Research and development of medium pore molecular sieve heavy oil catalytic cracking process with low green coke, low energy consumption and high olefin yield

The catalytic cracking process for the highly selective cracking of heavy hydrocarbons into olefins has achieved a significant improvement in the selectivity of olefin products. Catalytic cracking units are also gradually shifting to producing more light olefin products.

The average relative molecular weight of catalytic cracking feedstock oil is generally above 300, the initial boiling point is greater than 350 °C, and the molecular carbon number is generally greater than 30, most of which are around 60, which are divided into saturated fractions, aromatic fractions, gums and asphaltenes.

If gasoline is the target product, the molecular carbon number (distillation range) of gasoline is mainly concentrated in C5~C12 (40~200 ℃); if low-carbon olefin is the target product, the molecular carbon number of the product is mainly concentrated in C2~C4.

The composition of catalytic cracking feedstock oil is extremely complex. If low-carbon olefins are used as the target product, only saturated hydrocarbons or aromatic side chains in heavy hydrocarbons can be cut into olefin molecules, while aromatic hydrocarbons and aromatic cores are almost impossible to be cut into olefin molecules. It is also difficult to open an alkane ring to an alkene molecule.

Under the action of the solid acid catalyst, the carbocation R2+ will extract hydride ions from the raw alkane molecules to undergo a hydride anion transfer reaction, and at the same time convert itself into the product alkane (R2H), the raw alkane molecules will form a new R1+, so that the entire The catalytic cracking reaction proceeds continuously.

In the variable diameter fluidized bed reactor and the corresponding reaction system, the heavy hydrocarbons first come into contact with the high temperature (about 700 °C) hot catalyst. At the moment of contact, a small amount of thermal cracking must be accompanied. Although this part of the thermal cracking reaction results in the formation of dry gas, especially methane, high temperatures are beneficial for the decomposition and vaporization of gums and asphaltenes in heavier hydrocarbons.

For the catalytic cracking process that takes light olefins as the target product, the cracking of heavy hydrocarbons to generate olefin molecules requires a lot of heat, and the reaction is generally more severe.

At the moment of contact between the hot catalyst and oil and gas and in the early stage of the reaction, the active components of the molecular sieve in the catalyst have strong enough cracking activity to promote the rapid cracking of heavy hydrocarbon molecules to generate olefin molecules. But at the same time, the active components of the molecular sieve have strong enough hydrogen transfer activity, so that the cracked olefin molecules are converted into alkanes.

In this stage, it is better to suppress the hydrogen transfer reaction of olefins through higher reaction temperature, and further promote the cracking of olefins into smaller molecular olefins, but too high reaction temperature will cause more dry gas to be generated.

With the increasing demand for low-carbon olefins, a variety of technologies for producing low-carbon olefins have emerged, mainly including methanol-to-olefins, olefin cracking, propane dehydrogenation and other technologies.