Renewable resources are now become a vital sector to be discovered and utilised since the world are facing the scarcity of the most important energy sources which has been used for decades of time. Biodiesel became the central of attention when this topic is discussed. Commonly manufactured through transesterification of animal fats and vegetable oil with alcohol in the present of particular catalyst, biodiesel is produced along with a side product called glycerol. Originally can be extracted from fats and oil, this colourless liquid has now turned to be an abundant source that flooding the market since it is produced as much as 1/10 (w/w) for every amount of biodiesel. From the established research works, by-product glycerol is proven to be a potential carbon sources for a number of microorganisms that contribute to the microbial production of many chemicals such as 1,3-propanediol, pyruvate, succinic acid, citric acid, alcohols and lipid. By-product glycerol also contributes into cleaner gasoline production. Through processes like oxidation, hydrothermal conversion, catalytic conversion, dehydration and reforming reaction, glyceric acid, lactic acid, triacetin, acrolein and hydrogen gas are produced respectively. This so called industrial wasted glycerol can also be exploited as a green solvent and helping the production of granular sludge. The disadvantages and disadvantages of all these applications are evaluated briefly in this dissertation.
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Having carbon chain in its molecular structure makes C3H8O3 or glycerol an organic compound with various usages. Also known as glycerine and propane-1,2,3-triol, glycerol is originated from animal fats or vegetable oil. In 1779, glycerol was first discovered through saponification of olive oil with lead oxide (Barbara Elvers, 1989). Physically appear as a liquid in room temperature, pure glycerol has no specific colour and smell.
Figure : Glycerol molecular structure. (source: Organics, 2010)
Besides saponification of oil and fats in soap manufacturing, glycerol can also be produced from microbial fermentation, synthetics production process and petrochemical feedstocks (Ampaitepin Singhabhandhu, 2010). However, there is other industrial process that produces glycerol, indirectly as a co-product or by-product.
Since decades ago, people start to find the replacement of non-renewable fossil fuel to be used in automobile and other industries. This is because of the scarce of fossil fuel which will one day come to an end, plus the pollution like carbon emission caused by this type of fuel. Biodiesel manufacturing becomes famous and more researches are conducted to optimise the production and usage of biodiesel. Biodiesel can be produced from animals fats and vegetable oil through several process to be mentioned; microemulsions, thermal cracking and transesterification. Most commonly used method is transesterification which then produced 10%(w/w) of glycerol as co-product (Fangrui Ma, 1999). Mathematically, it means that for every 100kg of biodiesel manufactured, 10kg of glycerol is obtained as a side product. The chemical reaction of transesterification method of producing biodiesel is illustrated in Fig. 2. For every one mole of glyceride reacted with 3 moles alcohol with help of particular catalyst, one mole of esters and glycerol are produced (Ampaitepin Singhabhandhu, 2010).
Figure : Production of glycerol as by-product from transesterification method in biodiesel production. (source: Ampaitepin Singhabhandhu, 2010)
The method use to produce the by-product glycerol will determine the physical and chemical properties of the by-product. Some biodiesel is having used cooking oil as their feedstock to reduce the cost (Fangrui Ma, 1999).The glycerol fraction produced in large installation has to undergo purification for further use. Unfortunately, chemical purification of the by-product in small agriculture biodiesel installations is unprofitable and also contaminated. The quality of the glycerol depends on the technology used. Crude glycerol as the by-product of biodiesel has some impurities that make it high toxic, and less valuable compared to the pure one. This by-product is composed of up to 60% glycerol and up to 27% each of water and methanol (Striugas N., 2008). The waste that is dispose can sometimes correspond to a threat to the natural environment. Researchers have found out that one way to counter this problem is to use nonpurified glycerol as a liquid fuels in combustion plants, producing heat energy (Striugas N., 2008).
Glycerol is isolated in biodiesel manufacturing using gravitational settling or centrifuging. While the demand of renewable energy resource is increasing by times, the production of biodiesel is growing all over the world. This is in turn mounting the production of glycerol. Glycerol production form biodiesel manufacturing is forecasted to increase by times. In certain places, glycerol manufacturing industries have to shut down because glycerol produced from biodiesel production can now fulfil the market demand of glycerol. In some countries in Europe, the production of glycerol has already exceeded the total consumption of this product (Ampaitepin Singhabhandhu, 2010). This will not also cause an abundant of glycerol in market but also lowered the price. Moreover, many biodiesel companies are now searching for any other possible alternatives to use up all the by-product glycerol.
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This critical literature survey will focus on the possible applications of glycerol as the by-product of biodiesel production suggested by literatures. This dissertation will also discuss the ups and downs of each suggestion and its practicality to be carried out in an industrial scale.
2.1. Glycerol in Biodiesel Plant
Glycerol cannot be blend back directly in biodiesel plant because of its composition that full of impurities. Further purification process in biodiesel plant will cost extra money. Considering this fact, (Ampaitepin Singhabhandhu, 2010) investigated the pros and cons of adding glycerol purification process in biodiesel plant. The glycerol purification process begins with neutralization of glycerol with acidic solution. Then, neutralized glycerol is purified by removing alcohol such as methanol, water and other substance to give a 80-88% of pure glycerol. Some purification process is including distillation which then gives as high as 99% of pure glycerol. Ampaitepin et al. proposed a cost-benefit and sensitivity analysis of incorporation the glycerol purification process in biodiesel plant considering 'business as usual' (BAU) and alternative scenarios for low-cost and high cost plant. They suggested that, with government subsidy, it is beneficial to incorporate glycerol recovery and purification process in biodiesel plant since it helps to produce a better grade of glycerol and increase its market value.
In my opinion, more money will be spent to get purest glycerol from biodiesel production, since further step like distillation has to be added which is not cheap. The process occurs in a very high temperature and this means that more heat energy is needed. Adding energy is equivalent to adding more money to the process. (J. C. Thompson, 2006) reported the characterization of non-purified crude glycerol obtained from biodiesel production of various feedstock such as vegetable oil, canola seed, mustard, rapeseed, soybean and even wasted cooking oil. Fatty acid profile, nutritional value, macro elements, viscosity, and heat combustion were tested and analysed and it is claimed that even without further purify the glycerol, it is still worthy and valuable by-product for various usage (J. C. Thompson, 2006).
2.2. Glycerol as a source of life for other organisms
Glycerol if we referred to any definition, it is a must to have 'sweet-tasting liquid' as one of the properties to describe this simple compound. In fact, the name glycerol is said to be taken from Greek word glucose. Plus having carbon chain, glycerol has high chances to become one of the potential carbon sources for the other organisms. As a non-polar molecule, there is not difficult for glycerol to be absorbed into cytoplasmic membrane of organisms. As reviewed in (Gervasio Paulo da Silva, 2009), glycerol molecule can be a source of nutrient of living things. In Escherichia coli glycerol can be taken via facilitated diffusion across inner membrane of the microorganisms (Voegele RD, 1993). While in other microorganism Saccharomyces cerevisiae, passive diffusion of glycerol into the membrane takes place (Wang ZX, 2001). There are many established literatures proving that several microorganisms can grow on glycerol. Citrobacter freundii, Yarrowia lipolytica, Cryptococcus curvatus, Enterobacter agglomerans, Lactobacillus reuteri are just some to be named of many other microorganisms that can get benefit from glycerol. All these microorganisms have their own function whether in glycerol fermentation to produce other products or for the growth of the microorganisms itself to exported for other usage in microbiological technology.
2.3. Glycerol to Gases
Apart from biodiesel, hydrogen is one of the most popular renewable sources of energy. For example, fuel cell used in automotive industries that employs hydrogen as its source of electron to energize the automobile. One of the possible applications of the abundant by-product glycerol is production of hydrogen. Compare to pure glycerol, by-product glycerol from biodiesel could produce lower hydrogen and methanol gases due to high salt composition. Besides being a renewable source, glycerol can also be used in biofuel cell. Research done by (Robert L. Arechederra, 2007) proved that glycerol has an advantage compare to methanol and ethanol, since it would not swell the Nafion membrane use in fuel cell. This will lengthen the life of the proton exchange membrane. Conversion of glycerol to hydrogen could be done via glycerol reforming process. Taking place in one or more reactor called reformer, reforming reaction is a process that occur in high temperature condition to form a reaction that will produce desired gases. (Prakash D. Vaidya, 2009) has reviewed and compared a few routes of getting hydrogen from this by-product glycerol. The general equation of getting hydrogen from glycerol reforming process according to (Aurelien M.D. Douette, 2007) is:
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C3H8O3 + x H2O + y O2 a CO2 + b CO + c H2O + d H2 + e CH4 (eq. 1)
Methane and carbon monoxide are two undesired product produced glycerol reforming. Using aqueous instead of steam reforming could reduce production of CO but reduce selectivity of H2 plus requirement of higher pressure compare to steam reforming. Side reaction that produce methane is likely favoured in high pressure condition. Also reviewed in (Prakash D. Vaidya, 2009) is the catalytic side of glycerol reforming reaction. Ni, Pt and Ru are proven by many literatures as catalysts that favoured the production of hydrogen gas. However there are still limitations in using these metals considering catalyst deactivation when reaching certain point. But still further experiment could be done to improve this situation.
(Esteban A. Sanchez, 2010) has done a research on catalytic production of hydrogen from by-product glycerol from biodiesel using Ni/Al2O3 catalyst on steam reforming. γ-Al2O3 and nickel nitrate hexahydrate are used to prepare the Ni/Al2O3 catalyst for the process. Temperature-programmed reduction(TPR), Fourier-Transform infrared spectroscopy(FTIR), and ammonia temperature-programmed desorption (NH3-TPD) are used to characterized the surface species reducibility, size and acidity strength. With molar ratio of water:glycerol 16:1, it is reported in his result that the highest selectivity of hydrogen is 99.7% at 650°C, after 4 hours in process. However, as high as 80% methane is produced as by-product and few more hydrocarbon such as ethane and propane. As reported by (Adhikari S, 2007) over other alumina supported metal catalysts, Ni/Al2O3 gives the better results in terms effective conversion and product (hydrogen) selectivity even when using lower molar ratio than the above study.
Glycerol from biodiesel production contains at most 30% methanol. Considering this condition, (Suthida Authayanun, 2010) conducted a research on thermodynamic analysis of autothermal reforming of crude glycerol. This research proved that the production of hydrogen will depends on the percentage of methanol present in the glycerol. More methanol means more methane will be produce as by-product and less hydrogen obtained. However, too little methanol to glycerol ratio will favour carbon monoxide production. This can be reduced by adding enough oxygen in the process. Apart from lessen the production of CO as it will be converted to CO2, oxygen is necessary to maintain the energy needed for the process. However, to much oxygen will reduce the hydrogen selectivity.
Besides reforming reaction, there is other method to synthesis hydrogen from glycerol known as microbial conversion. It is discovered by (Takeshi Ito, 2005) that by-product glycerol that usually contains impurities can be converted into hydrogen and ethanol gases by employing Enterobacter aerogenes HU-101 and the highest H2 obtained is reported to be 63 m mol/l/h. To simply describe the process, diluted glycerol and E. aerogenes are cultivated in a sterilized medium prepared from various portion of chemicals and cultured continuously in a packed bed reactor with self-immobilized cells. The reaction occurred then produces hydrogen gas. The formation of hydrogen gas from glycerol can be described through the following equation (Shinsuke Sakai, 2007):
C3H5(OH)3 H2 + CO2 + C2H5OH (eq.2)
Using Enterobacter aerogenes NBRC 12010 and electrochemical reactor instead of packed bed reactor, (Shinsuke Sakai, 2007) take a further step in improving this discovery by using thionine, a crystalline thiazine base, as the electron mediator. His new method is proven can increase the consumption of the glycerol used. However, no commercial or industrial scale work has been done for this experiment and further research and development are required to improve the process.
2.4 Glycerol to other commercialised chemicals
Pure glycerol can be directly used in various sectors as mentioned in the introduction. By-product is usually associated as undesired product. This is because, unlike the pure compound, a by-product from a certain process usually contaminated with various substances and impurities. Being downgraded as less valuable chemical compound, by-product glycerol need to go through certain processes before it can enters the market with other name instead of glycerol. In other to fully utilised crude glycerol which its production keep increasing with the increasing of biodiesel demand, scientist and chemist started to search of new applications of glycerol by transforming it to other chemicals products that can possibly adds its commercial values. Many chemicals that usually extracted or manufactured from natural resources can now be produced synthetically from other product. Glycerol is now become one of the most interested renewable source or substrate of other demanded chemicals.
Figure : Possible routes of getting other chemical products from glycerol. (source: Gervasio Paulo da Silva, 2009)
Also known as propylene glycol, 1,2-propanediol is commonly found in toiletries, cosmetics, baby products, paint, tyre sealent, adhesive, wall paper, antifreeze,stain remover, coolant and even fabric softener. This valuable and multi use chemical can be one of the final product of by-product glycerol. The process is known as hydrogenolysis. There are many research works about this process. Recently, (Zhenle Yuan, 2010) with reference of so many published work has conducted a research on catalytic hydrogenolysis of gylcerol derived from biodiesel production using Cu/MgO catalysts. From (Alhanash A., 2008; Chaminand J., 2004; Furikado I., 2007; Maris E.P., 2007), hydrogenolysis of glycerol can take place with the present of either acidic or alkaline condition. The reaction pathways that happen in both condition is reprented by the following schemes:
C3H8O3 - H2O C3H6O2 +H2 C3H8O2 (eq.3)
C3H8O3 - H2 C3H6O3 -H2O C3H4O2 +2H2 C3H8O2 (eq. 4)
In equation 3, glycerol is first dehydrogenate and then hydrogenate to form 1,2-PDO. A little bit different dehydrogenation and dehydrogenation take place in the equation 4, but same product is produced. Even though the second scheme have to go through three steps to produce 1,2-PDO it is found that the presence of alkali help to improve the catalyst performance. For the experiment in (Zhenle Yuan, 2010), catalysts Cu/MgO were prepared by two method. First, Na2CO3 was added in amixed solution of CuCl2.2H2O and MgCl2.6H2O and the resulted solution was the precipitated yielding the catalysts required. This method is called coprecipitation. While the second method is impregnation where MgO was impregnated in Cu(NO3)2.3H2O and dried to yield Cu/MgO catalysts. The catalyst were distinguished based on different loading of copper. Aqueous solution of 75%wt glycerol was used in this experiment. The catalysts which already reduced with H2 were added together with the glycerol into the reactor which is a stainless steel autoclave. The reaction took place in temperature 180°C and pressure of 3.0 MPa with presence of H2. When the reaction was done, the resulted product was cooled and analysed. As the result, Cu-15/MgO catalyst (15 referred to percentage of copper loading) prepared by coprecipitatio showed the best performance with 72.0% conversion of glycerol and 97.6 selectivity of 1,2-PDO. It is also found that small addition alkali NaOH contribute in larger amount of glycerol consumption in this experiment. It is also found that the smaller the particle of the Cu and MgO in the catalyst, the better the performance.
C3H8O3 (glycerol) can be converted into C3H8O2 (1,3-Propanediol or PDO) through glycerol fermentation. Having same chemical formula but different structure makes PDO different from 1,2-propanediol in term of usage. 1,3-PDO is an important organic compound in various chemical industries. There are three known ways of producing 1,3-PDO from by-product glycerol (Arno Behr, 2008); heterogeneous catalysts, homogeneous catalysts and biocatalysts. Using homogeneous rather than heterogeneous catalyst is said can yield higher product selectivity.
Biocatalyst process is basically a microbial fermentation of glycerol to PDO. As mentioned in section 2.2., several microorganisms can live by having glycerol as their source of carbon. Of all, Klebsiella, Enterobacter, Clostridium, Citrobacter, Lactobacillus and Bacillus are manage to help out in converting glycerol to PDO (Thomas Willke, 2008) (S. Vollenweider, 2004) (D.C. Cameron, 1998). In 1987, Cecil W. Forseberg reported that as high as 61% glycerol was successfully converted into PDO using Clostridium species. By referring to figure 3, glycerol undergoes two stages before yielding 1,3-PDO. Enzyme glycerol dehydratase catalyzes a reverse reaction that transforming glycerol into 3-Hydroxypropionaldehyde and water. Then, NADH2 or nicotinamide adenine dinucleotide is consumed to reduce 3-Hydroxypropionaldehyde to form 1,3-Propanediol (Mario Pagliaro, 2007).
Pyruvate is another useful chemical compound that can be synthesised from by-product glycerol. Apart from supplying energy to human body, pyruvate will contribute to a helathy body by helping reducing fat and cholestrol in blood. Referring to figure 3, there are four steps in getting pyruvate from glycerol by biological fermentation. Enzyme glycerol dehydrogenase, NAD+ , ADP and ATP are necessary for this conversion to happen. Pyruvate later can be reduced to other valuable chemical such as acetate, 2,3-Butanediol, ethanol, butanol, butyrate and propionate as the end products as illustrated in Appendix A.
2.4.4 Citric acid
Pharmeceuticals, food industry, detergents and cosmetics are just a few sectors to be named that widely use citric acid in many of their products. Besides can be extracted from natural resource like citrus fruits, citric acid can now be synthesised from glycerol with help of certain microorganisms or bacterias. This application can at least gve further hope in utilising the abundant of by-product glycerol from biodiesel production since citric acid will never find an end in biochemical process industry. (Rymowicz W, 2006; Holz M, 2009; Imandi SB, 2009; Papanikolaou S, 2002; Waldemar Rymowicz, 2010) have done a few researchs on how Yarrowia lipolytica, a yeast from eukaryotes group, can be used to produce citric acid from glycerol. To improve the limitations in previous studies, (Waldemar Rymowicz, 2010) has used Yarrowia lipolytica from mutant A-101-1.22 and studies the effect of different reactors; batch, repeated batch and recycle regimes, in cultivating the yeast to produce citric acid from glycerol. The crude glycerol used in this experiment containing only 79% glycerol and the rest are salt, methanol, metals, heavy metals, organic materials and water. Same medium growth containing the by-product glycerol, the yeast, bacto peptone and tap water were prepared for every reactor. After days of cultivation, repeated batch culture gave the most successful result by yielding the highest citric acid production which is 124.2 g l-1 citric acid with production rate of 0.85 g l-1 h-1.
2.4.5 Glyceric acid, sodium glycerate and tartronic acid
As mentioned in introduction section, glycerol has three hydroxyl group. The oxydation of each group will yield different product. Glyceric acid, sodium glycerade and tartronic acid are produced by aerobic oxidation of the first or primary hydroxyl group of glycerol. 92% selectivity of sodium glycerate can be produced form glycerol over Carbon-supported Au catalyst (S. Carrettin, 2004). While glyceric acid 70% can be obtained from glycerol using Pt/Cu catalyst at temperature of 50°C (R. Garcia, 1995). 40% yield of tartronic acid is produced by oxidation of glycerol over Pt supported CeO2 catalyst (Gallezot, 1995). For oxydative dehydrogenation of glycerol supported by metal-catalyst, certain by-product is produced along the reaction because of the low stability of the catalyst. To minimize this problem, a systematic control has to be carried out during the reaction (Mario Pagliaro, 2007).
2.4.6. Lactic acid
Derived from Greek word 'lactose' which is protein from milk, lactic acid is originally extracted from milk. It is widely use in food industry, lactic acid is actually can be produced from by-product glycerol. This has been done in (A.Yuksel, 2009; Yuksel A., 2010; Shen Z., 2009; Kishida H., 2005). However, small yield of lactic acid became a setback that have to be improved in the next research experiment. (Camilo A. RamÄ±rez-Lopez, 2010) has developed a further on the process of producing lactic acid from by-product glycerol by alkaline hydrothermal conversion. As suggested by (Kishida H., 2005), using sodiumhydroxide as the alkaline in this experiment, the reaction that takes place in the reactor is represented by the following equation:
C3H8O3 + NaOH C3H5O3Na + H2 + H2O (eq. 5)
A mixture of glycerol and NaOH in aquoeous solution is prepared in a batch reactor which then will be heated and stirred continuously. The effect of different glycerol to NaOH molar ratio,glycerol concentration a range of reaction time and the reaction temperature to the lactic acid yield were investigated in (Camilo A. RamÄ±rez-Lopez, 2010). The products of the reaction were analysed using high performance liquid chromatoghraphy (HPLC). The experiment was first carried out using a 99.9% pure glycerol, and as the result, 89.9% acid lactic was produced from a 3M concentration of pure glycerol. Crude glycerol from biodiesel manufacturing was then used in the experiment. A 2.5M concentration of crude glycerol (85%) is used and the same operating condition as previous experiment, 280°C temperature and 90 min reaction time, are mantained. As the result, 85.5% yield of product with 192g/L concentration is is produced. Even though in term of productivity, the experiment shows a quite promising result if the process is carried out in industrial scale, there are still a few predicaments that have to be solved. First is the corrosion of the reactor in the long term usage since the reaction is concducted in a high temperature and even stainless steel is exposed to the risk of corrosion such condition. Material like glass linen reactor is very expensive compare to stainless steel. The second problem is the separation of lactic acid. Besides lactic acid, there are a few by-product produced along the reaction. They are formic acid, acetic acid and acrylic acid. According to (Camilo A. RamÄ±rez-Lopez, 2010) who conducted this experiment, further research has to be done sort out this matter.
2.4.7. Triacetin/ glycerol triacetate
Triacetin exist as an colourless oily liquid with fatty smell, bitter taste and combustible. It is naturally exist in cod oil, and any natural fats like butter (Grant, 1972). Triacetin is use as plasticizer in cigarette filter and chewing gum, in cosmetics as solvent and perfumes, in food industry as additive and humectants (Triacetin, 2008). There are many research conducted in studying the process of synthesising triacetin from glycerol using catalyst such as SnCl4.5H2O/C, phosphotungstic acid, aminosulfic acid, sulfonic acid, and solid sulfated Fe2O3/TiO2 as proposed respectively in (Zhang, 1999), (Zhang M., 2001), (Hou J., 1998), (Melero J.A., 2007) and (Dong Z., 2003). However, there is a little different approach taken by Jordi et.al 2009 in investigating the process of producing triacetin from glycerol. Using kinetic modelling to determine the best way to carry out the process in a large scale, the experiment was carried out without catalyst. Reactive distillation was found as the most practical way to carry out the reaction. The process is simulated using AspenPlus and 99% pure components is selected for the simulation. Basically, two feed stream consist of glycerol and acetic acid are fed into a distillation column producing two product stream; water in distillate product stream and purely triacetin in bottom product stream. Even though mono and diacetin are also produced in this reaction, these two co-product are then eventually converted into triacetin as the water is continuously being removed in distillate stream (Jordi Bonet, 2009). This process can possibly give non only high purity but also high yield of triacetin. However, further research have to be conducted to make this simulation a reality.
Acrolein, prop-2-enal or acrylic aldehyde is another valuable chemical that can be synthesised from by-product glycerol. It is used in control plants, algae, molluscs, fungi, rodents and microorganisms. It is also use as other chemical intermediates, military gas poison, manufacturing of colloidal metals and leather tanning (Medical Management Guidelines for Acrolein, 2008). Acrolein usually obtained through oxidation of propylene. Since the natural resource of acrolein is obtained from fossil fuel, it is beneficial to provide a new source of this chemical based on renewable compound such as glycerol. The process of getting acrolein via dehydration of glycerol has been investigated in many literatures. (W. Bühler, 2002) reported on quite non-feasible economical process of getting acrolein from by-product glycerol due to small selectivity and glycerol conversion while (S. Ramayya, 1987) gave a little hope on this process by reporting that the addition of acid during the dehydration process can improved the production of acrolein. A quite successful experimental procedure has been conducted and reported in (L. Ott, 2006) about the dehydration of by-product glycerol in sub- and supercritical water with help of zinc sulphate. The experiment was conducted in a plug flow reactor with reaction temperature of 300-390°C and pressure 25-34MPa for one minute or less reaction time. As the result, as high as 75% selectivity of acrolein is obtained at the temperature of 360°C and 25MPa pressure, and the maximum zinc sulphate required for the reaction is 470 ppm. It is also claimed in (L. Ott, 2006) that the use of subcritical rather than pure water can enhance the acrolein production, plus it is economically feasible based on the rough cost analysis. It is also proposed in the same paper, the possible industrial plant process for this method which can be referred in appendix B. In another literature (Ágnes Zsigmond, 2010) proposed a different condition of glycerol dehydration which is claimed to be an environmental benign process. A heteropolyacid (HPA) supported catalysts were prepared using Al2O3, phosphotungstic acid hydrate (PTA) and methanol. Among other catalysts, H3[P(W2O10)4]/Al2O3 gave the best result with maximum acrolein selectivity which is 92.8% and a 100% conversion of glycerol. It is also recommended in (Ágnes Zsigmond, 2010) that the heterogeneous catalyst used can be recycled for the process. This experiment is said to be environmental benign process because it is carried out in a mild condition, converting all of the reactant and the overall process only have material loss of 10%.
2.5. Lipid Production
It is always mention in any article regarding biodiesel that it is produce from animal fats and vegetable oils. We might forget that microorganisms such as bacteria, fungi, and yeast can also be classified as animal or plant which can produce their own oil. The need small amount of labour, short cycle of life, they are easy to scale up and not really affected by place, climate and environmental season (Li Q. D. W., 2008; Li Q. W. M., 1997) give benefits to microbial compare other vegetable or cooking oil that make them potential and promising source of oil to be use in biodiesel production. Moreover, the use of microbial oils to produce biodiesel will not clash with the supply of oil for food and cooking as what happen to vegetable oil. Seraphim Papanikolaou et al. investigated the production of lipid on by-product glycerol using microorganism Yarrowia lipolytica while Yanna Liang et al. experimented the potential of oleaginous yeast Cryptococcus curvatus to produce lipid from industrial waste glycerol. In (Seraphim Papanikolaou G. A., 2002), growth medium containing glycerol and Yarrowia lipolytica are cultivated in batch cultures prepared in conical flasks and also continuous cultures in bioreactor. Different concentrations of glycerol were tested and as the result, single-stage continuous culture gave the higher lipid yield compare to flasks culture. The production of lipid is 43% (w/w) with 1.2 g l-1 h-1 production rate. The amount of citric acid produced as co-product also considerably low. While in (Yanna Liang, 2010), batch and fed-batch cultures were tested for the fermentation of glycerol using Cryptococcus curvatus. For batch stage, glycerol samples were autoclaved, added with C. curvatus and supplied with nitrogen. While for fed-batch culture, a 2-1 fermentor with stirrer was used, glycerol and nitrogen were added at several time point during the cultivation. The glycerol conversion and biomass density produced were analysed during 12 days of cultivation. The result obtained suggested that two stage fed-batch cultures are better than the batch stage in term of glycerol consumption and lipid production with maximum 52% of lipid content (Yanna Liang, 2010). Apart from contributing to the utilization of the abundant by-product glycerol, this lipid production also provide a promising source of biodiesel industry.
2.6. Cleaner gasoline production
Another potential process for oxygenate glycerol is etherification. Regarding this reaction, isobutylene (IB) reacts with glycerol in the presence of acid catalyst to develop a mixture of mono-, di-, and tri-tert-butyl ethers of glycerol (called MTBG, DTBG and TTBG, respectively) (R.S. Karinen, 2006; K. Klepacova D. M., 2007; K. Klepacova D. M., 2005). These ethers can be used to reduce the emissions, mainly particulate matters, carbon oxide and carbonyl compounds in exhaust gases (H.S. Kesling, 1994). Notably, glycerol ether oxygenates can also reduce the cloud point of diesel fuel when blended with biodiesel (Noureddini, 2000). It is also reported that Karinen et al. etherified glycerol with IB in liquid phase catalyzed by an acidic ion exchange resin catalyst, Amberlyst 35. It was founded that DTBG and TTBG are the best due to their good combination properties with diesel fuel. Melero et al. examine the etherification of glycerol with IB using several sulfonic acid modified mesostructured silicas; i.e. prpylsulfonic and arenesulfonic acid functionalized silica (Pr- and Ar-SBA-15) and reveals theat Ar-SBA-15 shows excellent catalytic behavior in the etherification of glycerol with IB (J.A. Melero, 2008). Moreover, Klepacova et al. investigated etherification of glycerol with IB and TBA over Amberlyst type, zeolites H-Y and H-β. They stated that zeolites H-Y and H-β provided lower selectivity than ion-exchange resins; in addition, water structure in the case of using TBA as reagent has an inhibition outcome on glycerol tertbutylation (K. Klepacova D. M., 2005). Even though ethers of glycerol are largely studied as diesel fuel additives, It is stated in (R.S. Karinen, 2006) that they also offered high octane numbers. It is reported that the mixture ethers from IB and glycerol resulted in octane numbers to be 112-128 of BRON and 91-99 of BMON, offering alternatives for conventional octane enhancer. However, testing and investigation are limited with only two etherification agents; i.e. IB and TBA for etherification with glycerol.
The entire fluidized catalytic cracking (FCC) gasoline is etherified with glycerol mainly with methanol (E. Pescarollo, 1993) and ethanol (W. Kiatkittipong, 2008) using three commercial catalyst; i.e. Amberlyst 16, Amberlyst 15 and β-zeolite. It is proven that Research octane number (RON) of FCC gasoline etherified with glycerol is higher than that of original FCC gasoline. Furthermore, it also gave a reduction in blending Reid vapor pressure implying that it gives an advantage especially for near tropical countries or in the summer period.
In can be conclude that the etherification of FCC gasoline with glycerol in the presence of acid catalysts was proven to be a potential process for gasoline quality improvement and glycerol can also be utilize as fuel extender.
2.7. Green Solvent
Solvent is very important for any chemical reaction that occurs in a liquid phase. It plays role as a contact medium between the reactants, place for the intermediate and transition states and helps to stabilise or destabilise them, and also it is depends on the solvent for how the experimental procedure is going to work (Kerton, 2009). While for a solvent to be classified as a green or organic solvent, it has to be non-flammable, none or low in toxic, non-volatile, non-mutagenic, easily available, cheap and can be recycle (C. Capello, 2007). All these criteria are to ensure that this green solvent is safe and environmental friendly. Glycerol has become one of the potential candidates to fulfil all these criteria. The by-product glycerol is renewable, abundant in existence and thus it is cheap in price that makes glycerol a special interest for an organic solvent. (Yanlong Gu, 2010) reported the potentiality of by-product glycerol as a sustainable green solvent in term advantages, disadvantages and how it can be utilized in chemical process. As mentioned in the introduction section, besides of having no colour or smell and exist as a liquid in room temperature, glycerol is a polar solvent which is totally miscible in water and partly miscible in certain organic compound. The pro behind this fact is, glycerol can be a good medium of inorganic, polar solutes, and compound that hardly dissolve in water. This property will help the removal of the product by just liquid phase separation. High boiling point (290°C) and non-volatile at atmospheric pressure is another advantages for glycerol since it can be used as a solvent for reactions that require high temperature such as separation by distillation. As a non-flammable and low toxicity, glycerol is proven safe to widely use as a solvent. There also a few down side of glycerol to be a universal solvent such as high viscosity, reactive hydroxyl group and coordinating properties that might be a problem for a particular reaction. However, glycerol still is a promising candidate for a green solvent based on many established researches. (A. Wolfson, 2007) reported that many organic synthesis are going very successful with high yield by using glycerol as the solvent. While (Yanlong Gu J. B., 2008) reported that glycerol has special advantages over other popular solvent as it need no catalyst to enhance the reaction for a certain reaction, it helps the extraction of the liquid product and it is not volatile and remain as a solvent. Moreover, glycerol can improve the selectivity of the reaction, a good solvent for recycle and catalysts based reaction, and also suitable for separation reaction and materials chemistry (Yanlong Gu F. J., 2010).
2.8. Granular sludge
In recent years, researches have been looking into the possibility of granular sludge production using this by-product as an organic carbon source. The biogranulation is in the form of self-immobilization of microorganisms. In this method cell-to cell interface occur resulting in the formation of dense microbial consortia called granules. The appropriate selection of the operating conditions e.g., short sedimentation time and a high volumetric exchange rate for the waste treatment process will encourage the production of the granular biomass. The major difference with activated sludge is characterized by a better settling ability, longer biomass usual time and a higher biomass concentration. Glucose, acetate, phenol, dairy effluents or wastewater are common range of substrate use in cultivating granules in bubble airlift or bubble column reactor with normal height to diameter ratio of 10 (Liu, 2004).
Furthermore, from an engineering angle, setting up the technologies based on granular sludge requires smaller surface area and clarifier dimensions and are more economical in contrast with technologies based on activated sludge. The main drawback of granular sludge technology is the long time (usually a few months) needed for the cultivation. However, this time can be reduced by seeding the reactor with existing granular sludge cultivated on organic carbon sources such as glycerol fraction.
Impurities contained in glycerol derived from biodiesel production cause this by-product to be classified a potentially toxic to living certain microorganisms. Aerobic degradation of by-product glycerol using activated sludge was inhibited and cannot be carried out. However by granular sludge, this activity is quite promising and efficient since microorganisms in the granules are trapped in extracellular polymetric substance (EPS). These microorganisms can survive longer in hostile condition (Agieszka Cydzik-Kwiatkowska, 2010).
The morphology of granules depends strongly on the carbon source introduced to the reactor. The by-product is a complex combination of substrates with glycerol as the main compound. Granules with fluffy surface are obtained in granules cultivation when the reactor is fed with this substrate. Granules with a similar fluffy surface were observed by Tay et al. (Tay J.H., 2001) and Beun et al. (Beun J. J., 1999) in reactors fed with glucose and ethanol as the main source of organic carbon. The development of filamentous microorganisms in granules deteriotes the settling properties of biomass, intensify the effluent total suspended solid (TSS), and finally results in biomass washout and reduce the TSS in reactor (Liu Y., 2006). The highest granular sludge production for this experiment is 1.1 ± 0.27 g COD/g TTS per cycle. It is proven that by-product glycerol can be used for granular sludge production since the granules was cultivated efficiently in the reactors (Agieszka Cydzik-Kwiatkowska, 2010).
The idealistic and the realistic of suggested applications in large scale
Despite of hundreds applications that can be thought and mentioned about glycerol, not all are applicable in a large scale technology to consume the surplus of glycerol manufactured from biodiesel industry. Most of the researches are still in experimental procedure. Taking into account the cost, risks, and further research that required for the dreams comes true, more and more money are needed. Being a multiuse, environmental friendly and renewable resource, by-product glycerol from biodiesel manufacturing still a subsidiaries project of government in most of the countries all around the world even in developed countries like Japan (Ampaitepin Singhabhandhu, 2010). However, there is no end in finding the solutions of this matter. As dependency on biodiesel become more crucial, the urgency of getting crude glycerol to be used will give chance for the applications to be conducted in industrial scale.
There are so many applications of glycerol which already available in various sectors or yet to be discovered. However not every application is compatible with glycerol that produced as a by-product from biodiesel manufacturing. Certain applications need as high as 99% purity of glycerol which is impossible to be obtained from a by-product from a process like transesterification especially if the feedstock for the biodiesel production is taken from used oils that already has been contaminated with impurities.
In spite of this fact, there are so many works that have been done by researches and scientist to find out the most promising ways to use up all the surplus of glycerol derived from biodiesel manufacturing. The effort made by those researchers are in fact give a promising path in the future of by-product glycerol when more new possible applications were found which are not only compatible with the properties of a derived product like glycerol but somehow became a solution for other problems.
From common chemical processes like oxidation, dehydration, hydrogenation, to more complex route like microbial fermentation and reactive distillation, glycerol can be converted to so many new commercialised products. Many type of alcohol, acid, lipid and even synthetic gases like hydrogen can be manufactured from glycerine or glycerol. By-product glycerol is proven to be one a of the most wanted renewable substrate for chemicals if further research and development are successfully conducted in bringing the utilization of by-product glycerol up to industrial scale. Furthermore, glycerol also can be employed as a solvent for various chemical reactions.
However, there are still more to be improved in term of technological advance to facilitate the utilization of by-product glycerol from biodiesel production.