Phase Transitions in a Biodiesel Reactor Using Supercritical Methanol

Abstract

Recent experimental studies on noncatalytic transesterification have shown that high reaction rates, which justify the commercial application of this process, can be obtained if the operating conditions are above the critical pressure and temperature of methanol. Although it is generally agreed that temperatures of >550 K and high methanol/oil ratios are required to obtain high conversions and high reaction rates, there is disagreement in the literature regarding the operating pressure, the assumed phase conditions, the use of co-solvents, and the justification of the sudden increase of rate of reaction with temperature. In the present work, the reactor phase transitions are directly observed in a double-windowed cylindrical reactor and the conversion to methyl esters is measured. From direct observations and the modeling of the phase behavior, a better understanding of the supercritical methanol transesterification process is obtained.

Process intensification in green synthesis

“…This process exhibits advantages over the alkaline catalysis method, as it is a simple noncatalytic process, avoiding the removal of catalyst from the product and reducing wastewater. Hegel [14,15] reported that methanol and oil forms a single phase in the supercritical state in a tubular reactor. Bertoldi et al [16] carried out production of fatty acid ethyl esters from soybean oil transesterifi cation in supercritical ethanol and carbon dioxide as a cosolvent in a tubular reactor.…”

Transesterification in Supercritical Conditions

“…The phase behavior of soybean oil-methanol with propane as a co-solvent was reported first (Hegel et al, 2007), followed by that for soybean oilmethanol and soybean oil-ethanol with carbon dioxide as the co-solvent (Anitescu et al, 2008). The study of the phase behavior of supercritical transesterification, when performed in a high-pressure view cell, revealed that the liquid -liquid (LL) alcohol -triglycerides mixture transforms to a vapor -liquid -liquid (VLL) phase equilibrium.…”

“…Finally, the VL equilibrium merges to a one-phase supercritical at nearly the estimated critical point of mixture. The transition temperature of the VLL to VL equilibriums decreases with increasing methanol to oil molar ratios (Anitescu et al, 2008;Hegel et al, 2007). For example, the reaction mixture of soybean oil and methanol are partially miscible up to temperatures close to 350 ºC at a methanol to oil molar ratio of 24:1, while the two liquid phases of soybean oil and methanol become completely miscible at 180 ºC and 157 ºC with a methanol to oil molar ratio of 40:1 and 65:1, respectively.…”

“…Moreover, the transition temperature of the two-phase VL to a one-phase supercritical could be reduced by the addition of gaseous co-solvents, such as carbon dioxide and propane. For instance, the addition of 24% by weight of propane decreased the transition temperature of soybean oil-methanol, at a methanol to oil molar ratio of 65:1, from 315 ºC to 243 ºC (Hegel et al, 2007). Therefore, the addition of gaseous co-solvents is able to reduce the original severe conditions due to their ability to lower the transition temperature from a VL system to a one-phase supercritical system.…”

“…In 2009, carbon dioxide was applied to supercritical transesterification with ethanol to reduce the operating conditions (Bertoldi et al, 2009). From 2007 to 2010, numerous additional studies, such as vapor-liquid equilibria of binary systems (Anitescu et al, 2008;Fang et al, 2008;Shimoyama et al, 2008;Shimoyama et al, 2009;Tang et al, 2006), phase behavior of the reaction mixture (Glišic & Skala, 2010;Hegel et al, 2008;Hegel et al, 2007), thermal stability of unsaturated fatty acids in supercritical methanol (Imahara et al, 2008) and process simulation and economic analysis (Busto et al, 2006;D'Ippolito et al, 2006;Deshpande et al, 2010;Diaz et al, 2009;van Kasteren & Nisworo, 2007) were reported, leading to a better understanding of the supercritical transesterification process.…”