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WHEN A REFINERY ADDS a new ultra-low-sulfur hydrodesulfurization (HDS) plant to meet clean-fuels requirement for gasoline or diesel fuel, the change can put a strain on related units—even if fuel throughput remains essentially the same as before. The sharp increase in sulfur removal requires more hydrogen from the hydrogen unit and sends more sulfur gases to the amine treaters and downstream sulfur units.
In typical medium to large refineries, each clean-fuels complex—gasoline or diesel—involves multiple support units of each type (Figures 1 and 2). For example, in addition to the HDS plant, such a complex might have two small hydrogen plants, five amine absorber units, two amine regeneration units, two liquid-liquid coalescers for product haze removal, and three sulfur units. Any of these auxiliaries may become bottlenecks, with a number of potential adverse consequences.
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In many cases, such bottlenecks can be relieved with surprisingly little investment. To understand the possibilities, first consider the problems that may arise from implementing production of ultra-low-sulfur gasoline or diesel fuel (ULSG or ULSD):
1. Insufficient output from the hydrogen plants to sustain full throughput in the HDS plant. In order to meet low-sulfur requirements, the HDS plant has to be operated below capacity.
2. Product haziness due to water emulsion that is difficult to remove. Hazy gasoline or diesel cannot be immediately used for final product blending. Instead, it may have to sit in tanks for several days while the haze settles to the bottom. This situation can create costly shipment delays or tankage availability constraints.
3. Increased sulfur in hydrocarbon off gas from the amine treaters due to insufficient absorber capacity. Because this gas is used for fuel, environmental regulations limiting sulfur oxides (SOX) in stack gas may require reduction in clean-fuels unit throughput.
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4. Increased carryover of amines in the off gas. This has three consequences: damage to the fuel-gas compressors by pitting corrosion and amine salt deposits, plugging of burner tips in refinery heaters fueled by the off gas, and greatly increased make-up rate for expensive amine solvent.
5. High concentration of pentane and heavier liquids in the gas feed to the amine treaters due to inadequate removal by liquid-gas separators in the HDS unit. These liquids interact with the amines in the absorber and can cause severe foaming. Sulfur absorption is reduced, and amine carryover increases.
6. Sulfur plant capacity limitation. With greater throughput, incoming sulfur gases (mostly hydrogen sulfide, H2S) cannot be completely converted to elemental sulfur, and liquid sulfur may be entrained in tail gas. The result is an increased flow of sulfur to the tail-gas incinerator, so that sulfur oxide emissions in the stack gas increase to unacceptable levels.
Each of these problems will be considered in turn.
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