## 1 Introduction  
Glycerol, also known as propane-1,2,3-triol, is a polyhydric alcohol with three hydrophilic hydroxyl groups, each attached to the three carbon atoms. The hydroxyl groups are mainly responsible for the properties and applications of glycerol [1, 2]. Glycerol is soluble in water and other polar solvents. It is a clear, colourless, odourless, viscous, non-toxic, and hygroscopic liquid with a high boiling point (290 \({}^{\circ}\)C) and low melting point (18 \({}^{\circ}\)C) [3, 4, 5]. Glycerol has versatile applications in medical, pharmaceutical, personal care, and food industries as sweetener, flavour, preservative, emulsifier, humectant, antioxidant, and additive. It is also used in the automotive and chemical industries as a plasticizer, lubricant, and cryoprotectant [6, 7, 8]. The distribution of some of these applications varies in different literature reports. The percentage distribution, as found in various literature articles, is shown in Figure 1[9]. It has been reported that glycerol is among the 12 most important bio-based chemicals worldwide [10], with the global market expected to reach USD 2.52 billion by 2020 [11]. Glycerol can be obtained via synthetic or natural means. The synthetic route is usually through propylene oxidation or chlorination, while the natural route involves the hydrolysis of vegetable oil and or the transesterification of fatty acids or vegetable oil as well as the fermentation of yeast [4; 12; 13]. The synthesis of glycerol via propylene is based on petroleum feedstock (Scheme 1) and is no longer popular due to the global shift to less carbon emission. The hydrolysis of oil, also known as saponification, involves the reaction of triglyceride with alkaline hydroxide (Scheme 2). It is an old method that dates back to 2800 BC and accounted for most of the commercial needs over the years [14; 15]. However, production through this method is no longer meeting the global need. The fermentation method is limited in industrial applications. The transesterification method involves the reaction of triglyceride (oil) with alcohol in the presence of a catalyst, which results in the production of biodiesel (fatty acid esters) and glycerol (Scheme 3). The transesterification method is now the most popular because of the large quantity of biodiesel produced. Biodiesel is considered to be an environmentally benign renewable energy that is biodegradable, non-toxic, and renewable with less obnoxious emissions [11; 16]. The transesterification method also produced glycerol as a by-product, which is known as crude glycerol due to its low-grade purity. The recent increase in biodiesel production has led to a proportional increase in the crude glycerol. It has been reported that for every 100 kg of biodiesel produced, 10 kg (10%) of glycerol is obtained as the by-product leading to surplus in the market [12; 17; 18]. Recent literature indicates that by the year 2020, the global production of glycerol will move up to 41.9 billion litres [19]. This number is expected to increase with the recent prediction that biodiesel will account for over 70% of the global transportation fuel by 2040 [20]. This forecast is supported by many deliberate policies put in place by different countries and organizations to encourage biodiesel production and its utilization, such as the implementation of mandatory biodiesel blending targets, tax exemptions, government support, investment subsidies, and research and development programs [20]. Figure 2 shows the progressive increase in biodiesel production for the last ten years [21]. The above policies have led to the diversification of biodiesel feedstocks depending on the country, as indicated in Table 1. The impact of excess glycerol produced from the transesterification process is already felt in the market [22]. Figure 1: Distribution of major applications of glycerol in various literature reports [9].