Unit Operations: Introduction - PowerPoint PPT Presentation

Loading...

PPT – Unit Operations: Introduction PowerPoint presentation | free to download - id: 404830-OWU3M



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Unit Operations: Introduction

Description:

Unit Operations: Introduction What is chemical engineering? Chemical Engineering is a group of industrial processes in which row materials are changed or separated – PowerPoint PPT presentation

Number of Views:1795
Avg rating:3.0/5.0
Slides: 79
Provided by: lsar1
Learn more at: http://www.see.ed.ac.uk
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Unit Operations: Introduction


1
Unit Operations Introduction
What is chemical engineering? Chemical
Engineering is a group of industrial processes
in which row materials are changed or separated
into useful products Historical development
As the Industrial Revolution steamed along
certain basic chemicals quickly became necessary
to sustain growth - Example Sulfuric acid
was first among these "industrial chemicals".
Chemistry - to create a new substance - to
study its properties - to investigate all
possible pathways from one substance to
another
Chemical Engineering - to design the most
optimal technology for production of a
specified substance from row materials - to
develop and discover new technological
applications for materials
http//www.pafko.com/
2
Process flowsheet Example 1
3
Process flowsheet Example 2
4
Comparison of two processes
Units - Heaters/heat exchangers - Pumps -
Distillation units - Reactors -
Actions - Heat exchange - Material transport -
Separation - Mixing -
5
Unit Operations
- Unit Operations is a method of analysis and
design of chemical engineering processes in
terms of individual tasks/operations - It is a
way of organizing chemical engineering knowledge
into groups of individual tasks/operations -
A unit operation basic step in a chemical
engineering process
6
Unit Operations Classification
Fluid flow processes - fluid transport -
solids fluidization - mixing Heat transfer
processes - heating/cooling -
evaporation/condensation Mass transfer
processes - absorption - distillation -
extraction - adsorption - drying
Thermodynamic processes - liquifaction -
refrigeration Mechanical processes -
crushing - sieving - solid transportation
7
Section Lectures Tutorial
1. Introduction, revision, binary distillation with non-constant molar overflow
2. Separation of multicomponent mixtures
3. Separations in packed columns. Absorption
4. Adsorption processes
5. Humidification processes
6. Drying processes
7. Revision and problems
1.1 Introduction to Unit Operations Equilibrium stage operations (L1)
1.2 Thermodynamics of distillation (L2)
1.3 Binary distillations review (L3)
2.1 Multicomponent Distillation Flash distillation (L4)
2.2 Multicomponent Distillation Short Cut Methods (L5)
2.3 Multicomponent Distillation Short Cut Methods (L6)
3.1 Mass transport theories review (L7)
3.2 Mass transport theories review (L8)
3.3 Packed bed columns (L9)
3.4 Packed bed columns (L10)
4.1 Principles of adsorption (L11)
4.2 Principles of adsorption (L12)
5.1 Principles of humidification (L13)
5.2 Methods of humidification (L14)
6.1 Principles of drying (L15)
6.2 Methods of drying (L16)
7.1 Revision (L17)
7.2 Revision (L18)
http//www.see.ed.ac.uk/lsarkiso/UnitOps/syllabus
.htm
8
Total 18 lectures and at least 6 tutorials
Tutorials We will have a number of tutorials focusing on specific examples of unit operations.
Assessment Unit operations 1.5h exam. 2 questions
Text books 1) Warren L. McCabe, Julian C. Smith and Peter Harriot, Unit Operations of Chemical Engineering, (Seventh Edition). McGrawHill, 2005. 2) Robert E. Treybal, Mass Transfer Operations (McGraw-Hill Classic Textbook Reissue Series) (Paperback) 3) J.D. Seader and Ernest J. Henley, Separation Process Principles, John Wiley Sons, 1998.
9
Chemical separation processes required
background
- How do we know that at pressure P and
temperature T, vapour and liquid phase are
present in the system? - What is the composition
of the phases?
Chemical engineering thermodynamics
- How do we know the amount of mass exchanged
by two phases? - What is the new composition of
the phases?
Mass transfer methods
10
Chemical separation processes
- play a central role in chemical engineering
11
Chemical separation processes
Objective take a mixture of components and
produce one or more products with desired
composition/purity
n1
- A and B an be somehow different - boiling
points - size - polarity - etc.
A
B
AB
nF
B
split factor
A
n2
split ratio
separation power
12
Distillation process design
- Separation utilizes the difference in
volatility of components
Step 1 Thermodynamics data and methods to
predict equilibrium phase
compositions
13
Thermodynamic considerations and
phase equilibria Binary fluids
T
Tb(B)
V
L
Tb(A)
y
x
xA
14
Thermodynamic considerations and
phase equilibria Binary fluids
V
L
T, P
Pconst
15
Thermodynamic considerations and phase equilibria
T
V
Tb(B)
Tb(A)
L
y
xA
x
Pconst
16
Thermodynamic considerations and phase equilibria
Distribution of a component among the two phases
can be characterized with a K-value
T
V
Tb(B)
KA yA/xA
KB yB/xB (1-yA)/(1-xA)
Tb(A)
L
Relative volatility
y
xA
x
17
Thermodynamic considerations and
phase equilibria Binary fluids
Lets consider a binary mixture AB, where B is a
heavy component (high boiling point) and a is a
light component (low boiling point). A T-x phase
diagram of AB mixture, where x is a mole
fraction of component a might look like this at
some constant pressure P. This phase diagram can
be also transformed in y-x diagram where
composition of vapour phase in terms of
mole fraction of component A is plotted as
function of the liquid phase composition.
18
Thermodynamic considerations and
phase equilibria Binary fluids
P4
P1ltP2ltP3ltP4
T
P3
Tb(B)
P2
P1
Tb(A)
xA
19
Thermodynamic data for mixtures
- graphs (T-y-x, P-y-x, y-x), tables (usually
limited to binary mixtures) - K-values,
relative volatilities DePriester charts -
Analytical methods (part of most chemical process
design software) ASPEN Tech See a brief
Thermodynamics of multicomponent phase
equilibria file Promax - Simplified
models Ideal gas/Ideal solution
20
Thermodynamic considerations and
phase equilibria multicomponent mixtures
For multicomponent mixtures simple graphical
representations of vapour-liquid equilibria data
do not exist Most often such data (including
binary systems) is represented in terms of K
values defined as correlated empirically or
theoretically in terms of temperature pressure
and composition The ratio of two K-values, or
relative volatility, indicates the relative
ease or difficulty of separating components i and
j
Ki yi/xi
21
Light hydrocarbon mixtures DePriester
charts (1953)
22
Light hydrocarbon mixtures DePriester
charts (1953)
23
Thermodynamic data for mixtures Simplified
models
V
Raoults law (Ideal solution/ideal gas)
pi is the partial pressure of component i
L
T, P
Daltons law (Ideal gas)
K-value for ideal gas/ideal solution system
Relative volatility for ideal gas/ideal solution
system
Antoine equation
24
Thermodynamic calculations using K-values
P
  • Bubble point
  • Dew point
  • Two phase systems
  • Given P, T, V/(VL), find
  • x, y
  • Given P, T, x, y , find
  • V/(VL)

T
V
Tb(B)
Tb(A)
L
y
x
xA
Easy for 2 component system, if T-x-y diagram is
available (remember the lever rule?) What about
the multi-component system?
25
Thermodynamic calculations using K-values
Bubble point
- Model system binary mixture A, B - Consider
the process in the figure we start with a
mixture of composition 1 and temperature T1 and
start increasing the temperature - As we
increase the temperature we are going to reach
a point where the first bubble forms - The
vapour in this bubble obeys - On the other
hand - Thus as we increase the temperature we
put new K-values in the above equation until
this condition is met
1
26
Thermodynamic calculations using K-values
Bubble point
Procedure a) Select T b) Ki(T) c) d)
if T is too high e)
Adjusting T g) Final composition can be
corrected using
27
Thermodynamic calculations using K-values
Dew point
- Model system binary mixture A, B - Consider
the process in the figure we start with a
mixture of composition 1 and temperature T1 and
start decreasing the temperature - As we
decrease the temperature we are going to reach
a point where the first drop of liquid forms -
The liquid in the droplet obeys - On the
other hand - Thus as we decrease the
temperature we put new K-values the above
equation until this condition is met
1
28
Thermodynamic calculations using K-values
Dew point
Procedure a) Select T b) Ki(T) c) d)
if T is too low e)
Adjusting T g) Final composition can be
corrected using
29
Two phase system
  • Given the overall composition,
  • How do you know that you a 2-phase system?
  • How much vapour do you have per mole of the
    system?
  • What is the composition of the vapour and liquid
    phases?

30
Isothermal flash separation
  • - The liquid mixture is partially vaporized
  • in a boiler (or vapour condensed in a cooler)
  • How do you know that you a 2-phase system at a
    given T and P?
  • How much vapour did you form per mole of feed?
  • What is the composition of vapour and liquid
    phases?

31
Isothermal flash separation
Objective find D, B, and their compositions
32
Isothermal multicomponent flash separation
Objective find D, B, and their compositions
Rachford-Rice equation - drums, condensers,
reboilers etc
33
Isothermal multicomponent flash separation
Objective find D, B, and their compositions
Procedure
1) Check the feasibility of the process do you
have two phases in coexistence at given T,
P? a) all Ki gt 1 superheated vapour b) all
Ki lt 1 subcooled liquid c) some Kigt1 and some
Kilt1, then try
subcooled liquid
overheated vapour
34
Isothermal multicomponent flash separation
Objective find D, B, and their compositions
Procedure
2) solve for
a) Guessing
35
Isothermal multicomponent flash separation
Objective find D, B, and their compositions
Procedure
2) solve for
b) Newton-Raphson
36
Multicomponent flash separation (Adiabatic)
- Liquid feed is heated under pressure and then
adiabatically flashed through the pressure
reducing valve
37
Multicomponent flash separation (Adiabatic)
- Liquid feed is heated under pressure and then
adiabatically flashed through the pressure
reducing valve
38
Multicomponent flash separation (Adiabatic)
Objective find D, B, and their compositions
Procedure
1) Guess T1 2) Isothermal flash
procedure 3) Validate
not
39
Multicomponent flash separation (Adiabatic)
Objective find D, B, and their compositions
Procedure
1) Guess 2) Isothermal flash procedure find
temperature of the flash drum so that 3)
Validate
not
40
Distillation processes
Distillation is a process where a feed mixture of
two or more components is separated into
products, of compositions different from the
feed. This process takes advantage of the
differences in distribution of components between
the vapour and liquid phase.
F, zf
41
Distillation processes
The feed is material is introduced at one or more
points along the column. Liquid runs down the
column from tray to tray, where as vapour is
ascending along the column. At each tray vapour
and liquid contact and mix with each other
F, zf
42
Distillation processes
Liquid at the bottom of the column is partially
vaporized in a heated reboiler. The boil-up is
send back to the column. The rest is withdrawn
as bottoms, or bottoms product
F, zf
43
Distillation processes
Vapour at the top of the column is cooled and
condensed in the overhead condenser. Part of
this liquid is returned back to the column and
the rest is withdrawn as distillate or overhead
product
F, zf
44
Distillation processes
At each stage of the column two phases come in
contact with each other, mix, approach thermal
and composition equilibrium to the extent which
depends on the efficiency of the contact stage
F, zf
45
Definition of a stage in a process
A single stage is a device or a subunit of the
process, where two (or more) phases of a
different composition come in contact with each
other, exchange and leave with new compositions
- Mass balance Overall Components
- Energy balance
Q
46
Equilibrium stage processes
Streams leaving the stage are in
thermodynamic equilibrium with each other
Streams coming to the stage not in equilibrium
The idea is then to consider a hypothetical
column, composed of equilibrium stages This
idealistic design can be converted to the actual
design via analysis of tray efficiency
F, zf
47
Distillation processes
The lighter component tends to accumulate in the
vapour phase The heavier component tends to
accumulate in the liquid phase
F, zf
48
Distillation processes
In general, the overall separation process
depends on - relative volatilities - number
of contacting stages - ratio of liquid and
vapour flowrates
F, zf
49
Distillation processes
If the feed is introduced at one point, it
divides the column into a rectifying and
stripping sections But usually there are
multiple feed location and various side streams
F, zf
50
Distillation process design
Step 1 Thermodynamics data and methods to
predict equilibrium phase
compositions Step 2 Design of equilibrium stage
separation Design problem type 1 To
determine the number of equilibrium stages
required to accomplish the desired
separations Design problem type 2 Given a
particular column design, determine
separation that can be accomplished Step 3
Develop an actual design by applying the stage
efficiency analysis to equilibrium
stage design
51
Design of equilibrium stage distillation Binary
Mixtures Review
Va, ya
La, xaxd
condenser
Ln-1 xn-1
Overhead product D, xd
Vn yn
Ln xn
Vn1 yn1
F, zf
Lm-1 xm-1
Vm ym
Bottom product B, xb
Lm xm
Vm1 ym1
boiler
Vb, yb
Lb, xN
52
Operating lines
Va, ya
La, xaxdy1ya
condenser
Rectifying section
Ln-1 xn-1
Overhead product D, xd
RLa/D
Vn yn
Ln xn
Vn1 yn1
F, zf
Lm-1 xm-1
Vm ym
Bottom product B, xb
Lm xm
Vm1 ym1
boiler
Vb, yb
Lb, xN
53
Operating lines
This equation is a straight line (Vconst,
Lconst, L/Vconst) if - Two components have
similar and constant molar enthalpies of
vaporization (latent heats) - Component
sensible enthalpies changes and heats of mixing
are negligible (compared to latent heats) -
The column is well insulated (adiabatic) -
Pressure is uniform throughout the column
54
Operating lines
Reflux ratio
55
Operating lines
slopeR/(R1)
xD
56
Operating lines
Va, ya
La, xa
condenser
Ln-1 xn-1
Overhead product D, xd
RLa/D
Vn yn
Ln xn
Vn1 yn1
F, zf
Lm-1 xm-1
Vm ym
Bottom product B, xb
Lm xm
Stripping section
Vm1 ym1
boiler
Vb, yb
Lb, xN
57
Operating lines
slope
yB
xB
58
Operating line equation Stage-by-stage
Va, ya
La, xa
y
Plate 1
L1, x1
ya
Plate 2
Plate 3
xa
x1
x
59
Operating line equation Stage-by-stage
Va, ya
La, xa
y
Plate 1
L1, x1
ya
Plate 2
Plate 3
xa
x1
x
60
Operating line equation Stage-by-stage
Va, ya
La, xa
y
Plate 1
V2, y2
L1, x1
ya
Plate 2
Plate 3
Using the operating line equation we can
calculate y2 from x1. This step is depicted by
think green line in the graphs. This process can
be continued to calculated the number of
theoretical stages. This method of graphical
construction of theoretical stages is
called McCabe Thiele method
xa
x1
x
61
Feed stage considerations
62
Feed stage considerations
63
Feed stage considerations
q0
q1
F
qgt1
qlt0
0ltqlt1
64
Feed stage considerations
F
L
V
Feed line
65
qgt1
y
Feed line behavior
yx
q1
0ltqlt1
q0
xzf
xzf
qlt0
yzf
x
66
Complete picture
y1
zf
yB
zf
xB
xD
xN
67
Complete picture
68
Limiting cases
RL/D
slopeR/(R1)
xD
69
Limiting cases
RL/D
slopeR/(R1)
70
Total reflux
Total refluxMinimum number of stages
Total reflux
If RL/D 8 then R/(R1)1 also LV
71
Total reflux
Total refluxMinimum number of stages
Total reflux
If RL/D 8 then R/(R1)1 also LV
72
Minimum number of stages
a) Graphical methods
D, xD
RL/D
F, z
B, xB
b) Short cut methods Fenske Equation
73
Fenske Equation
Fenske equation
74
Limiting cases minimum reflux
RL/D
slopeR/(R1)
xD
75
Limiting cases minimum reflux
If we decrease the reflux ratio, then
RL/D
slopeR/(R1)
76
Limiting cases minimum reflux
If we decrease the reflux ratio, then
y1
zf
yB
zf
xB
xD
xN
77
Limiting cases minimum reflux
If we decrease the reflux ratio, then we are
arriving at a condition where both the
rectifying, stripping and feed line intersect at
the equilibrium line. In order for this
process to take place we need an infinite number
of plates
y1
zf
The minimum reflux ratio condition
yB
zf
xB
xD
xN
78
Limiting cases minimum reflux
y1
At this point xnx and yn1y
y
zf
yB
zf
xB
xD
xN
x
About PowerShow.com