Removing Sulfur in a Refinery: Hydrotreating and the Claus Sulfur Recovery Unit
Key takeaways
- Modern specifications limit diesel sulfur to about 10 to 15 ppm, while untreated diesel may contain several thousand ppm.
- A hydrotreater mixes the fuel with hydrogen over a CoMo or NiMo catalyst, converting sulfur into H2S gas.
- The amine unit absorbs H2S from the gas, then releases it concentrated in the regenerator to feed the Claus unit.
- The Claus unit burns roughly one third of the H2S to SO2, then the two gases react to give sulfur and water.
- H2S is lethal at low concentrations and paralyses the sense of smell, so never rely on the odour.
Educational content only: procedures and operating values vary between companies and sites. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.
Why remove sulfur?
Crude contains sulfur compounds, more so in heavy and sour crudes. If sulfur stays in the fuel, it burns in the engine to SO2, polluting the air, causing acid rain and damaging the exhaust after-treatment of modern vehicles. That is why many specifications now limit diesel sulfur to about 10 to 15 ppm.
Sulfur also does harm inside the refinery: it poisons the platinum catalyst in the reformer and increases corrosion. So it is removed from most streams before they reach the final product.
The hydrotreater: how is sulfur removed?
The idea is simple: add hydrogen to the fuel over a catalyst, and hydrogen strips the sulfur atoms out of the molecules, forming H2S. The same happens to nitrogen, which becomes ammonia (NH3).
The stream path through the unit:
- The fuel (naphtha, kerosene or diesel) is mixed with hydrogen-rich gas.
- The mixture is heated in exchangers and a fired heater to about 300 to 400 °C.
- It passes a reactor filled with cobalt-molybdenum (CoMo) or nickel-molybdenum (NiMo) catalyst. Pressure depends on the feed; diesel units may run at about 30 to 70 bar or more.
- The reactor effluent is cooled, and wash water is injected to dissolve ammonium salts that could plug the coolers.
- In the high-pressure separator, hydrogen-rich gas separates from the liquid. The gas passes an amine scrubber to remove H2S, then returns to the front of the unit through the recycle compressor.
- The liquid enters a stripper that removes dissolved H2S and light gases, leaving a low-sulfur fuel.
The amine unit: an H2S sponge
An amine solution (such as MDEA or DEA) absorbs H2S and CO2 from gas at low temperature and high pressure. Sour gas enters the bottom of the absorber and lean amine enters the top. Sweet gas leaves the top and rich amine, loaded with H2S, leaves the bottom.
The rich amine is then heated in the regenerator, which releases the H2S in concentrated form. This concentrated gas is called acid gas and goes to the Claus unit, while the lean amine returns to absorb again. Sour water is treated in its own stripper, which releases H2S and ammonia.
The Claus unit: turning H2S into sulfur
The sulfur recovery unit works in two stages.
Thermal stage: acid gas is burned in a furnace with a measured amount of air, enough to burn only about one third of the H2S, at temperatures that can reach 1,000 to 1,400 °C:
H2S + 3/2 O2 → SO2 + H2O
The remaining H2S then reacts with the SO2 formed:
2 H2S + SO2 → 3 S + 2 H2O
Much of the sulfur condenses in a condenser after the furnace.
Catalytic stage: the gas is reheated and passes two or three reactors filled with alumina catalyst, with a sulfur condenser after each one. The same reaction continues at lower temperature.
A Claus unit typically recovers about 95 to 98% of the sulfur. With a tail gas treating unit added, recovery can exceed 99.9%. What remains is burned in an incinerator before the stack.
A diesel unit processes 5,000 t/d of feed containing 1.0 wt% sulfur, and the product must meet 10 ppm.
S in feed = 5,000 × 0.010 = 50 t/d
S in product = 5,000 × 0.000010 = 0.05 t/d
About 49.95 t/d of sulfur is removed. Almost all of it becomes H2S, whose mass is larger by the ratio 34 to 32:
H2S = 49.95 × 34 ÷ 32 ≈ 53.1 t/d
If the Claus unit is 97% efficient, it makes about 49.95 × 0.97 ≈ 48.5 t/d of liquid sulfur. This only illustrates the idea: a real balance includes every stream in the refinery.
H2S hazards and safety
- H2S kills: it smells like rotten eggs at low concentration, but it paralyses the sense of smell at around 100 ppm, which is also the level immediately dangerous to life and health (IDLH). At much higher concentrations a person can collapse after a few breaths.
- Do not trust your nose: always wear a personal H2S monitor, know the wind direction and muster point, and never enter to rescue a casualty without breathing apparatus.
- Hydrogen: leaks from high-pressure units ignite easily, and hydrogen at high temperature and pressure can damage steel from the inside, known as high temperature hydrogen attack.
- Iron sulfide: when reactors and vessels are opened, FeS deposits can ignite spontaneously in air, so they are kept wet or chemically treated before opening.
- Liquid sulfur pits: liquid sulfur releases H2S into the space above it, which must be ventilated and monitored.
- Confined space work: catalyst change-out inside a reactor is confined space entry, often under a nitrogen atmosphere, and needs a permit and special procedures.
Takeaway for the engineer
Sulfur enters the refinery with the crude and does not disappear: the hydrotreater moves it from the fuel into H2S gas, the amine unit collects it, and the Claus unit turns it into saleable sulfur. Any weakness in this chain means off-spec fuel, SO2 emissions, or an H2S risk to people.
Test your understanding
Related lessons
- Conversion units: FCC, hydrocracker, reformer
- Crude distillation unit (CDU/VDU)
- Permit to Work and Job Safety Analysis for a Slickline Job: A Practical HSE Guide for Engineers and Supervisors
Sources
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