A violent explosion occurred in a refinery's crude oil hydrodesulfurization unit. The unit's alarm is immediately triggered in the control room, the operator activates the internal emergency plan. The site's firefighting team went to the site and brought a fire under control. The external fire fighters service did not need to intervene. The accident caused two fatalities and to five injuries among the workers. The explosion is heard at 12 km distance.The operation of the unit affected by the accident were stopped as well as the neighbouring thermal cracking unit, while the operation of 30 units of the refinery have continued their operation. An internal investigation committee was set up to find the root cause of the accident. Also the administration in charge of environmental protection opened an investigation to define the preventive measures that the operator must put in place to avoid this type of accident. According to a preliminary analysis based on the first elements known, the cause seems to have been a hydrogen leak on a compressor for the re-circulation of the hydrogen followed by its explosion.