fractional distillation

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Fractional Distillation

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    Mutawara Mahmood Baig

    Amin Durrani

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    Distillation - Purify a Liquidto separate a liquid from a Simple

    nonvolatile contaminant Distillation

    (e.g. to purify water which contains dissolved salts)

    to separate a mixture of Fractional

    Liquids Distillation

    (e.g. used in an oil refinery)

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    GENERAL PRINCIPLE The separation operation called distillation utilizes

    vapor and liquid phases at essentially the sametemperature and pressure for the coexisting zones.

    Various kinds of devices such as random or structuredpackings andplates or trays are used to bring the twophases into intimate contact.

    Trays are stacked one above the other and enclosed in

    a cylindrical shell to form a column. The column may be operated continuously or in batch

    mode depending on a number of factors such as scaleand f lexibility of operations and solids content of feed.

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    Simple Vs Fractional DistillationSimple distillation Fractionaldistillation

    Advantages

    simpler setup than fractional faster distillation times consumes less energy than fractional

    distillation

    much better separation betweenliquids than simple distillation

    can more readily purify complex

    mixtures than simple distillation

    Disadvantages

    requires the liquids to have large boilingpoint differences (>70oC)

    gives poorer separation than fractionaldistillation

    only works well with relatively pure liquids

    more complicated setup than simpledistillation

    takes longer for liquids to distill consumes more energy than simple

    distillation

    Best used for: separating relatively pure liquids with

    large boiling differences or liquids with

    solid impurities

    separating complex mixtures of

    liquids with smaller boiling point

    separations.

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    Industrial Applications1. Separate hydrocarbons in crude oil with

    fractionating tower (refining)

    2. Separate mixtures of volatile liquids not separable bysimple dist. (bp< 50 C)

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    Separate mixtures of volatile liquidsGoals:

    Separate a mixture of ethyl acetate and butyl acetate

    Compare results from fractional to simple.

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    DISTILLATION CURVE plots temperature vs volume of distillate

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    DISTILLATION CURVEWhich graph below represents the most efficient

    separation of a binary mixture? Why?

    Which most likely represents simple distillation andwhich fractional distillation? Why?

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    Separate hydrocarbons in crude oil The various components of crude oil have different

    Sizes

    Weights and Boiling temperatures

    Because they have different boiling temperatures, theycan be separated easily by a process called fractional

    distillation.

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    Cont.

    heatthe mixture of two or more substances (liquids)

    with different boiling points to a high temperature.

    The mixture boils, forming vapor (gases); mostsubstances go into the vapor phase.

    Thevaporenters the bottom of a long column that isfilled with trays or plates. The trays have many holes or

    bubble caps in them to allow the vapor to passthrough. They increase the contact time between thevapor and the liquids in the column and help to collectliquids that form at various heights in the column.

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    Cont

    When a substance in the vapor reaches a height where

    the temperature of the column is equal to thatsubstance's boiling point, it will condense to form aliquid.

    The trays collectthe various liquid fractions.

    The collected liquid fractions may pass to condensers,

    which cool them further, and then go to storage tanks,or they may go to other areas for further chemicalprocessing.

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    Separate hydrocarbons in crude oil

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    CHEMICAL PROCESSINGYou can change one fraction into another by one of

    three methods:

    Breaking large hydrocarbons into smaller pieces(cracking)

    Combining smaller pieces to make larger ones(unification)

    Rearranging various pieces to make desiredhydrocarbons (alteration)

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    CRACKING Cracking takes large hydrocarbons and breaks them into

    smaller ones. There are several types of cracking:

    Thermal - heat large hydrocarbons at high temperaturesuntil they break apart. Steam - high temperature steam (816 oC) is used to break ethane,

    butane and naptha into ethylene and benzene, which are used tomanufacture chemicals.

    Visbreaking- residual from the distillation tower is heated (482oC),cooled with gas oil and rapidly burned (flashed) in a distillationtower. This process reduces the viscosity of heavy weight oils andproduces tar.

    coking - residual from the distillation tower is heated totemperatures above 482 oC until it cracks into heavy oil, gasoline andnaphtha. When the process is done, a heavy, almost pure carbonresidue is left (coke)

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    CRACKINGCatalytic - uses a catalyst to speed up the cracking

    reaction. Catalysts include zeolite, aluminumhydrosilicate, bauxite and silica-alumina.

    fluidized catalytic cracking- a hot, fluid catalyst (538oC) cracks heavy gas oil into diesel oils and gasoline.

    hydrocracking- similar to fluid catalytic cracking, butuses a different catalyst, lower temperatures, higherpressure, and hydrogen gas. It takes heavy oil and cracksit into gasoline and kerosene (jet fuel).

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    UNIFICATION Sometimes, you need to combine smaller

    hydrocarbons to make larger ones -- this process iscalled unification. The major unification process iscalled catalytic reforming and uses a catalyst(platinum, platinum-rhenium mix) to combine lowweight naphtha into aromatics, which are used inmaking chemicals and in blending gasoline. A

    significant by-product of this reaction is hydrogen gas,which is then either used for hydrocracking or sold.

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    ALTERATION Sometimes, the structures of molecules in one fraction

    are rearranged to produce another. Commonly, this isdone using a process called alkylation. In alkylation,low molecular weight compounds, such as propyleneand butylene, are mixed in the presence of a catalystsuch as hydrofluoric acid or sulfuric acid (a by-productfrom removing impurities from many oil products).

    The products of alkylation are high octanehydrocarbons, which are used in gasoline blends toreduce knocking.

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    TREATING AND BLENDING THE

    FRACTIONS Distillated and chemically processed fractions are treated to

    remove impurities, such as organic compounds containingsulfur, nitrogen, oxygen, water, dissolved metals and inorganicsalts

    Treating is usually done by passing the fractions through thefollowing:

    A column of sulfuric acid - removes unsaturated hydrocarbons(those with carbon-carbon double-bonds), nitrogen compounds,oxygen compounds and residual solids (tars, asphalt)

    An absorption column filled with drying agents to remove water

    Sulfur treatment and hydrogen-sulfide scrubbers to removesulfur and sulfur compounds

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    Cont

    After the fractions have been treated, they are cooled and

    then blended together to make various products, such as:

    Gasoline of various grades, with or without additives

    Lubricating oils of various weights and grades (e.g. 10w-40,5w-30)

    Kerosene of various grades

    Jet fuel

    Diesel fuel Heating oil

    Chemicals of various grades for making plastics and otherpolymers