distillation lecture 1
DESCRIPTION
DistillationTRANSCRIPT
Distillationin Design
Terry A. RingChEUniversity of Utah
www.che.utah.edu/~ring
Use of Separation Units
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
What are you going to learn next year?
• Column sizing– Diameter of Column– Size of trays– Height of packing
• Column Costing• Optimization of column with respect to cost to run
(capital cost and operating cost)• How to develop a distillation train.• How to set up side streams in multi-component
distillation.
Marginal Vapor Rate
• Annualized Cost~ Marginal Vapor Rate• 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
Direct Distillation Sequence
Column Sequences
• No. of Columns– Nc=P-1
• P= No. of Products• No. of Possible Column Sequences
– Ns=[2(P-1)]!/[P!(P-1)!]• P= No. 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
No. of Possible Column Sequences Blows up!
How do I evaluate which is best sequence?
Marginal Vapor Rate
• Annualized Cost~ Marginal Vapor Rate• 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
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
Problem
Reactor Flash Distillation Train
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
Direct Sequence Indirect SequenceDistillate Flow Distillate Flow Distillate Flow Distillate Flow
Liquid Column 1 Column 2 Column 1 Column 2kmole/hr
Hydrogen 2 x x xMethane 18 x x xBenzene 264 x x xToluene 115 x xBiphenyl 3Total 402 284 115 399 284Sequence Total 399 683
R assumed to be similar for all columnsV~D
Simplified Marginal Vapor Flow Analysis
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– Gives Bottoms Temperature
• P > Atm, Pressure generated by system.• For Vacuum, how is it that generated?
• Not Near Critical Point for mixture
Steam Ejector Generates the Vacuum.
High PressureHigh VelocitySteam
VacuumBernoulli’s Equation
Velocity > Mach 1
Design Issues• Packing vs Trays• Column Diameter from flooding consideration
– Trays, DT=[(4G)/((f Uflood π(1-Adown/AT)ρG)]1/2 eq. 14.11
• Uflood= f(dimensionless density difference), f = 0.75-0.85 eq. 14.12– Packed, DT =[(4G)/((f Uflood πρG)]1/2 eq. 14.14
• Uflood= f(flow ratio), f = 0.75-0.85 eq. 14.15• Column Height
– Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK] eq. 14.1– N=Nmin/ε
• Tray Height = N*Htray
• Packed Height = Neq*HETP – HETP(height equivalent of theoretical plate)– HETPrandom = 1.5 ft/in*Dp eq. 14.9
• Tray Efficiency, ε = f(viscosityliquid * αLK,HK) Fig 14.3• Pressure Drop
• Tray, ΔP=ρLg hL-wier N• Packed, ΔP=Packed bed
Tray Efficiency
μL * αLK,HK
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 (sometimes longer (empty) tower), condensate accumulator
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– DSTWU – Winn-Underwood-Gilliland Method
• Min # stages, Rmin – Fenske-Underwood
• Min # stages vs R - Gilliland
– Distil – short cut Edmister Method
• Then go to more complicated one for sizing purposes– RadFrac – rigorous method– Sizing in RadFrac
Eric Carlson’s Recommendations
E?
R?
P?
Polar
Real
Electrolyte
Pseudo & Real
Vacuum
Non-electrolyte
Braun K-10 or ideal
Chao-Seader,Grayson-Streed or Braun K-10
Peng-Robinson,Redlich-Kwong-Soave,Lee-Kesler-Plocker
Electrolyte NRTLOr Pizer
See Figure 2Figure 1
Polarity
R?Real or pseudocomponents
P? Pressure
E? Electrolytes
All Non-polar
P?
ij?
ij?
LL?
(See alsoFigure 3)
P < 10 bar
P > 10 bar
PSRKPR or SRK with MHV2
Schwartentruber-RenonPR or SRK with WSPR or SRK with MHV2
UNIFAC and its extensions
UNIFAC LLE
PolarNon-electrolytes
No
Yes
Yes
LL?No
No
Yes
Yes
No
WILSON, NRTL,UNIQUAC and their variances
NRTL, UNIQUACand their variances
LL? Liquid/Liquid
P? Pressure
ij? Interaction Parameters Available
Figure 2
VAP?
DP?Yes
NoWilson, NRTL,UNIQUAC, or UNIFAC* with ideal Gas or RK EOS
Wilson NRTLUNIQUACUNIFAC
Hexamers
DimersWilson, NRTL, UNIQUAC, UNIFAC with Hayden O’Connell or Northnagel EOS
Wilson, NRTL, UNIQUAC, or UNIFAC with special EOS for Hexamers
VAP? Vapor Phase Association
Degrees of PolymerizatiomDP?UNIFAC* and its Extensions
Figure 3
Distillation Problems
• Multi-component Distillation– Selection of Column Sequences– Selection of tray for side stream
• Azeotropy– Overcoming it to get pure products
• Heat Integration– Decreasing the cost of separations