**4. Refining of crude biodiesel**

After transesterification is completed, many contaminants can still be present in the biodiesel product depending on the technology of transesterification used (Table 2). Removal of these impurities will be treated separately in the next subsections.

#### **4.1 Glycerides**

Removal of 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 bound glycerol. The ASTM D6751 and EN 14214 standards establish a maximum amount of 0.24-0.25% bound glycerol. Main problem with these compounds is that when heated they tend to polymerize forming deposits. They also increase the cloud point of biodiesel and they complicate the operation of liquid-liquid phase splitting units due to their amphiphilic nature.

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

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

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

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

After transesterification is completed, many contaminants can still be present in the biodiesel product depending on the technology of transesterification used (Table 2).

Removal of 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 bound glycerol. The ASTM D6751 and EN 14214 standards establish a maximum amount of 0.24-0.25% bound glycerol. Main problem with these compounds is that when heated they tend to polymerize forming deposits. They also increase the cloud point of biodiesel and they complicate the operation

Removal of these impurities will be treated separately in the next subsections.

of liquid-liquid phase splitting units due to their amphiphilic nature.

longer filtration cycles.

content in the cake can be as high as 40%.

stability (absence of metals and FFA).

**4. Refining of crude biodiesel** 

units.

**4.1 Glycerides** 


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

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

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

Adsorption in Biodiesel Refining - A Review 435

during the process of oil extraction or biodiesel production. Certain metals, such as cobalt, manganese and chromium, but particularly iron and copper, exhibit a prooxidant effect in oil. The manifestations of oxidation are flavor, color and odor deterioration. Copper is perhaps the most active catalyst, exhibiting noticeable oxidation properties at levels as low as 0.005 ppm (Flider & Orthoefer, 1981). Though flavor, color and odor deterioration are

For soaps, salts and metals, adsorption on silica adsorbents seems the most suitable means of removal (Welsh et al., 1990). Clays offer only a small adsorption capacity for soaps and an

FFAs have negligible values in biodiesel produced by the alkaline method. Depending on the efficiency of esterification they can be present in non-negligible amounts in biodiesel produced by the acid-catalyzed method or the supercritical method. Manuale et al. (2011) reacted different feedstocks with acidities ranging from 0.08 to 23.6% and found that the esterification with supercritical methanol (280 °C, 20=methanol-to-oil ratio) reduced the FFA content to 1-2.5% after 1 h and 0.4-0.6% after 1.5 h of reaction time. Reduction of the FFA content to values lower than those of the international norms can be done by washing. Adsorption however can prove simple, robust and efficient. For these application silicas are found to be superior than other adsorbents in both bleaching capacity and bleaching rate.

*Bleaching time,* 

*Adsorption capacity,* 

*gFFA gads-1*

*min* 

probably not an issue for biodiesel, oxidation stability is indeed required.

*mass %* 

*Virgin activated carbon* 5 720 6.0 *Mg doped activated carbon* 5 720 5.0 *Diatomaceous earth* 1 30 10.1 *Silica gel* 0.36 90 140.0 Table 3. Adsorbents capacity for FFA removal from biodiesel (Manuale et al., 2011).

In the last years there has been a great progress in adsorbent design and cyclic adsorption process developments, thus making adsorption an important separation tool (King, 1980). Adsorption is usually performed in columns packed with adsorbent but it can also be performed in stirred tanks with the adsorbent in suspension. The latter are usually known as bleachers since their most common application is the bleaching of edible oils with clays. The high separating power of the chromatographic effect, achieved in adsorbent-packed columns, is a unique advantage of adsorption as compared to other separation processes. The high separating power is caused by the continuous contact and equilibration between the fluid and sorbent phases. If no diffusion limitations are considered, each contact is equivalent to an equilibrium stage (theoretical plate) and several hundreds or more of such equilibrium stages can be achieved within a short column. Adsorption is thus ideally suited

The adsorptive separation is achieved by one of three mechanisms: adsorption equilibrium, steric effect and kinetic effect. Most processes, especially those in solid-liquid phase, operate with the principle of adsorption equilibrium and hence they are called equilibrium

almost null capacity for metals.

*Adsorbent Adsorbent conc.,* 

for purification applications and difficult separations.

**4.4 Free fatty acids** 

**5. Adsorption** 

reduce the complexity of the process by reducing the methanol-to-oil ratio and the amount of recycled methanol. (iii) MG and DG should be the focus for reducing bound glycerol. Steps in the direction of (ii) have been hinted by D'Ippolito et al. (2007) and Manuale et al. (2011) for the supercritical method. The first proposed using 2 reaction steps with a low methanol-to-oil ratio (6-10), retaining glycerol and glycerides in packed bed adsorbers and recycling them to the reactor. The second indicated that the combination of 1-step reaction, a methanol-to-oil ratio of 15-20 and silica refining could produce EN14214 grade biodiesel.

#### **4.2 Glycerol**

Liquid-liquid equilibrium studies of biodiesel-methanol-glycerol mixtures have been undertaken in the past by Kimmel (2004), Negi et al. (2006) and Zhou & Boocook (2006). They determined that the equilibrium glycerol content in biodiesel depends strongly on the residual content of methanol acting as a cosolvent. When methanol is completely removed the free glycerol content depends only on the temperature, being approximately 0.2% at 25 °C and increasing linearly with temperature (Kimmel, 2004). Even if methanol is not present hydrophilic glycerol can be solubilized in the oil phase by amphiphilic MG and DG. These glycerides can separate from the oil during storage and precipitate as a result of temperature changes or long residence times. Glycerol then precipitates as a consequence of the reduced solubility, leading to the formation of deposits. Soluble glycerol is also a problem because glycerol polymerizes on hot surfaces (cylinders, injectors) with formation of deposits or "tarnishes". For all these reason glycerol should be thoroughly removed.

Glycerol removal by adsorption was early performed by Griffin and Dranoff (1963) using sulfonic resin beads. Glycerol adsorption over polar surfaces is favored if dissolved in organic media that have little affinity for the adsorbent. Nijhuis et al. (2002) reported that adsorption of organic esters (e.g. biodiesel) over polar surfaces such as those of silica and Nafion resins, is negligible. Yori et al. (2007) studied the reversible adsorption of glycerol from biodiesel and reported that silica has a great capacity for glycerol removal, its saturation capacity being 0.13 g of glycerol per gram of adsorbent. When operated in packed beds, for a glycerol concentration of 0.11−0.25% typical of biodiesel streams issuing from gravity settling tanks, an effluent limit of *C/C0*=0.01 and an entrance velocity of 11 cm min-1, a 2 m high silica bed with 1/8" beads would have a net processing capacity of 0.01−0.02 m3 biodiesel kgsilica-1. Much of the good performance of silica is related to the favorable thermodynamics of adsorption, since glycerol-silica displays an almost irreversible, square isotherm (Yori, 2008).

#### **4.3 Soaps, salts and metals**

Soaps are produced by the reaction of FFAs during the first steps of caustic refining of the fatty feedstock or by the reaction of the remaining FFAs with alkaline homogeneous catalysts in the transesterification reactor. These reactions lead to the formation of estearates, oleates, palmitates, etc. of sodium and potasium, that are amphiphilic substances that bring phase separation and plugging problems downstream the reactor. Other salts of sodium or potasium come from the neutralization of acid homogeneous catalysts in the acid-catalized process. These inorganic salts lead to corrosion in lines and vessels and they must also be completely eliminated in the final biodiesel product because of quality issues.

Metals are minor components in all oils as they are present as oligoelements in highly specialized molecules such as chlorophylls (magnesium) and porphyrins (magnesium, iron, manganese). Other sources of metals are the contamination from iron and copper surfaces during the process of oil extraction or biodiesel production. Certain metals, such as cobalt, manganese and chromium, but particularly iron and copper, exhibit a prooxidant effect in oil. The manifestations of oxidation are flavor, color and odor deterioration. Copper is perhaps the most active catalyst, exhibiting noticeable oxidation properties at levels as low as 0.005 ppm (Flider & Orthoefer, 1981). Though flavor, color and odor deterioration are probably not an issue for biodiesel, oxidation stability is indeed required.

For soaps, salts and metals, adsorption on silica adsorbents seems the most suitable means of removal (Welsh et al., 1990). Clays offer only a small adsorption capacity for soaps and an almost null capacity for metals.
