High Temperature Corrosion Quantification in Renewable Diesel and Sustainable Aviation Fuel Production Applications
Sridhar Srinivasan and Jeremy Staats
Production of Renewable Diesel (RD) and Sustainable Aviation Fuels (SAF) from biological sources (natural oils) has seen exponential growth in recent years, stemming from worldwide government mandated climate change initiatives alongside the need for carbon capture and sequestration. Significant, rapid investments have occurred in retrofitting / adapting existing refinery hydroprocessing infrastructure to process natural oils or coprocess natural oils blended with crudes to produce RD and SAF. Such investments are driven by the fact that natural oils have the hydrocarbon (HC) molecular structures to fit within the mid-distillate fuel product such as diesel and aviation fuel as well as that hydroprocessing units are optimized for removal of unwanted Sulfur and Oxygen removal.
In these modified hydroprocessing applications, high temperature decomposition of triglycerides (TRG) leads to production of RD and SAF through hydroprocessing of esters and free fatty acids (FFA). The resulting oxygen free-RD and SAF products are completely fungible with petroleum hydrocarbons. Hydro-processing of refined natural oils has its own unique corrosion problems.
Over the last few years, the authors introduced a molecular mechanistic model to quantify simultaneous high temperature naphthenic acid and sulfidation corrosion (CorrExpert®-Crude) in refinery CDU/VDU operations. This model has been adapted to address high temperature FFA corrosion, given that FFA are carboxylic acids, akin to naphthenic acids found in conventional fossil-fuel based crude unit process streams.
A key aspect of modeling corrosion for FFA is the inhibitive role of hydrogen in the presence of Iron sulfide species. While natural oils do not contain sulfur compounds, presence of reactive sulfur species such as thiols and sulfides in coprocessing applications provides an easy pathway to provide for the formation of a potentially protective nano barrier layer of FeS. Further, the presence of FeS acts as a catalyst towards dissociation of molecular H2 to atomic H and subsequent reduction of FFA through atomic hydrogen.
A threshold H2 partial pressure is required to ensure hydrogen reduction of FFA is kinetically dominant when compared to acid corrosion of Fe. Residence time of acid is another key parameter that will impact propensity for corrosion and / or H2 inhibition and is considered in the development of the prediction model. A framework incorporating the effects of H2 partial pressure, residence time and reactive S concentration is proposed for assessing FFA corrosion for various commonly utilized natural oils in renewable applications.
The prediction model described herein represents a first-of-its-kind solution to address the complex questions of assessing FFA corrosion risk and metallurgical performance in RD / SAF Units, and provides an easy-to-use tool to evaluate unit piping and equipment reliability. https://becht.com/engineering-solutions/strategic-business-planning-2/