school energy&environment

Treatments for special hydrocarbons

In Estonia, oil shale is burnt directly in thermoelectric power plants, however this is one of the few examples of direct utilization of non conventional oils. Normally, heavy hydrocarbons cannot be utilized in conventional refinery plants: they are too dense and viscous and contain large quantities of substances such as sulphur or heavy metals. These require a prior treatment, known as upgrading, that transforms them into lighter hydrocarbons, which also purifies them from the more harmful substances. Thus the large family of so-called “syncrude” oils (or SCO, synthetic crude oil) is born, the raw materials that are produced by synthesis (i.e. by chemical treatment) from other compounds and that, besides the products of the “non conventional” hydrocarbons, also include the raw materials produced from the liquefaction of coal or liquid hydrocarbons produced from the condensation of natural gas. A family that is rather costly at present, but which will give an ever increasing contribution to the production of our energy requirements and will be of great help in the difficult and long transition from fossil sources to renewable sources. Since high viscosity oils are rich in carbon, and lack H, the upgrading processes to obtain low viscosity raw material that can be used in the conventional refineries imply three main phases: breaking up of the macromolecules (cracking) and elimination of excess carbon atoms, by means of a process called coking, addition of hydrogen to compensate the excess of carbon (hydrogenation) and removal of the sulphur, nitrogen and heavy metals. In the coking process, the heated viscous oil is vaporized in a low pressure chamber, and the residues produced are coke, carbon mixed with various minerals (5%) and with sulphur (6-8%). The coke can be utilized as fuel (which is however not advisable as it is one of the “dirtiest” fuels) or in the steel production process. The coking process produces large amounts of CO2 and therefore there is a trend to decrease its application in favour of hydrogenation. All the more viscous hydrocarbons have a very high content of sulphur (a percentage that varies from 0.1 to 0.2 % up to 4 – 8 %) that is eliminated with the hydrogenation process which, besides producing lighter oils, extracts S, transforming it into H2S. This is again transformed later on into elementary S, and is then opportunely disposed of or stocked. There are many processes that improve the characteristics of non conventional oils, transforming them into good quality syncrude, and these are rapidly evolving. In fact this is one of the research sectors of the oil industry in which investments and efforts are mostly focussed. Even though non conventional hydrocarbons are not particularly abundant in Italy, it is in the vanguard in research in this field. In fact , ENI and SNAM research laboratories have obtained remarkable results, and have also developed interesting upgrading technologies with which it has become possible to eliminate the intermediate production of heavy fuel oils and coke: this process is known as the ENI Slurry Technology or EST. The upgrading phase does not always take place at the extraction site, and for this reason the viscous oils must also be treated, by diluting them with lighter oils, so that it is possible to transport them in pipelines: in fact, products that are too viscous cannot be transported in pipelines.