Boling Point Curves

Boling Point Curves

Boling Point CurvesPrepared by:Engr. Adeel ur RehmanAssistant ProfessorDepartment of Chemical EngineeringUniversity of KarachiBoiling Point CurvesThe first step in refinery is distillation in which the crude oil separated into fractions according to its boiling point.

There are at least four types of distillation curves or ways of relating vapor temperature and percentage vaporized.

a) True-boiling-point (T.B.P): (Fractional, run only on crude oil, batch)b) Equilibrium or Flash Vaporization (EFV).c) ASTM or no fractionating distillation: (no fractional , run on fractions)d) Hempel: (Semi fractional).2Boiling Point Curvesa) True-boiling-point (T.B.P): (Fractional, run only on crude oil, batch) .Distillation characteristics of a crude are assessed performing a preliminary distillation called "True Boiling Point" analysis (TBP). This test enlightens the refiners with all possible information regarding the percentage quantum of fractions, base of crude oil and the possible difficulties beset during treatment operation etc.True boiling point (TBP) and gravity-mid percent curves can be developed from U.S Bureau of mines crude petroleum analysis data sheet Fig. (1) which is reported in two portions: The first is the portion of the distillation at atmospheric pressure and up to 527 0F (275 0C) end point, the second at 40 mm Hg total pressure to 572 0F (300 0C) end point.34Boiling Point Curves

Boiling Point Curvesb) Equilibrium or Flash Vaporization (EFV). The feed material is heated as it flows continuously through a heating coil.

As vapor is formed it kept cohesively with liquid at some temperature and a sudden release of pressure quickly flashes or separates the vapor from the mixture without any rectification.

By successive flash evaporation like this the stock can be progressively distilled at different increasing temperatures. a curve of percentage vaporized vs. temperature may be plotted.5Boiling Point Curvesc) ASTM or no fractionating distillation: (no fractional , run on fractions) .

It is supposed to be like EFV, a non fractionating distillation system, distinguishing itself as differential distillation.

It is a simple distillation carried out with standard ASTM flasks 100,200,500 ml flasks. The data obtained is similar to TBP data6Boiling Point Curvesd) Hempel: (Semi fractional).It is considered as a semi-fractionating type of distillation like Saybolt 's, Where TBP data is insufficient , this can be used.7ASTM Distillation ApparatusCondenserMeasuring CylinderHeating SourceEngler FlaskBoiling Point CurvesMinimum fractionation occurs in ASTM distillation, and components in the mixture do not distil one by one in the order of their boiling points, but rather as mixtures.

Nevertheless, because ASTM distillations are quickly conducted and have been successfully automated, require only a small sample, and are quite reproducible, they are widely used.

Because the degree of separation for a TBP distillation test is much higher than that of the ASTM distillation test, its IBP is lower and its EP is higher than those of the ASTM test.

8Boiling Point CurvesThe following equation suggested by Riazi and Daubert (1980) and published by the API (1993) is used for the inter conversion, referred to as API method:TPB = a(ASTM D86)bwhere a and b are constants varying with percent of liquid sample distilled

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Boiling Point CurvesTBP is true boiling point temperatures at 0, 10, 30, 50, 70, 90, and 95 volume percent distilled, in degrees Rankin.

ASTM D86 is the observed ASTM D86 temperatures at corresponding volume percent distilled, in degrees Rankin. Average error between the calculated and measured TBP is in the range of 5 C (41 F).10Boiling Point CurvesMore recently, Daubert (1994) published a new method for distillation curves interconversion using the following equations:

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90959590Boiling Point CurvesThe symbols T and T stands for ASTM D86 and TBP temperatures, respectively, both in F.

The subscripts 0 and f stand for the initial and final temperatures, respectively.

The reported average error for this method is about 3 oC (37.4 oF).

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Boiling Point Curves13



Boiling Point CurvesProbability Chart16






Boiling Point CurvesPseudo-ComponentsSince most of the actual components are not known, the petroleum fractions are characterized as a mixture of discrete pseudo-components with defined boiling point ranges or cut points on the TBP distillation curve.

Each pseudo-component corresponds to several unknown actual compounds (e.g. paraffins, naphthenes and aromatics) which boil in a given temperature range.

Usually each pseudo-component is characterized by an average normal boiling point, specific gravity and molecular weight.

Once these parameters are determined, the pseudo-components can be treated as any defined component for the calculation of thermophysical and thermodynamic property like enthalpy, entropy, and transport properties such as viscosity, thermal conductivity and diffusivity.22Boiling Point CurvesBreakup of TBP Curve into Pseudo-componentsTBP cut point ranges are used to define pseudo-components with the average temperature of the cut or the mid point NBP.The number of such pseudo-components depends on the boiling point range of the whole petroleum fraction.

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Boiling Point CurvesProblemDivide the TBP curve of the petroleum cut as calculated from API method into 20 pseudo - components. Calculate the liquid volume percentage of each pseudo-component.

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Boiling Point CurvesSolution

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Boiling Point CurvesThe end point of the cut is 218.2 oC (425 oF) and the IBP is -5.4 oC (22 oF). Therefore each pseudo-component has a temperature interval of (218.2+ 5.4)/20 or 11.2 oC. Then the curve can be cut as follows. The EBP of the first cut is IBP + 11.2 oC or 5.8 oC. The average boiling point for the first pseudo-component is (5.4 + 5.8)/2 or 0.2 oC. The vol% is 2.84 as read from the TBP curve. The second cut has an end boiling point of 17 oC and an end volume percent of 6.19.Therefore, the vol% of this cut is 6.19 - 2.84 or 3.36 vol%. The average boiling point for the second cut is (17 + 5.8)/2 or 11.4 oC.27Boiling Point Curves28

Boling Point Curves

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