Around the globe, refiners are seeking to convert an increasing portion of existing Hydroprocessing capacity to process renewable feedstocks. Facilities face economic pressure to bring renewables production online quickly, in order to take advantage of the current government incentives (credits) for renewable fuels. Simultaneously, however, materials and corrosion engineers face a lack of data-based guidance regarding the appropriate materials selection and expected damage mechanisms for renewables processing units. This paper discusses many of the major damage mechanisms which are considered most relevant in different areas of the RDU – Renewable Diesel Unit (e.g., feed, hot effluent, cooled effluent, etc.). Significant attention is devoted to fatty acid corrosion in the feed system, and lower-temperature aqueous acid corrosion in the cool effluent system. Specific considerations must be made when an existing unit is converted into an RDU, compared to new construction. The source and precise blend of renewable feeds and the extent (if any) co-processing with conventional petroleum feeds impact the susceptibility to certain damage mechanisms throughout the unit. Upstream pretreatment processes can have unique implications on the expected corrosion in the RDU.
As governments around the world seek to promote the adoption of lower-carbon fuels, credits are available for fuels which satisfy various low-carbon or renewable fuels standards. In the United States, the most common standards discussed include California's Low Carbon Fuels Standard1 and the US EPA's Renewable Fuel Standard2. These standards define the acceptable methods (called pathways) for conversion of renewable feeds into consumer fuel products. In order for a producer to be eligible for credits, one of the acceptable pathways must be used. The most common pathway presently being used or considered by most facilities is hydrotreating.
A critical factor in determining the profitability of a new renewable diesel unit (RDU) is the speed with which the producer can bring their products to market in states or countries offering renewable fuels credits. Currently, credits incentivizing RDU projects are readily available, and the volume of renewable feedstocks being diverted from traditional consumers (namely livestock feed) has not significantly impacted feedstock prices. One of the advantages of hydrotreating renewable diesel over other low-temperature renewable fuel pathways (e.g., transesterification into biodiesel) is that the hydrotreating reaction is less sensitive to poor feed quality aspects, such as elevated organic chloride, free fatty acid content, or peroxidized (spoiled) lipids. As a result, RDU operators can take advantage of lower-cost feeds which are not suitable for other uses. A thorough discussion can be found in the next section.