IntroductionChEn 3603

Mass Transfer & Separations

Administrative Stuff

Class web page: www.che.utah.edu/~sutherland - bookmark this!

• contains syllabus, lecture notes, homework, etc.

Homework ThoughtsGetting “an answer” is not the goal.

• Critically thinking about the answer to get insight is the goal.

• Answers ⇒ insight & understanding.

• Computers are just tools to help us get insight. The means, not the end!

How you present your results is frequently as important as the result itself!

• Interpretation, with supporting data, is key & why engineers make $

• Plots must always be labelled.

• Numbers without units are dangerous (sometimes meaningless). ‣Mars rover

Learning takes time. • homework is a critical part of this process.

Key ChEn TechnologiesReactors (Converting chemicals into more valuable products) • kinetics, mixing (fluid dynamics, mass transfer),

heat transfer

Separations (purifying products) • thermodynamics, heat transfer, mass transfer

Process safety & control • What happens when one unit goes offline? ‣ surge capacity, fail-safe design

• Control the process to ensure that it continues to run efficiently ‣need to handle upsets in feedstock conditions,

environmental conditions, etc.

Working with Other DisciplinesChemists:

• Fundamental kinetics understanding

• thermophysical properties of chemicals

Materials scientists - creating materials that can handle: • high temperature environments

• corrosive environments

• high compressive or tensile forces

• …

Mechanical engineers • pumps, compressors, pressure vessels, tanks, pipes,

• mechanical separation for solids handlingBusinessmen: businesses are motivated by $$ • risk mitigation (lawsuits=$,

downtime=$, fines=$, bad press=$)

• just because we can do something doesn't mean we should.

• just because we can do something better doesn't mean we should.

• better for the environment isn’t necessarily better for bottom line.

Ethics & Safety - Everyone’s Responsibility!

Just because it doesn't make sense economically doesn't mean we should ignore it. Many problems occur on the "border" of disciplines (e.g. Mech. Eng. / Chem. Eng.) East Cleveland, Ohio, October 20, 1944

• 2:30 PM: Liquified Natural Gas storage tank begins leaking vapor from a faulty seam (poor design)

• NG vapor mixes with sewer gas & ignites, blowing manhole covers skyward & shooting flames

• 3:00 PM: a second tank ruptures & explodes • 130 people killed, 1 square mile destroyed

Hindenburgh & R101 Airships • Hindenburgh: 35/97 killed. R101: 48/54 killed. • large flammability limits, storage challenges (easily

diffuses through containers, makes steel brittle, etc.)

• a hydrogen economy???

Cleveland, Ohio, Friday, October 20, 1944

Bhopal, India (December 1984)Pesticide plant, with methyl isocyanate (toxic gas) as an intermediate, stored in "surge" tanks Plant located in very close proximity to shanty towns (slums) in India. Systems operating out of design envelope (unsafe). Employees tried to act: • "No maintenance supervisor was placed on the night shift and instrument readings were taken

every two hours, rather than the previous and required one-hour readings. Workers made complaints about the cuts through their union but were ignored. One employee was fired after going on a 15-day hunger strike. 70% of the plant's employees were fined before the disaster for refusing to deviate from the proper safety regulations under pressure from the management." (from wikipedia)

Methyl isocyanate somehow mixed with water, causing a runaway exothermic reaction. This was further exacerbated by contaminants including iron oxide from rusty vessels. Relief valves vent toxic gas, which settles to the ground • Toxic gas leak results in ~8,000 deaths and over 500,000 injuries.

• stillbirth rate increases 300x

PEPCON (Henderson Nevada, May 4, 1988)

Manufactures Ammonium Perchlorate • ingredient in solid rocket motors.

• Lots of extra on hand after the space shuttle Challenger disaster, not enough storage capacity for surplus.

• Stored surplus in plastic bins in a parking lot.

Welding accident led to a building fire, which spread to the plastic drums. • Subsequent explosions killed two

people.

Marshmallow factory close by (roasted marshmallows, anyone?)

Key Chemical Engineering Process “Ingredients”

Reactors • convert some feedstock into more

valuable products

• vary operating conditions to improve product yield, reduce operating cost, etc.

Separation Units • often precede and/or follow a reactor.

• improve purity of a product

• remove “contaminants” that could foul downstream processes

• “unmix” a feed stream to use its constituents better

ChEn 3553 Chemical Reaction Engineering

ChEn 3603 ( ChEn 3853 - thermodynamics,

ChEn 3353 - fluid dynamics, ChEn 3453 - heat transfer )

Process design (senior-level) ties everything together.

Overview of Common Separation Techniques

Factors to consider when choosing a separation technique Molecular properties Thermo. & Transport Properties

• Diffusivity • Vapor pressure

• Solubility • Adsorptivity

• Molecular weight • Molecular shape • Electric charge

• Van der Waals volume • Van der Waals area • Dipole moment

• Dielectric constant • Radius of gyration

• DistillationPhase Creation

Phase 1

Feed

Phase 2

• Absorption/Stripping • Liquid-Liquid Extraction

Barrier

Phase 1

Feed

Phase 2

• Membranes

Force Field or Gradient

Phase 1

Feed

Phase 2

• Electrophoresis • Centrifugation

Solid Agent

Phase 1

Feed

Phase 2

• Adsorption

Phase Addition

Phase 1

Feed

Phase 2MSA

(absorbant)

Term

inol

ogy

• adsorption - adhesion of molecules to a surface

• absorption - molecules enter a phase volume

Note:

These show mass flow. Most separation processes also involve significant energy

input to the system!

SHR §1.2-1.6

PhaseCreation Example: Distillation

Distillation: Terminology & Key Features

condenser (condenses vapor from top of column)

V/L

L

L

reflux drum (holds tops reflux to recycle

back to top of column)

bottoms product (heavies)

overhead (tops/lights) product

reboiler (vaporizes bottoms to feed

back into bottom of column)

feed (to be separated)

thermal energy out

thermal energy in

mechanical energy in

mechanical energy in

mechanical energy in

mechanical energy in

Distillation Example

Molar Flow Rate, lbmol/h1 2 3 4 5 6 7

Component Feed to C1 C5 rich Feed to C2 C3 Feed to C3 iC4 nC4-richC2H6 0.6 0 0.6 0.6 0 0 0C3H8 57 0 57 54.8 2.2 2.2 0iC4H10 171.8 0.1 171.7 0.6 171.1 162.5 8.6nC4H10 227.3 0.7 226.6 0 226.6 10.8 215.8iC5H12 40 11.9 28.1 0 28.1 0 28.1nC5H12 33.6 16.1 17.5 0 17.5 0 17.5

C6+ 205.3 205.3 0 0 0 0 0Total 735.6 234.1 501.5 56 445.5 175.5 270

SHR Figure 1.8

Variations on the Theme

Distillation with partial condenser

L

V

L

MSA

Stripping

V

V

L

MSA

Absorption

V/L

V

L