API GRAVITY: SCALE AND CLASSIFICATION OF CRUDE OIL - PHYSICAL - 2025

The API gravity is a specific gravity scale used by the American Petroleum Institute (API) to express the relative density of the crude oil and other petroleum derivatives. Although by definition it is a dimensionless quantity, as we will see shortly, it is always expressed in degrees API.

By means of this scale crude oil is classified as light, medium and heavy. This is very important when determining the value of crude in the market, since a light one requires less refinement than a heavy one.

Figure 1. Light crudes have a higher value in the market, since they require less refinement and are easier to transport. Source: Pixabay.

The quality of the crude oil depends on the composition and the proportions of the hydrocarbon mixture, which present a great variety of molecular weights. Although API gravity does not take into account the actual chemical composition of crude oil, but rather classifies them according to their ease of being transported and refined.

For the classification, the following empirical formula is used:

ºAPI = (141.5 / γ _T) - 131.5 (fluids lighter than water)

γ _T is the specific gravity of the fluid under standard temperature conditions, that is, at 15,556 ° C or 60 ° F.

This is the standard temperature for any operation with crude oil and its derivatives, be it chemical or commercial. For other fluids, a temperature of 4ºC is considered standard.

The specific gravity of a fluid

Specific gravity, also called specific density or relative density, is a dimensionless quantity that compares the density of a fluid to that of water.

Density depends on temperature, and since crude is sold by volume, temperature has a decisive influence, since it is capable of producing appreciable variations that are reflected in the price.

For this reason, the oil industry established a standard temperature of 60ºF to determine the density, and all the quantities related in some way with it.

The specific gravity γ _T is mathematically defined as:

γ _T = density of fluid / density of water at reference temperature (60ºF)

Both the density of the fluid and that of the water must be in the same system of units and have been measured under the same conditions of pressure and temperature. Thus, specific gravity has no units.

The hydrometer

The hydrometer (hydrometer in English, erroneously translated as hydrometer, a device used to measure flow), is a device to measure the relative density or specific gravity of a liquid.

Its invention is attributed to the mathematical Hypatia of Alexandria (315-345). The apparatus consists of a hollow cylinder made of glass with a weight or ballast at one end, which helps it float vertically.

It is carefully introduced into the liquid whose density is to be measured and left at rest until it stabilizes.

The density of the liquid depends on how much the hydrometer sinks into it. In light liquids the instrument sinks more than in denser liquids. Therefore, the instrument provides a scale to perform the reading.

There are hydrometers specifically designed for certain fluids, such as milk (lactometers) or wine (breathalyzer), to name a few. For crude oils, the API density meter is used, whose scale is already graduated in API degrees to avoid having to make the conversion. For example: a reading greater than 10, means a crude oil lighter than water, which will float on it.

And since temperature is a very important parameter, there are hydrometers that already come with a built-in mercury thermometer. They are known as thermo-hydrometers and are very suitable for field tests, especially.

The scheme is shown in the following figure:

Figure 2. Diagram of the density meter to measure the API degrees of a sample. Source: Mott, R. Fluid Mechanics.

Crudes and their API gravity

The API grades in oils range from 10 to 80, with the majority in the 20-70 API range.

From the formula given at the beginning, the specific gravity is solved as:

γ _T = 141.5 / (131.5 + ºAPI)

The most common ranges are from 0.93 to 0.70. Note that heavier oils correspond to lower API values. In contrast, higher API gravity values ​​indicate oils of higher commercial value, because they are easier to process and transform.

This is the norm up to 45 degrees API, since beyond this value, in crude oils there is a preponderance of short hydrocarbon chains, which makes processing difficult.

Crude oil classification

According to the value it shows on the API scale, crude is classified as:

- Light or light, with an API grade greater than 31.1, in which low molecular weight hydrocarbons predominate. It is an easy-to-transport crude oil that is highly sought after to make gasoline, diesel, and kerosene.

- Medium or medium, whose API grade is in the range of 29.9 - 22, has a good concentration of low molecular weight hydrocarbons, which also makes it easy to transport.

- Heavy, API grade between 21.9 and 10, abundant in hydrocarbons of medium molecular weight, which makes transport difficult. From this type of crude oil, customs, oils of various kinds, paraffins and polymers are obtained.

-Extra-heavy, whose API is 10 or less, is more complicated to transport and process, therefore it has less commercial value.

The API gravity varies according to the region, for example Latin American oil has an average of 25.1º API, while in the Middle East crude oils are light, with 34º API.

Experimental determination of API gravity using the hydrometer

These are the main steps to follow to obtain the API degrees of a sample, by using a hydrometer:

- The temperature is adjusted according to the nature of the sample, important if it is volatile substances.

- The temperature of the test tube must be the same as that of the sample.

- Transfer the sample carefully to the clean cylinder without splashing. It is necessary to avoid the formation of air bubbles and to minimize evaporation in the case of volatile liquids.

- If bubbles have emerged, they can be removed by gently touching the surface with a filter paper.

- It is necessary to make sure that the ambient temperature does not undergo appreciable changes, not greater than 2 ºC.

- The hydrometer is submerged slowly, when it is well centered it sinks a little more and is released, trying to keep the rest of the tube dry. It should be kept away from the walls of the specimen.

- Wait some time for all the air bubbles to reach the surface, especially if the sample is very viscous.

- Take note of the temperature before reading. To do this, it is carefully stirred with the thermometer, completely submerging the mercury column. If the thermo-hydrometer is used, it is also stirred with the instrument, using gentle vertical movements.

- The closest mark, where the liquid surface intersects the scale, can now be read on the device.

- Record the temperature immediately after reading. If there was a noticeable variation, the measurement should be repeated.

- In opaque liquids a correction must be made. The corrected reading is the API severity sought.

References

1. AmSpec. A stands for API Gravity. Recovered from: amspecllc.com.
2. ICCT. Introduction to petroleum refining and ultra-low sulfur gasoline and diesel production. Recovered from: theicct.org
3. Mott, R. 2006. Fluid Mechanics. 4th. Edition. Pearson Education.
4. Sencamer. Crude oil and its derivatives. Determination of API Gravity. Hydrometer method. Recovered from: sencamer.gob.ve.
5. UNAM. Petroleum and its derivatives. Recovered from: professors.fi-b.unam.mx