Formation of Nickel Carbonyl in Hydroprocessing Reactors
Nickel Carbonyl or Nickel Tetracarbonyl Ni(CO)4 is is a highly toxic, potentially fatal if inhaled, rapid skin absorption, highly flammable, and a known carcinogen. Primarily it can cause lung damage. Exposure to even low concentrations may cause severe illness or death even though initial symptoms may be very moderate. The OSHA permissible exposure limit is an eight-hour time-weighted average of 0.001 parts per million by volume ppmv (i.e. 1 ppbv) and a maximum spot exposure of 0.04 wt-ppm (40 wt-ppb). Great care must be taken while opening a hydroprocessing reactor containing Nickel catalyst and Carbon Monoxide.
Nickel carbonyl, Ni(CO)4, is described as a colorless liquid at ambient conditions, which is reported to have a musty smell but a very low boiling point (42.5°C at 1 atm). Thus, even at ambient temperature and pressure, there is a significant vapor pressure over a sample of the liquid. The reaction equilibrium of Nickel Carbonyl follows first-order kinetics.
Ni + 4 CO = Ni(CO)4
In hydroprocessing catalysts, Ni(CO)4 is formed in the presence of Nickel catalyst and CO (carbon monoxide) and is favored by high pressure and low temperature i.e. below 200 °C. Under normal operating conditions, Ni(CO)4 does not form but it can be formed during abnormal conditions start-up, shutdown, or regeneration when the catalyst temperature is below 200 °C.
Sources of Carbon Monoxide in Hydroprcoessing Units
CO, the second component required for Ni(CO)4 is present in the refinery hydrogen circuit produced in the reforming and hydroprocessing reactions. In the reactions environment of reforming and hydroprocessing units, the Oxygenates are converted to CO in the presence of hydrocarbons. But the biggest source of CO is the incomplete combustion of Hydrocarbons during regeneration of hydrotreating catalyst. So, there are more chances of Ni(CO)4 after the catalyst has been regenerated. Further, when the Hydrotreating catalyst is being shut down for the removal of spent catalyst, then Ni(CO)4 is likely to exist and it is always better to assume that spent hydroprocessing catalyst contains nickel carbonyl.
Guidelines to Avoid Formation of NI(CO)4
The best way to prevent exposure to nickel carbonyl is to avoid its formation that can be avoided by stopping the presence of CO on the Nickel catalyst. The following guidelines are recommended to avoid the formation of NI(CO4).
- The recycle gas stream in the hydroprocessing must not contain any CO when cooling below 260°C. To remove this from the reactor, purge the reactor circuit with nitrogen until the system contains less than 1 mol% hydrogen and hydrocarbons. Further, CO must not be detected in the gas stream.
- When the hydrotreating catalyst bed is being cooled below 80°C, then avoid any air ingress to the catalyst. This will prevent the burning of the catalyst bed due to its pyrophoric nature. The catalyst burning could lead to high localized temperatures and the formation of nickel carbonyl if CO is present.
- During regeneration of hydrotreating catalyst, complete combustion of carbon material over the catalyst should be ensured by keeping sufficient oxygen flow. Further, all CO and CO2 must be purged from the system before any maintenance activity on the hydrotreating reactor.
- Maintain high temperatures of reactor until CO is purged from As the reaction temperatures are lowered, if there is any CO present in the recycle gas then NI(CO)4 can be formed. Therefore, for any shutdown prior to reactor entry, it is critical to purge any CO present in the recycle gas below 10 ppm.
Precautions While Maintenance Activity on the Hydrotreating Reactor
Persons who enter the confined spaces where they might expose to NiCO4 must have live breathing apparatus with a lifeline. Analytical instruments for continuous monitoring of the atmosphere should also be applied. Further, a proper covering suit to avoid its absorption into the skin is also mandatory.
Workers suspected to have exposure to Ni(CO)4, should be immediately removed from the area. Contaminated clothes should be removed. Fresh air should be administrated to the affected person and call for medical assistance. The Urine should be tested for Nickel concentration.
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