Separations

ChEN 4253 Design IChapter 19Terry A. RingUniversity 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 reaction2B Biphenyl+H2

Reactor EffluentT=1,350FP = 500 psia

Reactor Effluent

Component kmole/hrHydrogen 1292Methane 1167Benzene 280Toluene 117Biphenyl 3Total 2859

Reaction ConditionsT=1,350FP = 500 psia

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

Separation

• Vapor Separation– CH4 from H2

• Liquid Separation

Further SeparationWhat 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

• 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

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

– 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• Uflood= f(dimensionless density difference), f = 0.75-0.85 eq. 19.12

– Packed, DT =[(4G)/((f Uflood πρG)]1/2 eq. 19.14• Uflood= f(flow ratio), f = 0.75-0.85 eq. 19.15

• Column Height – Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK] eq. 19.1– N=Nmin/ε

• Tray Height = N*Htray

• Packed Height = Neq*HETP – HETP(height equivalent of theoretical plate)– HETPrandom = 1.5 ft/in*Dp eq. 19.9

• Tray Efficiency, ε = f(viscosityliquid * αLK,HK) Fig 19.3• Pressure Drop

• Tray, ΔP=ρLg hL-wier N• Packed, ΔP=Packed bed

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,

condensate accumulator

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

Tray Efficiency

μL * αLK,HK

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 and 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