Preventing corrosion resulting from renewables conversion

Many oil and gas companies are repurposing refineries to produce renewable diesel, sustainable aviation fuel, and other renewable biofuels and products. The replacement of fossil fuels will help to decrease greenhouse gas (GHG) and CO₂ emissions.

Integrated Global Services

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Article Summary

There are several incentive schemes available to refineries to increase the production of renewable fuels. For example, the United States offers the Renewable Fuel Standard (RFS) program, which provides tax credits for the production of certain renewable fuels, including biodiesel and ethanol.

Process equipment designed to refine crude oil products will now be faced with new chemical compositions, pressures, and temperatures. This new processing environment leads to new corrosion mechanisms which can all detrimentally affect existing carbon steel, stainless steel or internally clad reactors, drums, and other process equipment.

New corrosion mechanisms
Corrosion mechanisms in renewable diesel processing are unique. Conventional base materials of columns, towers, and reactors, as well as past corrosion mitigation strategies, are unsuitable in these new operating environments.

The processing of renewables, whether co-processing or switching to 100%, means that critical equipment, such as reactors, become susceptible to high-temperature free fatty acids (FFAs). These are a type of carboxylic acid and will contribute to the acidification of the feed, increasing the total acid number (TAN), and leading to corrosion. Other types of corrosion include CO₂ corrosion, wet chloride corrosion, sulphidation and stress corrosion cracking.

Repurposed equipment is faced with risks to asset integrity due to new corrodents and damage mechanisms. The original equipment design and/or mitigation strategy may no longer be sufficient to deal with any combination of free fatty acid (FFA), naphthenic acid, carbonic acid, chlorides etc. in the effluent and associated streams. Revised mitigation strategies must account for the differences between the old and new damage mechanisms, for both the asset base material and cladding (if installed). Depending on the process conditions, 3XX SS alloy overlays may not be resilient enough for these more aggressive service conditions.

Cladding materials based on alloys with a known tolerance (e.g. NiCrMo/W/XX), can provide the necessary corrosion resistance for renewable fuel processing.
To protect the existing equipment base material and pressure boundary, a metallurgy upgrade is required to prevent corrosion and potential asset integrity failure. There are several options available to achieve this.

Available solutions
This option involves replacing the existing assets with newly-built equipment designed for the latest operating environment. Replacement of small items can be relatively quick and cost effective, but when operators consider medium/large processing equipment, such as pressure vessels, drums, reactors, and columns, replacement becomes prohibitively expensive and slow, with lead times for the required high-nobility clad equipment often being many years.

Field applied weld metal overlay (WMO)
Welding is a commonly used solution in the wider oil and gas industry, both for rebuilding degraded areas of wall thickness and for providing a corrosion-resistant alloy barrier.

However, welding carries some fundamental drawbacks. A common issue is the potential damage of the vessel shell or any existing internal cladding, due to the heat input required for the welding process (preheating, welding (Heat Affected Zones) and post weld heat treatment or bake out). The process also necessitates additional mechanical support for the equipment during the process to mitigate the structural integrity risk, for example, loosening any flanged connections, cranes or laying the column down horizontally. A high degree of stress gets added during welding, especially on thinner wall vessels, which can cause distortion or failure. Additionally, the weld procedure, code, or environmental conditions will typically require heat treatment prior to and after the application, adding further time and cost to the repair solution.

There is also the question of time and cost. Welding is a relatively slow process with an application time of 10-16 ft² (1 – 1.5m²) per weld head per shift and can cause additional delays in bringing the asset back into service. Depending on the time frame available for converting to renewables, using the method of welding for corrosion protection can have a significant financial impact.

Field-applied High Velocity Thermal Spray (HVTS)
Developed by corrosion mitigation specialists Integrated Global Services (IGS) in the early 2000s following decades of field application experience, HVTS is designed to protect the base metal in high corrosion environments and involves the simple application of a non-porous high nobility metal alloy. This application upgrades the metallurgy of base materials, protecting them from new operating environments. Since the early 2000s HVTS has been successfully installed and the performance verified through inspection in hundreds of critical O&G process assets over thousands of square metres of internal surface.

The application process is considerably faster than welding and there are no stresses imposed on the base material during the application process, effectively for the substrate the application is a cold process. Furthermore, HVTS application does not generate any dilution (the process does not require fusion or a metallurgical bond).
Top tips for upgrading vessel internal metallurgy for renewables conversion
Prequalifying the solution

When selecting a contractor to protect mission-critical equipment from new processing environments, there are several things to consider. Firstly, the contractor should work with the EPC, asset owner and the licensor to deliver a comprehensive pre-qualification engineering package to govern the quality of the applied surface technology solution and ensure that the protection solution is suitable for the new harsh corrosion mechanisms. Rigorous testing should be performed by the provider to verify the suitability of the selected solution in advance of the application.

Delivering a technical package
A good surface solutions provider should deliver a complete technical package of services ensuring a turnkey site application within agreed timescales, including:
λ Project plan
λ Method statement
λ Project safety analysis
λ Risk assessment and mitigation plan
λ Job specific safety data sheets
λ Material selection
λ Surface preparation
λ Utilisation of the application process
λ Critical QA/QC Controls
λ Post project reports

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