Questions and Answers
What is the purpose of the gas absorber in the recycle process?
What is the chemical reaction between DEA and H2S?
What is the function of the stainless steel ballast rings in the absorber?
Where does the lean DEA solution enter the absorber?
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What happens to the rich DEA solution from the bottom of the absorber?
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What is the purpose of the recycle gas compressor?
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What happens to the stripper overhead vapor?
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What percentage of the H2S formed by the desulfurization reactions is removed in the high-pressure absorber?
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What is the difference in the hydrocracking feed between the current refinery and the Kuwait Refinery?
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What is the purpose of the High Pressure Separator in the hydrocracker unit?
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At what pressure and temperature does the High Pressure Separator operate?
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Why can't hydrogen, hydrogen sulfide, and ammonia be liquids at the High Pressure Separator conditions?
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What happens to the mixed liquid stream after leaving the High Pressure Separator?
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What is the purpose of the Low Temperature Separator?
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What happens to the sour water after leaving the Low Temperature Separator?
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What determines the amount of H2S that can react in the Recycle Gas Absorber?
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What is the purpose of the stripper column?
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What is the function of the reflux drum?
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How many trays are in the rectifying section of the fractionator?
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What is the purpose of the feed heater H-102?
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What happens to the vapor leaving the top tray of the column?
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What is the purpose of the pumparounds?
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What is the function of the overhead condenser?
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What happens to the liquid removed from the tower in the pumparounds?
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What is the primary function of a tray weir in a fractionator?
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What is the purpose of operating a hydrocracker unit in one-stage with recycle mode?
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What reacts in the first stage of a two-stage hydrocracking process?
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What is the primary difference between a one-stage and a two-stage hydrocracking process?
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What is the purpose of a downcomer in a fractionator?
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In a two-stage hydrocracking process, what type of reactions occur in the second stage?
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What is the range of conversion for one-stage hydrocracking operation?
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What are the main parameters to consider when choosing a process configuration for a hydrocracker unit?
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What is the purpose of pumparounds in a fractionator?
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Where is the superheated steam injected in the fractionator?
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What happens to the pumparound return stream?
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What type of trays are used in the fractionator?
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What happens as the superheated steam passes up through the stripping section?
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How many trays are in the portion of the column below the flash zone?
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What is the function of the downcomer in a Sieve tray?
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What happens to the residual liquid from the flash zone?
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Study Notes
Hydrocracker Unit Operation: Two Stages
- The hydrocracker unit operation differs from Kuwait Refinery in terms of feed composition, which consists of atmospheric gasoil, vacuum light gasoil, vacuum heavy gasoil, and coker gas oil.
Two Stages Operation Mode
- The operation mode involves two stages: high-pressure separator and low-pressure separator.
- In the high-pressure separator, the recycle gas (unreacted hydrogen, H2S, and NH3) is separated from the mixed liquid stream (sour water and liquid hydrocarbons) at 140°F and 2280 psig.
High-Pressure Separator
- Hydrogen, H2S, and NH3 cannot be liquids at this high pressure due to their critical pressures (188.2 bar for H2, 1309 psi for H2S, and 1647 psi for NH3).
Low-Temperature Separator
- The liquid is depressurized to 550 psig before being flashed in the low-pressure separator.
- Sour water (water + H2S) is separated from the liquid hydrocarbons and sent to sour water treating facilities, while the hydrocarbon stream is fed to the H2S stripper T-101.
Recycle Gas Absorber
- DEA (diethanolamine) absorption is used to remove H2S from the recycled gas.
- The reaction proceeds to the right at low temperatures, favoring the removal of H2S.
- About 90% of the H2S formed by desulfurization reactions is removed in a high-pressure absorber by scrubbing the gas with aqueous diethanolamine solution.
Recycle Gas Compressor
- Recycled gas is circulated by the recycle gas compressor C-101, driven by a steam turbine.
- The largest portion of the recycled gas stream joins the oil feed stream before the feed effluent exchangers, while a portion flows to temperature control for interbed quenching.
H2S Stripper
- Hydrocarbon liquid is pre-flashed to remove light ends and H2S before being stripped in the H2S stripper T-101.
- The stripper column contains packed sections and two sieve trays, and stripping is achieved with steam, which removes H2S and light ends.
Fractionator
- The fractionator feed is heated to column temperature by the feed heater H-102, and then partially vaporized and introduced into the flash zone.
- The vapor passes up through the rectifying section, containing approximately 27 trays, and is condensed in overhead condensers.
- The condensed overhead vapor is separated into hydrocarbon and water phases, with part of the hydrocarbon recovered as overhead product and the rest sent back to the column as reflux.
Pumparounds in Fractionator
- Pumparounds (circulated side streams) provide extra liquid refluxes to enhance separation and provide heat for the reboiler and feed preheat.
- Pumparounds are taken from higher temperature regions and returned to lower temperature regions, condensing more vapors and increasing reflux below that point.
Fractionator Trays
- Sieve trays, valve trays, and bubble cap trays are used in the fractionator.
Operation Modes of Hydrocracker Units
- Hydrocracker units can be operated in single-stage (once-through-mode), single-stage with partial or total recycling, and two-stage operation.
- The choice of operation mode depends on the catalyst system, feedstock quality, product quality required, and investment and operating costs of the unit.
Two-Stage Hydrocracking
- The first stage involves main reactions such as hydrodesulfurization (HDS), hydro-denitrification (HDN), and aromatic hydrogenation (HDA).
- The second stage involves main reactions such as aromatic hydrogenation (HDA), hydro-isomerization (HDI), and hydrocracking (HDC).
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Description
This quiz covers the operation and safety of hydrocracker units in a refinery, including the differences in feed composition and two-stage operation mode.