Separation Distillation

Separation

Separation TechniquesAli [email protected]

OutlinesIntroductionDistillationCrystallizationCentrifugationExtractionFlotation Particle separationChromatographyAffinity separationsMembrane separationsPresentations 2ReferencesClifton E. Meloan, Chemical Separations,Principles, Techniques, and Experiments, John Wiley & Sons, 1999Michael Cooke, Encyclopedia of separtion science, Academic press, 2000J. D. Winefordner, Sample Preparation Techniques in Analytical Chemistry, John Wiley & Sons, 2003Douglas A. Skoog, F James Holler, Stanley R. Crouch, Principles of Instrumental Analysis, 6th ed., Thomson, 2007 3

EvaluationStudents presentationsFinal exam4IntroductionImportant feature of modern life The need for separations as a means for isolation, purification or analysis of substancesVast industry has arisen to provide the equipment and instrumentation to perform and control these essential processesImpossible to envisage our world without separations5IntroductionSeparation science was first recognized as a distinct area of physical and analytical chemistry in the 1960sFirst was coined by the J. Calvin GiddingsProcesses of any scale by which the components of a mixture are separated from each other without substantial chemical modification

6IntroductionBasically, all separation techniques rely on thermodynamic differences between components Kinetic factors determine the speed at which separation can be achieved7Separation techniquesDistillationCrystallizationCentrifugationExtractionFlotation Particle separationChromatographyAffinity separationsMembrane separations Mass spectrometry8DistillationDistillationThe most commonly used method for the separation of homogeneous fluid mixturesSeparation exploits differences in boiling point, or volatility between the components in the mixtureRepeated vaporization and condensation of the mixture allows virtually complete separation of most homogeneous fluid mixturesRequires the input of energy10DistillationDisadvantageHigh energy usageAdvantagesThe ability to handle a wide range of feed flow rateThe ability to separate feeds with a wide range of feed concentrationsThe ability to produce high product purityIt is a versatile, robust and well-understood technique11Single-stage separationDistribution of components between the vapor and liquid is dictated by thermodynamic vapor-liquid equilibrium

Fugacity is a thermodynamic pressure for non-ideal systemsIt can be thought of as an escaping tendency12

Single-stage separation13Single-stage separation14Single-stage separationAlternativelyliquid-phase activity coefficient models

At moderate pressures15Single-stage separationWhen the liquid phase behaves as an ideal solutionRaoults law

Relative volatility for two components16Single-stage separationVapor-liquid equilibrium for a binary mixture of benzene and toluene at a pressure of 1 atm17

Single-stage separationDiagonal line Concentration in the vapor and the liquid are equalThe phase equilibrium behavior, shows a curve above the diagonal lineBenzene has a higher concentration in the vapor phase than the tolueneBenzene is the more volatile component in this case18Single-stage separationSeparation in a single equilibrium stage

Once the feed conditions and compositions, the system pressure and the relative flow rates of the vapor and liquid have been fixed, then the temperature and compositions of the exit streams are unique19

Single-stage separationAssumption Molar flow rates of the liquid L and molar flow rates of the vapor V are constant This assumption is known as constant molar overflow and is true if:Sensible heat effects are smallMolar latent heats of vaporization of the components are equalHeat of mixing is negligibleThere are no heat losses or gains20Single-stage separationMass balance

Component mass balance21

Single-stage separationwhich can be rearranged to22

Single-stage separationThe mass balance line is a straight line which depends on the liquid and vapor flow rates and the feed compositions of liquid and vaporIn this figure the liquid and vapor feeds are not in equilibriumFollowing the mass balance line until it crosses the equilibrium line allows us to predict the vapor and liquid composition at the exit of the stageSeparation which is achieved on a single equilibrium stage is limited23Cascade of separation stagesTo extend the amount of separation24

Cascade of separation stagesIt is assumed that streams leaving each stage are in equilibriumUsing a cascade of stages in this way allows The more volatile components to be transferred to the vapor phaseThe less volatile components to be transferred to the liquid phase and a greater degree of separation to be achieved than for a single stage25Cascade of separation stagesMass balance on a countercurrent cascade

Mass balance in constant molar overflow26

Cascade of separation stagesOverall mass balance for component i across the cascade

Mass balance over m stages

After rearrangementComposition stream after m stages

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Cascade of separation stagesCascade in terms of a more conventional representation in a distillation columnAt the top of the columnWe need Liquid to feed the cascade:Produced by condensing and returning some of the vaporAt the bottomWe need Vapor to feed the cascade:Vaporizing and returning some of the liquid leaving the bottom28

Distillation column29

Designing a distillation columnSet the Product specificationsSet the Operating pressureDetermine the Number of theoretical stages required and the energy requirementsDetermine the Actual number of trays or height of packing needed and the column diameterdesigne Column internalsSpecific dimensions of the trays, packing, liq. And vap. distributionCarry out Mechanical design Wall thicknesses, internal fittings, etc.30Binary distillationSimple columnColumn has one feed, two products, one reboiler and one condenserOverall mass balance31

Binary distillationComponent balance

Overall mass balance for the rectifying section32

Binary distillationReflux ratio

These expressions can be combined to give an equation which relates the vapor entering and liquid flows leaving stage n

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Binary distillationOn an x-y diagram for component i, this is a straight line starting at the distillate composition with slope R/(R+1) and which intersects the diagonal line at X D,i34

Binary distillationAt distillate composition X D, the composition of the vapor in equilibrium with the distillate, y 1As we step between the operating line and equilibrium line, we follow the change in vapor and liquid composition through the rectifying section of the column35Binary distillationOverall mass balance for the stripping section around stage m gives

Component mass balance36

Binary distillationPlot of this line in x-y plotIt is a straight line with slope L/V which intersects the diagonal line at xB Starting from the bottom composition, xBA vertical line to the equilibrium line gives the composition of the vapor leaving the reboiler, yB37

Binary distillationCombining the rectifying and stripping sectionsThe intersection of the operating lines is the correct feed stageThe feed stage necessary to minimize the overall number of theoretical stages38

Binary distillationTwo important limits which should be considered for distillationTotal reflux No products are taken and there is no feedSmallest number of stages required for the separation39

Binary distillationMinimum refluxThe reflux ratio is chosen such that the operating lines intersect at the equilibrium lineAs this stepping procedure approaches the q-line from both ends, an infinite number of steps are required to approach the q-line40

Binary distillationq: Reflux ratio

For a saturated liquid feed q=1 and for a saturated vapor feed q=041

Volatility and Fenske equationEfficiency of a columnNumber of plates required to separate two componentsNumber of theoretical plate

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Distillation equipmentTwo broad classes of internals used for distillationTrays Packing43TraysA: Conventional trayB: high capacity tray44

TraysConventional traySieve trayThe most common arrangement usedIt is cheap, simple, and well understood There are many other designs Simple valve arrangements in the holes can be used to improve the performance and the flexibility of operation to be able to cope with a wider variety of liquid and vapor flow rates in the columnDisadvantage Downcomer arrangement makes a significant proportion of the area within the column shell not available for contacting liquid and vapor45TraysOverall tray efficiency, Eo46

PackingRandom or dumped packingPieces of ceramic, metal or plastic which, when dumped in the column, produce a body with a high voidage47

PackingStructured packingManufactured by sheets of metal being preformed with corrugations and holes and then joined together to produce a preformed packing with a high voidage

Height of packing required to be calculated by relating to the theoretical stages48

Distillation arrangementsBatch distillationAzeotropic distillationSteam strippingIntermediate reboiling and condensingMultiple feedsMultiple productsColumn sequences49Batch distillationFeed is charged as a batch to the base of the distillation columnContinuous vaporizationUnlike continuous distillation, the overhead product will change with timeThe first material to be distilled will be the more volatile componentsAllows different fractions to be taken from the same feedBy careful control of the reflux, it is possible to hold the composition of the distillate constant for a time until the required reflux ratio becomes intulerably large.50Batch distillationReflux ratio can be varied in to maintain overhead product purity51

Azeotropic distillationA mixture in which the vapor and liquid have the same composition cannot be separated by conventional distillation52

Azeotropic distillationTwo means of separationUsing two columns operating at different pressures Adding a mass separation agent (known as an entrainer or a solvent)Changes the relative volatility of the original mixture53Steam strippingLive steam is added at the base of the distillation column Steam acts as an inert carrier(for separation of hydrocarbon), the presence of which decreases the partial pressure of the components in the vapor phase54Intermediate reboiling and condensingIt is possible to add or reject heat at intermediate points within the column55Multiple feedsUsed to separate two or more streams with the same components but with different compositions56Multiple productsSometimes possible to withdraw more than two products from the same column57Column sequencesIf a feed mixture needs to be separated into more than two products, then more than one distillation column will usually be required58

Industrial applications of azeotropic distillation59

Extractive distillation page:1014BasisIncrease of relative volatility between the close-boiling components caused by introducing a selective solvent, which has stronger affinity with one type of the components in the mixtureIn some cases, liquid-phase separation may occur in the ED tower, especially in the upper portion of the tower where less polar components are concentratedThe liquid in the solvent-rich phase is defined as the extract liquid phase60Extractive distillation61

Extractive distillation62

Laboratory scale distillationDistillation on a scale ranging from research quantities as small as ten milligrams to multikilogram lots is commonly encountered in the laboratoryPhysical and chemical properties of the substanceAttendant impurities Quantity of impure product to be distilled 63Laboratory scale distillationBroad classes of distillationSteam distillationFlash or simple distillationFractional distillation64

Sublimation page:1114Sublimation is not a procedure that is generally regarded as an analytical techniqueIt is a process, by which compounds can be purified or mixtures separated and as such can be of value as a single step or as an integral part of a more complex analytical methodIt is applicable to a range of solids of inorganic or organic origin in a variety of different matrices and can be particularly useful when heatlabile materials are involved65SublimationAs a method of sample purification To produce high-purity materials as analytical standardsRemoval of water from heat-labile materials in the freeze-dryingAs a separation technique Purification of samples for analysis Removing undesirable components of the matrix Removing the analyte from the matrix66SublimationSublimation is the direct conversion of a solid to a gas or vaporSolid+ heat gas or vapor (heat= Hsubl)67

SublimationEnhancing the sublimation processHeating sample Application of a vacuum Selectively cooling part of the apparatus Using an entraining gas68Apparatus69

Freeze-dryingFreeze-drying is a process, in which a product is first frozen and then dried by sublimation of the iceUnstable product in water is transformed into a dry, stable productDemands on the finished productIts volume remains that of the frozen substanceThe structure and the biological activity of the dried solid correspond as far as possible to those of the original substanceThe dried product remains stable during storage, if possible at temperatures up to 40C and for up to 2 yearsWith the addition of water the original product is quickly reconstituted70Freeze-dryingStepsFreezingMain drying (MD)Sublimation of the iceSecondary drying (SD)Desorption of the water bound to the solidPacking in containers to exclude absorption of water and/or oxygen from the atmospher71Freeze-drying72