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  • Why sulphiding of Diesel and Vaccum gas oil hydrotreaters catalyst sulphiding is done in 2 stages, one at 225degC for a break through of 3000ppm H2S and other at 330degC for a break through of 1.5-2%?

    Dec-2022

Answers


  • Luis Miguel Rodriguez-Otal, Pemex, lmrotal@gmail.com

    The catalyst sulphiding reaction is not a simple substitution reaction, it is a series of complex reactions in which not only the oxygen is substituted by the sulphur. The moly changes is oxidation state and a mophological reorganization take place as well. At a low temperature (around 225 -230 oC) the oxygen is substituted by the sulphur and the oxidation state of the moly is modified from (IV) to (V), once the moly is saturated with sulphur, in order to accelerate the last step of the activation, it is required a higher temperature (second step at 330 oC) and at this condition the moly oxidation state changes from (V) to (IV) and the morphological change finished. The H2S concentration in recycle gas is a good signal that most moly is saturated with sulfur and to ensure and accelerate the last reaction is necessary to increase the H2S concentration. It is important to consider that the activation involves several chemical reactions thus they follow the kinetic and thermodynamic rules, so the H2S is necessary to avoid the reduction reaction.

     

    Dec-2022

  • Randy Alexander, Reactor Resources, ralexander@reactor-resources.com

    Sulphiding of all hydrotreating catalyst involves conversion of oxidic metals to the active metal sulphide phase. These metals, typically molybdenum, cobalt, nickel, and/or tungsten, are impregnated on the alumina catalyst support.  As mentioned, the sulphiding process is normally carried out in two stages. For the first sulphiding plateau, the temperature is normally held below 260C (500F) until H2S breakthrough occurs. If oxidic catalysts are exposed to hydrogen at temperatures above this level, reduction of the catalytic metals can occur.  Once the metals are reduced, it is almost impossible to convert them to the active sulphide phase.  

    Breakthrough of H2S (H2S > 3000 ppm) indicates that the catalyst bed is saturated with sulphur which protects the catalytic metals from reduction once the reactor temperature is raised.  Higher reactor temperatures are needed to complete formation of the sulphide phase and to distribute the moly-sulphide molecules evenly across the surface of the catalyst support.  The kinetic rate of the sulphiding reaction doubles for every 15C (30F) of temperature increase.

    The H2S concentration should be held in the range of 0.5-1.5% after breakthrough in order to maintain a sufficient amount of sulphur in the system, protecting the newly formed active sites.  H2S concentrations above 1.5% will not help speed up the sulphiding process and can cause other issues such as:

    - High recycle gas density that could require excess purging of the sour recycle gas (wasting H2S and increasing the amount of spiking agent required)

    - High H2S concentrations are dangerous should leaks occur

    - Excess corrosion from moist sour gas

    The second sulphiding plateau involves a temperature hold in the range of 320C (608F) to 330C (626F). This hold ensures all of the active sulphide molecules are formed and that the distribution of active sites is optimized. The exact temperature and length of the hold depends on the catalyst manufacturer and the specific catalyst being sulphided.

    Note that we now recommend a reactor temperature above 235-240C (455-464F) for the first sulphiding plateau in order to ensure complete decomposition of DMDS to H2S. Below that threshold, some non-decomposed mercaptans (eg. methyl sulphide and dimethyl sulfide) will still be present in the recycle gas. Purging of gas containing these non-decomposed mercaptans can lead to sulphur emmision issues downstream since they will not be absorbed by the gas scrubbers.

     

    Dec-2022