Refining methods for straight-run products from atmospheric and vacuum distillation units and key points for on-site operation

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In the operation of atmospheric and vacuum distillation units, products such as straight-run gasoline, kerosene, diesel, and jet fuel all require refining processes in order to meet the quality standards required for release. The most commonly used refining methods in refineries today are mainly divided into three categories: chemical refining, adsorption refining, and hydrogenation refining. Chemical refining further includes specific techniques such as alkaline washing, acidic washing, electrorefining, and fixed-bed deodorization; different products and different impurities require different refining approaches. As a frontline operator, it is essential to thoroughly understand the principles of each refining method, the products suitable for use, the key operational steps, and the precautions involved, in order to ensure that the products meet quality standards from the first attempt and that operations proceed smoothly. I. Chemical refining – the most widely used and mature traditional refining method. Chemical refining relies on chemical reactions between reagents and impurities such as sulfides, acids, and gums present in oils, thereby removing these impurities or converting them into harmless substances. It is currently the primary method used for refining straight-run products in refineries, and it is employed most frequently in actual operations. 1. Alkali washing and refining: Alkali washing makes use of sodium hydroxide solutions with a concentration of 3% to 10% in order to remove acidic and corrosive impurities such as hydrogen sulfide, thiol compounds, naphthenic acids, and phenols from petroleum products. Alkali washing is commonly used for straight-run gasoline and diesel. In practical operations, free alkali can easily remain in the oil after alkaline washing; therefore, water washing is necessary. Otherwise, it can lead to problems such as unsatisfactory copper sheet corrosion, turbidity of the oil, and emulsification. For oils with high viscosity, high naphthenic acid content, and a tendency to emulsify, simple alkaline washing results in slow sedimentation and poor separation effects. In such cases, high-voltage electric refining is generally used on-site, with the voltage controlled between 15,000 and 25,000 V. Under the influence of this electric field, oil and water separate rapidly, and the reaction products aggregate and settle quickly, resulting in higher refining efficiency and more stable outcomes. 2. Acid pickling and refining: Acid pickling is primarily applied to kerosene intended for use in lamps, produced from Shengli crude oil, using concentrated sulfuric acid with a concentration of 93%–98%. Sulfuric acid can cause esterification and condensation reactions of olefins, aromatics, resins, etc., in kerosene; the resulting products dissolve in the acid sludge and are discharged together with it. Special attention must be paid on-site: after pickling, the oil has a high acidity level, so it is necessary to carry out a second alkaline wash followed by a water wash in order to completely remove any free acids and residual alkalis. Only in this way can the acidity, corrosiveness, performance in the lighting test, and the height of the smokeless flame of the kerosene meet all the required standards. During the operation, strict control must be exercised over the acid concentration, dosage, and reaction temperature to prevent excessive acid washing from causing discoloration of the oil product and a decrease in yield. 3. Gasoline fixed-bed deodorization: Fixed-bed deodorization is basically used for the deodorization of straight-run gasoline in China; it is a mature, stable, and easy-to-operate process. After being premixed with air, the pre-alkaline-washed gasoline enters an activated carbon bed containing titanium cobalt sulfide; under the action of the catalyst, adsorption oxidation occurs, converting the malodorous and corrosive thioethers into odorless and non-corrosive disulfides. This reaction process can be monitored directly from the on-site control system; once operation stabilizes, the thiol removal rate can exceed 96%, and the levels of gasoline corrosion, odor, and thiol content all meet the required standards at once. It is currently the most widely used method for gasoline refining. II. Adsorption purification – Utilizing molecular sieves and activated carbon for thorough impurity removal. This method does not involve the use of large amounts of acidic or alkaline chemicals; instead, it relies on adsorbents such as molecular sieves and activated carbon to selectively remove polar impurities in oils, including sulfur, nitrogen, oxygen, aromatics, and gums. It is a clean, environmentally friendly approach that ensures stable product quality. 1. Kerosene used for lamps is refined using CaY-type molecular sieves. Lamp kerosene is required to be smoke-free, free of soot, and odorless; strict limits are imposed on aromatics and gums, and CaY-type molecular sieves are commonly used on-site for adsorption purification. Molecular sieves have a strong adsorption capacity for polar compounds; they can firmly adsorb sulfur, oxygen, nitrogen compounds, and aromatic hydrocarbons. After saturation, they can be regenerated by using water vapor, allowing for reuse. They feature low operating costs and stable performance. 2. Desulfurization and denaturation of jet fuel using CuX molecular sieves: The jet fuel produced from Daqing crude oil generally employs the CuX molecular sieve process for desulfurization and denaturation. The fuel oil and compressed air are mixed in a venturi tube before entering the reactor, where thiols are removed thoroughly under the action of molecular sieves. After the reaction, the mixture is cooled to 40–50°C, and then passes through an activated carbon decolorization tank and a glass wool filter; as a result, the level of thiols can be kept below 10 μg/g, fully meeting the quality requirements for aviation fuel. This system is easy to operate and has minimal fluctuations, making it highly suitable for continuous production. III. Hydrofining – Advanced refining to meet high-standard oil specifications. Hydrofining is the highest-level refining method available; under the action of hydrogen and catalysts, impurities such as sulfur, nitrogen, and oxygen are completely removed, unsaturated hydrocarbons are saturated, and the stability, corrosion resistance, and color of the oil are all improved significantly. It is suitable for processing straight-run oils with high sulfur content and complex impurities, and is especially appropriate for producing diesel fuel that meets National VI standards as well as high-quality jet fuel. Hydrorefining resolves all issues such as sulfur, corrosion, acidity, gums, and stability in one go; it is a key method for modern refineries to improve product quality. The disadvantages are high equipment costs and stringent operating conditions, as it is necessary to control various parameters such as temperature, pressure, hydrogen-to-oil ratio, and space velocity. IV. Key points of refinement control that must be mastered in hands-on operations: 1. For alkali washing and water washing, it is necessary to properly control the ratios, temperature, and settling time to prevent emulsification; emulsification makes separation difficult, which directly leads to defective products. 2. For electrorefining, it is necessary to maintain a stable voltage and check the integrity of the electrodes; abnormal electric fields can significantly reduce the separation efficiency. 3. To maintain catalyst activity in fixed-bed deodorization, it is necessary to regularly check the bed temperature and space velocity to ensure complete conversion of thiols. 4. For molecular sieve adsorption, it is necessary to control the flow rate and temperature properly, and carry out regeneration on schedule to prevent impurity penetration due to adsorption saturation. 5. All refined oils must be sampled and analyzed in accordance with regulations, with particular attention paid to key indicators such as sulfur content, thiolic sulfur, copper strip corrosion, acidity, water-soluble acids and bases, and the light-burning test. 6. It is strictly prohibited to add or mix acids and bases indiscriminately; the concentration of chemicals and the amount added must be strictly in accordance with the process specifications, to avoid over-treatment or under-treatment. V. Conclusion The refining of straight-run products represents the final step to ensure the quality of the products produced by atmospheric and vacuum distillation units. The three commonly used methods on site—chemical refining, adsorption refining, and hydrorefining—cover all straight-run products such as gasoline, diesel, kerosene, and jet fuel. Alkali washing, acid washing, electro-refining, and fixed-bed deodorization are simple to operate and highly versatile ； Molecular sieve adsorption for purification is clean, environmentally friendly, and effective at removing impurities to a high degree ； Hydrorefining offers the highest standards and the most stable quality. As a operator, only by understanding the applicable scenarios, key control points, and precautions for each method can one achieve precise adjustments, ensure success on the first attempt, and maintain stable operation over a long period of time, thus truly ensuring control over product quality.