“Chemical Engineering Equilibrium Separations”Lectures 14

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17 Oct 2012

Overview

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• AspenPlus:o Shortcut methods: DSTWUo Rigorous method: RADFRAC

• Efficiencies• Introduction to multicomponent distillation (Chapter 9)

Multicomponent Distillation (Introduction)

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• In binary distillation we could specify xD and xB ….• Now: define “Key Components”

Decreasing Relative Volatility Keys

1  2 Light3 Heavy4  

Most of LK obtained in distillate productMost of HK obtained in bottoms product

Today, most multicomponent systems are solved rigorous simulation.

But need to do shortcut methods to get good starting point (FUG-Kirkbride).

Multicomponent Shortcut Methods

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Minimum number of stages: Fenske Equation

HKLK

BotLK

HK

DistHK

LK

xx

xx

N

ln

ln

min

31

BotFeedDistBotDist or

ln

11ln

min

BotHK

HK

DistLK

LK

FRFR

FRFR

N

Multicomponent Shortcut Methods

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Minimum reflux ratio: Underwood Equation

1,

,

,

,

min

HKLK

FHK

DHKHKLK

FLK

DLK

FzDx

FzDx

FL

Case A:NKs don’tdistribute D

LR minmin

Case B & C: NKs distribute, or there is a “sandwich” NK: see Wankat; numerical iterative procedures can be involved.

R = factor * Rmin

Approximate number of equilibrium stages (N): Gilliland correlation

Gilliland Correlation

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61 Data points over ranges:

1. No. components: 2 to 112. q : 0.28 to 1.423. P : vacuum to 42.4 bar4. : 1.11 to 4.055. Rmin : 0.53 to 9.096. Nmin : 3.4 to 60.3Molokanov Eqn:

5.0

12.11711

4.541exp1XX

XXY

Seader & Henley, 2006

Multicomponent Shortcut Methods

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Optimum feed stage location (NF): Kirkbride Equation

206.02

,

,

DB

zx

zz

NN

DHK

BLK

FeedLK

HK

S

R

DSTWU (AspenPlus)• Uses Winn, Underwood, and Gilliland methods to find Nmin, Rmin, & N.• Specify LK and HK recoveries in the distillate product stream• If input -1.2 for reflux ratio; it finds N at R = 1.2 * Rmin.

• N given by DSTWU is number of equilibrium stages (includes partial condensers and/or partial reboilers)

In-Class Problem

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SHORTYFEED

D

B

benzene (17 mol%)toluene (66 mol%)m-xylene (17 mol%)

F = 100 kmol/ssat’d liquid1 atm

DSTWU

  1.0135 bar Pi star [bar]Component Tbp oC 80.1 oC 123 oCBenzene 80.1 1.0135 3.217Toluene 110.7 0.387 1.42m-Xylene 139.1 0.151 0.645

In-Class Problem

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In-Class Problem

10

In-Class Problem

11

In-Class Problem

12

By hand calculations first.

Then use to verify AspenPlus results…

In-Class Problem

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In-Class Problem

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Homework Problem

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SHORTYFEED

D

B

benzene (17 mol%)toluene (66 mol%)m-xylene (17 mol%)

F = 100 kmol/ssat’d liquid1 atm

Xbz = 99 mol%

Xbz = 0.1 mol%

DSTWU

  1.0135 bar Pi star [bar]Component Tbp oC 80.1 oC 123 oCBenzene 80.1 1.0135 3.217Toluene 110.7 0.387 1.42m-Xylene 139.1 0.151 0.645

Overview

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• AspenPlus:o Shortcut methods: DSTWUo Rigorous method: RADFRAC

• Efficiencies• Introduction to multicomponent distillation (Chapter 9)