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Petroleum Processing

Elemental Analysis and Ternary Classification of Crude Oils


Elemental Analysis and Ternary Classification of Crude Oils

Despite a wide variety of crude oil found in different parts of the earth, the elemental composition of most crude oils changes in narrow ranges, as shown in Table 2.2.

Table 2.2: Elemental Composition of Crude Oils
Element % Wt
C 84-86%
H 11-14%
S 0-6%
N 0-1%
O 0-2%

With such narrow ranges of change in elemental contents, elemental composition does not have much utility for classification of crude oil. Instead, variations in hydrocarbon composition (paraffins, naphthenes, and aromatics) are used to classify crude oils, using a ternary diagram, shown in Figure 2.2. Each apex of the triangle represents 100 percent weight of the corresponding compounds, and 0% of this particular type of hydrocarbons on the side of the triangle across from the apex. For example, the side at the bottom of the triangle (across from the apex of 100% aromatics) represents binary mixtures of paraffins and naphthenes.

Triangle diagram. Top: 100% Aromatics, Bottom left: 100% Paraffins, Bottom right: 100% Naphthenes
Figure 2.2. A ternary diagram for classification of crude oils
Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

If you need to refresh your memory on reading ternary diagrams, you may check "Reading a Ternary Diagram", or consult other sources. The list below shows the six classes of crude oil that are defined using a ternary diagram. These classes are shown as areas on the ternary diagram for paraffins, below. It is generally accepted that Class 1 (rich in paraffins) represents the most desirable type of crude oil because refining these crudes would readily lead to high yields of light and middle distillates that constitute the fuels such as gasoline, diesel fuel, and jet fuel which are in high demand. Extensive refining would be required to produce high yields of distillate fuels from aromatic crudes (e.g., Class 4-6). Class 1 crudes tend to have high °API and low sulfur contents and tend to be more expensive than the other types of crude oils.

Ternary Classification of Crude Oils

  1. Paraffinic Crudes
    paraffins + naphthenes > 50%
    paraffins > naphthenes
    paraffins > 40%
  2. Naphthenic Crudes
    paraffins +naphthenes > 50%
    naphthenes > paraffins
    naphthenes > 40%
  3. Paraffinic-Naphthenic Crudes
    Aromatics < 50%
    paraffins < 40%
    naphthenes < 40%
  4. Aromatic-Naphthenic Crudes
    Aromatics > 50%
    naphthenes > 25%
    paraffins < 10%
  5. Aromatic-Intermediate Crudes
    Aromatic > 50%
    paraffins > 10%
  6. Aromatic-Asphaltic Crudes
    Naphthenes < 25%
    paraffins < 10%

Video: Lesson 2 Paraffins (2:47)

Placement of Class 1 Paraffinic Crudes on the ternary diagram.
Click here for transcript of Lesson 2 Paraffins

Ternary diagrams could be very useful tools for classifying crude oils. Here you see a ternary diagram, a triangle in essence. On the corners of the triangles, we can see pure hydrocarbons. At the top, where you see 100% aromatics, that is just one point at the top represents pure aromatic compounds.

On the left-hand side at the bottom, 100% paraffins point on the corner, and on the right, 100% naphthenes. The lines that connect these points represent binary mixtures. For example, if you connect aromatics corner with the paraffins on that line, you will only have aromatics and paraffins.

As an example, let us define the region in this ternary diagram for the group 1 or classification 1 crude oils that are paraffin liquid oils. You see the horizontal line with an arrow pointing downward. So below that line, the contents of paraffins and naphthenes is greater than 50%. Above that line, obviously, aromatics are greater than 50%.

Now, establish the second boundary line for the group 1 crude oils. That is paraffinic crude oils. You see the vertical line right in the middle of the triangle separating the triangle into two areas. To the left of that vertical line, we will have paraffins contents greater than naphthenes everywhere to the left of this point.

So with these two boundary lines then, one horizontal, one vertical, we have established a region where the contents of paraffins and naphthenes are greater than 50%, and paraffin contents are greater than naphthenes.

To establish the region for the type 1 crude oils, we need the third boundary line. So you see here the line that is designating paraffin contents greater than 40%. So to the left of that line in the triangle, the paraffin contents is greater than 40%. So all these three lines then designate the group 1 or type 1 crude oil or region in the ternary diagram that is the paraffinic crude oils region.

Credit: Dutton Institute © Penn State is licensed under CC BY-NC-SA 4.0

Video: Lesson 2 Aromatics (2:34)

Placement of Class 4, 5, and 6 Crudes on the ternary diagram.
Click here for transcript of Lesson 2 Aromatics

Let us continue with demonstration of using ternary diagrams for crude classification. This time, we will place the type 4 aromatic-naphthenic crudes on the ternary diagram. You'll see the boundaries for this type of crude. That is, aromatics greater than 50%, naphthenes greater than 25%, paraffins less than 10% in type 4 crude oils.

See the horizontal line right in the middle of the triangle, which designates the first boundary? That's aromatics greater than 50%. So above that line, in the orange region in the triangle, the aromatic content is greater than 50%. That is the first boundary.

The second boundary line defines an area where naphthenes are greater than 25%. You see the line on the triangle to the right of that line? [? Everywhere ?] is naphthenes greater than 25%. With these two boundary lines now, aromatics greater than 50% and naphthenes greater than 25%, we are confined to that little triangle, the orange triangle in the ternary diagram.

Now we need to place the third boundary line. The third boundary line for this type is paraffins less than 10%. You see the gray shaded region, which is essentially type 4 aromatic-naphthenic crudes region, as we have established.

We can now identify the region for group 5 crude oils. That is aromatic-intermediate crudes. That is bound by two lines.

So you can see the purple region, purple triangle, bounded by paraffins greater than 10% and aromatics greater than 50%. This is number 5 type aromatic-intermediate crudes. With these boundary lines, we can now identify the region for type 6 aromatic-asphaltic crudes. So you can see the orange region which is bound by naphthenes less than 25% and paraffins less than 10%.

Credit: Dutton Institute © Penn State is licensed under CC BY-NC-SA 4.0