Conversion of crude and pure glycerol into derivatives: A feasibility evaluation
Introduction
Glycerol is a colorless, odorless, viscous, and hygroscopic liquid substance with a slightly sweet taste. It is the simplest trihydric alcohol that can be reacted as an alcohol but can remain stable under most conditions. Glycerol was first discovered by C.W. Scheele in 1779 through the saponification of olive oil with lead oxide. The name “glycerol” was first used by M.E. Chevreul in 1813; until the 1930s, glycerol was mainly produced via a fat-splitting process. Pasteur (1857) showed that glycerol, together with succinic acid, can be produced from sugars via a biochemical pathway called alcoholic fermentation. In World War I and II, glycerol was also produced through fermentation or carbohydrate hydrogenolysis [1], [2], [3], [4]. Glycerol has also been synthetically produced from petrochemical feedstock since 1943 (I.G. Farben); synthetically produced glycerol accounted for approximately 60% of the total market in 1965 [3]. However, the use of synthetic glycerol has lost popularity over renewable-derived glycerol because of cost-ineffective production [5]. Three common pathways have concurrently generated excess agriculture-based glycerol: hydrolysis, saponification, and biodiesel transesterification [6], [7].
The three hydrophilic alcoholic hydroxyl groups (−OH) explain the hygroscopicity of glycerol and its solubility in water. Glycerol is characterized by a density of 1.261 kg/l, a melting point of 18.2 °C, and a boiling point of 290 °C under pure anhydrous condition and normal atmospheric pressure [3], [5], [7], [8], [9]. In terms of ecological toxicity, the thermal decomposition of glycerol at 280–300 °C can produce poisonous acrolein. Even approximately 2 ppm of acrolein exhibits strong toxicity. The lower exposure limit of acrolein recommended by an ecological report is as low as 0.09 ppm [10]. Table 1 presents the relevant physicochemical properties and toxicity data of glycerol reported in different studies [1], [9], [11]. Glycerol is a vital solvent in pharmaceutical preparations and flavour extracts; thus, considerable studies have been conducted on the solubility of numerous solvents in glycerol [12]. Furthermore, the solubility of glycerol in various organic solvents is a useful reference of azeotropic distillation and analytical studies on glycerol derivatives. The solubility of glycerol in various organic solvents is summarized in Table 2.
The quantity and quality of glycerol generated from three major commercial productions are elucidated comprehensively in this review. This review reveals that excess crude glycerol (CG) is attributed not only to biodiesel production but also to alternate chemical routes employed in the oleochemical industry. Fig. 1 illustrates the outline constructed in this review. This study also presents further insights into the utilization of CG and pure glycerol (PG) as starting materials of different valuable glycerol derivatives. To the best of our knowledge, studies have rarely investigated the feasibility of the direct conversion of unpurified biodiesel-derived CG form into different valuable derivatives via chemical pathways. Thus, the effects of impurities on direct conversion from CG and PG in different routes are critically evaluated. Important approaches in the utilization of CG as feedstock are also proposed in this review.
Section snippets
Glycerol production routes and quality
Glycerol that has not undergone chemical treatment, purification, or separation is known as CG. The quality of CG strongly depends on processes and materials. Different impurities, such as monoglycerides, diglycerides, alkali metals, fatty esters (FAEs), soaps, salts, or diols are formed with their corresponding processing technologies. Thus, the common processes of glycerol production (hydrolysis, saponification, and biodiesel transesterification) and their operating conditions are presented.
Biodiesel as a major source of glycerol
Market statistics has revealed that the highest glycerol production capacity is attributed to biodiesel manufacturing, followed by fatty acid splitting, and fatty alcohol separation [23]. The types of sources involved in glycerol production and their estimated capacities from 2011 to 2014 are presented in Fig. 5. Considerably few synthetic and saponification glycerol sources were involved in glycerol supply. Glycerol production capacity enhanced simultaneously with biodiesel production
Strategies to transform CG and PG
The conventional glycerol commodity market is very narrow, and any increase in biodiesel production causes a sharp decrease by more than 50% of its current value [35]. As such, glycerol derivatives can potentially occupy a large segment in the current market. In terms of value comparison for CG and PG, the price of CG ranged from $0.04/kg to $0.33/kg from 2001 to 2011; by contrast, the price of PG varied from $0.70/kg to $1.70/kg in the same years [5], [9]. According to a recent publication,
Solvent for reaction
CG was used as a green solvent for the aldol condensation of n-valeraldehyde and carbon-carbon coupling reaction of iodobenzene and butyl acrylate [132]. The aldol condensation of n-valeraldehyde is conducted without an extra dose of fresh KOH because CG contains a trace amount of KOH residue; this reaction yields 27% conversion. Conversely, an extra replenishment of fresh KOH in the condensation reaction results in a slightly higher conversion of 41.1%. This increase is attributed to the
Feasibility of the direct conversion of CG to glycerol derivatives
A previous study [85] assessed the feasibility of the direct conversion of CG to glycerol derivatives. Ideally, impure CG can be transformed into any desired value-added products. However, two vital conditions should be considered: (i) purification of CG before the reaction occurs to obtain high yield conversion of the target components and (ii) direct conversion of CG fractions to derivatives, followed by the separation and purification of the target components. Table 15 shows the feasibility
Conclusion
This review provided relevant information on the following: (i) biodiesel production exhibited a higher capacity in glycerol production than splitting and saponification do; (ii) different CG compositions could be produced through splitting, saponification, and conventional biodiesel production; (iii) advances in MEs in the oleochemical industry proposed possible glycerol sources. Although biodiesel-derived CG produced the highest and complicated impurities, the direct preparation of impure
Acknowledgments
The authors are thankful to the High Impact Research Grant (HIR Grant no.: UM.C/625/1/HIR/MOHE/ENG59) from the Ministry of Education Malaysia. The SBUM scholarship is gratefully acknowledged.
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