Design and Application of Control Valves in Hydroprocessing Units

With increasingly stringent environmental regulations limiting pollutants such as sulfur, heavy metals, and polycyclic aromatic hydrocarbons, hydroprocessing has become a widely adopted process in the refining industry. This process removes harmful impurities from feedstocks through selective reactions with hydrogen in a heated catalyst bed, converting heavy components into lighter fractions and providing significant economic benefits. Although numerous hydroprocessing units have been constructed in China, the performance of high-pressure differential control valves directly influences the safety and reliability of these systems. For many years, these critical valves have mainly relied on foreign brands such as Masoneilan and Fisher—particularly in residue hydroprocessing units—leaving domestic manufacturers with limited opportunities. In recent years, however, advances in localization have enabled Chinese enterprises to master key valve technologies and successfully replace imported products. This article focuses on control valves installed at critical locations within hydroprocessing units, where they operate under harsh conditions characterized by high temperatures, large pressure differentials, and severe flashing, cavitation, erosion, and corrosion. Accurate calculation of valve flow capacity is crucial for ensuring effective gas–liquid separation, stable unit operation, and prolonged service life. The paper presents the operating principles and key parameters of the hot high-pressure separator level control valve, cold high-pressure separator level control valve, feedstock booster pump outlet recirculation valve, rich and lean amine level control valves, and the sour water flash tank inlet level control valve. It also provides a theoretical basis for the selection and calculation of key control valves, offering practical guidance to ensure safe, reliable operation and accurate valve selection.

increasingly stringent environmental regulations on pollutants such as sulfur, heavy metals, and polycyclic aromatic hydrocarbons, hydroprocessing has become a widely adopted process in the refining industry. Hydrotreating removes impurities—including sulfur, nitrogen, and heavy metals—from petroleum feedstocks and converts heavy components into lighter fractions through selective reactions with hydrogen in a heated catalyst bed, offering significant economic benefits. China has constructed numerous hydrotreating units; however, the performance of high-pressure differential control valves in these systems directly affects operational safety, placing stringent demands on valve reliability. Since the introduction of hydrotreating technology, these valves have been largely dominated by internationally recognized brands such as Masoneilan and Fisher, particularly in residue hydrotreating units, leaving domestic suppliers with limited opportunities to bid. Over the past decade, with the advancement of localization efforts, China has gradually mastered the technologies required for key valves in hydroprocessing units, enabling domestic production and replacement of imported equipment. Several critical locations within a hydroprocessing unit require control valves that operate under high-pressure differentials in liquid service, making them susceptible to wear and damage. The most representative example is the hot high-pressure separator level control valve. For example, in residue hydroprocessing units, this valve operates under extremely high temperatures (above 400 °C) and large pressure differentials (over 13 MPa), while also being subjected to flashing, cavitation, erosion, and corrosion—resulting in exceptionally severe service conditions. Therefore, accurate valve design and precise calculation of flow capacity are critical at the design stage. Insufficient flow capacity may fail to meet process requirements and compromise effective gas–liquid separation, whereas excessive capacity can force the valve to operate at low openings, increasing the risk of malfunction or damage and undermining long-term stable operation. This paper examines the selection and calculation methods for control valves installed at critical positions in hydroprocessing units. The results provide both a theoretical and practical foundation for accurate valve sizing and selection, ensuring reliable and efficient plant operation.

To understand the application of control valves at critical positions, it is first necessary to understand the process flow of a hydrogenation unit. The feedstock enters a feed buffer tank, where it is pressurized by a feed pump. After heat exchange, the feedstock is further heated in the feed heater. Fresh hydrogen is compressed in the make-up hydrogen compressor and then mixed with recycled hydrogen from the recycle hydrogen compressor. This hydrogen–feedstock mixture then enters the main reactor, where critical reactions—such as hydrodesulfurization, denitrification, olefin saturation, and aromatic ring-opening saturation—occur. The reaction products exit from the bottom of the reactor and, after passing through a heat exchanger, enter the high-pressure separator for gas–liquid separation. The hot liquid phase discharged from the bottom of the separator is depressurized by the level control valve and then directed to the product distillation tower (or to the hot low-pressure separator for further separation). The gas phase from the top of the separator is cooled in a heat exchanger and then enters the cold high-pressure separator for three-phase separation of gas, oil, and water, with the aqueous phase subsequently treated in the sour water flash tank. The gas phase from the cold high-pressure separator (circulating hydrogen) flows in countercurrent through the circulating hydrogen amine scrubber for desulfurization. It is then recompressed by the recycle hydrogen compressor and returned to the reactor system. The detailed process flow is shown in Figure 1, with control valves at critical positions highlighted. The functions of these valves are described below.

The level control valve at the bottom of the hot high-pressure separator is a key component of a hydrogenation unit. Reactor effluent passes through a heat exchanger before entering the hot high-pressure separator, where hydrogen-rich gas is flashed off. The lower portion—consisting of liquid hydrocarbons, catalyst particles, and impurities—is depressurized through this valve and routed to the medium-pressure system or the distillation section. The primary function of this valve is to maintain the proper liquid level in the high-pressure separator, ensuring efficient separation of gas and liquid phases. Any malfunction can lead to poor separation efficiency, hydrogen entrainment in the recycle gas, or high-to-low pressure crossover, severely affecting unit performance and potentially causing major safety incidents. Because the process medium contains hydrogen sulfide, the valve is exposed to chemical corrosion. Moreover, multiphase flow cavitation and severe erosion from catalyst particles subject the valve to some of the harshest operating conditions in the entire hydrogenation unit.

The cold high-pressure separator level control valve is also critical in the hydrogenation unit. Its function is similar to that of the hot separator valve, regulating the liquid level within the cold high-pressure separator. If the valve’s control performance is inadequate, an excessively high liquid level can result in liquid carry-over into the recycle hydrogen stream, damaging downstream equipment, while a low liquid level can trigger high-to-low pressure crossover incidents. More importantly, this valve indirectly regulates system pressure by controlling the liquid level in the cold high-pressure separator, which serves as the reference pressure point for the entire reactor system. Consequently, the cold high-pressure separator level control valve is essential for the stable and safe operation of the entire hydrogenation unit.

The feedstock booster pump outlet recirculation valve—also called the minimum-flow valve—is installed at the pump discharge to prevent damage from overheating, excessive noise, instability, or cavitation when the pump operates under low-flow conditions. By recirculating a small portion of the fluid during low-load operation, this valve ensures that the pump maintains its minimum required flow, promoting stable and reliable performance. The key feature of this valve is its capability to ensure stable operation even under high differential pressure conditions.

The rich amine liquid at the bottom of the circulating hydrogen amine scrubber absorbs a significant amount of hydrogen sulfide. When depressurized by the level control valve, some of the hydrogen sulfide is released and expands. Under high-pressure conditions, hydrogen sulfide can corrode the valve. When exposed to large pressure differentials, severe flashing and cavitation may occur, causing significant erosion and potential valve failure. If the rich amine level control valve fails, the liquid level in the circulating hydrogen desulfurization tower cannot be properly maintained. Conversely, if the liquid level is too low, degassing may occur, allowing high-pressure gas to flow backward, which could overpressurize and damage low-pressure equipment. Therefore, this valve is also a critical component of the hydrogenation unit.

The lean amine booster pump discharge recirculation valve functions similarly to the crude oil booster pump discharge recirculation valve, ensuring stable pump operation. Therefore, it will not be discussed in detail here.

The primary function of this valve is also to regulate the liquid level, and its operating characteristics are similar to those of the level control valves described above. However, the difference lies in the nature of the medium—this valve handles sour (acidic) water. Under large pressure drops, sour water is highly prone to cavitation and severe corrosion. Therefore, special attention must be paid to selecting appropriate valve materials. Although the principles of various oil hydrogenation units are similar, their process parameters differ significantly. In particular, valves used in the residual oil hydrogenation process are subjected to the most severe service conditions and therefore have the highest performance requirements. Table 1 presents the operating parameters of key control valves in a typical residual oil hydrogenation unit.

Table 1 Operating Parameters of Key Control Valves in a Hydrogenation Unit