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Ind. Eng. Chem. Res. 2008, 47, 8412–8415
Thermal Decomposition of Waste Linear Alkylbenzene Sulfonate Hyung J. Lee,† Kyun Y. Park,*,† Surk-Sik Moon,‡ and Jong-Gi Lee§ Departments of Chemical Engineering and Chemistry, Kongju National UniVersity, 182 Shinkwandong, Kongju, Chungnam 314-701, Korea, and Central Research Laboratory, Aekyung Specialty Chemicals Co., Ltd., 217-2, Shinseong-Dong, Yooseong-Gu, Daejeon 305-345, Korea
A thermal decomposition is proposed to convert a waste linear alkylbenzene sulfonate (LAS) into a solid material that is environmentally more favorable and disposable at a lower cost. The waste LAS is a viscous liquid containing sulfur as high as 13.3 wt %. The waste LAS was heated in a nitrogen atmosphere at 200-300 °C for 1-4 h. The thermal treatment at 250 °C for 2 h decomposed the waste into vapors and a solid material or residue, 2.0 wt % in sulfur content. The vapors were cooled with water to form two condensate layers in the receiving cylinder. The upper layer was identified to be a linear alkylbenzene (LAB) and the lower one a mixture of water and organic sulfur oxy compounds. The uncondensed vapor that left the condenser was determined to be a mixture of SO2, SO3, and H2SO4. A material balance shows that the mass of the waste LAS charged was distributed in the decomposition products as follows: the solid residue, 65.3%; the LAB, 5.6%; the mixture of water and organic sulfur compounds, 5.6%; the mixture of SO2, SO3, and H2SO4, 21.5%; unidentifiable loss, 2.0%. 1. Introduction Linear alkylbenzene sulfonate (LAS), which is widely used as a biodegradable surfactant, has been produced by reaction of linear alkylbenzene (LAB) with gaseous sulfur trioxide, about 5%, in diluent air. The main reaction with overall stoichiometry is represented by ArH + SO3 ) ArSO3H
(1)
where ArH is LAB. Side reactions are known to occur simultaneously to form LAS anhydride, diaryl sulfone, and sulfuric acid:1 2ArH + 3SO3 ) ArSO2OSO2Ar + H2SO4
(2)
2ArH + 2SO3 ) ArSO2Ar + H2SO4
(3)
Figure 1 shows a flow diagram for LAS production in Aekyung Specialty Chemicals Co., Ltd., Korea. The sulfonation is carried out at 40-50 °C in a falling film reactor. The number of aliphatic carbons in the LAB feed ranges from 9 to 14. The product stream containing LAS, excess sulfur trioxide, and air is led to a vertical drum, where the liquid LAS is separated by gravity from the gaseous stream. A considerable amount of LAS droplets is entrained in the gas leaving the drum. These droplets are subsequently collected in a cyclone and an electric precipitator in series. The gas is then passed through a NaOH scrubber for the acidic components or sulfur oxides to be removed from the gas. The color of LAS is an important specification; faintly yellow to pale brown is acceptable.2 The LAS collected in the cyclone and electric precipitator is, however, dark brown to black and has been outsourced for disposal through burning in an incinerator. The incineration produces a large amount of sulfur dioxide because the waste LAS contains sulfur as high as 13.3 wt %. As the environmental regulation on air quality is becoming more stringent, the outsourcing cost is rising and the * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical Engineering, Kongju National University. ‡ Department of Chemistry, Kongju National University. § Aekyung Specialty Chemicals Co., Ltd.
LAS producer is seeking for an alternative solution. A neutralization of the waste LAS with NaOH was attempted. However, the resulting filtrate after the removal of the precipitate formed by neutralization was too dark to be discharged to the sewer; no effective means of removing the color of the filtrate has been found, yet. A thermogravimetric analysis indicated a considerable weight loss upon heating the waste LAS up to 300 °C in a nitrogen atmosphere. This experimental finding motivated us to explore the possibility of desulfurizing the waste through thermal decomposition. The desulfurization was studied using an apparatus capable of decomposing 50 g of the waste LAS per batch. With varying decomposition temperature and time, decomposition products were analyzed for identification, mass, and sulfur content. 2. Experimental Section The waste LAS was supplied by Aekyung Specialty Chemicals Co. Ltd. and used as-received. Figure 2 shows the experimental apparatus. About 50 g of the waste LAS was charged to a 100 mL round-bottom flask covered with an electric mantle and heated to a predetermined temperature with a
Figure 1. Process flow diagram for production of linear alkylbenzene sulfonate by sulfonation of linear alkylbenzene with sulfur trioxide.
10.1021/ie800382n CCC: $40.75 2008 American Chemical Society Published on Web 10/10/2008
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Figure 2. Experimental apparatus for thermal decomposition of waste linear alkylbenzene sulfonate.
Figure 3. Temporal variation in mass of waste linear alkylbenzene charged for thermal decomposition with varying decomposition temperature.
nitrogen flow of 100 cm3/min (STP). The temperature of the flask was variable from 200 to 300 °C. The vapor generated upon heating was passed through a water condenser. The obtained condensate was collected in a graduated cylinder, while the uncondensed vapor was bubbled through two NaOH bottles in series. IR spectroscopic studies were carried out to determine the structures of the condensates, using a Specord M80 (Karl Zeiss, Jena) IR spectrophotometer. The moisture content of the condensate was measured with a Karl Fischer titrator (Mettler Toledo DL31). The sulfur dioxide and sulfur trioxide in the gas stream leaving the condenser were determined with the EPA method 8.3 The sulfur contents of the waste LAS charged and of the liquid and solid decomposition products were measured using an elemental analyzer (FISONS EA-1108). The same analyzer was used for the determination of carbon and hydrogen in an organic compound. 3. Results and Discussion Figure 3 shows temporal variations of the mass of the waste LAS charged for decomposition, with the decomposition temperature varied from 200 to 300 °C and the decomposition time from 1 to 4 h. At the temperature of 200 °C, the mass decreased to 80.6% of the initial value in 1 h, 71.8% in 2 h, and 67.6% in 4 h. As the temperature was increased to 250 °C, the mass decreased to 69.0% in 1 h, 65.3% in 2 h, and 62.2% in 4 h. The mass decreased further with temperature increase to 300
Figure 4. Comparison of infrared spectra between transparent condensate and linear alkylbenzene (solid line, linear alkylbenzene; dotted line, transparent condensate). The transparent condensate was obtained through thermal decomposition of the waste linear alkylbenzene sulfonate for 2 h at 250 °C.
°C; the decreasing rate was, however, apparently slowed down. The waste LAS, which is initially a liquid, was turned into a solid by the decomposition. The vapor generated in the decomposition flask and passed to the condenser partially condensed to form two condensate layers in the receiving cylinder. The upper layer was transparent and the lower one cloudy. A considerable volume of the vapor was observed to be uncondensable. The thermal decomposition of the waste LAS thus produced a solid residue, a transparent condensate, a cloudy condensate, and an uncondensable vapor. 3.1. Formation Mechanisms and Analyses of Decomposition Products. The decomposition products were analyzed for identification and sulfur content, and how those products were formed are discussed. The decomposition temperature is fixed at 250 °C and the time at 2 h, unless stated otherwise. Transparent Condensate. The IR spectra of the transparent condensate were compared with those of the LAB, as shown in Figure 4. The two spectra are very similar to each other. We conclude that the transparent liquid is a linear alkylbenzene similar in structure to that used for the manufacture of the LAS. The alkylbenzene emerged probably through the reverse reaction of the sulfonation in eq 1; upon heating of the LAS, the equilibrium should move in the reverse direction because the sulfonation reaction is exothermic.4 The sulfur content of the transparent condensate was measured to be 0.4 wt %, indicating that the obtained linear alkylbenezene is not pure, but contains sulfur compounds in small quantity. The possibility of the obtained alkylbenzene being thermally cracked at the operating temperature of 250 °C was examined by estimating the cracking rate using the kinetic parameters reported for pentadecylbenzene.5 The estimated rate indicates that the cracking probability is very low. In gas chromatographic analyses of alkylbenzene components, the column temperatures close to or even higher than the current operating temperature were used.6,7 This supports the fact that the alkylbenzene is stable at the temperature used in the present study. Cloudy Condensate. Figure 5 shows the IR spectra of the cloudy condensate. The broad peak around 3500 cm-1 is due to the hydrogen-bonded OH. The peaks observed between 3000 and 3100 cm-1 correspond to aromatic rings. The peaks around 2900 cm-1 are assigned to the C-H stretch vibrations. The peaks at 1470, 1380, and 720 cm-1 are indicative of a longchain linear aliphatic structure. The peak at ∼1200 cm-1
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Figure 5. Infrared spectra of cloudy condensate. The cloudy condensate was obtained at the same decomposition conditions as those for Figure 4.
represents the presence of sulfur oxy compounds.8 The peak around 1650 cm-1 is due to the hydration of the SO3- group.9 A Karl Fischer titration of the condensate showed the presence of bulky water. It is concluded that the cloudy condensate is a mixture of water and organic sulfur oxy compounds. The mass fraction of the sulfur oxy compounds was determined to be 0.1 and the sulfur content of the cloudy condensate was measured at 0.45 wt %. The sulfur oxy compounds were formed probably through a recombination of the SO3 and alkylbenzene vapors passed from the decomposition flask. The recombination may be similar to the sulfonation and have occurred in the zone where the temperature is 40-50 °C, favorable for such reaction. The emergence of water may be due to the reaction of LAS with alkylbenzene, by which sulfone and water are produced,10 and to the reaction of alkylbenzene with H2SO4,2 which is assumed to have been available through the reaction of water and SO3. Uncondensable Vapor. The uncondensable vapor that left the condenser was analyzed to be a mixture of SO2 and SO3 plus H2SO4, using the EPA method 8. SO3 and H2SO4 were not differentiable. The molar ratio of SO3 plus H2SO4 to SO2 was determined to be 7.3. The SO3 must have come from the desulfonation or the reverse reaction of the sulfonation and the H2SO4 from the reaction of SO3 and water vapor. The mechanism for the SO2 emergence is not clearly known. A source for SO2 may be the extrusion of SO2 from the sulfones11 initially present in the waste LAS and those formed in the course of the decomposition process. The reaction between the regenerated LAB and SO3 could be another source for SO2.2 Figure 6 shows a temporal variation of the incremental mass of the NaOH absorber due to the accumulation of the sulfur oxides absorbed in the alkaline solution. The incremental mass is expressed in wt % of the mass of the waste LAS charged for decomposition. It was 20% in 1 h, 22% in 2 h, and leveled out thereafter, implying that the desulfurization was nearly completed in 2 h. Solid Residue. The solid residue was black and similar to petroleum cokes in appearance. The IR transmittance was too small for identification. Figure 7 shows the variation of the sulfur content with decomposition time. The sulfur content decreased in 2 h from the initial value of 13.3 to 5 wt % at 200 °C, 2.0 wt % at 250 °C, and 1.0 wt % at 300 °C. The sulfur content was nearly invariant after 2 h. The bond dissociation energies for the desulfonation by which SO3 comes off and for the sulfone extrusion resulting in the
Figure 6. Temporal variation in mass gain of NaOH bottle due to the absorption of sulfur oxides in the NaOH solution. The mass gain is normalized and expressed in % of the mass of the waste linear alkybenzene sulfonate charged for thermal decomposition.
Figure 7. Temporal variation in sulfur content of the solid residue with varying decomposition temperature. Table 1. Mass and Sulfur Balances for the Decomposition at 250 °C for 2 h total mass (g)
sulfur (g)
44.7
5.95
2.5 2.5 9.6 29.2 0.9
0.01 0.01 4.19 0.58 1.16
Input waste LAS Output transparent condensate cloudy condensate uncondensable vapor solid material unaccountable
evolution of SO2 were reported to be 175-1952 and 272 kJ/ mol,12 respectively. The initial desulfurization appears to have been governed by the desulfonation having the lower dissociation energy. This is supported by the experimental evidence that the ratio of SO3 to SO2 in the sulfur oxides gas was higher in the early stage. The sulfone extrusion was probably more influential in the later stage and with increasing temperature. 3.2. Mass and Sulfur Balances. Table 1 shows the mass and sulfur balances for a waste LAS feed of 44.7 g. The mass of the waste LAS charged was distributed in the decomposition products as follows: the solid residue, 65.3%; the transparent condensate, 5.6%; the cloudy condensate, 5.6%; the uncondensable vapor, 21.5%; unaccountable loss, 2.0%. 70.4% of the
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sulfur present in the waste LAS was removed as uncondensable vapor composed of SO2, SO3, and H2SO4; 0.2% of the sulfur was transferred to the transparent condensate and to the cloudy condensate, respectively; 10.0% of the sulfur remained in the solid residue; the unaccountable loss for sulfur amounts to 19.5%. The lack of consistency by employing two different sulfur-determination methods appears to be a reason for the greater unaccountable fraction in the sulfur balance. 3.3. Economic Incentives. The solid residue resulting from the thermal decomposition is estimated to be disposable at 250 U$/ton. Considering that 1 ton of the waste LAS produced 0.65 ton of the residue, the disposal cost of the residue amounts to 162.5 U$/ton of waste LAS. The treatment cost of the sulfur oxides is estimated at 60 U$, whereas the recovered alkylbenzene is valued at 130 U$/ton of waste LAS. The net cost for treating the decomposition products would thus be 92.5 U$/ton of waste LAS, approximately one-sixth the current outsourcing cost of the waste LAS, 600 U$/ton. It may be too early to make a capital cost estimate of considerable significance because the present study is primary and laboratory scale. 4. Conclusion The thermal decomposition of the waste LAS in nitrogen at 250 °C for 2 h reduced the sulfur content from 13.3 to 2.0 wt %, transformed the phase from liquid to solid, and reduced the mass to 65.3%, generating vapors. The vapors partially condensed in a condenser to form a transparent and cloudy condensates. The transparent condensate is similar in structure to the linear alkylbenzene used for the manufacture of the LAS, and potentially recyclable. The cloudy condensate is a mixture of water and organic sulfur oxy compounds. The uncondensable vapor is a mixture of SO2, SO3, and H2SO4. The masses of the transparent condensate, the cloudy condensate, and the uncondensable vapor were determined to be 5.6, 5.6, and 21.5%, respectively, of the initial mass of the waste LAS. Considering that the resulting solid is environmentally more favorable and disposable at a lower cost than the waste LAS and that the uncondensable sour vapor could be treated with minimal extra cost using the existing NaOH scrubber in the sulfonation unit, the thermal decomposition on site is suggested as an alternative to the outsourced incineration.
Acknowledgment This research was financially supported by the Ministry of Commerce, Industry and Energy (MOCIE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation. Literature Cited (1) Roberts, D. W. Sulfonation Technology for Anionic Surfactant Manufacture. Org. Process Res. DeV. 1998, 2, 194–202. (2) Roberts, D. W. Optimization of the Linear Alkyl benzene Sulfonation Process for Surfactant Manufacture. Org. Process Res. DeV. 2003, 7, 172– 184. (3) Determination of Sulfuric Acid Mist and Sulfur Dioxide Emissions from Stationary Sources. Method 8-CFR Promulgated Test Methods; U.S. Environmental Protection Agency: Washington, DC, 2000. (4) James Davis, E.; van Ouwerkerk, M.; Venkatesh, S. An Analysis of the Falling Film Gas-Liquid Reactor. Chem. Eng. Sci. 1979, 34, 539–550. (5) Savage, P. E.; Klein, M. T. Asphaltene Reaction Pathways. 2. Pyrolysis of n-Pentadecylbenzene. Ind. Eng. Chem. Res. 1987, 26, 488– 494. (6) Cohen, L.; Vergara, R.; Moreno, A.; Berna, J. L. Sulfonation with SO3: Relative Reactivity of Commercial Alkylbenzene Components. J. Am. Oil Chem. Soc. 1995, 1, 157–159. (7) Molever, K. Monitoring the Linear Alkylbenzene Sulfonation Process using High-Temperature Gas Chromatography. J. Surfactants Deterg. 2005, 2, 199–202. (8) Coates, J. Interpretation of Infrared Spectra, A Practical Approach. Encyclopedia of Analytical Chemistry, Meyers, R. A., Ed., John Wiley & Sons Ltd.: Chichester, 2000. (9) Jamroz, D.; Marechal, Y. Hydration of Sulfonated Polyimide membranes. I. Na+ and NH+(C2H5) 3 Homopolymers. J. Mol. Struct. 2004, 693, 35–48. (10) Austin, G. T. ShreVe’s Chemical Process Industries, 5th edition; McGraw-Hill: Singapore, 1984. (11) Samperi, F.; Puglisi, C.; Ferreri, T.; Messina, R.; Cicala, G.; Recca, A.; Restuccia, C. L.; Scamporrino, A. Thermal Decomposition Products of Copoly(arylene ether sulfone)s Characterized by Direct Pyrolysis Mass Spectrometry. Polym. Degrad. Stab. 2007, 92, 1304–1315. (12) Molnar, G.; Botvay, A.; Poppl, L.; Torkos, K.; Borossay, J.; Mathe, A.; Torok, T. Thermal Degradation of Chemically Modified Polysulfones. Polym. Degrad. Stab. 2005, 89, 410–417.
ReceiVed for reView March 8, 2008 ReVised manuscript receiVed July 16, 2008 Accepted July 25, 2008 IE800382N
