fractional distillation
DESCRIPTION
Fractional DistillationTRANSCRIPT
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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