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This post was last edited by LQ198619 on 2017-5-23 08:15. Chapter 4: Basic Knowledge of Hydrogen Production Units I. Overview of the Unit The process principle of the dry gas hydrogen production system involves using catalytic dry gas as the raw material (with light naphtha as a temporary alternative), applying hydrogenation technology to saturate the olefins in the raw material into alkanes, and converting impurities such as organic sulfur and organic chlorine into inorganic sulfur and inorganic chlorine. Subsequently, HCl and H2S are removed through dechlorination and desulfurization reactors, so that the sulfur content in the refined gas is less than 0.5 ppm, the chlorine content is less than 1 ppm, and the olefin content is less than 0.1% (v). The refined feedstock is subjected to a steam reforming process to convert hydrocarbons into H2, CO, and CO2. The CO present in the reformate gas is then converted into H2 and CO2 through medium-temperature shift reaction. Impurities such as residual CO, CO2, and CH4 in the reformate gas are removed using pressure swing adsorption (PSA) technology, thereby yielding product hydrogen gas with a purity of 99.9% (v) and a CO+CO2 concentration of less than 20 ppm. The dry gas hydrogen production process involves steps such as feedstock purification, hydrocarbon steam reforming, and medium-temperature shift of CO. During raw material purification, the raw gas is subjected to certain temperature, hydrogen pressure, and space velocity conditions; with the help of a hydrogenation catalyst, sulfides and chlorides in the raw gas are removed, reducing the sulfur content in the gas to 0.2 ppm and the chlorine content to 1 ppm, thereby ensuring the proper operation of the subsequent catalysts. When the refined feed gas resulting from the conversion of hydrocarbon vapors reacts under certain pressures, temperatures, space velocities, water-to-carbon ratios, and in the presence of a catalyst, the hydrocarbons and vapors are converted into gaseous hydrogen and carbon monoxide, with CO2 and a small amount of residual CH4 being produced as by-products. In the medium-temperature shift of converted gas, under certain temperature, pressure, space velocity, water-to-gas ratio, and catalyst conditions, CO reacts with water to produce hydrogen and CO2. The chemical reaction mechanism for hydrogen production from dry gas involves sulfur compounds present in the feed hydrocarbons, which exist in various forms. These are generally divided into inorganic sulfides and organic sulfides. Organic sulfides cannot be removed directly through reaction with zinc oxide desulfurization agents; they must first be converted into inorganic sulfides through hydrogenation before they can be removed via oxidation and adsorption by zinc oxide. Organic sulfides in the feed materials include thiolues, thioethers, disulfides, and cyclic sulfides, among others. The majority of the sulfur compounds in the feed gas are organic sulfides. The hydrogenation process also involves the conversion of organic chlorines into inorganic chlorines; high-activity metal oxides are used as active components. The dechlorinating agent reacts with hydrogen chloride and is fixed on a carrier, thereby achieving the removal of chlorides. Thiol hydrogenation: R-SH + H2 = RH + H2S. Thioether hydrogenation: R-S-R’ + H2 = RH + R’H + H2S. Thiophene hydrogenation: C4H4S + 4H2 = C4H10 + H2S. Carbon disulfide hydrogenation: CS2 + H2 = CH4 + H2S. Zinc oxide desulfurization: H2S + ZnS + H2O. Steam reforming of hydrocarbons involves converting hydrocarbons and steam into H2 and CO, with small amounts of residual CH4 remaining; CH4 + H2O = CO + H2; CO + H2O = CO2 + H2. Medium-temperature reforming involves further reacting the CO in the reformate gas with water vapor to produce CO2 and hydrogen. Pressure swing adsorption (PSA) is a process for separating and purifying gas mixtures; it relies on the physical adsorption of gas molecules by the surface of porous solid materials. Impurity components in the mixed gas have a higher adsorption capacity at high pressures and a lower adsorption capacity at low pressures; it is this principle that is utilized for the adsorption by the adsorbent, enabling a cycle of adsorption and desorption. The order of adsorption of general gas molecules by molecular sieves is: H2＜N2＜CH4＜CO＜CO2. The order of adsorption of general gases by activated carbon is: H2＜N2＜CO＜CH4＜CO2. In the dry gas hydrogen production process, the catalytic or coked dry gas coming from outside the plant passes through a raw gas liquid separation tank to remove liquids, and then through a filter to eliminate impurities in the gas; thereafter, it enters a raw gas compressor where its pressure is increased to 2.8 MPa. The raw gas is pressurized and then sent to the raw gas heater, where it is heated to 260°C using medium-pressure steam. The heated feed gas enters the hydrogenation isothermal reactor for olefin saturation reaction; the heat of reaction is absorbed by the boiler water outside the reaction tube, generating saturated steam at a pressure of 2.6 MPa. The dry gas coming out of the hydrogenation isothermal reactor enters the hydrogenation adiabatic reactor, where the unreacted olefins continue to undergo hydrogenation, and organic sulfur is converted into inorganic sulfur; by the time all the olefins are saturated, the temperature is around 360°C. This feed gas enters the dechlorination tank and the zinc oxide desulfurization tank to remove chlorine and hydrogen sulfide. Zinc oxide desulfurization tanks can be operated in series or in parallel. To prevent fluctuations in the raw material dry gas, this unit also uses light naphtha as a temporary backup material. The light naphtha is pressurized by a pump at the liquefied gas station before being fed into the unit; there it mixes with hydrogen coming from the hydrogen compressor in the hydrogenation unit. This mixture is then heated to 205°C using medium-pressure gas, and subsequently sent to the start-up furnace where its temperature is raised to 350°C for the hydrogenation reaction. After that, sulfur and chlorine are removed through dechlorination and desulfurization tanks. The saturated steam at 2.6 MPa produced by the waste heat boiler is superheated to 450°C in the convective section II of the converter, after which it is mixed with the desulfurized feed gas to achieve a water-to-carbon ratio of 3.75. This mixture then enters the convective section I of the converter, where it is heated to 500°C before entering the conversion tubes of the converter. There, the feed material undergoes hydrocarbon steam conversion reactions under the action of nickel catalysts. The conversion gas with a methane content of less than 5% (V) and a temperature of 800°C enters the waste heat boiler to recover heat, thereby generating saturated steam at 2.6 MPa (A). The conversion gas, after being cooled to 360°C in the waste heat boiler, enters the medium-shift reactor for the carbon monoxide conversion reaction. The medium-shift gas with a CO content of less than 3.0% (by volume) and a temperature of 410°C passes successively through a feed gas heater, a waste heat recovery unit, a boiler water preheater, and a demineralized water preheater to have its heat recovered; after this, its temperature is reduced to 148°C. Finally, it is cooled to 40°C using a medium-shift gas air cooler and a medium-shift gas water cooler. Medium-pressure gas with a pressure of approximately 1.7 MPa (A) and a temperature of 40°C is fed to the pressure swing adsorption (PSA) unit. The transformed gas enters the adsorber from bottom to top, and under the action of the adsorbent, all impurities other than the hydrogen component are removed, thereby producing product hydrogen with a concentration of over 99.9% (V), at a pressure of approximately 1.6 MPa (A) and a temperature of 40°C. The desorbed gas discharged from the pressure swing adsorption system has a pressure of approximately 0.15 MPa (A). This desorbed gas, along with the catalytic dry gas used as a hydrogen production fuel, enters the converter as fuel. The hydrocracking system (since the principle of hydrogenation for gasoline is the same as that for diesel and paraffin oils, it will not be explained separately) – The process principle of hydrocracking is a technique used in the petrochemical industry. During oil refining, under high pressure and temperature, hydrogen gas, acting through a catalyst, causes heavy oils to undergo hydrogenation, cracking, and isomerization reactions, thereby converting them into lighter oils (such as gasoline, kerosene, diesel, or raw materials for catalytic cracking or pyrolysis to produce olefins). It differs from catalytic cracking in that, during the catalytic cracking reaction, a hydrogenation reaction of hydrocarbons also takes place simultaneously. Hydrocracking is essentially a combination of hydrogenation and catalytic cracking processes; it enables heavy oil products to be converted into lighter oils such as gasoline, kerosene, and diesel through catalytic cracking reactions. It also prevents the formation of large amounts of coke, removes impurities such as sulfur, nitrogen, and oxygen from the feedstock, and saturates the olefins. Hydrocracking features high yields of light oils and excellent product quality. The reaction direction and extent of hydrocarbon chemicals under hydrocracking conditions depend on the composition of the hydrocarbons, the performance of the catalyst, and the operating conditions. The main types of reactions that occur include cracking, hydrogenation, isomerization, cyclization, desulfurization, denitration, deoxygenation, and demetallization. 1. Hydrocracking of alkanes. Under hydrocracking conditions, alkanes primarily undergo C-C bond cleavage reactions as well as hydrogenation of the resulting unsaturated fragments; in addition, isomerization reactions can also occur. 2. Hydrocracking of cycloalkanes. During hydrocracking, the reactions of naphthenes are influenced by factors such as the number of rings, the length of side chains, and the properties of the catalyst. Monocyclic cycloalkanes generally undergo reactions such as isomerization, chain scission, and dealkylation of side chains ； Bicyclic naphthenes and polycyclic naphthenes are first isomerized into pentacyclic derivatives, and then the rings are broken. 3. Hydrocracking of olefins. Under hydrocracking conditions, olefins are easily hydrogenated to form saturated hydrocarbons; in addition, reactions such as polymerization and cyclization also occur. 4. Hydrocracking of aromatic hydrocarbons. For aromatic hydrocarbons with side chains containing three or more carbon atoms, the side chain first breaks off to form the corresponding aromatic and aliphatic hydrocarbons; in some cases, the aromatic hydrocarbons may also undergo hydrogenation to form cycloalkanes. The hydrocracking of bicyclic and polycyclic aromatic hydrocarbons occurs in stages: first, one of the aromatic rings is hydrogenated to form a naphthoaromatic hydrocarbon; then the naphthene ring breaks apart to yield an alkylaromatic hydrocarbon, after which the reaction continues. 5. Hydrocracking of non-hydrocarbon compounds. Under hydrocracking conditions, non-hydrocarbon compounds containing sulfur, nitrogen, and oxygen heteroatoms undergo hydrogenation to produce corresponding hydrocarbons as well as hydrogen sulfide, ammonia, and water. The hydrocracking reaction mechanism primarily involves hydrogenation for desulfurization and denitration, hydrogenation of unsaturated hydrocarbons to saturate them, hydrogenation of heterocyclic and polycyclic hydrocarbons to saturate them as well as their cracking. Ultimately, this leads to desulfurization and denitration, saturation of unsaturated hydrocarbons, and, depending on the requirements of the desired product, fragmentation and hydrogenation saturation of large molecular hydrocarbons. All the aforementioned processes are highly exothermic processes. In this process, the catalyst acts as a catalyst to accelerate the reaction rate. Hydrofining 1. At higher H2 partial pressures, the HDN reaction is controlled by kinetic equilibrium and is irreversible. 2. HDN is very similar to HDS, but the breaking of the C-S bond directly produces H2S, whereas the breaking of the C-N bond results in the hydrogenation and saturation of the nitrogen heterocycle; the breaking of the C-N bond also yields amines, which then undergo hydrolysis to produce NH3 and hydrocarbons. HDN from easy to difficult: quinoline—pyridine—indole—pyrrole. 3. Five-membered N-heterocyclic compounds: indole, pyrrole, etc., which are non-basic nitrogen compounds and account for 2/3 of the total N. Hexagonal N-heterocyclic compounds: pyridine, quinoline, etc., are basic nitrogen compounds and account for 1/3 of the total nitrogen. 4. H2S inhibits hydrogenation reactions, but it promotes the cleavage of C-N bonds. It indicates that Ni and Mo sulfide-supported catalysts have two types of active centers: hydrogenation centers and hydrolysis centers (acidic centers). 5. The hydrogenation of N-heterocyclic compounds is relatively easy, but the cleavage of C-N bonds is difficult due to the lower activation energy of the former. 6. The bond energy of the C-N bond is 12–38 KJ/mol higher than that of the C-S bond, so more energy is released when S is removed compared to when N is removed. 7. The CoMo/Al2O3 catalyst is very effective for HDS, while the NiMo/Al2O3 catalyst is more effective for HDN than CoMo/Al2O3. The addition of P significantly enhances the activity of the NiMo/Al2O3 catalyst. (MoO3/NiO 24/4m%, P 0.9m%) Hydrocracking 1: The catalyst must possess good hydrogenation activity and resistance to N. 2. The catalyst has a dual function: a hydrogenation component and an acidic component. 3. The effect of nitrides on acidic centers and poisoning (shielding); acidic centers have a strong adsorption capacity for incoming N nitrides. 4. Generally, the N content in the feed should be below 10 μg/g, but an increase in the feed’s N content can be compensated for by raising the temperature. 5. Amorphous hydrocracking catalysts feature high selectivity for middle-distillate fractions and high liquid yield; moreover, the distribution and quality of the products remain relatively stable at the beginning and end of the catalyst’s service life. However, amorphous catalysts have low activity and require higher initial reaction temperatures. To improve the cracking activity of the catalyst without excessively compromising its oil selectivity, it is necessary to add a certain amount of molecular sieve to the amorphous silica-alumina carrier, and this molecular sieve must have good anti-N properties. Therefore, amorphous silicoalumina is generally used as the main carrier, with a certain amount of molecular sieves with good anti-N properties added to enhance the cracking and isomerization activity of the catalyst. 6. The key control factors for the performance of hydrocracking catalysts are the matching of hydrogenation activity and acidity (cracking activity), as well as the pore structure characteristics of the catalyst. 7. Catalysts with high hydrogenation activity and relatively low acidity have lower cracking activity, enable higher liquid yields, and also exhibit better resistance to nitrogen ； Conversely, it has high cracking activity, low liquid yield, and high selectivity for naphtha fractions. 8. Since the hydrocracking reaction of hydrocarbons is a diffusion-controlled multiphase catalytic reaction, the pore size of the catalyst should be sufficiently large and well-distributed, in order to achieve high activity, good medium-oil selectivity, and stability. 9. The order of hydrogenation activity for the four components is: W-Ni > Mo-Ni > Mo-Co > W-Co (primary elements: W/Mo, auxiliary elements: Ni/Co). 10. The cracking activity of hydrocracking catalysts stems from the acidity of the carrier. 11. For every increase of 10 μg/g in the N content of the feed, the reaction temperature needs to be increased by approximately 1°C. 12. Molecular sieves and amorphous silica-alumina: Amorphous silica-alumina is a gel-like porous solid, whereas molecular sieves are crystalline silicoaluminates formed by various arrangements of silicon oxide and aluminum oxide tetrahedra; this three-dimensional network structure is filled with uniform micropores, resulting in a large internal surface area. Their acidity is much higher than that of amorphous silica-alumina. Adding molecular sieves to the catalyst can significantly enhance its cracking activity. 13. Reaction pressure mainly affects the smoke point of aviation fuel, the cetane number of diesel, and the aromatic content of the product. Hydrocracking process flow: There are various types of industrial hydrocracking units, which can be further classified into single-stage and two-stage methods based on the function of the reactor. The two-stage process involves two reactors; the first stage serves as a hydrorefining section to remove nitrogen and sulfides from the feed oil. The second stage is the hydrocracking reaction section. A series reactor has only one or several reactors in parallel. The process flow of a fixed-bed hydrocracking unit in French style involves mixing feed oil, recycle oil, and hydrogen, which is then heated before being fed into the reactor. The reactor is filled with granular catalyst, and the reaction products pass through high-pressure and low-pressure separators to separate the liquid product from the gas; thereafter, the liquid product is distilled in a distillation tower to yield petroleum fractions. A certain stage of fluid catalytic cracking has a lower degree of cracking; it generally uses vacuum wax oil as raw material to produce mainly intermediate distillates. The two-stage cracking process achieves a greater degree of cracking, and is generally used primarily for gasoline production. The vast majority of hydrocracking processes use fixed-bed reactors. Depending on the properties of the feedstock, the requirements for the products, and the volume of material to be processed, hydrocracking units generally operate according to two types of processes: single-stage hydrocracking and two-stage hydrocracking. In addition to fixed-bed hydrocracking, there are also processes such as fluidized-bed hydrocracking and slurry-bed hydrocracking. ① The first-stage hydrocracking process in a fixed-bed system is primarily used to produce liquefied gas from crude gasoline, as well as aviation kerosene and diesel from vacuum wax oil and deasphalted oil. In a single-reactor hydrocracking unit, the hydrorefining of the feed oil and the hydrocracking take place in the same reactor; the upper part of the reactor is used for refining, while the lower part is used for cracking. Take the first-stage hydrocracking of straight-run diesel fractions (330–490°C) as an example. The crude oil is pressurized to 16.0 MPa by a pump, mixed with fresh hydrogen and recycled hydrogen for heat exchange, then fed into a heater for heating, before entering the reactor for reaction. The feed temperature of the reactor is 370–450°C, and the feedstock reacts at a reaction temperature of 380–440°C, with a space velocity of 1.0 h⁻¹ and a hydrogen-to-oil volume ratio of approximately 2500. The reaction products and raw materials are heat-exchanged to around 200°C, and softened water is added to dissolve substances such as NH3 and H2S in order to prevent the formation of hydrates that could block the pipes; after that, the mixture is cooled to 30–40°C before being fed into the high-pressure separator. Cyclic hydrogen is taken from the top, pressurized by a compressor, and then returned to the system for use ； Oil is produced at the bottom; after the pressure is reduced to 0.5 MPa, it enters the low-pressure separator where water is removed and some of the dissolved gases (fuel gas) are released. The produced oil is heated and then fed into a stabilizer column, where liquefied gas is distilled off at a pressure of 1.0–1.2 MPa. The liquid at the bottom of the column is heated to 320°C and sent to a distillation column, from which light gasoline, aviation kerosene, low-flash-point diesel, and residue oil are obtained. Hydrocracking can be operated using three approaches: single-pass feed, partial recycle of tail oil, and full recycle of tail oil. ②Fixed-bed two-stage hydrocracking process: The two-stage hydrocracking unit consists of two reactors, each equipped with catalysts of different properties. The first reactor is primarily used for the refining of crude oil, with highly active catalysts being employed to pre-treat the crude oil ； The second reactor is primarily used for hydrocracking reactions; cracking and isomerization reactions take place on catalysts with high cracking activity, in order to produce as much gasoline and intermediate distillates as possible. There are two operation schemes for two-stage hydrocracking: refining in the first stage and hydrocracking in the second stage ； In the first stage, in addition to refining, partial cracking is also carried out, while in the second stage hydrocracking takes place. The two-stage hydrocracking process has a high degree of adaptability to feedstocks and is relatively flexible in operation. ③Fixed-bed series hydrocracking process: A fixed-bed series hydrocracking unit consists of two reactors connected in series, with different catalysts filled in each reactor; the first reactor is equipped with a hydrogenation catalyst that has good desulfurization and denitration capabilities, while the second reactor uses a molecular sieve hydrocracking catalyst that is resistant to ammonia and hydrogen sulfide. The other parts are the same as those in a hydrocracking process. Compared to the same hydrocracking process, the advantage of the series process is that it allows for maximum production of gasoline, aviation kerosene, or diesel by simply adjusting the operating conditions. ④Boiling bed hydrocracking: The boiling bed hydrocracking process utilizes the flow velocity of fluids to move catalyst particles of a certain size, thereby creating a three-phase bed of gas, liquid, and solid. This allows hydrogen, feed oil, and catalyst to come into full contact with each other, enabling the hydrocracking reaction to take place. This process can handle feedstocks with high metal content and residual carbon levels (such as vacuum residue), and enables the deep conversion of heavy oils. However, the operating temperature of this process is relatively high, generally ranging from 400 to 450°C. ⑤Suspension bed hydrocracking process: The suspension bed hydrocracking process can be used with very low-quality feedstocks, and its principle is similar to that of the fluidized bed. The basic process involves pre-mixing the fine-powdered catalyst with the raw materials, which are then fed into the reactor along with hydrogen and flow from bottom to top, where the hydrocracking reaction takes place. The catalyst remains suspended in the liquid phase and exits the reactor from the top along with the reaction products. II. Key areas to inspect in the hydrogenation unit 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, and the temperature of the fluids involved is relatively high; in addition, there are open flames in the heating furnaces. Therefore, this area poses a high level of risk, with fire and explosion being the main hazards, making it a key area that requires special attention from a safety perspective. ⒉The high-pressure separator and the high-pressure air cooling area contain a high-pressure separator as well as high-pressure air coolers; if the liquid level of 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 compressor building: The hydrogen compressor building houses 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. Therefore, this area presents a high level of potential danger, with the main risks being fire, explosion, and poisoning. It is a zone that requires special attention from a safety perspective. 4. The distillation tower area contains a large number of equipment units; the media involved are mostly flammable and explosive substances. High-temperature hot oil pumps are devices that require special attention in terms of safety, as any leakage of such hot oil could lead to fire accidents. There is a large amount of fuel gas, liquid hydrocarbons, and oils in this area, and an accident there could have severe consequences. In addition, 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. III. Main equipment of hydrogenation units 1. Hydrogenation reactor The hydrogenation reactors are mostly fixed-bed reactors; 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 insulating linings; instead, they use double-layer weld-on linings, typically made of 2×1/4Cr—1M0. 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, which 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. The outlet temperature of the high-pressure heat exchanger reactor is relatively high, and it possesses a high heat content; therefore, this heat should be recovered as much as possible. Hence, hydrogenation units are equipped with high-pressure heat exchangers that are used to exchange heat between the reactor outlet stream 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 counterflow heat transfer, which improves heat exchange efficiency and reduces the area required for the high-pressure heat exchanger. Tube boxes typically use threaded lock-type 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 coolers: These units operate under high pressure and in a hydrogen-rich environment; they are important equipment in hydrogenation plants. In a medium-pressure hydrocracking unit at an oil refinery in North China, leaks occurred twice in the high-pressure air coolers, forcing the shutdown of the unit for repairs. Therefore, attention should also be paid to the design, manufacturing, and use of high-pressure air coolers. 4. High-pressure separator: The functional role of the high-pressure separator is to carry out the 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 result. Therefore, from a safety perspective, the high-pressure separator is a very important device. ⒌The operating conditions of the reaction heating furnace, also known as the hydrogenation reaction heating furnace, are high temperature, high pressure, exposure to hydrogen, and the presence of an open flame; these conditions are extremely severe, making it an important piece of equipment 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 usually a double-sided radiant heating furnace with a pure radiation chamber. This design is intended to increase the heat intensity of the radiant tubes, reduce the length of the tubes as well as the number of bends, thereby minimizing 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 generally 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, such compressors cannot operate for long periods of time, and spare units are usually provided. Reciprocating compressors are generally driven by electric motors, connected through 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 circulating 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 relieve pressure. The cyclic hydrogen compressor performs work in a cyclic manner 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 (8000–10,000 r/min) 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. The hydrogenation feed for the automatic backwash filter contains mechanical impurities; if these are not removed, they will deposit at the top of the reactor, causing an excessive pressure difference in the reactor and forcing it to shut down, thereby 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 ≥25/1μm 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. IV. Main factors affecting the hydrogenation process in hydrogenation units The main factors that influence the efficiency of hydrogenation refining include: reaction pressure, reaction temperature, space velocity, hydrogen-to-oil ratio, feedstock properties, and catalyst. 1. Reaction pressure: The effect of reaction pressure is manifested through the hydrogen partial pressure. The hydrogen partial pressure in the hydrogenation unit system is determined by the operating pressure, hydrogen-to-oil ratio, purity of the recycled hydrogen, and the vaporization rate of the feedstock. For the hydrodesulfurization of sulfides and the hydrosaturation of olefins, a high degree of conversion is achieved at relatively low pressures. When gasoline is hydrotreated at a hydrogen partial pressure of 2.5–3.0 MPa, the degree of purification is not governed by thermodynamic equilibrium, but rather by the reaction rate and reaction time. Under hydrorefining conditions, gasoline is generally in the gas phase; increasing the pressure prolongs the residence time of gasoline, thereby enhancing the degree of its refining. When the hydrogen partial pressure exceeds 3.0 MPa, the hydrogen concentration on the catalyst surface reaches saturation. If the operating pressure remains unchanged and the hydrogen partial pressure is increased by raising the hydrogen-to-oil ratio, the degree of purification decreases, as this reduces the partial pressure of the feed oil. The reaction pressure for the hydrorefining of diesel fractions (180–360°C) is generally 4.0–8.0 MPa (hydrogen partial pressure of 3.0–7.0 MPa). The effect of pressure on diesel hydrorefining is more complex. Under hydrorefining conditions, diesel fractions can be in a vapor phase or a vapor-liquid mixture phase. In the vapor phase, increasing the pressure prolongs the reaction time, thereby increasing the reaction depth. Increasing the reaction pressure enhances the degree of purification, with a particularly significant improvement in the nitrogen removal rate, as the rate of nitrogen removal is relatively low ； It has little effect on the desulfurization rate, as the desulfurization rate is high, and sufficient reaction time is available at lower pressures. When refining feedstocks with a high nitrogen content, it is necessary to increase the pressure or reduce the space velocity in order to achieve a certain denitration rate. If all other conditions remain unchanged, increasing the reaction pressure to a certain value will result in the formation of a liquid phase in the reaction system; once this liquid phase appears, further increases in pressure will lead to a deterioration in the purification effect. In the presence of a liquid phase, the rate at which hydrogen diffuses across the liquid film toward the catalyst surface is often the controlling factor affecting the reaction rate; this diffusion rate is proportional to the hydrogen partial pressure and decreases as the thickness of the liquid layer on the catalyst surface increases. Therefore, after the formation of a liquid phase, increasing the reaction pressure thickens the liquid layer on the catalyst surface, thereby reducing the reaction rate. If the pressure remains constant and the hydrogen partial pressure is increased by raising the hydrogen-to-oil ratio, the degree of purification reaches a maximum value. The reason for this phenomenon is that, before the feedstock is completely vaporized, increasing the hydrogen partial pressure facilitates the vaporization of the feedstock and reduces the liquid film on the catalyst surface; it also helps to reduce the liquid film of hydrogen on the catalyst surface and promotes the diffusion of hydrogen toward that surface. Therefore, before the feed oil is completely vaporized, increasing the hydrogen partial pressure (while keeping the total pressure constant) helps to increase the reaction rate. Increasing the hydrogen partial pressure after complete vaporization reduces the feedstock partial pressure, thereby slowing down the reaction rate. It can be seen that, in order to achieve the best results in diesel hydrorefining, a hydrogen partial pressure favorable at the moment of complete vaporization should be selected. Under normal conditions, an appropriate hydrogen partial pressure can be obtained by using a hydrogen-to-oil ratio of 120~600 Nm3/m3 when the reaction pressure is 4.0~8.0 MPa. The hydrogen sulfide partial pressure also affects the hydrogenation efficiency. The hydrogen sulfide partial pressure in the system depends on the concentration of hydrogen sulfide in the recycled hydrogen. Studies have shown that excessively high hydrogen sulfide concentrations can inhibit the hydrodesulfurization reaction, a phenomenon that is particularly evident in deep desulfurization processes. When the cyclic hydrogen changed from having no hydrogen sulfide to a hydrogen sulfide concentration of 2.0 v%, the relative volume activity for desulfurization decreased from 100 to 57; when the hydrogen sulfide concentration rose to 2.8 v%, the relative volume activity for desulfurization dropped further to 49. It can be seen that the effect of hydrogen sulfide concentration is very significant. The decrease in the desulfurization activity of the catalyst due to high hydrogen sulfide concentrations in the recycled hydrogen can be attributed to two reasons. First, the decrease in the purity of hydrogen in the recycled hydrogen leads to a reduction in the hydrogen partial pressure, which in turn affects the catalyst’s ability to carry out hydrodesulfurization, resulting in a lower desulfurization rate ； Secondly, from the perspective of the reaction mechanism of hydrodesulfurization, the hydrogen sulfide generated competes with the sulfides acting as reactants for adsorption at the active sites on the catalyst surface, thereby affecting the hydrodesulfurization of the sulfides. It is thus evident of the importance of desulfurizing the recycle hydrogen during the deep desulfurization process. 2. Reaction temperature: Increasing the reaction temperature accelerates the rate of hydrorefining reaction. Under hydrorefining conditions, at a certain reaction pressure, for specific feed oils and catalysts, the activation energy of the reaction remains constant. For different raw materials and catalysts, the activation energy of the reaction varies, and therefore the extent to which increasing the reaction temperature boosts the reaction rate also differs. The higher the activation energy, the faster the increase in reaction rate upon temperature rise. However, since the hydrorefining reaction is an exothermic reaction, from a chemical equilibrium perspective, increasing the reaction temperature reduces the equilibrium conversion rate of the forward reaction, which is unfavorable for it. As mentioned earlier, at the typical operating temperatures used in hydrorefining, the desulfurization reaction is not governed by thermodynamics; therefore, for the hydrodesulfurization of distillate oils, increasing the temperature accelerates the overall desulfurization rate ； For denitrification and aromatic saturation reactions, it is necessary to conduct a specific analysis to determine whether they are controlled by thermodynamics or kinetics under certain reaction conditions. In industry, the reaction temperature in hydrogenation units is directly related to the unit’s energy consumption and hydrogen usage; the optimal reaction temperature is the lowest temperature at which the properties of the product meet the required standards. Therefore, in practical applications, the appropriate reaction temperature should be selected based on the properties of the raw materials and the requirements of the product. 3. Space velocity: Space velocity refers to the amount of feed that passes through a unit of catalyst per unit of time, and it reflects the processing capacity of the plant. A high space velocity means that more feedstock passes through the catalyst per unit of time; as a result, the residence time of the feedstock on the catalyst is short, and the degree of reaction is shallow ； Conversely, a low space velocity means a longer reaction time; therefore, from both the perspective of reaction rate and chemical equilibrium, reducing the space velocity is beneficial for increasing the conversion rate of the reaction. However, a lower space velocity means that more catalyst is required for the same processing capacity, the reactor size must be larger, and the capital cost for installing the facility (including the costs of the reactor and catalyst) increases, which is not desirable in industrial applications. Therefore, the selection of the space velocity in industrial hydrogenation processes must be determined through comprehensive consideration of various factors such as plant investment, catalyst activity, raw material properties, and product requirements. 4. Hydrogen-to-oil ratio: A high hydrogen partial pressure needs to be maintained in the hydrogenation system, as it is thermodynamically favorable and can also suppress the condensation reactions that lead to carbon deposition. Maintaining a high hydrogen partial pressure is achieved through the circulation of large amounts of hydrogen gas. Therefore, the hydrogen-to-oil ratio used in the hydrogenation process often **exceeds the value required for the chemical reaction. Increasing the hydrogen-to-oil ratio means an increase in the hydrogen partial pressure (with the total pressure remaining constant), which is advantageous for the reaction in many aspects. However, this requires an increase in the flow rate of the circulation compressor, leading to higher energy consumption and increased operating costs; therefore, it is necessary to select an appropriate hydrogen-to-oil ratio based on specific circumstances. Furthermore, the hydrogenation process is an exothermic reaction, and a large amount of recycled hydrogen can increase the heat capacity of the reaction system, thereby reducing the magnitude of changes in reaction temperature. 5. Properties of raw materials: The properties of raw materials significantly affect the efficiency of hydrorefining. The hydrogen desulfurization of sulfides, the hydrogen denitration of nitrides, and the hydrogen saturation of olefins and aromatics mentioned earlier are certainly applicable to the hydrogenation process of petroleum fractions. However, petroleum distillates are complex mixtures of various hydrocarbon and non-hydrocarbon compounds; it is difficult to fully characterize the properties of these materials using a single factor, and it is equally hard to use a single factor to determine the ease of their hydrogenation. However, by combining certain basic properties of the raw material, its fundamental characteristics can be roughly described. For the hydrorefining process of distillate oils, the ease of hydrogenating a given feedstock can be determined based on factors such as its density, sulfur and nitrogen content, refractive index, bromine value (or olefin content), aromatic composition, ash content, and boiling range. Generally, the higher the density of the raw material, the higher the contents of sulfur, nitrogen, aromatics, and gums, making it more difficult to refine. Taking the hydrorefining of diesel fractions as an example, catalytic diesel has a higher density than straight-run diesel, higher levels of sulfur, nitrogen, and aromatics, as well as a lower cetane number ； Compared with catalytic diesel, coker diesel has a lower density, more alkanes, less aromatic compounds, and a higher cetane number; it is therefore a good component for blending diesel. However, its sulfur and nitrogen contents (especially basic nitrogen) as well as its gum content are very high, resulting in poor stability. As the quality of the raw materials deteriorates, the severity of hydrorefining increases, especially for the denitration reaction. The quality of raw materials has a direct impact on the effectiveness of hydrorefining, while the type of crude oil, the processing methods for heavy oils, and the degree of processing all affect the properties of distillates (especially those resulting from secondary processing). Therefore, different hydrogenation conditions should be adopted for different raw materials in order to optimize the efficiency of the hydrogenation process. 6. Catalysts: The hydrorefining catalysts used in industry for distillate oil, regardless of the variations in their types, all belong to the same category. They use sulfides of Group VIII metals such as Ni and Co as promoters, sulfides of Group VI metals such as Mo and W as the main metal, and active alumina or aluminum silicate as carriers. A hydrogenation refining catalyst containing molecular sieves has now been successfully developed. Different combinations of these two metal sulfides exhibit varying activities in different chemical reactions. The most commonly used catalysts include: Co-Mo/γ-Al2O3, Ni-Mo/γ-Al2O3, Ni-W/γ-Al2O3, etc. The Co-Mo/γ-Al2O3 catalyst exhibits high activity in breaking C-S bonds, is active in saturating C=C bonds and breaking C-N bonds, but has very low activity in breaking C-C bonds, which are undesirable during oil refining. Under the action of this catalyst, almost no polymerization or condensation reactions occur at normal operating temperatures. Therefore, the Co-Mo/γ-Al2O3 catalyst features a long service life, good thermal stability, high yield of liquid products, low hydrogen consumption, and slow carbon deposition. Therefore, this series of catalysts has long been considered excellent hydrodesulfurization catalysts among hydrogenation refining catalysts. However, in recent years, due to the trend of increasingly heavier feedstocks for hydrorefining, nitrogen removal from these feedstocks has become very important. Since Ni-Mo series catalysts exhibit greater activity in breaking C-N bonds compared to Co-Mo series catalysts, there is currently a trend in many processes to replace Co-Mo series catalysts with Ni-Mo series catalysts. Another type of hydrofining catalyst is the Ni-W series; these catalysts have a higher desulfurization activity than Co-Mo series catalysts, and their denitration and aromatic saturation activities are higher than those of Ni-Mo catalysts. In addition to the chemical composition of the catalyst affecting its activity, its physical properties such as specific surface area, pore volume, particle size, and shape also influence the performance of the active components. For example, in order to eliminate the diffusion-controlled phenomena that occur during the hydrorefining of heavy oils, it is necessary to shape the catalysts into specific particle forms. Most of the catalysts currently produced are in the form of thin strips with a trilobal or tetralobal shape, and their particle diameter is around 1.5 millimeters. Too small a diameter will increase the pressure drop across the bed, causing difficulties in normal operation. It is obvious that the physical properties of a catalyst are closely related to its preparation method; for more details, see the chapter on catalysts. As can be seen from the above, the properties of the catalyst directly affect the efficiency of hydrorefining, while the properties of the feedstock and the hydroprocessing conditions determine the choice of catalyst. Therefore, appropriate catalysts should be used for different raw materials and hydrogenation processes to optimize the efficiency of the hydrogenation process. For example, in hydrogenation processes aimed solely at hydrodesulfurization, such as the hydrogenation of straight-run gasoline (when the nitrogen content is low), straight-run kerosene, and straight-run diesel, a Co-Mo catalyst is sufficient ； For hydrogenation processes that require not only thorough desulfurization but also certain nitrogen removal and aromatic saturation capabilities, such as the hydrogenation of secondary processed distillates and heavy distillates, Ni-Mo or Ni-W catalysts are a better choice.