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The product composition for the six methyl ester sulphonate examples are presented in Table 2. The MES products were analyzed principally based on the methods described by Battaglini.17 Analysis of the sodium methyl sulfate content using the prescribed method yielded incorrect results for the lower molecular weight examples of MES in this study. The phenol red titration does not quantitatively determine the di-salt content. For the coconut and palm kernel based MES examples the results in Table 2 for the di-salt are presented as determined and the results for the sodium methyl sulfate are calculated by difference from 100 percent.
The residual methanol content in the MES product was determined using a headspace sampling - gas chromatography technique utilizing internal standard and standard additions. The water content was determined by Karl Fischer potentiometric autotitration. The Klett color was determined using a Klett colorimeter with a #42 filter and a 40 mm path length at 5% active concentration. Note that in Table 2 the pure component - sodium methyl ester sulfonate- is designated as α-Mes, as opposed to the composite MES.
Table 2: Methyl Ester Sulphonates
The coconut and palm kernel products were once-through steam stripped to a low level of methanol in pilot plant equipment. Typical desired level of residual methanol is 0.5 wt% or less. The steam to MES ratio is the primary factor in controlling the final methanol concentration in the stripped MES. Stripping MES versus drying leaves most of the water in the product, which results in a paste unless cooled to crystallization temperature. Drying low molecular weight (C12-14) MES produces a viscous paste that is difficult to crystallize and is not easily flaked or needled in the pure form. Typically, C12-14 MES is used in light duty liquid detergent products.18 The stripped product of the coconut MES made in this study is of sufficiently low moisture for most formulation needs and can be diluted to a clear, low viscosity solution if necessary. Coconut MES bleaches effectively at a relatively low hydrogen peroxide addition rate to very low color (30 Klett). This is comparable to sulfonates of good quality linear alkyl benzene. It has been observed that MES of equal Klett reading to LAS looks significantly lighter to the naked eye. The palm kernel MES was also stripped rather than dried, based on its low average molecular weight and carbon-chain distribution.
This particular palm kernel ME has a high IV (1.4) and the resultant stripped MES has a Klett color of 310, Table 2. In other respects such as the PEX and di-salt content the product quality is equivalent to the coconut-based product. Unrefined palm kernel based MES may be of interest for formulating into detergent products in situations where feedstock costs are paramount. In terms of color the C16 Palm ME produced the best quality MES. Prior to hydrogenation C16 ME is low in unsaturates and a low level of hydrogenation further reduces these resulting in very light colored MES. The palm stearin MES example shown in Table 2 has low color, low di-salt, and low PEX and with these parameters it can be readily formulated into both light and heavy-duty products, dry and liquid. In spite of the IV of 0.3 this feedstock produced very light colored MES. As a comparison the tallow based ME with an IV of 0.1 produced much darker MES (180 Klett). Previous work has noted that as the molecular weight of the ME increases so does the difficulty in bleaching.19
Several factors influence digested MESA color, including minor constituents in the ME,20 SO3 to ME mole ratio, post–sulfonation MESA digestion time and digestion temperature, degree of unsaturation measured as IV in the ME, and ME molecular weight.21 The color of digested MESA is somewhat correlated with the IV of the starting ME, Figure 3. The color of the digested MESA is determined by sample dilution in 2-propanol to an appropriate concentration for an on-scale meter reading on the Klett colorimeter. The log fit of the data suggests that additional starting unsaturation does not linearly increase the color of the acid. The same effect is seen in reverse with bleaching, as the unsaturation is reduced, the color decreases more slowly as the bleaching progresses.
Figure 3: Effect of Iodine Value on Digested MESA Color
The same factors that impact digested MESA color also effect the color of the final product. Additional factors affecting the product color are the level of bleaching agent (hydrogen peroxide preferred) addition, the level of methanol addition, the bleaching temperature and time, the neutralization temperature, and the dryer process temperatures. All of these process factors are well controlled in the technology available today. Figure 4 is a plot of the IV of the starting ME versus final product color for the six MES examples. The data shown in Figure 4 (MES product) shows more variability compared to the data shown in Figure 3 (digested MESA), this is due to the additional factors affecting the color of the MES product. Still the overall trend suggests that higher IV is deleterious to final product color.
Figure 4: Effect of Iodine Value on Stripped / Dried MES Color
Sulfonation of methyl ester feedstocks in the pilot plant is performed in a falling film reactor at a rate of about 0.1 kg-mole per hour. The reactor inlet SO3 gas concentration is 7 mole %. The reactor inlet gas temperature is about 42°C. The ME feedstock is supplied to the reactor at a temperature ranging from 40°C to 56°C, well above the freeze point of the particular ME feedstock. The reactants are mass flow controlled to maintain a fixed mole ratio of SO3 to ME (MR). For the six feedstocks the MR ranged from 1.2 to 1.3. The choice of MR is dependent on the expected selectivity of the particular ME to side-reactions and byproduct formation. These include oxidation of the alkyl chain by SO3, sulfonation of some of the resulting olefin sites,22 formation of methyl sulfuric acid, and hydrolysis of the ester to form di-salt. The MESA is transferred to the acid digester system where it rapidly reaches the desired digestion temperature. After the MESA is digested the methanol (35 to 40 wt%, digested MESA basis) and 50% hydrogen peroxide are ratio added with the MESA into the MES bleacher. A significant amount of exothermic reaction occurs in the bleacher. The acid bleaching step requires 1 to 1.5 hours. More bleaching time is better in terms of degree of color reduction as long as not all of the available peroxide is consumed. The excess methanol effectively limits the production of di-salt, and significantly reduces the viscosity of the mixture, which improves mixing and heat transfer through the bleaching process. Bleached MESA is forwarded to the neutralizer where a controlled proportion of 50% sodium hydroxide is admixed with the bleached MESA and a large recycle stream of neutralized paste. Neutralized MES paste continuously discharges to the stripper / dryer pilot plant, where the excess water and methanol are removed. The dryer pilot plant functions as a distinct processing system, and processes concentrated pastes at rates of about 60 kg/hr. For the stripping process the MES inlet temperature is about 100°C and the separation vessel operates under vacuum conditions at 200 Torr or greater. For the drying process the MES inlet temperature is about 145°C and operates under vacuum conditions at 120 to 200 Torr. No significant increase in di-salt occurs at the elevated process temperatures of the dryer, and the product color is stable.
Production of high quality MES requires more sophisticated equipment than linear alkylbenzene sulphonate or fatty alcohol sulfate but it has strong appeal because ME is an inexpensive, natural and renewable feedstock. Methyl ester sulfonates provide superior surfactant properties at low cost, and therefore a strong economic incentive to substitute MES for traditional surfactants in many applications. Production capacity of MES will increase in the next few years. More formulation work can be expected utilizing different types of ME feedstock based on the concentrated and ultra-concentrated forms of MES. There will be many new competitive products introduced into the marketplace.
- Fabry, B., Kratzel, U., Schmidt, W., Kreienfeld, G., “Process for the Production of Light-Colored Washing-Active alpha-Sulfofatty Acid Lower Alkyl Ester Salts, U.S. Patent No. 5,391,786 (1995)
- Steber, J.C., Wierich, P., Tenside, 26, pp. 406-411 (1989).
- Chow, C.S., Palm Oil tech. bul., 4, pp 4-7 (1998)
- Hovda, K., “Sulfonation of Fatty Acid Esters”, U.S. Patent No. 5,587,500 (1996).
- Foster, N.C., Hovda, K., “Manufacture of Methyl Ester Sulfonates and other Derivatives” in Presentation Outlines of the Soaps, Detergents and Oleochemicals Conference and Exhibit, AOCS Publication, (1997).
- MacArthur, B.W., Brooks, B., Sheats, W.B., Foster, N.C., “Meeting the Challenge of Methyl Ester Sulfonation”, Proceedings of World Conference on Palm and Coconut Oils for the 21st Century: Sources, Processing, Applications, and Competition, AOCS Press, pp 54-63 (1999).
- Ibid 4
- Duvall, L.R., Brooks, B., Jessup, W., U.S. Patent No. 5,723,433 (1995).
- Brooks, B., Jessup, W., MacArthur, B.W., U.S. Patent Pending Serial No. 09/032,303 (filed 1998).
- Hovda, K., “Effect of Methylester Feedstock on sulphonate Quality”, Proceedings of 1994 International Seminar on Surfactants and Detergents, ISSD, pp 306-313 (1994).
- Yamada, K., Matsutani, S., J. Amer. Oil Chem. Soc., 73, pp 121-125 (1996).
- Ahmad, S., Ismail, Z., Samsi, J., J. Oil Palm Res., 10, pp 15-34 (1998).
- Schambil, F, Schwuger, M.J., "Physico-Chemical Properties of alpha-Sulpho Fatty Acid Methyl Esters and alpha-Sulpho Fatty Acid Di-Salts", Tenside, 27, pp 380-385 (1990)
- Dieckelmann, G., Heinz, H.J., The Basics of Industrial Oleochemistry, Peter Pomp GmbH, p. 30 (1989)
- Yusof, M, Palm Oil Dev., 28, pp 1-20 (1998).
- ibid 14
- Battaglini, George, Larson-Zobus, J., Baker, T.G., J. Amer. Oil Chem. Soc., 63, pp 1073-1077 (1986).
- Drozd, J.C., "Use of Sulfonated Methyl Esters in Household Cleaning Products", Proceedings of World Conference on Oleochemicals into the 21st Century, AOCS, pp 256-268 (1990).
- ibid 10
- Schmid, K., Stein, W., Baumann, H., “Preparation of Light-Colored Wash Active α-Sulfofatty Acid”, U.S. Patent No. 4,671,900 (1987).
- Ibid 10
- Yamada, K., Matsutani, S., Separation and Identification of Colored Substances in Sulfonated Fatty Acid Methyl Ester, presented at 84th AOCS (1993).
W. Brad Sheats, Dr. Brian W. MacArthur
The Chemithon Corporation
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