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L09-Separations and Column Simulation.pptx
1. CH EN 5253
Process Design II
Lecture 09
Separations: Distillation Columns and Trains
January 27, 2020
2. Books
Product and Process Design Principles: Synthesis, Analysis and Evaluation
by J. D Seader, and Warren D. Seider and Daniel R. Lewin,
• Chapter 9
2
12. 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
13. How To Determine the Column Pressure
• Cooling Water Available at 90°F
• Distillate Can be cooled to 120°F min.
• Calculate the Bubble Pt. Pressure of Distillate
Composition at 120°F
– equals Distillate pressure
– Bottoms pressure = Distillate pressure + 10 psia ΔP
• Compute the Bubble Pt. Temp for an estimate of the
Bottoms Composition at Distillate Pressure
– Give Bottoms temperature
• Not Near Critical Point for mixture
14. Design Issues
• Packing vs Trays
• Column Diameter from flooding consideration
– Relations in Ch 17 of Towler (Design I)
• Column Height
– Relatiopns in Ch 17 of Towler
– N=Nmin/ε (or 2 Nmin/ ε)
• 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)
15. Column Operating Window
• Plate design must ensure good contacting between phases
– Coning: vapor bypasses liquid
– Weeping: liquid drains through to tray below
• Usually design to operate near (~ 70 to 80% of) flooding limits
so as to allow for turn-down
Liquid rate
Vapor
rate
Jet flooding
Downcomer flooding
Weeping
Coning
Excessive
entrainment
Area of satisfactory
operation
16. 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
21. Further Separation
What separation units should be used?
• Liquid Separation
– Toluene, BP = 111°C
– Benzene, BP = 80°C
– What happens to the Methane (BP = –162 °C) and Biphenyl
(BP = 256°C) impurities?
• Gas Separation
– Hydrogen
– Methane
– What happens to the Toluene and Benzene impurities?
23. Column Sequences
• Number of Columns
– Nc = P – 1
• P = Number of Products
• No. of Possible Column Sequences
– Ns = [2(P–1)]! / [P!(P–1)!]
• P = Number of Products
P = 3, Nc = 2, Ns = 2
P = 4, Nc = 3, Ns = 5
P = 5, Nc = 4, Ns = 14
P = 6, Nc = 5, Ns = 42
P = 7, Nc = 6, Ns = 132
Number of possible
column sequences
becomes very large
very quickly!
24. Example
• P = Number of Products = 4 ( A, B, C, D)
• Number of Columns
– Nc = P – 1= 4-1=3
• No. of Possible Column Sequences
– Ns = [2(P–1)]! / [P!(P–1)!]
= [2(4–1)]! / [4!(4–1)!] )
= [2X3]! / [4!X3!]
= 6! / [4!X3!]
= 720 / [24X6]
= 720 / 144
= 5
24
28. Marginal Vapor Rate
• Marginal Annualized Cost Marginal Vapor Rate
• Marginal Annualized Cost proportional to
– Reboiler Duty (Operating Cost)
– Reboiler Area (Capital Cost)
– Condenser Duty (Operating Cost)
– Condenser Area (Capital Cost)
– Diameter of Column (Capital Cost)
• Vapor Rate is proportional to all of the above
~
29. Selecting Multiple Column Separation Trains
• Minimum Cost for Separation Train will occur
when you have a minimum of Total Vapor
Flow Rate for all columns
R = 1.2 Rmin
V = D(R+1)
V = Vapor Flow Rate
D = Distillate Flow Rate
R = Recycle Ratio
31. Direct Sequence Indirect Sequence
Distillate Flow Distillate Flow Distillate Flow Distillate Flow
Liquid Column 1 Column 2 Column 1 Column 2
kmole/hr
Hydrogen 2 x x x
Methane 18 x x x
Benzene 264 x x x
Toluene 115 x x
Biphenyl 3
Total 402 284 115 399 284
Sequence Total 399 683
R assumed to be similar for all columns and R > 1
Simplified Marginal Vapor Flow Analysis
(First two columns only)
D=
32. Separation Train Heuristics
1. Remove thermally unstable, corrosive, or chemically reactive
components early in the sequence.
2. Remove final products one by one as distillates (the direct
sequence).
3. Sequence separation points to remove, early in the sequence, those
components of greatest molar percentage in the feed.
4. Sequence separation points in the order of decreasing relative
volatility so that the most difficult splits are made in the absence of
the other components.
5. Sequence separation points to leave last those separations that give
the highest-purity products.
6. Sequence separation points that favor near equimolar amounts of
distillate and bottoms in each column.
