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Separations

Separations. ChEN 4253 Design I Chapter 19 Terry A. Ring University of Utah. Simple Separation Units. Flash Quench Liquid-liquid decantation Liquid-liquid Flash Sublimation Solid/Vapor Flash Crystallization Filtration. Use of Separation Units.

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Separations

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

  2. Simple Separation Units • Flash • Quench • Liquid-liquid decantation • Liquid-liquid Flash • Sublimation • Solid/Vapor Flash • Crystallization • Filtration

  3. Use of Separation Units

  4. Separation ReactionHydrodealkylation of TolueneT+H2B+CH4side reaction2B Biphenyl+H2 Reactor Effluent T=1,350F P = 500 psia

  5. Reactor Effluent Reaction Conditions T=1,350F P = 500 psia

  6. After Flash to 100F @ 500 psia Recycled Reactants

  7. Separation • Vapor Separation • CH4 from H2 • Liquid Separation

  8. Further SeparationWhat separation units should be used? • Liquid Separation • Toluene, BP=110.6ºC • Benzene, BP=80.1ºC • What happens to the Methane (BP= -161.5ºC) and Biphenyl (BP=255.9ºC) impurities? • Gas Separation • Hydrogen • Methane • what happens to the Toluene and Benzene impurities?

  9. Direct Distillation Sequence

  10. 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 Criteria for the Selection of a Separation Method Phases I and II, Components 1 and 2 (light key and heavy key)

  11. Distillation

  12. Distillation

  13. Bubble Cap Tray Sieve Tray Plate Types

  14. Packed Towers • Random Packing • Structured Packing Note: Importance of Distributor plate

  15. Distillation α=KL/KH • Relative Volatility • Equilibrium Line

  16. Distillation • Rectifying Section • R= reflux ratio • V=vapor flow rate • Stripping Section • VB= Boil-up ratio • Feed Line

  17. Minimum Reflux Ratio

  18. McCabe-Thiele

  19. Step Off Equilibrium Trays

  20. 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

  21. 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 @ N/Nmin=2 or see Fig 19.1 • V=D (R+1) • V= Vapor Flow Rate • D= Distillate Flow Rate (=Production Rate) • R=Reflux Ratio

  22. Figure 19.1

  23. 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

  24. Design Issues • Packing vs Trays • Column Diameter from flooding consideration • Trays, DT=[(4G)/((f Uflood π(1-Adown/AT)ρG)]1/2 eq. 19.11 • Uflood= f(dimensionless density difference), f = 0.75-0.85 eq. 19.12 • Packed, DT =[(4G)/((f Uflood πρG)]1/2 eq. 19.14 • Uflood= f(flow ratio), f = 0.75-0.85 eq. 19.15 • Column Height • Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK] Fenske eq.19.1 • N=Nmin/ε (or 2 Nmin/ ε) • Column Height = N*Htray • Tray Height = typically 1 ft (or larger), 2 inch weir height • Packed Height = Neq*HETP (or 2 Neq*HETP) • HETP(height equivalent of theoretical plate) • HETPrandom = 1.5 ft/in*Dp Rule of thumb eq. 19.9 • Tray Efficiency, ε = f(viscosityliquid * αLK,HK) Fig 19.3 • Pressure Drop • Tray, ΔP=ρLg hL-wier N • Packed, ΔP=Packed bed (weeping)

  25. Tray Efficiency 19.3 μL * αLK,HK

  26. Costing

  27. Column Costs • Column – Material of Construction gives ρmetal • Pressure Vessel Cp= FMCv(W)+CPlatform • Height may include the reboiler accumulator tank • Tray Cost = N*Ctray(DT) • Packing Cost = VpackingCpacking + Cdistributors • Reboiler CBα AreaHX • Condenser CBα AreaHX • Pumping Costs – feed, reflux, reboiler • Work = Q*ΔP • Tanks • Surge tank before column, reboiler accumulator, condensate accumulator • Pressure Vessel Cp= FMCv(W)+CPlatform

  28. CPI

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

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

  31. 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

  32. Simulation Methods- ProMax • Start with 10 trays (you may need up to 100 for some difficult separations) • set ΔP on column, reboiler, condenser and separator • set ΔT 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 αLK,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/Nmin=2 (above and below feed tray) • R/Rmin=1.2

  33. Figure 19.1

  34. Tray Efficiency μL * αLK,HK

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