Mass Transfer Operations-2 Previous Year Question Paper With Solution

These questions of Mass Transfer Operations are taken from the previous year’s GTU papers. One can get good marks in the examination of chemical engineering by preparing these questions and these questions given with solutions.

What is the mass transfer?

Mass transfer is the net movement of mass or molecules from one location (phase) to another location (phase) due to the concentration difference. Mass transfer occurs in many processes such as distillation, evaporation, drying, etc.

3 or 4 Marks Questions:

1. Classify gas-liquid contact equipment.

→ Generally, gas-liquid contact equipments are classified based on gas and liquid phase dispersion.

  1. Based on gas dispersion
    • Tray tower
    • Sparger/ Bubble vessel
    • Mechanically agitated vessel
  2. Based on liquid dispersion
    • Packed tower
    • venturi scrubber
    • Spray tower
    • Wetted wall column

2. Write industrial application of crystallization

→ The industrial application of crystallization is given below:

  • Crystallization is used in the process like purification, production or recovery of solid materials. It is mainly used in the purification process.
  • It is used in the Purification of drugs.
  • it is used in the separation of Alum crystals from impure samples.
  • It is used in sucrose production from sugar beat.
  • In the food industry, it is used in the production of salt, sugar, etc.
  • It is also used in the production of detergents, fertilizer, etc
  • It is used in the chemical industry to produce high quality chemicals and reagents and to separate different chemicals from their solution.
  • It is also used in the production of fertilizers such as ammonium nitrate and potassium chloride.
  • Also used in the purification of seawater.

3. Draw the sketches of different Reboilers.

→The sketches of different reboilers are given below:

1.) Jacketed Kettle Reboiler

2.) Internal Reboiler

3.) External or Kettle type Reboiler

4.) Thermosyphon Reboilers

4. Explain various methods to achieve supersaturation.

→Unless a solution is supersaturated, neither nucleation nor crystal growth occurs. Thus for crystallization to occur, supersaturation can be generated by any one of the following methods:

  • By cooling a concentrated hot solution through indirect heat exchange.
  • By evaporating a part of the solvent or by evaporating a solution.
  • By adiabatic evaporation and cooling.
  • By adding a new substance that reduces the solubility of the original solute, for example:- salting
  • By a chemical reaction with a third substance.

5. Define Raoult’s law and its application.

→ For an ideal solution the equilibrium partial pressure of a component at a given temperature is equal to the product of its vapour pressure at the same temperature and its mole fraction in the liquid phase.

The mathematical formula of Raoult’s law is given below:

PA*=pA . X

where,

PA* = Partial pressure of A component.

PA = Vapour pressure of A component.

X = Mole fraction of liquid phase.

Applications of Raoult’s law:

  • Ideal Solutions: Describes the behavior of ideal solutions where the vapor pressure of each component is directly proportional to its mole fraction in the solution.
  • Vapor-Liquid Equilibrium: Predicts the vapor pressures of volatile components in liquid mixtures, essential for distillation processes.
  • Azeotropes: Helps identify azeotropic mixtures, where the vapor and liquid compositions are the same, which can affect separation processes.
  • Chemical Process Design: Used in designing processes involving volatile components, such as in the petrochemical and pharmaceutical industries.
  • Solvent Selection: Guides the choice of solvents in processes like extraction and separation, based on their vapor pressure behavior.
  • Air Pollution Modeling: Applied to estimate the volatility of chemicals in the atmosphere, aiding in air quality assessments.
  • Colligative Properties: Contributes to understanding colligative properties like vapor pressure lowering in solutions, influencing areas like cryoscopic and ebullioscopic measurements.
  • Phase Diagrams: Helps predict the compositions of liquid and vapor phases in phase diagrams of mixtures.
  • Quality Control: Used to assess the purity of liquids and the consistency of mixtures in industries like pharmaceuticals.
  • Chemical Engineering: Applied in various engineering fields involving liquid-vapor equilibrium, including separation processes and reactor design.
  • Environmental Chemistry: Useful for understanding the behavior of volatile compounds in natural systems, such as the release of volatile organic compounds into the atmosphere.

or

Define Henry’s law and its application.

→ Henry’s law is a gas law which states that the amount of gas that is dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid when the temperature is kept constant.

The mathematical formula of Henry’s law is given below:

P  ∝  C

P =KH.C

where,

P = the Partial pressure of the gas in the atmosphere above the liquid.

KH = the Henry’s law constant of the gas.

C = the concentration of the dissolved gas.

Applications of Henry’s Law:

  • Carbonation of beverages: Explains how carbon dioxide dissolves under pressure, giving carbonated drinks their fizz.
  • Deep-sea diving: Informs the effects of dissolved nitrogen at high pressures, crucial for diver safety.
  • Aquariums and fish tanks: Regulates gas concentrations for aquatic life well-being.
  • Oxygen therapy: Governs how dissolved oxygen in the blood depends on inhaled air’s oxygen pressure.
  • Water treatment: Used to remove volatile organic compounds from water through air stripping.
  • Air-sea gas exchange: Describes how gases like oxygen and CO2 move between oceans and the atmosphere.
  • Gasoline and oil refining: Influences dissolved gas behavior during refining processes.
  • Environmental monitoring: Helps assess the impact of gases on aquatic ecosystems and climate change.
  • Bubble formation: Dictates the rate and size of bubbles formed in liquids.
  • Volcanic activity: Explains gas release from magma, contributing to volcanic eruptions.

6. Write the principle and applications of ion exchange.

The principle of ion exchange:

The principle of ion exchange is a chemical process in which ions from one solution are exchanged with ions of the same charge from another solution. This process occurs on a solid material known as an ion exchange resin, which has a high affinity for certain ions. The primary principle behind ion exchange is electrostatic attraction and the balance of charges.

The applications of ion exchange:

  • Water Softening: One of the most common applications of ion exchange is water softening. Hard water contains high concentrations of calcium and magnesium ions, which can lead to scale buildup in pipes and appliances. Ion exchange resins with sodium ions are used to replace calcium and magnesium ions with sodium ions, resulting in softened water that is less likely to cause scaling.
  • Water Purification: Ion exchange is used in water purification to remove specific contaminants such as heavy metals (e.g., lead, mercury), nitrates, and other harmful ions. Anionic and cationic ion exchange resins are employed to selectively remove these contaminants from water sources.
  • Wastewater Treatment: Ion exchange is used to treat industrial wastewater by removing toxic or unwanted ions. This helps industries meet regulatory standards and reduce environmental impact.
  • Hydrometallurgy: In metal recovery processes, ion exchange is used to selectively extract valuable metals from solutions. Certain ion exchange resins can capture metal ions from solutions, enabling the separation and concentration of metals like uranium, gold, and rare earth elements.
  • Food and Beverage Industry: Ion exchange resins are used in the food and beverage industry for decolorization, deionization, and removal of impurities. They help improve the quality of products such as fruit juices, sugar, and wine.
  • Pharmaceuticals: Ion exchange is used in pharmaceutical manufacturing to purify and separate compounds. It’s also employed in the formulation of certain drugs and in the preparation of purified water for pharmaceutical processes.

7. Explain the effect of temperature on adsorption.

→ The effect of temperature on adsorption is given below;

  • Adsorption Capacity:
    • Increased Temperature: Generally leads to a decrease in adsorption capacity for physical adsorption (physisorption). Higher temperatures provide molecules with more thermal energy, allowing them to move more freely and reducing their likelihood of being captured by the adsorbent surface.
    • Decreased Temperature: Tends to increase adsorption capacity as lower temperatures result in slower-moving molecules with a higher probability of adhering to the adsorbent surface.
  • Energetics of Adsorption:
    • Exothermic Adsorption: If adsorption releases heat (exothermic), increasing temperature reduces adsorption. Higher temperatures cause the system’s overall energy to rise, diminishing the significance of the heat released during adsorption, leading to reduced adsorption capacity.
    • Endothermic Adsorption: When adsorption absorbs heat (endothermic), raising the temperature can enhance adsorption. The increased thermal energy compensates for the energy required by the endothermic process, allowing more molecules to overcome the energy barrier and adsorb.
  • Chemisorption: In chemisorption (chemical bonding between adsorbate and adsorbent), temperature effects are more complex. Higher temperatures can enhance reaction rates and, thus, the rate of chemisorption, which may lead to higher adsorption capacity.

8. Write a short note on Mier’s supersaturation theory for crystallization.

Meir’s Supersaturation theory
  • According to Mier’s Supersaturation theory, there is a definite relationship between the concentration and temperature at which crystals will spontaneously form in a pure solution.
  • This relationship is represented by the super solubility curve which is approximately parallel to the solubility curve. Both curves are shown in the above figure.
  • The curve AB is the solubility curve and the curve PQ is the super solubility curve.
  • The curve AB represents the maximum concentration of solutions that can be achieved by bringing solid solute into equilibrium with a liquid solvent.
  • If a solution having the composition and temperature indicated by point C is cooled in the direction shown by the arrow it first crosses the solubility curve AB and we would expect crystallization to start.
  • Actually, if we start with initially unseeded solutions, crystal formation will not begin until the solution is supercooled considerably past the curve AB.
  • According to Mier’s theory crystallization will start in the neighborhood of point D and the concentration of the solution then follows roughly along the curve DE.
  • For an initially unseeded solution, the curve PQ represents the limit at which spontaneous nuclei formation begins and consequently, crystallization can start.
  • According to Miers’s theory under normal conditions, nuclei cannot form and Crystallization can’t then occur in the area between the solubility curve and the super solubility curve i.e. at any position short of point D along line CD.
  • Meir’s theory is useful for discussing the qualitative aspects of nucleation from seeded and unseeded solutions.

9. Describe Flash Vaporization.

  • Flash Vaporization is also known as equilibrium distillation. It is normally carried out in a continuous manner.
  • In this method, a liquid mixture is partially vaporized, the vapor and liquid are allowed to attain equilibrium by providing a sufficient contact time and finally withdrawn separately.
  • Feed is heated in a tubular heat exchanger. The hot liquid mixture is then fed to a separator via a pressure-reducing valve(PRV) whereby pressure is reduced and the vapour is formed at the expense of liquid adiabatically.
  • The liquid is withdrawn from the bottom of the separator and the equilibrium vapour leaves the separator from the top which is then liquified in a condenser.
  • Flash distillation or vaporization is commonly used in the petroleum industry, handling multi-component systems in the pipe stills.

10. Draw a neat sketch of Swenson Walker Crystalliser.

→ The Swenson-Walker crystalliser is a cooling type, continuous, jacketed trough
crystalliser. It is an example of the scrapped surface crystalliser and is probably the most
widely used crystalliser.

Swenson-Walker Crystaliser

11. Explain minimum reflux ratio and optimum reflux ratio.

Minimum reflux ratio:

The minimum reflux ratio is that reflux ratio at which an infinite number of
plates are required for a desired separation.

Optimum reflux ratio:

The optimum reflux ratio is defined as that reflux ratio at which the total cost of
operation (the sum of the fixed charges and the cost of heating and cooling) is minimum.

12. Discuss ventury scrubber with diagram.

  • Venturi scrubbers are similar to ejectors, the gas is drawn into the throat of a venturi by a stream of absorbing liquid sprayed into the convergent duct section.
  • The device is used especially where the liquid contains a suspended solid, which would plug the otherwise more commonly used tray and packed towers, and where low gas-pressure drop is required.
  • These applications have become increasingly important in recent years, as in the absorption of sulfur dioxide from furnace gases with slurries of limestone, lime, or magnesia . Some very large installations are in service for electric utilities.
  • The device also used for removing dust particles from gases.

7 Marks Questions:

1. Discuss the operating problems of the tray tower. or

Describe the flooding, coning, and dumping problems of the tray tower.

→The operating problems of the tray tower are given below:

  1. Flooding
  2. Priming
  3. Excessive entrainment
  4. dumping
  5. coning
  6. weeping
Operating characteristics of tray

Flooding: High-pressure drop may lead to the condition of flooding.

When the liquid flowrate is very high, in comparison with gas flowrate the liquid fills the downspout as well as an entire tray space and eventually the entire column. This phenomenon is
called flooding.

Priming: For gas-liquid combination high gas velocity may lead to the priming condition, in this foam Persist through the space between the tray and a large amount of liquid is carried out by the gas from one tray to the above tray.

Dumping: At very low gas rate, none of the liquid reaches the downspout.

Coning: If the liquid rates are too low, the gas rising through the opening of tray may push liquid away and the contact of the gas and liquid is poor.

Weeping: If the gas rate is too low, much of the liquid may rain down through the opening of the tray tower, thus failing to obtain the benefits of complete flow over trays.

2. Write short notes on types of packing. or

Explain different types of packing.

→There are two types of packing,

  1. Random packing
  2. Regular packing

1. Random packing:

  • Random packings are simply dumped into the tower during installation and allowed to fall at random.
  • In the past such readily available materials as broken stone, gravel, or lumps of coke were used, due to small surface and poor fluid flow characteristics they were replaced by rasching rings, Lessing rings, Intalox saddles, etc.
  • They are made of ceramic materials, metals, and plastic.
  • Generally, the random packings offer larger specific surface in smaller sizes, but they cost less per unit volume in the larger sizes.
  • During installation, the packings are poured into the tower to fall at random and in order to prevent breakage of ceramic or carbon packings, the tower may first be filled with water to reduce the velocity of fall.

2. Regular Packing:

  • In Regular packing, there is an organized manner in which packings are arranged in the tower.
  • It provides low pressure drop for gas and a higher fluid flow rate comparatively.
  • It has low installation cost compared to random packing.

(a) Rasching ring

(b) Lessing ring

(c) Partition ring

3. Explain criteria for solvent selection for gas absorption.

→While selecting a particular solvent for gas absorption operation, the following properties of
the solvent are considered :

  1. Gas solubility
  2. Volatility
  3. Corrosive nature
  4. Viscosity
  5. Cost and availability
  6. Miscellaneous
  1. Gas solubility:
    • The solubility of a solute gas in a solvent should be high, i.e., the solvent should have a
      high capacity for dissolving the desired solute gas so that the less amount of the solvent will
      be required for a given absorption duty.
    • In general, for good solubility, a solvent of chemical nature similar to that of the solute to be absorbed must be searched out and used.
    • The solvent selected should have a high solubility for the solute to be absorbed.
  1. Volatility :
    • As the gas leaving an absorption unit is generally saturated with the solvent, there will be
      a loss of the solvent with the gas leaving the unit operation.
    • Hence, to minimise the solvent loss (in the gas leaving), the solvent should be less volatile, i.e., it should have a low vapour pressure under given operating conditions.
  2. Corrosive nature :
    • The solvent should not be corrosive (as far as possible) towards common materials of
      construction so that the construction material for an absorption equipment will not be too
      expensive.
  3. Viscosity :
    • The solvent should have a low viscosity for rapid absorption rates, low pumping cost and
      better heat transfer. The solvent should be non-viscous.
  4. Cost and availability :
    • The solvent should be cheap and readily available. Losses are less costly with the cheap
      solvent.
  5. Miscellaneous :
    • The solvent should be non-toxic, non-flammable, non-foaming, and chemically stable
      from a handling and storage point of view.

For reference purpose you can download the papers given below
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3.w2020-3350502
4.w2019-3350502

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