Chapter 1 Introduction
1 Scope of the course
2. Basic Concepts
Chemical processing industries ( Process, foods, petrochemicals etc… industries)
deal mainly with the transfer and change of energy and the transfer and change of
materials primary by physical means but also by the physical-chemical means, known
as ‘unit operations’. There exist several unit operations.
- Fluid flow : concerns principles that determine the flow or transportation of
any fluid from one point to another
- Heat transfer; this operation deals with principles that govern accumulation
and transfer of heat and energy from one place to another
- Evaporation
- Drying
Some of these units have certain fundamentals and basic principles in common, eg,
diffusion, mass transfer (drying, adsorption, distillation), heat transfer (drying,
distillation, evaporation etc…). Therefore there exists a classification more
fundamental based on transfer or transport processes, which is as follows
a) Momentum transfer: This is concerned with the transfer of momentum which
occurs in moving media ( fluid flow, sedimentation, mixing)
b) Heat transfer concerned with the transfer of heat from one phase to another.
c) Mass transfer, where mass is transferred from one phase to another distinct
phase.
3. Introduction to Momentum transfer
The flow and behavior of fluids is important in many of the unit operations used in
process engineering. A fluid may be defined as a substance that does not permanently
resist distortion and will change its shape. Usually gases, liquids, vapors are
considered to have characteristics of fluids and obey many of the same laws.
In process industries, many of the materials are in fluid form and must be stored,
handled, pumped and processed, so it is necessary to know principles that govern
fluid flows
In momentum transfer, we treat the fluid as a continuous distribution of matter
(statistical averages of properties are valid).
The study of momentum transfer or fluid mechanics can be divided into two branches
a) Fluid static or fluid at rest
b) Fluid dynamics or fluids in motion
If a fluid is not affected by changes in pressures, it is said to be incompressible
(liquids).Gas are considered to be compressible fluids. However, in small changes in
pressures and temperatures, gas can be considered incompressible.
A fluid is composed of a large number of molecules per unit volume. In engineering,
we are mainly concerned with the bulk or macroscopic behavior of fluid rather than
individual molecular or microscopic behavior.
4 Notions of temperatures, compositions, ideal gas
4.1 Temperatures
There are two temperatures scales in common use in chemical engineering. These
are Fahrenheit (F) and Celsius ( C). It is often necessary to convert from one scale to
the other. Both use the freezing point and boiling point of water at 1 atmosphere
pressure as base points. Often temperatures are expressed as absolute degrees (K) or
degrees (R).Table shows the equivalences of the four temperatures scales.
Centigrade
Boiling water
Melting ice
Absolute Zero
100 C
0C
-273.15C
Fahrenheit
212 F
32F
-459.7F
Kelvin
313.15K
273.15K
0K
Rankine
671.7R
491.7R
0R
Celsius
100C
0C
-273.15C
The following equations can be used to convert from one scale to another
F= 32 + 1.8 ( C)
C = 1/1.8 (F – 32)
R = F + 460
K = C + 273.15
4.2 Compositions
a) Moles fractions
Mixtures of A, B
Nt= Na + Nb
Nt= Total number in the mixture
Na: Number of moles of A
Nb: Number of moles of B
Xa = Na/Nt
Xa: Mole fraction of A in the mixture
b) Mass or weight fraction
Mt = Ma + Mb (Kg)
Mt= Total mass of mixture
Ma= mass of A (Kg)
Mb= mass of B (Kg)
wa = Ma/Mt
wa: mass fraction of A
c) Concentration
- When liquid is mixed with another liquid, the volumes are not additives; the
composition is expressed as weight or mole percent (gmole/liter or
lbmole/cuft).
- Most common method of expressing total concentration/unit volume is density
(kg/m3, lb/ft3, etc…)
4.3 Ideal Gas law
- Gas obey simple laws, and can be considered as ideal gas.
- Ideal gas are supposed to obey the following conditions: made of rigid
spheres, occupy no volume, do not exert force on one another)
- No real gases obey these laws exactly.
- But we can suppose that these laws are sufficient accurate for engineering
problems.
Ideal Gas law is expressed as follows,
PV = n R t
P= absolute pressure (N/m2)
T= absolute temperature (K)
V= Volume of the gas (m3)
n= Kg moles of the gas
R= Gas law constant = 8314.3 kg-m3/kg mole –s
Standard conditions: P=1atm= 101.325kPa, T=273K, V=22.4m3/kgmole
4.4 Ideal gas mixture
Dalton’s law for mixtures of ideal gas
Total Pressure = Pa + Pb + Pc = Pt
Where P is total pressure and Pa, Pb, Pc are the partial pressures of the
components A,B,C in the mixture.
Since the number of moles of a component is proportional to its partial
pressure, the mole fraction of a component is
Xa = Pa/P = Pa/ (Pa+ Pb + Pc)
The volume fraction is equal to the mole fraction.
Gas mixtures are almost always represented in terms of mole fraction and not
weight fractions.
For engineering purposes, Dalton’s law is sufficiently accurate for actual
mixtures and at total pressures of a few atmospheres or less.
Pa= Partial Pressure of A