Methylester Sulphonate Products (1):Food Industry News

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INTRODUCTION

Much of the recent work in the area of methyl ester sulfonates (MES) has focused on highly hydrogenated and refined methyl ester feedstock.1s As the market for MES matures, developing a product database for a wide variety of feedstocks is of interest, especially when one considers the manufacturing costs associated with improving feedstock quality. Less refined feedstocks from natural and renewable sources are particularly interesting. With this in mind two of the feedstocks discussed have iodine values greater than 1.0 as well as interesting carbon-chain distributions. This study was performed with process equipment using a series of dominant bath and plug flow operations. Sulfonation was accomplished in a Chemithon Annular Falling Film reactor (AFFR) with a dominant bath recycle loop followed by plug flow digestion at elevated temperature. The color of the dark sulfonic acid that results from the digestion is then reduced in a continuous hydrogen peroxide acid bleaching system where the components are admixed in a loop and further bleached in a plug flow vessel.

The light colored bleached methyl ester sulfonic acid is neutralized to a controlled pH and sent to a stripper or dryer as required. MES concentrated pastes may be dried to form flakes or needles (>90% active), which are easy to store and transport and are readily processed to a powder by milling or grinding. The powdered MES has excellent properties for agglomeration in detergent granules and tablets. The process technology to make concentrated and ultra-concentrated forms of the product has been commercialized and is becoming more widely applied as economic and environmental factors are understood.2 Availability of raw materials for conversion to MES is increasing, particularly palm oil which is growing at a rate of almost 3% per year. Demand for palm oil is expected to exceed soya as number one shortly after 2005.

PROCESS

The preferred continuous process for manufacturing concentrated and ultra-concentrated MES is illustrated in Figure 1 and described in detail elsewhere.4,5,6 Sulfur trioxide (SO3) diluted with air is typically supplied from a sulfur-burning unit coupled to a sulfur dioxide (SO2) to SO3 converter. The sulfonation is done in a Chemithon AFFR with a sulfonic acid recycle system that reduces the effluent gas particulate load by utilizing a high-efficiency cyclone.

The reactor is continuously cooled with a large flow of tempered water through external jacketing. The sulfonation is carried out at process temperatures higher than are typical when making other anionic surfactants. The methyl ester sulfonic acid (MESA) is discharged from the loop and passes through a plug-flow digester where the MESA temperature is maintained at 85°C for 0.7 hours.

Figure 1. Continuous Process for the Manufacture

of (Ultra-)Concentrated MES

The aging process in the digester completes the sulfonation reaction but causes a four-fold increase in the color of the MESA. The digested MESA, methanol and dilute hydrogen peroxide are combined in a loop and the reaction mixture is passed through a methanol refluxing vessel where the bleaching of the MESA is completed. The process temperature in the bleaching vessel is controlled by heat transfer surfaces submerged in the reactants. The process pressure is independently controlled. The non-condensable vapors discharged from the bleaching vessel are processed in an effluent gas treatment system. The bleached MESA is continuously neutralized with 50% sodium hydroxide to a pH of 5.5 to 7.5. Any residual peroxide can be decomposed by the addition of a molar equivalent of sodium sulfite.7 The neutral paste is transferred to a concentrating and / or methanol removal system. MES based on a methyl ester below a molecular weight of 245 is stripped rather than dried to remove the methanol making the concentrated product. Higher molecular weight MES is dried, removing both methanol and water, making the ultra-concentrated solid product forms. The recovered methanol is distilled and recycled back to the bleaching process.

The stripper / dryer consists of a supply system, a preheater, a proprietary dryer, a separation vessel, a product discharge device (typically a plodder), a vacuum system and an overhead condenser, Figure 2.8,9 The MES product is powdered or diluted as appropriate for application in liquid, bar, granular, and tablet detergent products.

Figure 2. Dryer for Ultra-Concentrated MES

METHYL ESTER

The coconut, palm and tallow based methyl ester (ME) feedstocks used in this study are typical of commercially available material. The distillation cuts are narrow, mostly incorporating just two carbon numbers, Table 1. The iodine value (IV) for these four feedstocks is 0.3 or less. The IV of methyl esters is used as a measure to predict intermediate digested acid color and final bleached product color10 but other components besides unsaturation have a significant influence.11 The palm stearin methyl ester feedstock used in this study is a typical sulfonation feedstock for use in laundry detergent formulations.12 For this palm stearin ME the C16 to C18 ratio is 2:1. MES with this carbon –chain distribution has a minimum Krafft point (17°C) and thus maximum solubility as compared to any other combination of C16 and C18.13 This is a useful feature for formulating low temperature laundry detergents. For the tallow methyl ester feedstock used in this study the C16 to C18 ratio is 1:2. The Krafft point of the resulting sulfonate would be closer to 27°C. The coconut methyl ester feedstock used in this study has a C12 to C14 ratio of 2.5:1. The palm kernel methyl ester feedstock used in this study, Table 1, has an IV of 1.4 indicating about a 90% reduction in unsaturation by hydrogenation since the palm kernel oil is up to 19% oleic.14 This particular feedstock has the complete carbon-chain distribution of the palm kernel oil. The C10 and lower methyl esters are usually stripped from the feedstock,15 as the sulfonates of the corresponding methyl esters should not have good detergent properties. The unrefined nature of this feedstock would be reflected in its cost so it was of interest. The soya methyl ester feedstock used in this study, Table 1, has an IV of 1.1 indicating about a 99% reduction in unsaturation since soybean oil is about 30% oleic, 50% linoleic and 7% linolenic.16 As expected the composition of this feedstock is almost 90% C18. This feedstock requires a relatively large amount of hydrogenation to reduce the IV to a sulfonatable level, but it represents a renewable source of predominately C18 methyl ester so it was of interest.

Table 1: Methyl Ester Feedstocks and Quality Comparison