Bài giảng hóa học dầu mỏ ( hóa dầu ) Chương 3 xúc tác cho quá trình chế biến dầu mỏ

Xúc tác cho trình chế biến dầu mỏ Catalytic Reforming *Increasing the octane number of a low-octane naphtha fraction * the key process for obtaining benzene, toluene, and xylenes (BTX) Reformer Feeds •heavy naphtha fraction •naphtha from other sources Reforming Catalysts The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier The two types of sites are necessary for aromatization and isomerization reactions Aromatization Aromatization Aromatization Isomerisation Isomerisation Hydrocracking Hydrocracking is a hydrogen-consuming reaction that leads to higher gas production and lower liquid yield This reaction is favored at high temperatures and high hydrogen partial pressure The following represents a hydrocracking reaction: Bond breaking can occur at any position along the hydrocarbon chain Hydrocracking long-chain molecules can produce C6, C7, and C8 hydrocarbons that are suitable for dehydro-cyclization to aromatics Hydrodealkylation Hydrodealkylation is a cracking reaction of an aromatic side chain in presence of hydrogen Like hydrocracking, the reaction consumes hydrogen and is favored at a higher hydrogen partial pressure This reaction is particularly important for increasing benzene yield when methylbenzenes and ethylbenzene are dealkylated Catalytic Cracking Catalytic cracking (Cat-cracking) is a remarkably versatile and flexible process Its principal aim is to crack lower-value stocks and produce higher-value light and middle distillates The process also produces light hydrocarbon gases, which are important feedstocks for petrochemicals Feeds The feeds to catalytic cracking units vary from gas oils to crude residues FCC (fluid catalytic cracking) feedstocks are often pretreated to decrease the metallic and asphaltene contents Residium fluid catalytic cracking (RFCC) has gained wide acceptance due to a larger production of gasoline with only small amounts of low-value products Cracking Catalysts Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics These catalysts have both Lewis and Bronsted acid sites that promote carbonium ion formation Bronsted acid sites Cracking Reactions Cracking Reactions The most important cracking reaction, is the carbon-carbon beta bond scission A bond at a position beta to the positively-charged carbon breaks heterolytically, yielding an olefin and another carbocation.This can be represented by the following example: Hydrotreatment Catalysts and Reactions Hydrodesulfurization, HDS The catalysts should be sulfur-resistant The cobalt-molybdenum system supported on alumina was found to be an effective catalyst NiO/MoO3 and NiO/WO3 Hydrotreatment Catalysts and Reactions Alkylation Process ... cobalt-molybdenum system supported on alumina was found to be an effective catalyst NiO/MoO3 and NiO/WO3 Hydrotreatment Catalysts and Reactions Alkylation Process