Co-processing renewables is a very attractive and cost-effective way of reducing the carbon intensity of liquid products from refineries. The cost of revamping existing units in terms of $/BPD for co-processing is substantially lower than installing new facilities that process only renewable feeds. The ease with which certain renewables can be co-processed has helped the adaptation of co-processing to gain traction among refineries. Co-processing, along with carbon-capture in supporting refinery units like hydrogen generation, helps refineries take steps towards the goal of carbon-neutrality. The most convenient route for co-processing is in a common refinery process unit, which is middle distillate hydroprocessing. Hydroprocessing units can be converted to co-process plant-based oils and animal fats with minimal changes to processing scheme.
This article explores the modifications and the general pitfalls to consider when co-processing plant and animal-based fats in an existing distillate/ diesel hydrotreater. Due to the inherent nature of the fats, co-processing with distillate fossil feed lends itself to the production of diesel that meets ASTM D975 specifications. The plant and animal fats consist of triglycerides which are C17-C20 chains of paraffins attached to a glycerol backbone. The paraffin molecules fall in the diesel boiling range, and the catalyst can be tailored to ensure the final product meets the requirement for the intended grade of diesel. [1]
Some of the main by-products of processing triglycerides are water, carbon dioxide and carbon monoxide. The commonality of these by-products is their oxygen content, which results from oxygen removal from the triglyceride chain. There is also an increased quantity of propane due to the cleavage of the glycerol backbone which consists of a C3 chain. Due to hydro-deoxygenation and decarboxylation reaction, co-processing of bio feedstocks is inherently more exothermic than fossil feed hydroprocessing. Hence, careful consideration must be given to the design of catalyst and quench requirements. It is possible that co-processing increases the quench gas requirements, which in turn leads to a revamp of the recycle gas compressor.
Generally, diesel and distillate hydrotreater reactor effluents are dry and there is minimal water with the feed and recycle gas. However, due to the generation of water and carbon dioxide while co-processing, there are additional corrosion concerns that are not present for traditional fossil feed hydrotreating. The water injection for most hydroprocessing units is upstream of the reactor effluent air cooler. This is due to the relatively dry nature of the product from the reactors. With co-processing, the water injection may need to be shifted upstream to the reactor effluent exchanger circuit. This has knock on effects on the heat integration due to the decrease in temperature that occurs with water injection.
Carbon dioxide in the product combines with water to form carbonic acid, increasing the possibility of carbonic acid corrosion in the water phase along with the ammonium chloride and bisulphide salts. A proper assessment of metallurgy is required for the reactor effluent circuit at lower temperatures.
In addition to corrosion concerns, potential fouling in the feed circuit needs to be addressed when considering co-processing. In the absence of hydrogen, bio feed stocks lend themselves to degradation at temperature in the reactor effluent circuit due to the presence of small quantities of phospholipids, which tend to polymerize at higher temperatures [2]. At higher fractions of co-processing, this might require addition of bio feedstock directly upstream of the reactor to mitigate fouling. Depending on the type of renewable feedstock, there is a threshold temperature beyond which fouling in the heat exchanger circuit becomes a real concern.
The other process considerations include increased purge requirement and increased gas production. The purge gas increase is directly related to maintaining the purity of the recycle gas stream and is a function of the percentage of renewable feed stock that is processed. The increase in gas purge will require rebalancing the fuel gas system. This may require revamping of downstream units to ensure the existing processing facilities are adequate to handle the changing fuel gas composition.
If a primary amine is used for the scrubbing of purge gas, carbon dioxide will be carried over to amine regeneration, sour water stripper and sulphur plant, which requires careful evaluation of those facilities. For example, additional carbon dioxide in sour water might lead to carbonic acid corrosion that will need to be addressed. [3] [4]
This paper has highlighted that in addition to the hydroprocessing unit there are ripple effects to various parts of the refinery. Hence, the options for processing bio feedstocks in a hydrotreater should be carefully considered and designed. Fluor has worked closely with owners and licensors to provide technical expertise in implementing a fit-for-purpose design that meets the requirements of a project. Fluor’s engineering knowledge and experience with revamps, along with project management skills, can be leveraged to tackle the unique challenges that are part of modifying an existing unit into a co-processing operation.
References:
- Peter Andreas Nymann and Pronit Lahiri, Topsoe. Co-processing renewables in a hydrocracker. Petroleum Technology Quarterly, March 2023.
- Sara Green, Ph.D., Customer Solutions Advisor, Catalysts & Licensing, ExxonMobil Chemical Company Key considerations for design and operation of a renewable diesel unit. Internet Publication
- Le Grange, P, Tekebayev, K., Goettler, L., Kiebert, J. Sulphur Experts, Sheilan, M. Amine Experts. Impact of biofeed retrofits, coprocessing on refinery amine units, SWSs and SRUs – Part 1. Hydrocarbon Processing, January 2022.
- Le Grange, P, Tekebayev, K., Goettler, L., Kiebert, J. Sulphur Experts, Sheilan, M. Amine Experts. Impact of biofeed retrofits, coprocessing on refinery amine units, SWSs and SRUs – Part 2. Hydrocarbon Processing, February 2022.