Chemical Processes

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Chemical Processes

• What is Engineering?
• July 25, 2007

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Chemical Processes Outline

• Motivations
• Reactions
• Separations
• Calculations using Conservation of Mass and
  Energy
• Distillation

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Chemists vs Chemical Engineers

• Chemists
• Design reaction pathways to produce a chemical
  from raw materials
• Work in the laboratory setting to produce
  material on the gram to kilogram scale

• Chemical Engineers
• Design a process to scale the chemists process
  to mass produce the product
• Work in a chemical plant to produce material in
  the ton and beyond range

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Why do we care about Chemical Engineering?
Chemicals Are All Around
Dyes
Hydrogen
Toothpaste
Shampoo
Gasoline
Food additives
Fertilizer
Soap
Decaffeinated Coffee
Cosmetics
Polymers
Sugar
Paint
Pharmaceuticals
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If that isnt 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/startin
g_salaries/index.htm
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Molecules that Chemicals Engineers work with

• 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

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How to Produce Chemicals

• Two methods to obtain a desired chemical
• Design a reactor to produce a chemical from raw
  materials
• To isolate the compound that exists in
  combination with other substances through
  separation processes

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Chemical Reactions
Reactor
Products Raw Materials Byproducts Energy Catalysts
Raw Materials Energy Catalysts
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Possible Problem with Exothermic Reactions
L
Energy Produced by reaction is proportional to
reactor volume L3
Reactor
Energy Removed is proportional to surface area
L2
AB-gtC
Possible Scale up Problem
Water Bath
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Separations
Exploits Differences of Material Properties

• Molecular Property
• Boiling Point
• Freezing Point
• Particle size
• Affinity to a stationary phase
• Density
• Selective affinity to solid particles

• Separation Process
• Distillation
• Crystallization
• Filtration
• Chromatography
• Centrifuge
• Adsorption

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Separations Unit Operations

• Use separation processes to
• Purify raw materials
• Purify products
• Purify and separate unreacted feed.
• Most common types
• Distillation
• Flash distillation
• Batch distillation
• Column distillation
• Absorption
• Stripping
• Extraction
• Chromatography

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Mass and Energy Balances

• Balance Equation
• Input generation Output Accumulation

Control Volume
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Mass and Energy Balances

• For non-reacting systems Generation 0
• For systems operated at steady state
  Accumulation 0
• Mass and Energy Balances reduce to
• Input Output

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Separations Calculation
V moles 40 C2H5OH
100 moles 10 C2H5OH 90 H2O
Magic Separating Machine
80 moles x C2H5OH
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Separation Calculation
V moles 40 C2H5OH
Magic Separating Machine
100 moles 10 C2H5OH 90 H2O
80 moles x C2H5OH
Conservation of total Moles 100 (V80) 0 V
20 Conservation of moles of C2H5OH 100.1
(.4Vx80) 0 x 2.5
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Separations Distillation
(Distillation Column)
Equilibrium Stages
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Distillation
Separates liquids based on differences in
volatility!
Consider a liquid mixture of A and B
Boiling point of A 70 C Boiling point of B 100 C
As mixture begins to boil, the vapor phase
becomes richer in A than the liquid phase!
Condense vapor phase to get a mixture with a
higher concentration of A!
As temperature increases, the concentration of B
in the vapor phase increases.
What would be the composition of the vapor phase
if the entire liquid mixture vaporized?
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Distillation
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Distillation Equilibrium Stages
A) Phases are brought into close contact B)
Components redistribute between phases to
equilibrium concentrations C) Phases are
separated carrying new component
concentrations D) Analysis based on mass balance

• L is a stream of one phase V is a stream of
  another phase.
• Use subscripts to identify stage of origination
  (for multiple stage problems)
• Total mass balance (mass/time) L0 V2 L1
  V1 M

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Distillation
Represent vapor liquid equilibrium data for more
volatile component in an x-vs-y graph
Pressure constant, but temperature is changing!
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Distillation McCabe-Thiele Calculation
Calculation of theoretical number of equilibrium
stages
xD
Operating Line
xF
xB
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Distillation McCabe-Thiele
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Distillation

• Benefits
• Applicable for many liquid systems
• Technology is well developed
• High Throughput

• Drawbacks
• High heating and cooling costs
• Azeotropes

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Azeotrope
Separations limitation
Due to molecular interactions. Composition of
vapor equal to composition of liquid mixture.
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Distillation
Batch distillation apparatus only one
equilibrium stage!
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Conclusions

• Chemicals are produced by reactions or
  separations
• The driving force for separations are property
  differences
• Mass and Energy are Conserved
• Distillation is the workhorse of separations

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Todays Laboratory

• Three Parts
• Energy Transfer
• Chromatography
• Batch Distillation
• (One equilibrium stage)

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Todays Laboratory 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?

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Todays Laboratory 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.

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Todays Laboratory 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.

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Assume three components A dye, B oil, C
water xA mass fraction of A in stream
L yA mass fraction of A in stream
V (e.g., L0 xA0 mass of component A in
stream L0 ) Component mass balance
(mass/time) L0 xA0 V2 yA2 L1 xA1 V1
yA1 M xAM L0 xC0 V2 yC2 L1 xC1 V1 yC1
M xCM (equation for B not necessary because
xA xB xC 1) Suppose the following V is
oil (B) contaminated with dye (A). L is water
(C) which is used to extract the dye from the
oil. When V comes in contact with L, the dye
redistributes itself between the V and L. L and
V are immiscible (i.e., two distinct liquid
phases).
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Oil flow V(1 - yA) V constant Water
flow L(1 - xA) L constant Then, for mass
balance of the A component Another assumption
dye concentrations yA1, xA1 come into
equilibrium according to Henrys Law yA1 H
xA1 , where H depends on the substances A, B, C.
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Specific problem 100kg/hr of dye-contaminated
oil (1 by weight) is mixed with 100 kg/hr of
water to reduce the dye concentration in the oil.
What is the resulting dye concentration in oil
after passing through the mixing stage if dye
equilibrium is attained and Henrys constant H
4? Soln L 100kg/hr V 100 (
1 - .01) 99 kg/hr xA0 0 (no dye in
incoming water) yA2 .01 (initial
contamination in oil) yA1 4 xA1 (equilibrium
concentration of dye between oil and water)
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xD
Rectifying operating line

zF
q-line
y-int 0.36
Stripping operating line

xB
Nideal 6 2/3