Catalysis 2023 Issue

• Ammonia is hazardous to handle and difficult to pump due to cavitation issues • Reactor runaway if nitrogen is not injected at sufficient levels • Ammonia will react with H 2 S in the vapour phase to form ammonium bisulphide (NH 4 SH). This not only consumes some of the H 2 S needed for sulphiding (wasting DMDS), but NH 4SH is very corrosive, can precipitate out in fin fans and exchangers, and can migrate through the process, causing more problems downstream. To enhance control of nitrogen injection and to mitigate the previously mentioned issues, an inline pH monitor- ing system was developed that links up with Reactor Resources’ web-connected pump telemetry (see Figure 1 ). The pH instrument quickly and accurately measures the pH change of separator water so that the nitrogen-compound injection rate can be adjusted to optimum levels and nitro- gen breakthrough can be detected as soon as it occurs. This instrument also minimises the chance of exposing opera- tors to sour water containing unsafe amounts of H 2 S and ammonia. Case study In 2021, a US Gulf Coast refiner with a history of innovation utilised three analyser systems to successfully sulphide and start up a large hydrocracker. In normal operation, this unit processes 60K BPD of highly reactive light cycle oil (LCO). During start-up, ULSD was circulated through the unit to reduce the chance of an exotherm. The pretreat reactors were loaded with a Type II nickel- molybdenum (Ni/Mo) catalyst and a highly active bulk metal catalyst, while zeolitic-supported catalysts were used in the cracking beds. The catalyst vendor recommended that additional nitrogen be brought into the system to attenu- ate hydrocracking activity. It was agreed that this approach would also help control the hyperactivity of the bulk metal catalyst that had been installed. After thorough drying of the system to remove any residual moisture, ULSD was fed to the unit at a reactor temperature of 300°F (150°C). DMDS spiking of the feed began once the reactor inlet temperature rose above 440°F (227°C) (hour 1.5 on Figure 2 ). Over the next three hours, the DMDS injec- tion was increased to 10 gpm (2,271 LPH) while the reactor temperature was gradually allowed to rise to 460°F (238°C). The proprietary online H 2 S analyser and the hydrogen purity analyser continually streamed gas concentration data back to the control room via a secure web page. Reactor temperatures were held constant for 8.5 hours until the recycle gas H 2 S concentration exceeded 3,000 ppm. H 2 S breakthrough indicated that the catalyst bed was sufficiently sulphided, and the first (or Low Reactor Temperature) sulphiding step was complete. With sulphur protecting all the active sites, the reactor temperature could then be increased without the risk of reducing the catalytic metals with hot, high-pressure hydrogen. Near the end of the first sulphiding step, the hydrogen purity continued to drop. As the purity approached 70%, the operator reacted quickly, purging the optimum amount of sour hydrogen and adding fresh make-up hydrogen only

Figure 1 Portable pH analyser system

Controlling hydrocracker exotherms A recently developed inline instrument also makes hydro- cracker start-ups safer and ‘smarter’. In this case, the con- cern 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. With nitrogen passivation, the start-up can proceed without the risk of uncontrolled cracking reactions and exotherms. Nitrogen passivation of hydrocracking catalysts has some drawbacks, such as: • The catalyst can absorb too much nitrogen if the passiv- ating agent is over-injected, causing lower catalytic activity and reduced yields that can last for days to weeks while the excess nitrogen is desorbed from the catalyst

Catalysis 2023