Tuesday, January 27, 2009

Hydroprocesses

The use of hydrogen in thermal processes was perhaps the single most significant advance in refining technology during the twentieth century. The process uses the principle that the presence of hydrogen during a thermal reaction of a petroleum feedstock will terminate many of the coke-forming reactions and enhance the yields of the lower-boiling components, such as gasoline, kerosine, and jet fuel. Hydrogenation processes for the conversion of petroleum fractions and petroleum products may be classified as destructive and nondestructive. See also: Hydrogenation

Destructive hydrogenation (hydrogenolysis or hydrocracking) is characterized by the conversion of the higher-molecular-weight constituents in a feedstock to lower-boiling products. Such treatment requires severe processing conditions and the use of high hydrogen pressures to minimize the polymerization and condensation reactions that lead to coke formation. See also: Hydrocracking

Nondestructive hydrogenation is used for improving product quality without appreciable alteration of the boiling range. Nitrogen, sulfur, and oxygen compounds undergo reaction with the hydrogen, forming ammonia, hydrogen sulfide, and water, respectively. Unstable compounds that might lead to the formation of gums or insoluble materials are converted to more stable compounds.

Hydrotreating

Hydrotreating (nondestructive hydrogenation) is carried out by charging the feed to the reactor, together with hydrogen in the presence of catalysts, such as tungsten-nickel sulfide (W-NiS), cobalt-molybdenum-alumina (Co-Mo-Al2O3), nickel oxide-silica-alumina (NiO-SiO2-Al2O3), and platinum-alumina (Pt-Al2O3). Most processes use cobalt-molybdena catalysts that generally contain about 10% molybdenum oxide and less than 1% cobalt oxide supported on alumina (Al2O3). The temperatures used are in the range of 300-345°C (570-655°F), while the hydrogen pressures are about 500-1000 psi (3447-6895 kPa).

The reaction generally takes place in the vapor phase but, depending on the application, may be a mixed-phase reaction. Generally it is more economical to hydrotreat high-sulfur feedstocks prior to catalytic cracking than to hydrotreat the cracking products. The advantages of hydrotreating are that (1) sulfur is removed from the catalytic cracking feedstock, and corrosion is reduced in the cracking unit; (2) carbon formation during cracking is reduced, resulting in higher conversions; and (3) the cracking quality of the gas oil fraction is improved.

Hydrocracking

Hydrocracking (destructive hydrogenation) is similar to catalytic cracking with hydrogenation superimposed and with the reactions taking place either simultaneously or sequentially. Hydrocracking was initially used to upgrade low-value distillate feedstocks, such as cycle oils (high aromatic products from a catalytic cracker which usually are not recycled to extinction for economic reasons), thermal and coker gas oils, and heavy-cracked and straight-run naphthas. Catalyst improvements and modifications have made it possible to yield products from gases and naphtha to furnace oils and catalytic cracking feedstocks. The hydrotreating catalysts are usually cobalt plus molybdenum or nickel plus molybdenum (in the sulfide) form impregnated on an alumina (Al2O3) base. The hydrotreated operating conditions are such that appreciable hydrogenation of aromatics will not occur [1000-2000 psi (6895-13,790 kPa) hydrogen and about 370°C (700°F)]. The desulfurization reactions are usually accompanied by small amounts of hydrogenation and hydrocracking.

In a hydrotreating process, the feedstock is heated and passed with hydrogen gas through a tower or reactor filled with catalyst pellets. The reactor is maintained at a temperature of 260-425°C (500-800°F) at pressures from 100 to 1000 psi (690 to 6895 kPa), depending on the particular process, the nature of the feedstock, and the degree of hydrogenation required. After leaving the reactor, excess hydrogen is separated from the treated product and recycled through the reactor after the removal of hydrogen sulfide. The liquid product is passed into a stripping tower where steam removes the dissolved hydrogen and hydrogen sulfide; after cooling, the product is stored or, in the case of feedstock preparation, pumped to the next processing unit.

The most common hydrocracking process is a two-stage operation (Fig. 1) that maximizes the yield of transportation fuels and has the flexibility to produce gasoline, naphtha, jet fuel, or diesel fuel to meet seasonal demand. The processes also operate at higher temperatures than the hydrotreating processes and use a more temperature-resistant catalyst.

Fig. 1 Two-stage hydrocracking process. (After J. G. Speight, ed., The Chemistry and Technology of Petroleum, 3d ed., Marcel Dekker, New York, 1999)

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