as low as possible within the capabilities of the feed treating systems. The major feed
contaminants normally found in alkylation feeds are water or oxygenates, sulfur
compounds, nitrogen compounds, noncondensables, and diolefins.
HF Alkylation Maintenance
Because HF is highly corrosive to most materials, careful control of metallurgy and
proper maintenance of equipment is required. Carbon steel is the primary material
used for vessels and piping, and it can be used only because of a corrosion barrier
layer of iron fluoride that forms on carbon steel surfaces exposed to HF. The iron
fluoride layer is tenacious and serves as a barrier against further carbon steel
corrosion as long as the iron fluoride layer remains undisturbed. Under certain
conditions, such as mechanical wear or elevated water content in the HF acid, this
iron fluoride scale can soften or break off leading to both fouling and corrosion
issues. In severe cases, this can lead to unscheduled shutdowns as well as potential
safety issues. Most refiners aggressively monitor their equipment’s remaining
corrosion allowance and use regularly scheduled valve and flange replacement to
avoid problems. For maintenance during a turnaround, many refiners choose to
dissolve all the iron fluoride scale by using a chemical cleaning process.
There are other areas within an HF alkylation unit where conditions are too
severe for carbon steel. Alloy 400 (Monel) is the primary metallurgy for such areas
because it has good resistance to acid with high water content, especially at higher
temperatures. Temperature is an important variable for the corrosion rate of both
carbon steel and for alloy 400. Alloy 400 is used for moving parts or where
machined surfaces are mated together because the use of carbon steel would
cause cementing of the parts together by the iron fluoride scale that is formed on
the steel surfaces.
HF Safety and Volatility Suppression
Through the American Petroleum Institute, the industry developed a comprehensive document for the safe operation of HF alkylation units entitled “Recommended
Practice for Safe Operation of Hydrofluoric Acid Alkylation Units,” API
Recommended Practice 751, first published in June 1992 and most recently revised
(4th edition) in May 2013.
A consortium of refiners and HF manufacturers led by Amoco developed HF
dispersion models and water spray mitigation with extensive, large-scale testing to
design release control systems. Together, cosolvents and water spray mitigation can
reduce downwind HF concentrations by over 95 % should a release occur.
UOP offers the ReVAP
TM and Alkad
TM processes, which introduce an additive
to the catalyst that significantly reduces catalyst volatility. The ReVAP process was
jointly developed by Mobil Oil and Phillips Petroleum, while Alkad was jointly
developed by Texaco and UOP. Introduction of the additive, also referred to as
446
D.A. Nafis et al.
contaminants normally found in alkylation feeds are water or oxygenates, sulfur
compounds, nitrogen compounds, noncondensables, and diolefins.
HF Alkylation Maintenance
Because HF is highly corrosive to most materials, careful control of metallurgy and
proper maintenance of equipment is required. Carbon steel is the primary material
used for vessels and piping, and it can be used only because of a corrosion barrier
layer of iron fluoride that forms on carbon steel surfaces exposed to HF. The iron
fluoride layer is tenacious and serves as a barrier against further carbon steel
corrosion as long as the iron fluoride layer remains undisturbed. Under certain
conditions, such as mechanical wear or elevated water content in the HF acid, this
iron fluoride scale can soften or break off leading to both fouling and corrosion
issues. In severe cases, this can lead to unscheduled shutdowns as well as potential
safety issues. Most refiners aggressively monitor their equipment’s remaining
corrosion allowance and use regularly scheduled valve and flange replacement to
avoid problems. For maintenance during a turnaround, many refiners choose to
dissolve all the iron fluoride scale by using a chemical cleaning process.
There are other areas within an HF alkylation unit where conditions are too
severe for carbon steel. Alloy 400 (Monel) is the primary metallurgy for such areas
because it has good resistance to acid with high water content, especially at higher
temperatures. Temperature is an important variable for the corrosion rate of both
carbon steel and for alloy 400. Alloy 400 is used for moving parts or where
machined surfaces are mated together because the use of carbon steel would
cause cementing of the parts together by the iron fluoride scale that is formed on
the steel surfaces.
HF Safety and Volatility Suppression
Through the American Petroleum Institute, the industry developed a comprehensive document for the safe operation of HF alkylation units entitled “Recommended
Practice for Safe Operation of Hydrofluoric Acid Alkylation Units,” API
Recommended Practice 751, first published in June 1992 and most recently revised
(4th edition) in May 2013.
A consortium of refiners and HF manufacturers led by Amoco developed HF
dispersion models and water spray mitigation with extensive, large-scale testing to
design release control systems. Together, cosolvents and water spray mitigation can
reduce downwind HF concentrations by over 95 % should a release occur.
UOP offers the ReVAP
TM and Alkad
TM processes, which introduce an additive
to the catalyst that significantly reduces catalyst volatility. The ReVAP process was
jointly developed by Mobil Oil and Phillips Petroleum, while Alkad was jointly
developed by Texaco and UOP. Introduction of the additive, also referred to as
446
D.A. Nafis et al.