Mitigating Atmospheric Distillation Unit Corrosion through Machine Insitu Methods
The crude oil supply chain
Fossil fuels substantially impact every aspect of our daily lives. Crude oil, specifically, is the basis for a breadth of products from cosmetics to fuel and arguably the most important commodity on the planet today. In 2019, U.S. petroleum consumption averaged about 20.64 million barrels per day (b/d), which included about 1.1 million b/d of biofuels. In order utilize these products at the consumer levels, crude oil must be drilled, transported and refined into products such as gasoline, naphtha, and numerous other petrochemicals.
Crude oil is extracted throughout the world but the largest volume is primarily supplied from Brent Crude, West Texas Intermediate (WTI), Dubai-Oman, and the OPEC basket. The crude oil supplies are classified by API gravity, the density of petroleum liquid compared to water, and sulphur content. Using these classifications, crude oil stocks can be determined to be light or heavy and therefore command various values in the market. For example, WTI is considered to be sweet due to its low sulphur content and light due to its relative density and therefore very high quality. The light sweet grade crude oils are the most desirable because they can be refined with far less sophisticated and energy intensive processes. According to OPEC, more than 70 million barrels are produced worldwide every day, with Reliance Industries’ Jamnagar Refinery being the largest in the world refining 1.24 million barrels per day (bpd).
The crude distillation process origins, operation
The complexity of refineries vary by location, but virtually all refineries share a basic process for initially separating products by their boiling point, this process is crude distillation. Referred to as a pipe still, crude unit, distillation unit or simply a still, it is the oldest means for processing oil. Pioneered by Samuel M. Kier in the 1850’s, the first versions of the distillation unit held 5 barrels of oil. Distillation allows for the materials to be separated without being subjected to conditions that would cause cracking or decomposition. The distillation unit feeds the rest of the refinery therefore any outages or upsets potentially affect the availability of the entire refinery.
The crude distillation column is a vertical pressure vessel constructed of carbon steel material and lined with a thin layer of <12%, martensitic stainless steel for corrosion resistance. The crude columns internals, typically matched in alloy to the lining material, are horizontal trays used for separating and collecting the various liquid cuts. Products ranging from uncondensed fixed gases at the top to heavy fuel oils at the bottom can be taken continuously from a column. The crude distillation process begins with desalted crude feedstock being moved through fired heaters and fed into the vertical column at lower levels. The pressures are slightly above atmospheric and temperatures range from 650 to 700°F. Any additional heating above these temperatures could cause thermal cracking. Most of the crude feed flashes into vapor with the exception of the heaviest cuts such as fuel oil and asphalt that are removed from the bottom. As the vapor rises in the column, temperatures are reduced with convection allowing for condensation and subsequent collection of the product at the various tray levels. Using a system of perforations and bubble caps, vapors rise and liquids fall eventually condensing at the temperature of that tray. The crude distillation unit distills crude feedstock into fractions or cuts at different boiling ranges, each of which can be additionally processed or pooled in the other refinery units.
Corrosion challenges in the distillation process
The global refining industry is processing more highly acidic crude oils, primarily based on lower costs and greater availability of high sulphur feedstocks. In some cases, refinery feedstocks are alternating between acid rich sweet crudes and lower acid sour crudes. These alternating feedstocks can lead to accelerated corrosion levels in carbon steel and low-alloy steels. Corrosion is most often severe in the areas of liquid and vapor phase flows as well as high velocity areas. Sulphidation takes place in highest temperature ranges of the process. Sulfidation is a competing and also complimentary mechanism that must be considered in most situations with naphthenic acid. In areas where thinning is occurring, it may be difficult to distinguish between naphthenic acid corrosion and sulfidation. High-nickel alloys are susceptible to sulfidation and should be avoided as a metallurgical upgrade. Increasing chromium in welded alloys will improve sulfidation resistance. Naphthenic acid corrosion (NAC) causes localized corrosion and pitting in the atmospheric distillation process. “NAC is typically associated with hot dry hydrocarbon streams that do not contain a free water phase but if the acids have the opportunity to condense in a water phase, wet corrosion can and will ensue.” 317L stainless steel offers good resistance under most circumstances by increasing the amounts of molybdenum in comparison to low alloy steels. Lastly, sulfur promotes iron sulfide formation and has an inhibiting effect on NAC. High velocity flow and turbulence areas of the process may create erosion-corrosion cycles by removing the iron sulfide scales on the surface of metals.
Distillation Column Corrosion Mitigation Utilizing AZZ Machine Technology
AZZ Specialty Welding can develop code acceptable corrosion resistant overlay (CRO) repair strategy with machine applied weld deposit to upgrade metallurgies in lieu of plate replacements, sheet lining, window cut outs in the vessel, or high dilution manual weld deposits. Shell base material evaluation and preparation are critical to the long-term reliability of the overall repair. Cladding removal using specialized techniques, advanced technology and skilled craftspeople is best practice. After removing the cladding and preparing the base material, AZZ’s proprietary mechanized Gas Metal Arc Welding (GMAW) overlay process becomes ideal for confined space applications because of the custom automatic proportional torch height control. This feature allows for our machines to electronically measure the tip to work distance over 200 times a second in order to ensure uniform deposition thickness and minimal weld dilution. Furthermore, the weld nozzle has a stroke capability of 9” in order to address a variety of corrosion depths experienced during repair applications. These systems utilize state-of-the-art waveform controlled welding power supplies running optimized welding parameters developed by AZZ engineers to minimize dilution, reduce spatter, and provide a smooth and consistent weld deposit.
AZZ Specialty Welding – The World Leader in Pressure Equipment Life Extension
Whether the pressure vessel needs to be repaired emergently or during a planned turnaround, AZZ has unparalleled innovative leadership backed by proven results. AZZ has the world’s largest portfolio of pressure equipment life extension projects and qualified procedures to handle distillation column specific challenges. Find out more about our pressure vessel life extension solutions and other engineered technologies at azz.com/WSI or follow us on LinkedIn.
 United States Energy Information Administration. Oil and Petroleum Products Explained. www.eia.gov
 Organization of the Oil Exporting Countries. www.opec.org
 Speight, James. 2014. Corrosion by High Acid Crude Oil.