Sulfiding

Chapter from the Hydroprocessing Operations Guide.

CATALYST ACTIVATION (SULFIDING) PROCEDURES

Sulfiding Overview

Hydroprocessing catalysts are delivered in oxide form (CoO-MoO3 or NiO-MoO3 on alumina). They must be converted to the active sulfide form (CoMoS or NiMoS) before exposure to hydrocarbon feed.

Sulfiding Reaction:
MoO3 + 2H2S + H2 → MoS2 + 3H2O (simplified)
CoO + H2S → CoS + H2O

Sulfiding Agents:
• DMDS (dimethyl disulfide): most common, decomposes to H2S + CH4 at ~350°F
• TNPS (di-tert-nonyl polysulfide): higher sulfur content, decomposes at higher T
• Spiked feed: feed doped with sulfiding agent
• Pre-sulfided catalyst: vendor pre-treats catalyst (ex-situ sulfiding)

Types of Sulfiding:
• Gas-phase: H2 circulation + sulfiding agent injection, dry catalyst bed
• Liquid-phase: oil circulation + sulfiding agent, catalyst wetted with oil first
Liquid-phase is now dominant — better heat management, more uniform sulfiding

Liquid-Phase Sulfiding Procedure (Typical)

Step	Temperature	Action	Key Parameters	Duration	Monitoring	Critical Limits
1. Oil wetting	Ambient → 250°F	Establish oil circulation through reactor beds	Oil flow, catalyst bed temps	4-8 hours	Uniform wetting, no channeling	ΔT across bed <25°F
2. Begin DMDS injection	250°F hold	Start sulfiding agent injection into oil	Injection rate per vendor recipe	Continuous	H2S in recycle gas (begin measuring)	Injection rate ±10% of target
3. First ramp	250 → 450°F	Ramp temperature at 25-50°F/hr	Exotherm management	4-8 hours	Bed ΔTs, H2S breakthrough	Max bed ΔT <50°F per bed
4. First hold	450°F	Hold until H2S breaks through	DMDS injection continues	4-12 hours	H2S in product gas → target >1 mol%	Do NOT advance until breakthrough
5. Second ramp	450 → 600°F	Ramp to final sulfiding temperature	Continue injection	3-6 hours	Temperature control, H2S level	Max exotherm 50°F
6. Final hold	600-650°F	Hold at final temperature for soak	Maintain H2S >1 mol%	4-8 hours	H2S stability, sulfur uptake calc	Total sulfur uptake meets target
7. Transition to feed	600-650°F	Reduce DMDS, introduce hydrocarbon feed	Smooth transition	2-4 hours	Product quality, temperatures	Maintain H2S above 500 ppm minimum

Common Sulfiding Problems

Problem	Symptom	Cause	Fix	Prevention
Delayed H2S breakthrough	No H2S in recycle after expected time	Insufficient DMDS, low temperature, channeling	Increase injection, check bed temps	Follow vendor procedure precisely
Excessive exotherm	ΔT >50°F in single bed	Too fast ramp, too much DMDS, channeling	Cut injection rate, reduce ramp rate	Control ramp and injection together
Temperature runaway	Uncontrollable temperature rise	Massive exotherm, all beds reacting at once	Emergency: cut DMDS, increase gas rate, quench	Staged ramp with holds
Incomplete sulfiding	Low total sulfur uptake	Insufficient DMDS, short hold times, bypassing	Extend hold time, increase injection	Calculate required DMDS from stoichiometry
Oil coking on catalyst	High ΔP, hot spots	Temperature too high before sulfiding complete	Lower temperature, improve sulfiding first	Sulfide at lower T before advancing

Source: Hydroprocessing_Ops_Guide_v1.xlsx · sheet “Sulfiding”

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