Nov-2021

Key considerations for design and operation of a renewable diesel unit

Design of the process configuration and operation of the unit are fundamental aspects in the production of renewable diesel. dstocks 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

Dr. Sara Green
ExxonMobil Chemical Company

Article Summary

However, the process for making  renewable  diesel  has some specific challenges that differentiate it from traditional hydroprocessing. The feedstock itself contains impurities and introduces corrosion concerns not present with traditional crudes, and the hydrotreating chemistry produces side products 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 addressing these challenges, and enabling a safe and  efficient  operation.  ExxonMobil  Catalysts and Licensing (“ExxonMobil”) recently announced its ExxonMobil Renewable Diesel Process, EMRD™, a process technology that exemplifies ExxonMobil’s expertise in design and operation of hydroprocessing units.²

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.^{3, 4}

The majority of these contaminants must be removed via pre-treatment 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 pre-heat train as well as the hydrotreater. These compounds are removed via a process called degumming that solubilizes the phospholipids using water, acid, or enzymes.⁴

This process also removes metals like phosphorus, alkali metals, and alkaline earth metals, which 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, or TAN, and leads to corrosion of the feed delivery system. Once the renewable feedstock is mixed with hydrogen and in the presence of the hydrotreating catalyst, these free fatty acids are hydrotreated and the concern is no longer present. The presence of metals, but especially phosphorus, is a concern for deactivation of active catalysts and reactor fouling that leads to high pressure drop. Free fatty acids, metals, and phosphorus are removed via chemical or physical refining processes and bleaching/adsorption steps.3 Beyond the aforementioned contaminants, certain feedstocks present unique challenges like the presence of polyethylene found in animal fats and used cooking oils.⁵

At high concentrations, polyethylene 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 pre-treated feedstock or investing in its own pre-treat system. While renewable feed pre-treatment 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 mitigates fouling and catalyst deactivation and preserves 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 have a tendency to decompose at high temperatures, increasing the concentration of free fatty acids and the total acid number (TAN).

A renewable diesel process must take into consideration the proper process configuration to ensure 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 pre-heating. Appropriate materials selection for the metallurgy of the feed pre-heat section is critical for avoiding corrosion in the event that high acid concentrations are encountered.

Once the feed has entered the hydrotreating reactors, the free fatty acids are reacted and no longer a corrosion concern. However, hydrotreating renewable feedstocks results in the formation of CO and 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 appropriate design and operation of the hydrotreating section,^6,7 and materials selection for this section of the unit can further mitigate potential corrosion. Additional corrosion concerns include 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 comprised of compounds known as triglycerides.^6,7

These compounds have three fatty acid chains connected by a propane backbone. Different feedstocks have variation 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 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.^6,7

Both of these 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 all 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 of these 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.

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.