Archives of Waste Management and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 12 nr 1 (2010), p-57-66
Problem Zagospodarowania Frakcji Glicerynowej – Produktu Ubocznego Procesu Produkcji Biopaliw z Olejów Roślinnych
Sulewski M.,1 Urbaniak W.,1 Wasiak W,2
1Faculty of Chemical Technology and Engineering, University of Technology and Life
Sciences, Seminaryjna 3, 85-326 Bydgoszcz, Poland tel.+480523749050, fax+48523749327
e-mail: m.sulewski@wp.pl
e-mail: wlodzimierz.urbaniak@amu.edu.pl
2
Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland
tel. +48618291365, fax +48618291505 e-mail: wasiakw@amu.edu.pl
Streszczenie
Frakcja glicerynowa powstająca jako produkt uboczny w procesie produkcji biopaliw z olejów roślinnych nie składa się, jak to wynika z wielu popularnych opracowań, praktycznie wyłącznie z gliceryny, lecz jest złożoną mieszaniną wielu substancji, także toksycznych, a zawartość gliceryny z reguły nie przekracza 50 %. W pracy przedstawiono wyniki badań wpływu rodzaju surowca, ilości i rodzaju katalizatora oraz warunków prowadzenia procesu transestryfikacji na ilość i skład frakcji glicerynowej.
Abstract
On The Utilization Of The Glycerin Fraction – A Side Product Of Biofuel Production From Vegetable Oils
The glyceric fraction, forming as a side product in the process of production of biodiesel from plant oils, does not consist, how it results from many popular papers, of practically glycerine only but is a complex mixture of many substances, including toxic ones, and the glycerine content does not usually exceed 50%. In this paper the results of research on the influence of the kind of raw material, the quantity and kind of catalyst and the conditions of transesterification process on the quantity and composition of glyceric fraction.
1. Introduction
The pressing necessity of protection of the natural environment has aroused increasing interest in production of energy by the methods ensuring the least pollution. It has been considered vital that the contribution of renewable energy sources in energy production must increase. One of the solutions proposed for production of energy in combustion
engines is the addition of liquid fuels obtained from vegetable oils to traditional mineral fuels. In petrol engines it is possible to use dehydrated alcohols (methylene and ethylene) and ethers of these alcohols (MTBE, ETBE, TAME). The solutions to be applied for diesel engines have brought much more controversies. Much attention has been devoted to the use of fatty acid methyl esters (FAME) [1,2], the viscosity and cetane number of FAME are similar to those of diesel oil [3]. The use of esters as additions to diesel oil has been proved to bring no harm to the engines [4-6]. It has been evidenced that the addition of esters reduces the emission of toxic combustion products to the atmosphere [7,8]. In particular, the emission of sulfur oxides and polycyclic aromatic hydrocarbons, classified as carcinogenic, has been substantially reduced [9]. Production of FAME, their effect on the exploitation and use of combustion engines and the related problems of emission of pollutants have been rather intensely studied. In this study the subject of our interest is the side product of the biodiesel production that is the glycerin layer. In many popular books or articles the process of biodiesel production is described by the following reaction:
C H2 C H O O C H2 O COR COR COR O COR C H3 C H2 C H O O C H2 O H H H CH3OH KOH
+
+
3 3This reaction gives the biofuel and glycerin that can be used in the pharmaceutical or cosmetic industry. Unfortunately, the above reaction is an oversimplification and can be misleading. In fact the actual process can be described by the three-stage equilibrium process given below.
C H2 C H O O C H2 O COR COR COR O COR C H3 C H2 C H O O C H2 O COR COR H CH3OH KOH
+
+
C H2 C H O O C H2 O COR COR H O COR C H3 C H2 C H O O C H2 O COR H H CH3OH KOH+
+
C H2 C H O O C H2 O COR H H O COR C H3 C H2 C H O O C H2 O H H H CH3OH KOH+
+
The transesterification of triglycerides (TG) with methanol runs through a series of reversible intermediate reactions, leading to diglycerides, monoglycerides, glycerin and esters of fatty acids [10]. Thus, in the process of biodiesel production it is not glycerin (an important substrate for production of pharmaceutical, cosmetic and explosives) but a mixture of many substances whose main component is glycerin. The components of the glycerin fraction include: water, free fatty acids, methanol, catalyst and a number of other substances such as phospholipids, color mono- and di-derivatives of proteins, polysaccharides and nitrogen-organic compounds, whose contribution depends on the raw
Archives of Waste Management and Environmental Protection, vol. 12 issue 1 (2010) 59 product used. Therefore, prior to further processing of glycerin obtained it must be purified. The necessity of purification increases the cost of biofuel production and reduces the attractiveness of the glycerin obtained as a side product in the process of biodiesel production.
It is generally assumed that glycerin (1,2,3-propanotriol) is a valuable raw product used in many branches of industry and that the glycerin obtained as a side product in the process of biofuel production will be fully used. However, analysis of the use of glycerin in particular branches of industry (Fig. 1 [11]) reveals that to be attractive for industry glycerin must satisfy certain quality demands and the glycerin fraction obtained in the process of biodiesel production does not meet these demands.
cosmetics and drugs 28% ester derivatives 13% polyglycerols 12% direct sale 14% foods 8% resins 6% coatings 5% tobacco industry 3% others 10% paper industry 1%
Fig. 1. Demand for glycerin by different branches of industry (paper industry, cigarette industry, cellulose coating production, alkide resins production, food production, cosmetic industry, pharmaceutical industry, other uses).
According to the above diagram, the greatest amounts of glycerin are used by the pharmaceutical and cosmetic industries, but these industries have high demands on the quality of the products they use. In cosmetic industry glycerin is used as the softening ingredient, soothing ingredient, solvent for extracts and dehydrator, in ointments or creams glycerin is used as a means protecting against drying. Glycerin is also used in toothpastes, while a glycerin derivative – dihydroxyacetone – is an ingredient of sun-tan preparations. Glycerin is added to fodder and to fluid organic fertilizers. In pharmaceutical industry glycerin has a whole gamut of applications: ester of nitric acid with glycerin is known as nitroglycerine, a widely used medicament for cardiac muscle, glycerin is an ingredient of elixirs, alcohol extracts, syrups and ointments. It is used as a plasticizer for spray coating of tablets, pellets, granules, etc. Condensed glycerin gives polyglycerin and the esters and derivatives pf polyglycerin are used as emulators for production cosmetics and household chemistry products as technical fluids and components of lubricants [12]. In plastic industry
glycerin derivatives are used for production of trifunctional polymers giving polyurethane foams in the reaction with diizocyanates. In the reactions with dicarboxylic acids (e.g. phthalate acid) glycerin makes alkide resins used in coatings and paints. Raw glycerin can be transformed in biochemical process into 1,3-propanodiol – a component of polyurethanes. This alcohol is used as replacement of the toxic ethylene glycol in production of polyesters. Production of 1,3–propanodiol by the way of biochemical conversion increased from a few tones at the beginning of the 1990s to hundred thousand tones in 1997. Glycerin is also widely used in food industry as moistening agent, dye solvent, in taste and odorizing agents used in food products and as sweetener. Glycerin gives smoothing effect and in the processes of crystallization and freezing it delays crystallization of sugar. In deep freeze products it gives the sensation of warmth in contact with human body. Glycerin derivatives such as polyglycerols and their esters are important additives in production of margarine. Moisturizing properties of glycerin are used in cigarette industry; it is sprayed over the tobacco leaves to prevent their crumbling. Glycerin acetates are used as plasticizers in cigarette filters.
The above examples illustrate the wide range of application of pure glycerin, devoid of impurities. The glycerin fraction obtained in the process of biodiesel production either needs to be purified prior to further use or some other ecologically friendly ways of its utilization must be developed. It has been suggested that the fraction could be used as additive to fluid organic fertilizers (e.g. liquid manure) or as a surfactant for surface cleaning [13], however these propositions must be approached with caution.
Because of the variation in composition and the content of many chemical compounds (including toxic methanol), the processing of the glycerin fraction should be trusted to authorized firms to guarantee the quality and type of the product desired [14]. Purified glycerin from the fraction can be used as ecological component of food products and for production of explosives [15]. The cost of obtaining raw glycerin as a side product of transesterification of vegetable oils should be lower than that of obtaining it by fat separation methods. As has been established, the yield and quality of biodiesel depend on the technological process of its production, type and composition of the oil used and even – as has been found for rape – on the method of its fertilization [16]. Therefore, the same factors are expected to affect the quality of the glycerin fraction.
2. Methods of study
The process of transesterification was performed in a thermostated reactor equipped with a mechanical stirrer and thermometer. Rape oil was heated to the temperature of the reaction and was introduced into the reactor. Then a mixture of methanol and catalyst was slowly added. The content of the reactor was vigorously stirred at a constant rate and at a constant temperature in the reactor. When the reaction was complete, the contents of the reactor were released to the sedimentation tank to separate the phases. Samples for chromatographic analysis were prepared by dissolving a weighted portion of the glycerin fraction in 2-propanol. The glycerin and esters were identified on a gas chromatograph made by Hewlett Packard with MS detection. The phase separation was made in a chromatographic column HP-FFAP; the temperature program was: 1000C for 2 min – 100C/min – 2400C for 5 min. Quantitative analysis of the glycerin fraction was made in
Archives of Waste Management and Environmental Protection, vol. 12 issue 1 (2010) 61 GC-FID system applying the same procedure as for qualitative analysis. The content of glycerin was estimated on the basis of the calibration curve determined for analytically pure glycerin as a standard. The analytical problems related to chromatographic analysis of the products of transesterification of vegetation oils have been considered in earlier papers [17-19]. The NMR spectra on protons were taken on an NMR spectrometer OXFORD at 200 MHz, made by Varian, the solvent was CDCl3.
3. Discussion of results
The amount of the glycerin fraction formed depends on many factors such as the type and amount of catalyst, the quality of vegetable oil and parameters of the process of biodiesel production. The effect of the type and amount of the catalyst on the amount of glycerin fraction obtained was analyzed. The homogeneous catalysts were sodium and potassium hydroxides, while the heterogeneous catalyst was anhydrous potassium carbonate. The process was performed at 30 or 600C. The results are presented graphically in Figs 2 and 3.
0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 100,00 0 5 10 15 20 25 30 g/dm3 of oil th e a m o u n t o f g ly c e ri n e f ra c ti o n [ % v /v ] NaOH KOH K2CO3
Fig. 2. The amount of glycerin fraction formed at 30oC over catalysts of different types. When sodium hydroxide was used in an amount greater than 5 g/l of oil, the amount of biofuel produced significantly decreased at the expense of the increasing amount of the glycerin fraction containing great contributions of soap. The lowest amount of the glycerin fraction was obtained over the heterogeneous catalyst, but the biofuel obtained over this catalyst had a lower content of methyl esters, as shown by the data displayed in Fig. 4.
0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 100,00 0 5 10 15 20 25 30 35 40 g/dm3 of oil th e a m o n t o f g ly c e ri n e f ra c ti o n [ % v /v ] NaOH KOH K2CO3
Fig. 3. The amount of glycerin fraction formed at 60oC over catalysts of different types. Analysis of the results indicates that the optimum catalyst is potassium hydroxide. Its use permits obtaining biofuel of satisfactory parameters and the glycerin fraction in the volume of a few percent of the post-reaction mixture. The amount of the glycerin fraction depends also on the process temperature, with increasing temperature the amount of the glycerin fraction increases by a few percent. The plot in Fig. 5 also shows that the use of the waste oil (post-fry) as the raw product for biofuel production results in an increase in the amount of the glycerin fraction by 5-6% relative to that obtained when using the raw vegetable oil or refined oil.
In the optimum conditions the glycerin fraction makes from 12 to 15 % vol of the post-reaction mixture. Assuming that the addition of esters in the biofuel is only of 5%, production of each liter of biofuel is accompanied by formation of 7 – 8 cm3 of the glycerin fraction. It seems a small amount but in view of the large scale production, a single refill of a motor truck (about 600 l.) means production of 4 – 5 litres of glycerin fraction. This amount of the side product cannot be wasted and must be utilized. The content of free glycerin in the glycerin fractions obtained when using different types of rape oil varied in a wide range; from 33 do 61 %. The content of pure glycerin in the glycerin fraction has been found to vary depending on the type of oil used and technological conditions of the transesterification process (Fig. 6).
Archives of Waste Management and Environmental Protection, vol. 12 issue 1 (2010) 63 60 65 70 75 80 85 90 95 100 0 5 10 15 20 25 30 35 40 g/dm3 of oil F A M E c o n te n t (% w e ig h t) NaOH KOH K2CO3
Fig. 4. The content of methyl esters in % in the biofuel obtained in the process run over different catalysts at 60oC. 10 12 14 16 18 20 22 24 25 30 35 40 45 50 55 60 65 temperature (C) c o n te n t o f g ly c e ri n e f ra c ti o n i n p ro d u c ts m ix tu re [ % v o l. ] refined oil raw oil waste oil Wielom. (waste
Fig. 5. The amount of glycerin fraction formed at different temperatures of the process of biofuel production
0 10 20 30 40 50 60 70 35 40 45 50 55 60 65
reaction tem perature (C)
fr e e g ly c e ro l c o n te n t (% w e ig h t)
Fig. 6. The contents of free glycerin in the glycerin fractions obtained at different temperatures of the process of biofuel production.
As follows from analytical results, the glycerin fraction contains on average slightly over 50% wt of free glycerin, so the claim that the side product of biodiesel production is glycerin is far from the truth. The glycerin fraction does not meet the requirements of glycerin used for production of pharmaceuticals, cosmetics or food products. The presence of pollutants also seriously limits the use of raw glycerin fraction as a substitute of glycerin in the industrial processes. According to our results, the glycerin fraction separated in a few hours after the completion of the transesterification process contains considerable amounts of methyl esters of fatty acids that have not been transformed into the ester phase. The esters were identified on the basis of the H1 and C13 NMR spectra of the biofuel and the glycerin fraction. Besides the signals assigned to glycerin and methanol, the spectra revealed the signals typical of the fatty acid esters -C= (127 – 132 ppm), C-O-CH3 (51,4 ppm) and protons of hydrocarbon chains from fatty acids. The esters present in the glycerin fraction were identified on the basis of GC-MS as methyl ester of oleic acid (C18:1), linolic acid (C18:2), linolenic acid (C18:3), palmitinic acid (C18:0) and stearinic acid (C16:0). According to our results, to ensure exact separation of the fatty acid esters from the glycerin fraction the post-reaction mixture should remain in the separator for a minimum of 16 hours. Variation in the contents of fatty acid esters in the glycerin fraction was analyzed versus the process temperature and the type of oil used. The contents of different oils were linearly correlated, which means that the ratio of different (saturated and unsaturated) esters is constant, see Fig. 7. In contrast to the content of glycerol, the contents of esters in the glycerin fraction depend on temperature; in general, the contents of esters transformed into
Archives of Waste Management and Environmental Protection, vol. 12 issue 1 (2010) 65 the glycerin fraction increase with increasing temperature. Interestingly, the scatter of the contents of the esters obtained in the same temperature is the greatest at 500C.
Fig. 7. The scatter of the mean contents of methyl oleate in the glycerin fractions obtained from different oils versus the temperature of synthesis.
4. Summary
The contents of free glycerin in the glycerin fraction show considerable variations, from 32 to 61%, depending on the type of oil used and the conditions of transesterification process. The remaining part of the fraction is made of impurities. In the time of up to 20 hours after cessation of the reaction the separation of the esters from glycerin is incomplete, which means that some amount of the esters is dissolved in the glycerin fraction. This leads to reduced yield of the main product (biofuel) and to contamination of the glycerin fraction, increasing the difficulty of its utilization. Increased temperature of the process leads to the increased content of the esters in the glycerin fraction. Although the increased temperature of the process accelerates the reaction but in view of the increasing content of the esters in the glycerin fraction the effect of temperature should be carefully analyzed and to establish its optimum value. The great variation in the contents of the side product of biofuel production from vegetable oil indicates the need of detail analysis of the relations between the parameters of the initial oils, the conditions of the process and the quantity and quality of the glycerin fraction obtained. Taking into regard the necessity of increased use of
biofuels, the results of the analysis presented in this paper also point to the need of developing economical and environmentally friendly process of utilization of the glycerin fraction obtained in the process of biofuel production.
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