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
Separation Reaction Hydrodealkylation of Toluene
T+H2B+CH4 side reaction 2B Biphenyl+H2 Reactor Effluent T=1,350F P = 500 psia
Reactor Effluent Reaction Conditions T=1,350F P = 500 psia Component
Hydrogen Methane Benzene Toluene Biphenyl Total
kmole/hr 1292 1167 280 117 3 2859
After Flash to 100F @ 500 psia Effluent Vapor Liquid Component kmole/hr kmole/hr kmole/hr Hydrogen 1292 1290 2 Methane 1167 1149 18 Benzene 280 16 264 Toluene 117 2 115 Biphenyl 3 0 3 Total 2859 2457 402 Recycled Reactants
Separation • Vapor Separation – CH4 from H2
• Liquid Separation
Further Separation What 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 I 1
C SF
C
I 2
C
II 2
II 1
C
• 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)
Distillation
Distillation
Plate Types • Bubble Cap Tray
• Sieve Tray
Packed Towers • Random Packing
• Structured Packing Note: Importance of Distributor plate
Distillation α=KL/KH
• Relative Volatility • Equilibrium Line
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 or see Fig 19.1
– V=D (R+1) • V= Vapor Flow Rate • D= Distillate Flow Rate (=Production Rate) • R=Reflux Ratio
Figure 19.1
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
– Packed, DT =[(4G)/((f Uflood πρG)]1/2
eq. 19.14
• Uflood= f(dimensionless density difference), f = 0.75-0.85 eq. 19.12 • Uflood= f(flow ratio), f = 0.75-0.85
eq. 19.15
• Column Height
– Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK] – N=Nmin/ε (or 2 Nmin/ ε)
Fenske eq.19.1
• 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
• Tray Efficiency, ε = f(viscosityliquid * αLK,HK) • Pressure Drop • Tray, ΔP=ρLg hL-wier N • Packed, ΔP=Packed bed (weeping)
eq. 19.9
Fig 19.3
Tray Efficiency
19.3
μL * αLK,HK
Costing
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 CB α AreaHX • Condenser CB α AreaHX • Pumping Costs – feed, reflux, reboiler – Work = Q*ΔP
• Tanks – Surge tank before column, reboiler accumulator, condensate accumulator – Pressure Vessel Cp= FMCv(W)+CPlatform
CPI
Distillation Problems • Multi-component Distillation – Selection of Column Sequences
• Azeotropy – Overcoming it to get pure products
• Heat Integration – Decreasing the cost of separations
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
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
Figure 19.1
Tray Efficiency
μL * αLK,HK