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Chemical Processes. What is Engineering? July 25, 2007. Chemical Processes Outline. Motivations Reactions Separations Calculations using Conservation of Mass and Energy Distillation. Chemists Design reaction pathways to produce a chemical from raw materials

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Chemical Processes

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Chemical Processes

What is Engineering?

July 25, 2007

Chemical Processes Outline

  • Motivations

  • Reactions

  • Separations

  • Calculations using Conservation of Mass and Energy

  • Distillation


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 chemist’s process to mass produce the product

Work in a chemical plant to produce material in the ton and beyond range

Chemists vs Chemical Engineers

Why do we care about Chemical Engineering?

Chemicals Are All Around







Food additives


Decaffeinated Coffee






If that isn’t reason enough

  • In the United States

    • 170 Major Chemical Companies

    • $400 Billion a year

    • Employs more than a million workers

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

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

Chemical Reactions



Raw Materials




Raw Materials



Possible Problem with Exothermic Reactions


Energy Produced by reaction is proportional to reactor volume L3


Energy Removed is proportional to surface area L2


Possible Scale up Problem

Water Bath

Molecular Property

Boiling Point

Freezing Point

Particle size

Affinity to a stationary phase


Selective affinity to solid particles

Separation Process








Exploits Differences of Material Properties

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

Mass and Energy Balances

Balance Equation

Input + generation – Output = Accumulation

Control Volume

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

Separations Calculation

V moles

40% C2H5OH

100 moles

10% C2H5OH

90% H2O




80 moles

x % C2H5OH

Separation Calculation

V moles

40% C2H5OH




100 moles

10% C2H5OH

90% H2O

80 moles

x % C2H5OH

Conservation of total Moles 100 – (V+80) = 0

V =20

Conservation of moles of C2H5OH 100*.1 – (.4*V+x*80) = 0

x = 2.5%




Separations: Distillation

(Distillation Column)

Equilibrium Stages


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?


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


Represent vapor liquid equilibrium data for more volatile component in an x-vs-y graph

Pressure constant, but temperature is changing!

Distillation: McCabe-Thiele Calculation

Calculation of theoretical number of equilibrium stages


Operating Line



Distillation: McCabe-Thiele


Applicable for many liquid systems

Technology is well developed

High Throughput


  • Drawbacks

    • High heating and cooling costs

    • Azeotropes


Separations limitation

Due to molecular interactions. Composition of vapor equal to composition of liquid mixture.


Batch distillation apparatus – only one equilibrium stage!


  • 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

Today’s Laboratory

  • Three Parts:

    • Energy Transfer

    • Chromatography

    • Batch Distillation

      • (One equilibrium stage)

Today’s 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?

Today’s 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.

Today’s 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.

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).

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 Henry’s Law: yA1 = H xA1 , where H depends on the substances A, B, C.

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 Henry’s constant H = 4?


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)



Rectifying operating line




y-int ~ 0.36

Stripping operating line



Nideal = 6 2/3

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