Requirements for Raw Material Oil in Carbon Black Production

Carbon black, also known as carbon black, is a quasi-graphite structured substance consisting of colloidal particles approximating spheres formed by the incomplete combustion or thermal cracking of hydrocarbon compounds, primarily petroleum derivatives. These particles exist in aggregates, appearing as pure black powder or granules. The particle size, structure, and surface activity of carbon black vary greatly depending on the raw materials and production processes used. Carbon black is insoluble in water, can be acidic or alkaline, and produces carbon dioxide when burned. The primary component of carbon black is carbon, with trace amounts of hydrogen, oxygen, sulfur, ash, tar, and moisture^[1]^.

Raw Material Oil for Carbon Black Production

Currently, there are two main types of raw material oil used in carbon black production in China:

1.1 Ethylene Tar (ET)

Ethylene tar is the residue oil (C10+) from ethylene cracking units, commonly referred to as cracking fuel oil or ethylene bottom oil in domestic ethylene plants. The quality of ET varies depending on the raw materials used in the cracking process, which include diesel, naphtha, and light hydrocarbons. The higher the density of the cracking raw material, the higher the yield and aromatic content. Ethylene tar has a high aromatic content but also a relatively high asphaltene content^[5]^.

1.2 Coal Tar and Its Distillates

Coal tar is one of the products obtained from the high-temperature dry distillation of coal and is a by-product of coking plants. The distillates of coal tar (CT) include anthracene oil (including mono-anthracene oil and di-anthracene oil), creosote oil, etc. These distillates have a higher aromatic content than ET and are high-quality raw materials for carbon black production. In China, coal tar and its distillates are the primary raw materials for carbon black production^[5][6]^.

Quality Requirements for Raw Material Oil

Carbon black raw material oil is primarily composed of alkanes, aromatics, and asphaltenes, and its properties significantly influence the quality and yield of carbon black. The key requirements for raw material oil in carbon black production are high aromatic content, low asphaltene and impurity levels. The evaluation of raw material oil primarily considers the following aspects:

2.1 Aromatic Content

Aromatic content is one of the most important quality indicators for carbon black raw material oil, often characterized by the correlation index and hydrogen-to-carbon ratio. Generally, the aromatic content of carbon black raw material oil should be high, ideally above 70%. Aromatic hydrocarbons, consisting of one or more benzene rings, are the main components in the formation of carbon black, as they undergo dehydrogenation and condensation reactions at high temperatures, resulting in the shortest process time for carbon black production^[2]^.

2.2 Asphaltene and Resin

Asphaltene, which is not easily vaporized, decomposes into coking materials above 300°C, causing coking in reactors and pipelines. The asphaltene content in ethylene tar is typically around 25%, and it is also relatively high in coal tar. Therefore, the asphaltene content in processed raw material oil should ideally be within 8%. Resin, a low-condensation unsaturated cycloalkane-aromatic compound containing oxygen, nitrogen, and sulfur, can transform into asphaltene and insoluble coke particles after prolonged oxidation and heating. Generally, carbon black raw material oil contains 1% to 10% resin^[2]^.

2.3 Free Carbon Content

Free carbon in raw material oil is produced during the coking process and exists as fine particles (typically 50-100 μm) that are difficult to remove. It remains unchanged during carbon black production but can significantly reduce the mechanical and physical properties of rubber products. Generally, the free carbon content should not exceed 10%^[2]^.

2.4 Sulfur Content

Sulfur exists in oil products as organic sulfides, which can generate SO2, SO3, and H2S during the carbon black reaction process. High sulfur content can accelerate equipment corrosion, reduce carbon black yield, increase free sulfur content in carbon black, cause premature vulcanization of rubber compounds, and degrade product performance. Additionally, excessive sulfide emissions can harm the environment. Generally, sulfur content should not exceed 2% for carbon black production applications and 0.5% for environmental considerations^[2]^.

2.5 Alkali Metal Content

Primarily sodium and potassium ions, high alkali metal content can increase the ash content and reduce the structure of carbon black products. Potassium has a 10-fold greater effect on reducing carbon black structure compared to sodium. Generally, sodium ion content should not exceed 10.0 ppm, and potassium ion content should not exceed 1.0 ppm^[2]^.

2.6 Moisture Content

High moisture content can lower reactor temperatures, disrupt combustion and reactions, affect the normal operation of carbon black production equipment and processes, reduce product quality and yield, and increase oil processing costs. Generally, moisture content should not exceed 2.0%, with a modern production process requirement of below 0.5%^[2]^.

2.7 Density

Density is a simple yet important physical property indicator for carbon black raw material oil. It can help determine oil type and roughly assess oil quality. Depending on the variety, density typically ranges between 1.0 and 1.2 g/cm³. Generally, higher oil density corresponds to higher aromatic content but lower calorific value, requiring more process heat for combustion and cracking. Excessively high density can lead to high asphaltene and viscosity levels^[2]^.

2.8 Distillation Range and Average Boiling Point

The distillation range refers to the temperature range from the initial boiling point (when the first drop of oil is distilled) to the final boiling point (when distillation is complete). A narrow distillation range is ideal. The distillation range of Chinese coal tar and ethylene tar oils is approximately 200-400°C, with a general requirement of 180-500°C. The distillation range is significant for evaluating raw material oil as it can indicate range width and allow calculation of the volume-average boiling point and correlation index^[2]^.

2.9 Correlation Index and Hydrogen-to-Carbon Ratio

The correlation index is an indicator of aromatic content in carbon black raw material oil. When asphaltene and resin content is not too high, the correlation index is proportional to aromatic content. A correlation index between 120 and 155 is desirable. Too low a correlation index can reduce product yield and structure, while too high can indicate excessive asphaltene and resin, leading to coking and high sieve residue in products^[2]^. The hydrogen-to-carbon ratio (H/C) is the ratio of hydrogen to carbon atoms in the oil. Lower H/C ratios indicate higher aromatic content, making oils with low H/C ratios suitable for carbon black production^[2]^.

2.10 Viscosity

Viscosity is an important quality indicator for carbon black raw material oil, affecting pipeline transportation and combustion performance. High viscosity can reduce oil flowability, increase energy consumption, and impair atomization. Generally, viscosity should be below 4.0 (E804)^[2]^.

Relationship Between Raw Material Oil and Carbon Black Quality

The quality of raw material oil is closely related to carbon black quality, significantly impacting production and determining carbon black quality.

3.1 Impact on Carbon Black Basic Properties

Raw material oils used in carbon black production are composed of various hydrocarbons with different cracking processes and reaction times. Mixed hydrocarbon raw material oils can complicate the carbon black formation process, adversely affecting particle size uniformity and structure. Using single-hydrocarbon raw material oils or oils with a narrow distillation range is optimal for carbon black production^[2]^.

3.2 Impact on Vulcanized Rubber Properties

The reinforcing effect of carbon black on vulcanized rubber depends on its basic properties, including particle size and structure. Uniform particle size distribution enhances vulcanized rubber reinforcement. Mixed hydrocarbon raw material oils and oils with a wide distillation range are not ideal for improving vulcanized rubber reinforcement. Asphaltene and resin content should be minimized as they can generate fine coke particles during reactions, reducing carbon black's reinforcing effect on vulcanized rubber^[2]^.

Selection of Carbon Black Raw Material Oil

Currently, China's carbon black industry does not have specialized raw material oil producers like those in the United States. Raw material oil sources are diverse, and market supply is insufficient, resulting in complex quality issues. Raw material oils used in carbon black production are often a mix of oils from different regions, producers, processing techniques, and raw materials^[2]^.

As coal tar and its processed oils dominate China's carbon black raw material oil supply (over 85%), the quality of "crude" coal tar has changed with advancements in coking technology. Its density is now close to or above 1.2, containing mostly asphalt oil and soft asphalt, making it unsuitable for carbon black production. Ideal carbon black raw material oils are aromatics (C10-C14), especially polycyclic aromatics with three or more rings. When selecting coal tar-based raw material oils, processed oils like creosote oil, anthracene oil, and di-anthracene oil should be preferred over "crude" coal tar, especially those with densities close to or above 1.2^[5][6]^.