Removal of sulfur from oil products is mandated by the increasing quality requirements to motor and heating fuels and environmental concerns. Oil refining distillate products have different content of sulfur compounds. The type content of organosulfur compounds, extracted from mid fractions of Arlan oil shows practically no merkaptan compounds, and 85-95% of the compounds are cyclic saturated sulfides, as well no more than 15% of poly alkyl-substituted and cycloalkyl-substituted thiophenes, dialkyl sulfides, benzthiophenes, thiohydrindens and alkylcycloalkyl sulfides. The diesel fraction of oil from Romashkin field, the prevailing substances are di-, tri-, polythiacylanes and aromatic sulfides, hence the different approaches to methods and conditions of their purification.
Light fractions (ligroin – benzene) contain mostly low molecular sulfur compounds, some of which are hydrogen sulphide and light mercaptans. Hydrogen sulphide and mercaptans, as well as part of the other sulfur compounds, can be removed by relatively simple, as far as technology is concerned, chemical processes (e.g. alkali treatment).
The drawbacks of the alkali process is the unrecoverable loss of expensive reagent and formation of sulfuric alkali waste, difficult in terms of disposal.
Heavier fractions (kerosene, diesel fractions, vacuum gasoil) contain mostly cyclic and polycyclic high molecular sulfuric compounds, which require deep and complex purification methods.
In the general case, the methods of oil and oil distillate purification methods can be divided into two groups: 1) destruction of organosulfur compounds and their removal from fuel, and 2) selective extraction of organic sulfur compounds with simultaneous purification of oil fractions.
The first group includes:
- adsorptive and catalytic sulfur removal from oil fractions in the presence of adsorbents and catalysts;
- removal of sulfur from oil and oil products by microorganisms.
The second group includes:
Catalytic hydrofining. This is a process of hydrocracking of the bonds between carbon and heteroatom with nearly complete transformation of sulfur, nitrogen and oxygen containing organic compounds into saturated hydrocarbons with simultaneous formation of easily removable H2S, NH3 and water vapor; hydrogenation of unsaturated hydrocarbons. Heavy metals also precipitate on the hydrocracking catalyst. Volatile sulfur, nitrogen and oxygen compounds, formed in the process of hydrocracking, are removed by steam stilling in distilling columns. Hydrocracking is a typical reaction for the entire organosulfur compound group:
The rate of hydrocracking reaction increases as follows:
mercaptans > disulfides > sulfides = thiophanes > thiophenes,
which corresponds to their stability. When hydrocracking conditions change, i.e. hydrogen pressure increase, more active catalyst, temperature change, the hydrocracking of organosulfur compounds becomes efficient with formation of the corresponding hydrocarbons and hydrogen sulfide.
Hydrocracking is performed at 300-380°С. Some of the typical catalysts are: alumina-cobalt-molybdenum (9-15% МоО3, 2-4% СоО) or alumina-nickel-molybdenum (up to 12% NiO, up to 4% СоО), carrier – Аl2О3, sometimes with addition of zeolites, alumina silicates etc. Content of hydrogen in the hydrogen containing gas is up to 90% by volume, hydrogen consumption 0.1-1.0% of raw material mass. Liquid product yield is usually 96-99%, total output of hydrocarbon gases, benzene, H2S, NH3 and Н2О vapor is 1-4%. The general process of hydrocracking is as follows: mixing the raw material with the hydrogen-bearing gas and preheating of the mix in a heat exchanger; heating of the mix in a pipe-still heater, the actual hydrocracking, in one or multi-section reactor (a steel cylinder), cooling of the hydrogenation product, separation of the hydrogenation product from the hydrogen-bearing gas and then from hydrocarbon gases in high and low pressure separators respectively with subsequent fractionation into target products, removal of H2S, NH3 and water vapor from gases.
The process of hydrodesulfurization of oil fractions is accompanied by the reactions of hydrocracking, dehydration of naphthenetic and dehydrocyclization of petroleum paraffinic hydrocarbons. The process of hydrofining of light and medium distillates is highly effective and is widely used in the industry; the main difficulties being the hydrofining of heavy fractions and cut back. Successful development of hydrodesulfurization of cutback products is defined by the progress in the field of catalyst development.
Biodesulfurization. Microbial desulfurization of oil includes aerobic and anaerobic transformation of organosulfur compounds, which lead to formation of easily removable water soluble products. The main condition is to selectively remove sulfur without destruction of the rest of of oil components. American and Japanese companies have the lead in the area of industrial desulfurization technologies.
Extraction. The most effective physico-chemical method of desulfurization is extraction of organosulfur compounds by mineral and organic acids. This process is of interest in the field of production of sulfur concentrates. Purified oil products are a byproduct of this.
Sulfuric acid purification. This is one of oldest methods of oil product purification, which involves mixing the product with a small amount of 90-93% sulfuric acid at normal temperature. Concentrated sulfuric acid is a strong oxidant, therefore sulfur compounds sulfonate and oxidize: mercaptans to disulfides, sulfides to sulfoxides, thiophanes and thiophenes to sulfoxides.
The mechanism of interaction of aliphatic and cyclic sulfides is the formation of acidic sulfates of sulfonium salts.
Organosulfur compounds, being weak bases, easily form sulfonium compounds with sulfuric acid.
The protonation of organosulfur compounds with sulfuric acid is an exothermic process, limited by the thermodynamic equilibrium. The process is significantly influenced by temperature, the nature of proton acceptor and solvent. The proton acceptor ability decreases with the growth of the molecular mass of organosulfur compounds in the row
It should also be noted, that using concentrated sulfuric acid to remove sulfuric compounds is complicated. Beside formation of sulfur compounds, there are undesired processes of oxidation, gumming, sulfonation and solution of sulfur compounds in the acid, which causes some of them to break down and others to be lost as high-polymer compounds. Therefore, extraction of sulfur compounds from oil and oil products is not done with concentrated acid, but with 50-80% acid. The drawbacks of this method is destruction and loss of sulfides, and also large consumption of sulfuric acid. To obtain 1 kg of sulfide concentrate, about 13 – 16 kg of concentrated sulfuric acid is required. A more effective extracting agent is perchloric acid. However, explosion hazard and high cost of this material make it unfeasible in comparison with sulfuric acid.
Some of the other extracting agents used are phenol, furfurol, diethylene glycol, sulfurous anhydride, sulfolane and hydrogen fluoride. The degree of sulfur compound extraction is significantly influenced by their nature. The effect of all extracting agents is based on the weak dipole-dipole interaction between polar molecules of the extracting agent and organosulfuric compounds.
Along with extraction methods, especially in laboratory practice, sorption processes are widely used, of which the most significant is adsorption chromatography. Some of the adsorbents used are silica gels, alumina oxides and mineral clays. Adsorption of organosulfur compounds increases as follows: thiophenes > aliphatic mercaptans > aliphatic disulfides > aromatic and cyclic thiols > aromatic disulfides > aliphatic and cyclic sulfides.
Oxidation methods. These methods are based on functional group modification and have long since been used for removal of sulfur compounds from oil products due to easy separation of hydrocarbons from oxygenous sulfur compound oxidation products. The feasibility of oxidation method is due to the possibility of practical use of the resulting sulfoxides and sulfones.
It can be assumed that the feasibility of a desulfurization method is defined largely by the ability of qualified application on industrial scale. The search for ways to use organosulfur compounds of petroleum origin, in particular sulfides, sulfoxides and sulfones, will develop in two directions:
- using these compounds and their derivatives as bioactive material;
- in the direction of using the beneficial qualities of chemical, physical and other properties of such compounds.