ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 3, pp. 299−302. © Pleiades Publishing, Ltd., 2014. Original Russian Text © A.V. Kurzin, A.N. Evdokimov, A.D. Trifonova, 2014, published in Zhurnal Prikladnoi Khimii, 2014, Vol. 87, No. 3, pp. 333−336.
ORGANIC SYNTHESIS AND INDUSTRIAL ORGANIC CHEMISTRY
Desulfurization of Tall Oil Pitch A. V. Kurzin, A. N. Evdokimov, and A. D. Trifonova St. Petersburg State Technological University of Plant Polymers, ul. Ivana Chernykh 4, St. Petersburg, 198095 Russia e-mail:
[email protected] Received January 31, 2014
Abstract—Method for desulfurization of tall oil pitch is suggested. The method is based on successive treatments with hydrogen peroxide, sulfuric acid, and aqueous solutions of acetonitrile. As a result, the total content of sulfur was reduced from 3800 to 220 ppm. The fuel characteristics of the desulfurized pitch were determined. The purified pitch can be used in the conventional areas of its application, in which restrictions are imposed on the total content of sulfur. DOI: 10.1134/S1070427214030094
composition of sulfurous compounds in the pitch are scarce. For example, according to [1], sulfur-containing lignin is present in the pitch in amounts of up to 12%, with the total content of sulfur being 0.180–0.228%. It has been found that organosulfur compounds in tall oil are represented by thiols, sulfides, disulfides, sulfoxides, sulfones, and sulfonic acids [10, 17]. The total sulfur can be determined in tall products by the methods that are applied in petrochemistry [18] and are based on various physical principles, the most widely used of which are the energy-dispersive X-ray fluorescence and UV spectroscopy of products formed in high-temperature oxidation. The applicability of these methods is limited, e.g., by halogens and phosphorus compounds present in a sample being analyzed (in the case of UV spectroscopy) or because of the matrix effect and content of halogens, phosphorus, and some other elements (X-ray fluorescence method). When the content of sulfur in various tall oil and pitch samples was determined simultaneously by these two methods, it was found that the values thus obtained are reproducible, close, reliable, and homogeneous. The goal of our study was to develop a method for diminishing the content of total sulfur in tall oil pitch and determine its fuel characteristics. The method we suggest for desulfurization is based on successive treatments with a 20% hydrogen peroxide solution, concentrated sulfuric acid, water–acetonitrile
Tall oil pitch, a fusible nonvolatile residue from distillation of crude tall oil, is a large-tonnage by-product of the sulfate cellulose industry [1, 2]. It is composed of esters of fatty and rosin acids, their oligomers and oxidized derivatives, high-boiling neutral substances (hydrocarbons, alcohols, including phytosterols, etc.), and lignin. Being an accessible and inexpensive renewable raw material, tall oil pitch is used to obtain sitosterol. In addition, the pitch and its derivatives are used in manufacture of sizing substances for some kinds of cardboard, corrosion inhibitors, paint-and-varnish and building materials, general rubber products and adhesives, components of drilling muds, and in road making [1–9]. A considerable part of tall oil and some of its fractions (fatty acids and pitch), produced in the world is used as a biofuel [10–12]. At present, the environment protection regulations are aimed to reduce the content of sulfur in boiler and motor fuels. To reduce the content of sulfur in the pitch used as a biofuel, it is diluted with various oil products, including masut and middle-distillate fractions. Because of the steady increase in the price of hydrocarbon raw materials and products of its processing, development of methods for pitch desulfurization is a topical task for chemical engineering. Thus, the concentration of sulfurous compounds in tall oil pitch is one of the most important characteristics. Information about the composition of tall oil pitch has been published [1, 13–16], but data on the quantitative and qualitative 299
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mixtures. Treatment with hydrogen peroxide favors oxidation of sulfides to sulfoxides and sulfones. Sulfuric acid causes formation of soluble sulfonium salts, on the one hand, and oxidation to the above-mentioned sulfones and sulfoxides, on the other. Acetonitrile and its aqueous solutions are widely used in petrochemistry in desulfurization of oxidized oil distillates to remove sulfones and sulfoxides. In addition, washing with a sodium hydrocarbonate solution removes sulfonic acids from the pitch (in the form of sodium salts). EXPERIMENTAL We used in the study commercial reagents (sulfuric acid, n-hexane, acetonitrile, hydrogen peroxide) and tall oil pitch manufactured by Segezhskii TsBK (pulp-andpaper combine) OAO. The total content of sulfur in the pitch was determined in conformity with the ASTM D4294/EN ISO 8754 and ASTM D5453/EN ISO 20846 standards with Oxford Lab-X 3500 and ECS/TN/TS 3000 (Thermo Euroglas B.V.) instruments. Tall oil pitch was desulfurized as follows. A 60-g portion of the pitch was mixed with 300 g of n-hexane at a temperature of 55°C. After the insoluble part was separated, 20 g of a 25% hydrogen peroxide solution was added to the resulting solution and the mixture was agitated for 2 h at 60°C. After that, the reaction mass was washed with water (3 × 200 mL), the organic layer was separated, 50 g of 92% sulfuric acid was added to this layer, and the mixture was agitated for 0.5 under cooling (10°C). After that, the hexane layer was separated and washed with cold (5°C) 0.5% solution of NaHCO3 and water to neutral reaction. Further, it was dried with calcium chloride, the solvent was evaporated, and the residue was treated with aqueous solutions of acetonitrile by the method previously employed [19] for purification of fatty acid contained in tall oil. The yield of the purified product was 52 g, and its sulfur content was 220 ppm. RESULTS AND DISCUSSION The suitability of tall oil pitch as a boiler fuel can be assessed on considering its physicochemical properties in comparison with the requirements to the boiler fuel based on hydrocarbon raw materials. The main parameter that affects the loading-unloading and pipeline transport conditions and the fuel spraying
by nozzles (and the fuel efficiency) is the viscosity. The kinematic viscosity of tall oil pitch at 50°C may reach values of 1000 mm2 s–1 and more. To control the viscosity, the commercial tall oil pitch can be diluted with distilled tall oil or fatty acids of tall oil, which yields viscosities of 400 mm2 s–1 and less [for masut of MTU-380 (IFO-380) brand, the maximum rated viscosity is 380 mm2 s–1]. The viscosity largely determines the sedimentation rate of mechanical admixtures in storage and the ability of a fuel to settle out water. It should be remembered that the viscosity is not an additive parameter and mixtures of such substances as boiler and heavy motor fuels, as well as the pitch under consideration, are structured systems and non-Newtonian fluids, and, therefore, their rheological properties are to be taken into account in their characterization. The viscosity substantially decreases with increasing temperature. Of interest in this regard is one more parameter, the flow point and the related pour point. Its value depends on the standard measurement procedure by which it is determined. The temperature found in conformity with GOST (State Standard) 20287 is, as a rule, than that by ASTM D 97; for the pitch samples under study these values were 22 and 23°C, respectively. The pour point characterizes, as also does the viscosity, the conditions of transportation, loading-unloading, and pumping of fuel and fuel conditioning. For example, the pour point y GOST 20287 for M-100 masut is 25°C. The pour pint is strongly affected by the heating temperature, cooling rate, switched on or off agitation, and content of n-paraffins. The effect of depressor additives, which modify the structure of crystallizing paraffin and thereby preclude formation of a strong crystal lattice, is associated with the last factor. The efficiency of a depressor additive is inversely proportional to the content of n-paraffins and their melting points and directly proportional to the content of asphaltic-resinous substances. Tall oil pitch hardly contains any n-paraffins, and, therefore, use of tall oil itch together with hydrocarbon residual fuels will improve the effect of depressor additives. An important fuel parameter affecting the operation of burners and furnaces is the content of sulfur. Heavy fuels, as also tall oil pitch, contain no mercaptan sulfur, and, therefore, their combustion products are less corrosionactive than those of light sulfurous oil products. The combustion of sulfur and its compounds yields sulfur oxides SO3 and SO2. Presence of SO3 in gases makes higher the dew point. With the content of sulfur in a fuel
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increasing from 0 to 2%, the dew point grows by 50°–60°. And, because the temperature of air heaters, economizers, and other devices of this kind is equal to the dew point of fume gases, sulfuric acid is condensed on their surfaces and causes corrosion. At the same time, sulfur oxides react with molten metal in metallurgical furnaces and thereby adversely affect its quality. For these and also ecological reasons, the content of sulfur in masuts is limited in a number of foreign countries by 0.5–1.0%. Thus, use of a tall oil pitch with lowered content of sulfur (0.02%) as a boiler fuel will make it possible to solve a number of technological and ecological problems. The coking properties of boiler fuels affect the cake deposition at nozzle mouths, which distorts the torch shape and impairs the atomization of the fuel and its efficiency. This parameter is under regulations only for light fuels F-5 and F-12 (not more than 6%); for heavy boiler fuels M-4.0 and M-100, there is no limitation on this parameter at all; for MTU-380 (IFO-380) masut it should not exceed 18%; the coking capacity of tall oil pitch, determined by the ASTM D 4530 method, is 10.4%. By contrast, the minimum value is limited for furnace fuels. For example, the coking capacity limit for furnace fuels of MP and MPVA brands should not be less than 8%. A characteristic feature of tall oil pitch is that its acid number is not lower than 20 mg KOH/g. For residual fuels and, in particular, for M-100 masut, this parameter is not specified; for the desulfurized tall oil pitch, the acid number determined by the ASTM D 664 method did not exceed 5 mg KOH/g. The fuel density determines the possibility of stratification with water that gets into a fuel in its steam-heating or water transport. The limit on density (not more than, kg m–3) is 1015 for M-100 boiler fuels and 991 for MTU-380 (IFO-380) fuel. The density of tall oil pitch is 910 kg m–3. However, in contrast to hydrocarbon residual fuels, it is necessary to take into account that, tall oil pitch, there are compounds that can form true solutions or emulsions with water. Tall oil pitch subjected to desulfurization hardly contains any compounds of this kind. Important technological parameters determining both the fuel characteristics and the fire safety are the flash points in a closed or open crucible. The flash point of tall oil pitch in a closed crucible, found by the ASTM D 93(B) method, is 207.0°C. The self-ignition point is 370°C. To detrimental impurities affecting the fuel characteristics are attributed ash, mechanical admixtures, and water. The ash content determined by the ASTM D 482
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method does not exceed 1.5% in the starting tall oil pitch and 0.08% for the desulfurized pitch (the upper limit for M-100 masut is 0.14%), with its main part constituted inorganic compounds of sodium and iron. It is known that vanadium pentaoxide exhibits the highest capacity for corrosion of boiler tubes made of alloyed steel; the content of vanadium determined by the ICP-AES and ICP-OES methods does not exceed 3 ppm (its typical content in IFO-380 masut is 70–190 ppm), and the content of aluminum and silicon does not exceed 7 ppm. The content of mechanical admixtures should not exceed 1.5% for M-100 masut; their content in the desulfurized tall oil pitch, determined by the IP 375 method, is 0.12%. Depending on the application place of the fuel, water may be either a useful element, e.g., in ship motor fuels where it is a component of the water–fuel emulsion, or a detrimental component in boiler and furnace fuels because it reduces the combustion heat, impairs the combustion stability, and promotes formation of acids. The content of water in M-100 masut should not exceed 1.5%. The content of water in the desulfurized tall oil pitch is 0.30% according to ASTM D 95. The combustion heat (heat content) is one of the most important characteristics of a fuel. This parameter depends on the elemental composition of the fuel and is determined by the relative contents of carbon and hydrogen and the ash content, and also by the content of sulfur and water. The combustion heat of desulfurized tall oil pitch is 37.5 MJ kg–1, which is smaller than the standard value for low-sulfur residual fuels (40.5 MJ kg–1). However, with medium-distilled products of oil processing as a viscosity-controlling agent, the heat content of the mixture increases to 40.9 MJ kg–1. CONCLUSIONS (1) The oxidative-extractive method used to purify tall oil pitch diminished its total sulfur content from 3800 to 220 ppm. (2) The purified tall oil pitch can be used as a biofuel with low sulfur content. REFERENCES 1. Sandermann, W., Naturharze, Terpentinol, Tallol (Chemie und Technologie), Berlin (Gottingen), Heidelberg: Springer Verlag, 1960. 2. Golovin, A.I., Trofimov, A.N., Uzlov, G.A., et al., Lesokhi-
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