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ChE 422 Advanced

Separation Process

Introduction Multi-Component Distillation

Lecture 1

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Objectives

Key Questions Distillation Separation

Define Light Key, Heavy Key, Light Non-key,

Heavy Non-key multi-component distillation.

Methods Available for Multi-ComponentDistillation.

Define the concepts of equilibrium stages and

unit operations Constant Molal Overflow (CMO) Assumptions

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Binary Distillation

A

A

B

B

TA < T B

A lightcomponent

B heavycomponent

H

Stage N

Column Height

H?

Column Diameter?

Feed Location?

Number of Tray?

Key

Questions

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McCabe-Thiele Diagram

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Multi-Component Distillation

AB

C

A

BC

B

CABC

AB

AB

A

BC

TA < TB < TC

A light key, B heavy key

C

heavy non-key

A

light non-key, B

lightkey

C heavy key

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Multi-Component Distillation

A

A

BCD

CD

C

D

AB

B

A- light non-key (non-distributing)

B - light key

C heavy key

B- heavy non-key (non-distributing)

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Multi-Component Methods

Shortcut Methods Rigorous Methods

Smith-Brinkley

Fenske min platenumber

Underwood

minreflux

Gilliland (Erbar -Maddox)

FUG -Approach

Thiele -Geddes

Lewis -

MathesonRatingMethod

DesignMethod

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Concept of Equilibrium

Pliq

Tliq

XA

XB

Pvap Tvap

yA yB

vapour pressure,

liquid pressure

vapour temperature,

liquid temperature

liquid mass fraction of A,

vapour mass fraction of A

liquid mass fraction of B,

vapour mass fraction of B

At equilibrium;Pvap = PliqTvap= Tliq

Rate condensation = Rate evaporation

Equilibrium Relationship;yA = KAXA

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Equilibrium Staged Separations

Equilibrium stage concept applicable - process can

be constructed as a series of discrete stages

Where the two phases are contacted and thenseparated.

The separated phases are assumed to be in

equilibrium with each other.

E.g. in distillation, a vapor and a liquid are commonly

contacted on a metal plate with holes in it.

With equilibrium assumption - can calculateconcentrations & temperatures without detailedknowledge of flow patterns and heat and mass transfer

rates.

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Multi-Component Distillation Design

Unit Operation

Equilibrium Data

Column Height

Column Diameter

Number of Trays

Methods

Assumptions

Key equilibrium

data Relative

Volatility

Balances i.e.material andenergy

Feed Location

Operating Conditions

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Constant Molar Overflow (CMO):

Assumption

The column is adiabatic.

The specific heat changes are negligiblecompared to latent heat changes.

The heat ofvaporization per mole is constanti.e. independent of concentration.

The saturated liquid and vapour lines on theenthalpy composition diagram are parallel.

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Lewis Matheson vs. Graphical McCabe-

Thiele Diagram

Both methods based on CMO assumptions.

Graphical method is faster.

Graphical method presents very clear picture,

easier to understand than analytical method. Graphical method is a useful tool to help determine

what the effect of changing variables; i.e. diagnostic.

McCabe-Thiele method is tedious when designing

column with large number of stages.

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Summary

Equilibriumconcept key aspect distillationcolumn design

Multi-component distillation more than 2

components

light key, heavy key, light non-key & heavy non-key

Constant molar overflow assumption !

Graphical method

faster, clear picture &easy to understand; but difficult to apply

large number of stages.