Corrosion risks in co-processing of rapeseed oil

Abstract

With an increased knowledge about greenhouse gas emissions and climate change, the interest in renewable energy sources are increasing remarkably. At Preem’s refinery in Lysekil an ongoing project is the refining of a more renewable diesel by initially adding 2 10 wt% of rapeseed oil into the already present Synergetic Saturation (SynSat) unit. The added rapeseed oil is blended in with fossil oil in a so called co-processing technique. Rapeseed oil consists of triglycerides, which in turns consists of different types of fatty acids which can cause fatty acid corrosion. This occurs when triglycerides decompose into free fatty acids. This master thesis investigated the decomposition of triglycerides at temperatures of 290, 310 and 330 °C together with a mixture of 5, 10, 15 and 100 wt% of rapeseed oil for each temperature. The pressure was set to 59 bar and LHSV to 1.5 h−1 in hydrogen atmosphere. TAN values concluded that none of the samples caused a risk of corrosion. By comparing the TAN values with a theoretical chemical kinetic model over degradation of vegetable oil in a hydrogen free environment it was concluded that the presence of hydrogen gas is of great importance. All samples tested in a hydrogen environment showed a TAN value of < 3 mg KOH/g whereas all theoretical samples in a hydrogen free environment gave a TAN value of > 39 mg KOH/g. With time an increase in added amount of rapeseed oil into the SynSat unit is planned to 20 wt% or more. Since vegetable oils contains much more chlorides than fossil oil there is a risk of deposition of NH4Cl in the upstream system of the revamped SynSat unit, which can cause fouling or corrosion in pipes of heat exchanger or air coolers. Calculations in combination with published diagrams showed that there in fact is a risk of NH4Cl salt deposition in the upstream system of the SynSat unit. Therefore an water injection point is necessary at this stream to avoid such deposition. The degradation of rapeseed oil forms unwanted by-products like CO2, which in contact with water is corrosive. Other products such as H2S and NH3 are formed as well in the process. The addition of an water injection point to avoid salt deposition of NH4Cl, can therefore cause sweet or sour corrosion in the amine recovery system. When investigating the system it showed to be of sour corrosion dominating type, where a risk of corrosion was found to be possible in some streams. It was however not possible to predict the type ofcorrosionorcorrosionrateofsuchasystemduetothecomplicatedreactionmechanisms of sour corrosion.