Process Flow of Hydrotreating

There are two t)q)es of processes. One is called a single stage process, while another is termed a two or multiple stage process. The single stage 

The present authors proposed a new type of reactor as shown in Fig. 14, where fractions of a gas oil were reacted separately in upper and lower parts of the catalyst bed. Hydrogen was charged from the bottom of the reactor. H2S inhibition on the heavier fraction can be avoided. The optimum catalysts can be apphed for the respective parts of the bed. Pressure drop is a possible worry at the lower part of the reactor, where the hydrogen is up flow while liquid feed is down flow. A honeycomb type catalyst bed is now available for this problem. 

By lowering the end point of the starting diesel fuel, hydrotreating the lower end point diesel fuel feed to ultra low sulfur levels is much easier. For example, when the 90% distillation point (T90) of diesel fuel is lowered by 20 °C, the required reactor size is only about half that needed for the full range feed. This diesel with lower T90 has another merit. It will produce less particulate matter in diesel exhaust gas. The downside of this approach would be the requirement for increasing the cracking capacity of the refinery to produce the required volume of diesel fuel with this T90. One solution is to revamp a VGO hydrotreater to mild hydrocracking service. 

The color of finished diesel oil is a stringent requirement in some countries. Presently, hydrodesulfurization of faintly yellow diesel oil feedstocks produces colorless and transparent products at 500 ppm S. However, when severe conditions are used for deep HDS, the diesel oil becomes, a fluorescent yellowish green product. High hydrogen pressures suppress color formation, while a high reaction temperature conversely retards hydrogenation and enhances color formation.  

Finally, we must emphasize the advantage of fractionated HDS for LCO. ° We may be able to avoid the real difficulty for HDS of refractory sulfur species in the presence of aromatic inhibitions. 

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