油气储运专业毕业设计文献翻译(附原文)---原油的加工工艺.doc

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 the 500-600 foot range have been built, one of 800 feet in the USA) so that the SO2 can be dispered widely in the atmosphere thus avoiding localised pollution. The ideal solution would be to desulphurise all parts of the crude oil. Unfortunately, although the desulphurisation of distillates boiling up to about 550 cab be relatively easily accomplished, the treatment of heavy crude-oil residues poses many difficult problems. With increasing boiling-point the difficulty of desulphurisation increases and also the proportion of molecules containing sulphur becomes high (possibly up to 50 per cent) which means that a high proportion of the molecules present must be decomposed. Trace metals in the oil tend to deactivate the most effective desulphurisation catalysts and high pressures (up to 170 bar) must bs used. All these factors result in high costs for fuel-oil desulphurisation. Also recent developments in the crude-oil sopply situation worldwide have placed a stong emphasis on energy conservation. Consequently, fuel intensive processes would be employed only as alast resort when alternative means of miniming pollution are not viable.In a refinery where desulphurisation is used extensively the production of H2S can easily reach 100 tonnes per day. Although the H2S could be burnt to SO2 and vented from all stacks,it is very undesirable because of the atmospheric pollution caused and additional plant is instslled to recover the sulphur. The H2S is burnt to SO2 with the oxygen supply limited so that about one-thirt of the H2S burns. This gives a mixture of two-thirds H2S and one-third SO2 which will combine to form sulphur and water: 2H2S+SO2=3S+2H2OThe sulphur is collected and is usually sold to chemical companies mainly for the manufacture of sulphuricacid.Thermal CrackingWhen hydrocarbons are heated to temperatures exceeding about 450 they begin to decompose. The large molecules breaking or “cracking” into smaller ones. Paraffins are the most easily cracked followed by naphthenes, aromatics being extremely refractory . At one time thermal-cracking processes were widely used to improve the octane number of naphthas or to produce gasoline and gas oil from heavy fractions. The quality of the gasoline from the thermal cracking of naphtha is not high enough for present motor gasolines and the process has fallen out of use. The products from heavy oil cracking requirements and while at present the process is little used it could be of interest should conversion of heavy distillates (up to 550) to gas oils be required. One thermalcracking process presently in common use is visbreaking which is the thermalcracking of viscous crude-oil residues to reduce their viscousity by breaking down the large complex molecules to smaller ones.A satisfactory fuel oil can them be made without the necessity of using gas oils or kerosine to blend with the viscous residue.Another thermal-cracking process presently employed is Delayed Coking, which is normally applied toatmospheric or vacuum residues from low sulphur crudes for the production of electrode grade coke (used mostly in aluminium production). The residue is heated to about 500 and passed to the bottom of a large drum where the cracking reaction proceeds which breaks down the high-boiling materials, The lower boiling materials formed vaporise at the high temperature in the drum and pass out of the top to the fractionation system where they are separated into gas, gasoline and gas oil and leave behind in the drum a porous mass of coke. When the drum is full of coke the feed is switched to another drum which is filled while the full one is steamedout and the coke removed. As in the thermal-cracking process the liquid products require hydrogenation for use as naphtha or gas oil.Catalytic CrackingThermal cracking of heavy distillates for gasoline production is not selective and produces substantial quantities of gas and fuel oil together with the gasoline, which is also not of very good quality. About thirty years ago it was found that fullers earth and simillar materials could act as cracking catalysts and give a good yield of high octane number gasline (catalytic cracking). Unfortunately, the fullers earth became quickly covered in carbon and no longer acted as a catalyst was returned to its previous activity and thus to operate the process continuously it was necessary to devise methods of alternatively using and regenerating the catalyst continuously on a large scale.One of the most successful methods of achieving this depends on the use of fluidisation. When a gas is passed up through a bed of fine power the behaviour of the power depends on the velocity of the gas. If it is high (about 1 m/sec.) the particles are moved about by the gas and the bed of power acts like a fluid and can be transported, find its own level, ect. just like a liquid. By using this property, the catalyst, in power form, can be circulated continuously between a reaction stage and a regeneration stage.The reactor and regenerator vessels are each designed so that the upward vapour velocity is sufficient to fluidise the catalyst. The oil feed (normally boiling 350-550) meets hot (620-740) regenerated catalyst, which is substantially free of carbon, and the vaporised oil and catalyst pass through a transfer line to the reactor, where the catalyst forms a fluidised bed. The cracking reaction proceeds as soon as catalyst meets the oil and is completed within the reactor at 480-540, depositing carbon on the catalyst. The spent catalyst is steam stripped to remove entrained hydrocarbons and returned to the regenerator where air is used to burn the carbon from the catalyst. The oil products leave the reactor, via cyclones to reduce catalyst entrainment, and are separated into fuel gas C3/C4, gasoline and gas oils. The gasoline octane number can be as high as 95.The catalyst loses activity as a result of hydrothermal deactivation and the accumulation of metals from the feed, which can contain up to 1 p.p.m. of vanadium plus nickel. Catalyst activity is maintained by continuous sddition oof fresh catalyst and withdrawal of equilibrium catalyst to maintain a constant inventory.In addition to straight-run and vacuum gas oils, coker gas oil,etc. the feed to a catalytic cracker can include atmospheric residue provided the metals content is low enough. The Kellogg heavy oil cracking (HOC) process is designed for high metals content atmospheric residue.Great improvement have been made in the manufacture of catalysts which now incorporate molecular sieve materials (zeolite) and have a very high activity. It has been found that the long residence time of the vapours in the reactor gives rise to secondary reactions which reduce the selectivity of the conversion to gasoline and produce more gas and coke. New designs dispese with the fluid bed in the reactor and carry out the reaction in the transfer line; the oil and catalyst are quickly separated at the end of the transfer line, for instance in a cyclone, and the catalyst drops into a stripper as previously to the remove entrained hydrocarbons before transfer to the regenerator. This is called Short Contact Time (SCT) cracking and has markedly improved the yield of gasoline obtainable in the process.HydrocrackingAs hydrocarbons increase in the number of carbonatoms they contain, so there is a decrease in the ratio of the number of hydrogen atoms per carbon atom, e.g. methane, CH4, has a ratio of 4; pentane, C5H12, has a ratio of 2.4; decane, C10H22, has a ratio of 2.2. If we wish to produce low-boiling hydrocarbons (e.g. gasoline containing 5-10 carbon atoms) from highboiling hydrocarbons containing say 20 carbon atoms we must find some means of increasing the ratio of hydrogen to carbon. In thermal cracking olefines (which have a lower hydrogen/carbon ratio than paraffins) are produced and also carbon eliminated by deposition in the catalyst . The alternative approach is to add hydrogen and this is done in the hydrocracking process by cracking at a very high pressure in hydrogen.This process, which is very flexible and can produce high yields of either gasoline or gas oil from wax distillaate, or gasoline from gas oil, operates at pressures of 150-170 bar and temperatures of around 430. Reactors capable of withstanding these severe conditions may be 150-200 mm thick and pose many difficult engineering problems in design and construction. Hydrogen requirements for hydrocracking are very high, up to about 300 m3 per m3 of oil processsd which is far in excess of that available from catalytic reformers so that a large hydrogen production plant must be built to supply the hydrocracker.When designed to produce gas oil a hydrocracker will use one reactor and the basic-flow diagram appears very similar to a hydrofiner, but two reactors containing different catalyst are used when gasoline production is required . Unreacted feed is recycled to the reactor so that complete conversion of the feed to lower boiling products may be achieved. It will be appreciated that the severe operating conditions required in this process necessitate high-duty equipment to withstand the high temperatures and pressures, large gas compressors and pumps, and a hydrogen production unit which makes the capital cost very high.Catalytic ReformingCatalytic reforming is now one of the most important processes for the production of motor gasolines taking straight-run materials in the boiling range of about 70-190 as feed and raising the octane number from about 40 to 95-100 .The main reactions taking place are: Dehydrogenation of naphthenes to aromaticsCH2 CH3 CHsCH2 CH CH2 CH CH3+3H2CH2 CH2 CH2 CH CH2 CH2a paraffin isomerisation n hex