Impurity
Effect on process/equipment
Pretreatment needed
Phosphorous compounds (phosphatides or phospholipids)
Gum or gum-like material that creates serious fouling issues in process equipment and impacts catalyst activity Corrosion of process equipment that is not specifically selected for HVO processing (e.g. when existing hydroprocessing unit is revamped) Shorter catalyst lifetime and/or increased
Various methods of de-gumming, such as chemical or enzymatic
hydration
Free fatty acids (FFA)
Neutralisation or steam stripping
Metals, soap residues, chlorides,
Different operations, such as washing, bleaching, and filtering
polyethylene
equipment fouling
Table 1 Typical impurities in bio-based feedstocks, their effect on process and equipment, and pretreatment processes required for their removal
as equipment fouling and corrosion, as well as to reduced catalyst cycle length and selectivity. PTU configuration 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
Phosphorous compound
Ease of hydration
Phosphatidylcholine (PC) Phosphatidylinositol (PI)
Acceptable to high
PI calcium salt Phosphatidylethanolamine (PE) PE calcium salt
Vegetable oil
Phosphorous content, ppm
Coconut and palm oil
<30
Not possible through normal hydration
Groundnut, sunflower and corn Cottonseed, soya and rapeseed
200-800
Phosphatidic acid (PA)
PA calcium salt
<1,400
Table 2 Typical phosphorous content in various vegetable oils
Table 3 Ease of hydration depending on type of phosphorous compound
www.decarbonisationtechnology.com
38
Powered by FlippingBook