Raw material cracking issue

The weight of a raw material is relative; lighter raw materials have smaller molecules and higher bond energies, requiring more energy to break the bonds. The cracking reaction mainly involves the conversion of large molecules into smaller ones, and of long chains into shorter chains. The raw material is light and not prone to cracking. Indicators used to measure the properties of raw materials: The properties of crude oil are usually assessed using the following indicators. 1. Fraction composition: The fraction composition can determine the density of the raw material and the width of its boiling range. When the chemical composition types of the crude oil are similar, the heavier the fractions, the easier they are to crack ； The lighter the fraction, the less likely it is to crack. Due to the rational use of resources, catalytic cracking using pure wax oil has become increasingly rare in recent years. 2. Hydrocarbon composition: The hydrocarbon composition is usually expressed in terms of the contents of alkanes, cycloalkanes, and aromatics. The composition of the raw materials varies depending on their source. Paraffin-based feedstocks are prone to cracking, resulting in lower yields of gasoline and coke, but higher yields of gas ； Naphthenic feedstocks are the easiest to crack, yielding high gasoline production with a high octane rating, while gas production is lower ； Aromatic feedstocks are difficult to crack, resulting in low gasoline yield and excessive coking. Analyzing the hydrocarbon composition of heavy crude oil is difficult, and such analysis is rarely carried out in daily production; it is only performed during plant calibration. Normally, judgments are made indirectly by measuring physical properties such as density, specific factor, and aniline point. (1) Density: The higher the density, the heavier the raw material. If the distillate composition is the same, the density is high, and there is a high content of naphthenes and aromatics ； It has a low density and a high content of alkanes. (2) Characteristic factor K: The characteristic factor is related to density and distillate composition. A high K value of the raw material indicates a high content of alkanes, while a low K value indicates a high content of aromatics (see Table 3-1). The K value of the raw material can be calculated from Enneberg distillation data and density. It can also be obtained by consulting a chart using density and aniline point. (3) Aniline point: The aniline point is an indicator of the aromatic content in oils; the lower the aniline point, the higher the aromatic content in the oil. 3. Residue: The residue value of crude oil is one of the main indicators for assessing the properties of the crude oil. It is related to factors such as the composition of the raw materials, the width of the distillation fractions, and the contents of resins and asphaltenes. If the residual carbon value of the raw material is high, more green coke will be produced. The carbon residue value in conventional catalytic cracking feedstocks is low, generally around 6%. Heavy oil catalytic cracking involves adding a portion of vacuum residue to the feedstock or processing whole-range atmospheric residue directly; as the feedstock becomes heavier, the levels of gum and asphaltenes increase, along with the carbon residue value. 4. Metals: Among the heavy metals in crude oil, vanadium, nickel, iron, and copper have the greatest impact on catalyst activity and selectivity. During the catalytic cracking process, vanadium tends to deposit on the catalyst. During regeneration, vanadium moves to the location of the molecular sieve and reacts with it to form an eutectic compound with a melting point of 632°C, which destroys the crystal structure of the catalyst and causes it to become permanently inactive. Nickel deposits on the catalyst and moves to the positions of the molecular sieve, but it does not damage the molecular sieve; it only partially neutralizes the acidic sites of the catalyst, with little impact on the catalyst’s activity. Since nickel itself is a dehydrogenation catalyst, the dehydrogenation reaction can take place under the temperature and pressure conditions of catalytic cracking, thereby increasing hydrogen yield and reducing the amount of liquid product. Alkali metals such as sodium and calcium in the feedstock also affect the catalytic cracking reaction. The deposition of Na on the catalyst affects its thermal stability, activity, and selectivity. With the development of heavy oil catalytic cracking, more and more attention has been paid to the hazards of Na. Na not only causes acidic poisoning of the catalyst but also forms a low-melting-point sodium vanadate eutectic with the vanadium oxides deposited on the catalyst surface; this eutectic becomes molten at the high temperatures used for catalyst regeneration, thereby destroying the molecular sieve lattice and reducing its activity. The degree of this poisoning becomes more severe as the temperature rises (see Table 3-5). Therefore, for heavy oil catalytic cracking, the Na content in the feedstock must be strictly controlled, generally kept below 5 mg/kg. 5. Sulfur and nitrogen contents: High levels of nitrogen-containing compounds in the raw materials, especially basic nitrogen compounds, can cause catalyst poisoning and reduce its activity. Studies show that adding 0.1% (by mass) of basic nitrides to the cracking feed reduces the cracking reaction rate by approximately 50%. In addition, basic nitrogen compounds are one of the main reasons for the discoloration of product oils and the deterioration of their oxidative stability. Sulfur-containing compounds in the feedstock had no significant effect on catalyst activity; in the experiments, feedstocks containing 0.35% to 1.6% sulfur were found to have no impact on the rate of catalytic cracking reactions. However, sulfur increases equipment corrosion, raises the sulfur content in the products, and also pollutes the environment. Therefore, during the catalytic cracking production process, attention should be paid to the sulfur and nitrogen contents in both the feedstock and the products; if these levels are too high, pre-refining treatment is necessary.

Just like tree branches, the longer they get, the more likely they are to break in the middle.