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