Advances in catalyst sulphiding and passivation
By continuously measuring H2S and hydrogen levels, online analysers provide real-time analysis of a hydrocracking reactor’s recycle gas and a data stream to the control room.
Randy Alexander and Paul Temme
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When a hydrotreater or hydrocracker is reloaded with fresh catalyst, the catalyst must be activated by converting the metal oxides on the catalyst to the active metal sulphide form by means of a process called sulphiding. The process involves exposing the catalyst to hydrogen sulphide (H2S) in the presence of hydrogen at adequate temperatures and pressures to carry out the desired chemical conversion.
To provide H2S needed for the reaction, one and/or two sulphur spiking agents are injected into the reactor once it meets a specified temperature. The primary spiking agents used by the refining industry are dimethyl di-sulphide (DMDS) or tertiary-butyl polysulphide (TBPS). When introduced to a hot reactor pressurised with hydrogen, these compounds will readily decompose to form H2S, which reacts with the metal oxides on the hydrotreating catalyst, forming the metal sulphides that carry out hydrodesulphurisation and hydrocracking reactions once fresh feed is brought into the unit.
The sulphiding reaction also produces byproduct water that is removed in a separator downstream. Once the sulphiding steps are completed, and the unit has reached normal operation, sulphur spiking can be stopped.
Thorough sulphiding of the fresh catalyst load is critical to assure peak catalyst performance, resulting in:
• The highest catalytic activity and stability, delivering on-spec product and the best product quality parameters (such as low aromatic content and/or high cetane for diesel)
• A low start-of-run temperature (WABT)
• A longer catalyst run length
• Fewer catalyst change-outs.
Several factors must be controlled during sulphiding to ensure the catalyst is completely activated, the start-up is safe, and the chemical spiking agents are used efficiently. The primary parameters to monitor are:
• Reactor temperature
• Delta temperature (dT) across the reactor
• Spiking agent injection rate
• H2S concentration of the recycle gas
• Hydrogen purity.
The sulphiding reaction is exothermic, so the rate of sulphur chemicals injected into the unit is adjusted to help control reactor bed temperatures. Reactor temperatures are straightforward to monitor from the control room, but operators and engineers also need to control the H2S concentration and hydrogen purity of the gas circulating through the unit. Very few units are equipped to measure H2S and/or hydrogen during start-up, so in the past, gas samples were taken every hour or so for measurement with colorimetric tubes and gas chromatographs.
With limited data available, engineers tended to over-inject sulphiding chemicals to ensure there was a sufficient amount of H2S in the system to maintain the sulphiding reactions until the next data point was received an hour later. This approach had several negative consequences, including:
• H2S concentrations could become dangerously high, potentially exposing personnel to hazardous H2S levels
• The recycle gas density could reach unacceptably high levels and lead to compressor trips that caused total de-pressure of the unit to the flare system. Sour gas sent to the flare could result in sulphur emission environmental exceedances and regulatory fines
• Chemical spiking agents were wasted since a concentration of 1 wt% H2S is typically sufficient to maintain an adequate sulphiding kinetic rate. In the past, H2S levels would often well exceed 2 wt%. This was not only wasteful but potentially hazardous to unit operations personnel if a leak developed
• Byproduct methane from the decomposition of DMDS would cause the hydrogen purity to drop below the minimum recommended level (normally 70%), slowing sulphiding kinetics and opening up the potential for excess coke formation in the catalyst bed.
Fortunately, these problems can now be avoided by using online analyser systems. By continuously measuring H2S and hydrogen levels, online instruments provide real-time analysis of the recycle gas and instantly provide a data stream to the control room via the cloud. The continuous flow of accurate data allows for optimisation of the spiking agent injection rate and eliminates issues associated with chemical over-injection. Furthermore, online analysers significantly lower the risk of exposing personnel to life-threatening levels of H2S and give the ‘sulphiding team’ greater control of the sulphiding process.
Controlling hydrocracker exotherms
A recently developed inline instrument also makes hydrocracker start-ups safer and ‘smarter’. In this case, the concern relates to the tendency of hydrocracking catalysts to be hyperactive at start-up, making it extremely difficult to control temperature exotherms when liquid feeds are brought into the unit. Highly active bulk metal catalysts are also difficult to control during the start-up phase.
To avoid this issue, some catalyst vendors recommend starting up a hydrocracker in the gas phase (hydrogen only) so that there are no reactive hydrocarbons in the system. However, this approach can take much longer to complete due to the reduced heat capacity and lower heat transfer rate of hydrogen gas vs liquid feeds.
A quicker approach is to start the hydrocracker up in the liquid phase with the addition of a nitrogen compound such as ammonia or methyl diethanolamine (MDEA) to temper cracking activity. Liquid-phase start-ups allow for faster heat up of the unit, while the nitrogen atoms provided by chemical injection will temporarily passivate acidic sites on the zeolitic substrate.
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