Hydroprocessing

Hydroprocessing is the conversion of various feedstocks using the physical aspects of temperature, residence time, and the presence of hydrogen under pressure. Hydroprocessing is more conveniently subdivided into hydrotreating and hydrocracking. 

Hydrotreating is defined as the lower-temperature removal of hetero-atomic species by treatment of a feedstock or product in the presence of hydrogen. On the other hand, hydrocracking (Fig. 2.6) is the thermal decomposition (in the presence of hydrogen) of a feedstock in which carbon-carbon bonds are cleaved in addition to the removal of hetero- 

All the reactions that take place during hydroprocessing can be classified into four large groups  

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The hydroprocessing of sulfur containing compounds proceeds with the formation of hydrogen sulfide. An example of this reaction is shown in the reaction equation (6.44). 

Sulfides and disulfides are hydrogenated in two steps. The reactions for the hydrogenation of sulfides and disulfides are shown in the reaction equations (6.45) and (6.46). 

The hydrogenation of the cyclic sulfide proceeds with ring destruction I J + H2 - CH3-CH2-CH2-CH3 + H2S (6.47) 

Hydrotreating of the pyrolyzer stripper bottoms was carried out over a commercial nickel-tungsten on silica-alumina hydrotreating catalyst at 300°C, a liquid hourly space velocity of 1.5 h 13.4 MPa total pressure, and 880 standard cubic meters once-through H2 per cubic meter of feed. At these conditions, cracking of the feed was minimal. 

Isomerization of the waxy pyrolyzer stripper bottoms was carried out over a proprietary wax hydroisomerization catalyst. A commercial Pt-Pd on silica-alumina hydroflnishing catalyst was used in a second reactor downstream from the first to hydrogenate any unsaturated compounds to improve thermal and oxidative stability. 

Significant C-C Bond Breaking Dealkylation of aromatic rings -CH2-R + H2 -CH3 + RH -1.3 to-1.7 

Enthalpies for the reactions (AHr) can be grouped into three categories. For HDS, HDN, HDO, HDM, and aromatics saturation, AHr are about -2.5 to -3.0 kJ per standard cubic meter of consumed H2. For reactions that break carbon-to-carbon bonds, AHr are about -1.3 to -1.7 kJ per m of consumed H2. And for saturation of olefins, AHr are about -5.5 kJ per m of consumed H2. Isomerization reactions produce a small amount of heat, but this can be neglected. 

The following sections describe most of these reactions in more detail. In-depth information is provided in Chapter 20 by Michael Klein and Gang Hou, in Chapter 9 by I. Mochida and Ki- Hyouk, in Chapter 10 by Barry Cooper and Kim Knudsen, and in Chapter 11 by Chunshan Song and Xiaoliang Ma. 

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Aromatic and Nonaromatic Hydrocarbon Separation. Aromatics are partially removed from kerosines and jet fuels to improve smoke point and burning characteristics. This removal is commonly accompHshed by hydroprocessing, but can also be achieved by Hquid-Hquid extraction with solvents, such as furfural, or by adsorptive separation. Table 7 shows the results of a simulated moving-bed pilot-plant test using siHca gel adsorbent and feedstock components mainly in the C q—range. The extent of extraction does not vary gready for each of the various species of aromatics present. SiHca gel tends to extract all aromatics from nonaromatics (89). 

Typical COED syncmde properties are shown in Table 12. The properties of the oil products depend heavily on the severity of hydroprocessing. The degree of severity also markedly affects costs associated with hydrogen production and compression. Syncmdes derived from Western coals have much higher paraffin and lower aromatic content than those produced from Illinois coal. In general, properties of COED products have been found compatible with expected industrial requirements. 

Table 2. General Process Characteristics for Hydroprocessing Various Feedstocks...

Reclamation, Disposal, and Toxicity. Removal of poisons and inorganic deposits from used catalysts is typically difficult and usually uneconomical. Thus some catalysts are used without regeneration, although they may be processed to reclaim expensive metal components. Used precious metal catalysts, including automobile exhaust conversion catalysts, are treated (often by the suppHers) to extract the metals, and recovery efficiencies are high. Some spent hydroprocessing catalysts may be used as sources of molybdenum and other valuable metals. 

M0S2 is one of the most active hydroprocessing catalysts, but it is expensive, and the economical way to apply it is as highly dispersed material on a support, y-Al202. The activity of the supported catalyst is increased by the presence of promoter ions, Co " or Ni ". The stmctures of the catalysts are fairly well understood the M0S2 is present in layers only a few atoms thick on the support surface, and the promoter ions are present at the edges of the M0S2 layers, where the catalytic sites are located (100,101). 

Catalyst design is in a primitive stage. There are hardly any examples of tme design of catalysts (42). However, development of improved catalysts has been guided successfully in instances when the central issues were the interplay of mass transport and reaction. An example is catalysts used for hydroprocessing of heavy fossil fuels. 

Hydroprocesses Hydrogen is chemically stable and relatively unreactive at ordinary temperatures most processes utilizing it require a catalyst. Above 500°C it reacts readily with oxygen and confined flammable mixtures explode violently if ignited Main hazards fire, explosion, metallurgical problems arising from hydrogen attack... 

Hydrotreating/Hydroprocessing Heater stack gas (CO, SO, NO, hydrocarbons and PM), vents and fugitive emissions (HCs) and catalyst regeneration (CO, NO, SO,). 

Residuum Conversion This includes fluid coking, delayed coking, visbreaking, and residuum hydroprocessing. 

Catalytic Hydroprocessing of Petroleum and Distillates, edited by Michael C. Oballah and Stuart S. Shih... 

Pretreatment of FCC feedstock through hydroprocessing has a number of benefits including ... 

Desulfurization of FCC feedstocks reduces the sulfur content of FCC products and SOX emissions. In the United States, road diesel sulfur can be 500 ppm (0.05 wt%). In some European countries, for example in Sweden, the sulfur of road diesel is 50 ppm or less. In California, the gasoline sulfur is required to be less than 40 ppm. The EPA s complex model uses sulfur as a controlling parameter to reduce toxic emissions. With hydroprocessed FCC feeds, about 5% of feed sulfur is in the FCC gasoline. For non-hydroprocessed feeds, the FCC gasoline sulfur is typically 10% of the feed sulfur. 

Hydroprocessing reduces the Conradson carbon residue of heavy oils. Conradson carbon residue becomes coke in the FCC reactor. This excess coke must be burned in the regenerator, increasing regenerator air requirements. 

Harvey G, Matheson TW (1986) Hydroprocessing catalysis by supported ruthenium sulphide. J Catal 101 253-261... 

Carbons and Carbon Supported Catalysts in Hydroprocessing 2 Chiral Sulfur Ligands Asymmetric Catalysis... 

Hydroprocessing of Heavy Oils and Residua, edited by James G. Speight and Jorge Ancheyta... 

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