The problem removing water and particulate matter from high quality diesel fuel in the production process is quite common. Contamination of diesel fuel starts at the refinery and continues all the way down to storage facilities and fuel tanks. Water can be present in diesel fuel in solved or emulsified state. The latter is especially harmful to the product’s performance.
Modern oil refining companies are locked in fierce competition and have to make ever increasing effort to purify diesel fuel beginning with the refinery.
Most common devices and processes used in the refineries involve regular methods of emulsion destabilizing: gravity, centrifugal, electrical and chemical methods, as well as filtration through coalescing barriers. Special physical methods, such as ultrasonic or magnetic influences, are used rarely and only when the amounts of fuel are relatively small.
Settling can only be used as a preliminary purification stage. Centrifugal equipment to remove water and particulate matter is limited by its high costs and the high expenditure of electricity, as well as complexity of assembly, commissioning and servicing.
Particulate matter is commonly removed by regular fiber filters, however, such filters are nearly useless to remove the water in the fuel.
The recently popular coalescing filters, which separate water well, are not efficient enough when the diesel fuel contains a lot of solid impurities.
These filters clog easily and are difficult to restore.
Small amounts of water can be removed by vacuum drying. The fuel is sprayed inside the vacuum chamber through a nozzle. Free and solved water, as well as air, are removed from the oil. The fine dispersion of the fuel makes the drying process efficient. Dry fuel droplets fall to the bottom of the vacuum chamber. Efficiency is further improved by heating the product. The loss of product due to evaporation is insignificant. The vacuum drying machines are more reliable and efficient than centrifuges. Besides, the energy consumption of such machines is 3-4 times less. The limitation of this method is that it alone cannot remove solids from the fuel.
Drying petroleum products with calcium oxide (СаО) is also worth mentioning. Fresh high quality calk is used, which is easily hydrated. The moist fuel passes through an adsorbent tower with CaO at 18-20°С. Calk loss is 0.2% of the product volume. The flakes of Ca(OH)2 are separated from the fuel or oil with other solid impurities by settling or filtration. This method requires laboratory evaluation for specific fuel and CaO (for compatibility and consistent quality). This is evaluated by ash content. Oil ash after CaO treatment must not contain calcium salts. One of the limitation of this method is the fact that little research has been performed on its application with diesel fuel specifically.
There is a way of fuel drying by synthetic zeolite (a molecular sieve). The water captured by the zeolite behaves like absorbed water. When zeolite is heated, it is gradually removed, but even with total dehydration the structure of the medium remains intact. Both NaA and CaA zeolites are used with fuels. Both absorb moisture well and are highly selective.
Drying of oil fractions and specific hydrocarbons, such as transformer oils, olefins, alcohols etc, with NaA molecular sieves has received much attention from researchers. Compared to the existing methods of oil product dehydration, based on heating them to water evaporation point, molecular sieves enjoy several significant benefits. Zeolite dehydration is approximately three times less expensive than centrifuge dehydration. Research shows that after 2000 cycles of regeneration, zeolite retains up to 70% of its activity. The limitations of this method is reduced filtration rate and short filter cycle if the product contains impurities, complexity and costs of zeolite regeneration by heat. The problem of comprehensive petroleum product dehydration can be solved by using porous composite polymers. Such materials were first used as filter elements in fuel and oil filters in the 1970s. Much research and wide implementation of porous polymers demonstrated their ability to ctap water and solid impurities in the product. This material is highly efficient if used in fine filters to remove solid impurities and water from petrochemical products.
Keeping up the high quality of diesel fuel requires several measures to remove all or most contaminants.