basic definitions and correlations

8/3/2019 Basic Definitions and Correlations

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AN INTRODUCTION TO CRUDE OIL AND ITS PROCESSING 9

The next few sections ofthis chapter illustrate how the assay data and basic petroleumrefining processes are used to develop a process configuration for an oil refiningcomplex.

Other basic definitions and correlationsAs described earlier the composition of crude oil and its fractions are not expressedin terms of pure components, but as 'cuts' expressed between a range of boilingpoints. These 'cuts' are further defined by splitting them into smaller sections andtreating those sections as though they were pure components. As such, each of thesecomponents will have precise properties such as specific gravity, viscosity, moleweight, pour point, etc. These components are referred to as pseudo components andare defined in terms of their mid boiling point.Before describing in detail the determination of pseudo components and their appli-cation in the prediction of the properties of crude oil fractions it is necessary to definesome of the terms used in the crude oil analysis. These are as follows:Cut pointA cut point is defined as that temperature on the whole crude TBP curve that representsthe limits (upper and lower) of a fraction to be produced. Consider the curve shownin Figure 1.1 of a typical crude oil TBP curve.

Gas OilsKero

Full range Naphtha

Residue

Temp

% DistilledFigure 1.1. Cut points and end points .


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10 CHAPTER 1

A fraction with an upper cut point of 100F produces a yield of 20% volume of thewhole crude as that fraction. The next adjacent fraction has a lower cut point of lOOFand an upper one of 200F this represents a yieldof30-20% = 10% volume on crudeEndpointsWhile the cut point is an ideal temperature used to define the yield of a fraction, theend points are the actual terminal temperatures of a fraction produced commercially.No process has the capability to separate perfectly the components of one fractionfrom adjacent ones. When two fractions are separated in a commercial process someofthe lighter components remain in the adjacent lighter fraction. Likewise some oftheheavier components in the fraction find their way into the adjacent heavier fraction.Thus, the actual IBP of the fraction will be lower than the initial cut point, and its FBPwill be higher than the corresponding final cut point. This is also shown in Figure 1.1.Mid boiling point componentsIn compiling the assay narrow boiling fractions are distilled from the crude, and areanalyzed to determine their properties. These are then plotted against the mid boilingpoint of these fractions to produce a smooth correlation curve. To apply these curvesfor a particular calculation itis necessary to divide the TBP curve ofthe crude, or frac-tions of the crude, into mid boiling point components. To do this, consider Figure 1.2.For the first component take an arbitrary temperature point A. Draw a horizontal linethrough this from the 0% volume. Extend the line until the area between the lineand the curve on both sides of the temperature point A are equal. The length of thehorizontal line measures the yield of component A having a mid boiling point A "ERepeat for the next adjacent component and continue until the whole curve is dividedinto these mid boiling point components.Mid volume percentage point componentsSometimes the assay has been so constructed as to correlate the crude oil propertiesagainst components on a mid volume percentage basis. In using such data as this theTBP curve is divided into mid volume point components. This is easier than the midboiling point concept and requires only that the curve be divided into a number ofvolumetric sections. The mid volume figure for each of these sections is merely thearithmetic mean of the volume range of each component.Using these definitions the determination of the product properties can proceed usingthe distillation curves for the products, the pseudo component concept, and the assaydata. This is given in the following items:Predicting TEP and ASTM curves from assay dataThe properties of products can be predicted by constructing mid boiling point com-ponents from a TBP curve and assigning the properties to each of these components.


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PointA

Vol%Figure 1.2. Example of mid boiling points.

These assigned properties are obtained either from the assay data, known compo-nents of similar boiling points, or established relationships such as gravity, molecularweights, and boiling points. However, before these mid boiling points (pseudo) com-ponents can be developed it is necessary to know the shape of the product TBP curve.The following is a method by which this can be achieved. Good, Connel et al. (1)accumulated data to relate the ASTM end point to a TBP cut point over the light andmiddle distillate range of crude. Their correlation curves are given in Figure 1.3, andare self explanatory. Thrift (2) derived a probable shape of ASTM data. The proba-bility graph that he developed is given as Figure 1 0 4 . The product ASTM curve froma well designed unit would be a straight line from 0 %vol to 100 %vol on this graph.Using these two graphs it is possible now to predict the ASTM distillation curve of aproduct knowing only its TBP cut range.


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. . . . "",;" . . . ."';_,;_:" . . . .'~ ~-

40

200 300 400 500 60 0 70 0TBP Cut Point of

A End Points Vs TBP Cut Point for fractions starting at 200F TBP or LowerB End Points Vs TBP Cut Point for fractions starting at 300C End Points Vs TBP Cut Point for fractions starting at 400D End Points Vs TBP Cut Point for fractions starting at 500E & F ASTM End Points Vs TBP Cut Point 300 ml STD col & 5 ft Packed Towers.G 90% vol temp Vs 90% vol TBP cut (All Fractions).

Figure 1.3. Correlation between TBP and ASTM end points .

An example of this calculation is given below:Itis required to predict the ASTM distillation curve for Kerosene, cut between 387Fand 432F cut points on Kuwait crude.


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700

600

500

400

LL 3500~::J 300" CJ)0..E 250CJ)I-

200

150

100


50+--+-r++---+-+-+-+~t-+-I-++~++-I-~~-+---+-I--+~--~IBP 2 345 10

Solution:

20 30 40 50 60 70 80 90 95 97Vol Percent Over

Figure 1.4. ASTM distillation probability curves.

Yield on crude = 3.9% volCut range = 27.3-31.2% vol on crude.90%Vol of cut = 30.81 which is = 430FFrom Figure 1.3, curve B ASTM end point = 432 - 13F = 419FFrom Figure 1.3, curve G ASTM 90% point = 430 - 24F = 406F

EP

These two points are plotted in Figure 1.4 and a straight line drawn through them todefine the probable ASTM distillation of the cut. This is plotted linearly in Figure 1.5


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14 CHAPTER 1

Lab Data 420

Calculated400

380

360

340~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~o 10 20 30 40 50 60 70 80 90 FBP

%VolFigure 1.5. Comparison between calculated ASTM curve and lab data.

and can be seen to compare well with laboratory results of the actual product from acrude distillation unit.

Developing the TEP curve and the EFV curve from the ASTM distillation curveUsing a product ASTM distillation curve developed as shown above the TBP curveis developed as follows.Converting theproduct ASTM distillation to TEPMost crude distillation units take a full range naphtha cut as the overhead product.This cut contains all the light ends, ethane through pentanes, in the crude and of coursethe heavier naphtha cut. All the light ends are in solution, therefore it is not possibleto prepare a meaningful ASTM distillation on this material directly. Two routes canbe adopted in this case, the first is to take naphtha samples of the heavy naphtha anddebutanized light naphtha from downstream units. Alternatively the sample can besubject to light end analysis in the lab such as using POD apparatus (Podbielniak)and carrying out an ASTM distillation on the stabilized sample. Itis the second routethat is chosen for this case.There are two well-proven methods for this conversion. The first is by Edmister (3)and given in his book Applied Thermodynamics and the second by Maxwell (4) in hisbook Data Eook on Hydrocarbons. The correlation curves from both these sources


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ASTM Temperature Difference of0 10 20 30 40 80 90 100 110 120 130 140 150 160 170 180210

200

190 ASTM Temp Dif fvs180 TBPTemp D if f

170Reference:

160 Edolater-Okanato , Pel , Ref .Number 3, pp 11 7- 129 ( 1150

140

130

120

110

100

90 70

80 60

70 50

60 40AST M 50% Temp

50 vs 30TBP 50% Temp40 20

30 10

20

10 -10-20

100 200 300 400 500 600 700 800 900ASTM 50% Temperature of

Figure 1.6. ASTM-TBP correlation-Edmister method.

are given as Figures 1.6 and 1.7. In this exercise Edmister's method and correlationwill be used.The ASTM distillation is tabulated as the temperature for IBP, 10%, 20% throughto the FBP. IB P is the Initial Boiling Point (equivalent to 0% over) and the FBP isthe Final Boiling Point (equivalent to 100% vol over). The multiples of 10% reflectthe volume distilled and the temperature at which each increment is distilled. UsingFigure 1.6 the 50% vol TBP point (in degrees Fahrenheit) is calculated from the 50%vol point of the ASTM distillation.


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CHAPTER 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1Prediction of Flash Reference LineFrom Distillation Reference Lines

Reference:S. D.Maxwell. "Data on Hydrocarbons"pp 222-228. Van Hostrand Company.New York. 1950.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 10

60: : : JIT:LL 40. . . JIT:05 i 203 : : i

0 0

0.8uj(5, . , 0.6'"Cs3 : : i 0.4AJ::C 0.2'"~~ 0 0

2 3 4 5 6 7 8 9 10 11 12Slope of Crude Assay (TBP) Distillation Reference Line cF/%

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 Prediction of FRL

50% Point

2 3 4 5 6 7 8 9 10 11 12Slope of Crude Assay (TBP) Distillation Reference Line cF/%

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Prediction of Flash Curvefrom its Reference Line

Crude Assa TBP Disti lla tion

10 20 30 40 50 60 70 80 90 100 110 120Percent off

NOTE: Flash and disti llation reference l ines (FRL and DRL) are straight l ines through the 10%and 70% points. The temperature of the 50% points refer to these reference l ines. i lt ' isthe deparature of the actual flash and disti llation curves from their respectivereference l ines. While the individual ( il t' )'s may be either plus or minus, the ratiois always poistive.

Figure 1.7. EFV- TBP correlation-Maxwell method.


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Table 1.2. Converting ASTM to TBP distillationASTM (Lab Data) TBP (from Figure 1.6)of ,A.F ,A.F of

IBP 424 29 61 36110%vol 453 31 52 42330 %vol 484 18 52 47550 %vol 502 50770 %vol 504 2 31 53890 %vol 536 32 41 579FBP 570 34 40 619

Figure 1.6 is then used to determine the TBP temperature difference from the ASTMtemperature difference for the O-lO% vol, 10-30% vol, 30-50% vol, 50-70% vol,70-90% vol, and 90-100% vol. Moving from the established 50% vol TBP figure andusing the temperature differences given by Figure 1.6 the TBP temperatures at 0, 10,30,50, 70, 90, and 100% vol are obtained (Table 1.2).

Developing the equilibrium flash vaporization curveThe Maxwell curves given as Figure 1.7 are used to develop the equilibrium flashvaporization curve (EFV) from the TBP. The EFV curve gives the temperature atwhich a required volume of distillate will be vaporized. This distillate vapor is alwaysin equilibrium with its liquid residue. The development of the EFV curve is alwaysat atmospheric pressure. Other temperature and pressure related conditions may bedetermined using the vapor pressure curves or constructing a phase diagram.The TBP reference line (DRL) is first drawn by a straight line through the 10% volpoint and the 70% vol point on the TBP curve. The slope of this line is determinedas temperature difference per volume percent. This data are then used to determinethe 50% volume temperature of a flash reference line (FRL). The curve in Figure 1.7relating I1t50 (DRL-FRL) toDRL slope is used for this. Finally, the curve on Figure 1.7relating the ratio of temperature differences between the FRL and flash curve (EFV)from that for the TBP to DRL is applied to each percent volume. From this theatmospheric EFV curve is drawn.A sample calculation for the compilation of the EFV curve follows. Note the TBPcurve is used to define product yields while the EFV curve is used to define tempera-ture/pressure conditions in distillation. This example uses the TBP curve developedabove as a starting point (Table 1.3).The resulting TBP curves and EFV curves are shown in Figure 1.8.


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18 CHAPTER 1

Table 1.3. Converting TBP to EFV disti llationf'..t (Flash - FRL)

% Volume f'..t (TBP - DRL), OF f'..t (TBP - DRL) f'..t (Flash - FRL), OF Flash, OF0 -46 0.2 -9.2 45310 0 0.4 0 46920 9 0.38 3.4 48230 14 0.37 5.2 49140 13 0.37 4.8 49850 7 0.37 2.6 50760 4 0.37 1.5 51170 0 0.37 0 51480 -2 0.37 -0.8 52390 0 0.37 0 531100 22 0.37 8.1 547

Predicting product qualitiesThe following paragraphs describe the prediction of product properties using pseudocomponents (mid boiling point) and assay data. A diesel cut with TBP cut points 432 ofto 595F on Kuwait crude (Figure 1.9)will be used to illustrate these calculations. Theactual TBP of this cut is predicted using the method already described. The curve isthen divided into about six pseudo mid boiling point components as described earlierand is shown as Figure 1.10.Predicting the gravity of theproductUsing the mid boiling point versus specific gravity curve from the assay given inthe Appendix, the SG for each component is obtained. The weight factor for eachcomponent is then obtained by multiplying the volume percent of that component bythe specific gravity. The sum of the weight factors divided by the 100% volume totalis the specific gravity of the gas oil cut. This is shown in Table 1 0 4 .The prediction ofproduct sulfur contentThe prediction of sulfur content is similar to the method used for gravity. First the TBPcurve for the product is determined and split into pseudo boiling point components.The weight factor is then determined for each component as before. Note that sulfurcontent is always quoted as a percent weight. Using the relationship of percent sulfurto mid boiling point given in the assay the sulfur content of each component is readoff. This is multiplied by the weight factor for each component to give a sulfur factor.The sum of the total sulfur factors divided by the total weight factor gives the weightpercent sulfur content of the fraction. For example, using the same gas oil cut asbefore its sulfur content is determined as shown in Table 1.5.


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/'Y ///' (/~

./ -;:::/-;7--::~ - -.,::?-- \EFV -/ . . . . . . . . . . - : : ASTM-:/;/I///

60 0

50 0

40 0

o 10 20 30 40 50 60 70 80 30 090 100%Vol

Figure 1.S. Types of distillation curves.

Viscosity prediction from the crude assayUnlike sulfur content and gravity, viscosity cannot be arithmetically related directlyto components. To determine the viscosity of a blend of two or more components, ablending index must be used. A graph ofthese indices is given in Maxwell "Data Bookon Hydrocarbons," and part of this graph is reproduced as Figure 1.11. Using the


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20 CHAPTER 1

70

80 0 80

API70 0

600 60

APITemperature

~ 500 50!'!:J" QlQ_E 40~ 400

300 30 3.0

20 0 20 2. 0.2:;rJ)

Sulfur ~- ; { ! _

10 1. 0 010 0

% Vol Disti lled or Mid vol

Figure 1.9. Typical crude assay curves (based on Kuwait crude).


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22 CHAPTER 1

Table 1.4. Calculating the SG of a cutComponent Volume % Mid-BPt, OF SG@60F Weight factor

13.0 410 0.793 10.32 16.5 460 0.801 13.23 21.0 489 0.836 17.64 18.0 520 0.844 15.25 18.5 550 0.846 15.76 13.0 592 0.850 11.1Total 100.0 83.1SG of cut = ~~.~= 0.831.

indices do not warrant this. These properties are therefore read off directly from themid boiling point of the whole product. Considering the gas oil used in the previousexample, its mid boiling point is about SlOoF, from the crude assay its pour pointis -SOF and cloud point is +4F. Determining pour point for a blend of two ormore products is rather more difficult. In this case blending indices are used for thispurpose. A graph of these indices is given as Figure 1.12. Itis self explanatory andits application is explained in Table 1.7.Flash pointsThe flash point of a product is related to its ASTM distillation by the expression:

flash point = O.77(ASTM S% in F-lS0F)Thus for the gas oil product in the above the example the flash point will be:

flash point = 0.77(420 - I S O ) = 208F.

Table 1.5. Calculating the sul fur content of a cutComponent Weight factor Mid BPt, OF Sulfur, %wt Sulfur factor

10.3 410 0.2 2.062 13.2 460 0041 50413 17.6 489 0.84 14.784 15.2 520 1.16 17.635 15.7 550 1.35 21.26 11.1 592 1.5 16.65Total 83.1 77.73Sulfur % weight = ~n 100 = 0.935 %wt. (actual plant data gave 0.931 %wt.)


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Table 1.6. Calculating the viscosity of a cutComponent Volume % Mid BPt OF Viscosity Cs lOOF Blending index Viscosity factor

(A) (B) (A x B)13.0 410 1.49 63.5 825.5

2 16.5 460 2.0 58.0 9573 21.0 489 2.4 55.0 1,1554 18.0 520 2.9 52.5 9455 18.5 550 3.7 49.0 906.56 13.0 592 4.8 46.0 598Total 100.0 5,387.0Overall viscosity index = 5i~~7 ~53.87.From Figure 1.8 an index of 53.87 = 2.65 Cs (actual plant test data was 2.7 Cs).

Blending products of different flash pointsAs with pour points and viscosity, the flash point of ablend oftwo ormore componentsis determined by using a flash blending index. Figure 1.13 gives these indices. Againthe indices are blended linearly as in the case of viscosity. Consider the followingexample:2,000 BPSD of Kerosene with a flash point of 120F is to be blended with 8,000BPSD of fuel oil with a flash point of 250F. Calculate the flash point of the blend(Table 1.8).

Predicting the mole weights ofproductsThe prediction of molecular weights of product streams is more often required forthe design of the processes that are going to produce those products. There are othermore rigorous calculations that can and are used for definitive design and in buildingup computer simulation packages. The method presented here is a simple method bywhich the mole weight of a product stream can be determined from a laboratory ASTMdistillation test. The result is sufficiently accurate for use in refinery configurationstudies and the like.A relationship exists between the mean average boiling point of a product (commonlydesignated as MEABP), the API gravity, and the molecular weight of petroleumfractions. This is shown as Figure 1.14.Using a gas oil fraction as an example, the MEABP of the product is calculated fromits ASTM distillation in degrees Fahrenheit given below:


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./V / / / v/ / 1/ / /V / / // / // V v // /// // /v /// /V/-50 -40 -30 -20 -10 0 0.810 20 30 40 50

Pour Points ofFigure 1.12. Pour point blending index.

10090807060504030

20

109876543

2

1O .0.10.20.30.40.50.6

0.7


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% Vol OF0 40610 44730 46950 48770 50790 538100 578

rO F @ 90% - rOF@10%The slope of the curve is calculated by 80538 - 447= = 1.14F/%80

Volume average boiling point rO F @ 10% + (2 x rO F @ 50%) + rO F @90%4447 + 974 + 538 = 4900F

4From the upper series of curves given in Figure 1.14 the correction to the volumetricaverage boiling point (VABP) to obtain the Mean Average (MEABP) is -5E Thus,the MEABP is

490 + (-5) = 485FThe API of the stream from the calculation for gravity is 38.8. Using this figure andthe MEABP in the lower series of curves in Figure 1.14 a molecular weight of 20 1 isread off.

Table 1.7. Calculat ing pour points of a cutComposition ASTM dist Pour point

Components BPSD Fraction 50%OF Factor Pour point Index Factorof

Gas oil 2,000 0.33 500 85.8 -5 5.8 1.9Waxy dist 4,000 0.67 700 249 30 12.7 8.5Total 6,000 1.00 334 22 10.4The pour point of the blend is read from Figure 1.9 where the ASTM 50% point is 334Fand the index is 10.4. In this case the pour point is 22F.


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basic definitions and correlations

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