OBJECTIVES •
To know and understand the fundamental principles of distillation as a unit operation
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To To have a grasp on the mechanisms of a basic distillation distillation process
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To learn and familiarize the applications of distillation in the industry
BASIC CONCEPTS
In the eld of Chemical Engineering – Engineering – separation process is the transfer transfer of any mass that converts the substance mixture into distinctive product mixtures In some cases! a separation may fully divide the mixture into its pure constituents "eparations are carried out based on di#erences in chemical properties! or physical properties such as size! shape! mass! density! or chemical a$nity! between the consti constitue tuents nts of a mixtur mixture! e! and are are often often classi classied ed accor accordin ding g to the partic particula ularr di#erences they use to achieve separation In most cases one single di#erence is used to attain a desired separation% however multiple processes are implemented in combination to attain the desired solution &part from a few cases! compounds and elements are found naturally in an impure state and these impure materials needs to be separated for purication There are a large number of types of separation processes! including distillation! extraction! absorption! membrane ltration! and so on Each of these can also be used for purication! to varying degrees Distillation is the process of heating a li'uid until it boils! then condensing and collecti collecting ng the resultan resultantt hot vapors vapors This This proces process s is used to separa separate te a pure pure li'uid from a mixture of li'uids It works when the li'uids have di#erent boiling points (wwwessentialchemistrycom)
*istil *istillat lation ion was was proba probabl bly y rst used used by ancien ancientt &rab &rab chemis chemists ts to isolat isolate e perfumes +essels with a trough on the rim to collect distillate! called diqarus ! date back to ,-.. /C *ist *istil illa lati tion on is the the olde oldest st and and the the most most univ univer ersa sall proc proces ess s of chem chemic ical al technology technology and other branches of industry industry incorporat incorporating ing separation separation of mixtures mixtures It has substantial advantages over other processes applied in order to separate a mixture! such as extraction! crystallization! semipermeable membranes! etc &s a rule! it is the most cost0e#ective process! so it may be used for mixtures with very
diverse properties This process is based on the fact that the composition of the boiling li'uid and that of the vapor over it di#er Thus! if the boiling temperature is low! it is necessary to use low0temperature refrigerants and conduct the process at a higher pressure If it is high! high0temperature heat carriers or re preheating have to be used and the process is run under vacuum The following are the general types of distillation2 •
Simple distillation - In this process! the li'uids are separated by evaporating and capturing them at various points in their cooling cycle! and then immediately channeled into a condenser "imple distillation is used for a mixture in which the boiling points of the components di#er by at least 1-345 (6.4C) It is also followed for the mixtures contaminated with nonvolatile particles (solid or oil)! and those that are nearly pure with less than 1. percent contamination
"ource2 http277wwwotecws7pure8water8productshtml 5igure 91 Desalination of water process thro!h simple distillation •
Vacm distillation – This form of distillation relies on the basic principle that boiling occurs when vapor pressure exceeds ambient pressure for a li'uid The pressure above the li'uid being distilled is reduced via vacuum to below its vapor pressure! causing it to boil and rise through the column This may or may not re'uire heating
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5igure 99 Vacm Distillation in Petrolem "e#nin! "ource2 wwwbritannicacom •
$ractional distillation – *istillation process where chemical components are separated into :fractions; based on their properties
"ource2 http277pythagorasandthatcouk 5igure 9, $ractional Distillation of %i&id Air to prodce Nitro!en 'as •
Steam distillation – *istillation system used for temperature sensitive materials or materials that begin eroding at sustained high temperatures /y adding water or steam to the distillation process! the boiling temperatures of the other substances are brought down! allowing lower temperatures to be used
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"ource2 http277herb0 educationeu7angoldvd7gi,html 5igure 9< E(traction of Essential Oils thro!h Steam Distillation T)PES O$ DISTI%%ATION CO%*+NS
There are many types of distillation columns! each are designed to perform specic types of separation! and each design di#ers in terms of complexity =ne way of classifying distillation column type is to look at how they are operated and are as follows2 1 /atch Columns The feed to the column is introduced batch0wise That is! the column is charged with a >batch? and then the distillation process is carried out @hen the desired task is achieved! a next batch of feed is introduced 9 Continuous Columns Continuous columns process a continuous feed stream Ao interruptions occur unless there is a problem with the column or surrounding process units They are capable of Bandling high throughputs and are the most common of the two types Classi#cation of continos Colmns , The natre of feed that the. are processin! a /inary Column – feed contains only two components b ulti0component Column – feed contains more than two components / The nm0er of prodct streams the. ha1e
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a ulti0product Column – column has more than two product streams 2 To where the e(tra feed e(its when it is sed to help with the separation a Extractive distillation 0 where the extra feed appears in the bottom product stream b &zeotropic distillation – where the extra feed appears at the top product stream 3 The t.pe of colmn internals a Tray Column – where trays of various designs are used to hold up the i'uid to provide better contact between vapour and li'uid! hence better separation b Dacked Column – where instead of trays! :packings; are used to enhance contact time between vapour and li'uid BASIC DISTI%%ATION E4*IP+ENT AND OPE"ATION
+a5or Components of a T.pical Distillation Colmn
1 9 , < -
+ertical "hell – the separation of li'uid components is carried out Trays7 Dlates7 Dackings – used to enhance component separation eboiler 0 provide the necessary vaporization for the distillation Condenser – To cool and condense the vapour leaving the top of the column eFux *rum – to hold the condensed vapour from the top of the column so that a li'uid (reFux) can be recycled back to the column
BASIC OPE"ATION AND TE"+INO%O')
The li'uid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray The feed tray divides the column into a top (enriching or rectication)
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section and a bottom (stripping) section The feed Fows down the column where it is collected at the bottom in the reboiler Beat is supplied to the reboiler to generate vapour The source of heat input can be any suitable Fuid! although in most chemical plants this is normally steam In reneries! the heating source may be the output streams of other columns The vapour raised in the reboiler is re0introduced into the unit at the bottom of the column The li'uid removed from the reboiler is known as the bottoms product or simply! bottoms The vapour moves up the column! and as it exits the top of the unit! it is cooled by a condenser The condensed li'uid is stored in a holding vessel known as the reFux drum "ome of this li'uid is recycled back to the top of the column and this is called the reFux The condensed li'uid that is removed from the system is known as the distillate or top product Thus! there are internal Fows of vapour and li'uid within the column as well as external Fows of feeds and product streams! into and out of the column
E4*IP+ENTS *SED AND +EC6ANIS+
The following is an illustration of a simple distillation setup
"ource2 http277chemistrytutorvistacom7inorganic0 5igure ,1 Apparatses inclded in a Simple Distillation Setp chemistry7distillationhtml
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5or the mechanism of a simple distillation! imagine a mixture of three li'uids! &! /! and C & has a boiling point of 3G45% / has a boiling point of 1.<45% and C has a boiling point of 19945 The three0li'uid mixture described above is added to a distillation 7as8 The mixture in the Fask is heated by a Bnsen 0rner Hi'uid & begins to boil when the temperature in the Fask reaches 3G45 It turns into a vapor at that temperature! rises in the distilling Fask! and passes out of the 7as8 arm into the condenser The condenser consists of a long tube surrounded by a larger tube The outer tube contains water! which enters near the bottom of the condenser and leaves near the top The water passing through the outer acket of the condenser cools the vapor passing through the inner tube The vapor loses heat and condenses It Fows out of the condenser and into a recei1in! container The li'uid is now known as the distillate! or the product of the distillation eanwhile! the temperature in the distilling Fask has not changed! as indicated by the thermometer in the mouth of the Fask Beat added to the li'uid mixture is used to vaporize li'uid &! not to raise the temperature of the mixture That temperature will begin to rise only when li'uid & has completely boiled away /y watching the thermometer! therefore! an observer can know when li'uid & has been completely removed from the li'uid mixture &t that point! the receiver containing pure li'uid & can be removed and replaced by a new receiver =nce li'uid & has boiled away! the temperature in the distilling Fask begins to rise again @hen it reaches 1.<45! li'uid / begins to boil away! and the se'uence of events observed with li'uid & is repeated Eventually! pure samples of &! /! and C can be collected 5or the summary! the following is the se'uence of events in distillation2
6EATIN' CONDENSIN'
EVAPO"ATIN'
COO%IN'
'ENE"A% T)PES O$ DISTI%%ATION CO%*+NS •
Dacked /ed *istillation Column
Dacked bed columns are often used to recover solvents *i#erences in concentration cause the less0volatile components to transfer from the vapor phase to the li'uid phase The packing increases the time of contact! which increases the separation e$ciency The exiting vapor contains the most volatile components! while the li'uid product stream contains the least volatile components
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Copyright2 "ulzer Chemtech Htd! "witzerland 5igure ,9 Pac8ed Bed Colmns sed in an ethanolamine plant •
echanism The feed mixture contains components of di#erent volatilities! and enters the column approximately at the middle &fter the feed mixture enters the column li'uid Fows down the column through the packing countercurrently and contacts the rising vapor stream The li'uid at the bottom enters a reboiler Two streams exit the reboiler% a vapor stream! which returns to the column! and a li'uid product stream The vapor stream Fows upward through the packing! picks up the more volatile components! exits the column! and enters a condenser &fter the vapor condenses! the stream enters a reFux drum! where it is split into an overhead product stream! known as the distillate! and a reFux stream that is recycled back to the column
"ource2 http277encyclopediacheenginumichedu 5igure ,, Parts of Pac8ed Bed Distillation Colmn
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•
Dlate or Tray Column
The plate! or tray column is the most widely used type of distillation column The number of trays! or stages in the column is dependent on the desired purity and di$culty of separation The number of stages also determines the height of the column
Copyright2 =dfell! Aorway 5igure ,< 39-Tra. Colmn sed for mineral oils •
echanism The feed enters a plate column towards the middle of the column &fter the feed mixture enters the column li'uid Fows down the column and across the trays in either crossFow or countercurrent Fow & reboiler at the bottom separates the stream into a vapor stream that returns to the column and a li'uid product stream The vapor stream Fows upward through the trays! and contacts the down0Fowing li'uid stream! allowing the separation to take place &t the top of the column! the vapor is condensed in a condenser The condensed stream is split into an overhead product stream! known as the distillate! and a reFux stream that returns to the top of the column
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"ource2 http277encyclopediacheenginumichedu 5igure ,- +echanism of Tra. Colmn CO%*+N INTE"NA%S 5or Tray and Dlates 1 B00le Cap Tra.s 0 & bubble cap tray has riser or chimney tted over each hole! and a cap that covers the riser The cap is mounted so that there is a space between riser and cap to allow the passage of vapour +apour rises through the chimney and is directed downward by the cap! nally discharging through slots in the cap! and nally bubbling through the li'uid on the tray 9 Val1e Tra.s 0 In valve trays, perforations are covered by liftable caps. Vapour flows lifts the caps, thus self-creating a flow area for the passage of vapour. The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays. Sieve Trays - ieve trays are simply metal plates with holes in , them. Vapour passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters.
Dackings 1 9 ,
andom Dacking Krid Dacking "tructured Dacking
PAC:IN' VS T"A)S
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& tra. colmn that is facing throughput problems may be de0bottlenecked by replacing a section of trays with packings This is because2 1 packings provide extra inter0facial area for li'uid0vapour contact 9 e$ciency of separation is increased for the same column height , packed columns are shorter than trayed columns Pac8ed colmns are called continuous0contact columns while trayed columns are called staged0contact columns because of the manner in which vapour and li'uid are contacted DISTI%%ATION P"INCIP%ES "eparation of components from a li'uid mixture via distillation depends on the di#erences in boiling points of the individual components &lso! depending on the concentrations of the components present! the li'uid mixture will have di#erent boiling point characteristics Therefore! distillation processes depends on the vapour pressure characteristics of li'uid mixtures VAPO" P"ESS*"E AND BOI%IN' The vapour pressure of a li'uid at a particular temperature is the e'uilibrium pressure exerted by molecules leaving and entering the li'uid surface Bere are some important points regarding vapour pressure2 1 energy input raises vapour pressure 9 vapour pressure is related to boiling , a li'uid is said to >boil? when its vapour pressure e'uals the surrounding pressure < the ease with which a li'uid boils depends on its volatility - li'uids with high vapour pressures (volatile li'uids) will boil at lower temperatures G the vapour pressure and hence the boiling point of a li'uid mixture depends on the relative amounts of the components in the mixture 6 distillation occurs because of the di#erences in the volatility of the components in the li'uid mixture T6E BOI%IN' POINT DIA'"A+
The boiling point diagram shows how the e'uilibrium compositions of the components in a li'uid mixture vary with temperature at a xed pressure Consider an example of a li'uid mixture containing 9 components (& and /) 0 a binary mixture This has the following boiling point diagram The boiling point of & is that at which the mole fraction of & is 1 The boiling point of / is that at which the mole fraction of & is . In this example! & is the more volatile component and therefore has a lower boiling point than / The upper curve in the diagram is called the dew0point curve while the lower one is called the bubble0point curve The dew-point is the temperature at which the saturated vapour starts to condense
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The 000le-point is the temperature at which the li'uid starts to boil
This di#erence between li'uid and vapour compositions is the basis for distillation operations APP%ICATIONS •
Keneral &pplications
*istillation is a powerful tool both for the identi#cation and the pri#cation of or!anic componds The boiling point of a compound is one of the physical properties used to identify it *istillation is used to purify a compound by separating it from a non0volatile or less0volatile material @hen di#erent compounds in a mixture have di#erent boiling points! they separate into individual components when the mixture is carefully distilled •
"pecic &pplications •
Laboratory Scale Applications
*istillation has long been used as the separation process in the chemical and petroleum industries because of its reliability! simplicity! and low0capital cost It is employed to separate 0en;ene from tolene< methanol or ethanol from water! acetone from acetic acid! and many multicomponent mixtures •
"ample &pplication with Illustration
*istillation is commonly used to separate ethanol 0 the alcohol in alcoholic drinks 0 from water
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"ource2http277wwwbbccouk7schools7gcsebitesize7science7a'a7crudeoil7crudeoilrev9 shtml 5igure <1 6eatin! water and ethanol mi(tre
"ource2http277wwwbbccouk7schools7gcsebitesize7science7a'a7crudeoil7crudeoilrev9 shtml 5igure <9 E1aporation and Condensation of ethanol
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"ource2http277wwwbbccouk7schools7gcsebitesize7science7a'a7crudeoil7crudeoilrev9 shtml 5igure <, Total separation of ethanol from water •
Industry scale applications
*istillation is widely used in industry! for example in the manfactre and pri#cation of nitro!en< o(.!en and the rare !ases Bowever! one of its best known uses is the re#nin! of crde oil into its main fractions! including naphtha! kerosene and gas oil •
"ample &pplications with Illustration
"ource2http277wwwessentialchemicalindustryorg7processes7distillationhtml 5igure << $ractional Distillation of Crde Oil
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"ource2 http277allcarspwhtml7 5igure <- Distillation of =ater for In5ection Other Fields of Application a Dharmaceutical b 5ood Drocessing c =il ening d Cosmetics e Droduction of +arious Chemicals
T"IVIAS
1 &lchemists believed that distillation released the essence or :spirit; of a substance
"tradamus (Circa 1--.) 5igure -1 Ancient alchemists performin! distillation 9 Labir / Bayyan described distillation using an alembic in the 3th century
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"ource2 http277wwwmontgomerydistillerycom7our0process7distilling7
5igure -9 Ara0ic Distillation , @hiskey and whisky are both made from grain @hiskey is the usual &merican spelling! especially for beverages distilled in the M" and Ireland @hisky is the spelling for Canadian and "cotch distilled beverages
"ource2 http277wwwaskmencom7
5igure -, $amos alcoholic drin8< Jac8 Daniel>s and Tennessee =his8e. < It was a Dersian doctor! &vicenna (J3.01.,6)! who was the rst to obtain the oil from Fowers! known as attar! by distillation /efore this revelation! perfumes were derived from the bark of twigs and shrubs in the form of resins Bis works were faithfully followed by other chemists! soon becoming general practice
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"ource2 http277wwwcrytalinkscom7
5igure -< Persian Doctor A1icenna
"E$E"ENCES
http277wwwbbccouk7schools7gcsebitesize7science7edexcel8pre89.117oneearth7fuels rev1shtml http277wwwscienceclariedcom7everyday7eal0Hife0Chemistry0+ol097*istillation0and0 5iltration0eal0life0applicationshtml http277sciencerankorg7pages791997*istillation0&pplicationshtml http277wwwessentialchemicalindustryorg7processes7distillationhtml http277orgchemcoloradoedu7Techni'ue7Drocedures7*istillation7*istillationhtml http277wwwslidesharenet7kumarsachin,3.17distillation0column http277wwwmitaoeacin7di#erent0types0list0of0separation0processes0in0chemical0 engineering http277wwwmontgomerydistillerycom7our0process7distilling7 http277wwwencyclopediacom7topic7distillationaspx http277sciencerankorg7pages791997*istillation0&pplicationshtmlNixzz<1ecusn6b http277wwwbuzzlecom7articles7types0of0distillationhtml http277encyclopediacheenginumichedu7Dages7"eparationsChemical7*istillationCol umns7*istillationColumnshtml http277articlescompressionobscom7articles7oileld01.17961.0distillation0columns0 towers0column0control0OstartP1
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