January 2022

Biofuels, Alternative/Renewable Fuels

Key considerations for the design and operation of a renewable diesel unit

Designs for the process configuration and operation of the unit are fundamental aspects in the production of renewable diesel.

Designs for the process configuration and operation of the unit are fundamental aspects in the production of renewable diesel. Renewable diesel feedstocks include vegetable oils, animal fats and used cooking oils.

A key advantage of the renewable diesel product is that it is fungible with conventional diesel and can be blended with no limitations.¹ However, the process for making renewable diesel has some specific challenges that differentiate it from traditional hydroprocessing. The feedstock contains impurities and introduces corrosion concerns not present with traditional crudes, and the hydrotreating chemistry produces side products that are atypical for conventional operations.

The severity of the operation also presents differences in heat release, hydrogen consumption and dewaxing requirements. Proper unit design and operation are key for producing a high yield of renewable diesel, while also addressing these challenges and enabling a safe and efficient operation.

Renewable feedstock contaminants

Renewable diesel feedstocks contain atypical contaminants, such as phospholipids and free fatty acids, as well as traditional contaminants like metals and chlorides.^2,3

Most of these contaminants must be removed via pretreatment steps before proceeding to hydroprocessing. Phospholipids consist of fatty acid chains, glycerol and a phosphate group, and have the tendency to polymerize at high temperatures, leading to fouling in both the feed preheat train and the hydrotreater. These compounds are removed via a process called degumming, which solubilizes the phospholipids by using water, acid or enzymes.³

This process also removes metals—such as phosphorus, alkali metals and alkaline earth metals—that are associated with the phospholipid. Free fatty acids are those in which one of the chains on the triglyceride has broken off the propane backbone to form a carboxylic acid. These compounds also have the tendency to polymerize, but, beyond that, they contribute to acidification of the feed. A high concentration of free fatty acids increases the total acid number (TAN) and leads to corrosion of the feed delivery system. Once the renewable feedstock is mixed with hydrogen and is in the presence of the hydrotreating catalyst, these free fatty acids are hydrotreated and the concern is eliminated. The presence of metals—especially phosphorus—is a concern for active catalyst deactivation and reactor fouling, which leads to high pressure drop. Free fatty acids, metals and phosphorus are removed via chemical or physical refining processes and bleaching/adsorption steps.² Beyond the contaminants, certain feedstocks present unique challenges, such as the presence of polyethylene (PE) found in animal fats and used cooking oils.⁴

At high concentrations, PE can cause fouling and catalyst deactivation and must be removed along with the other contaminants. A renewable diesel producer has the option of purchasing previously pretreated feedstock or further investing in the producer’s own pretreat system. While renewable feed pretreatment is outside the scope of the hydroprocessing unit, it is critical for maintaining the effectiveness of the hydroprocessing operation. Careful control and monitoring of the contaminant levels enable operators to mitigate fouling and catalyst deactivation and to preserve cycle length.

Once the renewable feed is introduced to the hydroprocessing unit, the remaining mitigation is the inclusion of demetallation catalysts and grading materials to remove residual contaminants before they reach the active catalysts. Proper selection of demet and grading materials and their stacking arrangement, as well as control of the operating conditions to maximize metals uptake and minimize fouling, are key for protecting downstream catalyst.

Corrosion mitigation

Renewable feedstocks tend to decompose at high temperatures, increasing the concentration of free fatty acids and TAN.

A renewable diesel process must take into consideration the proper process configuration to ensure that the feed streams are sufficiently heated prior to the inlet of the hydroprocessing reactors, while avoiding high acidity that can lead to corrosion. This concern increases throughout the cycle length as reactor inlet temperatures rise, requiring more feed preheating. Appropriate materials selection for the metallurgy of the feed preheat section is critical for avoiding corrosion if high acid concentrations are encountered.

Once the feed has entered the hydrotreating reactors, the free fatty acids are reacted and are no longer a corrosion concern. However, hydrotreating renewable feedstocks results in the formation of carbon monoxide (CO) and carbon dioxide (CO₂), which can lead to the presence of carbonic acid. This could contribute to corrosion of equipment between the outlet of the hydrotreating reactors and separator equipment, where the CO and CO₂ are removed from the system. The concentration of CO and CO₂ can be minimized through the appropriate design and operation of the hydrotreating section,^5,6 and materials selection for this section of the unit can further mitigate potential corrosion. Additional corrosion concerns include the potential for chloride stress cracking and high-temperature hydrogen attack, although these are also of concern for conventional hydroprocessing units.

Heat release and hydrogen consumption

Hydroprocessing of renewable feedstocks generates higher heat release and consumes more hydrogen than traditional diesel processing. Renewable feedstocks from vegetable oils, animal fats and used cooking oils are made up of compounds known as triglycerides.^5,6 These compounds have three fatty acid chains connected by a propane backbone. Different feedstocks have variations in both the length of the chains and the number of unsaturated bonds. In conventional hydrotreating, the desired chemistry is hydrodesulfurization, hydrodenitrification and aromatic saturation. In the hydrotreating of renewable feedstocks, the desired reactions are saturation of the double bonds on the fatty acid chains and hydrodeoxygenation to produce n-paraffins, water and propane.^5,6

Both chemistries result in extremely high heat release and hydrogen consumption. Catalyst selection and design of the catalyst stacking arrangement, quench capabilities, liquid recycle and process controls are critical to ensuring safe operation of the unit, preventing runaway reactions, and avoiding premature coking and deactivation of the catalyst.

Competing reactions to hydrodeoxygenation are decarbonylation, which produces CO and water as side products, and decarboxylation, which produces CO₂ as a side product. Both reactions lead to yield reduction, since carbon is lost as CO and CO₂, rather than retained on the paraffin chain. CO and CO₂ can react with hydrogen to form methane and water, generating additional heat that can lead to coking.

The catalyst selection and stacking arrangement, as well as process conditions (such as pressure and treat gas availability), impact the selectivity between hydrodeoxygenation, decarbonylation and decarboxylation and, therefore, are important for controlling the overall desired yield.

Temperature is another factor for selectivity, and, as the unit ages, the selectivity will shift for these three reactions. Hydrodeoxygenation maintains the highest yield of the carbon number of the feedstock; however, this reaction also consumes the highest amount of hydrogen (FIG. 1).^5,6 Therefore, renewable diesel processes require high treat gas availability and high makeup rates to ensure that sufficient hydrogen is present for the desired dominant reaction.

FIG. 1. Hydrodeoxygenation maintains the highest yield of the carbon number of the feedstock; however, this reaction also consumes the highest amount of hydrogen.

Managing treat gas quality

CO, CO₂ and water generated during hydrotreating must be removed before the hydrotreated stream passes onto the dewaxing reactor to ensure high dewaxing yields. Proper design and operation of the separator equipment and gas treating system are needed to manage the concentration of these contaminants. Specifically, a purge stream is needed to control the concentration of contaminants like CO, but it is important to minimize the flow of this stream to manage the purity of the treat gas and the required amount of makeup hydrogen.

Additionally, sulfur from dimethyl sulfide, hydrogen sulfide (H₂S) or another source is required to maintain the sulfide state of the hydrotreating catalyst. The gas treating system is responsible for balancing the level of H₂S to ensure that enough is present for the catalyst while avoiding it cycling up over time. Similarly, the gas treating and purge systems should remove sufficient concentrations of CO and CO₂ to ensure that these do not cycle up and lead to inhibition of hydrotreating catalyst activity.

Other components to consider are the concentration of propane, which is the backbone of the triglyceride molecules and which is generated during hydrotreating, and methane formed from methanation reactions. Although propane and methane will not inhibit catalyst activity, they can dilute the treat gas purity.

Cold flow improvement

Hydrodewaxing of renewable feedstocks is key for meeting cold flow properties. The composition of hydrotreated conventional diesel is a mixture of n-paraffins, aromatics and naphthenes—whereas, the composition of hydrotreated renewable diesel is mainly n-paraffins. N-paraffins have extremely poor cold flow properties, and must be either isomerized or cracked to meet diesel cold flow specifications.

For example, the cloud point of a conventional hydrotreated diesel may be approximately 5°C to –5°C, while the cloud point of a renewable hydrotreated diesel is typically closer to 25°C or higher. Therefore, to produce a –15°C diesel product, the conventional sample only requires 10°C–20°C of cloud point improvement, while the renewable sample requires 40°C of cloud point improvement. The higher dewaxing requirement is not a function of the design and operation of the hydrotreating section, but is rather due to the composition of the renewable feedstock. Although cracking reactions can improve cold flow properties, they can also lead to yield loss.

Therefore, isomerization chemistry, specifically that which adds multiple branches to the paraffin rather than a single branch, is preferred to maintain yield in the diesel boiling range. The higher the cloud point improvement, the more important it is to selectively isomerize vs. crack to maintain diesel yield.

The author’s company’s proprietary dewaxing catalyst^a is formulated to meet diesel cold flow specifications, even at deep delta cloud, while maintaining high liquid yields through its selectivity to multi-branch isomerization.⁷

Increasing interest in the production of jet fuel—or sustainable aviation fuel (SAF)—has the potential to shift the design and operation of renewable fuel processes. For co-production of diesel and jet fuel, this may require changes to the product separation equipment and operating severity, thereby introducing additional capital and operating expenses.

Renewable feedstocks typically have a narrow boiling range; therefore, the operating conditions need to drive molecules from the diesel boiling range to the jet fuel boiling range. The goal is to make this transition with the highest jet fuel and diesel total yield, while avoiding the production of naphtha and light ends. The proprietary dewaxing catalyst^a can help manage this balance (FIG. 2).

FIG. 2. The proprietary dewaxing catalysta can help promote multi-branch isomerization.

Takeaway

Production of renewable diesel includes technical challenges (including the removal of contaminants, corrosion concerns and metallurgy selection, as well as the management of high heat release and hydrogen consumption) beyond those of conventional processing, thus preventing catalyst activity inhibition and providing deep dewaxing, while maintaining high liquid yields. The design of the process configuration and operation of the unit are critical for the mitigation and management of these factors. The author’s company’s proprietary renewable diesel process^b features a two-stage unit design that uses the proprietary dewaxing catalyst^a and processes a range of renewable feedstocks to produce high yields of renewable diesel, with the flexibility to also produce nominal volumes of jet fuel. The use of two stages enables separate control of the hydrotreating and dewaxing chemistries, thus allowing refiners to better adjust to changing objectives compared to single-stage processes. The author’s company’s expertise as both an owner and operator of hydroprocessing units allows it to address all the key challenges noted in this article in the design and operation of the proprietary renewable diesel process and to deliver a high-performance technology that meets renewable product objectives. HP

NOTES

^a ExxonMobil’s Bio-Isomerization Dewaxing (BIDW™) catalyst
^b ExxonMobil Renewable Diesel (EMRD™) process

LITERATURE CITED

1. U.S. Energy Information Administration (EIA), “Renewable Diesel is a Biomass-Based Diesel Fuel,” August 18, 2020, online: https://www.eia.gov/energyexplained/biofuels/biodiesel-in-depth.php
2. Zhang, B., J. Wu, C. Yang, Q. Qiu, Q. Yan, R. Li, B. Wang, J. Wu and Y. Ding, “Recent Developments in Commercial Processes for Refining Bio-Feedstocks to Renewable Diesel,” Bioenergy Research, July 2018.
3. Sharma, Y. C., M. Yadav and S. N. Upadhyay, “Latest Advances in Degumming Feedstock Oils for Large-Scale Biodiesel Production” Biofpr, October 2018.
4. Van Gerpen, J., “Animal Fats for Biodiesel Production,” Farm Energy, April 3, 2019, online: https://farm-energy.extension.org/animal-fats-for-biodiesel-production/
5. Zhao, C., L. Wei, S. Cheng and J. Julson, “Review of Heterogeneous Catalysts for Catalytically Upgrading Vegetable Oils into Hydrocarbon Biofuels,” Catalysis, March 2017.
6. Huber, G. W., P. O’Conner and A. Corma, “Processing Biomass in Conventional Oil Refineries: Production of High Quality Diesel by Hydrotreating Vegetable Oils in Heavy Vacuum Oil Mixtures,” Applied Catalysis A: General, October 2007.
7. ExxonMobil Bio-Isomerization Dewaxing (BIDW™) Catalyst, online: https://www.exxonmobilchemical.com/en/library/library-detail/86785/horizontal_factsheet_bidw_dewaxing

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