A new process for catalyst-free production of biodiesel using supercritical methyl acetate
Introduction
Biodiesel fuel (BDF), which is an alternative of the diesel derived from oils and fats, has been recently receiving an intensive attention, and in the currently used alkaline catalyst method, triglycerides (TG) are trans-esterified in the presence of an alkaline catalyst with methanol and converted to fatty acid methyl esters (FAME). Therefore, what is referred to as BDF is in fact a mixture of various fatty acid methyl esters.
Although the alkaline catalyst method has the benefit of using moderate reaction conditions, several aqueous washings are needed to remove the catalyst after the reaction. Moreover, the oils and fats used may contain water and free fatty acids. The presence of water reduces the catalytic activity, while free fatty acids react with the catalyst to produce saponified products, risking a reduction in the yield of FAME. Therefore, manufacturing BDF from waste cooking oils by the alkaline catalyst method is not necessarily easy [1], [2].
Non-catalytic BDF production methods with supercritical methanol have been, therefore, developed in order to resolve the various problems in these techniques [3]. In one of these methods, the one-step supercritical methanol method (Saka Process), the trans-esterification reaction of TG without catalysts proceeds to produce FAME and glycerol by processing the raw oils and fats using supercritical methanol. FAME are also generated simultaneously by the esterification reaction of the free fatty acids [4], such that even if there is a high content of free fatty acids in the raw material oils and fats, FAME can be derived at a high yield, and no saponified products are produced. Moreover, the separation and purification are easy because of the non-catalytic process. Unlike the alkaline catalyst method, it can also be adapted to relatively long chain alcohols [5]. However, this method requires extreme temperature and pressure conditions of 350 °C and 43 MPa, respectively, and induces breakdown of unsaturated fatty acids and trans isomerization, leading to adverse effects on the fluidity of the fuel at low temperatures [6], [7]. Thus, Cao et al. reported that oils and fats can be converted into FAME successfully at lower temperature by supercritical methanol with co-solvent such as propane [8]. Furthermore, CO2 addition to supercritical methanol can make the reaction temperature lower [9].
Kusdiana and Saka developed the two-step supercritical methanol method (Saka-Dadan Process) as a BDF manufacturing process at very mild reaction conditions [10]. In this method, the hydrolysis reaction of the triglycerides is induced by subcritical aqueous processing and thus the triglycerides are converted to fatty acids and glycerol. After the reaction mixture is left undisturbed, the oil layer containing the fatty acids and the aqueous layer containing the glycerol are separated. Methanol is, then, added to fatty acids in the oil fraction which are transferred into FAME by the esterification reaction under supercritical condition. Because the reaction conditions in this method are relatively mild (270 °C and 7 MPa), no breakdown of unsaturated fatty acids or other changes are barely induced [6]. Therefore, the two-step method (Saka and Dadan Process) is more suitable for practical application, compared with the one-step method.
In the processes using the method mentioned above, the production of glycerol as a by-product could not be avoided. Therefore, with the production of BDF increased in the recent years, glycerol has been overproduced. However, in the alkaline catalyst method, the glycerol was recovered as along with a mixture of methanol, water, and alkaline catalyst. To produce pure glycerol, crude glycerol must be neutralized by acid such as sulfuric acid and this process forms a large quantity of salt. Thus, purification of crude glycerol is very complicated and needs much cost. The sales value of this crude glycerol becomes extremely low at approximately $0.1/kg when compared with the value of purified glycerol, at approximately $1.3–2.0/kg. When the cost of transportation is taken into consideration, the market price is in fact uneconomical [11].
For the future, the production of glycerol after BDF manufacturing is expected to increase, unless an effective method is established. Fabbri et al. have, therefore, proposed that oils and fats can be reacted with dimethylcarbonate in the presence of alkalis as catalysts to produce FAME and cyclic glycerol carbonate esters of fatty acids (FAGC), which can be utilized as BDF [12]. Although this method will not produce glycerol as a by-product, it utilizes the alkaline catalyst, which is difficult to be adapted for some waste oils and fats.
In this research, therefore, we focused on one of the trans-esterification reactions. Xu et al. have already reported the reaction by enzyme [13], but enzymatic reaction takes long time to convert oils and fats into FAME completely. Thus, we propose a non-catalytic BDF manufacturing process by using supercritical methyl acetate, investigating the reactivity of rapeseed oil with methyl acetate. Moreover, the characteristics of the fuel produced by mixing FAME and triacetin (TA) were evaluated to establish if its mixture is usable as BDF.
Section snippets
Materials
Rapeseed oil was used for the supercritical treatment of oils and fats, while oleic acid was used as the fatty acid (both manufactured by NACALAI TESQUE INC.), and methyl acetate was used as the reaction solvent (Tc = 234 °C; Pc = 4.69 MPa [14]) (manufactured by NACALAI TESQUE INC., 99%). Moreover, triolein was used as the standard for triglycerides and methyl oleate as that for FAME (both manufactured by Sigma–Aldrich, ⩾99%), while triacetin as that for the final product (manufactured by NACALAI
Trans-esterification of triglycerides with methyl acetate
Fig. 1 shows the FAME yield and recovered amount of BDF when rapeseed oil was supercritically processed with methyl acetate. The dotted line shows BDF as FAME only, while the solid line shows BDF as the aggregate of FAME and triacetin. At a reaction temperature greater than 320 °C, FAME is generated from the oil and methyl acetate, and trans-esterification reaction proceeds as shown in Eq. (1). Therefore, it is clear that the trans-esterification reaction of triglycerides with methyl acetate
Concluding remarks
In this research, the use of methyl acetate as the reaction solvent was investigated in the manufacture process of BDF, producing FAME and triacetin from rapeseed oil with the aim of solving the problem of the by-product glycerol in the conventional processes. As a result, it became evident that the non-catalytic trans-esterification reaction between methyl acetate and triglycerides proceeding in the supercritical process could result in a high yield of FAME and triacetin. Moreover, it was
Acknowledgment
This work has been done in NEDO “Development of Preparatory Basic Bioenergy Technologies” Project in FY2006-2007, for which the authors are highly acknowledged.
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2022, Chemical Engineering Research and DesignCitation Excerpt :Several alternatives of supercritical processes have been reported, mainly varying in the number of reactive stages and the kind of supercritical reactant. Among those variations, it can be mentioned the one-step supercritical methanol process (Saka and Kusdiana, 2001), the two-steps supercritical methanol process (Saka, 2005), the supercritical methyl acetate process (Saka and Isayama, 2009) and the sub-critical acetic acid plus supercritical methanol process (Saka et al., 2010). It has been reported that, after following a suitable design and analysis strategy, the one-step supercritical methanol process is the one with the lowest total annual cost and CO2 emissions inside the production scheme (Gómez-Castro et al., 2015).