Transcript
Separations
ChEN 4253 Design IChapter 19Terry A. RingUniversity of Utah
Simple Separation Units
• Flash– Quench
• Liquid-liquid decantation– Liquid-liquid Flash
• Sublimation– Solid/Vapor Flash
• Crystallization• Filtration
Use of Separation Units
Separation Reaction
Hydrodealkylation of Toluene
T+H2B+CH4
side reaction2B Biphenyl+H2
Reactor EffluentT=1,350FP = 500 psia
Reactor Effluent
Component kmole/hrHydrogen 1292Methane 1167Benzene 280Toluene 117Biphenyl 3Total 2859
Reaction ConditionsT=1,350FP = 500 psia
After Flash to 100F @ 500 psia
Effluent Vapor LiquidComponent kmole/hr kmole/hr kmole/hrHydrogen 1292 1290 2Methane 1167 1149 18Benzene 280 16 264Toluene 117 2 115Biphenyl 3 0 3Total 2859 2457 402
Recycled Reactants
Separation
• Vapor Separation– CH4 from H2
• Liquid Separation
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?
Direct Distillation Sequence
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
II
II
I
I
CC
CC
SF
2
1
2
1
Phases I and II, Components 1 and 2 (light key and heavy key)
Distillation
Distillation
Plate Types• Bubble Cap Tray • Sieve Tray
Packed Towers
• Random Packing
• Structured Packing
Note: Importance of Distributor plate
Distillation
• Relative Volatility
• Equilibrium Line
α=KL/KH
Distillation
• Rectifying Section– R= reflux ratio– V=vapor flow rate
• Stripping Section– VB= Boil-up ratio
• Feed Line
Minimum Reflux Ratio
McCabe-Thiele
Step Off Equilibrium Trays
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
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
– V=D (R+1) • V= Vapor Flow Rate• D= Distillate Flow Rate (=Production Rate)• R=Reflux Ratio
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
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] eq. 19.1– N=Nmin/ε
• Tray Height = N*Htray
• Packed Height = Neq*HETP – HETP(height equivalent of theoretical plate)– HETPrandom = 1.5 ft/in*Dp 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
Costing
Column Costs
• Column – Material of Construction gives ρmetal
– Pressure Vessel Cp= FMCv(W)+CPlatform
• Reboiler CB α AreaHX
• Condenser CB α AreaHX
• Pumping Costs – feed, reflux, reboiler– Work = Q*ΔP
• Tanks– Surge tank before column, reboiler accumulator,
condensate accumulator
CPI
Problem
• Methanol-Water Distillation
• Feed – 10 gal/min– 50/50 (mole) mixture
• Desired to get – High Purity MeOH in D– Pure Water in B
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
Tray Efficiency
μL * αLK,HK
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 and 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
Figure 19.1
Tray Efficiency
μL * αLK,HK
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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