**3. Refining of biodiesel feedstocks**

Depending on their degree of refining, biodiesel feedstocks might need some or all the refining steps common to the refining of edible oils: (i) Degumming, that is necessary if large amounts of phosphatides are present in the feedstock, phosphoric acid and steam being used to swell the gums for further removal. (ii) Deodorization, that is used with feedstocks up to 30% FFA. It is basically a vacuum distillation at 240-270 °C and 2-5 mmHg, that removes aldehydes, ketones and smelly products, pesticides, fungicides, herbicides, etc. It also lightens up the product by destroying carotenoids. (iii) FFA reduction by many means, steam stripping, caustic stripping, solvent extraction, glycerolysis, acid esterification, etc. (iv) Bleaching, that is normally used to remove remaining impurities such as pigments, soaps, insolubles, peroxides, phospholipids and metals.

It must be noted however that biodiesel and edible oil have different quality specifications. This is especially true for color and odor, that are not an indication of technical quality of biodiesel. ASTM quality biodiesel can range from clear to black and have an unpleasant smell. This is more a consumer issue because it raises uncertainties. Color removal may need carbon filtration and bleaching while odor removal may need deodorization.

The degree of FFA reduction in biodiesel feedstocks needs special attention because it has a high dependence on the technology of biodiesel production used: (i) In the case of the non-

 FFAs and soaps. In the case of the alkali-catalyzed process, the dominant biodiesel technology, the presence of free fatty acids (FFAs) leads to the use of an increased amount of catalyst and other chemicals. It also increases the concentration of salt and water in the crude glycerol phase. Aside from the increased cost of chemicals, the presence of FFA causes a larger potential for soap formation and all the production issues associated with soap, including more difficult phase separations and more frequent cleaning of process vessels. Although FFA can be reacted in an acid-catalyzed reaction with methanol to form methyl esters, the amount of acid required is much higher than the amount of catalyst used in the transesterification of neutral oil. The reaction also does not go as far to completion as transesterification, which may lead to the resulting biodiesel product to be out of specification on FFA. Acid catalysis of FFA to methyl esters also results in higher salt and water formation. For all these reasons

 Unsaponifiable matter (UM) consists of plant sterols, tocopherols and hydrocarbons, with very small quantities of pigments and minerals. UM is limited in the feedstock of biodiesel processes mainly on the basis of its foaming properties that make separations difficult (see Table 1). The unsaponifiable matter is not affected by ester preparation, so it is likely to be present in similar amounts in biodiesel to its level in the crude feedstock. UM has no harmful effects except possibly for a change in the crystallization onset temperature (van Gerpen et al., 1996). For this reason UM is not limited in biodiesel norms. Some unsaponifiable compounds, such as the phytosterols, have antioxidizing capacities and they are useful for prolonging the storage life of biodiesel (Rabiei et al., 2007). A possible challenge for adsorption operations in this case could be the selective removal of impurities while not affecting these antioxidizing compounds. Water. Alkaline catalysts (NaOH, KOH, MeONa) react with water and oil to produce soaps. Acid catalysts (e.g. H2SO4) when hydrated reduce their effective acid strength and their catalytic activity. Water thus leads to deactivation and higher catalyst usage.

Depending on their degree of refining, biodiesel feedstocks might need some or all the refining steps common to the refining of edible oils: (i) Degumming, that is necessary if large amounts of phosphatides are present in the feedstock, phosphoric acid and steam being used to swell the gums for further removal. (ii) Deodorization, that is used with feedstocks up to 30% FFA. It is basically a vacuum distillation at 240-270 °C and 2-5 mmHg, that removes aldehydes, ketones and smelly products, pesticides, fungicides, herbicides, etc. It also lightens up the product by destroying carotenoids. (iii) FFA reduction by many means, steam stripping, caustic stripping, solvent extraction, glycerolysis, acid esterification, etc. (iv) Bleaching, that is normally used to remove remaining impurities such as pigments,

It must be noted however that biodiesel and edible oil have different quality specifications. This is especially true for color and odor, that are not an indication of technical quality of biodiesel. ASTM quality biodiesel can range from clear to black and have an unpleasant smell. This is more a consumer issue because it raises uncertainties. Color removal may need

The degree of FFA reduction in biodiesel feedstocks needs special attention because it has a high dependence on the technology of biodiesel production used: (i) In the case of the non-

carbon filtration and bleaching while odor removal may need deodorization.

feedstock specifications for FFA have low limits.

**3. Refining of biodiesel feedstocks** 

soaps, insolubles, peroxides, phospholipids and metals.

catalytic method that uses supercritical methanol at high temperatures and pressures, FFA content is not an issue, because triglycerides and FFAs react to form methyl esters with similar rates (Warabi et al., 2004). (ii) In the acid-catalized method feedstocks with up to 20% acidity can be completely reacted by acid catalysis with mineral acids though the kinetics are much slower (Lotero et al., 2005; Freedman, 1986). (iii) The alkali-catalized method (dissolved NaOH, KOH, MeONa, etc. catalysts) tolerates only 0.5% FFA in the feedstock (Table 1). However some producers accept feedstocks of up to 4% FFA and then use caustic stripping by the same catalyst in the reactor or caustic washing before the reactor to eliminate them from the reaction medium. The soap that goes into the glycerol phase or the wash water is hydrolized and reacted to biodiesel by acid-catalyzed transesterification in a separate reactor. (iv) In the acid-base method feedstocks with up to 20% acidity are first esterified in acid catalysis and then the reaction is continued with alkaline catalysis.

The use of adsorbents for the pretreatment of biodiesel raw materials is related to known techniques for edible oil refining. After pressing of oil seeds, and after degumming and caustic refining of the virgin oil, a step of bleaching is commonplace in order to improve the colour by adsorbing chlorophylls, carotenoids and other pigments, and the removal of other undesired components such as metals and free fatty acids, that contribute to the unstability of the oil under oxidizing conditions. Bleaching of oils can be done with natural clays such as bentonite, smectite, montmorillonite, etc., or activated clays produced by acid treatment (Foletto et al., 2011). Clays are mainly used for removing high molecular weight organic compounds but their affinity for polar compounds and metals is low. In this sense most part of the metals is eliminated during the caustic refining of edible oils and in the subsequent water washing steps, while bleaching with clays does not practically modify the metal content (van Dalen & van Putte, 1992; Farhan et al., 1988).

Fig. 1. Isotherms of adsorption of phosphatides on silica (left). Adsorption of chlorophyll on clay as a function of the silica adsorption pretreatment (right). Welsh et al. (1990).

Another adsorbent commonly used is silica, either alone or together with clays, though it is now accepted that best treatments should include some portions of both, since silicas adsorb preferably polar compounds and clays are more suitable for organic compounds. Treatment with silica has become incresingly widespread and silicas for edible oil refining have become highly tailored for this application, thus leading to the coining of the term "silica refining" (Welsh et al., 1990). The conditions for optimal silica refining can be

Adsorption in Biodiesel Refining - A Review 433

the catalyst

esterification.

Yes. Product of transesterification.

Yes. Product of transesterification.

Yes. Product of transesterification.

conversion.

Table 2. Contaminants in biodiesel product depending on transesterification technology.

In the specific case of monoglycerides (MG), diglycerides (DG) and triglycerides (TG), they are the raw materials and the intermediates of the transesterification reaction. This is an equilibrium reaction with an equilibrium constant close to unity (D'Ippolito et al., 2007), that dictates that a methanol excess must be used to shift the equilibrium to the right and to decrease the concentration of triglycerides and intermediates in the final product mixture. Noureddini & Zhu (1997) and Darnoko & Cheryan (2000) studied the kinetics of transesterification of oil and they reported that the conversion value for the 1-step reaction of transesterification of soy oil with methanol in a stirred tank reactor, using a methanol-tooil ratio of 6 was 80-87% at 1 h of time of reaction. Busto et al. (2006) indicated that in supercritical tubular reactors a methanol-to-oil ratio of 6 yields an equilibrium value of 94- 95% at high Péclet numbers. In the case of processes with two reaction steps, after the final step of glycerol removal, the amount of TG, MG and DG is sufficiently low to almost comply with the ASTM D6751 limits. It can be however deduced that this final content of bound glycerol is a function of the methanol-to-oil ratio used in the reaction and the number of reaction steps. For the alkali catalized process with two reaction steps this methanol-to-oil ratio is 6. In the case of the supercritical method with one reaction step (Goto et al., 2004) the adequate methanol-to-oil molar ratio is reported to be 42. The final adjustment of the glycerides content is made in the standard industrial practice by water washing. Some authors however propose separating the glyceride fraction (Goto et al., 2004; D'Ippolito et

One interesting issue is that of the relative concentration of MG, DG and TG in the final product. According to data of Noureddini and Zhu (1997) the equilibrium constants for the partial transesterification (producing 1 mol of FAME) of triglycerides, diglycerides and monoglycerides are K1=0.45, K2=0.18, K3=34.6. TGs would therefore be thermodynamically more stable. It is however found in practice, probably because of kinetic limitations, that MGs and DGs are main impurities (He et al., 2007). This points to the adequacy of adsorption treatments since MGs are efficiently removed by adsorption over silica, even in

Some points seem clear: (i) The final bound glycerol content is a function of the methanol-tooil ratio. (ii) An adequate separation/recycling or removal/disposal of glycerides could

Yes. Due to incomplete

After neutralization of

If feedstock treatment was uneffective.

If feedstock treatment was uneffective.

If feedstock treatment was uneffective.

Yes. Due to hydrolysis of the feedtock.

Yes. Due to incomplete

Yes. Product of transesterification.

Yes. Product of transesterification.

Yes. Product of transesterification.

conversion.

*Impurity Alkali-catalyzed Acid-catalyzed Supercritical* 

*Soaps* Yes. By neutralization

*Metals, P* If feedstock treatment

*Monoglycerides* Yes. Product of

*Diglycerides* Yes. Product of

*Glycerol* Yes. Product of

*Triglycerides* Yes. Due to incomplete conversion.

al., 2007) and recycling it to the reactor.

the presence of water and soaps (Mazzieri et al., 2008).

of FFA with catalyst

*FFAs* No Yes. Due to incomplete

was uneffective.

transesterification.

transesterification.

transesterification.

summarized as follows: (i) Oil temperature is raised to 70-90 °C. (ii) Silica is added at atmospheric pressure to the vessel contaning the oil. (iii) The moisture content of the oil is reduced to 0.2-0.5% by evaporation, preferably in a vacuum. (iv) The contact time between the silica and the oil should be 10-15 min. (v) The moisture in the oil plays an important role in the mechanism responsible for transporting polar compounds from the oil to the silica, where they are trapped. (vi) After the removal of the polar contaminants the oil should be further dried if clays are to be used in the bleacher. During the vacuum drying process water is removed from the silica and the weight is reduced to even 40% of its original value; the solid reduces also in size and so does the load on the filters downstream the bleachers, which can then be operated at higher filtration flowrates and longer filtration cycles.

As silicas are far more efficient adsorbents for polar contaminants, if colour is not an issue (like in the case of biodiesel fuel) they can easily replace bleaching clays. If colour reduction is necessary then clays can be used in a second step after silicas have removed the polar contaminants. This reduces the amount of adsorbent used and enhances the quantity of oil produced because a lower quantity of filter cake is produced and oil losses are reduced. In this sense, a common industry perception is that 20-25% of oil is present in the filter cake but as oxidized and polymerized oil are not extracted in the extraction tests, the typical oil content in the cake can be as high as 40%.

The claimed advantages of silica (Grace, 2011) for refining biodiesel feedstocks are: (i) Lower costs of residue treatment by means of the reduction of effluents. (ii) Lower costs by elimination of washing steps. (iii) Lower product losses. (iv) Higher yield of the biodiesel fuel precursor. (v) Lower demand of catalyst in the transesterification reactor due to a lower FFA content. (vi) Lower consumption of acid for neutralization of the catalyst. (vii) Higher yield of biodiesel due to an enhanced separation of the glycerol and biodiesel phases (absence of soaps and glycerides). (viii) Purer glycerol due to a low content of impurities. (ix) Lower costs of production of biodiesel. (x) Quality improvement due to an enhanced stability (absence of metals and FFA).

One additional benefit of silica addition in the case of the caustic refining for oil treatment (e.g. for biodiesel alkaline processes of low FFA tolerance) is that water-wash centrifuges can be eliminated because silicas efficiently remove residual metals, phospholipids and soaps. These must be otherwise washed away to prevent them reaching the bleaching units.
