Hydrocracking Flashcards
What is hydrocracking?
Hydrocracking is the high-pressure hydrogenation of petroleum fractions to produce saturated, lower-boiling materials like light distillates.
What are the main objectives of hydrocracking?
- Producing light distillates from heavier crude fractions.
- Achieving high yields of saturated hydrocarbons.
What are the key reactions in hydrocracking?
- Polycyclic aromatics are partially hydrogenated rapidly.
- Naphthene rings in polycyclic compounds are opened and cracked.
- Single-ring naphthenes and paraffins resist cracking.
- Single-ring aromatics are difficult to hydrogenate.
What type of catalysts are used in hydrocracking?
Dual-function catalysts are used, providing:
- Acid Function: Supported by materials like HF-treated Montmorillonite clay, alumina, silica alumina, and zeolite.
- Hydrogenation Function: Supplied by metals like nickel (Ni), tungsten (W), platinum (Pt), or palladium (Pd).
How does nitrogen content in feedstock affect hydrocracking?
Nitrogen compounds neutralize the acid sites of the catalyst, reducing activity. Feedstocks with high nitrogen content require a denitrogenation stage.
How is heat managed during the highly exothermic hydrocracking reactions?
- Heat is removed by injecting cold recycle gas or liquid.
- Some systems use interbed exchangers, although they present design challenges under severe operating conditions.
What are the major by-products in hydrocracking of high sulfur and nitrogen feedstocks?
- Hydrogen sulfide (H2S)
- Ammonia (NH3)
How are H2S and NH3 removed from recycle gas?
Water is injected under high pressure to dissolve H2S and NH3, and the solution is processed further to recover these compounds.
What are the challenges in designing hydrocracking reactors?
- Managing high pressures and temperatures.
- Handling high concentrations of H2, H2S, and NH3.
- Maintaining reaction temperature within narrow limits using quench systems or exchangers.
How are reactors designed to minimize mechanical stress and corrosion?
- Using liners to contain the catalyst and protect the reactor shell.
- Ensuring makeup gas flows through the annulus for cooling and minimizing corrosive contact with the main shell.
What process is used for hydrogen production in hydrocracking units?
Steam reforming is the most common process, involving the reaction of hydrocarbons with steam to produce hydrogen.
What catalyst is used in steam reforming?
Nickel-based catalysts, sometimes promoted with alkaline substances to prevent carbon deposition.
How is CO and CO2 removed in hydrogen production?
- CO is converted to CO2.
- CO2 is removed using absorption in an amine solution.
- Residual CO and CO2 are eliminated through methanation.
How is sulfur removed from feedstocks for steam reforming?
- Desulfurization for naphthas involves H2 treatment over Co-Mo catalysts.
- LPG feedstocks with low sulfur content are treated with ZnO
Why is heat transfer important in steam reforming furnaces?
The reforming reaction is highly endothermic. Uniform heat distribution is crucial to avoid hot spots that could damage catalyst or tubes.
