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Q&A on Hydrogen Production Technology – Feedstock Section 14. What are the hydrocarbon feedstocks suitable for steam reforming to produce hydrogen? 15. What are the principles for selecting raw materials for steam reforming to produce hydrogen? 16. Why cannot refinery secondary processed oil be easily used as a feedstock for conversion? 17. Are there secondary processed oils that can be used as raw materials for hydrogen production? 18. What impact does the group composition of light oil have on the conversion process? 19. What are the requirements for conversion catalysts when natural gas is used as a raw material for hydrogen production? 20. What are the requirements for conversion catalysts in various refinery gases that do not contain olefins? 21. How is pretreatment carried out when using refinery dry gas as a hydrogen production feedstock? 22. What are the requirements of various light oils for conversion catalysts? Raw Materials Section 14. What are the hydrocarbon raw materials suitable for steam reforming to produce hydrogen? Answer: Hydrocarbon feedstocks suitable for steam reforming are generally divided into gaseous hydrocarbons and liquid hydrocarbons. (1) Gaseous hydrocarbons include natural gas, liquefied petroleum gas, and various refinery gases. Natural gas generally includes associated gas from oil fields and gas field gas. The main component of natural gas is methane, along with small amounts of lower hydrocarbons such as ethane and propane. It also contains certain amounts of inert gases like nitrogen and carbon dioxide, as well as harmful impurities such as hydrogen sulfide. The natural gas from these gas fields is primarily methane, with trace impurities. Liquefied petroleum gas is a hydrocarbon composed mainly of propane and butane. It has two sources: one is natural liquefied gas that is produced alongside oil fields and natural gas fields, and the other is liquefied petroleum gas generated by refineries and petrochemical plants. Refinery gas refers to various off-gases produced as by-products during crude oil processing, including catalytic cracking gas, coking dry gas, catalytic reforming gas, thermal cracking gas, and high-pressure hydrocracking off-gases. The composition of various refinery gases varies greatly; those containing olefins are not suitable for direct use in steam reforming to produce hydrogen. However, after thorough olefin removal or hydrodesaturation, it can serve as an excellent feedstock for steam reforming to produce hydrogen, just like hydrogen-rich gas free of olefins. (2) Liquid hydrocarbons include straight-run naphtha, stripped oil, tail oil, and secondary processed oils. Straight-run naphtha is the fraction obtained from the atmospheric distillation of crude oil at temperatures below 210°C. The specific gravity is generally 0.63 to 0.77, the sulfur content is less than 0.05%, the paraffin hydrocarbon content is high, the aromatic hydrocarbon content is less than 13%, and the olefin content is less than 1%. The distillate oil used as a raw material for hydrogen production is generally reformation distillate oil, that is, the residue left after the extraction of aromatics through reformation. The dry point of the raffinate oil is generally between 130 and 150°C, with very low aromatic content. Hair oil is generally also a reformed hair oil, usually consisting of hydrocarbons below C5. Secondary processed oil refers to light oil obtained from heavy oil through a series of secondary processing treatments such as cracking, coking, and viscosity reduction. This type of light oil generally cannot be used directly as a raw material for hydrogen production. However, after hydrogenation, it is free of olefins, has a dry point of less than 100°C, and a low specific gravity; such oil can be blended with straight-run light oil as a feedstock for hydrogen production or used alone as a feedstock, but experimental evaluation is generally required to determine this. 15. What are the principles for selecting raw materials for steam reforming to produce hydrogen? Answer: (1) First, select raw materials that contain no olefins, have low levels of aromatics, and low levels of cycloalkanes. Because the tendency to form carbon deposits in hydrocarbons with the same number of carbon atoms is: olefins > aromatics > cycloalkanes > alkanes ； The C/H ratio of hydrocarbons with the same carbon count also generally follows this pattern. This principle is followed to reduce the tendency to carbon deposition and increase the hydrogen yield per unit of raw material. Generally, it is required that the olefin content in the raw materials be less than 1%, the aromatic content be less than 13%, and the naphthenic content be less than 36%. (2) Give priority to using lighter raw materials. Since the tendency for carbon deposition in homologous hydrocarbons increases with increasing molecular weight, and C/H also increases with molecular weight, it is therefore preferable to use gaseous hydrocarbons over liquid ones, and lighter fraction liquid hydrocarbons over heavier fraction ones. (3) Give priority to the use of low-toxicity raw materials. Regarding desulfurization, when the sulfur content is generally below 150 ppm, cobalt-molybdenum hydrogenation combined with ZnO desulfurization can reduce the sulfur content to below 0.5 ppm ； When the sulfur content is greater than 150 ppm, pre-desulfurization must be carried out first, followed by cobalt-molybdenum hydrogenation and ZnO desulfurization; this, however, increases energy consumption, costs, and investment. Therefore, low-sulfur raw materials should be given priority. Chlorine and arsenic cause significant poisoning of catalysts, so it is generally not advisable to use raw materials containing chlorine or arsenic. If selected, dechlorination and dearsenification units should be installed. (4) For hydrogen production from refinery gas, there are usually several available feedstocks, but their quantities are limited. In this case, in accordance with the aforementioned principles, saturated hydrocarbons with a low average molecular weight and low sulfur content should be given priority. When insufficient, a raw material with a slightly higher molecular weight is added. For example, the amount of liquid hydrocarbons at the top of the stabilizer in a wide-range reforming unit is small, but it constitutes an excellent raw material for hydrogen production. Similarly, the hydrogenated dry gas from refineries is also an excellent raw material for hydrogen production. It is highly unreasonable to burn the above two types of raw materials as fuel, while using straight-run light oil or vacuum residue as feedstocks for hydrogen production. Relationship between hydrogen-to-carbon ratio and theoretical gas production volume (m3/kg): H/C 2 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0; Theoretical hydrogen production volume: 4.8 4.89 4.98 5.06 5.15 5.23 5.31 5.38 5.46 5.53 5.60. 16. Why cannot refinery secondary processed oil be easily used as a feedstock for conversion? Answer: Refined oil is light oil produced from heavy oil or residue through processes such as cracking, coking, and viscosity reduction. After hydrogenation, properties such as specific gravity, boiling range, and olefin content are similar to those of straight-run light oil, allowing it to be used as fuel oil or as a feedstock for the petrochemical industry. However, it should be used with caution as a raw material for hydrogen production. Many manufacturers have caused damage to their conversion catalysts by using secondary oil. The reason is: (1) side-chain alkanes account for a large proportion in the secondary oil. (2) There are many cycloalkanes. (3) There are many aromatic hydrocarbons, especially heavy aromatics. (4) It contains a high amount of impurities such as gums and heavy metals. (5) The hydrogenation depth in the hydrogenation unit is insufficient, resulting in incomplete removal of sulfur, nitrogen, olefins, etc. These factors can easily cause catalyst poisoning and carbon deposition, rendering the catalyst inactive; therefore, they should not be readily used for secondary processed oils as raw materials for hydrogen production via conversion. 17. Are there secondary processed oils that can be used as raw materials for hydrogen production? Answer: Secondary processed oil cannot be easily used as a raw material for hydrogen production, but lightly processed oil that has undergone deep hydrodesulfurization can be considered as a suitable option. For example, light oils that have undergone high-pressure hydrocracking at around 16.0 MPa feature higher reaction temperatures and hydrogen-to-oil ratios compared to conventional hydrorefining processes. Although they still belong to the category of secondary processed oils, the hydrocracking reaction is more thorough, resulting in the effective removal of sulfur, chlorine, nitrogen compounds, as well as gums from the oil. Aromatics and cycloparaffins are also present in small amounts, and their physicochemical properties are similar to those of straight-run light naphtha. Under normal circumstances, it is acceptable to use high-pressure hydrogenation secondary oil with a dry point of less than 70°C as a raw material for hydrogen production. To date, there are precedents of several hydrogen production facilities successfully using hydrocracked light naphtha as a raw material for hydrogen production. If the dry point and specific gravity of the light oil are high, it is necessary to conduct oil evaluation tests and select appropriate process conditions to determine whether it can be used as a raw material for hydrogen production. One must be cautious when choosing reprocessed oil. Currently, several factories in China use reprocessed oil as a raw material for hydrogen production, and it is generally only adopted after passing strict testing and evaluation by the research institute of Sinopec Qilu Branch. Due to the differences in secondary processing techniques and the properties of the base crude oils across various plants, there are significant variations in the physicochemical properties of the light oils. Therefore, the specific type of oil to be used as a raw material must be determined through targeted testing and evaluation. Under normal circumstances, even when secondary processed oil is used, it should not be employed alone; instead, it must be mixed with straight-run light oil in a certain proportion to ensure a reasonable distribution of the individual hydrocarbons in the hydrogen production feedstock, thereby preventing an excessive concentration of the same type of hydrocarbon and avoiding uneven distribution of the conversion reaction load across the catalyst bed. 18. What impact does the group composition of light oil have on the conversion process? Answer: The more complex the family composition of light oil, the more complex the reactions on the conversion catalyst. For methane conversion, the conversion catalyst primarily facilitates the progress of the steam conversion reaction. However, for light oil conversion, due to the complex composition of the feedstock, the steam conversion reaction involves a series of processes such as the thermal cracking of higher hydrocarbons, catalytic cracking, dehydrogenation and hydrogenation, coking, and decoking. As a result, the catalyst must possess comprehensive properties suitable for handling such a complex reaction system. The most prominent issue is the tendency for carbon deposition; therefore, the light oil conversion catalyst must possess strong resistance to carbon deposition. Different hydrocarbons have varying rates of coking under operating conditions; the table below shows the relative coking rates of several hydrocarbons. Raw materials: Butane, n-Hexane, Cyclohexane, n-Heptane, Benzene, Ethylene. Carbonization rate: 2, 95, 64, 135, 532, 17500. Induction period / 107, 219, 213, 44