Interest in renewable diesel has increased significantly in recent years. In 2019, the estimated US renewable diesel consumption was 900 million gallons  followed by an estimated 960 million gallons in 2020. This trend is expected to continue in the coming years. Conversion of existing petroleum facilities to renewable diesel production provides an appealing path for many producers, but careful evaluation of the conversion must be performed to ensure that the facility is operating safely and in full compliance with PSM standards. While the existing relief and flare systems may have been adequate for the petroleum refinery service, these systems should be reviewed and redesigned along with the new process as the demand on these systems can significantly change.
Renewable diesel is produced by refining animal fats or plant oils rather than petroleum oil. It is chemically identical to petroleum diesel fuel and meets the ASTM D975 standard for petroleum diesel. As a result, unlike biodiesel, renewable diesel is not required to be blended with petroleum fuel before use and does not require modification to existing pipelines or storage facilities. Much of the increase in production capacity will be achieved through the conversion of petroleum refineries to renewable diesel service, which provides a faster and less costly path to bringing product to market.
The energy industry trend toward renewable diesel has been especially evident in the past 12 months. Disruptions due to the Covid-19 pandemic resulted in a drop in demand for oil as nonessential travel came to a halt and part of the workforce began to work from home, resulting in idled refineries across the country. For facilities that were idled due to this loss of demand, converting to renewable diesel product by reusing existing equipment and facilities creates an opportunity to begin production more quickly, while taking advantage of favorable government programs. Similarly, the existing relief valves and the flare disposal system can be revalidated more efficiently than designing grassroots relief systems. Depending on the scope of design changes to a particular unit, the effort to update existing relief systems design documentation can be significantly lower than greenfield design and documentation.
While there are multiple processes for converting the raw biomass to a diesel product, such as gasification and pyrolysis, hydrotreating is common to both renewable diesel production and petroleum fuel production. This process involves reacting the pretreated feed with hydrogen under elevated temperatures and pressures in the presence of a catalyst. Hydrotreating units in a petroleum refinery can be directly converted for use in renewable diesel processing.
When converting an existing petroleum facility to renewable diesel production, there are several things that should be considered when reviewing the relief devices and flare system to ensure that equipment is adequately protected. These include the following:
- Compared to petroleum service, the hydrotreating process for renewable diesel operates at a higher temperature and requires a significant recycle stream to provide cooling. This high recycle rate limits the feed and product rate units as the total flow through the hydrotreating section of the unit drives the capacity. The flow rate through equipment in the hydrotreating unit is expected to be significantly smaller in the feed and product sections when compared to petroleum service  and existing relief devices in these sections may be significantly oversized for blocked outlet scenarios. In addition, new recycle control valves can send high pressure products back to the lower pressure feed side of the unit. Failure open of these recycle valves can become the sizing case for the relief valves in the feed section.
- The higher temperature of the reactor train can impact the overpressure protection of the equipment. Loss of liquid recycle can send high temperature blowdown gas to the flare system through the emergency depressuring valves or the relief devices in the unit. This high temperature can reduce the capacity of the protecting relief and depressuring valves and needs to be considered when evaluating these systems. In addition, this impact should be evaluated on the flare system.
- As part of the conversion process, equipment may be removed or added to the facility, and equipment is likely to have different or additional feed sources than in the original design. As a result, existing relief device documentation may be out of date, and any change to the design should be evaluated to ensure that overpressure protection is adequate for any potential blocked outlet, failure of automatic controls, or other scenario. Changing pump lineups can create new flow paths that will need to be evaluated for the potential of reverse flow.
- Due to the hydrogen intensive nature of the renewable diesel hydrotreating process compared to petroleum diesel hydrotreating, the capacity of the facility’s hydrogen production unit may limit the production rate of the entire facility. Because of this, extra care should be taken when evaluating the relief requirement for rate dependent scenarios, such as closed outlets, in the hydrogen production unit to prevent artificial bottlenecking of the facility.
- The vegetable oils and animal fats that may be used as feedstock can become waxy or solidify at ambient temperatures. Because of this, heat tracing may need to be added to the feed processing units that was not previously required in petroleum service. Additional thermal relief valves may be needed to protect the heat traced piping. The relief valve discharge lines and flare systems may require heat tracing as well, to ensure the product does not solidify in the event the relief valve opens.
- As part of the conversion process, new alternate modes of operation may be created that must be evaluated, such as sending the unit into a full recycle, or a diesel flush mode to shut down the unit. Additionally, there may be new procedures created to start up and shut down the facility. A majority of process safety incidents occur during transient operations such as startup and shutdown of a facility. Because renewable diesel refineries undergo turnaround more frequently than petroleum service facilities, detailed analysis of these modes of operation are critical to evaluate the impact on the relief and flare systems.
This list is not meant to be exhaustive of all process safety considerations that should be made during the conversion of a petroleum refinery to renewable diesel service, but it does provide a few impacts on existing process safety infrastructure that should be expected for the new service. By carefully evaluating the changes to design and ensuring that all modes of operation are captured, any deficiencies can be identified and corrected early in the conversion process, preventing unsafe operation or expensive future rework.
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