GRAPHICAL CORRELATION CALCULATES DIESEL PROPERTIES FROM KNOWN VALUES
H. U. Khan, M. M. Mungali, K. M. Agrawal, G. C. Joshi
Indian Institute of Petroleum
Dehradun, India
A graphical correlation has been developed between pour point, density, n-alkane content, and volume composition of diesel oil blends made from four distillate fractions.
From the value of any two of these parameters, the remaining two properties, except volume composition, can be estimated with good precision.
For determining the volume composition of the blend, all three physical properties must be known.
The results of this study can be used in refinery blending schemes to meet pour point specifications.
BACKGROUND
Diesel fuel is produced by blending various distillate fractions, generally having a boiling range of 140-380 C. When diesel oil blends are developed for cold climates, cold-flow properties-cloud point, pour point, and cold filter plugging point (CFPP)are important parameters to consider.
These cold-flow properties are commonly believed to be predominantly controlled by the n-alkane content of the fuel.
Recent attempts have been made to develop mathematical correlations between single cold-flow properties and average molecular weight, n-paraffin content, or blend composition of diesel fuel.' I However, most of these studies have limited applications because they are generally based on one or two paraffins in the base fuel.
Earlier studies at the Indian Institute of Petroleum attempted to develop a graphical correlation between cloud point and volume composition of diesel fuel blends of three and four components.' The resulting correlation can be used for predicting diesel blend cloud point within the repeatability limits of the ASTM D-2500 method of cloud point determination.
The latest study was aimed at developing a graphical correlation between pour point, density, n-alkane content, and volume composition that could be used to determine any of these blend properties with good precision when the other properties are known.
EXPERIMENTS
Four distillate fractions, boiling in the ranges of 140-180 C., 180-230 C., 230-300 C., and 300-360 C., were obtained by distilling Bombay High crude oil. The physicochemical characteristics of these distillates are given in Table 1.
These fractions are heavy naphtha (Z1), kerosine (Z2), light gas oil (Z,3), and heavy gas oil (Z4).
Keeping Z, at 5 vol %, 10 blends were prepared containing different proportions Of Z2, Z3, and Z4.
One blend contained 95 Vol % Z3. In the other nine blends, Z2, Z3, and Z4 were varied so that there was 0 vol % of one component and 20, 50, or 80 vol % of a second component; the balance comprising the third component (Table 1).
The pour point, density, and n-alkane content of each blend was measured. The composition and properties of the blends are shown in Table 2.
NOMOGRAPH DEVELOPMENT
Three-dimensional coordinates were established from the experimental results, using n-alkane content, density, and pour point for the V Y, and Z axes, respectively. The experimental results for each of the 10 blends were plotted on the coordinates (Fig. 1) as follows. From points representing any two of the three parameters of a blend, lines parallel to the opposite axis were drawn.
From the intersection of these lines, a third line parallel to the remaining axis was drawn, having a length equal to the value of the property corresponding to that axis, thereby producing the required point in the three-dimensional plot.
Points representing the three blends containing 0 vol % Of Z2 fall on a straight line (BC). Likewise, points representing blends containing 0 Vol % Of Z3 and Z4 fall on lines AC and AB, respectively.
Extension of these three lines in both directions results in the formation of a triangular graph, or nomograph-ABC (Fig. 1). In this graph, the points A, B, and C correspond to 95 vol % Of Z2, Z3, and Z4, respectively.
It was presumed that in this triangle, lines parallel to AB, AC, and BC will correspond to a constant volume percent Of Z4, Z3, and Z2, respectively (each at 5 vol % of the Z1). Z2, Z3, and Z4 were varied by increments of 5 vol %, and the triangular graph ABC was completed by extrapolation (represented by dashed lines in Fig. 1).
The plot so obtained represents the correlation between these four parameters of the diesel oil blends: density (d4l5, as per IP 190/79); pour point, C.; n-alkane content, wt %; and volume composition.
This nomograph indicates that, by knowing the volume composition at 5 vol % Z, and any one of the three blend properties, the other two properties can be calculated. However, for determining the composition of the blends, all three properties must be known.
Additional nomographs can be constructed for other values of Z1, or heavy naphtha.
Each nomograph would represent the entire range of combinations of other fractions making up the blend.
NOMOGRAPH VALIDITY
To verify the validity of the nomograph, 16 additional blends containing 5 vol % Z1 and varying percentages of the remaining three fractions were prepared. These blends, along with their experimentally obtained densities, pour points, and n-alkane contents, are shown in Table 3.
Using the value of one of the properties and the blend composition, the remaining two properties were calculated from the nomograph (Fig. 1).
The blend compositions were also determined from the nomograph using the known values of the three parameters.
The values of all parameters calculated from the nomograph are shown in parentheses in Table 3. The calculated values are within experimental error of the experimentally obtained values, showing good correlation between the two.
NOMOGRAPH USE
The following examples illustrate the use of the nomograph.
Data given in Table 3 for blend numbers 4, 5, 10, and 14, are used in these examples.
A. Determination of pour point and n-alkane content of a blend, from known composition and density.
For a blend of 5 vol % heavy naphtha (Z1), 5 vol % kerosine (Z2), 60 vol % light gas oil (Z3), and 30 vol % heavy gas oil (Z4), the compositional point A' is established in the nomograph.
From this point, a line parallel to the Y axis is drawn and A'B', equal to the density of the blend (0.8454), is cut from this line.
Then from B', lines parallel to the X and Z axes are drawn, which meet these axes at points C' and D', respectively.
These points correspond to an n-alkane content of 39.92 wt % and a pour point of 2 C. for the blend.
B. Determination of pour point and density from known composition and n-alkane content.
For a blend containing 5 vol % Z1, 30 vol % Z2, 30 vol % Z3, and 35 vol % Z4, and having an n-alkane content of 38.79 wt %, the compositional point Al is established in the plot.
The point B, corresponding to the n-alkane content (38.79 wt %) is marked on the X axis.
Then from these points, lines parallel to the Y and Z axes, respectively, are drawn, which intersect each other at point C1. From C1, a line parallel to the X axis is drawn meeting the Z axis at D1.
The lines A,Cl and OD, (O being the origin) correspond to the density (0-8380) and pour point (4 C.) of the blend, respectively.
C. Determination of density and n-alkane content of the blend from its composition and pour point.
For a blend containing 5 vol % Z1, 25 vol % Z2, 50 vol % Z3, and 20 vol % Z4, the compositional point A2 is established as described in Example B.
Then from this point, a line parallel to the Z axis is drawn, from which A2B2, equal to the pour point (-3 C.), is cut.
From B2, lines parallel to the X and Y axes are drawn that meet these axes at points C2 and D2, which correspond to the n-alkane content (38.30 wt %) and density (0.8384) of the blend, respectively.
D. Determination of volume composition of the blend from its density, pour point, and n-alkane content
For a blend having known density (0.8418), pour point (-3 C.), and n-alkane content (38.87 wt %), the point corresponding to these properties is established as follows.
The points A3 and C3 on the Z and Y axes, respectively, are marked. From these points, lines are drawn parallel to the opposite axes, which intersect each other at point B3.
From B3, a line parallel to the X axis is drawn and B3D3, equal to the n-alkane content (38.87 wt %), is cut.
The volume composition corresponding to this point is then read from the nomograph.
The proposed graphical correlation method can thus be a valuable tool for estimating various properties of diesel oil blends with good precision.
Although the developed nomograph is quantitatively valid for particular streams derived from Bombay High crude, similar nomographs could be developed for other crude oil fractions as well.
ACKNOWLEDGMENT
The authors are grateful to CSIR for providing a senior research fellowship to M. M. Mungali, and to Dr. T. S. R. Prasada Rao, director of the Indian Institute of Petroleum, Dehradun, for permission to publish this article.
REFERENCES
- Tsang, C.Y., Ker, Y.S.F., Miranda, R.D., and Wesch, C.J., "Equation predicts diesel cloud points," OGJ, Mar. 28, 1988, pp. 33-36.
- Seglin, L., "Extension developed for NOVA/Husky cloud point," OGJ, Oct. 24, 1988, pp. 68-70.
- Khan, H.U., Mungali, M.M., Agrawal, K.M., and Joshi, G.C., "Graphical method simplifies diesel cloud point determinations," OGJ, Sept, 24, 1990, pp. 98-101.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.