Chapter 7 Distillate hydrocracking (p. 287-288) Adrian Gruia Hydrocracking is a versatile catalytic refining process that upgrades petroleum feedstocks by adding hydrogen, removing impurities, and cracking to a desired boiling range. Hydrocracking requires the conversion of a variety of types of molecules and is characterized by the fact that the products are of significantly lower molecular weight than the feed. Hydrocracking feeds can range from heavy vacuum gas oils and coker gas oils to atmospheric gas oils. Products usually range from heavy diesel to light naphtha. Hydrocrackers are designed for and run at a variety of conditions depending on many factors such as type of feed, desired cycle length, expected product slate but in general they will operate at the following range of conditions: liquid hourly space velocity (LHSV)—0.5 to 2.0, H2 circulation—5,000 to 10,000 SCFB (850–1,700 Nm3/m3), H2PP—1,500 to 2,000 psia (103–138 bars), and SOR temperatures ranging between 675.F and 725.F (357–385.C). Hydrocracking is particularly well suited to generating products that meet or exceed all of the present tough environmental regulations. Brief history While the first commercial installation of a unit employing the type of technology in use today was started up in Chevron’s Richmond CA refinery in 1960, hydrocracking is one of the oldest hydrocarbon conversion processes. Hydrocracking technology for coal conversion was developed in Germany as early as 1915 designed to secure a supply of liquid fuels derived from domestic deposits of coal. The first plant for hydrogenation of brown coal was put on stream in Leuna Germany in 1927, applying what may be considered the first commercial hydrocracking process. Conversion of coal to liquid fuels was a catalytic process operating at high pressures, 3000– 10,000 psig (207–690 bar) and high temperatures, 700–1000.F (371–538.C). Other efforts were undertaken subsequently to develop hydrocracking technology designed to convert heavy gas oils to lighter fuels. The emergent availability of Middle Eastern crude after World War II removed the incentive to convert coal to liquid fuels, so continuing the development of hydrocracking technology became less important. In the mid-1950s the automobile industry started the manufacture of highperformance cars with high-compression ratio engines that required high-octane gasoline. Thus catalytic cracking expanded rapidly and generated, in addition to gasoline, large quantities of refractory cycle stock that was material that was difficult to convert to gasoline and lighter products. This need to convert refractory stock to quality gasoline was filled by hydrocracking. Furthermore, the switch of railroads from steam to diesel engines after World War II and the introduction of commercial jet aircraft in the late 1950s increased the demand for diesel fuel and jet fuel. The flexibility of the newly developed hydrocracking processes made possible the production of such fuels from heavier feedstocks. The early hydrocrackers used amorphous silica alumina catalysts. The rapid growth of hydrocracking in the 1960s was accompanied by the development of new, zeolite based hydrocracking catalysts. They showed a significant improvement in certain performance characteristics as compared with amorphous catalysts: higher activity, better ammonia tolerance, and higher gasoline selectivity. While hydrocracking was used in the United States primarily in the production of high-octane gasoline, it grew in other parts of the world, starting in the 1970s primarily for the production of middle distillates. The amorphous catalysts remained the catalysts of choice for this application, though some‘flexible’ catalysts were developed that made it possible to maximize the yield of different products by using the same catalyst but changing the operating conditions. As of the beginning of 2001, there were more than 150 hydrocrackers operating in the world with a total capacity in excess of 3,800,000 B/D (500,000 MT/D). |
