Chemical Processes
What is Engineering?
July 8, 2009
Chemical Engineering
Look around you – nearly everything you see has parts designed by
chemical engineers!
Dyes
Hydrogen
Toothpaste
Gasoline
Fertilizer
Decaffeinated
Coffee
Paint
Shampoo
Soap
Cosmetics
Food
additives
Polymers
Sugar
Pharmaceuticals
Chemical Engineering
Chemists vs Chemical Engineers:
Chemists determine
reactions to make
new compounds in
a test tube.
Chemical Engineers
design processes to
make compounds at
a rate of 1000 L/min
that are efficient and
don’t explode.
Chemical engineers have backgrounds in
chemistry AND fluid dynamics, heat
transfer, materials science…
Chemical Engineering
If that isn’t reason enough
• In the United States:
– 170 Major Chemical
Companies
– $400 Billion a year
– Employs more than a
million workers
http://money.cnn.com/2006/02/13/pf/college/starting_salaries/index.htm
Chemicals: Raw Materials
• Small and Simple
Helium (He)
Ammonia (NH3)
Hydrogen Flouride (HF)
Trinitrotoluene (C6H2(NO2)3CH3)
• Large and Complicated
Insulin C257H383N65O77S6
• Large and Simple
Polyvinyl Chloride (-CH2-CHCl-)n
Chemicals: Measure in Moles
Moles are the SI unit for the amount of a substance.
Formally, amount of a substance that contains the same number of entities as
there are atoms in exactly 12 grams of Carbon-12 (6.022 x 1023).
Converting mass percent to mole percent:
Convert 20% C2H5OH, 80% H2O by mass into mole percents
Efficient Engineering
$ is the most important parameter in engineering!
Consider sulfuric acid production.
In 1997, 160 million tons consumed; cost $8 billion to
produce.
2S + 3O2 + 2H2O  2H2SO4
If improve efficiency of process by 1% (better
mixing, improved reactor design, etc), save $80
million.
Chemical Engineering: Unit Operations
Chemical Engineering: Two Processes
Chemical Engineering
Reactions
Separations
(Producing chemicals on from
raw materials on large scale)
(Producing chemicals by
isolating from a mixture)
Need to understand:
• Transport (flow & mixing of molecules)
• Thermodynamics (energy & heat)
• Material & Energy balances (conservation laws)
Material and Energy Balances
Balance Equation:
Input + Generation – Output = Accumulation
Control
Volume
Material and Energy Balances
• For non-reacting systems Generation = 0
• For systems operated at steady state (flow
rate in = flow rate out): Accumulation = 0
Balance equation reduces to: Input = Output
Material Balances
How quantify what happening in a chemical process.
We can solve for the flow rates and the mole fractions in the exit
stream
100 moles/min
5% C2H5OH
Mixing Process:
40% CH3OH
No reactions –
45% H2O
No accumulation
20 moles/min
? moles/min
?% C2H5OH
?% CH3OH
?% H2O
40% C2H5OH
60% H2O
mole fraction of i in a liquid stream: xi
mole fraction of i in a vapor (gas) stream: yi
Material Balances
What are the input streams and output streams?
How do we lose weight?
Reaction Engineering
Create desired compounds from raw materials via chemical reactions.
Raw Materials
Reactor
Energy
Products
Raw Materials
Byproducts
Catalysts
Energy
Catalysts
Cost considerations
• Byproducts:
• Could be expensive to dispose of
• Could be valuable – sell in addition to product
• Purifying product
Reaction Engineering
Digestive tract reactions
Catalytic Converter
Reaction Engineering
Possible Problem with Exothermic Reactions
L
Reactor
A+B->C
Water Bath
Energy Produced by
reaction is proportional to
reactor volume L3
Energy Removed is
proportional to surface
area L2
Possible Scale up Problem
Making Epoxy
Separation Problem
We have a ton of material composed
of sawdust, iron filings, 1” diameter
marbles, 2” marbles diameter, and
salt crystals.
Design a process to separate these
components into pure form.
Separations Engineering
Produce desired product by isolating it from a mixture.
Exploit differential properties of mixture components.
• Molecular Property
–
–
–
–
Boiling Point
Freezing Point
Particle size
Affinity to a stationary
phase
– Density
– Selective affinity to
solid particles
• Separation Process
–
–
–
–
–
–
–
–
Evaporation
Distillation
Crystallization
Extraction
Filtration
Chromatography
Centrifuge
Absorption
Boiling/Freezing Points
Temperatures at which substances change phase
Boiling: liquid  vapor
Freezing: liquid  solid
Depends on pressure!
@ atmospheric pressure (1 atm):
• Tb (H2O) = 100 °C
• Tf (H2O) = 0 °C
Density
How closely together atoms are packed together.
Mass

Volume
Liquid and solid atoms are packed together ~1000x vapor
atoms!
vs.
fluid
vapor
Volatility
Tendency for atoms or molecules in a liquid to evaporate and become
a vapor.
Always have atoms or molecules leaving liquid phase for
vapor phase and vice versa.
When rate of liquid  vapor
equilibrium
equals
vapor  liquid, system at
Mixtures of different volatilities
yi ≠ xi
(more volatile)
(less volatile)
Surface Tension
Force needed to stretch a film of atoms/molecules on surface of a
liquid.
Surface
Spheres minimize
surface area,
maximize molecular
interactions
Viscosity
Measure of how easy it is to stir a substance
Harder to stir, more viscous,
more energy needed to
move substance
mass
g
 ()

 centipoise
lt
cm s
 water  1 cp
Increasing viscosity 
Molecular Shape
Size, shape, polarization
 Determines molecule behavior (boiling point, viscosity, etc)
because of effect on intermolecular interactions
Solubility
Ability of one substance to dissolve into another
Can be:
• Solids into liquids (sugar into iced tea)
• Gases into liquids (CO2 into water – carbonated beverages)
• Liquids into liquids (oil not dissolving into water)
Solubility in 100 grams of water at 1 atm
Substance
Solubility (grams)
O2
0.0043
NH3
53
Salt
36
Separations Engineering
Produce desired product by isolating it from a mixture.
Exploits Differences of Material Properties of mixture’s components
• Molecular Property
–
–
–
–
Boiling Point
Freezing Point
Particle size
Affinity to a stationary
phase
– Density
– Selective affinity to
solid particles
• Separation Process
–
–
–
–
–
–
–
Distillation
Evaporation
Extraction
Filtration
Chromatography
Centrifuge
Absorption
Distillation
Utilizes differences in boiling points in liquids.
“Work horse” of chemical engineering.
Used extensively in petroleum industry to refine crude oil.
Consider liquid mixture of ethanol (Tb = 78.5 °C) and water (Tb = 100 °C):
Vapor (Top Product)
yEtOH, t
Liquid (Feed Mixture)
xEtOH,f
xH2O,f
Distillation
Column
yH2O, t
T = 90 °C
Liquid (Bottom Product)
xEtOH, b
xH2O, b
xi = liquid mole faction
yi = vapor mole fraction
Ethanol more volatile, therefore: yEtOH, t > xEtOH, f > xEtOH,b
yH2O, t < xH2O, f< xH2O, b
Distillation: McCabe Thiele Diagram
Represent vapor liquid equilibrium data for more volatile
component in an x-vs-y graph (McCabe Thiele Diagram):
(from experimental data)
Reference line
(y = x)
Pressure constant, but temperature is changing!
Distillation: McCabe Thiele Diagram
y1
*x
F
Distillation
(Distillation Column)
Separator
Engineering Design Problem:
How many stages needed in order
to get the desired concentration of
the volatile component at the top
of the column?
Equilibrium Stages – have
vapor and liquid phases in
equilibrium with each other
(In most cases, yi ≠ xi)
Distillation: McCabe Thiele Diagram
N=5
Distillation
• Benefits
– Applicable for
many liquid
systems
– Technology is well
developed
– High Throughput
• Drawbacks
– High heating and
cooling costs
– Azeotropes
Distillation: Azeotrope
Separations limitation
Azeotrope
Due to molecular interactions. Composition of vapor
equal to composition of liquid mixture.
Batch Distillation
Batch distillation
apparatus – only one
equilibrium stage!
Batch Distillation
Evaporation
Utilizes differences in volatilities in a mixture.
Desired product is the less volatile one.
Remove other components from mixture by vaporization.
Example: evaporation of sea water (undesired) to get salt
(desired).
Sometimes add volatile
components to give a temporary
property.
When no longer needed, volatile
component evaporates.
Gas Absorption/Desorption
Utilizes differences in solubility of components.
Transfer of a vapor to/from a liquid from a gas phase.
Absorption Example:
Burn coal, sulfur is a byproduct don’t want entering
atmosphere.
Pass vapor exhaust stream through liquid water, SO2
absorbs and is removed from vapor stream.
Called “scrubbing”
Extraction
Also utilizes differences in solubility of components.
Use one liquid to remove a component from another liquid.
Extracted component must be
soluble in both liquids!
Two liquids being transferred
between must be immiscible (oil &
water)!
More dense
liquid with
desired
product
Less dense liquid
with higher
solubility for
desired product
Solvent +
desired
Filtration
Utilizes differences in physical properties (size and shape)
Separating a solid from a liquid or a gas.
Example: Passing spaghetti and water through a
colander to get just spaghetti (desired product)
Chromatography
Utilizes varying degrees of affinity for a solvent and chromatography material.
Good for biology applications because is a “gentle” process (no high
temperatures or harsh chemicals)
Consider paper chromatography:
Solvent & solute travel up
absorbent paper.
Molecules that have a higher
affinity for solvent will travel
further up paper!
Impossible Separation #1
• Separating Uranium 235 from Uranium 238
Impossible Separation #2
• Separating (R)Thalidomide from (L)Thalidomide
Molecule has exact same boiling point, density,
freezing point, and only is slightly different in
geometry
Conclusions
• Chemicals are produced by reactions or
separations
• The driving force for separations of mixtures are
differences in component properties
• Systems can be analyzed by mass and energy
balances
• Distillation is the workhorse of separations
Chemical Processes Laboratory
• Three Parts:
– Energy Transfer
– Chromatography
– Batch Distillation
• (One equilibrium stage)
Energy Transfer
Want efficient transfer and conversion of energy ($$)
In lab, will be examining energy transfer in the form of
heat: warming a pot of water with a hot plate –
what is the efficiency of energy transport from
electricity to the water?
Chromatography
• Separation technique that takes advantage of
varying affinities of solutes for a given solvent
traveling up a filter paper.
– Solutes: colored dyes
– Solvents: water, methanol, 2-propanol
• Measure the distance traveled by the solutes
and solvents!
**Methanol and 2-propanol are poisons! Wear safety
goggles, do not ingest or inhale and rinse skin
immediately if spilled.
Distillation
• Using distillation to separate a liquid mixture of
ethanol and water
– Ethanol is the more volatile material (it will boil first)
• Take samples of distillate with time to determine
the concentration of ethanol in the mixture!
**Ethanol is a poison! Wear safety goggles, do not
ingest or inhale and rinse skin immediately if spilled.
Bilinear Interpolation
What is the mass fraction of alcohol at a temperature of 22C and a specific
gravity of 0.9820
%Alcohol by weight
%
10C
0
1
2
3
4
5
.99098
6
.98946
7
801
8
660
9
524
10
393
11
267
12
145
13
026
14
.97911
Temperature
15C
20C
0.99823
636
453
275
103
.99032 .98938
.98877 780
729
627
584
478
442
331
304
187
171
047
041
.97910
.97914 775
790
643
25C
0.99708
520
336
157
.98984
817
656
500
346
193
043
.97897
753
611
472
30C
0.99568
379
194
014
.98839
670
507
347
189
031
.97875
723
573
424
278
35C
0.99406
217
031
.98849
672
501
335
172
009
.97846
685
527
371
216
063
40C
0.99225
034
.98846
663
485
311
142
.97975
808
641
475
312
150
.96989
829