Heavy Oil Upgrading
Hydrocarbons are subjected to a variety of physical and chemical processes to produce higher value products. These processes include fractionation, isomerization, bond dissociation, reformation, purification and increasing hydrogen content. These processes tend to require high pressures and temperatures. Catalysts are employed in these processes for various reasons including, but not limited to, reducing the temperatures and pressures at which the hydrocarbon conversion reaction takes place.
Petroleum or crude oil is a naturally occurring mixture of hydrocarbons and smaller amounts of organic compounds containing heteroatoms such as sulfur, oxygen, nitrogen and metals, generally nickel and vanadium. The petroleum products obtained from processing may vary considerably according to market demand, crude oil quality, and refinery objectives. In current industrial practices, crude oils are submitted to distillation under atmospheric pressure and vacuum. The distillation fractions including the residual fractions, undergo further catalytic refining processes in order to produce high value products.
The Hydrogen content of petroleum products is an important aspect of their economic value because the values of petroleum products are directly related to their hydrogen contents. Therefore, the effective hydrogenation of products is highly desirable in all stages of petroleum refining. To remove undesirable heteroatoms, desulfurization, denitrogenation and demetalization processes are also accomplished using hydroprocessing methods.
In conventional hydrocracking and hydrotreating processes, the hydrogenation reactions of aromatic compounds play a crucial role. As heavy residual compounds are normally aromatic in nature, therefore, the complete or partial saturation of these compounds, by hydrogen addition, is an important step in their cracking into smaller, more valuable compounds. Conventional heavy oil hydrocracking processes require relatively high temperature e.g. greater than 400 OC, and very high pressure e.g. greater than 1000 psi. In current hydrotreating and hydro reforming processes, supported Ni-Mo and Co-Mo sulfide catalysts become active only at the high temperature range. In order to achieve good hydrogenation efficiency, reactions need to take place at a favorable lower temperature range for which expensive noble metal catalysts are usually used.
As the name implies, hydrocracking combines catalytic cracking and hydrogenation by means of a bifunctional catalyst to accomplish a number of favorable transformations of particular value for the selected feedstocks. In a typical bifunctional catalyst, the cracking function is provided by an acidic support, whereas the hydrogenation function is provided by noble metals or non-noble metal sulfides from Periodic Table Groups 6, 9, and 10 (based on the 1990 IUPAC system in which the columns are assigned the numbers 1 to 18). Hydrocracking is a versatile process for converting a variety of feedstocks, ranging from naphthas through heavy gas oils, into useful products. Its most unique characteristics involve the hydrogenation and breakup of polynuclear aromatics. Significant portions of these feedstocks are converted through hydrocracking into smaller sized and more useful product constituents. However, some of the large aromatic complexes within these feedstocks, once partially hydrogenated via hydrocracking, may proceed to dehydrogenate forming coke on the catalysts. Coke formation is one of the many deactivation mechanisms that reduce catalyst life.
In many refineries, the hydrocracker serves as the major supplier of jet and diesel fuel components (middle distillates). Because of the high pressure required and hydrogen consumption, conventional hydrocrackers are very costly to build and to operate.
To remove undesirable heteroatoms, desulfurization, denitrogenation and demetalization, processes are also accomplished using hydroprocessing methods. Because the values of petroleum products are directly related to their hydrogen contents, the effective hydrogenation of products is highly desirable in all stages of petroleum refining.
Metals, such as platinum, deposited on oxide supports such as alumina or silica, are widely used as catalysts for hydrocarbon reforming reactions. The deposited metal provides reactive sites at which the desired reactions can occur. However catalysts using these metals have the problem of being rendered inactive if heavy polyaromatic compounds build up and occupy or block the sites. The removal of sulfur and sulfur compounds are also a problem for these catalysts. Sulfur reacts with the catalytic sites of Pt or Pd metals and can also deactivate these sites by chemically binding to the metals. Successful catalysis requires that a suitable high local concentration of hydrogen be maintained during the catalytic process. Pressure and temperature conditions are selected to favor formation of the desired product, to provide a suitable rate of conversion and avoid rapid deactivation of the catalytic surface.
Attempts have been made to find new classes of catalysts that would significantly lower the process parameters, while increasing the hydrogenation efficiency in terms of deep reduction of aromatic content but the progress made thus far is mostly small improvements over existing catalyst systems.