Hydrocracker Unit Operation in Refinery

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Questions and Answers

What is the purpose of the gas absorber in the recycle process?

To remove H2S from the recycled gas (correct)

To cool the recycled gas

To compress the recycled gas

To mix the recycled gas with the oil feed stream

What is the chemical reaction between DEA and H2S?

R2NH2 + S ⇌ R2NH + H2S

R2NH + H2S ⇌ R2NH2 + S

2R2NH + H2S ⇌ 2(R2NH2) + S (correct)

2R2NH2 + S ⇌ 2R2NH + H2S

What is the function of the stainless steel ballast rings in the absorber?

To compress the recycled gas

To heat the recycled gas

To mix the DEA solution with the recycled gas

To pack the absorber vessel (correct)

Where does the lean DEA solution enter the absorber?

Through the top of the absorber Signup and view all the answers

What happens to the rich DEA solution from the bottom of the absorber?

It is sent to the H2S recovery unit Signup and view all the answers

What is the purpose of the recycle gas compressor?

To compress the recycled gas Signup and view all the answers

What happens to the stripper overhead vapor?

It is partially condensed in an air cooler Signup and view all the answers

What percentage of the H2S formed by the desulfurization reactions is removed in the high-pressure absorber?

90% Signup and view all the answers

What is the difference in the hydrocracking feed between the current refinery and the Kuwait Refinery?

The Kuwait Refinery uses a combination of atmospheric gasoil, vacuum light gasoil, vacuum heavy gasoil, and coker gas oil Signup and view all the answers

What is the purpose of the High Pressure Separator in the hydrocracker unit?

To separate the mixed liquid stream from the recycle gas Signup and view all the answers

At what pressure and temperature does the High Pressure Separator operate?

140°F and 2280 psig Signup and view all the answers

Why can't hydrogen, hydrogen sulfide, and ammonia be liquids at the High Pressure Separator conditions?

Because their critical pressures are lower than the operating pressure Signup and view all the answers

What happens to the mixed liquid stream after leaving the High Pressure Separator?

It is depressurized to 550 psig and then flashed in the Low Temperature Separator Signup and view all the answers

What is the purpose of the Low Temperature Separator?

To separate the sour water from the liquid hydrocarbons Signup and view all the answers

What happens to the sour water after leaving the Low Temperature Separator?

It is sent to sour water treating facilities Signup and view all the answers

What determines the amount of H2S that can react in the Recycle Gas Absorber?

The operating conditions Signup and view all the answers

What is the purpose of the stripper column?

To remove H2S and light ends from the low-pressure separator oil Signup and view all the answers

What is the function of the reflux drum?

To send sour gas to the amine contactor Signup and view all the answers

How many trays are in the rectifying section of the fractionator?

27 Signup and view all the answers

What is the purpose of the feed heater H-102?

To bring the fractionator feed to column temperature Signup and view all the answers

What happens to the vapor leaving the top tray of the column?

It is condensed in the overhead condensers Signup and view all the answers

What is the purpose of the pumparounds?

To cool the tower by exchanging heat Signup and view all the answers

What is the function of the overhead condenser?

To condense the overhead vapor Signup and view all the answers

What happens to the liquid removed from the tower in the pumparounds?

It is cooled by exchanging heat Signup and view all the answers

What is the primary function of a tray weir in a fractionator?

To channel liquid downcomer flow Signup and view all the answers

What is the purpose of operating a hydrocracker unit in one-stage with recycle mode?

To enhance conversion Signup and view all the answers

What reacts in the first stage of a two-stage hydrocracking process?

Hydrodesulfurization, hydro-denitrification, and aromatic hydrogenation Signup and view all the answers

What is the primary difference between a one-stage and a two-stage hydrocracking process?

The number of reactors used Signup and view all the answers

What is the purpose of a downcomer in a fractionator?

To channel liquid downcomer flow Signup and view all the answers

In a two-stage hydrocracking process, what type of reactions occur in the second stage?

Aromatic hydrogenation, hydro-isomerization, and hydrocracking Signup and view all the answers

What is the range of conversion for one-stage hydrocracking operation?

40%-80% Signup and view all the answers

What are the main parameters to consider when choosing a process configuration for a hydrocracker unit?

Feedstock quality, product quality, and operating costs Signup and view all the answers

What is the purpose of pumparounds in a fractionator?

To provide extra liquid refluxes and heat for the reboiler and feed preheat Signup and view all the answers

Where is the superheated steam injected in the fractionator?

Below the bottom tray Signup and view all the answers

What happens to the pumparound return stream?

It condenses more of the vapors coming from the lower tower section and then increases reflux below that point Signup and view all the answers

What type of trays are used in the fractionator?

Sieve trays only Signup and view all the answers

What happens as the superheated steam passes up through the stripping section?

It strips light components from the residual liquid Signup and view all the answers

How many trays are in the portion of the column below the flash zone?

Five trays Signup and view all the answers

What is the function of the downcomer in a Sieve tray?

To allow liquid to flow down to the next tray Signup and view all the answers

What happens to the residual liquid from the flash zone?

It is stripped of light components by the superheated steam Signup and view all the answers

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.