Can tail oil recycling improve diesel quality?

It is crucial in the operation of the vacuum distillation tower to prevent the recycled oil from carrying diesel components back into the cracking reactor; regular sampling of the recycled oil should be carried out along with tests using actual boiling point distillation to ensure that the diesel content is less than 3% by volume. As refineries process an increasing proportion of sulfur-containing and high-sulfur crude oils year by year, the sulfur content in the feedstock processed by hydrocracking units also rises. This accelerates corrosion in the distillation system, resulting in high levels of iron ions in the oil fed into the cracking reactors. Consequently, fouling (in the form of ferrous sulfide) builds up in the first bed of the cracking reactors, causing a rapid increase in pressure drop and an increased frequency of shutdowns for cleaning. The key to controlling Fe is to enhance the circulating hydrogen desulfurization system to address corrosion upstream. Additionally, the cracking reactor can use a large-pore-size Fe-removing protective agent, which is filled in stages together with the catalyst. In hydrocracking units with some cyclic operating procedures, since tail oil is discharged frequently, there is no issue of accumulation of heavy poly-nuclear aromatics (HPNA). However, in hydrocracking units that operate using a full-cycle process, large-molecule polycyclic aromatic hydrocarbons are formed and accumulate during the reaction process; these compounds exist in liquid form within the reaction stream at higher temperatures and lower concentrations ； When the temperature drops, it precipitates in a solid state; this affects the catalyst’s activity and it can also deposit in heat exchangers, impacting heat transfer efficiency.

The quality of the tail oil is good, and it is rich in branched-chain alkanes; these compounds can easily break apart under cracking to produce lighter components, thereby improving the quality of diesel. Additionally, the sulfur and nitrogen content in tail oil is low, and recycling it alters the properties of the raw material, resulting in better diesel characteristics.