Hydrocracker vs. Hydrotreater: Picking the Right Unit

People conflate these two. They shouldn't. A hydrotreater (HDT) removes heteroatoms and saturates olefins. A hydrocracker (HCR) does that and breaks carbon-carbon bonds to convert heavy to light. Which one you need depends entirely on what you want out the other end.

One-line distinction: HDT changes the chemistry of your feed; HCR changes the boiling range. If you need the product lighter, it's HCR.

Quick Decision Matrix

Feed / Goal	Unit	Why
Naphtha desulfurization to < 1 ppmS for reformer	Naphtha HDT (NHT)	Sulfur poisons Pt/Re reformer; no conversion needed
Kerosene/jet sweetening to JP-8 or Jet A spec	Kerosene HDT	Mercaptan removal, saturation, limited hetero removal
ULSD production (ULSD 15 ppmS, Euro VI 10 ppm)	Diesel HDT (DHT) / ULSD HDT	Deep HDS, some aromatic sat; no cracking
Heavy diesel aromatics saturation for cetane lift	DHT with dearom stage (2-stage)	HDA catalyst in 2nd stage
VGO to naphtha + distillates (max gasoline or max distillate)	Hydrocracker (HCR)	Real bond breaking; conversion > 40%
Atmospheric resid or vacuum resid upgrade	Residue Hydrocracker (RDS, LC-Finer, H-Oil)	High pressure, ebullated bed; CCR-tolerant
FCC feed pretreat (lower S, N, and CCR)	FCC Feed HDT (FFHT/VGOHT)	Kills S and N ahead of catalytic cracker; no cracking wanted
Renewable diesel / SAF from triglycerides (FOG, UCO)	Renewable HDT (Hydrotreated Esters & Fatty Acids, HEFA)	Decarboxylation / HDO; isomerization downstream

Key Operating Differences

Parameter	HDT (typical)	HCR (typical)
Reactor WABT	600 - 720 F	700 - 780 F (single stage) / 750 - 830 F (2nd stage)
Reactor pressure	500 - 1200 psig (distillate HDT) 600 - 900 psig (naphtha)	1500 - 2500 psig (mild HCR) 2000 - 3000 psig (full HCR)
H2 partial pressure	400 - 900 psia	1500 - 2500 psia
H2 consumption	100 - 400 SCF/BBL	1500 - 2500 SCF/BBL (often 2000+)
LHSV	1.5 - 4.0 hr-1	0.4 - 2.0 hr-1
Cycle length	24 - 48 months	18 - 36 months; 2nd stage is the shorter
Catalyst	CoMo (HDS-selective) / NiMo (HDN, HDA)	NiMo/NiW + zeolite or amorphous SiO2-Al2O3 cracking
Conversion (to lower boiling range)	< 5 vol%	30 - 95 vol%
Product slate	Same as feed, cleaner	Lighter than feed (naphtha, kero, diesel, LPG)
Metallurgy	1-1/4 Cr - 1/2 Mo (reactor), CS piping post-reactor	2-1/4 Cr - 1 Mo - 1/4 V (reactor); SS 347/321 internals

What Actually Happens in the Reactor

Hydrotreater

Feed + H2 cross over the catalyst. Heteroatom-removal reactions dominate:

HDS (hydrodesulfurization): R-S-R' + 2H2 -> RH + R'H + H2S
HDN (hydrodenitrogenation): R-N + H2 -> R-H + NH3 - requires higher T/P, ring saturation first.
HDO (hydrodeoxygenation): R-O-H + H2 -> RH + H2O - important on renewable/biobased feeds.
HDM (hydrodemetallization): organic-Ni/V + H2 -> Ni/V sulfides on guard bed.
HDA (aromatic saturation): ArH + 3H2 -> cycloparaffin - exothermic, limited thermodynamically above 700 F.

All mild exotherms (10-25 F per stage). No significant C-C bond cleavage. Carbon skeleton is preserved.

Hydrocracker

Everything HDT does plus bifunctional cracking:

Dehydrogenation (metal site): paraffin -> olefin.
Protonation (acid site, zeolite): olefin -> carbenium ion.
Beta-scission (acid site): C-C bond break, smaller olefin + smaller carbenium.
Isomerization (acid site): skeletal rearrangement before cleavage -> high-iso paraffins.
Hydrogenation (metal site): olefin back to paraffin.

Exotherm is large: typical 40-80 F per bed, managed by H2 quench between beds. Conversion is tunable via reactor temperature. Runaway risk is real; emergency depressuring is a required safeguard.

Catalyst Selection Cheat Sheet

Service	Catalyst	Rationale
Straight HDS (naphtha, distillate)	CoMo on gamma-Al2O3	Highest HDS activity per H2 mole
Deep HDS + HDN + HDA (ULSD, kero)	NiMo on gamma-Al2O3	Better on refractory S and N; higher H2 consumption
Resid HDM + HDS guard	NiMo on alumina with large pore/macropore structure	Metal accommodation, diffusion limited
Single-stage max-distillate HCR	NiMo + amorphous SiO2-Al2O3	Lower acidity = lower gas yield, more diesel
Single-stage max-gasoline HCR	NiW/NiMo + Y-zeolite	High acidity, high conversion to light product
Mild HCR on VGO (10-40% conv)	NiMo on amorphous support	Extend cycle, moderate conversion
HEFA (renewable diesel)	NiMo pretreat + Pt/Pd dewaxing	HDO then iso for cloud point spec

Why the Pressure Matters So Much

Hydrogen partial pressure controls aromatic saturation equilibrium and catalyst coking rate. At HDT pressures (400-900 psia H2), you can't push HDA past about 50% on a heavy feed - thermodynamics caps it. At HCR pressures (1500-2500 psia), you can push past 95% HDA and break the aromatic rings.

Pressure also controls cycle length. Every 100 psia of H2 partial pressure buys roughly 3-6 months of cycle on distillate HDT, more on HCR. Every dollar spent on compressor work buys cycle length back.

Common Design Mistakes

Assuming HDT will meet future specs. 500 ppm ULSD turned into 15 ppm, then 10 ppm. Existing units often need revamp to 2-stage HDT or ULSD dearom reactor. Build in pressure margin on new units.
Sizing H2 plant for HDT only, then adding HCR. HCR H2 consumption is 5-10x HDT. Retrofit H2 is the single biggest budget miss on combined HPU+HCR projects.
Picking CoMo for a deep-HDN requirement. CoMo is weak on HDN. Use NiMo. Expect 20-30% more H2 consumption.
Treating HCR second-stage like HDT on metallurgy. Severe HTHA territory: 2-1/4 Cr - 1 Mo - 1/4 V is minimum. API 941 Nelson curves are not optional here.
Skipping silicon trap on coker naphtha feed. Antifoam silicone poisons HDT catalyst fast. A 6-ft polymer-support layer is cheap insurance.
Over-converting in HCR to meet a naphtha yield target. Catalyst activity drops steeply past 85-90% conversion; cycle collapses.

How to Sanity-Check a Proposed Unit

If someone proposes either unit, get these numbers from the engineering package before you approve:

Feed sulfur, nitrogen, metals, CCR, and endpoint. The "feed is VGO" is not a spec.
Required product sulfur. "ULSD" is ambiguous - US 15, Euro 10, CARB 15 ppm are different tests.
H2 partial pressure at reactor inlet and outlet. Watch for the recycle ratio and purity.
Catalyst cycle length assumption. 24, 36, or 48 months changes reactor size.
Hydrogen source - SMR, PSA off-gas, or imported. Hidden cost driver.
Utilities hookup - HP steam, cooling water, fuel gas, purge compression. Each one is a standalone project risk.
PSVs sized for loss-of-quench scenario. HCR is a runaway hazard; this case usually governs relief.

Bottom Line

HDT is cheap, small, and reliable - but it does not break bonds. HCR is expensive, large, runs hot, and is metallurgically demanding - but it turns heavy into light. Pick the cheaper unit that meets the actual product spec; don't spend HCR money for an HDT problem. And when you do need HCR, build the H2 plant and the metallurgy at full spec the first time. Retrofits are brutal.