Mass Transfer — Part 1 MCQs with Explanations

The correct answer is (A) Chemical properties . Here's why: Let's break down what influences mass transfer: * Mass Transfer: Think of it like sugar dissolving in water. Mass transfer is the movement of a substance from one location (like a solid sugar cube) to another (the water). * Physical Properties: These are things like density, viscosity, and diffusivity. These properties *directly* affect how easily a substance can move and dissolve. Imagine trying to stir honey versus water; the viscosity makes a big difference in how quickly things mix. * Degree of Turbulence: Turbulence means more mixing. More mixing means the substance is carried away faster from the interface, and new material is brought in to be dissolved. More turbulence leads to a higher mass transfer rate. * Interfacial Area: The interfacial area is the area of contact between the two phases (e.g., the surface area of the sugar cube in contact with the water). A larger interfacial area means more opportunities for the substance to dissolve or transfer. * Chemical Properties: While chemical properties *can* indirectly influence mass transfer (e.g., solubility if a chemical reaction is occurring), the *rate* of mass transfer itself is not directly determined by the chemical properties of the phases in the absence of a chemical reaction. The other options (physical properties, turbulence, and interfacial area) have a more *direct* impact on the speed at which mass transfer occurs.

Co-current absorbers are generally used in cases where the gas to be dissolved in the liquid is sparingly soluble. Here's why: Option A: Sparingly soluble → Correct. Co-current absorbers are effective for gases that are not easily soluble in the liquid. In such cases, the concentration of the gas is higher in the inlet gas stream and decreases as it moves through the absorber. The liquid and gas move in the same direction, allowing more contact time to dissolve the gas into the liquid, which works well for sparingly soluble gases. Option B: Highly soluble → Incorrect. For highly soluble gases, other types of absorbers (such as counter-current absorbers) are preferred, as they provide a more efficient method for absorbing gases that dissolve easily into liquids. In co-current absorbers, the driving force for mass transfer decreases along the length of the absorber, which is not ideal for highly soluble gases. Option C: A pure substance → Incorrect. While pure gases can be absorbed in co-current absorbers, the design of the system is typically optimized for sparingly soluble gases rather than pure substances. The solubility factor plays a key role in determining the type of absorber. Option D: A mixture → Incorrect. For gas mixtures, especially when the components have different solubilities, counter-current absorption or other specialized systems are usually preferred to ensure more efficient separation and absorption. Co-current absorbers are not the best choice for handling mixtures with significantly different solubilities. Thus, the correct answer is Option A: Sparingly soluble , as co-current absorbers are most effective when dealing with gases that are not highly soluble.

The laboratory determination of the diffusivity of vapor refers to the process of measuring how a vapor diffuses through a medium. The following methods are considered for this purpose: Option A: Wetted wall column → Correct. The wetted wall column method is a standard laboratory technique used to measure the diffusivity of vapors. It involves a column where liquid is continuously circulated over the surface, allowing the vapor to diffuse through the liquid. By measuring the rate of vapor absorption, the diffusivity can be determined. Option B: Gilliland's → Incorrect. Gilliland's method is typically used to determine the efficiency of packed towers or to predict the performance of absorption and distillation columns. It is not specifically used for measuring diffusivity of vapor. Option C: Winkelmann's → Incorrect. Winkelmann’s method is not a standard method used for the laboratory determination of vapor diffusivity. It is more related to other fluid dynamics and heat transfer processes. Option D: Wike's → Incorrect. Wike’s method is not a widely recognized method for determining vapor diffusivity in laboratory settings. Thus, the correct answer is Option A: Wetted wall column , which is specifically designed for determining vapor diffusivity in laboratory conditions.

Given: Drying occurs in the linear drying rate regime Initial moisture content = X o Final moisture content = X F Critical moisture content = X o (same as initial moisture) Drying rate becomes zero at X = 0 This implies that the entire drying happens in the constant (linear) rate period , because the critical moisture content is equal to the initial moisture content. In the linear drying regime, the drying time (θ) is given by: θ = (L / A R c ) × (X o - X F ) Let M = L / A R c Therefore, θ = M (X o - X F ) Hence, the correct answer is: Option A: M (X o - X F )

A humidity chart (psychrometric chart) is a graphical tool used to study the properties of air-water vapor mixtures in processes like condensation, vaporisation, and air conditioning . At a given dry bulb temperature, the value of humidity that is read directly from the vertical axis of the chart is the humidity ratio (also called absolute humidity ), which represents the mass of water vapor per unit mass of dry air. This corresponds to molal humidity when expressed in molar terms (moles of water vapor per mole of dry air). Relative humidity is also present on the chart as curved lines and can be inferred or estimated, but it is not the direct vertical-axis value at a fixed dry bulb temperature. Percentage humidity is a vague and non-standard term not used in technical interpretations of the chart. Therefore, the value of humidity obtained directly at a given dry bulb temperature from the chart is in terms of molal humidity. Correct Answer: Option A: Molal Option B: Relative Humidity ❌ Relative humidity is shown on the psychrometric (humidity) chart as a series of curved lines , not as values that are directly read from the main axes. To determine relative humidity at a specific dry bulb temperature, you need another parameter like wet bulb temperature, dew point, or enthalpy. Hence, while relative humidity is present on the chart, it is not the direct value obtained from a dry bulb temperature alone. Option C: Percentage Humidity ❌ Percentage humidity is a vague and imprecise term. It is sometimes used interchangeably with relative humidity, but that usage is technically incorrect . The psychrometric chart does not use the label "percentage humidity" as a direct measure. Also, the chart doesn't plot any axis explicitly labeled with this term, so it cannot be considered a correct direct reading. Option D: None of These ❌ This is incorrect because Option A (Molal humidity) is valid. The humidity chart allows direct reading of the humidity ratio or absolute/mass-based humidity (kg water vapor per kg dry air), which is conceptually aligned with molal humidity when expressed in moles. Therefore, saying “none of these” ignores the valid and correct option available in the list. Conclusion: The only correct and directly readable form of humidity from the chart (at a given dry bulb temperature) is molal humidity , making Option A the right answer.

For a solid sphere dissolving in a large volume of quiescent (stationary) liquid, the system is governed by diffusion alone since there is no convective flow. Under these conditions, the mass transfer process is described by a steady-state diffusion model. According to the solution of Fick’s second law for a stationary medium around a dissolving sphere, the mass transfer coefficient (k c ) is given by: k c = D/a , where D = diffusivity of the solute in the liquid a = radius of the solid sphere This expression assumes that the diffusion is radial and the concentration at the surface of the sphere remains constant. Option B (D/2a) is incorrect because it does not match the standard solution for diffusion from a sphere. Option C (Proportional to √D) applies in some unsteady-state or film-type approximations, not for steady diffusion from a sphere. Option D (Dependent on Reynolds number) is incorrect here, since the liquid is quiescent and there is no bulk motion , making Reynolds number irrelevant. Therefore, the correct answer is D/a. Correct Answer: Option A: D/a

Option D: All of the above is the correct answer. Option A: Highest temperature is near the feed plate This is true because the feed plate is generally located between the rectifying and stripping sections. Since the bottom of the column is hotter (to vaporize the liquid) and the top is cooler (to condense vapors), the feed plate lies between these extremes and usually has a relatively high temperature, especially in comparison to the overhead. Option B: Driving force for the liquid flow is its weight This is correct. In a distillation column, the trays are arranged vertically, and gravity causes the liquid to flow from one tray to the next lower one through downcomers. Thus, the liquid flow is driven by its weight. Option C: Vapors are not always at their dew points Also true. Vapors in the column can be superheated or in equilibrium with liquid depending on the location and thermal gradients. Therefore, they are not necessarily always at their dew point. Since all the individual statements are correct, the correct answer is: Option D: All of the above.

Given: Dry bulb temperature = 60°C Dew point temperature = 40°C Total pressure = 101.3 kPa Vapour pressure of water at 40°C = 7.30 kPa Vapour pressure of water at 60°C = 19.91 kPa Humidity (H) is calculated using: H = 0.622 × (P v / (P - P v )) Where: P v = partial pressure of water vapour = vapour pressure at dew point = 7.30 kPa P = total pressure = 101.3 kPa Now substituting the values: H = 0.622 × (7.30 / (101.3 - 7.30)) H = 0.622 × (7.30 / 94.0) H ≈ 0.622 × 0.07766 H ≈ 0.0482 kg water/kg dry air Therefore, the correct answer is Option A: 0.048

Higbie developed the film theory. The film theory, also known as the Higbie's film theory, is used to describe mass transfer processes occurring at the interface between a fluid and a solid surface. It is widely applied in chemical engineering for analyzing processes such as absorption, adsorption, and evaporation.

Binary diffusivity in liquids primarily depends on the temperature, and for liquids, it is directly proportional to the temperature . The relationship of binary diffusivity with temperature differs between gases and liquids. For gases, binary diffusivity (D) is proportional to T 3/2 /P , where P is pressure. However, in liquids, binary diffusivity is proportional to T as temperature changes have a more direct impact due to the close molecular interactions in liquid phases.

The Schmidt number (Sc) is a dimensionless number in mass transfer operations. It is defined as the ratio of momentum diffusivity (kinematic viscosity, μ/ρ) to the mass diffusivity (D) . In mathematical terms, Sc = μ / (ρ D) . This number helps compare the relative effects of momentum and mass transport within a fluid.

Marginal decrease in the number of is the correct answer to “For ethanol-water system, the lowering of distillate quality from 95% to 92% will cause __________ plate requirement.”. This question is standard in competitive examination syllabi for Mass Transfer.

1 is the correct answer to “The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are same for, N Pr = N Sc = __________”. This question is standard in competitive examination syllabi for Mass Transfer.

500 is the correct answer to “The interfacial area per unit volume of dispersion, in a gas-liquid contactor, for fractional hold up of gas = 0.1 and gas bubble diameter = 0.5 mm is given by (in m 2 /m 3 )”. This question is standard in competitive examination syllabi for Mass Transfer.

Perfectly mixed is the correct answer to “The ratio of Murphree plate efficiency to point efficiency is 1 in a __________ flow model.”. This question is standard in competitive examination syllabi for Mass Transfer.

Water-hydrochloric acid is the correct answer to “Which of the following binary systems is an example of a maximum boiling azeotrope”. This question is standard in competitive examination syllabi for Mass Transfer.

Both A & B is the correct answer to “Capacity of a rotary dryer depends on its”. This question is standard in competitive examination syllabi for Mass Transfer.

The entire amount of C is transferred to solvent B is the correct answer to “In a liquid-liquid extraction, 10 kg of a solution containing 2 kg of solute C and 8 kg of solvent A is brought into contact with 10 kg of solvent B. Solvent A and B are completely immiscible in each other whereas solute C is soluble in both the solvents. The extraction process attains equilibrium. The equilibrium relationship between the two phases is Y* = 0.9X, where Y* is the kg of C/kg of B and X is kg of C/kg of A. Choose the correct answer.”. This question is standard in competitive examination syllabi for Mass Transfer.

Water from petrol is the correct answer to “Removal of __________ exemplifies an adsorption unit operation.”. This question is standard in competitive examination syllabi for Mass Transfer.

Reynolds number in mass transfer is analogous to Grashoff number in heat transfer is the correct answer to “Pick out the wrong statement pertaining to the analogy between equations of heat and mass transfer operations.”. This question is standard in competitive examination syllabi for Mass Transfer.

Increasing the gas film co-efficient is the correct answer to “The overall mass transfer co-efficient for the absorption of SO 2 in air with dilute NaOH solution can be increased substantially by”. This question is standard in competitive examination syllabi for Mass Transfer.

2.09 × 10-4 is the correct answer to “The diffusion co-efficient in m 2 /s. of acetic acid in benzene (liquid in liquid) is”. This question is standard in competitive examination syllabi for Mass Transfer.

Both B and C is the correct answer to “The relative saturation of a partially saturated mixture of vapour and gas can be increased by __________ of the mixture.”. This question is standard in competitive examination syllabi for Mass Transfer.

Mass transfer co-efficient is the correct answer to “Experimental determination of __________ is done by wetted wall column method.”. This question is standard in competitive examination syllabi for Mass Transfer.

None of these is the correct answer to “Powdery materials can be guarded against caking tendency on storage by”. This question is standard in competitive examination syllabi for Mass Transfer.

Wetted wall is the correct answer to “Measurement of the interfacial area of mass transfer is achieved easily & accurately in case of a __________ coloumn.”. This question is standard in competitive examination syllabi for Mass Transfer.

Same is the correct answer to “The equilibrium liquid composition compared to the vapor composition in case of azeotropic mixture is”. This question is standard in competitive examination syllabi for Mass Transfer.

Pure gas is the correct answer to “When the gas to be dissolved in liquid is a/an __________ then normally co-current adsorber are used.”. This question is standard in competitive examination syllabi for Mass Transfer.

Hot water is the correct answer to “Coffee is prepared from coffee beans by leaching with”. This question is standard in competitive examination syllabi for Mass Transfer.

Increase in pressure drop is the correct answer to “Flooding in a distillation column is detected by a sharp”. This question is standard in competitive examination syllabi for Mass Transfer.

Increases is the correct answer to “With increase in the liquid flow rate at a fixed gas velocity in a randomly packed counter current gas-liquid absorption column, the gas pressure drop”. This question is standard in competitive examination syllabi for Mass Transfer.

Lewis reltionship is the correct answer to “Which of the following is directly concerned with psychrometry”. This question is standard in competitive examination syllabi for Mass Transfer.

Both A & B is the correct answer to “The 'shanks system' of leaching (i.e., counter-current multiple contact leaching) is used for”. This question is standard in competitive examination syllabi for Mass Transfer.

Froude number is the correct answer to “Which is not concerned directly with mass transfer”. This question is standard in competitive examination syllabi for Mass Transfer.

Film is the correct answer to “Calculation of mass transfer co-efficient is mostly/normally done using __________ theory.”. This question is standard in competitive examination syllabi for Mass Transfer.

There is equimolar counter diffusion is the correct answer to “Component A is diffusing in a medium B. The flux N A relative to a stationary point is equal to the flux due to molecular difusion, if”. This question is standard in competitive examination syllabi for Mass Transfer.

The fraction vaporised but not the product composition is the correct answer to “One mole feed of a binary mixture of a given composition is flash vaporised at a fixed P and T. If Raoult's law is obeyed, then changing the feed composition would effect”. This question is standard in competitive examination syllabi for Mass Transfer.

Liquid extraction is the correct answer to “The recovery of pencillin from the acidified fermentation broth is done by”. This question is standard in competitive examination syllabi for Mass Transfer.

> 1 is the correct answer to “Absorption factor, for a fixed degree of absorption from a fixed amount of gas should be”. This question is standard in competitive examination syllabi for Mass Transfer.

Selectivity is the correct answer to “Polar organic compounds are normally used as separating agents for the azeotropic and extractive distillation. Which of the following is the most important factor to be considered for the choice of the separating agent for extractive distillation”. This question is standard in competitive examination syllabi for Mass Transfer.