As a replacement for conventional fossil fuels to meet energy demands, a new wave of research on biodiesel production technologies has commenced for the development of alternate energy sources worldwide. These include fatty acid methyl esters (FAMEs), i.e., biodiesel, having characteristics similar to petrodiesel oil, allowing its use in compression motors without any engine modification [1
]. FAMEs are commonly obtained by (1) the transesterification of vegetable oils, i.e., triglycerides (TG) of fatty acids (FAs), or (2) esterification of free fatty acids (FFA), with lower alcohols [2
]. Generally, triglycerides can be classified into two groups: simple and mixed. The simple triglyceride is composed of three identical fatty acid chains, whereas fatty acid chains of a mixed triglyceride are not identical. Natural oils produced from oil-bearing crops comprise 97% of various triglycerides and 1–5% of free fatty acids (FFA). Along with simple triglycerides, vegetable oils consist of mixed triglycerides containing different fatty acid chains; e.g., C12:0 (lauric acid chain), C14:0 (myristic acid chain), C16:0 (palmitic acid chain), C18:0 (stearic acid chain), C18:1 (oleic acid chain), and C18:2 (linoleic acid chain). Their compositions are known to vary with oil sources and growth conditions [3
]. Recently, alternative feedstocks such as waste/used cooking oils, and non-edible feedstocks such as jatropha, pongamia, castor and microalgal oils are used to produce biodiesel fuels, to reduce the high prices of biodiesel fuel.
Transesterification of triglycerides with homogeneous acid or base catalyst requires its neutralization and recovery from the reactor products. Increased purification and recovery steps can, eventually, affect product costs and the market. Also, the base catalyst results in the production of undesirable products due to the saponification reaction. Alternatively, heterogeneous catalysts can be separated from the liquid effluents and re-used easily [4
]. Solid acid-catalyzed transesterification reactions have been explored to circumvent the problems associated with the conversion of low quality feedstocks (containing free fatty acids) to biodiesel, and thus, are preferred over base catalyzed transesterifications. Also, non-catalytic transesterifications have shown promising reaction rates for commercial application using supercritical methanol (>250 °C, 19–45 MPa) [5
]. The partial miscibility of the oil and methanol phases at moderate temperatures and pressures hinders the rate of reaction.
Supercritical processes do not require neutralization, washing, and drying steps, allowing waste oils to be processed without these expensive pretreatment steps [5
]. Supercritical carbon dioxide (Sc-CO2
) (critical point at 31 °C and 7.3 MPa) and methanol (critical point at 240 °C and 7.95 MPa) used in a single supercritical phase for the transesterification resulted in higher reaction rates and lesser time duration [6
]. But, the monophasic system can suffer from high energy requirements and the need for downstream separation of glycerol from the product [7
]. Operation at moderate temperatures (~80–100 °C) and pressures (8–10 MPa) in a multi-phase liquid-vapor system may allow for the same benefits without high energy burdens. Sc-CO2
(supercritical carbon dioxide) acts as a co-solvent and can increase the rate of the reaction by eliminating or reducing the transport resistance and increasing the solubility of methanol in triolein and vice-versa [8
In previous work Soh et al. [7
], demonstrated experimentally that mixed carbon dioxide (CO2
) and methanol (MeOH) successfully transesterifies triolein into methyl-oleate at moderate pressures and temperatures below 100 °C in the presence of a heterogeneous acid catalyst, Nafion NR50. Additionally, high-pressure CO2
was experimentally found to be effective and selective in separating algae oil triglycerides [9
], with new separation approaches currently under development. Silva et al. [10
] simulated a batch reactor involving six chemical species; viz., triolein, methanol, CO2
, glycerol, FAME, and water. The comparison of simulation results using the RK-ASPEN EOS (with no binary parameters) gave reasonable agreement with VLLE (vapor-liquid-liquid equilibrium) experimental results, and thus, the RK-ASPEN EOS was used in thermo-kinetic reactor model (see Section 3
below). A custom-written FORTRAN®
subroutine in a USER2 block of ASPEN PLUS was used that integrates the mass balance ordinary differential equations (ODEs) and checks the multiphase equilibrium, at various time intervals, to incorporate the effect of the phase behavior on the reaction kinetics periodically using the FLSH_FLASH subroutine [10
]. FLSH_FLASH is an ASPEN PLUS subroutine that performs only flash calculations (without reactions). Rate constants were regressed using the bulk concentrations in the experimental 50 mL, agitated reactor vessel [9
However, since the motivation behind these reactions was to evaluate only the effect of phase behavior without assessing yields of methyl-oleate converted and a constant amount of methanol on the reaction yield, the molar ratio of methanol/triolein used was quite high (1087). Because of its impact on the FAME yield [12
], these experiments have been extended to methanol/triolein molar ratios of 50, 100, 300, and 550 herein. Then, using the batch-reactor simulation model [10
], the predicted FAME yields are shown to compare favorably with the experimental data. A key objective of this verification, is to show that the FAME yields can be optimized by varying the methanol/triolein molar ratio, together with operating temperature and pressure. This manuscript focuses on this verification of the laboratory data.
For the laboratory experiments (carried out at Lafayette College), corn oil and methanol at the four molar ratios were transesterified using solid-acid heterogeneous catalyst (Nafion NR50) in the presence of supercritical CO2
at 95 °C and 9.65 MPa for 4 h in a batch reactor to yield FAME. Then, a FORTRAN®
USER2 block in ASPEN PLUS V10, prepared by Silva et al. [10
], was used to carry-out dynamic simulations of the batch reactor. As the mass-balances were integrated, using the kinetic model in Section 3
, the FLSH_FLASH subroutine was used to compute 3-phase equilibria using the RK-ASPEN EOS. Given the concentrations of FAME in the vapor and two liquid phases, and the phase volumes, the yield of methyl-oleate (g) was computed.