Corrosion in biodiesel production process using high free fatty acid feedstocks.

摘要

Recently, biodiesel has received worldwide attention due to its renewable, biodegradable, and non-toxic nature. It can be produced from vegetable oils and animal fats through a 'transesterification reaction' using alcohol and a catalyst. The use of virgin vegetable oil as a raw material to produce biodiesel could divert agriculture from food to fuel production, which in turn raises food prices and leaves some parts of world vulnerable to famine. The use of cheap raw materials, such as waste/used vegetable oils, animal fats, and non-edible oils, could help avoid imbalance in food chains and enhance the utilization of waste oils. The production process using these high free fatty acid feedstocks involves sulphuric acid as a catalyst to convert oil and fatty acid to biodiesel, typically methyl esters. This can be achieved through an acid transesterification process or two step process wherein acid esterification is followed by an alkali transesterification process. However, the use of acid in the processes may lead to the corrosion of process equipment, as has been addressed in a number of publications. However, to date, no detailed study on corrosion in this process has been carried out.;This work investigates the corrosion of carbon steel in the acid catalyzed esterification process by carrying out electrochemical corrosion experiments using cyclic polarization and Tafel plot techniques and weight loss immersion experiments. Eleven process locations in the esterification process were simulated for corrosion testing, and their susceptibility to corrosion was reported in terms of corrosion rate and pitting tendency. Canola oil and oleic acid were used as oil feedstock and free fatty acid, respectively. Results show that carbon steel is suitable for five of the eleven process locations. The presence of a small amount of water in methanolic solution contributes to non- or low-corrosion rates in the methanol recovery flow line, while the presence of free fatty acid contributes to non- or low-corrosion rates in the storage tanks of oil feedstock, the esterification reactor, the glycerol feed to the stripping column, and the end-product recovery flow line. The other six process locations made of carbon steel are corrodible and require the application of effective corrosion control methods. These locations are the methanol storage tank, sulphuric storage tank, methanol and sulphuric acid storage tank, fresh and recovery methanol and acid mixing tank, inlet flow line to the vacuum distillation column, and vacuum distillation column. The vacuum distillation column and its inlet flow line are the most susceptible to corrosion due to the elevated temperature and the presence of methanol, sulphuric acid, glycerol, and water. The application of nitrogen blanketing to remove dissolved oxygen is effective for corrosion control in the methanol storage tank, and the use of corrosion resistant materials, i.e., stainless steel and fibreglass reinforced plastic, is effective for the rest of the corrosive process locations.