Petroleum Refining ProcessesDesulphurisationSulphur occurs in crude oils combined in a variety of ways, from the simplest compound H2S to complex ring structures. H2S is produced during distillation of the crude oil by decomposition of higher boiling sulphur compounds and appears in the LPG from which it must be removed because of its poisonous and corrosive nature. This is done by counter current washing with an amine (e.g.diethanolamine), the H2S being removed for sulphur recovery by heating the amine solution in a separate vessel thus regenerating the amine for recycle to the washing stage . Mercaptans can be considered derivatives of H2S, in which one hydrogen atom is replace by a carbon /hydrogen group, and share some of its unpleasant properties of bad smell and corrosivity . Those mercaptans boiling below about 80 are readily dissolved in alkaline solutions but the solubility decreases rapidly above that temperature .For LPG and light gasolines therefore the mercaptans can be removed by counter current washing with caustic soda solution.The UOP Merox process uses caustic soda to extract the mercaptans which are then oxidised with air to disulphides and the caustic soda regenerated for further use. The oxidation step is assisted by a metal complex catalyst dissolved in the caustic soda.The process can be represented as follow: 4C2H5SH+4NaOH= 4C2H5SNa+2H2O +2H2O+O2 2C2H5SSC2H5+4NaOHThe disulphides are not soluble in caustic soda and form an oil layer which can be removed.Mercaptans in fraction boiling between80 and 250 cannot be oxidised to disulphides in the Merox solution with air. The disulphides,which are non-corrosive and have little smell, remain dissolved in the oil so that no actual desulphurisation has been achived but the products have been “sweetened”. Another process for the oxidation of mercaptans uses copper chloride as a catalyst. Both processes can be used in the production of aviation jet fuels.As the cuts taken from crude oil increase in boiling point it is found that the sulphur increases. In the 250-350 range which is used for both diesel fuel and domestic central-heating fuel the sulphur content is about 1 per cent weight from most Middle East crudes. When this material is burnt the sulphur is oxidised to SO2 which, being easily oxidised to sulphuric acid, causes atmospheric pollution and corrosion of metals. The sulphur cannot be treated by the methods previously outlined as it is mainly combined with carbon and hydrogen in forms much more complicated than the simple mercaptans. These complex compounds have to be broken down to get at the sulphur which is done by passing the oil together with hydrogen at high temperature (320-420) and high pressure (25-70 bar), over a catalyst containing cobalt and molybdenum oxides on an alumina base, made in the form of small pellets or extrudates. The reaction is easier and the catalyst life better when the ratio of hydrogen to feed is several times higher than that necessary to complete the reaction chemically. Under these conditions the sulphur compounds decompose and the sulphur combines with the hydrogen to give H2S. Almost all of the sulphur compounds can be decomposed in this way without significantly affecting the remaining hydrocarbons.This process of desulphurisation, also called “hydrofining”, is effective in attacking all forms of sulphur compounds, and can be used to treat any part of crude oil.In principle the equipment used for all feeds is basically simlar and will contain means for carrying out the following steps:1 Supplythe feed and hydrogen to the reactor at the correct temperature and pressure.2 Cool the reactor product to condense the oil and allow the separation of the excess hydrogen so that it can be recycled to the reactor.3 Remove the H2S and small quantity (2-3 per cent) of low-boiling hydrocarbons producted in the reaction.A pump takes the feed and raises it to the repuired pressure and passes it through tubes in a furnace where it is heated to the required temperature before being mixed with the hydrogen and passing into the reactor. The reactor product is cooled, partially by the fresh feed in a heat exchanger to save fuel, and partially by water in another heat exchanger. Excess hydrogen is separated from the condensed oil in a drum and recirculated back to the reactor by a compressor together with fresh hydrogen to replace the amount consumed in the reaction. The liquid from the drum is passed into a distillation column where the H2S and low-boiling breakdown products are removed and the desulphurised oil taken from the bottom of the column.Much of the crude oil boiling above 350 is used to make heavy fuel oil for power-stations, ships and large industrial plants and can have a sulphur content of 2.5-4 per cent weight from most Middle East crudes. Buring this material releases SO2 and very high chimneys have to be used (a number in