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1. Introduction to equilibrium stagewise separations

Learning Outcomes

After completing this lecture and associated tutorial problems, you should be able to do the following:

o Appreciate that at the most basic level, one can only separate phaseso For a homogeneous mixture, consider the physicochemical properties of the system and try to

create a new phase to facilitate separation

Introduction

One often needs to separate materials e.g. tea leaves from brewed tea, iron or copper

minerals from ore roc!

"hemical reactors usually end up with unreacted reactants in final product plus byproducts

A # $ "

If we are dealing with a gas phase reaction then, in general, after reacting A % $, you get a mixture ofA, $ % "

&hase separation

At the most basic level, one can only separate phases : 'hese are usually:

(olid

)i*uid

+as

A # $

eaction

A # $ # "

A # $ "

&roductecycle

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(eparation and purification

o 8esigned to separate components of mixtures from one another

Examples of multiphase separation processes

o $rewing a cup of coffee

Leaching dissolving a component of a solid phase into a li*uid solution4coffee5

iltration separating the ground coffee from the brewed coffee

o emoval of (O9from a gas stream

$urning of a fuel containing sulphur combustion gas contains (O9 In the atmosphere, (O9combines with O9to form (Owhich in turn dissolves

and reacts with water vapour to form sulphuric acid acid rain

!bsorption or scrubbing (O9dissolves in the solvent % the clean

gas that remains is released to the atmosphere

o ecovery of methanol from an a*ueous solution

6ethanol has a higher "apour pressure than water , meaning it has a greatertendency to vapourise when a mixture of the two species is heated.

'his separation process is called #istillation 7xploits this difference in volatility by partially vapourising a li*uid

mixture, yielding a vapour rich in methanol and a residual li*uidrelatively rich in water

(ubse*uent partial condensationand vapourisation can be used to

recover almost pure methanol

ecovered methanol can berec$cledand reused resulting in

considerable savings in raw material costs

o (eparation of paraffinic % aromatic hydrocarbons

)i*uid paraffinic hydrocarbons pentane, hexane, heptane

)i*uid aromatic hydrocarbons ben3ene, toluene, xylene

0hat preferences in properties of paraffinic % aromatic hydrocarbons

can we exploit to separate mixtures of the two

o 8ifferent chemical characteristics e.g. li*uid paraffinic

compounds are almost immiscible in li*uid ethylene glycolwhereas aromatic compounds readily form homogeneousmixtures

o (eparation strategy

$lend 4mix5 a mixture of the two with ethylene glycol allow to settle 4i.e. stop mixing5

Aromatic compounds distribute between aparaffinic-rich phase and a glycol phase

9-phase system6ixture of A % $

$ 4almost pure5

A 4almost pure5

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o Liquid extraction

(ubse*uent processing separates aromatics from ethyleneglycol which may be recycled and reused for the extractionprocess

o (eparation of an isomeric mixtureo p-xylene, o-xylene, m-xylene

p-xylene is a constituent in the synthesis of polyesters and must beseparated from two of its isomers

o (eparation strategy ;

6olecular sieve pores large enough to accommodate p-xylene but not m-xylene and o-xylene !dsorption

o (eparation strategy 9

8ifference in free3ing points of the three isomers

p-xylene 4;."5, m-xylene 4-"5, o-xylene 4-9?.9"5

6ixture is cooled to a temperature at which p-xylene cr$stalli%esand thencan be separated physically 4e.g. using filtration5 from the li*uid containing m-

xylene and o-xylene

1.&. 'ome concluding comments

0hen a species transfers from one phase to another, the transfer rate generally decreases with timeuntil the 9ndphase is saturatedwith the species holing as much as it can hold at the prevailingprocess conditions.

0hen the concentration of all species in each phase no longer change with time, the phases are saidto be in phase equilibrium.

'he effectiveness of any separation process described above depends both on how species aredistributed between the phases at e*uilibrium and on the rate at which the process@system

approaches e*uilibrium from its initial state.

Test Yourself &

(uggest a method that might be suitable to achieve each of the following separations

;. (eparate crude oil into three fractionsa. a fraction containing volatile low molecular weight compounds 4napthas used to

ma!e gasoline and light chemicals5b. intermediate molecular weight compounds 4used for heating oils5c. non-volatile high molecular weight compounds 4used for lubricating oils5

9. emove water from an a*ueous slurry of bleached wood pulp. Obtain fresh water from sea water

4. (eparate from a mixture of 9, 9and . Ammonia is highly soluble in water, also itcondenses at -.