A hydrotreating unit sits in front of the processes that are least forgiving of impurities. Its task is straightforward to describe: contact a hydrocarbon stream with hydrogen over a catalyst at elevated temperature and pressure so that heteroatoms become removable compounds. Sulfur becomes hydrogen sulfide, nitrogen becomes ammonia, oxygen becomes water, and organometallic species deposit as metal sulfides on the catalyst. Depending on the service, olefins and part of the aromatic content may also be hydrogenated, which changes stream quality beyond contaminant removal.
What makes hydrotreating interesting is the matching. Two refineries can run similar units on similar feeds and still need different catalysts, different operating envelopes and different cycle strategies. Selecting a hydrotreating catalyst is a comparison between what the feed actually contains, what the unit can physically sustain, and what the downstream process needs the treated stream to be.
What Hydrotreating Is Designed to Achieve
Most units serve several of the following objectives at once:
- Specification compliance. Meeting a sulfur target in a product or intermediate, whether set by fuel specification, product slate or a downstream catalyst’s tolerance limit.
- Downstream protection. Reforming, isomerisation and other sensitive processes are poisoned or deactivated by sulfur, nitrogen and metals. Pretreatment protects the catalysts that follow.
- Product quality and stability. Colour, odour, oxidation stability, smoke point and aromatic content all respond to how far the reaction is pushed.
These objectives interact: deeper desulfurisation costs hydrogen and temperature, so a unit optimised for maximum cycle length may not deliver the tightest specification. Catalyst choice determines how much freedom the operation has to move between them over a run.
Why Feed Composition and Operating Objectives Matter
Selection begins with the feed, which is rarely as simple as its name suggests.
- Sulfur speciation. Mercaptans, sulfides and thiophenic compounds respond differently to the same catalyst under the same conditions, so total sulfur alone is not enough information.
- Nitrogen content and type. Basic nitrogen species inhibit hydrodesulfurisation activity and often limit how much desulfurisation is achievable at a given temperature.
- Boiling range and endpoint. Light, heavy and full range naphtha present different diffusion and residence-time problems and behave differently in a fixed bed.
- Metals and fines. Silicon, arsenic and iron accumulate on the catalyst and can end a cycle early if they are not anticipated.
- Hydrogen availability. Partial pressure and recycle gas purity often constrain achievable severity more tightly than the catalyst itself.
On the operating side the questions are practical: what temperature and pressure can the unit sustain safely, is the objective throughput, cycle length or deepest desulfurisation, and is regeneration available or is the catalyst replaced each cycle? Each answer narrows the field before any vendor comparison begins.
Activity, Selectivity, Stability and Lifecycle
Four properties are weighed against one another when a catalyst is evaluated.
Activity is how much conversion a given volume achieves at stated conditions. A more active catalyst can run at lower temperature for the same result, which usually slows deactivation, or deliver more conversion in a fixed volume. Activity is always quoted against a test condition, so it should be read alongside the temperature and space velocity behind it.
Selectivity is what else happens besides the target reaction. Excessive cracking reduces liquid yield, and hydrogenation beyond what the product requires increases hydrogen consumption. Selectivity keeps a hydrotreater from solving one problem by creating another.
Stability is behaviour over time. Coke laydown, metals accumulation and pore blockage reduce activity gradually, and the shape of that decline determines whether a cycle can be planned or must be managed reactively. Pressure drop growth frequently sets the real limit in units handling dirty or cracked feeds.
Lifecycle is the commercial dimension. Cost includes the material, loading labour, the start-up and sulfiding programme, the value of hydrogen consumed, and eventual regeneration, replacement and disposal. A cheaper catalyst that costs days of lost production may be the more expensive choice.
At a high level the industry families are well established: cobalt-molybdenum for conventional desulfurisation, nickel-molybdenum where higher nitrogen removal or greater severity is needed, and nickel-tungsten where aromatics saturation or very deep desulfurisation matters. Pore structure, support and forming method distinguish products within those families.
Matching the Catalyst to the Application
Different applications place emphasis on different properties.
Kerosene Hydrotreating (KHT)
Kerosene hydrotreating targets sulfur removal and quality improvement in the kerosene and jet boiling range, where smoke point, colour and thermal stability matter alongside the sulfur number. Endpoint control is important, because heavier material in the feed raises the severity required and shortens the achievable cycle.
Gasoline Hydrotreating (GHT)
Gasoline hydrotreating spans straight-run upgrading and the treatment of cracked streams. Olefin retention is often a consideration, since hydrogenating olefins reduces octane, which makes selectivity as important as activity.
Naphtha Hydrotreating (NHT)
Naphtha hydrotreating is typically a pretreatment step protecting reforming or isomerisation catalysts downstream. The feed defines the variant:
- LNHT (Light Naphtha Hydrotreating) handles the lightest cut, where the target is very low residual sulfur and nitrogen and the material is destined for isomerisation.
- HNHT (Heavy Naphtha Hydrotreating) operates on the heavier fraction, where higher temperatures are required and the protection duty for reforming is critical.
- FRNHT (Full Range Naphtha Hydrotreating) treats the whole naphtha boiling range in one unit, satisfying the light-end specification and the heavy-end duty simultaneously. That usually calls for a carefully considered catalyst loading and a clear view of the temperature profile.
Hydrodesulfurization (HDS) and Gasoline Kerosene Hydrodesulfurization (GKHDS)
Hydrodesulfurization is the general framing: sulfur removal from a defined stream at conditions set by the application. GKHDS describes a combined duty covering the gasoline and kerosene boiling ranges in one processing route. In both, the balance between activity and selectivity, achievable cycle length and sensitivity to feed change are the questions that matter during selection.
Loading, Start-Up and Monitoring
Performance is not determined by the material alone. How it is charged and commissioned frequently decides whether test results are reproduced in the unit.
Loading requires uniform, dense and consistent filling. Segregation of catalyst and inert material, wall channelling, or uneven reactivity between the top and bottom of the bed all produce temperature profiles that changing operating conditions cannot correct. Inlet grading captures fouling before it reaches the main catalyst.
Start-up is where most avoidable damage occurs. Moisture and carbon dioxide must be removed before high temperature is applied, and activation — usually sulfiding with an agent such as DMDS — needs controlled temperatures, controlled hydrogen partial pressure and sufficient circulation. A rushed sulfiding programme risks an under-activated catalyst and a shorter first cycle.
Once on stream, monitoring determines how well the cycle is managed. Reactor temperature profile, weighted average bed temperature, hydrogen partial pressure, recycle gas hydrogen sulfide concentration, pressure drop and product analyses together show whether the catalyst is performing to expectation and whether the unit can be pushed or should be conserved.
Why Technical Consultation Matters
Hydrotreating catalyst selection resists a simple catalogue answer because the inputs are plant-specific. A feed analysis from last month may not describe today’s slate, and a catalyst that solved one unit’s problem may underperform in another because hydrogen supply, reactor internals or operating philosophy differ.
A structured technical review — feed and process data assessment, definition of objectives, catalyst and grading selection, then a loading, start-up and monitoring plan the plant team can execute — usually surfaces these issues before they appear as lost run length, and produces documentation the operations team can rely on through commissioning.
Sarv Catalyst supplies hydrotreating catalysts across these applications, including KHT, GHT, NHT in its light (LNHT), heavy (HNHT) and full range (FRNHT) forms, HDS and GKHDS duties. Selection is supported by consultation, loading and unloading planning, commissioning assistance and monitoring through the cycle, described in our note on catalyst lifecycle support. For refineries whose main conversion unit drives the catalyst strategy, FCC and RFCC catalyst considerations cover the downstream side of the same question.
Review the range at the portfolio section of the homepage or bring your process conditions to the technical team through the consultation route.