1. Classification of Crude Oil
Petroleum generally refers to natural petroleum, which is crude oil extracted from the ground without any processing.
a. Composition of Petroleum
Crude oil is primarily composed of carbon and hydrogen compounds. The carbon content ranges from 83% to 87%, and the hydrogen content is between 11% and 14%. These compounds are known as hydrocarbons. Crude oil also contains elements such as sulfur, nitrogen, and oxygen, with concentrations ranging from 1% to 3%. These elements form compounds with carbon and hydrogen, constituting non-hydrocarbon compounds, which can account for 10% to 20% of the total content. Trace elements such as vanadium, nickel, lead, and calcium are also present in crude oil.
b. Classification of Crude Oil
Crude oil can be classified based on the composition of hydrocarbons and other properties.
a) By Hydrocarbon Structure:
Paraffinic hydrocarbons
Naphthenic hydrocarbons
Aromatic hydrocarbons
b) By Specific Gravity:
Light Crude Oil: Specific gravity less than 0.830
Medium Crude Oil: Specific gravity between 0.830 and 0.904
Heavy Crude Oil: Specific gravity between 0.904 and 0.966
Extra Heavy Crude Oil: Specific gravity greater than 0.966
c) By Sulfur Content:
Low Sulfur Crude Oil: Sulfur content less than 0.5%
Sour Crude Oil: Sulfur content between 0.5% and 2.0%
High Sulfur Crude Oil: Sulfur content greater than 2.0%
d) By Wax Content:
Low Wax Crude Oil: Wax content between 0.5% and 2.5%
Wax Crude Oil: Wax content between 2.5% and 10%
High Wax Crude Oil: Wax content greater than 10%
e) By Asphaltene Content:
Low Asphaltene Crude Oil: Silicon gel asphaltene content not exceeding 5%
Asphaltene Crude Oil: Silicon gel asphaltene content between 5% and 15%
High Asphaltene Crude Oil: Silicon gel asphaltene content greater than 15%
2. Petroleum Refining
a. Development of the Petroleum Refining Industry
The discovery, extraction, and direct utilization of petroleum have a long history. The processing and utilization of petroleum gradually formed the petroleum refining industry, which began in the 1830s. By the 1940s and 1950s, the modern refining industry had emerged, becoming one of the largest processing industries.
In the 1830s, petroleum distillation plants were established, primarily producing kerosene for lamps, with gasoline being discarded as waste due to its lack of utility. By the 1870s, lubricant factories were built, and high-boiling-point oils from distillation began to be used as boiler fuel. The invention of the internal combustion engine at the end of the 19th century greatly increased the demand for gasoline and diesel. Simple distillation of crude oil (primary processing) could not meet the demand, leading to the development of secondary processing techniques aimed at increasing the yield of gasoline and diesel and making comprehensive use of various crude oil components.
Key developments included thermal cracking in 1913, coking in 1930, catalytic cracking in 1930, and catalytic reforming in 1940. Subsequently, hydrogenation technology rapidly developed, forming the modern petroleum refining industry. After the 1950s, petroleum refining provided large quantities of raw materials for the development of chemical products, leading to the formation of the modern petrochemical industry. By 1996, the world's petroleum processing capacity had reached 3.8 billion tons, with large refineries having an annual processing capacity exceeding 10 million tons.
b. Overview of Major Refining Processes
a) Atmospheric and Vacuum Distillation
Atmospheric and vacuum distillation are collectively known as atmospheric-vacuum distillation. This process, essentially physical, separates crude oil into fractions with different boiling ranges in a distillation column. Some of these fractions are blended and treated with additives before being sold as products, while a significant portion serves as feedstock for further processing units. This is known as primary processing. The process includes three steps: desalting and dehydrating crude oil, atmospheric distillation, and vacuum distillation.
b) Desalting and Dehydrating Crude Oil
Also known as pretreatment, crude oil from oil fields often contains salts (mainly chlorides) and water (either dissolved in oil or in emulsified form), leading to equipment corrosion, scaling, and affecting the composition of finished products. Thus, these contaminants must be removed before processing. Common methods include adding demulsifiers and water to aggregate the water within the oil, separating it along with the dissolved salts, and then using a high-pressure electric field to remove the larger water droplets formed.
c) Catalytic Cracking
Catalytic cracking evolved from thermal cracking and is crucial for increasing the depth of crude oil processing and producing high-quality gasoline and diesel. The feedstock mainly consists of heavy oil fractions (350-540°C) from crude oil distillation or other refining units. The process includes three parts: catalytic cracking of feedstock, catalyst regeneration, and product separation. The cracked products are then fractionated to obtain gas, gasoline, diesel, and heavy distillates, with some oil recycled back into the reactor for further processing (referred to as recycle oil). Changes in operating conditions or feedstock can lead to fluctuations in product composition.
d) Catalytic Reforming
Catalytic reforming (or reforming) involves converting light gasoline fractions obtained from atmospheric distillation into high-aromatic reformate gasoline in the presence of a catalyst and hydrogen. Using an 80-180°C fraction yields high-octane gasoline; using a 60-165°C fraction primarily produces aromatics like benzene, toluene, and xylene. The process also generates hydrogen as a byproduct, which can be used in other refinery hydrogenation operations. Operating conditions include temperatures of 490-525°C and pressures of 1-2 MPa, with the process divided into feedstock pretreatment and reforming.
e) Hydrocracking
Hydrocracking occurs under high pressure and hydrogen presence, requiring a catalyst to convert heavy feedstocks into gasoline, kerosene, diesel, and lubricating oils. Due to the presence of hydrogen, the formation of coke is minimized, and harmful sulfur, nitrogen, and oxygen compounds are removed, making the process flexible and allowing product yield adjustments based on demand. The products have higher yields and better quality.
f) Delayed Coking
This process involves deep cracking of feedstock over extended reaction times to produce solid petroleum coke as the main product, along with gases and liquids. The feedstock mainly consists of high-boiling-point residues. Operating conditions include heating the feedstock to about 500°C and slightly positive pressure in the coke drum. Adjusting feedstock and conditions can alter the ratios of gasoline, diesel, cracking feedstock, and coke.
g) Refinery Gas Processing
Gases produced from primary and secondary processing units are collectively referred to as refinery gas. These gases primarily consist of hydrogen, methane, ethane and ethylene, propane and propylene, and butanes and butylenes. Their main uses include serving as feedstock for gasoline production, petrochemical feedstocks, and hydrogen and ammonia production. Effective utilization of refinery gas requires separation and purification to produce chemical feedstocks like ethylene for ethylbenzene production or propylene for polypropylene production.
h) Refining of Petroleum Products
Oils produced by the aforementioned units often require further refining to meet commercial standards. This involves blending, adding additives, and additional refining to remove impurities and improve properties. Common impurities include sulfur, nitrogen, and oxygen compounds, waxes, and resins, which can impart odors, deepen color, cause equipment corrosion, and make storage difficult. Refining methods include acid-alkali refining, deodorization, hydrogenation, solvent refining, clay refining, and dewaxing.
Acid Refining: Uses sulfuric acid to remove sulfur, nitrogen compounds, and resins.
Alkali Refining: Uses caustic soda solution to remove oxygen compounds and sulfur residues from acid refining.
Deodorization: Removes mercaptans causing foul odor in high-sulfur crude-derived products using catalysts, alkali, and oxidation.
Hydrogenation: Removes sulfur, nitrogen, oxygen compounds, and metals under hydrogenation conditions, improving storage, corrosion, and combustion properties.
Dewaxing: Removes wax from aviation kerosene and diesel to improve flow properties and prevent pipeline blockages.
Solvent Refining: Removes undesirable components from lubricating oils to improve composition and color, sometimes requiring dewaxing.
Clay Refining: The final refining step, using clay (mainly composed of silica and alumina) to adsorb harmful substances.
3. Definitions of Key Terms
a. Acid Refining
Acid refining involves treating oil products with sulfuric acid to remove certain sulfur-containing compounds, nitrogen-containing compounds, and resins.
b. Alkali Refining
Alkali refining uses caustic soda solution to treat oil products such as gasoline, diesel, and lubricating oils. This process removes oxygen-containing compounds and sulfides and eliminates residual sulfuric acid left from acid refining. Often, acid and alkali refining are combined, referred to as acid-alkali refining.
c. Deodorization
Deodorization targets gasoline, kerosene, and diesel produced from high-sulfur crude oil, which generate foul odors due to thiols. High thiol content can lead to resin formation, making storage difficult. This process typically involves treating the oil with alkali in the presence of a catalyst, followed by air oxidation.
d. Hydrogenation
Hydrogenation occurs at 300-425°C and a pressure of 1.5 MPa in the presence of a catalyst, adding hydrogen to the oil. This process removes sulfur, nitrogen, oxygen compounds, and metal impurities, improving storage properties, reducing corrosion, and enhancing combustion properties. It can be applied to various oil products.
e. Dewaxing
Dewaxing is primarily used to refine aviation kerosene and diesel. Oils containing wax can form wax crystals at low temperatures, impairing flow properties and potentially blocking pipelines. Dewaxing is critical for aviation fuel and can be achieved using molecular sieve adsorption. Lubricating oil refining often involves solvent refining to remove undesirable components, improving composition and color, and sometimes requires dewaxing.
f. Clay Refining
Typically, clay refining is the final refining step, using clay (mainly composed of silica and alumina) to adsorb harmful substances from the oil.
g. Lubricating Oil
Lubricating oil, derived mainly from crude oil distillation, is characterized by viscosity, stability, and lubricity. The production process primarily involves removing undesirable components from the feedstock, such as resins, asphaltenes, sulfur, nitrogen, oxygen compounds, waxes, and polycyclic aromatic hydrocarbons. These components affect viscosity, stability, and color. Methods include solvent refining, dewaxing, deasphalting, hydrogenation, and clay refining.
h. Solvent Refining
Solvent refining utilizes the different solubility of components in solvents to achieve purification, a method used in most lubricating oil production processes. Common solvents include furfural and phenol. The process is similar to the aromatic hydrocarbon extraction in catalytic reforming.
i. Solvent Dewaxing
Solvent dewaxing removes components from lubricating oil feedstocks that crystallize at low temperatures, mainly wax oil. The process uses cold crystallization. To overcome the challenge of high viscosity and small paraffin crystals at low temperatures, a mixed solvent that does not dissolve wax, such as toluene-methyl ethyl ketone, is often added. This process is thus also known as ketone-benzene dewaxing.
4. Conclusion
The above text provides an overview of the classification of crude oil and the basics of petroleum refining. If you are involved in the petroleum refining industry or in the processing of petroleum by-products such as wax oil, contact us to learn more about dewaxing and solvent dewaxing equipment.
