**2. The needs for refining of petrodiesel and biodiesel fuels**

The objectives of Diesel fuel refining operations are aimed at improving the fuel combustion performance, maximizing the power delivered to the motor, increasing the engine life and reducing the emission of noxious compounds. The relevant properties involved are cetane

Adsorption in Biodiesel Refining - A Review 429

resulting from hydrolysis of soaps and esters. Biodiesel acidity also reflects the degree of fuel ageing during storage, as it gradually increases due to hydrolytic cleavage of ester bonds. High fuel acidity of biodiesel has been discussed in the context of corrosion and the formation of deposits within the engine, particularly in fuel injectors, by catalyzing polymerization in hot recycling fuel loops (Refaat, 2009). However the main problem

*Carbonization properties.* Formation of carbon deposits in the injectors of a Diesel engine is undesired; the tendency of a fuel to form these deposits being measured by the Conradson Carbon Residue (CCR) test (ASTM D189). In the case of petrodiesel CCR is related to the presence of aromatic and polyaromatic compounds, and is favorably reduced by hydrotreatment. In the case of biodiesel deposits formed in the injectors are related to polymerization of glycerol and glycerides. These polymers undergo further decomposition to carbon deposits and tarnishes over injectors and cylinders. In this sense the needed biodiesel refining step is the removal of free and bound glycerol to minimum values. ASTM D6751 constrains the iodine number of biodiesel to less than 112 on the same basis because olefinic chains are also reactive for polymerization. However this is not an issue in european

*Cold flow properties.* In raw biodiesel the presence of wax-like, long acyl chains, poses the problem of crystallization when temperature is too low. Crystal nucleation is enhanced by the presence of MGs and DGs, mainly affecting the cloud point (van Gerpen et al., 1996). A first solution is to eliminate glycerides to negligible values. Other solutions for waxy FAMEs are not without drawbacks: (i) Catalytic dewaxing (Yori et al., 2006) decreases the cetane number and increases the viscosity. (ii) Winterization for removing the waxy saturated fraction also removes the fraction with higher cetane and oxidizing stability. (iii) Commercial pour point depressants are reported to reduce the pour point of biodiesel but usually do not reduce its cloud point nor improve its filterability at low temperatures (Dunn et al., 1996). Fortunately, biodiesel-petrodiesel blends have cloud and pour points closer to those of petrodiesel. The saturated portion eliminated by winterization can also be used as a

*Refinery operation issues.* Some specifications for feedstocks and intermediate streams in refineries are related to the correct functioning of process units. Sulfur reduction in the case of petroleum fuels is necessary not only to improve the quality of the final product but also to prevent the poisoning of catalysts in some hydroprocessing units (Ito and van Veen, 2006). In the case of biodiesel many undesired components are responsible for the

 Phosphorous, calcium, and magnesium are minor components typically associated with phospholipids and gums that act as emulsifiers or cause sediments, lowering yields during the transesterification process. Phosphorus typically leads to an increased difficulty in the separation of the biodiesel and glycerol phases (Anderson et al., 2003).

*Component Crown Iron Works (USA) Lurgi GmbH (Germany)* 

Table 1. Quality requirements for the feedstock of two alkali homogeneous catalyst biodiesel

*Moisture and volatiles* 0.05% max 0.1% max *Acidity* 0.5% max 0.1% max *Phosphorus total* 20 ppm max 10 ppm max

*Soap* 50 ppm max n.a. *Unsaponifiables* 1% max 0.8% max

production technologies (Anderson et al., 2003; Lurgi, 2011).

associated with acids is the formation of soaps as it will be discussed later.

norms (EN 14214).

"summer fuel" if massive storage is available.

malfunctioning of reactors and phase separators:

index, heat content, lubricity, viscosity, cold flow properties, oxidation stability and amount and kind of tailpipe emissions. Some properties are superior for biodiesel in comparison to petrodiesel and need no adjustment. This is the case of lubricity (93% film for biodiesel and 32% for petrodiesel), cetane index (45 for petrodiesel and 56 for biodiesel) and tailpipe emissions (Chang et al., 1996; Romig & Spataru, 1996). Other properties of biodiesel needing adjustment will be discussed in the next paragraphs.

*Viscosity.* Viscosity affects injector lubrication and atomization. Natural oils and fats (triglycerides) have excessive viscosity and cannot be easily injected; this is the main reason why they must be transformed into methyl esters. Even after transesterification the viscosity of biodiesel is higher than that of petrodiesel (5 cSt at 40 °C compared to 3 cSt), though it is considered enough low in international norms. A few reports have however indicated that additivations and chemical transformations could advantageously alter biodiesel viscosity. Noureddini et al. (1998) found that addition of GTBE in amounts as big as 22% could not only lower the pour and cloud points of biodiesel but also its viscosity by 8%. Yori et al. (2006) studied the acid-catalyzed isomerization of methyl soyate and found that isomerization decreased pour and cloud points but adversely increased the viscosity.

*Oxidation stability.* The stability of a diesel fuel is related to the occurrence of undesired reactions during storage. In the case of petrodiesel routine oxidation tests as performed by ASTM D2274 detect the formation of minor amounts of insolubles that are due to the precipitation of polar compounds, mainly polycyclic acids, after their reaction with iron particles or oxygen (Díaz & Miller, 1990). In the case of biodiesel the problem is worse because unsaturated fatty acid chains are main components of the fuel and they are active in oxidizing reactions. In contact with oxygen, peroxides are formed that promote the formation of organic acids, and then of polymers (gums) that plug fuel lines and filters. Oxidative degradation during storage can also compromise fuel quality with respect to effects on kinematic viscosity, acid value, cetane number, total ester content, and formation of hydroperoxides, soluble polymers, and other secondary products (Du Plessis et al., 1985; Bondioli et al., 2002; Thompson et al., 1998). The increased acidity and peroxide values as a result of oxidation reactions can also cause the corrosion of fuel system components, the hardening of rubber components and the erosion of moving parts (Tang et al., 2008). By now the only method for increasing the biodiesel resistance to oxidation is to add synthetic or natural oxidation inhibitors such as tocopherols and hydroquinones. Other alternative way is the hydrogenation of the unsaturated chains. Compared to untreated soybean oil methyl esters, partially hydrogenated products have shown superior oxidative stability and similar specific gravity, but inferior low-temperature performance, kinematic viscosity and lubricity (Moser et al., 2007). In order to raise the saturated fraction of biodiesel other efforts have been carried out by distillation and crystallization (Falk & Meyer-Pittroff, 2004) and it is conceivable that the same could be done by adsorption over suitable materials.

*Storage stability.* Also related to the stability of biodiesel, some other minor components of biodiesel, the monoglycerides (MGs) and diglycerides (DGs) can form crystals during storage at low temperatures and precipitate. These crystals not only can clog fuel lines and fuel filters but due to their amphiphilic nature, their absence in the solution causes the precipitation of other unstable solvatable impurities such as glycerol.

*Acidity.* Acidity in petrodiesel is mainly related to the presence of napthenic acids in the crudes. Acidity of biodiesel depends on a wider variety of factors and is influenced by the type of feedstock used and on its degree of refinement. Acidity can also be generated during the production process, e.g. by mineral acids introduced as catalysts or by free fatty acids

index, heat content, lubricity, viscosity, cold flow properties, oxidation stability and amount and kind of tailpipe emissions. Some properties are superior for biodiesel in comparison to petrodiesel and need no adjustment. This is the case of lubricity (93% film for biodiesel and 32% for petrodiesel), cetane index (45 for petrodiesel and 56 for biodiesel) and tailpipe emissions (Chang et al., 1996; Romig & Spataru, 1996). Other properties of biodiesel needing

*Viscosity.* Viscosity affects injector lubrication and atomization. Natural oils and fats (triglycerides) have excessive viscosity and cannot be easily injected; this is the main reason why they must be transformed into methyl esters. Even after transesterification the viscosity of biodiesel is higher than that of petrodiesel (5 cSt at 40 °C compared to 3 cSt), though it is considered enough low in international norms. A few reports have however indicated that additivations and chemical transformations could advantageously alter biodiesel viscosity. Noureddini et al. (1998) found that addition of GTBE in amounts as big as 22% could not only lower the pour and cloud points of biodiesel but also its viscosity by 8%. Yori et al. (2006) studied the acid-catalyzed isomerization of methyl soyate and found that

isomerization decreased pour and cloud points but adversely increased the viscosity.

conceivable that the same could be done by adsorption over suitable materials.

precipitation of other unstable solvatable impurities such as glycerol.

*Storage stability.* Also related to the stability of biodiesel, some other minor components of biodiesel, the monoglycerides (MGs) and diglycerides (DGs) can form crystals during storage at low temperatures and precipitate. These crystals not only can clog fuel lines and fuel filters but due to their amphiphilic nature, their absence in the solution causes the

*Acidity.* Acidity in petrodiesel is mainly related to the presence of napthenic acids in the crudes. Acidity of biodiesel depends on a wider variety of factors and is influenced by the type of feedstock used and on its degree of refinement. Acidity can also be generated during the production process, e.g. by mineral acids introduced as catalysts or by free fatty acids

*Oxidation stability.* The stability of a diesel fuel is related to the occurrence of undesired reactions during storage. In the case of petrodiesel routine oxidation tests as performed by ASTM D2274 detect the formation of minor amounts of insolubles that are due to the precipitation of polar compounds, mainly polycyclic acids, after their reaction with iron particles or oxygen (Díaz & Miller, 1990). In the case of biodiesel the problem is worse because unsaturated fatty acid chains are main components of the fuel and they are active in oxidizing reactions. In contact with oxygen, peroxides are formed that promote the formation of organic acids, and then of polymers (gums) that plug fuel lines and filters. Oxidative degradation during storage can also compromise fuel quality with respect to effects on kinematic viscosity, acid value, cetane number, total ester content, and formation of hydroperoxides, soluble polymers, and other secondary products (Du Plessis et al., 1985; Bondioli et al., 2002; Thompson et al., 1998). The increased acidity and peroxide values as a result of oxidation reactions can also cause the corrosion of fuel system components, the hardening of rubber components and the erosion of moving parts (Tang et al., 2008). By now the only method for increasing the biodiesel resistance to oxidation is to add synthetic or natural oxidation inhibitors such as tocopherols and hydroquinones. Other alternative way is the hydrogenation of the unsaturated chains. Compared to untreated soybean oil methyl esters, partially hydrogenated products have shown superior oxidative stability and similar specific gravity, but inferior low-temperature performance, kinematic viscosity and lubricity (Moser et al., 2007). In order to raise the saturated fraction of biodiesel other efforts have been carried out by distillation and crystallization (Falk & Meyer-Pittroff, 2004) and it is

adjustment will be discussed in the next paragraphs.

resulting from hydrolysis of soaps and esters. Biodiesel acidity also reflects the degree of fuel ageing during storage, as it gradually increases due to hydrolytic cleavage of ester bonds. High fuel acidity of biodiesel has been discussed in the context of corrosion and the formation of deposits within the engine, particularly in fuel injectors, by catalyzing polymerization in hot recycling fuel loops (Refaat, 2009). However the main problem associated with acids is the formation of soaps as it will be discussed later.

*Carbonization properties.* Formation of carbon deposits in the injectors of a Diesel engine is undesired; the tendency of a fuel to form these deposits being measured by the Conradson Carbon Residue (CCR) test (ASTM D189). In the case of petrodiesel CCR is related to the presence of aromatic and polyaromatic compounds, and is favorably reduced by hydrotreatment. In the case of biodiesel deposits formed in the injectors are related to polymerization of glycerol and glycerides. These polymers undergo further decomposition to carbon deposits and tarnishes over injectors and cylinders. In this sense the needed biodiesel refining step is the removal of free and bound glycerol to minimum values. ASTM D6751 constrains the iodine number of biodiesel to less than 112 on the same basis because olefinic chains are also reactive for polymerization. However this is not an issue in european norms (EN 14214).

*Cold flow properties.* In raw biodiesel the presence of wax-like, long acyl chains, poses the problem of crystallization when temperature is too low. Crystal nucleation is enhanced by the presence of MGs and DGs, mainly affecting the cloud point (van Gerpen et al., 1996). A first solution is to eliminate glycerides to negligible values. Other solutions for waxy FAMEs are not without drawbacks: (i) Catalytic dewaxing (Yori et al., 2006) decreases the cetane number and increases the viscosity. (ii) Winterization for removing the waxy saturated fraction also removes the fraction with higher cetane and oxidizing stability. (iii) Commercial pour point depressants are reported to reduce the pour point of biodiesel but usually do not reduce its cloud point nor improve its filterability at low temperatures (Dunn et al., 1996). Fortunately, biodiesel-petrodiesel blends have cloud and pour points closer to those of petrodiesel. The saturated portion eliminated by winterization can also be used as a "summer fuel" if massive storage is available.

*Refinery operation issues.* Some specifications for feedstocks and intermediate streams in refineries are related to the correct functioning of process units. Sulfur reduction in the case of petroleum fuels is necessary not only to improve the quality of the final product but also to prevent the poisoning of catalysts in some hydroprocessing units (Ito and van Veen, 2006). In the case of biodiesel many undesired components are responsible for the malfunctioning of reactors and phase separators:

 Phosphorous, calcium, and magnesium are minor components typically associated with phospholipids and gums that act as emulsifiers or cause sediments, lowering yields during the transesterification process. Phosphorus typically leads to an increased difficulty in the separation of the biodiesel and glycerol phases (Anderson et al., 2003).


Table 1. Quality requirements for the feedstock of two alkali homogeneous catalyst biodiesel production technologies (Anderson et al., 2003; Lurgi, 2011).

Adsorption in Biodiesel Refining - A Review 431

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.

content (van Dalen & van Putte, 1992; Farhan et al., 1988).

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

Fig. 1. Isotherms of adsorption of phosphatides on silica (left). Adsorption of chlorophyll on

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

clay as a function of the silica adsorption pretreatment (right). Welsh et al. (1990).

