Many refineries employ hydrocracking technology to convert heavy hydrocarbon oils into lighter and more valuable products. Here's a typical flow sheet for a single stage hydrocracking process.

Hydrocarbon liquid and hydrogen feed into the hydrocracker. Hydrocrackers are capable of processing a wide range of liquid hydrocarbon feed stocks, but typically process heavy oils such as vacuum gas oils and atmospheric residuals. The hydrogen-hydrocarbon feed blend is typically heated in a fired heater and sent to the reactors, where the cracking reaction occurs. After heat exchange, operators separate the hydrocarbon products from hydrogen and light gases in a series of separators and flash drums. They process hydrocarbon products further in a fractionation section. Both heavy hydrocarbon liquids and hydrogen are recyclable.

The reactions taking place in the hydrocracker process include cracking, whereby long-chain hydrocarbons break into smaller chains, and hydrogenation, where any free radicals or double bonds are saturated. The end result is a hydrocarbon product whose average molecular weight is much smaller than the molecular weight of the feed. The overall reaction set is significantly exothermic. Under some circumstances, heat generated in the reaction may increase the temperature of the catalyst bed, leading to increased reaction rates and more heat generation. This effect can spiral out of control and result in a potential loss of integrity of the reactor vessel or piping due to excessive temperature.
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The reaction occurs as liquid hydrocarbon contacts a fixed bed of catalyst with excess hydrogen at a high pressure. During normal operation, adding a cold hydrogen quench to sweep away the heat of reaction to the downstream heat exchangers controls temperature. In an emergency situation, depressuring the reactor can stop the reaction. When a depressuring occurs, the reactor pressure, and thus the partial pressure of hydrogen, decreases. The decrease in hydrogen partial pressure essentially decreases the concentration of reactant available, and in accordance with traditional chemical reaction kinetics, the reaction rate quickly falls off. The speed at which the reaction rate falls is a function of how fast the reactor pressure drops. Many hydrocrackers are equipped with two different means of depressuring: a slow system and a fast system. Obviously, the fast system can bring the process to a safe state more rapidly, but causes unwanted side effects such as intense flaring and equipment degradation due to hydrogen embrittlement. In an emergency scenario, an operator will first attempt to bring the process under control using the slow depressuring and only use the fast depressuring system if the other is not capable of stopping the runaway reaction from continuing.

Shortcut methods such as hazard matrices and risk graphs, commonly used for safety integrity level (SIL) selection, are effective in most situations. In some scenarios, selecting the SIL using these tools doesn't work-usually because the selected SIL was significantly higher than originally expected.