Questions
8 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Mass Transfer is a cornerstone of Chemical Engineering, focusing on the net movement of mass from one location to another, typically driven by concentration gradients. In PSU exams, this topic is high-yield, often appearing in calculations related to separation processes like distillation and absorption. Mastering this requires a firm grasp of both transport phenomena principles and empirical design methods.
Diffusion and Fick's Laws
Diffusion describes the movement of particles from high to low concentration regions. Understanding the difference between molecular diffusion and eddy diffusion is crucial for predicting transport rates in stagnant and turbulent systems.
- Fick's First Law: JA = -DAB * (dCA/dz)
- Steady-state diffusion through a stagnant gas: NA = (DAB * P / RT * z * PBM) * (pA1 - pA2)
- Schmidt Number (Sc) = μ / (ρ * DAB)
- Sherwood Number (Sh) = kc * L / DAB
- Diffusivity DAB is proportional to T^(3/2) in gases
Distillation (McCabe-Thiele)
The McCabe-Thiele method is a graphical approach to calculate the number of theoretical stages in binary distillation. It assumes constant molar overflow and is frequently tested through questions on reflux ratio and feed conditions (q-line).
- Rectifying line: y = (R/R+1)x + (xD/R+1)
- Stripping line: y = (Lm/Vm)x - (W*xw/Vm)
- Feed line equation: y = (q/q-1)x - (xF/q-1)
- Minimum reflux ratio (Rm) occurs at the pinch point
- Total reflux requires minimum number of plates
Absorption and Stripping
Absorption involves transferring a solute from gas to liquid, while stripping is the reverse. The operational design relies heavily on the operating line slope (L/V) and the equilibrium curve.
- Absorption factor: A = L / (m * V)
- Kremser equation for number of stages: N = ln[((y1 - mx2)/(y2 - mx2)) * (1 - 1/A) + 1/A] / ln(A)
- Slope of operating line is L/V
- Flooding occurs when pressure drop is excessive
Extraction, Leaching, and Drying
These processes involve solid-liquid or liquid-liquid phase contacts. Examiners often focus on the stage efficiency in extraction and the constant/falling rate periods in drying.
- Constant rate period: drying rate depends on surface area and humidity
- Falling rate period: drying rate limited by internal diffusion
- Critical moisture content defines the transition between periods
- Ternary diagram used for liquid-liquid extraction stage calculations
Formula Sheet
JA = -DAB * (dCA/dz)
Sh = f(Re, Sc)
y = (R/R+1)x + (xD/R+1)
A = L/(mV)
N = ln[((y1-mx2)/(y2-mx2))*(1-1/A) + 1/A] / ln(A)
t = (Wc / A * Rc) * (X1 - Xc) + (Wc / A * Rc) * (Xc - X2) * ln((Xc - X*) / (X2 - X*))
Exam Tip
Memorize the slope of the q-line for various feed conditions, as this is the most common point of confusion in graphical distillation problems.
Common Mistakes
- Confusing the q-line slope for different feed types (saturated liquid vs saturated vapor).
- Neglecting the log-mean pressure difference (PBM) term in gas diffusion calculations.
- Incorrectly assuming the operating line always lies above the equilibrium curve for absorption.
More Revision Notes
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