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Case Studies

Refinery Turnaround of Hydrotreating Unit

– H2Stopper 7106

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Background

Heavy oil from the refinery fluid catalytic cracking unit is normally hydrotreated to remove sulfur compounds and to upgrade the oil to reduce unsaturated molecules for additional downstream treatment and recovery. During the bi-yearly catalyst change out in the hydrotreating unit, the heavy oil is diverted to storage for sale as a low value bunker fuel oil.

During this period of several weeks, the oil must be treated by a H2S scavenger to avoid poisonous hydrogen sulfide gas from concentrating in the head space of the storage tank. Low levels of hydrogen sulfide will be released from the oil in storage as the oil stagnates and cools. It is very common for oil with only 10 ppm of H2S to have over 100 times this amount in the vapor space of the storage tank. The H2S concentration is the vapor space will vary with tank level, tank throughput and temperature but past experiences gave measurements of 300-1000 ppm, all dangerous levels. Personnel exposed to 500 ppm of H2S can loose consciousness and exposure levels from 700 to 1000 ppm can lead to death.

Objective

  • Treat the oil to less than 1 ppm H2S specification during the turnaround to keep tank headspace levels below 10 ppm H2S.

Procedure

  • Since the line from the fluid cracking unit to the storage tank is only 1-2 bar pressure, an injection pump with 4-5 bar discharge pressure was used to inject the H2Stopper7106 just before the oil departed the cracking unit. The optimum injection location is in the cross exchangers used to recover heat from the oil. The H2Stopper was injected at the inlet to an exchanger with the oil temperature of approximately 3500F. No injection quill was used as the turbulence through the shell and tube exchanger provided the necessary mixing.
  • The H2Stopper7106 was injected at 2 gallons pre 1000 barrel of oil.

Results

  • The H2Stopper7106 performed extremely well in this application. Measurements of H2S in the tank vents never exceeded 10 ppm during the turnaround and typical levels were 4-5 ppm H2S. The oil was sold as bunker oil with no hazard to personnel.