Deglycerolization of Biodiesel Streams by Adsorption Over Silica Beds

Abstract

A simple method for the almost complete removal of glycerol from methanol-free biodiesel streams coming out from industrial transesterification reactors is presented. The method is posed as a “dry” alternative to the conventional “wet” methods involving water washing. It is based on the use of silica beds and relies on the adsorption at room temperature to retain the small amounts of glycerol dissolved in the solutions of fatty acid methyl esters and adjust their content to the quality standards for biodiesel fuel. Fresh silica has a great processing capacity and the breakthrough of the bed depends mainly on the feed rate, the concentration of glycerol, and the mass of adsorbent. In the case of the silica gel used, the saturation capacity was found to be 0.13 g of glycerol per gram of silica. If the particle diameter is 1−1.5 mm, the breakthrough and saturation point almost coincide and the full capacity of the bed is used. However, industrial adsorption units with 1/8 in. silica beads suffer from mass-transfer limitations inside the pellet pores, and for this particle size, the breakthrough point (C/C 0 = 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 of 0.01−0.02 m3 biodiesel kgsilica -1. The breakthrough curves were studied using approximate solutions to the set of differential equations. Assuming a linear isotherm gives erroneous results; fitting the experimental breakthrough curves 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 irreversible isotherm. 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 succesfully regenerated eluting 4 bed volumes of methanol and drying in a nitrogen stream for 1 h. Temperature programmed oxidation tests of fresh, regenerated, and glycerol impregnated silica pellets indicated that desorption of glycerol was practically complete. In the industrial practice, the eluted volume can be recycled to the transesterification reactors with no waste of products or reactants. Evaporation of the adsorbed methanol during drying of the bed produced a decrease of the bed temperature and about 200 kJ kgsilica -1 should be provided in order to maintain the temperature.

Dry washing in biodiesel purification: a comparative study of adsorbents

“…[1][2][3] Biodiesel is currently produced by the base-catalyzed transmethylation of triglycerides, producing fatty acids methyl esters (FAME). 4,5 At the end of the reaction, the Faccini et al 559 Vol.…”

“…Removal of glycerol and glycerides from biodiesel is an important step of the process because key aspects of the quality of the fuel strongly depend on the content of free and bound glycerol. 1 Although it is slightly soluble in biodiesel, glycerol can be found dispersed as small droplets in biodiesel. 6,7 High concentrations of glycerol in biodiesel can cause problems during storage due to its separation, and in usage by forming deposits on injection nozzles and promoting an increase in aldehyde emissions.…”

“…It can be used to remove contaminants such as water, soap, free glycerol and glycerides from biodiesel. 1,13,15 Yori et al 1 and Predojevic 8 studied the removal of glycerol from biodiesel from waste frying oils with elevated acid values using silica and achieved high purity of the resultant biodiesel. 1,8 The organic resin Amberlite BD10 DRY ® is already being used in pilot industries, where biodiesel is purified through a column filled with the resin.…”

“…1,13,15 Yori et al 1 and Predojevic 8 studied the removal of glycerol from biodiesel from waste frying oils with elevated acid values using silica and achieved high purity of the resultant biodiesel. 1,8 The organic resin Amberlite BD10 DRY ® is already being used in pilot industries, where biodiesel is purified through a column filled with the resin. 18 Another ion exchange resin commonly employed is Purolite PD 206 ® , which is a selective special organic resin that has been developed to remove maximum amounts of waste water, soaps, salts, catalysts and glycerol during the biodiesel purification process.…”

“…It can be used to remove contaminants such as water, soap, free glycerol and glycerides from biodiesel. 1,13,15 Yori et al 1 and Predojevic 8 studied the removal of glycerol from biodiesel from waste frying oils with elevated acid values using silica and achieved high purity of the resultant biodiesel. …”