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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/

Process flowsheet Example 1

Process flowsheet Example 2

Comparison of two processes

Units - Heaters/heat exchangers - Pumps -

Distillation units - Reactors -

Actions - Heat exchange - Material transport -

Separation - Mixing -

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

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

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

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.

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

Chemical separation processes

- play a central role in chemical engineering

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

Distillation process design

- Separation utilizes the difference in

volatility of components

Step 1 Thermodynamics data and methods to

predict equilibrium phase

compositions

Thermodynamic considerations and

phase equilibria Binary fluids

T

Tb(B)

V

L

Tb(A)

y

x

xA

Thermodynamic considerations and

phase equilibria Binary fluids

V

L

T, P

Pconst

Thermodynamic considerations and phase equilibria

T

V

Tb(B)

Tb(A)

L

y

xA

x

Pconst

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

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.

Thermodynamic considerations and

phase equilibria Binary fluids

P4

P1ltP2ltP3ltP4

T

P3

Tb(B)

P2

P1

Tb(A)

xA

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

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

Light hydrocarbon mixtures DePriester

charts (1953)

Light hydrocarbon mixtures DePriester

charts (1953)

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

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?

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

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

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

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

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?

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?

Isothermal flash separation

Objective find D, B, and their compositions

Isothermal multicomponent flash separation

Objective find D, B, and their compositions

Rachford-Rice equation - drums, condensers,

reboilers etc

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

Isothermal multicomponent flash separation

Objective find D, B, and their compositions

Procedure

2) solve for

a) Guessing

Isothermal multicomponent flash separation

Objective find D, B, and their compositions

Procedure

2) solve for

b) Newton-Raphson

Multicomponent flash separation (Adiabatic)

- Liquid feed is heated under pressure and then

adiabatically flashed through the pressure

reducing valve

Multicomponent flash separation (Adiabatic)

- Liquid feed is heated under pressure and then

adiabatically flashed through the pressure

reducing valve

Multicomponent flash separation (Adiabatic)

Objective find D, B, and their compositions

Procedure

1) Guess T1 2) Isothermal flash

procedure 3) Validate

not

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

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

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

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

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

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

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

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

Distillation processes

The lighter component tends to accumulate in the

vapour phase The heavier component tends to

accumulate in the liquid phase

F, zf

Distillation processes

In general, the overall separation process

depends on - relative volatilities - number

of contacting stages - ratio of liquid and

vapour flowrates

F, zf

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

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

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

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

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

Operating lines

Reflux ratio

Operating lines

slopeR/(R1)

xD

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

Operating lines

slope

yB

xB

Operating line equation Stage-by-stage

Va, ya

La, xa

y

Plate 1

L1, x1

ya

Plate 2

Plate 3

xa

x1

x

Operating line equation Stage-by-stage

Va, ya

La, xa

y

Plate 1

L1, x1

ya

Plate 2

Plate 3

xa

x1

x

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

Feed stage considerations

Feed stage considerations

Feed stage considerations

q0

q1

F

qgt1

qlt0

0ltqlt1

Feed stage considerations

F

L

V

Feed line

qgt1

y

Feed line behavior

yx

q1

0ltqlt1

q0

xzf

xzf

qlt0

yzf

x

Complete picture

y1

zf

yB

zf

xB

xD

xN

Complete picture

Limiting cases

RL/D

slopeR/(R1)

xD

Limiting cases

RL/D

slopeR/(R1)

Total reflux

Total refluxMinimum number of stages

Total reflux

If RL/D 8 then R/(R1)1 also LV

Total reflux

Total refluxMinimum number of stages

Total reflux

If RL/D 8 then R/(R1)1 also LV

Minimum number of stages

a) Graphical methods

D, xD

RL/D

F, z

B, xB

b) Short cut methods Fenske Equation

Fenske Equation

Fenske equation

Limiting cases minimum reflux

RL/D

slopeR/(R1)

xD

Limiting cases minimum reflux

If we decrease the reflux ratio, then

RL/D

slopeR/(R1)

Limiting cases minimum reflux

If we decrease the reflux ratio, then

y1

zf

yB

zf

xB

xD

xN

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

Limiting cases minimum reflux

y1

At this point xnx and yn1y

y

zf

yB

zf

xB

xD

xN

x