Separations

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Separations

• ChEN 4253 Design I
• Chapter 19
• Terry A. Ring
• University of Utah

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Simple Separation Units

• Flash
• Quench
• Liquid-liquid decantation
• Liquid-liquid Flash
• Sublimation
• Solid/Vapor Flash
• Crystallization
• Filtration

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Use of Separation Units
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Separation ReactionHydrodealkylation of
TolueneTH2??BCH4side reaction2B??
BiphenylH2
Reactor Effluent T1,350F P 500 psia
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Reactor Effluent
Reaction Conditions T1,350F P 500 psia
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After Flash to 100F _at_ 500 psia
Recycled Reactants
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Separation

• Vapor Separation
• CH4 from H2
• Liquid Separation

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Further SeparationWhat separation units should
be used?

• Liquid Separation
• Toluene, BP110.6ºC
• Benzene, BP80.1ºC
• What happens to the Methane (BP -161.5ºC) and
  Biphenyl (BP255.9ºC) impurities?
• Gas Separation
• Hydrogen
• Methane
• what happens to the Toluene and Benzene
  impurities?

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Direct Distillation Sequence
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Criteria for the Selection of a Separation Method

• Energy Separation Agent (ESA)
• Phase condition of feed
• Separation Factor
• Cost

• Mass Separation Agent (MSA)
• Phase condition of feed
• Choice of MSA Additive
• Separation Factor
• Regeneration of MSA
• Cost

Phases I and II, Components 1 and 2 (light key
and heavy key)
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Distillation
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Distillation
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Plate Types

• Bubble Cap Tray

• Sieve Tray

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Packed Towers

• Random Packing
• Structured Packing

Note Importance of Distributor plate
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Distillation
aKL/KH

• Relative Volatility
• Equilibrium Line

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Distillation

• Rectifying Section
• R reflux ratio
• Vvapor flow rate
• Stripping Section
• VB Boil-up ratio
• Feed Line

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Minimum Reflux Ratio
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McCabe-Thiele
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Step Off Equilibrium Trays
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Marginal Vapor Rate

• Marginal Annualized Cost Marginal Vapor Rate
• Marginal Annualized Cost proportional to
• Reboiler Duty (Operating Cost)
• Condenser Duty (Operating Cost)
• Reboiler Area (Capital Cost)
• Condenser Area (Capital Cost)
• Column Diameter (Capital Cost)
• Vapor Rate is proportional to all of the above

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Short cut to Selecting a Column Design

• Minimum Cost for Distillation Column will occur
  when you have a
• Minimum of Total Vapor Flow Rate for column
• Occurs at
• R 1.2 Rmin _at_ N/Nmin2 or see Fig 19.1
• VD (R1)
• V Vapor Flow Rate
• D Distillate Flow Rate (Production Rate)
• RReflux Ratio

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Figure 19.1
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How To Determine the Column Pressure given coolant

• Cooling Water Available at 90ºF
• Distillate Can be cooled to 120ºF min.
• Calculate the Bubble Pt. Pressure of Distillate
  Composition at 120ºF
• equals Distillate Pressure
• Bottoms Pressure Distillate Pressure 10 psia
  delta P
• Compute the Bubble Pt. Temp for an estimate of
  the Bottoms Composition at Distillate Pressure
• Give Bottoms Temperature
• Not Near Critical Point for mixture

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Design Issues

• Packing vs Trays
• Column Diameter from flooding consideration
• Trays, DT(4G)/((f Uflood p(1-Adown/AT)?G)1/2 e
  q. 19.11
• Uflood f(dimensionless density difference), f
  0.75-0.85 eq. 19.12
• Packed, DT (4G)/((f Uflood p?G)1/2 eq. 19.14
• Uflood f(flow ratio), f 0.75-0.85 eq. 19.15
• Column Height
• Nminlog(dLK/bLK)(bHK/dHK)/logaLK,HK
  Fenske eq.19.1
• NNmin/e (or 2 Nmin/ e)
• Column Height NHtray
• Tray Height typically 1 ft (or larger), 2 inch
  weir height
• Packed Height NeqHETP (or 2 NeqHETP)
• HETP(height equivalent of theoretical plate)
• HETPrandom 1.5 ft/inDp Rule of thumb eq.
  19.9
• Tray Efficiency, e f(viscosityliquid aLK,HK)
  Fig 19.3
• Pressure Drop
• Tray, ?P?Lg hL-wier N
• Packed, ?PPacked bed (weeping)

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Tray Efficiency
19.3
µL aLK,HK
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Costing
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Column Costs

• Column Material of Construction gives ?metal
• Pressure Vessel Cp FMCv(W)CPlatform
• Height may include the reboiler accumulator tank
• Tray Cost NCtray(DT)
• Packing Cost VpackingCpacking Cdistributors
• Reboiler CB a AreaHX
• Condenser CB a AreaHX
• Pumping Costs feed, reflux, reboiler
• Work Q?P
• Tanks
• Surge tank before column, reboiler accumulator,
  condensate accumulator
• Pressure Vessel Cp FMCv(W)CPlatform

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CPI
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Distillation Problems

• Multi-component Distillation
• Selection of Column Sequences
• Azeotropy
• Overcoming it to get pure products
• Heat Integration
• Decreasing the cost of separations

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Problem

• Methanol-Water Distillation
• Feed
• 10 gal/min
• 50/50 (mole) mixture
• Desired to get
• High Purity MeOH in D
• Pure Water in B

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Simulator Methods - Aspen

• Start with simple distillation method
• DSDTW or Distil
• Then go to more complicated one for sizing
  purposes
• RadFrac
• Sizing in RadFrac
• Costing

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Simulation Methods- ProMax

• Start with 10 trays (you may need up to 100 for
  some difficult separations)
• set ?P on column, reboiler, condenser and
  separator
• set ?T on condenser
• Create a component recovery for HK in bottom with
  large
• Set Reflux ratio 0.1 (increase to get
  simulation to run w/o errors).
• May need pump around loop estimate.
• Determine aLK,HK, viscosity
• (use Plots Tab to determine extra trays)
  determine Nmin and feed tray
• Use Fig. 19.1 to determine Rmin from R, N from
  Nmin
• Redo calc with tray efficiency defined see Figure
  19.3 correlation.
• Recommendations for final design
• Use N/Nmin2 (above and below feed tray)
• R/Rmin1.2

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Figure 19.1
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Tray Efficiency
µL aLK,HK