There are four methods for producing bitumen: Straight Run, Air Blowing, Deasphalting, and Blending. Air Blowing is the most commonly used method.
Suitable Crude Oil for Producing Bitumen
It is possible to produce bitumen using crude oil that has a specific gravity greater than 0.9. This is because these crude oils contain a suitable amount of heavy molecules, such as asphaltene and maltene, which play a crucial role in bitumen production. For a more detailed explanation, please refer to the following section of this article. Typically, crude oils with these characteristics can potentially produce 20-50% bitumen. Additionally, these crude oils contain a sulfur content of over 1 weight percentage, which plays a vital role in polymerization and increases the viscosity of the bitumen. It is worth noting that sometimes the appropriate crude oil is obtained by blending different types of crude oil.
Bitumen Production Feedstock
Bitumen is a substance that is extracted from crude oil, which is a mixture of different materials with varying boiling points. To separate these components, distillation towers are used. In the tower, crude oil is heated, causing the components to separate based on their boiling points. Lighter components with lower boiling points separate first and their vapors are collected in trays placed at various heights in the tower. The heaviest components with high boiling points remain at the bottom. There are two types of distillation towers: atmospheric and vacuum. In an atmospheric tower, crude oil is heated at atmospheric pressure, while in a vacuum tower, the residue from the atmospheric tower is heated under near-vacuum conditions. The residue from the atmospheric tower is called the long residue. However, this residue cannot be heated at very high temperatures because it reacts with oxygen to produce coke. To avoid this reaction, the long residue is taken to the vacuum distillation tower to produce more products at higher temperatures without damaging it. Bitumen is produced from the residue of the vacuum distillation tower, which is called short residue or vacuum bottom. Sometimes a combination of different short and long residues is also used. It is worth noting that short residue or vacuum bottom should have a suitable penetration of between 35-300 dmm (deci-millimeter).
Methods of Bitumen Production
There are several methods for producing bitumen as described below.
1- Straight Run:
Vacuum bottom or short residue may possess bitumen properties, meaning it has a reasonable viscosity and penetration and can be directly used in paving work. However, usually, vacuum bottoms require different production stages.
2- Air Blowing:
Air blowing is the primary method of producing bitumen. This process involves different methods that are explained below. In air blowing, air at a constant temperature is blown through the feedstock (vacuum bottom) to produce Asphaltene molecules that increase the viscosity of the bitumen. It generates Ester molecules, leading to larger molecules (polymerization process). Oxygen atoms bond two different molecules and generate heavier molecules and asphaltene. Oxygen molecules react with the vacuum bottom, causing cyclic molecules to be generated (hydrogenation process). During air blowing, the penetration of the vacuum bottom is decreased, and/or its softening point is increased. Blowing is done in different ways, which are explained below.
Semi-Blowing or Air Rectification:
The semi-blowing process can be applied in two methods: Batch blowing or Continuous blowing.
In this process, a certain volume of feedstock needs to be heated before entering the reactor. Depending on the degree of penetration and viscosity of the demanded product, blowing lasts between 3 to 24 hours. During air blowing, oxidation reactions occur, leading to an increase in temperature. A cooling system is used in the tower to keep the temperature constant.
In continuous blowing, the vacuum bottom enters the reactor with a constant flow rate. The blowing temperature in the reactor is constant in a particular range. The temperature of the fresh feed is lower than the operating temperature. But the heat of the blowing process and oxidation reaction causes the bitumen to heat up. The advantages of the continuous method over the batch process are:
- Increases the production of bitumen per unit of time.
- Reduces the preheating cost of the vacuum bottom before entering the reactor.
- Due to its continuous nature, it is easy to control and process.
Fully Air Blowing:
In a fully blown process, air is blown through the feedstock more than in the semi-blown method. It is applied to a blend of vacuum bottoms with a relatively low viscosity property to reach oxidized bitumen. The amount of blowing required depends on the temperature in the column and the air-to-feed ratio.
3- Solvent Deasphalting:
One of the methods of bitumen production is to modify the vacuum bottom in propane deasphalting units. In these units, the vacuum bottom is mixed with propane (sometimes butane), and de-asphaltene oil is extracted from it. At the end of the process, the remaining product is bitumen. There are some differences between this bitumen and refined bitumen from the same feedstock. Propane deasphalting reduces residue further and can produce a bituminous product that has:
- Lower viscosity
- Higher ductility
- Better temperature resistance than other bitumen
Usually, this process is used on crude oil with a low content of heavy molecules.
Blending is not a direct method of producing bitumen. However, sometimes, it is used to achieve bitumen with desired properties. Several types of bitumen are mixed together to obtain the desired bitumen with the preferred properties. For example, penetration bitumen 40/50 is combined with penetration bitumen 200/300 to obtain the desired bitumen with a similar penetration to bitumen 80/100.
Prepared by Rumays specialists group.