Marcio Wagner da Silva, Petrobras, email@example.com
To elaborate this response I adopt as assumption that both hydrotreating units are operating with the same feedstock. In this case, the Axens processing unit which applies a combination of CoMo and NiMo catalysts operate under higher total pressure but lower H2/HC ratio considering the balance between the diferent catalysts. The NiMo catalyst is applied to promote harder hydrotreating reactions like the hydrodenitrogenation and aromatics saturation, in other words, heavier and high contaminants content feed stocks.
To promote the high severity reactions, the NiMo catalyst is highly active and an atmosphere with low hydrogen partial pressure tends to quickly coking the catalyst bed. For this reason, the processing unit designed by Haldor Topsoe which applies only NiMo as catalyst can operate under lower total pressure, but with higher H2/HC ratio in order to control the coke deposition and, consequently, the operating lifecycle of the processing unit.
In other words, the synergy between CoMo and NiMo catalysts in the Axens Processing unit allows operate under lower hydrogen partial pressure, but requires a higher total pressure to reach the same final product specification of the Haldor Topsoe processing unit which applies only NiMo catalyst which is highly active allowing operations with lower total pressure but with higher hydrogen partial pressure to put under control the coking rate of the catalyst bed.
Marcelo Tagliabue, Air Liquide, firstname.lastname@example.org
In the first place, to elaborate this answer, I suppose that the two hydrotreatments have the same food, referring specifically to their composition.
In the past, the most used catalysts were CoMo, and almost all technologies used it. NiMo-based catalysts then burst in. The latter were more expensive than the former and were preferably used when the feed contained olefins or sulfur compounds that were difficult to hydrogenate, as in the case of Thiophene.
Hydrotreatments, in terms of their severity, respond to the concentration of H2 in the feed as well as to the working pressure. In other words, they respond to the partial pressure of H2. It is for this reason that we can find different pressures and different concentrations of H2 to achieve the same result.
The configuration of the reactor with respect to its dimensions, space velocity, and the use of a catalyst or a mixture of both respond exclusively to each particular technology and to the characteristics of the catalyst that each one has at the time of designing the system. Let's not forget that in this case almost ten years passed between one technology and the other, and that period for progress in the formulation of catalysts is a long time. It is probable that the most modern HTSA catalyst will meet or improve the performance of the mixture used by Axens.