Glycerol sorbent selection and optimized conditions for ethyl biodiesel purification by response surface methodology
Abstract
The feasibility of using alternative and commercial sorbents for dry washing biodiesel was quantitatively examined. Rice husk ash (RHA) and Amberlite showed similar performance and their applications in adsorbing glycerol were determined by response surface methodology. A 22 face centered composite design was employed to analyze the combined effect of sorbent concentration and temperature on glycerol removal. The statistical analysis indicated that levels of 2.1% (RHA) and 3.7% (Amberlite) at 30 ºC maximized the glycerol removal with minimum loss of adsorbate. Under optimal conditions, the sorbents were used for purifying ethyl biodiesel samples from palm kernel oil. Both sorbents were quite efficient in removing glycerol, providing samples containing glycerol values lower than 0.01% that meet the required standard to be used as a fuel. The high quality of the purified samples was also demonstrated by the ethyl esters contents (> 98.0%), viscosity (< 4.0 mm2 s -1 ) and density ( 867 kg m-3 ) values. Rice husk ash was also able to remove monoacylglycerols and diacylglycerols, thus its structure, composition and adsorption kinetics were further assessed to better understand its property as sorbent
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Introduction
The transesterification of feedstocks based on triacylglycerols with monohydric alcohols yields crude biodiesel containing mono-alkyl ester as main product and glycerol as byproduct. Crude biodiesel must be submitted by a purification step to remove various impurities (glycerol, unconverted triacylglycerols, monoacylglycerols, diacylglycerols, free fatty acids, water, catalyst, soaps, among others) to prevent a significant damage to diesel engines [1,2]. Therefore, this purification process becomes very important to meet the ASTM D 6751 (USA) (American Society for Testing and Materials) and EN 14214 (European standard), quality standards for commercial biodiesel production [3].
The purification of biodiesel can be carried out by wet and/or dry washing. Until recently, water washing was the most common purification method. Nowadays, both methods are generally accepted and applied on a commercial scale [1,4,5]. Dry washing removes contaminants from crude biodiesel by adsorption, or by passing crude biodiesel through a packed bed running under specific flow rates. Different commercial adsorbent materials were developed specifically for treating crude biodiesel, such as Magnesol® (magnesium silicate) and ion exchange resins (Purolite PD206® , Amberlite Dry BD10 and Lewatit GF 202). In addition, alternative and low-cost sorbents (bentonite, starch derivatives, silica, among others) have already been investigated to reduce the overall cost of this procedure [6,7,8].
Berrios et al. [9] studied the purification of methyl biodiesel from fried oil using bentonite clay as sorbent in comparison with the performance attained by commercial resins, Magnesol and Lewatit. The results showed that, among all tested sorbents, bentonite could effectively remove soap, methanol and glycerol. The feasibility of using natural adsorbents such as cellulose and starch obtained from different sources (maize, potato, cassava and rice), for biodiesel purification was investigated by Gomes et al. [10]. All natural adsorbents studied showed good efficiency in removing impurities from biodiesel when compared to the aqueous washing technique and adsorption with a commercial resin, Select 450.
Conclusion
This study demonstrated the applicability of statistical designs to optimize the conditions that maximize the glycerol removal by dry washing. Under optimized conditions, the RHA showed promising results as a sorbent agent, providing high-quality biodiesel in accordance with the specifications established by ASTM D 6751 and EN 14214 standards. In addition, this sorbent showed similar performance to the conventional aqueous washing and adsorption with commercial resin, such as Amberlite. The adsorptive capacity of the RHA can be related to the high content of silica in its composition, as demonstrated by EDS, XDR and FT-IR analysis. RHA is a low-cost agricultural waste with high-availability features, hence being a promising material to be used in biodiesel purification processes.