Main equipment of hydrogenation units

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1. Hydrogenation reactors: Most hydrogenation reactors are fixed-bed reactors. The hydrogenation reaction is a three-phase trickle-bed reaction involving gas, liquid, and solid phases. There are two types of hydrogenation reactors: cold-wall reactors and hot-wall reactors. Cold-wall reactors have thermal insulation lining, and their material grade is relatively low； 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 hydrogenation plants. 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 output and the feed oil as well as the recycled hydrogen. Modern high-pressure heat exchangers are mostly U-tube type double-pass heat exchangers. Such heat exchangers enable pure counter-current 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. The operating conditions of high-pressure heat exchangers involve high temperature, high pressure, and hydrogen exposure. They have numerous static sealing points, making leaks prone to occur; thus, they are crucial equipment in hydroprocessing 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. Its operating conditions involve high pressure and exposure to hydrogen; the operating temperature is not high. In the presence of water and hydrogen sulfide, the corrosiveness of the materials increases, so this aspect needs to be given due attention during use. 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 pressure transfer from high pressure to low pressure can occur, with large amounts of recycle hydrogen rushing into the low-pressure separator. In such a situation, if the safety valve of the low-pressure separator fails to open or does not discharge enough fluid, 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, along with 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 pure radiation chamber with double-sided radiation heating. This design aims to increase the thermal intensity of the radiation tubes, reduce the length and number of bends in the furnace tubes, thereby decreasing the amount of furnace tubes required 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 supply and the pressure in the reaction system, 2 to 3 compression stages are usually used. Most components of a reciprocating compressor are parts that move back and forth, and the airflow is pulsatory; as a result, such 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 provide circulating hydrogen for the hydrogenation reaction. The cyclic hydrogen compressor is the “heart” of the hydrogenation unit. If the recycle hydrogen compressor stops operating, the hydrogenation unit can only be shut down urgently to release pressure. The cyclic 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, so centrifugal compressors are typically used. Due to the low molecular weight of circulating hydrogen, which results in a low energy head for single-stage impellers, circulating 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 extended periods without the need for backup units. Cyclic hydrogen compressors are often driven by steam turbines, as these turbines have high speeds and their speed can be adjusted. 8. Automatic backwash filter: 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 cycle 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 backwash mechanism is activated to carry out backwashing and remove the impurities from the filter.