Methanol-to-oil ratio

Increasing temperature has also an important role. Near complete conversion of soybean triglycerides requires only 3 h at 117 C, whereas comparable conversions at 77 °C require 20h. On the other hand, high temperatures and high methanol-to-oil ratios also imply high pressures and consequently plant construction costs are high. 

Acid-catalyzed reactions require the use of high alcohol-to-oil molar ratios in order to obtain good product yields in practical reaction times. However, ester yields do not proportionally increase with molar ratio. For instance, for soybean methanolysis using sulfuric acid, ester formation sharply improved from 77% using a methanol-to-oil ratio of 3.3 1 to 87.8% with a ratio of 6 1. Higher molar ratios showed only moderate improvement until reaching a maximum value at a 30 1 ratio (98.4%)." " " ... 

Transesterification using supercritical methanol is also possible [289-291]. No catalyst is needed and in 5 min over 90% conversion could be reached. Hovv ever, high temperatures (350 °C) and pressures (45 MPa) are necessary, as vell as a high methanol to oil ratio (>40). The advantage is that free fatty acids in crude oils and fats could also be converted into methyl esters. Hovv ever, the main limitations are the high investment and energy costs, and the use of excessive methanol. There are good advances to overcome these limits, but the industrialization is still far. 

Although the importance of the methanol-to-oil ratio was briefly mentioned. The yield of biodiesel is affected by other parameters, including reaction temperature, catalyst concentration, feed flow rate, transmembrane pressure (TMP), and membrane thickness and pore size (Hoong Shuit et al., 2012). Some of these factors are briefly discussed. 

As the methanol-to-oil ratio increases, the biodiesel production increases in a membrane reactor. The common molar ratio of methanol to oil used in biodiesel production is 24 1 in a membrane reactor in which the separation is based on oil droplet size (Baroutian et al., 2011 Cao et al., 2008a Cheng et al., 2010). 

CaO and ZrOj mixed oxides based heterogeneous catalyst was used for transesterification of waste cooking oil at 65° C with 2 h reaction time at 30 1 methanol to oil ratio yielded biodiesel in 92%. It was observed that the activity of catalyst increase with increasing Ca to Zr ratio but the stability of the catalyst decreased with increasing ratio of Ca to Zr and the optimum Ca to Zr molar ratio was found to be 0.5 [39]. 

Thus, the ratio of methanol to oil is a critical parameter in optimization studies. Various lipases have shown a different level of tolerance toward methanol. Most of the researchers have optimized the methanol to oil molar ratio in the range of 3 1 to 4 1 for lipase-catalyzed conversion. Some lipases have shown optimum activity at higher methanol to oil ratio. To overcome methanol inhibition, various alternatives have been suggested by researchers, including stepwise addition of methanol, the use of other acyl acceptors, use of solvent, and use of methanol-tolerant lipase (Kumari et al., 2007 CamUo Naranjo et al., 2010). 

The transesterification reactions were conducted in a sealed 250 ml autoclave equipped with a stirrer. The molar ratio of methanol to oil was 12 1, reaction temperature was 200 C-230°C, and the ratio of catalyst to oil was about 2 wt%. Samples were taken out from the reaction mixture and biodiesel portions were separated by centrifuge. 

Peterson and Scarrah 165) reported the transesterification of rapeseed oil by methanol in the presence of alkaline earth metal oxides and alkali metal carbonates at 333-336 K. They found that although MgO was not active for the transesterification reaction, CaO showed activity, which was enhanced by the addition of MgO. In contrast, Leclercq et al. 166) showed that the methanolysis of rapeseed oil could be carried out with MgO, although its activity depends strongly on the pretreatment temperature of this oxide. Thus, with MgO pre-treated at 823 K and a methanol to oil molar ratio of 75 at methanol reflux, a conversion of 37% with 97% selectivity to methyl esters was achieved after 1 h in a batch reactor. The authors 166) showed that the order of activity was Ba(OH)2 > MgO > NaCsX zeolite >MgAl mixed oxide. With the most active catalyst (Ba(OH)2), 81% oil conversion, with 97% selectivity to methyl esters after 1 h in a batch reactor was achieved. Gryglewicz 167) also showed that the transesterification of rapeseed oil with methanol could be catalyzed effectively by basic alkaline earth metal compounds such as calcium oxide, calcium methoxide, and barium hydroxide. Barium hydroxide was the most active catalyst, giving conversions of 75% after 30 min in a batch reactor. Calcium methoxide showed an intermediate activity, and CaO was the least active catalyst nevertheless, 95% conversion could be achieved after 2.5 h in a batch reactor. MgO and Ca(OH)2 showed no catalytic activity for rapeseed oil methanolysis. However, the transesterification reaction rate could be enhanced by the use of ultrasound as well as by introduction of an appropriate co-solvent such as THF to increase methanol solubility in the phase containing the rapeseed oil. 

For the alkali-catalyzed transesterification reaction of rapeseed oil, we investigated several operating conditions reaction temperature, type and amount of catalyst, molar ratio of methanol to oil, and reaction time. In alkali-catalyzed transesterification, the amount of free fatty acid was assumed to be below 0.5% on the basis of oil weight, in order to obtain high conversion yield (13). The conversion yield or percentage of conversion was calculated by dividing the amount of product by the maximum theoretical product. Because it has a high acid value, the activity of catalyst was diminished in the transesterification reaction. As reported in Table 1, the fatty acid content of rapeseed oil used for this experiment was 0.018%, which was lower than the proposed value (below 0.5%). 

Fatty acid methyl and ethyl esters derived from vegetable oils are considered to be a promising fuel for direct injection diesel engines. Moreover, they are valuable compounds for the production of fine chemicals for food, pharmaceutical and cosmetic products. Leclercq et a/.1711 showed that the methanolysis of rapeseed oil can be carried out with MgO, although its activity depends strongly on the pretreatment temperature of this oxide. Thus, with MgO pretreated at 823 K and a methanol to oil molar ratio of 75 at methanol reflux, a conversion of 37 % with 97 % selectivity to methyl esters was achieved after 1 h in a batch reactor. 

For homogeneous base catalyzed processes, reaction conditions are generally at ambient or slightly higher pressure, and a temperature of 65 °C-70 °C, in the presence of approximately 0.5% catalyst, with a 6 1 molar ratio of methanol to oil (Freedman et al, 1986). The process can be operated continuously (Noureddini et al, 1998) or in batch mode. Examples of continuous industrial processes include Ballestra, Connemann CD, and the Lurgi PSI process. 

Li et al. (2007) reported the use of dry biomass, Rhizopus oryzae (R. oryzae) IF04697, whole cell-catalyzed methanolysis of soybean oil for biodiesel (methyl ester) in rm-butanol system. Changing one separate factor at a time (COST), live-level-four-factor Central Composite Design (CCD) were used to evaluate the effects of synthesis conditions, such as tert-butanol to oil volume ratio, methanol to oil molar ratio, water content, and dry biomass amount. Biodiesel yields of 72% were obtained under the optimal conditions using the proposed model for prediction. 

Biodiesel was produced from pretreated tung oil. Reaction was performed using KOH as a catalyst at 80 °C for 20min. The final purity of tung biodiesel was 90.2wt% when the molar ratio of methanol to oil was 6 1 and KOH was 0.9% of oil and did not increase with increasing ratio of methanol to oil (Fig. 3). 

In this experiment, the effects of the methanol/oil molar ratio on rapeseed oil methanolysis oil were evaluated in a ratio range of 1 1 to 6 1, with a reaction temperature of 40 °C and 5% (w/w) Novoz)mi 435 on the basis of oil weight as a catalyst for 24h, the results of which are indicated in Fig. 3. The conversion was enhanced in a linear fashion with increases in the methanol/oil molar ratio within a ratio range below 2 1. From the results of the methanol/oil molar ratio study, it was determined that when the molar ratio of methanol to oil was in a range of 2 1-5 1, high conversions were achieved, and no... 

However, Sails et al. [15] reported that solvent-free synthesis of oleic acid short-chain alcohols reached 100% of triolein conversion after only 6h at 40 °C by immobilized lipase of Pseudomonas cepacia. Furthermore, Kim [16] reported that the high conversion of 98.5% was possible at 45 °C of reaction temperature with 4 1 of methanol to oil molar ratio and 1% (v/v) methyl glucoside oleic polyester as an emulsifier using Novozym 435 in the presence of cosolvent. However, several researchers have reported that the conversion of... 

Also, the higher the ratio of methanol to oil is, the better conversion is achieved without considering the operational principle of the membrane (Shi et al., 2010). 

Xie and co-workers (362) tested Mg-Al hydrotalcites as solid base catalysts in the reaction of transesterification of soybean oil with methanol and fovmd that a sample with a ratio Mg Al of 3.0 calcined at 773K presented the highest basicity and the best cataljdic activity for this reaction. Trakampruk and Pomtangjitlikit reported the transesterification of palm oil with methanol in the presence of 1.5% K/Mg-Al hydrotalcite. The catalyst was found to be effective that gives methyl ester content of 96.9% and yield of 86.6%, using a 30 1 methanol to oil molar ratio at 100°C for 6 h and 7 wt% of catalyst (368). However, leaching of K was not reported. 

As mentioned earlier, the quality of the feedstock has a significant effect on the reaction yield, where FFAs and moisture contents are key parameters in defining feedstock feasibility. For example, the alkali-catalyzed production of biodiesel from duck tallow at 65°C and a 6 1 methanol-to-oil molar ratio resulted in a 62% conversion (Chung et al 2009), whereas sunflower resulted in an 86% conversion using the same catalyst under the same conditions (Vicente et al., 2004). 

Rashid, W.N.W.A., et al., 2014. Synthesis of biodiesel fiom palm oil in capillary millicharmel reactor effect of temperature, methanol to oil molar ratio, and KOH concentration on FAME yield. Procedia Chemistry 9,165—171. Available at http //www.sciencedirect.com/ science/article/pii/S1876619614000217 (accessed 10.03.15.). 

Synthesis of fatty acid methyl esters from rapeseed oil was carried out in a fixed bed continuous flow reactor at 60°C with 1 g of catalyst pellets, a molar methanol to oil (triglyceride) ratio of 40 and at liquid hourly space velocity of 6.5 h. ... 

These authors used 6 1 and 30 1 alcohol-to-oil molar ratios for both methanol and butanol. As expected, a pseudo first-order reaction was found at large excess of alcohol for both alcohols. At low excess alcohol, however, the butanolysis reaction (30°C) was second-order, but the methanolysis reaction (40°C) was reported to be a combination of a second-order consecutive reaction and a fourth-order shunt reaction. The shunt reaction, in which three methanol molecules simultaneously attack a TG molecule, was adopted to better fit the kinetic data. However, such a reaction is highly unlikely. Nureddini et al. later found that the inclusion of a shunt mechanism was not necessary to fit the kinetic data of the transesterification reaction, and Boocock et al showed that the shunt reaction assumption came as a misinterpretation of the observed kinetics. At low temperatures (20 0°C) the multiphase methanolysis reaction... 

In a previous patent, Bayense et al. reported the use of ETS-10 and ETS-4 for the transesterification of TGs with alcohols.In their studies, both batch and fixed bed reactors were used to conduct the reaction. Under batch conditions, reactions were carried out at 220°C, 21 bar, and a catalyst loading of 0.23 wt% based on total autoclave content. Experiments with soybean oil and methanol, at a methanol/oil ratio of 4.2, resulted in 69.0% total oil conversion with an ester yield of 52.6% for ETS-10 and 96.9% conversion with an ester yield of 85.7% for ETS-4. When tallow was used under the same conditions, conversion and ester yield were 30.6% and 19.1% for ETS-10 and 44.1% and 29.6% for ETS-4, respectively. The slightly better activity shown by ETS-4 might imply that only basic sites at the pore opening of these zeolites were catalyzing the reaction. The effective radius of the TG molecule is such that it can not enter through the small pore openings of ETS-4 (diameter of 3.7 A) or even ETS-10... 

Alkaline earth metal oxides and hydroxides have also been tested in transesterification reactions. Ca(OH)2 did not show significant catalytic activity in the transesterification of rapeseed oil with methanol at conditions normally used to prepare biodiesel.Peterson et al. reported relative alcoholysis activities of a series of supported CaO catalysts under near reflux conditions of methanol-rapeseed oil mixtures at 6 1 molar ratios.Among the catalysts tested, the most active was CaO (9.2 wt% CaO) on MgO. For instance, in a 12 h reaction the total oil conversion using this catalyst was over 95%, similar to... 

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