Catalysis 2023 Issue

Advances in catalyst sulphiding and passivation By continuously measuring H 2 S 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 Reactor Resources

W hen 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 cata- lyst to hydrogen sulphide (H 2 S) in the presence of hydrogen at adequate temperatures and pressures to carry out the desired chemical conversion. To provide H 2 S 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 intro- duced to a hot reactor pressurised with hydrogen, these compounds will readily decompose to form H 2 S, which reacts with the metal oxides on the hydrotreating catalyst, forming the metal sulphides that carry out hydrodesul- phurisation 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 sul- phiding steps are completed, and the unit has reached nor- mal 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) 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 • H 2 S concentration of the recycle gas • Hydrogen purity. Avoiding over-injection The sulphiding reaction is exothermic, so the rate of sul- phur chemicals injected into the unit is adjusted to help • A longer catalyst run length • Fewer catalyst change-outs.

control reactor bed temperatures. Reactor temperatures are straightforward to monitor from the control room, but operators and engineers also need to control the H 2 S concentration and hydrogen purity of the gas circulating through the unit. Very few units are equipped to measure H 2 S 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 H 2 S 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: • H 2 S concentrations could become dangerously high, potentially exposing personnel to hazardous H 2 S 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 concen- tration of 1 wt% H 2S is typically sufficient to maintain an adequate sulphiding kinetic rate. In the past, H 2 S 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 mini- mum recommended level (normally 70%), slowing sulphid- ing 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 measur- ing H 2 S and hydrogen levels, online instruments provide real-time analysis of the recycle gas and instantly pro- vide 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 expos - ing personnel to life-threatening levels of H 2 S and give the ‘sulphiding team’ greater control of the sulphiding process.

Catalysis 2023