PrintPour Point Blending
Pour point is an important property for diesel and fuel oil blends. Pour point blending is also non-linear, and pour point blending indices were developed to enable reliable calculation of the pour points of the blends. Pour point blending indices for some distillate fuels are given in Table 12.7 of the textbook, and copied in Figure 9.11, where the blending indices are tabulated as a function of ASTM 50% temperature, °F (first horizontal listing) and Pour Point, °F (first vertical listing). For example, pour point blending index for a distillate that has an ASTM 50% temperature of 500°F and a pour point of 40°F would be 36. This index can be used in calculating the pour point of blends using this distillate as a component.
As an example of using blending indices to calculate the pour point of a blend, consider blending a straight-run gas oil (50% ASTM T = 470°F and pour point = -6°F) and a hydrotreated heavy gas oil (50% ASTM T = 620°F and pour point = 40°F). What would be the pour point of a binary blend that consists of 68.7 % vol of straight-run gas oil and 31.3% vol of hydrotreated heavy gas oil?
The procedure used to calculate the pour point of the blend (shown Figure 9.12) can be summarized as follows:
- Read pour point blending indices for the two distillates in the table (PPI column) shown in Figure 9.11, and list it in the table (Figure 9.12). Straight- run gas oil PPBI =6.3 (using double interpolation), and Hydrotreated heavy gas oil PPBI = 26.
- Multiply PPBI for individual distillates with their respective volume fraction to calculate the Pour Factor. For example, the Pour Factor for straight-run gas oil = 0.687 x 6.3 = 4.33. Add the pour factors for each component, to calculate the blending index for the blend = 12.47.
- Calculate blend 50% ASTM temperature (linearly additive) of the blend by multiplying the volume fraction with ASTM 50% T of each component and adding them together (0.687x470 +0.313x620 = 517).
- Using the blend ASTM 50% just calculated (517) and the blending index for the blend from the pour factors (12.47), interpolate/read the pour point of the blend from the table in Figure 9.11, as 15°F.
Note that if you would assume linear addition of the pour points, you would calculate a blend pour point as = 0.687x(-6) +0.313x40 = 8.4°F. A serious underestimation of the pour point! Thinking that your diesel fuel has a pour point 8.4 °F, you may try to start your diesel truck on a 12°F day, to no avail, not knowing that your fuel tank has a gel, and not a liquid that can be easily pumped to the combustion cylinder.
Similar to pour point, the viscosity of blends can also be calculates using blending index numbers, or plots developed for this purpose. For the same blend, we can use the Viscosity Blending Index Numbers in Table 12. 3. of your textbook [6], and the procedure shown in Figure 9.13 to calculate the viscosity of the blend. Try to verify the numbers given in Figure 9.13, and calculate a viscosity of the blend if viscosities were linearly additive to compare with the value calculated in Figure 9.13. Post your questions, if any, and comments in the Help Discussion Forum.
| Pour Point | Volume Fraction |
50% ASTM T, ºF |
Pour Point Index (PPI) |
Pour Factor (PPI x Volume Fraction) |
Blend (50% ASTM T x Volume Fraction |
|
| Straight-run gas oil | -6ºF | 0.687 | 470 | 6.3 | 4.33 | 323 |
| Hydrotreated heavy gas oil | 40ºF | 0.313 | 620 | 26 | 8.14 | 194 |
| Total | - | - | - | - | 12.47 | 517 |
Read chart using this information to get a pour point of 15ºF
| Volume | Viscosity | Viscosity Index (Table 12.3 in the textbook) | Factor (VI x Vol) | |
| SRGO | 0.687 | 2.0 cst | 0.290 | 0.199 |
| HHGO | 0.313 | 8.0 cst | 0.443 | 0.139 |
| Total | - | - | - | 0.338 |
From the table, you get: Viscosity = 2.9 cst
[6] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 12, Product Blending, p.263.