Strippers hydrocracking

FIGURE 11.12. Composite curves for the two-stripper hydrocracking unit. 

FIGURE 11.13. Grand composite curve for the single-stripper hydrocracking unit. 

For convenience, the discussion of materials for these various processes is divided into five chapters. Crude units and utilities are discussed in this chapter. FCCs, fluid cokers, delayed cokers, sour water strippers, and sulfur plants are covered in Chapter Two. Desulfurizers, reformers, hydrocrackers, and flue gas are discussed in Chapter Three. Hydrogen plants, methanol plants, ammonia plants, and gas treating are discussed in Chapter Four. Underground piping, pipelines, production equipment, and tankage associated with the refinery industry are covered in Chapter Five. Discussed throughout these chapters are many common environments and equipment (e.g., sour or foul water, distillation, etc.) that appear in the various types of refinery process plants. 

Several of the commercial simulation programs offer preconfigured complex column rigorous models for petroleum fractionation. These models include charge heaters, several side strippers, and one or two pump-around loops. These fractionation column models can be used to model refinery distillation operations such as crude oil distillation, vacuum distillation of atmospheric residue oil, fluidized catalytic cracking (FCC) process main columns, and hydrocracker or coker main columns. Aspen Plus also has a shortcut fractionation model, SCFrac, which can be used to configure fractionation columns in the same way that shortcut distillation models are used to initialize multicomponent rigorous distillation models. 

One example of a single variable control strategy is applied to a stripper in a hydrocracking unit. The main purpose of the stripper is to remove H2S and noncondensable components from the bottom product. One of the key indicators identified is the water dew point at the top of the stripper column. As a matter of fact, the dew point is a function of column overhead vapor composition and the amount of water. There was no monitoring capability available for the dew point temperature. If the column top temperature is lower than the dew point, the hydrogen sulfide will dissolve in the condensed water and cause corrosion to the column overhead system. 

A yield improvement option was identified from the hydrocracking unit (HCU) as a part of the energy optimization project. It was found that a large amount of diesel was lost into the unconverted oil due to poor fractionation efficiency in the main fractionators. To recover this lost diesel with an estimated value of 12 MM/year, a new stripper column will be installed. The stripper column will require stripping LP steam of 150t/day costing 1.0 MM per year. The cost for related piping would be estimated as part of HCU revamp project... 

Thermal processes such as coking, cat cracking, hydrodesulfurization, and hydrocracking ail produce both NF 3 and H2S. The resulting aqueous-phase ammonium hydrosulfide is antagonistic to carbon steel heat-exchanger tubes. Most of the ammonium hydrosulfide winds up in sour-water streams. Before disposal, the sour water must be steam stripped. The overhead condenser used for this stripper has an extremely corrosive environment. Titanium is preferred for this service. 

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