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Distillation CurvesDistillation Curves

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Distillation: Introduction

• What is Distillation

• Arrangement of chemical compounds in order of their boiling points.

• Why Distillation required

• Boiling pt changed →Chemical compound changed → property changed

• By distillation properties of crude oil & its products can be assessed.

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Types of Distillation

• There are various methods do carry out distillation. The mostcommon type of distillation curves frequently used in refinery are;

• TBP (True Boiling Point) • ASTM (American Society for Testing and Materials)• EFV(Equilibrium Flash Vaporization)

• The selection of distillation curve for sample (crude / petroleumproducts) is based upon the requirement in operation e.g. petroleumproducts (gasoline/ diesel) are used in internal engine and thedistillation/ evaporation phenomena which is occurred on heat ismost resembles with ASTM distillation

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Distillation Curves

• Arrangement of boilingpoint temperatureswith reference to theirvolumetric yield in theform of graph is calleddistillation curves.

• Very useful tounderstand propertiesof material under study

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Volumatic yield------------------>

Tem

pera

ture

-----

------

------

->

0 % 100 %

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TBP Distillation

• ASTM D2892 method is used for TBP distillation.• This method is a batch distillation type• It uses large number of stages, high reflux to distillate ratio so that the

temperature at any point on the temperature- volumetric yield curverepresents the actual (true) boiling point of the hydrocarbon materialpresent at the volume percentage point.

• This method is also referred to as 15/5 distillation, because of 15theoretical plates and a reflux ratio of 5 used in the distillation.

• TBP distillation is used to characterize crude oils and constitute a significantcomponent of crude assay

• A simulated distillation method described in ASTM D2887 may also be usedto obtain TBP data for crude oils.

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TBP Curves

Distillation curve for perfect separation of the components A and B in a binary mixture.

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If a mixture of A (70%) & B (30%) distilled via TBP with TBP curve will be

ASTM Distillation or Engler Distillation

• ASTM D86 and D1160 distillations are run in an Engler flask.• It is a Non fractionating batch distillation,• No packing is employed, and reflux results only from heat losses through

the neck of the flask.• ASTM is used for petroleum products• ASTM distillations are more widely used than TBP distillations because

• It is simpler,• less expensive,• require less sample,• And require only approximately one-tenth as much time.• ASTM distillations are standardized where as TBP distillations vary appreciably in

procedure and apparatus.

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ASTM D86

• This method is used for the distillation of motor gasolines, aviationgasolines, aviation turbine fuels, naphthas, kerosines, gas oils,distillate fuel oils, and similar petroleum products.

• It is carried out at atmospheric pressure.• An exposed thermometer is used.• ASTMD86 distillations are plotted in volume percent.

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ASTM Method D1160

• This method is used for heavy petroleum products which canbe vaporized partially or completely at a maximum liquidtemperature of 750 F at absolute pressures down to 1 mmHg and condensed at the pressures of the test.

• It is carried out at pressures between 1 mm Hg and 50 mmHg, absolute.

• Temperatures are measured with a thermocouple.• ASTM D1160 distillations are plotted in volume percent

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ASTM Distillation

TBP distillation curve for distillation of the same binary mixture.

Distillation curve for ASTM distillation of the binary mixture A and B.

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If a mixture of A (70%) & B (30%) distilled via TBP & ASTM then curve will be

EFV Distillation

• This is known as Equilibrium Flash Vaporization distillation.• This distillation can be run at atm pressure, above atmospheric as

well as under vacuum.• The EFV initial boiling point is the bubble point of the fraction under

study, and the EFV final boiling point is its dew point.• The EFV curve is used for determining phase behavior of liquids and is

usually of designer interest.• Limitation: EFV curves are rarely run because of the time and

expenses involved and are almost exclusively limited either to crudeoil or to reduced crude samples.

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EEFV Distillation CurveEquilibrium flash vaporizationinvolves heating a flowingfeed and the separation ofthe liquid and vapor in a flashdrum. A distillation curvemay be obtained byconducting this distillation atvarying heater outlettemperatures.

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Ta

Tb

Simulated Distillation ASTM D2887

• Simulated Distillation of Petroleum Products (ASTM D2887) by GC in less than two minutes

• Advantage of ASTM 2887• The determination of the boiling range distribution of petroleum fractions by

conventional GC is a rapid analytical tool, which is widely used to replaceconventional distillation methods for control of refining operations andspecification testing.

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Comparison between curves

• The purpose of running these distillations is to characterize thecomposition of material under study.

• TBP is theoretically indicative of actual composition• ASTM is the one used by both designers and operators in distillation.• EFV curve is used for determining phase behavior of liquids and thus

is interest of designers.

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Comparison of Distillation Curves

TBP ASTM EFV

Batch Type Batch Type Batch Type

Fractionating Non Fractionating Non Fractionating

Crude oil Petroleum Products Crude oilHigh Reflux No Reflux No Reflux

atmospheric pressure & Vacuum pressure

atmospheric pressure & Vacuum pressure

Above atmospheric pressure, atmospheric pressure & Vacuum pressure

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Comparison of Curves

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Comparison between curves

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Comparison between curves

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Conversion of Distillation Curves

• All the above mentioned boiling points can be inter convertable

• Both correlations & graphical methods are available for conversion.

• Conversion methods

• Using correlation

• Using Charts and graphs

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Correlation for ASTM D86 TO TBP

Where,T’ = Temperature of TBP test, [K]T= D86 temperature, [K]a, b = constant depending upon the fraction distilled.

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Question: Convert ASTM into TBP

% distilledvolume

Coefficienta

Coefficientb

TemperatureD 86, °C

TBP Temperature, oC

0 0.9177 1.0019 36.5 1410 0.5564 1.0900 54.0 3330 0.7617 1.0425 77.0 6950 0.9013 1.0176 101.5 10270 0.8821 1.0226 131.0 13590 0.9552 1.0110 171.0 18195 0.8177 1.0355 186.5 194

Sheet S01

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Conversion of curves by Graphical Method

• All the above mentioned boiling points can be inter convertible• Inter convertability of boiling points can be worked out by knowing

the slope of distillation curve of a fraction.

`TBP slop

• Where 70% and 10% are volumetric boiling points on vaporization curve

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• The conversion of TBP slope to ASTM or EFV slope can be done with the help of following figs.

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Y = 0.8334X − 0.0452

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Relationship b/w ASTM & TBP 50 Vol %

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ASTM = 1.05 × TBP

Relation b/w ASTM & TBP IBP & FBP

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Relation of T50% of EFV & ASTM/TBP

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Question

• Convert TBP data into ASTM & EFV• Overall basis

• Convert following TPB cuts into ASTM & EFV• 20-50 vol % cuts• 50-80 vol % cuts

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Common Terms Used

• Cuts: portion of whole crude/ part of whole crude/ fraction of whole crude, e.g: cut between 20 vol% to 50 vol %.

• Mathematically we can use word: “Range” for cut• Range have two parts/ limits: Initial value/ limit & Final value/ limit• Some Calculations:

• Mid Point of Cut/ T50vol%• What is mid volume % of cut having range 20-50 vol% = 35 vol%• What is 10 vol% of this cut = 23 vol%• What is 70 vol% of this cut = 41 vol %

• Yield: it is amount of cut produce , volume basis or mass basis• Yield calculation: final value of range – initial value of range

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