Sulfiding, pre-sulfiding, or pre-sulfurizing is the activation process of hydrotreating catalysts that converts catalyst metal oxides into metal sulfides by reacting with H2S (Hydrogen Sulfide) in the presence of hydrogen gas. The sulfided metal form is the most active state of the hydrotreating catalyst and will facilitate the hydrodesulfurization reactions.
The newly manufactured or regenerated catalysts are in the oxide state which is an inactive condition of the catalyst metals. They are converted into sulfide form, which is the active state of the hydrotreating catalysts. More about hydrotreating catalysts can be found in my previous blog “ Hydrotreating Catalysts”
Most of the time the sulfidation process of the hydrodesulfurization catalysts is, in-situ (at the same site) but the trend is also rising to adopt the pre-sulfided (ex-situ sulfiding) catalysts. The ex-situ sulfiding has the advantage of less downtime, minimum effects of pollutants or toxic chemicals, and efficient sulfiding results.
Types of Sulfiding
The Sulfiding process of hydrotreating catalysts can be accomplished either in the vapor phase or liquid phase. Both of these sulfiding methods are commercially utilized but the wet phase sulfiding in the presence of an organic sulfiding compound is most preferred.
Dry Phase Sulfiding or Gas Phase Sulfiding also called vapor phase sulfiding is executed in the presence of hydrogen by injecting a chemical (dimethyl disulfide (DMDS) or dimethyl sulfide (DMS) which can easily be decomposed to H2S and no oil is used.
Wet Phase Sulfiding or Liquid Phase Sulfiding: Liquid-phase sulfiding can be carried out with or without the addition of organo-sulfur compounds in the feed. In the case, the organic compound is not to be utilized then the feedstock is generally a gas oil-type with sulfur in ranges 1~2 weight %. When the organic compound is used then the H2S necessary for the activation of the catalyst is generated by the decomposition of the sulfur compounds.
The preferred sulfiding method in the industry is a liquid phase with a sulfur-containing compound because it requires less time. Another advantage of the liquid phase over gas-phase sulfiding is that all the catalyst particles are fully wet thus reducing the chance of catalyst bed channeling which can occur if the catalyst particles are allowed to dry out.
Further, in wet or liquid phase sulfiding, the quantity of sulfiding agent can easily be manipulated as per requirement thus giving control over sulfiding. Moreover, in the wet method liquid serves as a heat carrier during the sulfiding exothermic reactions and saves the catalyst from pre-mature coking.
The modern Type-II catalysts require the wet sulfiding method because of the presence of a chelating agent. These compounds in the presence of startup oil help to convert the active metals into multilayers.
Catalyst Sulfiding Reaction
At a temperature above 205 degrees C, the DMDS is decomposed into H2S and then facilitates the sulfiding reactions in the presence of Hydrogen as follows;
MoO3 +2H2S+H2 → MoS2+ 3H2O
Co8O9+8H2S +H2 → 8CoS + 9H2O
NiO2+H2S+ H2 → NiS+ 2H2O
The Sulfiding Agents
Many Sulfding agents are used in the market which includes DMDS (Dimethyl Disulfide), DMS (Dimethyl Sulfide), TBPS (Tertiary-butyl Polysulfide), TNPS (Di-tertiary nonylpolsulfide), etc. But sulfiding agent DMDS is widely used around the globe due to its high H2S content i.e. 68% as compared to other agents.
The required quantity of chemicals is determined by the amount of catalyst metal oxide. Its required quantity is calculated on a stoichiometric basis then the actual amount is maintained more than the theoretical amount to ensure completion. Please view the post for stoichiometric calculations of DMDS required for Sulfiding.
Note: Every catalyst provider has a specific procedure of sulfiding to achieve the maximum efficiency of the catalyst. Sulfiding services can also be hired to perform in situ sulfiding or ex-situ sulfiding. These are only general steps, so please follow specifically your vendor procedures and instructions.
Catalyst Dry Out and Purging
Before sulfiding, the catalyst is dried out and freed from Oxygen by pulling a vacuum and then purging with Nitrogen. Also, it can be dried out with Hydrogen gas at 115~150 oC temperature and normal operating pressure. Water is removed from the boot of the separator and low point drains at the reactor circuit. Gas circulation should continue until water stops appearing in the separator.
Pre-wetting is carried out to soak the catalyst with oil and remove the debris or dust produced during loading. Oil is introduced while increasing the reactor temperature to more than 100 oC, depending on the designer’s guidelines. After oil introduction, the heat of absorption will produce, and care should be taken to avoid sudden temperature rise. The oil used for the whole process should be straight run and with designed specifications. The heat of absorption is a one-time phenomenon and the temperature drops within minutes after the heatwave passes.
First Phase Sulfiding at Low Temperature
After completion of the pre-wetting step, the reactor temperature can be increased to the decomposition temperature of the sulfiding agent I.e. 180~230 oC in the case of DMDS. The temperature rate should not be more than the recommended, normally 15~20 oC per hour. Total sulfur content (including Sulfur in HC & Sulfiding agent) should be 1.5~2.0%. Initially, the rate should be minimum to avoid high exotherms.
Monitor H2S concentration in the recycle gas at regular intervals. Hydrogen Sulfide breakthrough will be observed if H2S concentration has increased abruptly in the recycle gas.
Second Phase Sulfiding
This step is performed at 315~345 oC while maintaining the injection rate i.e. 1.5~2.0 %. Initially with the increase of reactor temperature, H2S content will reduce below 1000 ppm, and then after 4~5 hours, H2S will rise to approximately the same as added. During this phase, H2S content is maintained in Recycle Gas at approximately 2000 ppm, maximum. At this stage, the catalyst will stop taking in the H2S and will pass through the reactor without reaction and the H2S content in the recycle gas will start to increase above 2000 ppm. The step will be considered complete when water in High-Pressure Separator stops appearing.
After the completion of this step, the normal operation of the unit can be continued.
The following parameters should be monitored at specific intervals, normally after an hour;
The dimethyl Disulfide sulfiding agent injection rate should be monitored closely to find the actual consumption in comparison to the theoretical. This is calculated by measuring the level reduction of the DMDS measuring drum.
Recycle Gas Purity
Hydrogen purity is continuously monitored using an online analyzer and is normally more than 80% in the Sulfiding process. H2S content in the recycle gas is monitored using the Dragger tube or by some other method.
The reactor temperature is a very critical parameter and uncontrolled rise may lead to premature coking of the new catalyst. So, temperature-rising rates are maintained at the recommended.
Water content in the Separator
Sour water is the by-product of the Sulfiding process if water continues to come then Sulfiding is in progress. The separator boot level is noted down at regular intervals.
Troubleshooting of the Sulfiding Process
Following are some general problems but not limited to, may arise during the sulfiding activity;
- Catalysts at normal ambient conditions can contain a moisture content of about 2~5% because of their hygroscopic nature. If this is still more than 1 %, it will restrict the sulfiding reaction. Further, at high temperatures it will convert into steam, causing physical damage to the catalyst. So, ensure that the maximum amount of water has been removed before sulfiding.
- During the sulfiding of the catalyst, if the dosing rate is higher and the catalyst temperature is lower then suddenly exotherms may appear when the catalyst reaches the decomposition temperature of the sulfiding agent. This high temperature may hammer the catalyst activity due to coking.
- The heating rate should not be more than the design, because a sudden increase in temperature may lead to coking. If a sudden temperature rise is observed at any stage then adjust or in extreme cases stop firing the heater.
- When there is a deficient atmosphere of H2S at a high temperature (greater than 200 oC ) in the presence of hydrogen, oxygen will convert to water and resulting in the reduction of catalyst metal. Reduction results in permanent activity loss of the catalyst.
- DMDS decomposition and the sulfiding reactions are both exothermic, hence heatwave will generate across the catalyst. Also, high heating rates will cause premature coking of the catalyst. In case of unexpected exotherms immediately hold your step or cut out the heat source until conditions normalized.
- To avoid hydrogen embrittlement of the reactor, pressure should never exceed the metallurgical limits. Besides, ensure minimum pressurization temperature before going above the critical pressure as advised by the manufacturer.
- After the completion of sulfiding, the catalyst is super active, and cracked stock may lay down the permanent scale or coke on the catalyst. Hence, for at least three days after sulfiding, straight-run oil should be used. Less reactive or straight-run oil will produce small soft coke on the catalyst surface and will decrease the super activity. Then cracked stock can be introduced.
- The sulfide injection rate should not be lower than the calculated and the total sulfur content should be up to 2 %, as the low amount of the H2S in the system increases the time duration of the sulfiding process.
- During sulfiding, recycle gas should not be bleed from the system to maintain the H2S in the system. System pressure can be maintained by using fresh makeup hydrogen. Also, the gas treating system i.e. Amine Scrubbing should be stopped.
- The last and most important point is about safety, as sulfiding agents contain a very high content of H2S, low flash point, ignition temperature, and are highly explosive. Strictly, follow the MSDS instructions and train the staff about the hazardous nature of the sulfiding agent. Also, take special care while sampling and use live breathing apparatus or some other method to avoid H2S exposure.