Category: Interview question

• Unit operations are the fundamental building blocks of chemical engineering processes.
• Each unit operation is a basic physical step (not chemical change) used in manufacturing.

Examples:

1. Mass Transfer Operations: Distillation, Absorption, Extraction, Drying.
2. Heat Transfer Operations: Evaporation, Heat Exchangers, Condensation.
3. Mechanical Operations: Filtration, Crushing, Grinding, Screening.
4. Fluid Flow Operations: Fluidization, Transportation of fluids.

1. Batch process
   • Raw materials loaded → reaction/operation takes place → product removed.
   • Time-dependent, flexible, but less efficient.
   • Example: Pharmaceutical manufacturing, specialty chemicals.
2. Continuous process
   • Raw materials fed continuously, products withdrawn continuously.
   • Steady-state, more efficient, used for large-scale production.
   • Example: Oil refining, fertilizer plants, petrochemical units.
3. Semi-batch process
   • Combination of both. Some reactants are fed continuously while others are charged batch-wise.
   • Example: Hydrogenation reactions (gas fed continuously, liquid reactants batch loaded).

Henry’s Law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid (at constant temperature). C=kH⋅PC = k_H \cdot PC=kH​⋅P

Where:

• CCC = concentration of gas in liquid
• kHk_HkH​ = Henry’s constant (depends on temperature & gas-liquid pair)
• PPP = partial pressure of gas

Applications in Chemical Engineering:

• Gas absorption (e.g., CO₂ absorption in water).
• Carbonated beverages (dissolving CO₂ in soft drinks).
• Environmental engineering (oxygen solubility in water bodies).
• Design of scrubbers & absorbers

• Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature.
• At boiling point: Vapor pressure = external (atmospheric) pressure.

Importance in distillation:

• Determines boiling temperature of liquids.
• Helps in separating mixtures based on relative volatility.
• Used in design of distillation columns.

Raoult’s Law describes the vapor pressure of a liquid mixture: PA=xA⋅PA0P_A = x_A \cdot P_A^0PA​=xA​⋅PA0​

Where:

• PAP_APA​ = partial vapor pressure of component A
• xAx_AxA​ = mole fraction of A in liquid phase
• PA0P_A^0PA0​ = vapor pressure of pure A

Applications:

• Distillation (predict vapor-liquid equilibrium, VLE).
• Solvent recovery in industries.
• Separation processes (e.g., ethanol-water distillation).

The Reynolds number (Re) is a dimensionless number used to predict fluid flow behavior. Re=ρvDμRe = \frac{\rho v D}{\mu}Re=μρvD​

Where:

• ρ = fluid density (kg/m³)
• v = velocity of fluid (m/s)
• D = characteristic dimension (e.g., pipe diameter in m)
• μ = viscosity of fluid (Pa·s)
• Re < 2000 → Laminar flow
• 2000 < Re < 4000 → Transition region
• Re > 4000 → Turbulent flow

Significance:

• Helps design pumps, reactors, and pipelines.
• Used in scale-up of chemical processes.
• Predicts heat & mass transfer coefficients.

• Laminar flow: Fluid moves in smooth, parallel layers with little mixing. Velocity is uniform at a given cross-section.
  • Occurs when Reynolds number (Re) < 2000.
  • Example: Flow of honey or oil in a narrow pipe.
• Turbulent flow: Fluid particles move chaotically with eddies and vortices. Mixing is high.
  • Occurs when Re > 4000.
  • Example: Flow of water in large pipelines, river water, air around an airplane wing.