Feedstock composition and properties of the catalytic cracking process
The raw material for catalytic cracking units is usually heavy distillate, but currently the raw material used in some units is becoming heavier and heavier. The proportion of blended residual oil is gradually increasing, and some of them directly use normal pressure heavy oil as the raw material for catalytic cracking.
The higher the boiling point, the easier it is to crack the catalytic cracking feedstock with similar family composition. However, for molecular sieve catalysts, the effect of fractionation is not important. The ease of feedstock cracking can be illustrated by the characteristic factor, where a high aromatic content and a small characteristic factor indicate a difficult feedstock cracking. At the same conversion rate, the gasoline and coke yields of paraffin-based feedstocks are lower and the gas yields is relatively high. The gasoline yield of cycloalkyl feedstock is lower than the gas yield, the gas has more hydrogen and methane, and the main components of the gas are C1 and C2. For aromatic-based feedstock, the gasoline yield is medium, the coke yield is high, and the gas has more hydrogen and methane.
If there are more thick-ringed aromatic hydrocarbons in the feedstock, the adsorption capacity of these thick-ringed aromatic hydrocarbons is strong, and there are more coke, and the reaction rate is slow, which affects the reaction of other The reaction of other hydrocarbons is affected. Tests have shown that the addition of 50% naphthalene or 3% anthracene to an alkane rich synthetic distillate (200-300°C) results in a slow reaction rate. anthracene, the catalytic cracking reaction rate decreases by 50%. Therefore, it is not desirable to have a feedstock with high levels of thick ring aromatics.
The nature of the feedstock is the most important of all operating conditions. The choice of catalyst, the development of the production program and the selection of operating conditions should be based on an understanding of the nature of the feedstock. The production requires a relatively stable feedstock. At the same time, when processing several raw materials with different properties, they should be blended in the raw material tank or pipeline before being sent to the plant. In addition, we should pay special attention to the tank dehydration, do not change the tank because of the dehydration is not clean, the water will be sent to the reactor, otherwise it will sharply reduce the reaction temperature, the reaction pressure will rise rapidly due to the vaporization of water, which can cause serious accidents.
