Strategies to maximise profitability in HVO complexes
A review of some of the challenges involved in processing hydrotreated vegetable oil and the available solutions to optimise plant profitability.
Jay Jeong, Eva Andersson and Bent Sarup
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Given the ever-increasing focus on reducing Scope 3 CO₂ emissions and closing the gap on carbon neutrality, the interest in biofuels and the pressure to increase their production are higher than ever. Fatty acid methyl esters (FAME) biodiesel has served the industry well for many years as an additive to petroleum-based diesel. However, it has significant limitations with respect to blending ratio, and it cannot be used for jet fuel. These factors restrict its potential to replace fossil fuels.
A more viable solution has emerged in the form of renewable diesel or sustainable aviation fuel (SAF), produced through hydroprocessing of fossil-free feedstock or so-called hydrotreated vegetable oil (HVO) processing. With HVO processing, production capacities of more than 10,000 bbl/day are achievable, and both renewable diesel and SAF can be produced with quality that is equal to or better than that of traditional petroleum-based fuels. Moreover, with an optimally designed pretreatment system, the HVO process can handle a wide variety of feedstocks, ranging from vegetable, animal or even waste fats and oils to second-generation feedstocks, such as pyrolysis or hydro-pyrolysis oils generated from biomass.
Due to these advantages, many existing refineries are now implementing HVO processing, whether through drop-in co-processing of bio-based feedstock in existing diesel hydrotreating units (DHT), by revamping an existing hydroprocessing unit or by integrating a new grassroots HVO processing unit into the existing plant. Additionally, new entrepreneurial companies are entering the fuel market and constructing their own stand-alone HVO complexes.
The investment in an HVO complex is typically higher than that for a traditional FAME plant by at least one order of magnitude. To achieve economies of scale, the complex is commonly designed for a higher capacity. In addition, for a complete stand-alone HVO complex, several process units are required:
• Pretreatment unit (PTU) for bio-based feedstock
• Hydrogen production unit (HPU)
• The HVO process unit itself
• Sulphur recovery unit (SRU), including amine treatment unit (ATU), tail gas treatment unit (TGTU) and sour water stripper (SWS)
• Wastewater treatment unit.
With the exception of a bio-based PTU, these processes are typical in most refineries. However, the feedstock in HVO processes is different, which poses new challenges for the refinery operators. To maximise plant profitability, feedstock flexibility, plant cycle length, and product yield must all be maximised. Meanwhile, the risk of equipment corrosion and fouling must be minimised, along with utility consumption (energy and water). Similarly, waste handling must be optimised.
The following sections will explore some of the challenges in HVO processing, highlighting solutions available to maximise plant profitability.
Feed pretreatment unit
One of the most important success factors for the HVO complex is the feed PTU. Without proper feed pretreatment, the impurities present in bio-based feedstocks can lead to issues such as equipment fouling and corrosion, as well as to reduced catalyst cycle length and selectivity.
Depending on whether the HVO plant is a revamped hydroprocessing unit or a new purpose-built process unit, and in order to maximise the feedstock flexibility, the PTU must be tailored with a different set of pretreatment processes. Additionally, the selection of feed pretreatment processes is influenced by environmental legislation, the value and cost of handling the by-products, and the cost and availability of utilities and labour.
The selection of a de-gumming process depends on the type and amount of phospholipids in the feedstock. Typically, the oil is treated with acid (phosphoric or citric) for a high conversion of oil-soluble phospholipids into their water-soluble form, which can then be removed efficiently by high-speed centrifugal separators as part of the heavy phase. For difficult-to-remove non-hydratable phospholipids, enzymatic de-gumming can be applied to convert the phospholipids into lysophospholipids (cutting off a fatty acid side chain) and increase their water-solubility. Enzymatic de-gumming is already widely applied as upstream feed pretreatment in FAME biodiesel production units.
If the acidity of the oil is high after de-gumming, and if the HVO process equipment is not upgraded to corrosion-resistant materials, neutralisation must be the next step in the PTU. If the free fatty acids (FFA) content is less than 2-3 w/w%, a chemical neutralisation process with caustic soda will be sufficient. However, if the FFA content is above 3-5 w/w%, a physical de-acidification process using steam stripping under vacuum will be required. This type of de-acidification produces a distillate by-product known as ‘soap stock’, which can be sold to processors who use a soap stock splitting process, in which the acidic oil is liberated through treatment with concentrated sulphuric acid.
For the feedstock to be acceptable for the HVO process, its phosphorous content must typically be less than 3 ppm. With many feedstocks, de-gumming is not enough to reach this level. In such cases, bleaching/adsorption is the next step in the PTU. Various qualities of adsorption clay/earth exist in the market, and there is usually a correlation between price and performance.
For most feedstocks, these three pretreatment steps are sufficient to remove or reduce impurities to a level acceptable for further HVO processing. However, if used cooking oil (UCO) or tallow is used as the feedstock, another washing step, aimed at removing water-soluble chlorides, must be added upstream of the de-gumming stage. In the case of tallow, a further adsorption step, aimed mainly at removing polyethylene, must be carried out prior to de-gumming.
Figure 1 summarises the many different feed pretreatment steps in one flow chart.
Effluents from the PTU
The de-gumming process produces a significant amount of wastewater that needs to be treated. The main effluents from the PTU are spent adsorption clay (around 0.5-2% of the oil flow, depending on the feedstock and the final quality of the pretreated oil) and wastewater from the different oil washing operations (typically in the range of 5-10% of the oil flow). How the effluents are handled depends on the availability of an outlet for by-products such as spent adsorption clay and soap stock, local site conditions such as spare capacity in an existing wastewater treatment facility, and the specifications for the cleaned wastewater discharge.
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