CHE 303: Mass Transfer I
Course Teacher:
Dr. Md. Iqbal Hossain
Assistant Professor
iqbalhossain@che.buet.ac.bd ,+8801927885215
Course Synopsis:
Introduction to equilibrium-staged separations, vapor-liquid phase
equilibrium, flash distillation, introduction to column distillation, column
distillation: internal stage-by-stage balances, advanced binary distillation:
McCabe-Thiele and Lewis analyses, batch distillation, staged column design,
absorption and stripping, immiscible extraction, washing and leaching, and
extraction of partially miscible systems.
Learning Outcomes:
ChE 303: Mass Transfer I is the first course on mass transfer and offered to
the students of level 3 and term 1. It is a compulsory course.
The main objective of this course is to impart in-depth knowledge and
understanding on the underlying theory/principle, application, practical
consideration and constraint, process and basic design calculations, and
operation of equilibrium-staged separations.
Text Book and Author
• Phillip C Wankat is a Distinguished Professor in Chemical Engineering and
Engineering Education at Purdue University
CHE 303: Mass Transfer I
Chapter 1:
Introduction to Equilibrium Staged Separations
Dr. Md. Iqbal Hossain
Assistant Professor
iqbalhossain@che.buet.ac.bd and +8801927885215
Objective:
• Importance of separations?
Why does chemical engineering require the
study of separations techniques?
Explain how separations are used in a typical
chemical plant.
• Define the concepts of equilibrium stages
and unit operations.
Importance of separations:
A typical chemical plant is a chemical reactor surrounded by separators
Chemical plants commonly have from 50% to 90% of their capital invested in
separations equipment.
Chemical plants commonly have from 40% to 70% of both capital and operating costs
in separations
Since separations are ubiquitous in chemical plants and petroleum refineries,
chemical engineers must be familiar with a variety of separation methods.
Concepts of equilibrium stages and unit operations:
The equilibrium stage concept is applicable when the process can be
constructed as a series of discrete stages in which the two phases are
contacted and then separated. The two separated phases are assumed to be
in equilibrium with each other.
Assuming that the stages are equilibrium stages, the engineer can calculate
concentrations and temperatures without detailed knowledge of flow
patterns and heat and mass transfer rates.
A second useful concept is that of a unit operation. The idea here is that
although the specific design may vary depending on what chemicals are being
separated, the basic design principles for a given separation method are
always the same. For example, the basic principles of distillation are always
the same whether we are separating ethanol from water, separating several
hydrocarbons, or separating liquid metals. Consequently, distillation is often
called a unit operation, as are absorption, extraction, etc.
A more general idea is that design methods for related unit operations are
similar. Since distillation and absorption are both liquid-vapor contacting
systems, the design is much the same for both. This similarity is useful
because it allows us to apply a very few design tools to a variety of separation
methods.
CHE 303: Mass Transfer I
Chapter 2:
Vapor-Liquid Phase Equilibrium
Dr. Md. Iqbal Hossain
Assistant Professor
iqbalhossain@che.buet.ac.bd and +8801927885215
Objective:
• Concept of vapor-liquid equilibrium
• Various approaches to present vapor-liquid
equilibrium data/relationship
Concept of Equilibrium:
When not at equilibrium, the liquid and the vapor can be at different pressures
and temperatures and be present in different mole fractions.
At equilibrium the temperatures, pressures, and fractions of the two phases
cease to change. Although molecules continue to evaporate and condense, the
rate at which each species condenses is equal to the rate at which it evaporates.
Although on a molecular scale nothing has stopped, on the macroscopic scale,
where we usually observe processes, there are no further changes in
temperature, pressure, or composition.
Tliq=Tvap (Thermal) ; Pliq=Pvap (Mechanical)
(Chemical Potential i)liq= (Chemical Potential i)vap (Chemical) ; Ci,liq≠Ci,vap
Degrees of freedom (intensive) from Phase rule (nonreacting) , F= C-P+2=2
Approaches and Form of Equilibrium data:
• Experimental data
– Table
– Graphical presentation
– De Priester Chart
• Relative Volatility
• Simple models for VLE
– Raoult’s law, Henry’s law constant
Experimental Data
VLE Data for ethanol water system at 1 atm
F= C-P+2=2
Graphical Representation
Ethanol/water Azeotrope: 0.8943 mol fraction, 0.955 wt fraction, 78.15°C
Enthalpy-composition Diagram
Ethanol water at 1 kg/cm2
De
Priester
Chart
K values for lower
hydrocarbons
K= y/x
De
Priester
Chart
Relative Volatility
• In order to separate a binary mixture using distillation process,
there must be a differences in volatilities of the components.
The greater the difference, the easier it is to do so. A measure
for this is termed the relative volatility.
• For a binary mixture of A and B, therefore:
Volatility of A = pA / xA
Volatility of B = pB / xB
where p is the partial pressure of the component and x is
the liquid mole fraction.
• Relative volatility is the ratio of volatility of A (MVC) over
volatility of B (LVC) and a measure of separability of A and B.
Special Case: Constant Relative
Volatility a
• If the value of a is known
and is constant, we can
use it to obtain the
equilibrium curve. This
can be done by
rearranging the equation
for relative volatility, to
obtain the function y = f(x)
or x = f(y).
SIMPLE MODELS FOR VAPORILIQUID
EQUILIBRIUM
• Raoult's Law
• The two major assumptions required to
reduce VLE calculations to Raoult's law are:
– The vapor phase is an ideal gas.
– The liquid phase is an ideal solution
Raoult's Law : Applicability
• it can be applied only to species for which a vapor
pressure is known, and this requires that the
species be "subcritical," i.e., that the temperature
of application be below the critical temperature
of the species.
• An important and useful feature of Raoult's law is
that it is valid for any species present at a mole
fraction approaching unity, provided only that the
vapor phase is an ideal gas. Chemical similarity of
the constituent species is not here a requirement.
Henry’s Law
• For a species present as a very dilute solute in
the liquid phase, that the partial pressure of
the species in the vapor phase is directly
proportional to its liquid-phase mole fraction.
Thus, according to Henry's
