For best product quality from vegetable oils should we hydrotreat them with conventional petroleum feed or use them as straight feed?Jun-2021
Robert Ohmes, Becht, ROhmes@Becht.com
Addressing this question requires review of multiple factors:
1. Type and quality of vegetable oils
2. Capabilities of existing hydroprocessing unit
3. Product blending system
For item 1, each of the vegetable oils (canola, soybean, corn, and so on) have their own distinct properties, both before and after hydroprocessing. After hydrotreating, these streams should meet or exceed typical specifications of ultra low sulphur diesel, such for specific gravity, distillation, cetane, and sulphur. The key quality that impacts this decision is cold properties. Given the highly paraffinic nature of the resultant products, the renewable diesel product will not meet cloud, pour, and/or CFPP without additional processing or blending. The primary options for meeting cold properties are 1) blending with conventional fossil fuel distillate streams, 2) cold property enhancement additives, or 3) use of dewaxing catalysts within the hydrotreater. Processing vegetable oils with conventional fossil fuel distillates in a hydrotreater is the most straight forward path to making renewable diesel, but the percentage of vegetable oil allowed may be dictated by the resultant cold properties. Blending of other hydrotreated fossil fuel distillate streams with the coprocessing unit’s product can allow for additional vegetable oil processing up to the same controlling limit. Maximising vegetable oil processing likely requires use of a dewaxing catalyst to isomerise the paraffins, improve cold properties, and meet final product blending requirements.
Building off this element is the capability of the existing hydroprocessing unit (item 2). The high organically bound oxygen content of vegetables oils (at least 10%+) requires significant hydrogen addition to meet final product specifications, along with breaking the backbone of the triglycerides. Hence, the hydrotreating unit must have the assets to manage and complete hydrodeoxygenation. For instance, the make-up hydrogen compressors, as well as hydrogen generation assets, must have sufficient capacity to meet the vegetable oil processing target. If either of these elements is limited, then the percentage of vegetable oil processed will be limited. Similarly, this high level of hydrogen consumption requires sufficient recycle gas capacity not only for sufficient treat gas into the reactor to ensure proper catalyst distribution, hydrogen availability for the reactions at the catalyst site, and a heat sink for the resulting exotherm, but also quench capacity to maintain reactor heat balance and avoiding runaway reactions. In addition, the metallurgy and contacting systems must be able to handle the CO, CO2 and water that are generated by the oxygen removal reactions. If the unit is designed for a significant percentage of vegetable oil processing, then the unit should have capacity for higher processing levels. However, if an existing asset was repurposed and moderately modified to allow for a small percentage of co-processing, then increasing that processing percentage will be a challenge. For existing assets, if some of the reactor catalyst volume was used for dewaxing catalyst, then the conventional fossil fuel processing and vegetable oil processing will be impacted as one balances improving cold properties while hydrotreating the renewable and conventional feeds to meet oxygen, sulphur, and cetane requirements. In summary, the asset capacity and capabilities of the hydroprocessing unit will impact this decision.
Finally, hydrotreated vegetable oils produces high quality diesel material — high API gravity/low SG, high cetane, low sulphur, and ideal distillation range. However, as stated above, the poor cold properties require processing or blending mitigations to meet specifications. The advantage of coprocessing and coblending of hydrotreated vegetable oils with conventional distillate streams is that the organisation can use each blendstock to create a product that meets all the diesel qualities with minimal giveaway. Hence, a typical refiner with cold property giveaway and ‘spare’ hydrotreater capacity can examine coprocessing of vegetable oils to not only take up that giveaway but generate renewable credits and reduce carbon intensity. Similarly, a facility that is cetane limited can coprocess or blend renewable diesel to create space within the cetane pool to allow for further optimisation of upstream fractionation and/or additional processing of low cetane feedstocks. Producing a pure renewable diesel product does give the most flexibility from a blend perspective and, in some regions, provides clearer accounting and capturing of renewable credits, but making pure renewable diesel does require a robust processing scheme and hydroprocessing unit as well as the ability to manage cold properties of the standalone stream.
In summary, the decision on achieving best qualities is multifactorial and is a combination of sources for vegetable oils, capability of the treating assets and auxiliary systems, blending infrastructure, ability to capture renewable credits, and target market. A holistic analysis is required to review the processing capabilities, limitations, feedstock sources, market potential, incentives, and blending approaches.
Tom Ventham, email@example.com, Unicat B.V / G. W. Aru LLC
Processing of vegetable based materials, such as FAME, is known to be a highly hydrogen intensive process. The incremental increase in hydrogen demand would also be seen if blending with conventional feeds. If considering adding vegetable oils to the hydrotreater diet, it is recommended to review the hydrogen balance to ensure demand can be met. Many refiners now are tasked with finding more hydrogen to satisfy the refinery hydrogen balance. That may be a result of newly processing vegetable feeds, increased hydrotreatment requirements to meet new road fuel specifications, changing crude slates, or even hydrogen production for fuel cells. The steam methane reformer remains the heart of the hydrogen production system and is also the most adaptable tool should hydrogen requirements increase for reasons such as that posed in the question. Unicat has a long history in the hydrogen catalyst market, and the recent acquisition of the Magma group means it is now the leading company to offer catalytic solutions to refiners who would like to increase hydrogen production from their SMR unit. With MagCat SMR catalyst technology, hydrogen yields can be increased by 15-20% purely through replacing the incumbent catalyst with pellet-like MagCat catalyst. Using a nickel based catalytical system but with a totally redesigned and reengineering pellet-type substrate, MagCat radically improves fluid flow properties in the SMR tubes. If increased hydrogen production is not the current driver, the same catalyst also significantly improves hydrogen production efficiency which is seen by the user as a reduction in energy or fuel requirements and a reduction in CO2 emissions. A reduction in both reformer tube wall temperature (TWT) and hot spots or banding increases tube lifetimes as a further cost and safety driver. Therefore, with a view of moving to a greener operation using vegetable feeds, MagCat catalyst from Unicat gives many synergistic benefits to improve the efficiency and effectiveness of the whole hydroprocessing operation in a simple and straightforward way that can be implemented immediately without the need for costly retrofits or redesigns.
Joris Mertens, KBC, Joris.Mertens@kbc.global
Product quality from vegetable oil hardly depends on whether the oils are co-processed or not.
Co-processing can provide a low capex option to upgrade vegetable oils. The value of the product is largely determined (and boosted) by legislation that incentivises low carbon intensity produce and by the availability of feedstocks that qualify as low carbon feedstock. There is a drive to only qualify waste vegetable oils (and animal fats) as possible low carbon feeds. In some regions legislation only incentivises pure vegetable oil processing, not co-processing.
Co-processing to up to around 5% may be feasible without investment at all but pretreatment normally is required for waste bio-feeds. At higher percentages the increase in hydrogen consumption (and reaction exotherm) will become very significant, as will catalyst activity inhibition due to CO formation, while the impact on the product cloud point will become notable as well.
A unit treating only vegetable oils requires significant changes in unit design i.e. The use of higher grade steels, high and/or liquid quenches to control exotherms, the requirement to remove CO2 and to isomerise the product (the latter in order to reduce cloud point), and a difference in product separation due to the difference in yield structure.