ARTICLE pubs.acs.org/EF
Using Deep Eutectic Solvents Based on Methyl Triphenyl Phosphunium Bromide for the Removal of Glycerol from Palm-Oil-Based Biodiesel K. Shahbaz,† F. S. Mjalli,*,‡ M. A. Hashim,† and I. M. AlNashef § †
Department of Chemical Engineering, University of Malaya, Kuala Lumpur, 50603, Malaysia Department of Petroleum and Chemical Engineering, Sultan Qaboos University, Muscat 123, Oman § Department of Chemical Engineering, King Saud University, Post Office Box 800, Riyadh 11421, Saudi Arabia ‡
ABSTRACT: Biodiesel is a remarkable alternative to the decreasing resources for fossil fuels. One of the critical steps in producing biodiesel is its purification from the byproduct glycerol. The content of glycerol permissible must pass the EN 14214 and ASTM D6751 international biodiesel standards. In this work, methyl triphenyl phosphunium bromide as salt and three different hydrogenbond donors, namely, glycerol, ethylene glycol, and triethylene glycol, were selected to synthesize three deep eutectic solvents (DESs). These DESs were employed as the solvent for the removal of glycerol from palm-oil-based biodiesel. The effects of DES type on the removal of free glycerol, bound glycerol, and total glycerol were investigated. The results revealed that the ethylene glycol DESs and triethylene glycol DESs were successful in removing all free glycerol from the palm-oil-based biodiesel. All tested DESs were able to reduce the content of monoglycerides (MGs) and diglycerides (DGs). Moreover, all used DESs had the tendency to reduce the DGs more effectively than removing MGs. The maximum removal efficiency of MGs and DGs was attained by triethylene-glycol-based DESs at a molar ratio of 3:1 (DES8:biodiesel) and 0.75:1 (DES8:biodiesel), with 37.9 and 53.4% removal, respectively. The total glycerol was reduced below the ASTM standards using all tested DESs. The DES:biodiesel molar ratios of 3:1 for DES4 and 3:1 for DES8 were found to be the most effective solvents for reducing total glycerol content, with 40 and 50% removal efficiency, respectively.
1. INTRODUCTION Biodiesel is a remarkable alternative to the ever decreasing fossil fuel resources. It can be synthesized from all sorts of plant oils or animal fats. There are several advantages in the use of biodiesel; a few of these to mention are being biodegradable and renewable, having high combustion efficiency and low sulfur and aromatic content, its cetane number and flash point being as high as fossil-based diesel, its agricultural and environmental benefits, and being of domestic origin, which reduces the dependency upon imported petroleum.13 The most common method for its synthesis is the chemically catalyzed (e.g., alkali and acid) transesterification reaction of the fatty acid alkyl esters (FAAEs) with an alcohol. This reaction takes place in a series of steps. The triglycerides (TGs) (which are the main components of vegetable oils) are first converted to diglycerides (DGs); this is followed by the conversion of DGs to monoglycerides (MGs) and finally to glycerol as a byproduct.4 This process has been generally used to decrease the viscosity of TGs, thereby enhancing the physical properties of biodiesel.5 Methanol is the most common alcohol because of its low cost and good physical and chemical properties.6 Potassium hydroxide is a favorable catalyst in industries because of its low price and high activity. It is used widely in the transesterification of vegetable oils.7 Glycerol has low solubility in methyl esters and is largely separated by settling or centrifuging at the end of the reaction. The produced biodiesel still contains a quantity of glycerol, which is not permissible as per the EN 14214 and ASTM D6751 r 2011 American Chemical Society
standard specifications. Consequently, another purification stage of biodiesel is necessary to remove the remaining glycerol in biodiesel before using it as an alternative fuel. The remaining glycerol in biodiesel is referred to as free glycerol, while the MGs, DGs, and TGs are referred to as bound glycerol. The sum of the bound glycerol and the free glycerol is referred to as total glycerol. The high viscosity, crystallization, and deposition of biodiesel in internal combustion engine injectors (carbon residue) are mainly due to the presence of glycerides in biodiesel. In addition, the presence of glycerol initiates settling problems in the engines and, on the long term, affects human or animal health by the emission of hazardous acrolein into the environment.8 Moreover, its presence in biodiesel reduces the quality of the produced biodiesel. To overcome these shortcomings, international standards have been put to control the quality and reliability of the finally produced biodiesel. According to the ASTM standards, glycerol and total glycerol content must comprise less than 0.02 and 0.24 wt %, respectively.9 The removal of glycerol may be achieved using several methods. The most conventional methods are wet washing, dry washing, and membrane extraction.8,10 The water-washing method has many disadvantages: considerable product loss because of retention in the water phase. Moreover, emulsion Received: December 1, 2010 Revised: May 10, 2011 Published: May 11, 2011 2671
dx.doi.org/10.1021/ef2004943 | Energy Fuels 2011, 25, 2671–2678
Energy & Fuels
ARTICLE
Table 1. Compositions of the Synthesized DESs salt
HDB
molar ratio (salt:HDB)
abbreviation
methyl triphenyl phosphonium bromide
glycerol
1:2
DES1
methyl triphenyl phosphonium bromide
glycerol
1:3
DES2
methyl triphenyl phosphonium bromide
glycerol
1:4
DES3
methyl triphenyl phosphonium bromide
ethylene glycol
1:3
DES4
methyl triphenyl phosphonium bromide
ethylene glycol
1:4
DES5
methyl triphenyl phosphonium bromide
ethylene glycol
1:5
DES6
methyl triphenyl phosphonium bromide
triethylene glycol
1:3
DES7
methyl triphenyl phosphonium bromide methyl triphenyl phosphonium bromide
triethylene glycol triethylene glycol
1:4 1:5
DES8 DES9
formation when processing used cooking oils or other feeds with high free fatty acid (FFA) content may take place because of soap formation. In addition, the wet-washing method is not environmentally benign because of a massive discharge of wastewater. The dry-washing method replaces the water with a magnesium silicate powder or an ion-exchange resin to remove the impurities. Although both methods are dry, they are applied differently.8,11 Both processes have the advantage of being waterless and, thus, eliminating the problems aforementioned, but other than some fairly sketchy advertising material, little is really known about their performance. The use of ion-exchange resin as a purification method is promoted by two resin manufacturers, Rohm and Haas (BD10 dry) and Purolite (PD206). Although sold as ion-exchange materials, neither supplier advocates regeneration in normal use and they are really acting as adsorbents. It has not been possible, for reasons of commercial confidentially, to obtain any information on the chemical composition of either of the resins. The use of magnesium silicate powder is promoted in the U.K. by Hydrotechnik and in the U.S. by the Dallas Corporation; again, this is purely an adsorbent, and the spent material has to be disposed of to a landfill or other applications.8 However, using these methods, none of the products from these processes fulfills the limits specified in the EN standard.12 Another method for the purification of biodiesel is by membrane extraction. Using this method, the final production cost increases and throughput decreases.12,13 In recent years, ionic liquids (ILs) with many unusual favorable properties have attracted more attention for use in a diversity of applications. They have great potential for use as “green” solvents for industrial processes.14 However, it is still a challenge for the large-scale applications of ILs in industry, because of complicated synthetic processes and the expensive raw material chemicals.15 Hence, eutectic mixtures with so-called deep eutectic solvents (DESs) have been recognized as a low-cost alternative of ILs. The reason that they have been termed DESs is because, when the two components are mixed together in the correct ratio, a eutectic point can be seen. In fact, DESs formed from mixtures of organic halide salts with an organic compound, which is a hydrogen-bond donor (HBD), are able to form a hydrogen bond with the halide ion.16 DESs have properties comparable to ILs, especially their potential as tunable solvents that can be customized to a particular type of chemistry.17 DESs have several advantages over traditional ILs. They are prepared easily in high purity at low cost. In addition, they are nontoxic, have no reactivity with water, and most importantly are biodegradable.18 Because of these attractive advantages and their potential as alternative environment friendly solvents, DESs are being
studied comprehensively in many research areas as media for organic and inorganic reactions and separations. In addition, the advantages of the DESs have the best answer for the industrial requirements for large-scale applications. Abbott et al.19 have shown that glycerol-based DES is an efficient extraction medium for glycerol from biodiesel based on rapeseed and soybeans. Previously, we have shown that a low-cost quaternary ammonium-salt-glycerol-based DES can be successfully used as a solvent for extracting the byproduct glycerol from palm-oil-based biodiesel.20 Lately, our research group has reported new DESs synthesized by the reaction of phosphunium-based salts with different HBDs.21 The aim of this work is to use this class of DESs as solvents to get rid of free glycerol and reduce the total glycerol from palm-oil-based biodiesel, whereby the free glycerol and total glycerol specifications are according to ASTM D6751 standard specifications. The effect of DES type and composition on the removal of free glycerol, bound glycerol, and total glycerol based on removal efficiency is investigated in this work. To justify the use of DES as a probable solvent for removal of glycerol from biodiesel, it is critical to approach the feasible recovery of salt from the used DES after extraction. There are many methods to recover the salt from the used DES extract phase. Distillation is a possible separation method; however, it is an energy-intensive process. In this case, vacuum distillation may be used instead of conventional distillation to reduce the energy requirements. Another possible separation method is to recrystallize the salt, by either cooling or the addition of an anti-solvent. However, more work is needed in this area.19,20
2. EXPERIMENTAL SECTION 2.1. Materials. Two different types of palm oil (FFM Sdn Bhd and PPb group Bhd) were purchased at a local food store. Methanol (99.8%), n-heptane [gas chromatography (GC) grade], and potassium hydroxide (98.9%) were obtained from SigmaAldrich, Malaysia. The reference standards of biodiesel comprising monoolein, diolein, triolein, glycerol, butanetriol (internal standard 1), and tricaprin (internal standard 2) at concentrations specified in the ASTM and EN methods were purchased from Agilent Technologies, Malaysia. The derivatization agent, n-methyln-(trimethylsilyl)trifluoroacetamide (MSTFA) was also purchased from Agilent Technologies. Methyl triphenyl phosphunium bromide (C19H18PBr) as salt and glycerol (C3H8O3), ethylene glycol (C2H6O2), and triethylene glycol (C6H14O4) as HBDs were supplied by Merck Chemicals with high purity (>99%) and used for the synthesis of DESs without further purification. Moreover, the water mass fraction in these chemicals, as per the guideline of the manufacturer, is
