Deglycerolization of Biodiesel Streams by Adsorption Over Silica Beds

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• 作者：J. C. Yori ; S. A. D'Ippó ; lito ; C. L. Pieck ; C. R. Vera
• 刊名：Energy & Fuels
• 出版年：2007
• 出版时间：January 2007
• 年：2007
• 卷：21
• 期：1
• 页码：347 - 353
• 全文大小：111K
• 年卷期：v.21,no.1(January 2007)
• ISSN：1520-5029

文摘

A simple method for the almost complete removal of glycerol from methanol-free biodiesel streams comingout from industrial transesterification reactors is presented. The method is posed as a "dry" alternative to theconventional "wet" methods involving water washing. It is based on the use of silica beds and relies on theadsorption at room temperature to retain the small amounts of glycerol dissolved in the solutions of fatty acidmethyl esters and adjust their content to the quality standards for biodiesel fuel. Fresh silica has a great processingcapacity and the breakthrough of the bed depends mainly on the feed rate, the concentration of glycerol, andthe mass of adsorbent. In the case of the silica gel used, the saturation capacity was found to be 0.13 g ofglycerol per gram of silica. If the particle diameter is 1-1.5 mm, the breakthrough and saturation point almostcoincide and the full capacity of the bed is used. However, industrial adsorption units with ¹/₈ in. silica beadssuffer from mass-transfer limitations inside the pellet pores, and for this particle size, the breakthrough point(C/C⁰ = 0.01) is located at about one-half of the time of full saturation. For a glycerol concentration of 0.11-0.25% typical of biodiesel streams issuing from gravity settling tanks and an entrance velocity of 11 cm min^-1,a 2 m high silica bed with ¹/₈ in. beads has a breakthrough point of 8 h and a net processing capacity of0.01-0.02 m³_biodiesel kg_silica^-1. The breakthrough curves were studied using approximate solutions to the set ofdifferential equations. Assuming a linear isotherm gives erroneous results; fitting the experimental breakthroughcurves produces underestimated values of the Henry's adsorption constant and of the mass-transfer resistances.Modeling the high dilution regime with the UNIFAC method gives more realistic values of the Henry's constant(1.1 m³ kg^-1). The experimentally measured saturation capacity is close to the monolayer capacity (13-15%w/w). These values give a Langmuir isotherm which can be fairly well approximated by a square irreversibleisotherm. Accordingly, breakthrough curves were fairly well predicted using an irreversible isotherm, a shrinking-core adsorption model, and common correlations for the mass-transfer coefficients. The silica bed was succesfullyregenerated eluting 4 bed volumes of methanol and drying in a nitrogen stream for 1 h. Temperature programmedoxidation tests of fresh, regenerated, and glycerol impregnated silica pellets indicated that desorption of glycerolwas practically complete. In the industrial practice, the eluted volume can be recycled to the transesterificationreactors with no waste of products or reactants. Evaporation of the adsorbed methanol during drying of thebed produced a decrease of the bed temperature and about 200 kJ kg_silica^-1 should be provided in order tomaintain the temperature.