Hydrogenation unit – Description of key equipment, hazard factors, and preventive measures

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Key Areas and Equipment 1. Key Areas 1. Heating Furnace and Reactor Area: The heating furnace and reactor area of the hydrogenation unit is equipped with devices such as hydrogenation reaction heaters, heating furnaces for the distillation section, high-pressure heat exchangers, etc. Most of these devices are high-pressure equipment; the temperature of the fluids involved is relatively high, and there are open flames in the heating furnaces. As a result, this area presents a high level of potential risk, with fire and explosion being the main hazards that require careful attention from a safety perspective. 2. High-pressure separator and high-pressure air cooling area: The high-pressure separator and high-pressure air cooling area contain a high-pressure separator as well as high-pressure air coolers; if the liquid level in the high-pressure separator is not properly controlled, serious problems can occur. The main hazards are fire, explosion, and H2S poisoning; therefore, this area is a priority for safety precautions. 3. Hydrogen compression unit building: The hydrogen compression unit building houses the cycle hydrogen compressors and hydrogen boosters. This area is exposed to hydrogen; the pressure of hydrogen in this area is high, and since the compressors are moving equipment, they are prone to failure. As a result, this area presents a high level of risk, with the main hazards being fires, explosions, and poisoning. It is therefore an area that requires special attention from a safety perspective. 4. Fractionation Tower Area: This area contains a large number of equipment units, and the media handled are mostly flammable and explosive substances. High-temperature hot oil pumps are devices that require special attention in terms of safety; a leak of such hot oil can lead to fire accidents. There is a large amount of fuel gas, liquid hydrocarbons, and oils in this area, and an accident in this region could have severe consequences. Additionally, the butane removal tower, along with its dry gas and liquefied gas, contain high concentrations of H2S, posing a risk of poisoning; therefore, this area also requires careful attention from a safety perspective. 2. Main equipment 1. Hydrogenation reactor: Hydrogenation reactors are mostly fixed-bed reactors, and the hydrogenation reaction takes place in a gas-liquid-solid three-phase trickle-bed system. There are two types of hydrogenation reactors: cold-wall reactors and hot-wall reactors; cold-wall reactors have an insulating lining, and the material used for these reactors is of lower quality ； Hot-wall reactors do not have an insulating lining; instead, they use a double-layer surfacing lining, typically made of 2×1/4Cr-1Mo material. The catalyst in the hydrogenation reactor must be packed in layers, with quenched hydrogen used in between; as a result, the structure of the hydrogenation reactor is complex. The reactor inlet is equipped with a diffuser that contains internal components such as a feed distribution plate, a scale collection basket, a catalyst support plate, cold hydrogen pipes, a cold hydrogen tank, a redistribution plate, and an outlet oil collector. The hydrogenation reactor operates under high temperature, high pressure, and in the presence of hydrogen, resulting in severe operating conditions; it is one of the most important devices in a hydrogenation plant. 2. High-pressure heat exchangers: The outlet temperature of the reactor is relatively high, and the material contains a large amount of heat energy; therefore, it is necessary to recover as much of this heat as possible. Hence, hydrogenation units are equipped with high-pressure heat exchangers that are used to exchange heat between the reactor outlet material, the feed oil, and the recycled hydrogen. Modern high-pressure heat exchangers are mostly U-tube type double-pass heat exchangers. Such heat exchangers enable pure counterflow heat transfer, which improves heat exchange efficiency and reduces the area required for the high-pressure heat exchanger. Tube boxes typically use threaded locking end caps, which offer the advantages of a compact structure, good sealing performance, and ease of installation and removal. High-pressure heat exchangers operate under conditions of high temperature, high pressure, and in the presence of hydrogen; they have numerous static sealing points, which makes leaks more likely to occur. They are important equipment in hydrogenation units. 3. High-pressure air cooling: The operating conditions for high-pressure air cooling involve high pressure and exposure to hydrogen; it is an important device in hydrogenation units. In a medium-pressure hydrocracking unit at a refinery in North China, China, leaks occurred twice in the high-pressure air cooling system, forcing the shutdown of the unit for repairs. Therefore, attention should also be paid to the design, manufacturing, and use of high-pressure air cooling systems. 4. High-pressure separator: The function of the high-pressure separator is to carry out three-phase separation of gas, oil, and water. It operates under high pressure and in the presence of hydrogen; the operating temperature is not high. The corrosivity of the materials increases in the presence of water and hydrogen sulfide, so proper attention must be paid when using this device. Furthermore, the liquid level in the high-pressure separator of the hydrogenation unit is extremely important; poor control of this level can lead to serious consequences. If the liquid level is too high, liquid may enter the recycle hydrogen compressor and damage it. If the liquid level is too low, an accident involving the flow of high-pressure gas into the low-pressure system can occur, with large amounts of recycle hydrogen rushing into the low-pressure separator. At this time, if the safety valve of the low-pressure separator fails to open or the discharge volume is insufficient, a serious accident will occur. Therefore, from a safety perspective, the high-pressure separator is a very important device. 5. Reaction heating furnace: The operating conditions of the hydrogenation reaction heating furnace are high temperature, high pressure, and exposure to hydrogen, in addition to the presence of an open flame; these conditions are extremely harsh, making it an important device in hydrogenation plants. The tube material for hydrogenation reaction heating furnaces is generally high-Cr, high-Ni alloy steel, such as TP347. The furnace type used in hydrogenation reaction heaters is mostly a double-sided radiation heating furnace with a pure radiation chamber. This design is intended to increase the heat intensity of the radiation tubes, reduce the length of the tubes as well as the number of bends, thereby decreasing the amount of tubes needed and lowering the system pressure drop. To recover the waste heat from flue gas and improve the thermal efficiency of the heating furnace, a waste heat boiler system is generally installed in hydrogenation reaction heating furnaces. 6. New hydrogen compressor: The function of a new hydrogen compressor is to pressurize the raw hydrogen gas and feed it into the reaction system. Such compressors typically have a large pressure difference between the inlet and outlet, with a relatively low flow rate; reciprocating compressors are commonly used for this purpose. The compression ratio per stage of a reciprocating compressor is generally 2–3.5; depending on the pressure of the hydrogen source and the pressure in the reaction system, 2 to 3 compression stages are usually used. Most components of a reciprocating compressor are components that move back and forth, and the airflow is pulsatory; as a result, reciprocating compressors cannot operate for long periods of time, and spare units are usually provided. Reciprocating compressors are generally driven by electric motors, connected via rigid couplings. The motors have high power but low speed, and synchronous motors are commonly used. 7. Circulating hydrogen compressor: The function of the circulating hydrogen compressor is to supply circulating hydrogen for the hydrogenation reaction. The cyclic hydrogen compressor is the “heart” of the hydrogenation unit. If the cyclic hydrogen compressor stops operating, the hydrogenation unit can only be shut down urgently to release pressure. The circulating hydrogen compressor performs cyclic work within the system; the pressure difference between its inlet and outlet is generally small, while the flow rate is relatively high, which is why centrifugal compressors are typically used. Due to the low molecular weight of recycled hydrogen, the energy head required by single-stage impellers is small; therefore, recycled hydrogen compressors generally operate at high speeds (8000–10000 rpm) and have a large number of stages (6–8 stages). Apart from the bearings and shaft end seals, a circulating hydrogen compressor has almost no components with relative friction. The seals used in such compressors are typically dry gas seals and floating ring seals. Coupled with a comprehensive set of instrumentation for monitoring and diagnosis, circulating hydrogen compressors can generally operate for long periods without the need for backup units. Cyclic hydrogen compressors are often driven by steam turbines, as these turbines have high rotational speeds and the ability to adjust those speeds. 8. Automatic backwash filter: The hydrogenation feed contains mechanical impurities; if these are not removed, they will accumulate at the top of the reactor, causing an excessive pressure difference that forces the reactor to stop operating and thus reducing the operational lifespan of the facility. Therefore, the hydrogenation feedstock needs to be filtered, and automatic backwash filters are now commonly used. The automatic backwash filter is equipped with Johnson filters, which can remove solid impurity particles of ≥25/1m in size. When the pressure difference between the inlet and outlet of the filter exceeds the set value (0.1–0.18 MPa), the backwashing mechanism is activated to carry out backwashing and remove the impurities from the filter. Risk factors and preventive measures 1. Risk factors and preventive measures during startup and shutdown 1. Drying and baking of the hydrogenation reaction system. The purpose of drying and baking the reaction system in a hydrogenation unit is to remove moisture from within the system, eliminate free water and crystalline water present in the refractory materials of the heating furnaces, and sinter these refractory materials thereby increasing their strength and service life. When heating the furnace with coal, it is necessary to introduce fuel gas into the system; before doing so, it is essential to ensure proper airtightness and isolation of the gas. Generally, the oxygen content in the fuel gas should be less than 1.0%. Prevent gas leakage and migration to other systems. When lighting a heating furnace, the furnace chamber must be thoroughly purged with steam to ensure that no flammable gases remain. When heating the furnace, the temperature should be increased and decreased strictly in accordance with the heating curve, to avoid rapid temperature rise, which could cause the moisture in the refractory materials to evaporate quickly and lead to the collapse of the furnace walls. 2. Catalyst loading in the hydrogenation reactor: The catalyst should be loaded strictly in accordance with the specified loading procedure. The quality of catalyst loading has a significant impact on the operation and service life of the hydrogenation unit. Before loading the catalyst, the reactor and its internal components should be carefully inspected, as well as the condition of the catalyst dust, to determine whether the catalyst needs to be sieved. Catalyst loading is preferably carried out on dry and sunny days to ensure even distribution of the catalyst; otherwise, flow imbalances or \"hot spots\" may occur in the reactor during operation, affecting the proper functioning of the plant. When loading the catalyst, workers need to enter the reactor to carry out the work; therefore, special attention must be paid to the protection of these workers and their safety. They must wear protective clothing and breathing masks that can supply oxygen or air. Workers entering the reactor are not allowed to bring any other items, in order to prevent foreign objects from falling into the reactor (generally, catalyst loading is carried out by professional personnel from specialized companies). 3. Purging of the hydrogenation reaction system The purging of the hydrogenation reaction system is carried out in two stages: first, the air environment is replaced with a nitrogen environment, and then the nitrogen environment is replaced with a hydrogen environment. When replacing the air environment with a nitrogen environment, it is important to note that after the replacement, the oxygen content in the system should