ABU GODWIN ITAKPE PSC0904473 TOPIC: DESULPHURIZATION OF KEROSENE LITERATURE REVIEW INTRODUCTION Refining of crude oil to final products requires desulphurization of the oil. Fuel specifications that govern transportation fuels have over the years become increasingly stringent with respect to sulphur content. Many petrochemical products are likewise
produced to be almost sulphur-free. The removal of sulphur from oil is consequently one of the central conversion requirements in most refineries and the price (and processing cost) of a crude oil is influential by its sulphur content. Kerosene is a clear, oily highly flammable liquid with strong odour, usually obtained by distillation of petroleum and used mostly as fuel and solvent (Merriam – Webster 2012).
It is burned in lamps, heaters, and furnaces and used as a fuel or fuel component for diesel and tractor engines, jet engines and rockets and as a solvent for greases and insecticides.
Kerosene is less volatile than petrol fraction boiling at the temperature range of 160 – 240oC and has a flash point of 38oC.
CHEMICAL COMPOSITION OF KEROSENE The chemical composition of kerosene depends on the different refining process. The major hydrocarbon contents in kerosene include aromatic, paraffinic naphthalenic compounds while the non-hydrocarbon components include sulphur, nitrogen and oxygen compounds (WL.Nelson, 1978).
Emphasis will be laid on the non-hydrocarbon content of kerosene as it relates to the subject matter. A. SULPHUR COMPOUNDS Kerosene distillate may contain from 0.3 to 0.5% of sulphur in the form of various organic compounds (Hobson, 1973) some of which are generated during distillation from higher original crude oil. The types of sulphur found in kerosene are mercaptans, sulphides, disulphudes and thiophenes (Greenwood and Earnshaw, 1997 ).
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Structural example of cyclic mercaptans in kerosene
refers to molecules containing
–SH functional group
with linkage C-S-C. Dissolved free sulphides (HS- and S2-) are very aggressive species for the corrosion of many metals such as steel, stainless steel and copper (Vaughan and Craig, 1978).
Sulphides and its corrosive properties is reduced by oxidation of the sulphide which forms thiosulphate as intermediate (S2O32-) while the medium is also acidified by production of sulphuric acid when the oxidation is more advanced (Greenwood and Earnshaw 1997).
The general formular of sulphides is RSR Structural examples of Sulphides in kerosene CH3 – S – CH3 H–S–H C2H5 – S – C2H5 Diethyl sulphide
Dimethy sulphides Hydrogen sulphide
Disulphides refers to the structural unit composed of linked pair of sulphur atoms (Greenwood and Earnshaw, 1997).
(hydrogenation) to give hydrogen sulphide and hydrocarbons as products. Structural examples of Disulphides in kerosene CH3 – S – S – CH3 Dimethy disulphide NITROGEN COMPOUNDS In crude oil distillates both basic and non-basic nitrogen C3H7 – S – S – C3H7 Dipropyl disulphide
compounds in kerosene appear and they usually have low nitrogen content. The occurrence of nitrogen compounds in kerosene degrades the colour of the product on exposure to sunlight. The nitrogen is easily removed by stripping or fractionation method. The basic nitrogen are nitrogens with alkyl substituted quinoline, pyridines, isoquinoline and its methyl derivative. The non-basic nitrogen are indoles, pyroles and carbozoles. They appear in kerosene but in trace amount. Structure of Basic Nitrogen Compounds
Non-Basic Nitrogen Compounds
OXYGEN COMPOUNDS (ACID COMPOUNDS) The oxygen compounds present in crude oil exists as organic acids or phenols. They are mostly concentrated in the gas oil fraction of the crude. The organic oxygen compounds are converted into water and hydrocarbons in a desulphurizer plant as show in the reaction below (William, 1994).
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Example of acid compounds in kerosine
ORGANIC COMPOUND (OIL) Sulphur in organic materials is conveniently determined by burning the sample in a stream of oxygen. The sulphur dioxide (as well as the sulphur trioxide) formed during the oxidation is collected by distillation into a dilute solution of hydrogen peroxide (Douglas, 2001) SO2(g) + H2O2 ® H2SO4
The sulphuric acid is then titrated with standard base (e.g. NaOH).
Alternatively, sulphur in inorganic compound can be detected or determined by adding freshly prepared sodium nitroprusside [Na2Fe(CN)5NO] (Wilson,1992 ).
The fading of the deep violet colour of the salts shows that sulphur is present.
WAYS OF REMOVAL OF SULPHUR (OR DESULPHURIZATION) OF PETROLEUM FRACTION (KEROSENE)
kerosene: Hydrodesulphurization is most commonly used method in the petroleum industry to reduce the sulphur content of crude oil as well as its fraction. In most cases hydrodesulphurization is performed by co-feeding oil and hydrogen to a fixed-bed reactor packed with an appropriate hydrodesulphurization hydrodesulphurization catalyst. catalysts are The standard and
CoMo/Al2O3 but there are many types available. During the hydrodesulphurization process, the sulphur in the
organosulphur compounds is converted to hydrogen sulphide (Topsoe et al. 1996).
Hydrotreating conditions typically range from 200 to 425oC and 1 to 18MPa, the specific conditions depending on the degree of desulphurization required and the nature of the sulphur compound in the feed. Aliphatic sulphur compounds are very reactive and can be removed completely during hydrodesulphurization according to the equations below; Thiols : R – SH + H2 ® R – H + H2S
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Sulphide : R1 – S – R2 + 2H2 ® R1 – H + R2 – H + H2S Disulphides: R1 – S – S – R2 + 3H2 ® R1 – H + R2 – H + 2H2S The sulphur contained in thiopheric rings is more difficult to remove because the lone pair electrons from sulphur
participates in the p – electrons structure of the conjugated C = C system. The resonance stabilization is around 120 – 130 KJmol-1 which is less than that of benzene (160 – 170KKmol-1) (Hochgensang, 1952) but still sufficient to make Two in the
hydrodesulphurization pathways of
equations below (Babich and Monlijn, 2003).
The least hydrogen intensive pathway is by hydrogenolysis meanwhile, the main hydrodesulphurization pathway requires saturation of the aromatic ring before hydrodesulphurization can take place.
Fig.1 Hydrogen Treatment Method 2. Extractive method: Desulphurization via extraction depends on the solubility of the organosulphur compounds in certain solvents. It is a liquid-liquid extraction process and the two liquid phase must be immisable. The elements of extractive desulphurization are shown in the diagram below. desulphurization or by solvent extraction
In the mixing tank the feedstock is mixed with the solvent and the organosulphur compounds are extracted into the solvent because of their higher solubility in the solvent. Then in the separator section the hydrocarbon is separated from the solvent. After separation the treated hydrocarbon with lower sulphur content can be processed further. During distillation the organosulphur compounds are separated from the solvent and then recovered solvent is recycled to the mixing tanks (Babich, 2003).
3. Oxidation Process (Oxidative Desulphurization Process): Oxidative desulphurization, as the name implies, involves a chemical reaction between an oxidant and sulphur that facilitates desulphurization. Oxidation process invariably
involves two conceptually different steps. The first step is the
sulphur oxidation, which changes the nature of the sulphur compound. The second step is the sulphur removal, which exploits the properties of the oxidized sulphur compounds to effect their removal (Marty, 2001).
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R – S – R’ + R – S – R’ + ½O2 ® O2 ® R – (SO) – R’ R – (SO2) – R’
The equations above shows oxidation of the sulphur in sulphide and thiophenic compounds to sulphoxides and sulphones (Plesnicar,1978).
The most common industrial application of oxidative
desulphurization is sweetening. Sweetening is a refining process that is employed for the conversion of thiols into disulphides shown in the equation below (Marty, 2001) : 2R – SH + 1/2 O2 ® R–S–S–R + H2O
Industrially, the thiols or mercaptans are converted in a basic medium to increase their reactivity to oxygen. For economic reasons, aqueous caustic soda (NaOH) is used as a base, air is the oxidant, and a homogenous phase catalyst is employed to increase the reaction rate.
The merox process is an acronym for mercaptan (thiol) oxidation. It is a catalytic chemical process developed by the universal oil products (UOP) company, used in oil refineries to remove
mercaptans (thiols) from kerosene by converting them to liquid hydrocarbon desulphuides (Meyers, 1999).
The Merox process for the removal of mercaptans in kerosene is a one-step process oxidation reaction take place in an alkaline environment as the feedstock kerosene, mixed with compressed air, flow through a fixed bed of catalyst in a reactor vessel. The catalyst consist of charcoal granules that have been impregnated with UOP’s proprietary catalyst Meyers, 1999).
The oxidation reaction that takes place is 4RSH + O2 ® 2RSSR + 2H2O
The Merox process also requires that feedstock be prewashed to remove hydrogen sulphide that would interfere with the sweetening. The reaction that takes place in the batch caustic prewash vessel is H2S + NaOH ® NaSH + H2O
Fig 2. The Merox Process 4. Colour Indicator Method : The water-white colour of kerosene has been associated with high quality and stable or unreactive type of the material. (Welcher, 1978).
Colour in kerosene may be caused by high boiling point molecules not properly separated by fractionation. Sulphur in coloured bodies in organic compounds may cause poor burning qualities or smoking. Treatment with sulphuric acid followed by caustic soda and a water wash and finally by percolation through Fuller’s earth (clay) may be
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needed for a particular petroleum fraction. Also, colour treatment in kerosene can be done by contracting kerosene with calcium oxide in Elenmeyer’s flask for a given contact time ( Welcher, 1978).
REFERENCES 1. 2. Merriam – Webster Dictionary, 11th edition, 2006. W.L. Nelson, Petroleum Refining Engineering, 4th edition, 1958.
G.D. Hobson, Modern Petroleum Technology, 2nd edition, 1973.
Greenwood, N.N. and Earnshaw A., Chemistry of Elements, 2nd edition, 1997.
ASTM Standard Test Method for Mercaptan, 2000, Pg 304 – 309.
Vaughan D.J. Craig T.R., Mineral Chemistry of Metal Sulphides, 1978.
William F. Bland and Robert L., Petroleum Processing Hand Book.
Douglas, Skoog Donald West James and Croneh, Analytical Chemistry. Analytical Chemistry.
L. Wilson, Comprehensive Analytical Chemistry. Topsoe H., Clausen B.S. Nassoble Fe (1996).
Welcher F.J. edition.
Standard Method of Chemical Analysis, 2nd
Hochgesang F.P., Thiophene and its derivations, 1952. Plesmear B. (1978), Oxidation with peroxy acids and other peroxides
Robert A. Meryers Hand Book of Petroleum Process, 3rd edition.