Transcript L7b-Tower Design in Promax and Aspen.ppt

Distillation
ChEN 4253 Design I
Chapter 19
Terry A. Ring
University of Utah
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
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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,
• Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK]
• Rmin~(F/D)/(α-1)
– 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
– 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)
eq. 19.1
• Column Height = N*Htray
• Tray Height = typically 1 ft (or larger)
• 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
μL * αLK,HK
Costing
Column Costs
• Column – Material of Construction gives ρmetal
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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
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
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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)
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Or R~1.2*(F/D)/(α-1)
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
Distillation Problems
• Multi-component Distillation
– Selection of Column Sequences
• Azeotropy
– Overcoming it to get pure products
• Heat Integration
– Decreasing the cost of separations