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. Spent adsorption clay contains residual oil of 20-25%. Potential outlets for this by-product include biogas manufacture or burning of the clay to utilise the oil’s calorific value, as well as extraction of the residual oil by another company. If there is no spare capacity in an existing wastewater treatment facility, there is potential to debottleneck or to add another treatment facility. Either can be done using well-known technologies, such as dissolved air flotation (DAF), sequencing batch reactors (SBR), membrane bioreactors (MBR), aerobic digesters,
Impurity
Content before/ after bleaching, ppm
Phosphatides
10 → <3 5 → 0 50 → 5 0.5 → 0
Soap from de-gumming
Metals
Moisture and volatile matter
Table 4 Impurity content before and after bleaching
thickeners, presses, decanters and high-speed centrifugal separators. However, wastewater can also be evaporated, thereby concentrating the waste stream to less than 2 w/w% of the original wastewater stream. Besides minimising the load on the wastewater treatment facility, this recovers more than 98% of process water for reuse in pretreatment processes. A wastewater evaporation plant can be run using low-pressure steam, which can be generated through waste heat recovery from the HVO process itself. This is described in the fractionator optimisation section. HVO processing The HVO process is, in fact, a series of processes. First, hydrotreatment (HDT) removes oxygen and splits the triglycerides into three chains of hydrocarbons. Next, the paraffins are converted into a mixture of hydrocarbons with the right cold flow properties, either through isomerisation for maximal diesel yield or mild hydrocracking (HCK) for maximal SAF yield. By-products from these reactions, such as propane, CO₂, and sour water, are removed in a stripper, while the final liquid products are separated in a downstream fractionation section. Figure 1 shows all HVO
processes in a simplified flow chart. The following sections focus on the
Crude palm oil (CPO)
Wastewater: Phosphorous compounds
Spent clay: Residual oil, soap, metals
Pretreated feedstock
Other seed oils
(Neutralisation) For high FFA content only
De-gumming Various types
Adsorption
Used cooking oil (UCO)
Wastewater or distillate (soap stock)
Chloride mitigation
Polyethylene removal
Tallow
Spent clay: Residual oil, metals, PE
Wastewater: Chlorides
Figure 1 A flow chart summarising the many different feed pretreatment processes
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