HCK Fundamentals

Chapter from the Hydroprocessing Operations Guide.

HYDROCRACKING FUNDAMENTALS

Hydrocracking Configurations

Configuration	Description	Conversion	Product Slate	Advantages	Limitations	Typical Application
Single-stage once-through	One reactor section, unconverted oil out	40-70%	Middle distillate + UCO	Simple, lower cost	Lower conversion, UCO must be used	Mild HCK, fuel oil reduction
Single-stage with recycle	One reactor + fractionator + recycle of UCO	90-99%	Maximum middle distillate	Near-complete conversion	Higher H2, larger reactor/frac	Maximum diesel/jet production
Two-stage	First stage: HDT/mild HCK; second stage: HCK	95-99+%	Flexible: naphtha, jet, diesel	Highest flexibility & conversion	Most complex, highest capital	Maximum flexibility, large scale
Series flow	Two reactors in series, single loop	80-95%	Balanced naphtha/distillate	Moderate complexity	Less flexible than two-stage	Intermediate conversion target
Reverse staging	UCO recycled to first stage (not second)	95+%	Heavy product quality optimization	Better first-stage catalyst life	Complex fractionation	Specialty configurations

Conversion & Selectivity Relationships

Conversion Definition:
Conversion (%) = (Feed 700°F+ - Product 700°F+) / Feed 700°F+ × 100
(Cut point may be 650°F or 700°F depending on licensor definition)

Key Relationships:
• Selectivity = desired product yield / conversion
- Selectivity decreases with increasing conversion (overcracking)
- Low conversion (40-60%): very high middle distillate selectivity
- High conversion (>90%): more naphtha and gas production

• Temperature Effect:
- Higher temperature → higher conversion but LOWER selectivity
- Each 10°F increase ≈ 3-5% conversion increase (depends on feed)
- Overcracking produces excess gas and light naphtha

• Space Velocity Effect:
- Lower LHSV → higher conversion AND better selectivity (longer contact time)
- Optimal balance between conversion and overcracking

• H2 Pressure Effect:
- Higher H2PP → less coking, longer catalyst life
- Moderate effect on conversion directly, but enables higher temperature operation

• Fresh Feed Properties:
- Nitrogen content is the #1 conversion inhibitor (poisons acid sites)
- Higher nitrogen → higher WABT required for same conversion
- Polynuclear aromatics are harder to crack than paraffins

Source: Hydroprocessing_Ops_Guide_v1.xlsx · sheet “HCK Fundamentals”

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