Converting Units to Process Renewable Feeds: Materials & Corrosion Concerns

Introduction

At present, government-driven credits are available for fuels that satisfy various low-carbon or renewable fuel standards. In the United States, the most commonly discussed standards include California’s Low Carbon Fuel Standard and the US Environmental Protection Agency’s (EPA’s) Renewable Fuel Standard. These standards define the acceptable methods (a.k.a, pathways) for conversion of renewable feeds into consumer fuel products. For a producer to be eligible for credits, one of the acceptable pathways must be used.

Credits are the major factor in determining the profitability of a renewable diesel unit (RDU). However, as renewable production ramps up and more RDUs come online, several shifts may be expected. Competition among producers for renewable fuel credits will increase, while the growth of RDU capacity will naturally lead to a decrease in government incentivization. Simultaneously, the more advantageous renewable feedstocks will realize improved demand, which will result in price increases amid a lagging rise in supply.

In light of this, speed to market is critical for RDU projects. Some refiners have chosen to modify existing heavy distillate petroleum hydrotreaters or hydrocrackers to avoid the full fabrication timeline associated with some of the longest lead-time components, such as heavy wall chromium-molybdenum alloy (CrMo) reactors. Some facilities with a longer-term position are building standalone units, particularly to serve markets such as Europe and the UK, where renewable fuel standards have been more widely adopted for longer.  

Materials Selection Mindset for Renewables Projects

Corrosion and materials subject matter experts (SME) will be tasked with specifying new materials of construction, equipment modifications, and possibly integrity operating windows for renewable fuel projects. Traditionally, when less aggressive project timelines are in effect, the SME identifies the lowest-cost alloy that effectively mitigates against the relevant damage mechanisms of the process. Given the need for speed to market, the traditional approach may not be ideal for a renewable conversion. The economic analysis will be unique in every case; however, installing a significantly over-alloyed component might make more sense if it allows renewable production to come online faster and helps the refiner leverage government incentives to recoup investment earlier. Conversely, installing a more available but less resistant material with expected higher corrosion rate and shorter life might prove advantageous.

Understanding Renewable Nomenclature

It is confusing that some of the terms relating to renewables processing are used interchangeably (and incorrectly) on a colloquial level. For consistency, it first helps to understand the nature of RDU feedstocks. Generally, renewable units can process either bio-oils or biomass oils.

Renewable Feedstocks

The term “bio-oil” typically refers to lipids from plants and animals. This includes vegetable oils, animal fats, greases, etc. Bio-oils will be a mixture of triglycerides and free fatty acids (FFAs). A triglyceride is simply a combination of three fatty acids attached to a glycerol to form a larger molecule. In some bio-oils, phosphorous-containing groups replace one of the fatty acids of the tryglyceride. These are referred to as phospholipids and are relevant since phosphorous can act to poison hydroprocessing catalysts.

Conveniently, in many vegetable and animal oils, the fatty acids are of a similar chain length to a distillate-range hydrocarbon obtained from petroleum (C12-C20). This lends them to use in producing diesel-range products. Bio-oils serve as the feedstock for nearly all renewable and bio-diesel production.

Biomass oils, in contrast, come from cellulosic material that does not have readily extractable lipids. Sources of cellulosic material for biomass oil include crop residues or tillage, woody biomass, dedicated energy crops, and other general plant-based agricultural waste. Biomass-based oils require different and additional pathways from those used to upgrade bio-oils. The most viable method is pyrolysis, which is high-temperature oxygen-free decomposition, similar to thermal cracking, to produce smaller molecules.

Biomass oils have different corrosion properties than bio-oils since the former contain lighter organic acids such as formic, acetic, propionic, etc., and the resulting oils vary in boiling point (LPG range through distillate range). Generally, biomass oil technologies have not been widely adopted at this point.

Renewable Fuel Products

Many are familiar with biodiesel, which uses a transesterification process to convert the fatty acid feed into an ester. Biodiesel must meet ASTM D6751 and is approved for blending with petroleum diesel in certain amounts. B5 (5% biodiesel) is the most common blend for existing diesel engines.

In contrast, renewable diesel is a “renewable hydrocarbon biofuel.” Multiple pathways are defined by the US Department of Energy, Alternative Fuels Data Center for renewable diesel production, however, the most common is hydrotreating. Renewable hydrocarbon biofuels are chemically identical to their petroleum-based counterparts: gasoline, diesel, or jet fuel. They must meet the same ASTM fuel quality standards as petroleum fuels (ASTM D975 for petroleum diesel) and can be used in existing engines and infrastructure without blending limits. For this reason, they are sometimes called “drop-in” biofuels.

Damage Mechanisms in Renewable Fuels Processing

The reader may correctly expect that damage mechanisms in a renewable unit will differ substantially from those in conventional petroleum hydroprocessing. With petroleum feeds, sulfur and nitrogen are the major impurities being removed. There may be some naphthenic acids if present, but naphthenic acid corrosion (NAC) is generally not a limiting damage mechanism. The presence of both sulfur and hydrogen in petroleum hydroprocessing usually mitigates concerns for NAC for the common materials of construction.

In contrast, renewable feeds are very low in sulfur and nitrogen. Fatty acids, and particularly FFAs, are the dominant corrodent in the feed. Depending on the source of bio-oil, the pre-processing performed, and the handling/shipping conditions, renewable feeds can contain an appreciable amount of chloride. Co-processing renewables with petroleum feeds have advantages and disadvantages depending on the ratio of each feed.

Comparison of Damage Mechanism Concerns in Petroleum vs. Renewables vs. Co-processing

It should also be emphasized that the RDU remains susceptible to many damage mechanisms typically associated with conventional hydroprocessing. During operation, high-temperature hydrogen attack (HTHA) and creep may affect components in the hot feed/effluent or reaction section of the unit. During downtime, brittle fracture (higher likelihood from the effects of temper embrittlement and hydrogen embrittlement) and polythionic acid stress corrosion cracking may apply. Renewables units may often rely on the same 300-series stainless overlaid CrMo equipment as conventional petroleum refineries. 

Fatty Acid Corrosion

Currently, this mechanism is the major focus of refiners seeking to repurpose existing equipment in renewable service. Companies want to understand how their equipment will handle co-processed or 100% renewable feeds. Most petroleum hydrotreaters were built to resist H2/H2S corrosion and make extensive use of type 321 or 347 stainless steel (grades that do not have molybdenum additions). In contrast, fatty acid corrosion resistance (much like NAC) requires Mo-bearing grades. These would include types 316 (2-3% Mo), 317L (3-4% Mo), or 317LM (4-5% Mo). Other applications may require higher-Mo austenitics or nickel alloys such as AL6XN or Alloy 625.

Even in petroleum environments, NAC is difficult to predict. There is generally even less available data for fatty acid corrosion in renewables plants. However, careful consideration of the factors that affect fatty acid corrosion may allow materials selection decisions to be made in confidence. Several critical variables must be considered, including temperature, acid content, alloy, velocity/flow, and the presence of other species such as hydrogen, water, and inorganic chlorides.

Temperature matters with respect to the acid boiling point since the general corrosivity of organic acids is greatest within about 50°F (10°C) of the boiling point. This conventional wisdom may not relate directly to the high-pressure hydrogenated conditions of an RDU. Similar to petroleum oils, the acid content of renewable feeds can be measured as a total acid number (TAN). It is also common to express the acid content simply as a percent of FFA if the molecular weight of the acid is known (or estimated). Flow will tend to exacerbate fatty acid corrosion. Higher velocities, or the presence of two-phase flow, will impart a large shear stress between the fluid and the wall of the pipe or equipment. This can remove semi-protective corrosion product films and bring additional acid molecules in contact with the metal. Hydrogen and sulfur (H2S), as well as water (where present in the lower temperature regions of a unit), can help to mitigate fatty acid corrosion while chlorides accelerate it.

Effluent Damage Mechanisms

A conventional petroleum unit converts organic sulfur and nitrogen in the feed into H2S and NH3. Analogously, the RDU reactor converts fatty acids and triglycerides in the feed into CO2 and H2O. As these effluent species are cooled in the feed effluent train and subsequent reactor effluent air coolers (REACs), condensation of liquid water can result in CO­2 corrosion (carbonic acid corrosion). In co-processing units, the presence of nitrogen may be sufficient to increase the pH of the condensed water and mitigate aggressive corrosion of carbon steel.

As mentioned, chloride contamination is a concern in renewable units, both from organic and inorganic chlorides. Inorganic chlorides are more easily removed by aqueous pretreatment operations; however, carryover of water in the pretreated lipid is still possible. Organic chlorides generally cannot be removed by pretreatment; however, in most virgin vegetable oils, these are low and consistently under 5 ppm. Waste cooking oil/used cooking oil (WCO/UCO) can have much higher organic chlorides, but no “representative” value is generally stated for all cases.

HCl corrosion is a possibility when the renewable feeds contain significant amounts of chloride. In co-processing, the relative ratios of ammonia and chloride in the renewable mix will dictate the separator water pH and dominant corrosion mechanism. When the hydrocarbon feeds contain significant nitrogen, ammonium chloride corrosion concerns will still dominate. Just as in a conventional petroleum unit, accurate chloride quantification in the overall mass balance can be used to identify dew point (or salt point) and either adjust temperature or water wash location accordingly.

Conclusions & Summary

Renewable processing presents a blend of damage mechanisms for which existing hydroprocessing equipment may not have been designed. FFA corrosion is a significant corrosion risk in the feed system and represents a large unknown for refiners converting units to process renewables. There are more damage mechanisms and additional considerations that cannot possibly be addressed in this short article.

As with refineries, no “one-size-fits-all” approach exists to evaluate all the specific concerns for a unit. Flexible materials selection is critical to bringing renewable products to market quickly. In combination with integrity operating windows (IOWs), the best solution can often be achieved.

For more information on this subject, be sure to check out our webinar with the author, "Materials, Damage Mechanisms, & Inspection Considerations for Renewable Fuels Units."