Renewable diesel technology is proving to be a top choice among the various means to satisfy a growing appetite for low carbon intensity fuels. The core process for producing renewable diesel is quite similar to hydrotreating processes common to most petroleum oil refineries. Renewable diesel has drop-in compatibility in the petro-diesel fuel marketplace. However, implementation of renewable diesel technology in petroleum oil refining facilities presents new challenges due to procurement and pretreatment requirements for the agricultural triglyceride feedstocks that are a crucial input in the renewable diesel value chain.
The overall renewable diesel production process can be divided into two major steps represented by the pretreatment unit (PTU) and the renewable diesel unit (RDU). The PTU reduces contaminants in the feedstock material to an acceptable level, typically dictated by catalyst life expectations within the RDU and/or metallurgical constraints for process equipment due to chloride content and acid number of the feedstock. The RDU converts the pretreated triglycerides into renewable diesel, renewable naphtha, and renewable liquefied petroleum gas (LPG). A refiner can choose to implement the RDU process only, purchasing pretreated feedstocks for conversion to renewable diesel. Another option is to expand the range of acceptable feedstocks and to control the pretreatment process with a project to install both a PTU and an RDU.
Challenge: Approach to Feedstock Pretreatment
The decision to perform feedstock pretreatment in-house or to outsource this processing step is perhaps the most challenging, complex and critical decision for a project team. The renewable diesel marketplace is rapidly changing. The feedstock market can be volatile. The marketplace for agricultural triglycerides has long been competitive. These feedstocks are valued and utilized in numerous food and consumer products. Moreover, these feedstocks are coproducts with other commodities, making supply and production inelastic relative to price. In other words, farmers will not raise more cattle to provide additional beef tallow as a renewable diesel feedstock; only consumer meat demand can drive more beef tallow production. Renewable diesel producers have little ability to drive increases in triglyceride production. Constructing a PTU is a considerable capital cost, but it widens the renewable diesel producer’s range of feedstocks available for purchase. Indeed, this is a complex decision.
Nested within the complexity of the decision on the inclusion of the PTU are the decisions of breadth and depth of pretreatment capabilities. Which feedstocks will be pretreated? Each feedstock requires specific pretreatment capabilities. Soybean oil, palm oil, corn oil, beef tallow, chicken fat and used cooking oil are just some of the triglyceride feedstocks to consider. Then also, what degree of pretreatment is to be provided? Each of these feedstocks can be procured with varying contaminant levels based on source and prior processing. For example, more than a dozen grades of tallow are commercially defined and available. These designations vary widely in contaminant levels.
Strategy: Consider PTU Early in Scope
The project team should evaluate the business case for inclusion of the PTU as soon as possible. Opportunities to impact the carbon intensity of the renewable diesel product will be a critical determining factor. Prospective RDU operators should identify feedstock suppliers and quality constraints early to help drive the decision on PTU inclusion. Consideration of impacts on wastewater treatment infrastructure will be important as well. Thoughtfulness and timeliness of this decision will drive the success for design and construction of the RDU and PTU as well as the long-term profitability.
Due to the relatively small cost of the PTU compared to the RDU, inclusion of the PTU should strongly be considered. The PTU should be designed for feedstock flexibility; plan for future changes in feedstock type and quality that might require expansions to the PTU. If the PTU is not initially included in project scope, engineering provisions for a future PTU would be a wise addition. A PTU may be the key to unlocking the value of the overall renewable diesel facility; renewable diesel cannot be produced without properly pretreated renewable feedstocks.
Challenge: Design and Operation of PTU Equipment
Once the project team has decided to include a PTU, there are still headwinds to successful engineering, construction and operation of the PTU. The RDU will be familiar to the refinery team. There are no novel unit processes, and the process flow diagram is almost indistinguishable from other hydrotreating flow sheets. The cost of the PTU, although not trivial, is typically much smaller than the RDU. The attention of project management, chemical process engineers, subject matter experts and others may gravitate to the RDU, leaving the PTU at risk of falling behind during engineering development.
The processing equipment in a PTU is not typical of refineries. The degumming process can be acid- or enzyme-catalyzed, each with costs and benefits to consider. Centrifuges and decanters separate solids and contaminant-laden water from the feedstocks. These pieces of equipment are often cost drivers and throughput bottlenecks for a PTU. Precise operation of this water separation is needed to minimize feedstock losses to the water phase. A typical PTU removes the last remaining contaminant metals via a bleaching reaction. Typically, this is a two-step reaction under “wet” and “dry” conditions, but there is variation in the process flow scheme, with different benefits under each.
Optimal design and operation of this process strikes a balance between consuming too much of the high-cost bleaching earth additive and allowing more catalyst-poisoning metals to pass onto the reactor. The depth and timing of chloride removal can help drive certain metallurgical decisions. Precoat filtration is normally utilized for removal of the bleaching earth. This process generates significant solid waste, requiring a combination of manually operated and automated solids handling equipment. These oil-laden solids have the potential to smolder due to spontaneous combustion, an alarming thought to most petroleum oil refinery personnel. The wet gums waste product from degumming might require further dewatering before disposal, usually performed via falling film evaporation technology. These processes using novel equipment may prove challenging to integrate with routine refinery practices for design and operation. Each decision point might prove crucial to project success.
Strategy: Build a Uniquely Experienced Team
Renewable diesel project teams should be built with the novel equipment challenges in mind. Select team members with an interest in novel processes and equipment. Work with a team that has experience with both processing agricultural feedstocks and refined petroleum products. Select vendors for pretreatment technologies who have worked with the petroleum refiners. Also, consider the benefits of technology vendors and contractors with robust commissioning and training programs, as well as access to pilot facilities to assist in bringing the PTU operations team up to speed.
As the diesel fuel market shifts to meet demand for low carbon intensity diesel fuels, there is a valuable opportunity for petroleum oil refiners to rapidly incorporate renewable diesel production into facilities. This opportunity certainly has its challenges, but they can be overcome with proper planning and forethought.
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