HDT Fundamentals

Chapter from the Hydroprocessing Operations Guide.

HYDROTREATING FUNDAMENTALS

Hydrotreating Reaction Chemistry

Hydrotreating removes heteroatoms (S, N, O, metals) and saturates olefins and aromatics using hydrogen over a catalyst at elevated temperature and pressure.

Primary Reactions (all consume H2, all exothermic):
• Hydrodesulfurization (HDS): R-S + H2 → R-H + H2S
Mercaptans > sulfides > thiophenes > benzothiophenes > dibenzothiophenes
Refractory sulfur (4,6-DMDBT) requires severe conditions
• Hydrodenitrogenation (HDN): R-N + H2 → R-H + NH3
Harder than HDS; requires ring saturation first
• Hydrodeoxygenation (HDO): R-O + H2 → R-H + H2O
Important for renewable feedstocks (triglycerides, fatty acids)
• Olefin Saturation: R=R + H2 → R-R
Very fast, highly exothermic — dominates in FCC naphtha hydrotreating
• Aromatics Saturation: aromatic + 3H2 → naphthene
Equilibrium-limited at high temperature; favored by high P, low T
• Hydrodemetallization (HDM): removes Ni, V from heavy feeds
Deposits on catalyst — irreversible deactivation

Key Process Variables

Variable	Effect of Increasing	Typical Range	Operational Consideration	Measurement
Temperature (WABT)	Increases conversion (HDS, HDN)	550-800°F	Higher T = faster deactivation	Reactor thermocouples, WABT calc
H2 Partial Pressure	Increases conversion, reduces coking	200-2,500 psig (depends on service)	Higher P = higher H2 consumption, better stability	System pressure × H2 purity
LHSV (Liquid Hourly Space Velocity)	Lower LHSV = more conversion	0.5-6 hr⁻¹	Lower LHSV = larger reactor or less throughput	Feed rate / catalyst volume
H2/Oil Ratio	Higher ratio = better H2 distribution	500-5,000 SCF/BBL	High ratio = more compressor power	Gas rate / liquid rate
Feed Quality (S, N, metals)	Higher impurities = more severity needed	Variable	Heavier feed = more H2 consumption	Lab analysis (S, N, metals, sim dist)
H2 Purity	Higher purity = more H2 partial pressure	70-99.9%	Purge to maintain purity, PSA recovery	Recycle gas chromatograph

Catalyst Types

Catalyst Type	Active Metals	Application	Operating Range	Regenerable?	Typical Life	Notes
CoMo/Al2O3	Cobalt-Molybdenum	HDS (sulfur removal primary)	Low-moderate severity	Yes (ex-situ)	2-5 years	Most common HDT catalyst
NiMo/Al2O3	Nickel-Molybdenum	HDN + deep HDS + aromatics sat	Moderate-high severity	Yes (ex-situ)	2-5 years	Better HDN than CoMo
NiW/Al2O3	Nickel-Tungsten	Aromatics saturation	High severity, high H2 PP	Limited	2-4 years	Specialty, high activity
CoMo/Al2O3 (high activity)	Cobalt-Molybdenum (proprietary support)	ULSD production	Moderate severity	Some grades	3-5 years	Type II active sites
HDM catalyst	Mo on large-pore alumina	Metals removal (front guard)	Low-moderate T	No (metals pore plugging)	6-18 months	Sacrificial, high metals capacity
Grading catalyst	Low activity, shaped catalyst	Pressure drop control, scale trapping	Reactor inlet	No	Replaced with main catalyst	Various shapes (rings, stars)

Source: Hydroprocessing_Ops_Guide_v1.xlsx · sheet “HDT Fundamentals”

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