HF – treat it with the utmost respect

Hydrogen fluoride (HF) is a very useful chemical compound which finds widespread application in laboratories, manufacturing facilities as well as large-scale industrial processes such as alkylation. Unfortunately, HF is at the same time an extremely hazardous substance, meaning that all process equipment needs to be designed and built to the highest of standards. Valve World reviewed the production, usage and handling of HF, and checked out how manufacturers engineer valves suited for HF alkylation units.

By KCI Editorial

Description

Pure hydrogen fluoride (HF) has a boiling point around 20°C (67°F). It therefore exists as a liquid at low temperatures, converting into a gas at about the standard room temperature. HF readily dissolves in water, creating hydrofluoric acid. Standard hydrofluoric acid commercial concentrations are around 50%, although stronger solutions (70%) are also available. Both hydrogen fluoride as well as hydrofluoric acid are highly corrosive.

From the chemical perspective there are differences between hydrogen fluoride and hydrofluoric acid. However, for practical purposes they are considered to be the same substance. This article will henceforth use the general term HF.

Uses

HF finds widespread use in a myriad of applications. The pharmaceutical sector for example uses it to help produce fluoxetine, which is better known as the antidepressant Prozac. HF is also used in the production of polytetrafluoroethylene, more familiar to cooks around the world as PTFE, found in non-stick frying pans.

In industrial applications, HF is widely used to etch glass and silicon wafers as well as metal extraction and stainless steel picking. HF is furthermore used in the production of refrigerants, herbicides, phosphate fertilisers, pharmaceuticals, aluminium, plastics, electrical components and fluorescent light bulbs. And, coming right up to date, HF is also used in chemical lasers.

HSE

Whilst HF is present in the air (from natural and industrial sources), the levels are low and not expected to cause adverse health effects. Incidents involving HF exposure are therefore typically associated with occupational settings. This could include breathing in the fumes or skin contact. Depending on the concentration, skin contact with HF can cause severe and painful burning. Note that burns on the skin may not be visible until 24 hours after exposure. Breathing in HF can irritate the eyes and nose, cause a sore throat, as well as coughing, headaches and confusion. Exposure to high levels of HF can lead to drowsiness and heart problems. Ingestion of just a small quantity of highly concentrated HF will affect major internal organs and may be fatal. Given the above, authorities typically draw up guidelines to protect worker safety. These include good working practices, hazard assessments and suitable personal protective equipment (PPE). For example, a long-sleeved shirt, long trousers, and closed shoes are recommended even when working with small quantities of dilute forms of HF. As the risks become more significant – such as from potential exposure to higher HF concentrations – then the use of protective goggles and a face shield are often mandatory. An HF-resistant full-body apron and chemical-resistant boots are recommended if there is a risk of splashing.

HF in refining

Concentrated HF is widely found in refineries, where it serves as a catalyst in the alkylation process (alkylation helps to boost octane level). Despite requiring tighter safety protocols, HF is often preferred to the alternative catalyst, sulphuric acid, as (i) the same output can be achieved with far less acid (ii) by-products are minimal and (iii) a wider range of feedstocks can be processed.

Note that for various reasons, alkylation plants are often ‘split’ into two separate areas, termed acidic and non-acidic.

Market projections

Data research companies offer positive yet varied perspectives on the size and growth in demand for HF valves for alkylation. One suggests that the market value for all HF valves combined will grow to USD 0.5 billion by 2030, whilst a second predicts that demand for HF globe valves alone will top USD 2 billion by the same year. Whatever the actual value, the numbers remain impressive, with ongoing growth in HF (and hence HF valves) ascribed to increasing demand for products such as gasoline (petrol), diesel and petrochemicals. In terms of geography, North America has for decades spearheaded investment in refinery alkylation facilities. However, current and forward-looking growth seems to be focused in Asia. Reports from 2023 indicate that China had the highest refinery alkylation capacity in Asia at around 200 million barrels per day (mbd). Other key countries included Japan and India (both around 100 mbd). With total 2023 Asian alkylation capacity put at a shade under 590 mbd, that figure is expected to increase sharply to 760 mbd in 2028.

Valves for alkylation – materials

At face value, the valves often found in HF service in alkylation facilities may sound and look familiar: gate valves, globe valves, piston check valves, swing check valves, plug valves, etc.

However, manufacturers clearly continue to work in close alliance with ‘the industry’ to develop, engineer and manufacture valves that are fully geared to the demanding requirements of handling such a highly corrosive and toxic medium. To give a quick example: the avoidance of dead spaces in valve designs which can help to prevent certain forms of corrosion. Speaking of corrosion, several manufacturers state that, for the most severe applications such as for use with hot/wet HF, they produce valves with solid bodies and trims from ‘exotic’ alloys like Monel or Hastelloy. For less critical environments solid Monel trims can be found in conjunction with carbon steel bodies (with an appropriate corrosion allowance).
Incidentally, double-seal welding is often noted in conjunction with Monel seat rings and carbon steel body pockets. This creates a long-term barrier to prevent seat pocket corrosion.

Special grades of Monel may be selected for the stems to minimise bending and/or twisting, whilst the seat rings and forged wedges may also be produced using specific Monel alloys to eliminate the risk of galling. Solid Monel again seems to be the norm for such components.

Manufacturers are also aware of the potential of stress corrosion cracking (SCC) due to the combined effects of mechanical stress and exposure to HF. This could be the case with bolts if HF were to leak through the flanges. Bolts destined for use on HF valves may be heat treated, which makes them less likely to crack after coming into contact with HF.

Attention is also given to sealant injection ports, with double ball isolation systems deployed to ensure integrity.

For on/off duty, plug valves with a PTFE liner seem to be a common selection. However, given the tools and training required to properly manufacture such valves, any repair work should only be conducted by authorised personnel.

And it goes without saying that testing is performed during all stages. Manufacturers often cite RT, UT and visual inspection. Finished valves are hydrotested with kerosene, as HF could become corrosive in the presence of water.

Valves for alkylation – emissions

Again, no surprises to hear how much thought has gone into preventing fugitive emissions with dedicated containment solutions for the valve flanges as well as the stems. Many valve makers refer to the combined application of several features to prevent stem leakages, such as multiple-walled bellows, gland packings and back seats.

As a further ‘leak prevention’ measure, a special colour coating is available for HF valves. Should any HF leak through a joint, on contact with the coating it will quickly trigger a colour change in the paint. This will alert on-site staff who can take immediate remedial action.