Diesel fuels contain significant quantities of cracked stocks, such as FCC light cycle oil (LCO), which contains a large concentration of aromatic sulfur compounds. The effective removal of these species are done in the diesel hydrotreater, which requires tailored catalysts and process conditions, including consideration of the feed nitrogen content and aromatics equilibrium.
Maximum reactor outlet temperature at end-of-cycle catalyst conditions is generally set at 725...750°F to avoid aromatics saturation equilibrium constraints. Reactor space velocity, hydrogen gas quantity and hydrogen partial pressure and reactor temperature are the variables that are optimized during the process design.
The highest level of accuracy and stability is required. For temperature, a type N thermocouple is linearized, isolated and converted to a standard 4...20 mA signal using the PR 5337D (HART 5/7) temperature transmitters. The 4...20 mA signal from a hydrogen pressure transmitter must also be measured and excited. The double channel 9106B HART transparent repeater safely excites and measures both signals from the 5337 temperature transmitter and the hydrogen pressure transmitter, providing isolation, fast response time and the IS barrier to the safe area.
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