Points to note in the process design of hydrogenation reactions

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The hydrogenation reaction is **one of the first hazardous chemical processes identified by the Work Safety Administration as requiring strict supervision. The dangers arise from the following points: 01▶ Dangers of hydrogen Hydrogen has characteristics that other flammable gases or organic solvent vapors do not possess: ① It is lighter than air ； ② Easy to diffuse ； ③ The explosion range in air is extremely wide (4%–75%) ； ④ Hydrogen has a very low ignition energy ； ⑤ Hydrogen has high chemical reactivity. 02▶ Hazards of hydrogenation reactions The hazards of hydrogenation reactions include: ① Hydrogenation reactions generally take place under high pressure; in some cases, the pressure can reach up to 10 MPa. Therefore, the reaction vessels must be specially designed ； ② The hydrogenation reaction is a highly exothermic reaction; improper control can easily lead to overpressure and accidents in the system ； ③ The catalysts used in hydrogenation processes have very high activity and can catch fire spontaneously in air; special care must be taken when using them. This paper analyzes the hazards of hydrogenation reactions and discusses the key considerations in the design of hydrogenation process systems, covering aspects such as material feeding, nitrogen purging, reaction process control, pressure relief after the reaction, and safe venting under abnormal operating conditions. 01 Points to note in the process design of hydrogenation reactions: 1) Feeding before the reaction and nitrogen purging. Catalysts used in hydrogenation reactions, such as Raney nickel and palladium on carbon, have dry powders that can catch fire spontaneously in air; therefore, the feeding operation must be carried out in a sealed feeding device under nitrogen protection. The hydrogenation reactor must be purged with nitrogen before starting the hydrogenation reaction, and the purging process is completed only when the oxygen content in the system is below 0.5%. The displacement methods include pressure displacement, evacuation displacement, and water injection exhaust method. 2) Reaction process control: The process and automatic control design must, based on the results of reaction safety risk assessment, define safe operating conditions. The design of process and safety facilities should cover aspects such as process design, instrument control, alarm and emergency intervention (safety instrumented systems), collection and protection of materials after release, as well as emergency response measures for the plant area and surrounding regions. 3) Pressure relief after the reaction: After the hydrogenation reaction is complete, it is necessary to release the pressure inside the tank; only after that can nitrogen be introduced to displace the gas before the contents can be removed. The exhaust pipe of the hydrogenation reactor tank must be installed separately, with its outlet positioned 2 meters above the roof; a flame arrester should be installed at the end of the exhaust pipe. A check valve should be installed after the exhaust pipe cut-off valve to prevent gas from flowing back. 4) Safety relief measures: The hydrogenation reactor should be equipped with safety relief devices such as safety valves or burst discs. When the reaction is intense, it is advisable to install two safety relief devices, with a certain gradient between their opening pressures or burst pressures; each of these safety relief devices must be capable of delivering the required maximum amount of release. The discharge pipe at the outlet of the safety valve or rupture disc should first be connected to an explosion vent tank before being released to the atmosphere. 02 Points to Note Regarding Civil Engineering, Electrical Systems, and Ventilation Conditions 1) The process engineering team should specify hydrogen as a special medium in the requirements provided to the civil engineering team; the civil engineering team, when designing rooms and steel platforms, must ensure that there are no structures that could hinder the diffusion of hydrogen. The upper space above the hydrogenation area should be well-ventilated, and the inner surface of the ceiling should be smooth to avoid dead corners and prevent hydrogen from accumulating in recessed areas at the top. Vents or exhaust openings should be provided on the top of the building or at the upper part of its exterior walls; these exhaust openings should be located at the highest point and face a safe direction. 2) The selection of electrical equipment in the hydrogenation area should not be lower than the rating and category of hydrogen explosive materials (IICT1). Equipment exposed to the outdoors, such as explosion vent tanks, roof-mounted fans and ducts, as well as the vent pipes of hydrogenation reactors, should be equipped with lightning protection measures. 3) Good ventilation should be ensured during hydrogenation reactions; in buildings equipped with mechanical ventilation, the air inlets should be located at the bottom of the building while the exhaust outlets should be placed at the top. The air exchange rate for normal ventilation during hydrogenation reactions must be no less than 3 times per hour, while the air exchange rate of the emergency exhaust system must be no less than 12 times per hour. The emergency exhaust system should be interlocked with the hydrogen detection device. In the relationship between production and safety, safety comes first; it must be given top priority. It is necessary to analyze potential hazards in advance, predict and assess dangerous and harmful factors, understand the patterns and changes in such hazards, and take appropriate preventive measures to eliminate them at their earliest stages.