Hydrofluoric Acid (HF) Alkylation remains one of the most important processes in petroleum refining, enabling the production of high-octane, clean-burning alkylate. Alkylate is a premium gasoline blending component, valued for its high Research Octane Number (RON >95), low Reid Vapor Pressure (RVP) and absence of sulfur and aromatics. In a global market driven by stricter fuel specifications and environmental regulations, alkylation continues to play a central role in ensuring fuel quality However, HF alkylation comes with challenges.
HF is highly toxic and corrosive; even small releases form dense vapor clouds that can cause catastrophic consequences. Safe and reliable operation requires strict adherence to metallurgy standards. In particular, Low Residual Element (Low RE) carbon steels are mandated by API RP 751 and NACE standards, while nickel alloys such as Monel 400 are used in HF+water service or high-velocity zones.
At the same time, global steelmaking is shifting away from ore-based blast furnace steel (naturally low in residual elements) to scrap-based Electric Arc Furnace (EAF) steel, which has higher levels of copper, nickel and chromium. This poses significant risks to HF units, where even small deviations in chemistry can compromise integrity. Supply chain fragmentation in fittings and forgings, combined with high cost premiums for compliant materials, further complicates procurement.
Looking forward, two major forces will shape the future of HF alkylation metallurgy:
(1) the rise of green steel technologies, particularly hydrogen-based Direct Reduced Iron (H₂-DRI), which offer both decarbonization and low residual element control; and (2) alternative alkylation processes, such as Chevron’s ISOALKY™ ionic liquid technology, which could reduce reliance on HF altogether.
This white paper explores the HF alkylation process, hazards, metallurgy requirements, material selection, steelmaking trends, supply chain challenges and the future outlook for this critical refining technology.
Alkylation is a cornerstone refining process that converts light olefins such as propylene and butylene into alkylate by reacting them with isobutane in the presence of a strong acid catalyst. The resulting alkylate has a high Research Octane Number (RON >95), very low Reid Vapor Pressure (RVP) and contains no sulfur or aromatics. These properties make it a highly desirable blending component for gasoline, especially in regions where environmental regulations mandate clean, low-sulfur and low-aromatic fuels.
Alkylation was first developed in the late 1930s and deployed on a large scale during World War II to produce aviation gasoline. Since then, it has become a mainstay in refineries around the world. Three major technologies dominate the landscape:
Sulfuric Acid Alkylation: Widely applied, easier metallurgy, but higher acid consumption.
Hydrofluoric Acid (HF) Alkylation: Lower acid consumption, higher octane, but more hazardous.
Ionic Liquid Alkylation: Emerging technology (e.g., Chevron ISOALKY™), offering potential safety and environmental benefits.
While sulfuric acid technology accounts for the majority of installed capacity worldwide, HF alkylation remains significant. Its advantages include reduced catalyst consumption, higher alkylate octane and operational efficiency. However, the use of HF introduces significant risks due to its toxicity and volatility, making safe design and metallurgy selection critical.
The HF alkylation process can be divided into three key sections:
Olefins (propylene, butylene) are mixed with excess isobutane in the presence of HF acid catalyst.
Reactions are conducted at 16–38 °C and moderate pressures.
High isobutane-to-olefin ratios are maintained to suppress polymerization and maximize alkylate yield.
Reactor effluent is distilled into product streams.
The alkylate product is separated from propane, butane and recycled isobutane.
Fractionation also separates polymeric byproducts and acid-soluble oils (ASO).
Propane and butane streams undergo defluorination using alumina beds.
This removes organic fluorides and residual HF, preventing downstream corrosion.
Several licensors provide HF alkylation technology, including:
UOP (Honeywell) – The most widely licensed HF alkylation process.
ExxonMobil – Variations on reactor and circulation design.
ConocoPhillips – Proprietary designs with enhanced safety features.
Produces superior-quality alkylate with higher octane.
Requires significantly less catalyst compared to sulfuric acid units.
Handles a wide range of feedstocks.
HF is highly hazardous; leaks pose catastrophic risks.
High metallurgy requirements and safety systems increase capital cost.
Regulatory and community pressures are rising against HF use.
HF acid is extremely toxic. When released, it forms dense, ground-hugging vapor clouds capable of traveling long distances. Even short exposures at low concentrations can be fatal. HF also penetrates skin and reacts with calcium in the blood, causing life-threatening hypocalcemia.
Because of these hazards, HF units require multiple layers of protection, including:
Specialized metallurgy (Low RE steels and nickel alloys).
Leak detection systems to rapidly identify releases.
Water spray systems to absorb and knock down HF vapor clouds.
Emergency response planning and training for operators.
Several incidents over the decades have highlighted the potential consequences of HF leaks. These incidents have shaped regulatory standards, particularly API RP 751, which serves as the principal guideline for HF alkylation unit safety and metallurgy.
HF presents multiple corrosion risks, including:
Uniform corrosion of steels in HF environments.
Hydrogen blistering and embrittlement, especially in the presence of moisture.
Localized attack promoted by copper, nickel, chromium and molybdenum.
To mitigate these risks, Low Residual Element (Low RE) carbon steels are required. API RP 751 and NACE Paper 03651 specify limits on residual elements.
|
Element |
Maximum Limit (wt%) |
|---|---|
|
Copper (Cu) |
0.15 |
|
Nickel (Ni) |
0.20 |
|
Chromium (Cr) |
0.15 |
|
Molybdenum (Mo) |
0.03 |
|
Combined (Cu+Ni+Cr+Mo) |
0.25 |
|
Sulfur (S) |
0.02 |
|
Carbon (C) |
0.30 |
These requirements apply to both base metal and weld consumables.
Material Test Reports (MTRs) with ladle and product analysis.
Positive Material Identification (PMI) with XRF or LIBS analysis.
Welding procedures qualified with Low RE filler metals.
Inspection programs during fabrication and turnaround.
Different equipment within an HF alkylation unit requires different materials depending on service conditions.
|
Service / Equipment |
Recommended Material |
Notes |
|---|---|---|
|
Vessels & piping |
Low RE carbon steel |
Standard for most HF service |
|
HF + water zones |
Monel 400 |
Excellent resistance to HF–H₂O |
|
Heat exchanger tubes |
Monel 400 / Inconel 600 |
Prevents thinning and pitting |
|
Condensers (dry HF) |
Low RE carbon steel |
Safe if HF remains dry |
|
Condensers (wet HF) |
Monel 400 |
Prevents localized corrosion |
|
Flanges & fittings |
Low RE CS / Monel trims |
Prevents localized attack |
|
Pumps |
Monel 400 |
Reliable under velocity |
|
Storage tanks |
Low RE CS + lining |
Brick or alloy lining in bottoms |
|
Instrumentation |
Monel / Inconel |
Reliability for small wetted areas |
|
Gaskets |
PTFE, graphite |
Resist HF permeation |
Stainless steels are generally not used in HF service because they are rapidly attacked.
The availability of Low RE steels is directly tied to steelmaking routes.
Uses iron ore and coke as feedstock.
Naturally produces steels with very low residual elements.
Emits ~2.0–2.2 tonnes CO₂ per tonne of steel.
Traditionally supplied most pipeline-grade steels.
Uses scrap steel and/or DRI/HBI.
Much lower CO₂ emissions (~0.4–0.6 tCO₂/t steel).
Higher residual elements due to scrap input.
Growing share of global steel capacity.
|
Feature |
BF–BOF |
EAF |
|---|---|---|
|
Feedstock |
Ore + coke |
Scrap + DRI/HBI |
|
Energy |
Coal (coke) |
Electricity |
|
Scale |
Large (>3 Mt/y) |
Flexible (0.5–2 Mt/y) |
|
Residuals |
Low |
High risk |
|
CO₂ emissions |
2.0–2.2 t/t |
0.4–0.6 t/t |
|
Decarbonization |
Difficult |
Easier (renewables) |
The shift from BF–BOF to EAF threatens the reliable supply of Low RE steels for HF service.
Ensuring consistent availability of Low Residual Element (Low RE) materials for HF alkylation units has become one of the most difficult aspects of refinery asset management. Historically, refiners could rely on major pipe and fitting suppliers whose steelmaking practices naturally aligned with supplemental Low RE specifications. Today, structural changes in the steel industry - particularly the shift from blast furnace/basic oxygen furnace (BF–BOF) to scrap-based electric arc furnace (EAF) production - have disrupted this balance. The following subsections review the challenges across major product categories.
In the past, most seamless pipe produced in North America and Europe originated from BF–BOF operations. Mills such as those in Germany, Brazil and the United States routinely delivered pipe that met the chemistry limits of ASTM A333 S2 and A106 S9 without refiners even needing to specify the supplemental restrictions. This was because ore-based steelmaking inherently resulted in low residual element content.
Over the last decade, however, several large mills have exited the seamless pipe business or idled capacity. Remaining producers are increasingly EAF-based, relying heavily on scrap feedstock. Scrap introduces elevated copper, nickel and chromium - precisely the elements that HF alkylation metallurgy aims to limit. This means that merely reviewing MTRs and performing PMI at the fabrication shop no longer guarantees compliance. Residual element control now has to be secured at the melt stage under binding purchase specifications.
EAF-based producers will supply Low RE heats, but only with large melt commitments. Typical minimums range from 100–400 tons per grade, with further minimums of 10–20 tons for each diameter and schedule. To illustrate: ordering 40 feet of 2” Schedule 160 pipe could require committing to several thousand feet, with a total purchase value approaching half a million dollars.
For distributors, this level of overbuy represents years of stock, tying up both cash and storage. Unsurprisingly, Low RE pipe commands a steep premium and many suppliers now prefer to outsource production on a project-by-project basis instead of holding inventory.
Modern mills often optimize by producing multiple sizes from a single billet diameter, simply varying billet length. While this works when pipe is ordered to supplemental Low RE specs, it is not valid when trying to qualify off-the-shelf stock by PMI. The implication is that true compliance can only be ensured when the pipe order is tied back to the melt.
Despite these challenges, compliant seamless pipe is still available from select mills in Europe and Asia (e.g., Tenaris, Nippon Steel), but refiners must plan early, budget for premiums and enforce tight QA/QC.
Among all components in HF alkylation units, butt-weld fittings are consistently the most difficult to source in compliant Low RE grades. Unlike seamless pipe, which is produced in large melts where chemistry can be carefully controlled, fittings are made in relatively small batches and across a wide variety of diameters and wall thicknesses. A 6-inch elbow, for example, may come from a completely different billet and forging route than a 6-inch reducer, making chemistry uniformity difficult to ensure.
Most fittings manufacturers operate on commodity billet purchases rather than melts specifically ordered to HF-grade supplemental requirements. In practice, this means billets are typically verified against broad ASTM chemistry ranges (e.g., A234 WPB) but not specifically to A106 S9 or A333 S2 levels. Unless a buyer explicitly stipulates supplemental clauses like A960 S50 (product analysis), there is no obligation for the forge to certify chemistry on the finished item. This creates major traceability gaps, especially when fittings are procured through multiple distributors.
Over the last decade, owner companies have introduced stricter chemistry requirements for fittings used in HF and other corrosive services. European operators, for instance, now impose cumulative limits on copper, nickel and chromium, even when not explicitly required by ASTM. Some North American refiners additionally insist on post-forge heat treatments such as normalization to ensure metallurgical stability. These requirements significantly reduce the number of fittings that qualify, since most commodity production is not normalized and does not control residuals tightly.
Korea and Japan: A few forging shops in these countries can certify normalized fittings with low residual chemistry, especially in sizes ≤12” and in standard or extra heavy wall thickness.
India and Middle East: While large volumes of commodity fittings originate here, residual element control is inconsistent and PMI failures are common when applying HF specifications.
Europe: Smaller boutique forges sometimes supply high-quality fittings but at much higher costs and with long lead times.
The result is that for larger diameters (≥16”) or heavy-wall fittings (SCH 120 and above), refiners often find that only one or two viable sources exist globally, creating long lead times and price premiums.
Because each fitting size and schedule often corresponds to a separate heat or forging batch, the probability of encountering non-compliant chemistry in at least part of an order is high. Even with PMI screening, the process becomes a game of “selection by elimination,” and the risk of scrap, delays and cost overruns is greatest in this category. For this reason, butt-weld fittings are often considered the weakest link in the Low RE supply chain for HF alkylation units.
In contrast to fittings, flanges have become one of the more manageable categories to procure in compliant Low RE grades. The reason lies in forging economics: the majority of 2–24 inch flanges can be produced from a relatively small number of billet sizes, typically five or six, which consolidates heats and improves traceability.
When billets are sourced from ore-based BF–BOF production routes, residual element levels are naturally low and compliance with HF alkylation metallurgy is relatively straightforward. Several European and North American forges have historically supplied Low RE flanges that consistently met Cu+Ni+Cr cumulative limits of ≤0.15%.
However, with the global shift toward EAF steelmaking, variability is increasing. Billets produced from scrap-based operations may show elevated residuals, which cannot always be screened out by PMI alone. This has led many refiners to adopt stricter purchasing specifications, requiring both ladle and product analyses, along with explicit supplemental clauses under A961 (e.g., S62 for LF2 flanges).
Europe: Specialty forges in Italy and Spain continue to produce normalized Low RE flanges, though volumes are limited.
North America: Some distributors maintain inventory of A105 normalized flanges tested to HF service limits, with acceptable toughness down to –20 °F.
Asia: Chinese and Indian flange producers dominate volume supply, but traceability is variable. Refiners often flag imported A105 flanges as high risk unless ordered to a tailored Low RE spec.
In general, flanges present fewer challenges than fittings because fewer heats are required to cover the dimensional range. Still, procurement must include chemistry certification, normalization and impact testing where specified, especially for LF2 grades.
Forged steel fittings: including socket weld couplings, threaded unions and small-diameter tees - occupy a middle ground in the Low RE supply chain. When purchased in large consolidated orders, forged fittings can be produced with reasonable traceability. For example, several Korean and Italian shops can cover a full range of ½–2 inch forged fittings using as few as half a dozen heats, reducing variability and PMI burden.
However, most forged fittings are not ordered this way. The market is dominated by small, job-specific orders for specific SKUs. Because demand is fragmented across thousands of part numbers (olets, couplings, crosses, etc.), production is spread across smaller forges and machine shops worldwide. These orders often rely on billets from EAF-based commodity production and traceability to the melt is limited.
Lead times vary widely - from a week for commodity couplings to several months for specialty olets. In this fragmented landscape, refiners have learned that QA/QC discipline must be highest for forged fittings, particularly when sourced from regions with weaker traceability practices.
Korea/Europe: Well-regarded for consistent billet sourcing and normalized products, especially in bulk ranges.
India/Asia: Strong volume supply but significant variability in chemistry; PMI essential.
North America: Production capacity is limited, but distributors sometimes carry long-term inventory certified to supplemental specs.
In HF service, forged fittings are not as universally problematic as butt-weld fittings, but they remain a medium-risk category, particularly where procurement relies on spot buys rather than planned, consolidated orders.
Valves represent a unique challenge because they are both forged and cast products, each route carrying different risks.
Forged carbon steel valves (e.g., ASTM A105N) face the same billet sourcing issues as other forged products. For small sizes (≤1 inch), billets can be shared across multiple product categories, spreading the tonnage and making compliance easier. The challenge arises with larger flanged valves (≥1½ inch), where each forging die requires billet diameters above 200 mm. Only a handful of mills globally can supply Low RE billets in these sizes and minimum heats are often several tons - impractically large when the final valve order may weigh only a few hundred kilograms. This mismatch leads to long lead times and high premiums.
Cast valves present different issues altogether. The primary concerns are inclusions, porosity and shrinkage defects. These are magnified in HF service, where even minor flaws can accelerate corrosion and lead to catastrophic failures. Owner companies and licensors have responded by tightening qualification requirements for cast valves, including radiographic inspection, ultrasonic testing and stricter acceptance criteria for porosity.
Industry groups, including an MSS task force, are actively working on HF valve quality. Recent initiatives include:
Development of welded-bonnet forged valves, which reduce casting dependence.
Expansion of forged valve ranges to 4 inches, improving billet efficiency.
Introduction of forged Monel valves up to 4 inches, offering higher corrosion resistance where economics justify.
Italy and Germany: Known for high-quality forged and cast valve production, though expensive and often backlogged.
China: High-volume producer but traceability and defect rates remain a concern.
North America: Smaller specialty shops are filling niche HF valve requirements, often at premium prices.
For valves, the greatest risks lie in larger sizes and custom designs, where billet or casting supply is thin. Smaller bore forged valves are increasingly manageable thanks to new product developments, but for larger HF valves, planning horizons of 12–18 months are often necessary to secure compliant supply.
|
Category |
Availability |
Cost Impact |
Risk Level |
Notes |
|---|---|---|---|---|
|
Pipe |
Still available but costly |
Very high (large melts, premiums) |
Medium |
Must lock orders early, melt-level control required |
|
Butt-Weld Fittings |
Very limited, especially in larger diameters |
Highest |
High |
Weakest link; PMI failures common |
|
Flanges |
Moderate; fewer billet sizes |
Moderate |
Low–Medium |
Easier to manage, but EAF billets need strict checks |
|
Forged Fittings |
Moderate for bulk sizes; poor for specials |
Variable |
Medium |
Traceability weaker for job orders |
|
Valves |
Scarce in larger forged/cast sizes |
Very high |
High |
Lead times 12–18 months for HF-grade valves |
Given the increasing complexity of securing Low RE steels for HF alkylation units, refiners must treat metallurgy sourcing as a strategic discipline rather than a transactional purchase. The following measures are recommended:
Engage Early with Mills and Distributors
Secure melt commitments years in advance of planned turnarounds.
Build long-term partnerships with mills known to maintain strong Low RE practices (including select European, Korean and Japanese producers).
Specify Supplemental Requirements in Contracts
Always order to A106 S9, A333 S2 or A960 S50/S78 where applicable.
Require product-level analysis in addition to ladle analysis.
Explicitly define Cu+Ni+Cr+Mo cumulative limits in purchase specs.
Implement Rigorous QA/QC and PMI
Require 100% PMI testing on fittings, flanges and forged components.
Use third-party inspectors at the melt stage for high-value orders.
Track and archive MTRs with heat-level traceability.
Prioritize High-Risk Categories
Recognize that butt-weld fittings and valves pose the greatest risk of PMI failures.
Allocate extra budget and lead time to these items.
Where possible, standardize fitting and valve sizes to reduce SKU fragmentation.
Plan for Premium Costs and Inventory Strategy
Accept that Low RE materials will command significant premiums.
Consider pooling purchases with other units or across corporate sites to meet melt minimums.
Distributors should balance inventory holding costs vs. outsourcing risks.
Adopt Multi-Sourcing Strategies
Avoid reliance on a single global supplier.
Maintain relationships with at least two qualified sources for each major category (pipe, fittings, flanges, valves).
Monitor Industry Trends
Track steelmaking capacity shifts, especially mills transitioning from BF–BOF to EAF.
Stay current with owner standards (Shell MESC, API RP 751 updates, MSS task force work on HF valves).
The challenges outlined in this section - from melt-level residual element control to the traceability gaps in fittings and valves - underline why specialized distributors are essential to safe HF alkylation operations. The Buhlmann Group has positioned itself as one of the few PVF distributors with a dedicated HF materials program, offering not just inventory but a technical assurance system that reduces metallurgical risk for refiners.
Buhlmann maintains inventory and supply agreements for a wide range of products qualified to HF supplemental specifications:
Seamless Pipe, Flanges and Fittings
Sizes: ½–24”
Pressure Classes: 150#, 300#, 600#
End Connections: Flanged, threaded, socket weld
Gate, Globe and Check Valves
Sizes: ½–24”
Classes: 150#–800#
End Connections: Flanged, threaded, socket weld
Plug Valves and Triple Offset Butterfly Valves
Sizes: ½–24”
Classes: 150#, 300#, 600#
End Connections: Flanged, threaded, socket weld
ASTM A106 Gr. B S9 – HF-N carbon steel pipe
ASTM A333 Gr. 6 S2 – low-temperature HF-N carbon steel pipe
ASTM A234 WPB & A960 S78 – HF-N butt-weld fittings
ASTM A420 WPL6 & A960 S78 – HF-N low-temp fittings
ASTM A105 & A961 S62 – HF-N carbon steel flanges and forgings
ASTM A350 LF2 & A961 S62 – HF-N carbon steel flanges and forgings
These grades align with the NACE Paper No. 03651 specification for HF service and with ASTM supplemental requirements referenced in API RP 751.
Because slight variations in copper, nickel or chromium can dramatically increase corrosion rates in HF service, Buhlmann operates an in-house quality assurance process that goes beyond standard distributor practice:
Dimensional inspections at receiving and before dispatch.
Certificate verification (ladle and product analysis).
Spectral analysis and PMI testing on every batch, using advanced portable analyzers capable of measuring down to 0.01% residual levels.
Heat traceability maintained through ERP systems, linking MTRs directly to customer deliveries.
Normalization and additional heat treatments offered through partnered forges where required by end-user standards.
Through its subsidiaries Unicorn and NRG Special Products, Buhlmann operates dedicated forging and machining facilities. These produce:
Reinforced branch outlet fittings, machined directly from HF-grade bar material.
Custom forgings for flanges and fittings where standard stock cannot meet chemistry limits.
Small-batch runs with product analysis and normalization per ASTM A960 S50/S78.
This vertical integration allows Buhlmann to control material from billet through finished component - something particularly valuable for fittings and valves, where traceability is often weakest.
Several leading refineries in Europe, North America and the Middle East have already qualified Buhlmann as a supplier for HF units. These refiners have emphasized two aspects in particular:
Consistent compliance with Low RE chemistry requirements across multiple product categories.
Reduced procurement risk during turnarounds, as critical components are already stocked and verified rather than being hunted on the spot market.
The value of a specialized distributor lies not only in product range but in acting as a quality gatekeeper:
Filtering out non-compliant heats before they enter refinery service.
Offering PMI-backed documentation packages aligned with refinery QA/QC programs.
Supporting engineering, procurement and turnaround planning with material expertise.
By aligning inventory, in-house forging and advanced PMI, the Buhlmann Group reduces the uncertainty that has come to dominate the Low RE supply chain. For refiners, this translates to higher confidence in metallurgy, safer operations and lower long-term risk.
Two major trends will shape HF alkylation metallurgy in the coming decades:
Green Steel
Hydrogen-based DRI (H₂-DRI) offers the ability to produce low RE steels directly from ore with minimal CO₂ emissions.
Early projects (e.g., in Europe) show promise for scaling this technology.
Alternative Alkylation Technologies
Ionic liquid alkylation (e.g., ISOALKY™) offers a safer alternative.
While HF units will remain dominant for decades, gradual replacement may reduce the risks associated with HF metallurgy.
Hydrofluoric acid alkylation remains one of the most important processes in refining, but it is also among the most demanding in terms of metallurgy and supply chain assurance. With the steel industry shifting away from BF–BOF production to scrap-based EAF operations, refiners can no longer assume that commodity pipe, fittings and valves will naturally comply with HF residual element restrictions. Proactive planning, supplemental specifications and robust QA/QC programs are essential to mitigating this risk.
Engage mills and distributors early to secure compliant melts.
Specify supplemental requirements such as A106 S9, A333 S2 and A960 S78/S62 at the contract stage.
Implement rigorous PMI testing across all categories, with particular focus on fittings and valves, where failures are most common.
Accept that compliant materials will carry a premium and plan procurement strategies accordingly.
Develop multi-source strategies to avoid reliance on a single global supplier.
Looking forward, the evolution of green steel technologies (H₂-DRI) may help align decarbonization goals with Low RE requirements, but these solutions are still years away from scale. In the interim, the safest approach is to partner with specialized distributors that maintain dedicated HF programs.
Distributors such as the Buhlmann Group provide a practical bridge between steelmakers and refiners - combining inventory of HF-specific grades, in-house forging and machining and advanced PMI testing. By filtering out non-compliant material and guaranteeing traceability, such suppliers reduce the risk profile of HF alkylation units and support safe, reliable operation in a tightening global supply environment.
API RP 751 – Safe Operation of Hydrofluoric Acid Alkylation Units
API 571 – Damage Mechanisms Affecting Fixed Equipment
NACE Paper 03651 (2003) – Specification for Carbon Steel Materials for HF Alkylation Units
ASTM Standards (A516, A106, A234, A960, A961)
Oil & Gas Journal Worldwide Refining Survey (2016)
Honeywell UOP, ExxonMobil, ConocoPhillips licensor documentation
ArcelorMittal & SSAB green steel publications
