First published in PTQ Q1 2021
Marion Guillevic, Technologist, Axens
Hydrogen consumption in a hydrotreater comes from hydrotreatment reactions but also from losses within the process.
Hydrogen consumption related to the hydrotreatment process is of course inevitable since it is directly linked to hydrogenation reactions targeted to achieve product specifications (hydrodesulphurisation or aromatics hydrogenation). One way to reduce catalytic hydrogen consumption is to use a specific catalyst adapted to the product specification targeted. For instance, for hydrodesulphurisation two paths are possible: hydrogenolysis or hydrogenation. Hydrogenolysis is favoured by CoMo catalysts where hydrogenation is favoured by NiMo catalysts. A CoMo type catalyst will be chosen to minimise hydrogen consumption when the hydrodesulphurisation target is driving. It should be noted that it is not possible to perform hydrodesulphurisation reactions without simultaneous minimum hydrogenation of aromatics at the same time.
Hydrogen consumption related to dissolution losses within the process can be minimised by adapting the operating temperature of high pressure separator drums to minimise hydrogen dissolution loss and hydrogen sent to the separation unit. If some minor revamping is envisioned for the unit, hydrogen losses can be minimised by minor modifications such as modification of the scheme (hot scheme versus cold scheme), or addition of membrane on the off-gas of the MP drum (if any).
Moreover, the addition of an amine absorber on the recycling loop can be envisioned to increase recycle gas purging and to avoid the need for purging.
If other diesel hydrotreatment units are present on site, or envisioned, optimisation of the feedstock blend routed to the raw unit can also be a solution to lower the aromatics content of the blend and reduce hydrogen consumption within the unit.
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