are not driving forces.

Azeotropic distillation is a specialized form of distillation, used when components form azeotropes (mixtures that boil at a constant composition). It still depends on boiling point differences (volatility), though with added complexity of entrainers or pressure change.

Pressure swing adsorption (খ): Works on selective adsorption capacity at different pressures, not boiling points.

Crystallization (গ): Relies on solubility differences, not volatility.

Ultrafiltration (ঘ): A membrane process based on size exclusion and sometimes charge, not thermal properties.

Liquid–liquid extraction relies on partitioning of the solute between two immiscible phases according to Nernst's distribution law.

**The distribution coefficient  = (Concentration of solute in extract phase) / (Concentration in raffinate phase).

**A high distribution coefficient means the solute strongly prefers the solvent, making extraction more efficient.

Additionally, the solvent should be immiscible or have minimal miscibility with the raffinate to ensure phase separation.

Option খ (Exact density match): This actually makes separation harder because gravity-based settling becomes inefficient. A density difference is desirable.

Option গ (High viscosity): Increases mass transfer resistance, slowing extraction kinetics—undesirable.

Option ঘ (Dissolve all components equally): That destroys selectivity, making separation impossible.

Gas absorption in a packed column is a process where a gas mixture is brought into contact with a liquid to transfer one or more components from the gas phase to the liquid phase.

This occurs because of:
1. Solubility difference: The target gas component is more soluble in the liquid than in the gas phase.
2. Mass transfer: Driven by the concentration (or chemical potential) gradient between the gas and liquid phases.

ক (Centrifugal force): Applies in cyclone separators, not in absorption columns.
গ (Gravitational settling): Relevant for particle removal in settling chambers, not gas absorption.
ঘ (Adsorption on the solid surface): Happens in adsorption columns with solids like activated carbon, not in liquid absorption.

In RO, the solvent (e.g., water) naturally tends to move from low solute concentration (pure water) to high solute concentration (saline water) because of osmosis, driven by the osmotic pressure difference.
**To reverse this natural process, we apply a hydraulic pressure (P) greater than the osmotic pressure (π) of the feed solution.
**This pressure difference modifies the chemical potential (μ) of the solvent across the membrane, making the solvent flow from the saline side to the pure water side, overcoming osmotic equilibrium.

খ (Electrostatic interactions): Relevant in electrodialysis or ion exchange membranes, not in RO where the mechanism is size exclusion and pressure-driven.
গ (Gravity-induced hydrostatic head): Negligible compared to the pressure (typically 10–80 bar) applied in RO.
ঘ (Capillary action): RO membranes are non-porous or very dense at the molecular level (polyamide thin-film composites), so there’s no capillary mechanism like in paper or soil.

Centrifugation separates particles or phases based on density differences by applying a centrifugal field much stronger than gravity.
The centrifugal acceleration increases the effective gravitational force, causing denser particles to move outward while lighter components remain closer to the center.

ক (Magnetic attraction): Only applies in magnetic separation, not in centrifugation.
গ (Brownian motion): Affects very small particles but does not cause phase separation.
ঘ (Surface tension): Plays a role in emulsions and droplets but is not the main separation principle in centrifugation.

Steam distillation is most common for essential oils because it prevents thermal degradation by reducing the effective boiling point.

Option ক (Vacuum distillation): Good for heat-sensitive compounds in liquid mixtures, but not ideal for essential oils from solid matrices like plant material.

Option গ (Molecular distillation): Used for very high boiling substances like vitamins, not typically for essential oils.

Option ঘ (Supercritical CO₂ extraction): An alternative for essential oils, but it’s costly and specialized, not the standard industrial practice compared to steam distillation.

Adsorption vs Absorption:
**Adsorption = Solute accumulates on the surface of a solid or liquid (surface phenomenon).
**Absorption = Solute penetrates and distributes within the bulk phase of a liquid or solid (bulk phenomenon).

The key deciding factor is where the solute needs to be retained:
**If separation is desired at the surface → adsorption.
**If separation requires bulk incorporation → absorption.

ক (Surface area requirements): Important for adsorption efficiency, but not the main criterion for choosing between the two processes.
গ (Temperature of the mixture): Affects equilibrium and kinetics but does not fundamentally decide between adsorption or absorption.
ঘ (Availability of membranes): Membranes are not inherent to adsorption or absorption processes.

Relative volatility (α) is a measure of how easily two components can be separated by distillation. It is defined as:

α_AB=K_A/K_B

where
K=y/x (the equilibrium ratio, vapor phase mole fraction divided by liquid phase mole fraction).

Interpretation:
**Higher α means components are more easily separable by distillation.
**If α=1, components are difficult or impossible to separate by ordinary distillation (as in azeotropes).

ক (Ratio of densities): Density has no direct role in vapor-liquid equilibrium or relative volatility.
খ (Ratio of mole fractions in feed): That’s composition, not volatility relationship.
ঘ (Difference in boiling points): Boiling point difference correlates with volatility, but α is not defined by temperature difference alone.

α = 1.5 means that at equilibrium, A’s vapor-liquid ratio is 1.5 times that of B, so A is more volatile.

Azeotrope (option খ) is not implied by α alone.

Option গ is false because α > 1 means separation is feasible without a third component.

Option ঘ is wrong because vapor and liquid compositions differ unless α = 1

*** Higher relative volatility → easier distillation. 

Option গ: Correct because Raoult’s law applies to ideal solutions, where activity coefficients (γ) = 1.

Option ক: Positive deviation means Raoult’s law fails (due to weaker A–B interactions).
Option খ: Misleading because high pressures generally increase non-ideality, not decrease it.
Option ঘ: Azeotropes represent strong deviations from Raoult’s law.

Raoult's law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present.

The McCabe-Thiele method is a graphical design method for binary distillation. It uses:

** Operating lines (which depend on reflux ratio, feed condition, etc.)
** Equilibrium curve, which represents vapor-liquid equilibrium (VLE) data.

The slope of the equilibrium curve in an x-y diagram (mole fraction of more volatile component in liquid vs vapor) is determined by how the components distribute between vapor and liquid phases, which is defined by VLE relationships.

খ (Feed temperature): Affects feed line, not the equilibrium curve.
গ (Column pressure drop): Affects actual column design, but equilibrium curve assumes constant pressure.
ঘ (Reflux ratio): Affects operating lines, not the equilibrium curve.

A minimum-boiling azeotrope forms when two liquids exhibit positive deviation from Raoult’s law (they have weaker intermolecular interactions when mixed than in their pure states). As a result:
The mixture boils at a temperature lower than either of the pure components → hence called minimum-boiling azeotrope.
Example: Ethanol–Water azeotrope at 95.6% ethanol boils at 78.1°C, which is lower than water (100°C) and slightly lower than pure ethanol (78.37°C).

ক (Forms due to negative deviation): Wrong → Negative deviation leads to maximum-boiling azeotropes, not minimum.
গ (Separation becomes easier above azeotropic composition): Incorrect → No matter the composition, distillation cannot break the azeotrope without special techniques.
ঘ (Exists only at high pressure): Wrong → Azeotropes exist at various pressures, including atmospheric pressure (pressure affects azeotropic composition but not its existence).

Reflux ratio (R) = L/V, where L = liquid returned as reflux and V = vapor going up the column.

When reflux ratio increases:
More liquid returns to the column → improves contact between vapor and liquid, giving better mass transfer.
This enhances separation efficiency, meaning fewer actual trays are needed for the same separation.
BUT energy consumption increases, because more liquid is condensed and reboiled in the reboiler and condenser. So operating cost increases.

ক (Increases number of theoretical stages): Wrong → Higher reflux ratio actually reduces the number of theoretical stages required for a given separation.
খ (Decreases separation efficiency): Opposite → Efficiency increases.
ঘ (Has no effect on operation): Completely incorrect → Reflux ratio is one of the most critical design and operating parameters.

Vacuum distillation is a process of separating mixtures based on differences in their boiling points under reduced pressure. This helps in lowering the boiling points of components, making it possible to distill them at lower temperatures.

This process is particularly beneficial in contexts where high temperatures might lead to thermal decomposition of the desired components. 
Some key characteristics of vacuum distillation include:
1. Lower pressure facilitates the distillation process.
2. Reduces the boiling point of substances.
3. Used frequently in the petroleum and chemical industries.

By reducing the pressure inside the distillation apparatus, vacuum distillation helps in the efficient separation of components that are heat-sensitive.

The Fenske equation provides the minimum number of theoretical stages required for a given separation under total reflux conditions (i.e., no product withdrawal, maximum internal recycling).
It is derived based on relative volatility (α) and light and heavy key compositions in the top and bottom products

খ) Reboiler duty: Calculated from energy balance, not Fenske.
গ) Column diameter: Determined from vapor-liquid traffic and flooding limits, not Fenske.
ঘ) Column pressure drop: Related to hydraulics and tray/packing design, not Fenske.

Weeping occurs when vapor flow rate is too low to support the liquid on the tray, causing liquid to drain through the perforations instead of flowing across the tray. This reduces mass transfer efficiency, as the liquid bypasses proper contact with rising vapor.

ক) Excessive liquid hold-up on plates → Related to flooding, not weeping.

খ) Vapor flow carrying liquid droplets upwards → That is entrainment, not weeping.

ঘ) High pressure drop across the column → Happens in flooding, not weeping.

In a tray (plate) column, downcomer seals are used to ensure that vapor does not enter the downcomer and bypass the trays. Here’s why:

Role of Downcomer Seal:
1. It maintains a liquid seal at the bottom of the downcomer.
2. Ensures vapor flows only through tray perforations, where it contacts liquid on the tray for proper mass transfer.
3. Prevents vapor short-circuiting, which would drastically reduce separation efficiency.

ক) Prevent entrainment of liquid droplets into vapor → This is controlled by entrainment separators or proper vapor velocity, not the seal.
গ) Reduce pressure drop across trays → The seal does not significantly affect pressure drop; it mainly prevents vapor leakage.
ঘ) Improve vapor-liquid equilibrium on trays → This happens due to contact area on trays, not because of the seal.

The turndown ratio in distillation refers to the ability of the column to operate efficiently over a wide range of vapor flow rates.

**Valve Trays are best for high turndown ratio because:
1. The valves automatically adjust opening size based on vapor flow.
2. At low vapor flow, valves partially close, preventing weeping.
3. At high vapor flow, valves lift fully, allowing more vapor through without flooding.

ক) Bubble Cap Tray:
Has good turndown but high pressure drop and expensive.
Better than sieve trays at low load, but still less flexible than valve trays.

খ) Sieve Tray:
Has fixed holes → prone to weeping at low vapor rates.
Poor turndown ratio (typically ~2:1).

ঘ) Packed Bed:
Used for low-pressure drop operations.
Turndown is limited because at low flows, liquid maldistribution occurs

Flooding is a hydraulic limitation in distillation columns that occurs when the vapor flow rate (or vapor velocity) becomes excessive, causing the following:
1. High vapor upward flow creates a large pressure drop across the tray.
2. Liquid cannot flow down the downcomers properly because upward vapor force resists downward liquid movement.
3. This results in liquid accumulation on trays, reduced mass transfer efficiency, and eventually column instability.

ক) Vapor velocity is too low:
This causes weeping, not flooding (liquid leaks through tray perforations).

গ) Reflux ratio is zero:
No reflux means no separation, but it doesn’t cause flooding.

ঘ) There is no pressure drop:
Flooding is associated with excessive pressure drop, not zero pressure drop.

In a plate column, vapor flows upward through the tray openings (sieve holes, valve openings, or bubble caps), while liquid flows across the tray and down through downcomers.
If vapor velocity is too high, the vapor exerts excessive force on the liquid:
**Liquid droplets get carried upward → entrainment occurs.
**If severe, flooding happens → liquid cannot flow down properly through downcomers.
This condition reduces tray efficiency, causes pressure drop, and may even shut down separation.

ক) Pressure drop across downcomers:
Downcomer design is important but not the primary limiting factor for vapor velocity.

গ) Heat duty in the reboiler:
Heat duty affects boil-up rate, but maximum vapor velocity is set by mechanical & hydraulic limits, not energy input alone.

ঘ) Tray spacing limitations:
Tray spacing affects column height and liquid head but does not directly limit vapor velocity.

High pressure drop per tray means the vapor flow is facing significant resistance, usually because liquid is backing up on the trays and restricting vapor passage. This condition is most often associated with flooding, which occurs when vapor velocity is so high that liquid cannot flow down through downcomers, causing accumulation of liquid on multiple trays.

Flooding results in:
**Increased column pressure drop.
**Poor mass transfer (lower efficiency).
**Possible operational shutdown if severe.

ক) Weeping:
Happens at low vapor rates, where liquid leaks through perforations instead of flowing across the tray. Pressure drop is low, not high.

গ) High separation efficiency:
Not true, because flooding reduces efficiency drastically.

ঘ) Low vapor velocity:
Low vapor velocity → low pressure drop, not high.

Theoretical plates (or stages) represent an ideal situation where each plate achieves complete vapor-liquid equilibrium (VLE).
In reality, trays are not 100% efficient because:
**Liquid and vapor may not fully mix.
**Heat and mass transfer limitations exist.

Murphree efficiency (E_M) accounts for this deviation from ideal behavior.

Other Options:

খ) Multiplying by Murphree efficiency:
This would make the actual trays fewer than theoretical, which is impossible (real columns need more trays).

গ) Adding 1 or ঘ) Subtracting 1:
These adjustments are only for minor design considerations (like adding a reboiler or condenser), not to convert theoretical plates to actual.

The McCabe-Thiele method is a graphical approach to determine the number of theoretical stages in a distillation column.
For the rectifying section (above the feed plate), the operating line equation is:

y = xR/(R+1) + x_D/(R+1)

Where:
y = vapor composition (mole fraction of more volatile component)
x = liquid composition
R = reflux ratio
x_D = distillate composition

Slope of the rectifying operating line = m = L/V = R/(R+1)

In McCabe-Thiele analysis, the feed condition (subcooled liquid, saturated liquid, mixture, saturated vapor, or superheated vapor) determines the slope of the q-line.

ক) Equilibrium curve → Represents VLE, not feed condition.
খ) Operating line → Represents rectifying and stripping sections, not feed type.
গ) Stripping line → Represents liquid-vapor balance below the feed plate.

In McCabe–Thiele analysis,
q = fraction of feed that is liquid at its boiling point.
If q = 1, the entire feed is liquid at its boiling point → saturated liquid.

Other cases for q:
q > 1: Subcooled liquid (needs extra heat to reach boiling).
q = 0: Saturated vapor (all vapor at boiling).
q < 0: Superheated vapor (extra heat above boiling point).
0 < q < 1: Mixture of liquid and vapor.

In McCabe-Thiele analysis:
**The q-line represents the feed condition (saturated liquid, vapor, mixture, etc.).
**The rectifying line and stripping line represent the two column sections.

The point where q-line intersects the operating lines:
**Indicates the point of feed introduction in the column.
**From this intersection, we start stepping off stages upward and downward.

Other options:
Minimum stages (ক) → Determined by total reflux condition (not this intersection).
Column height (গ) → Related to the number of actual trays and tray spacing, not this intersection.
Reflux ratio (ঘ) → Determined from the rectifying line slope, not the q-line intersection.

The enthalpy-concentration method (also called the Ponchon-Savarit method) is an advanced graphical approach for distillation analysis.

It is used instead of McCabe-Thiele when energy (heat) effects cannot be ignored, such as:
1. Variable latent heat of components
2. Significant heat duties at condenser and reboiler
3. Enthalpy balances need to be included along with material balances

In this method:
**Both mass and energy balances are plotted on an H-x diagram (Enthalpy vs. composition), unlike McCabe-Thiele which assumes constant molar overflow.

In enthalpy-concentration (H-x or H-y) diagrams, we plot enthalpy (H) against composition (x or y) to account for both material and energy balances in a distillation process.
When heat effects are negligible and latent heats are constant, operating lines can appear nearly straight (like in McCabe-Thiele).
However, when heat effects are significant and latent heats vary, the enthalpy changes are nonlinear, making the operating lines curved.
This is why the Ponchon-Savarit method (enthalpy-concentration method) is used—it accurately handles curved operating lines due to energy effects.

The McCabe-Thiele method works well under these idealized conditions:

1. Constant molar overflow (liquid and vapor flow rates remain constant in each section).
2. Constant relative volatility (α does not vary significantly with composition or temperature).
3. Negligible heat effects (latent heat roughly constant, no major heat of mixing).

For non-ideal systems (e.g., azeotropic or highly non-ideal VLE), these assumptions break down because:

1. Relative volatility varies significantly with composition.
2. Heat of mixing or large enthalpy changes distort the energy balance.

Pressure-swing distillation is specifically used for azeotropic mixtures whose composition changes with pressure. The feasibility relies on the fact that the relative volatility of the components varies significantly with pressure. By operating two distillation columns at different pressures, the azeotrope composition shifts, allowing separation.
​
Other Options:
Presence of an entrainer (খ) → Relevant for azeotropic distillation with entrainer, not pressure-swing distillation.
Equal molar latent heats (গ) → Not a determining factor for azeotrope separation.
Packed column vs trays (ঘ) → Column internals affect efficiency but don’t enable separation of an azeotrope.

In extractive distillation, the entrainer is added to increase the relative volatility of the components in a mixture that are hard to separate. 
​It is usually less volatile than the mixture, so it stays in the column. 
​It alters intermolecular interactions, making one component easier to vaporize. 
​Unlike azeotropic distillation, it does not form an azeotrope.
​
​Other options:
ক) Entrainer should not be highly volatile.
গ) It does not form an azeotrope.
ঘ) It does affect intermolecular interactions to work.

Molecular distillation is a type of short-path vacuum distillation used for thermally sensitive, high-boiling compounds (e.g., vitamins, oils, polymers).
   By operating at very low pressures (often <0.01 Torr), the boiling points of compounds are drastically reduced, allowing them to vaporize at much lower temperatures, avoiding thermal decomposition.
   The short distance between the evaporator and condenser further minimizes residence time, but the key factor is the ultra-low pressure.

Other options:
ক) Chemical entrainers → Relevant for azeotropic distillation, not molecular distillation.
খ) Countercurrent contact → Typical of conventional distillation columns, not molecular distillation.
গ) High turbulence → Helps mass transfer, but not the main reason for preventing decomposition.

Supercritical fluid extraction (SFE) uses fluids above their critical temperature and pressure. Commonly, CO₂ is used because it is non-toxic, inert, and has a relatively low critical point (31 °C, 73 atm).
    The solvent power of supercritical CO₂ increases with pressure and can be fine-tuned to selectively extract compounds.
    It operates at moderate temperatures, making it suitable for heat-sensitive compounds, unlike pyrolysis.

Other options:
ক) Very high temperatures → Incorrect; SFE uses moderate temperatures.
গ) Always uses water → Incorrect; water can be used, but CO₂ is most common.
ঘ) Unsuitable for heat-sensitive compounds → Incorrect; SFE is ideal for such compounds.

Gas separation membranes (like for O₂/N₂ separation) work primarily on selective permeability:
  Smaller or more diffusive molecules (O₂) pass through the membrane faster than larger or less diffusive molecules (N₂).
  This is called the solution–diffusion mechanism: gases dissolve in the membrane material and diffuse at different rates.

Other options:
খ) Capillary condensation → Relevant for adsorption in porous materials, not membranes.
গ) Charge-based electrostatic repulsion → Relevant for ion-exchange membranes, not neutral gases.
ঘ) Surface adsorption followed by chemical reaction → Relevant for reactive separation (e.g., facilitated transport), not standard gas separation membranes.

Pervaporation is a membrane-based separation process used to separate liquid mixtures, especially azeotropic or close-boiling mixtures.
​
Mechanism:
  1.The liquid feed contacts a dense, non-porous membrane.
  2.Selective sorption: One component preferentially dissolves in the membrane.
  3.Diffusion: That component diffuses through the membrane.
  4.Evaporation: It vaporizes on the permeate side, often under reduced pressure.

Other options:
ক) Vaporization under high vacuum without heating → This is vacuum distillation, not pervaporation.
খ) Filtration through microporous media → Describes microfiltration or ultrafiltration, not pervaporation.
ঘ) Phase inversion and crystallization → Irrelevant; relates to polymer processing or freeze separation.

High vacuum distillation is used to separate heavy petroleum fractions (e.g., vacuum gas oil) that would thermally crack if heated too much.
Thermal cracking occurs when hydrocarbons are exposed to high temperatures for too long.
To prevent this:
      1.Operate at very low pressures to reduce boiling points.
      2.Design the column and reboiler to ensure short vapor residence time.
      3.Use low metal surface temperatures to avoid local overheating.

Other options:
ক) Increasing pressure → Increases boiling points, worsening thermal cracking.
খ) Trays vs packing → Affects efficiency but not primary prevention of cracking.
গ) Increasing reflux ratio → Improves separation but doesn’t prevent decomposition.

In a packed absorption column, the capacity (amount of gas that can be effectively absorbed per unit time) is primarily determined by the efficiency of mass transfer between the gas and liquid phases.
    High specific surface area → Provides more contact area between gas and liquid.
    Good wetting characteristics → Ensures the liquid spreads over the packing uniformly, maximizing absorption.

Other factors:
ক) Column height → Affects contact time and separation efficiency, not directly the capacity.
গ) Type of liquid distributor only → Important for uniformity, but not the main factor.
ঘ) Pressure drop → Operational concern, not the primary determinant of capacity.

In an absorption column:
1.The minimum liquid-to-gas ratio (L/G)min is the theoretical minimum amount of liquid required to achieve a given separation.
2.It occurs when the operating line just touches the equilibrium line at one point.
     At this point, the column operates at the least liquid flow rate possible without violating equilibrium.
3.Using a higher L/G than the minimum increases the driving force but uses more solvent.

Other options:
খ) Reflux ratio → Relevant to distillation, not absorption.
গ) Solvent partially miscible → Affects solubility, not minimum L/G concept.
ঘ) Column pressure → Changes solubility and equilibrium but not the fundamental L/Gmin criterion.

In stripping operations (like steam stripping):
  1.Heat (usually as steam) is added to the liquid to increase the vapor pressure of the solute relative to the solvent.
  2.This increases the relative volatility, making the solute more likely to transfer into the vapor phase.
  3.As a result, the solute can be efficiently removed from the liquid.

Other options:
ক) Decrease volatility of solutes → Opposite of the purpose; volatility is increased.
খ) Increase partial pressure of carrier gas → Not the main purpose; steam provides vapor, not necessarily carrier gas.
ঘ) Reduce column diameter → Operational parameter, not the primary reason for adding heat.

In liquid-liquid extraction, selectivity refers to how preferentially the solvent dissolves the target solute over other components.
   1.The solvent’s polarity and dielectric constant determine how well it interacts with the solute via dipole-dipole, hydrogen bonding, or van der Waals forces.
   2.A solvent with polarity closely matching the solute will maximize solute transfer while leaving undesired components behind.

Other options:
খ) High density difference → Helps phase separation but does not determine selectivity.
গ) Low viscosity → Improves mass transfer but not selectivity.
ঘ) High vapor pressure → Makes solvent recovery easier but does not influence solute preference.

The Schmidt number (Sc) is defined as:

Sc = kinematic viscosity/mass diffusivity = μ/ρD

It represents the ratio of momentum diffusivity (viscous diffusion) to mass diffusivity.
A high Sc means momentum diffuses faster than mass, affecting mass transfer rates in processes like absorption, stripping, and extraction.

Other options:
ক) Controls boiling point elevation → Irrelevant; related to colligative properties.
খ) Determines the phase equilibrium constant → Sc does not affect equilibrium.
ঘ) Measures column pressure drop → Pressure drop depends on flow, packing, and viscosity, not directly on Sc.

Random packing:
Made of small shapes (rings, saddles) randomly dumped in the column.
Vapor and liquid flow through the packing randomly.
Simpler, more tolerant of solids in the feed, but less efficient.

Structured packing:
Made of ordered sheets or corrugated surfaces arranged in a regular pattern.
Creates well-defined flow channels for liquid and vapor.
Provides high surface area and controlled liquid distribution.

Reactive distillation combines chemical reaction and distillation in a single unit.
   1.The reaction occurs on catalyst-packed sections of the column.
   2.As the product forms, it is immediately removed by distillation, preventing it from reacting back.
   3.This shifts the reaction equilibrium toward more product formation, improving conversion and yield.

Other options:
ক) Increased relative volatility due to azeotrope formation → Incorrect; azeotrope formation is not the main mechanism.
গ) Lower pressure drop → Minor effect; pressure drop is more a function of packing design.
ঘ) Ability to operate without heat input → Incorrect; heat is usually required to drive both reaction and distillation.

Flooding in a packed column occurs when gas velocity is too high, causing liquid to accumulate and the column to lose efficiency. The main factors controlling flooding are related to the physical characteristics of the packing:
   Packing size: Smaller packing increases surface area for mass transfer but also increases resistance to vapor flow, making flooding more likely.
   Void fraction (porosity): Higher void fraction allows easier gas flow, reducing the risk of flooding.

Other options:
ক) Packing height → Affects separation efficiency but does not directly cause flooding.
খ) Column wall thickness → Structural factor, irrelevant to flooding.
ঘ) Type of feed distributor only → Affects liquid distribution but is not the main factor for flooding.

Osmotic pressure (π) is a colligative property that depends on the number of solute particles in a solution, not their type. In membrane processes like reverse osmosis:

π = i C R T

Where:
C = solute concentration
R = gas constant
T = temperature
i = van’t Hoff factor
Higher solute concentration → higher osmotic pressure, requiring more applied pressure to achieve filtration.

Other options:
ক) Pressure drop across the membrane → Affects pumping energy, not osmotic pressure.
গ) Viscosity of permeate → Influences flow resistance, not osmotic pressure.
ঘ) Temperature uniformity → Minor effect; temperature affects π linearly, but concentration is dominant.

Ultrafiltration (UF) is a pressure-driven membrane separation process that separates solutes based mainly on size (molecular weight):
   1.UF membranes have pore sizes typically 1–100 nm, allowing solvent and very small solutes to pass, while retaining larger molecules like proteins, polymers, and colloids.
  2. Rejection is defined as the fraction of solute prevented from passing through the membrane, and it primarily depends on whether the solute is larger than the membrane pores.
​
Other options:
ক) Solute diffusivity → Minor effect compared to size exclusion.
খ) Density differences → Not relevant in UF.
গ) Difference in boiling points → Relevant for distillation, not membrane processes.

Entrainment occurs when liquid droplets are carried upward by the rising vapor in a distillation column.
1.The main factor is vapor velocity:
    If the vapor moves too fast, it can shear off liquid from trays or packing, causing droplets to enter the vapor stream.
2.This leads to liquid loss, contamination of the overhead product, and reduced separation efficiency.

Other options:
ক) Decreasing vapor velocity → Reduces entrainment risk.
খ) Reducing column pressure → Affects boiling points, not directly entrainment.
ঘ) Increasing tray spacing → Affects column height and flooding potential, but not the primary cause of entrainment.

In absorption and stripping operations, the stripping factor is defined as:
S = L/mV

Where:
L = Liquid molar flow rate
V = Vapor molar flow rate
m = Slope of the equilibrium line (relating liquid and vapor compositions)

The stripping factor indicates the relative ease of removing a component from the liquid into the vapor phase.
A higher m (steeper equilibrium line) means the component prefers the vapor phase more, lowering the required L/V ratio.

Other options:
ক) Murphree efficiency → Measures tray efficiency, unrelated to S definition.
গ) Relative volatility → Related to distillation, not the direct definition here.
ঘ) Reflux ratio → Relevant for distillation, not stripping factor.

In distillation, energy consumption (reboiler and condenser duty) is closely related to the reflux ratio (R):
1. Minimum reflux ratio (Rmin): The lowest reflux ratio at which the desired separation is theoretically possible.
​Operating near Rmin reduces the amount of vapor and liquid circulation, minimizing energy consumption.
2. Trade-off: Operating exactly at Rmin is not practical because it requires an infinitely tall column. Usually, columns operate at 1.1–1.5 × Rmin for practical height and low energy.

Other options:
খ) High reflux ratio → Increases separation efficiency but consumes more energy.
গ) Maximum pressure → Can increase relative volatility for some systems but may also increase energy input.
ঘ) Very high boil-up rate → Increases energy consumption unnecessarily.