ANALYSIS AND SIMULATION OF DIETHYL CARBONATE SYNTHESIS VIA CO 2 AND ETHANOL CONVERSION

Objective: The objective of this study is to analyze and simulate the synthesis of diethyl carbonate (DEC) from the conversion of carbon dioxide and ethanol. The focus is on evaluating the best operating conditions, such as temperature and pressure, using the Aspen Plus software for process simulation. In addition, the study seeks to compare the results obtained with data from the literature and validate the simulation through previous studies. Methodology: To achieve the objective, thermodynamic models were used, such as the NRTL for the liquid phase and the Peng-Robinson equation state model for the vapor phase. The reactor chosen for the simulation was of the PFR type, and the catalyst used was cerium dioxide. Kinetic parameters and reactions were based on previous studies. The simulation was performed by varying the temperature and pressure of the system, feeding the data into the Aspen Plus software. Results and Conclusion: The simulation results indicated a maximum DEC production of 4.268 kmol/h, obtained at a temperature of 165°C and pressures of 150, 200 and 250 bar. Ethanol conversion reached 90.58% under these conditions. It was found that atmospheric pressure is not suitable for the reaction to occur. Comparison with previous studies and simulation validation showed satisfactory agreement. Phase analysis along the reactor revealed significant changes in the phase diagram as the reaction progressed. In short, this study contributes to the understanding of the DEC synthesis, highlighting the ideal operational conditions for its sustainable production.


INTRODUCTION
Concern for the environment is a recurring issue, and therefore there are numerous studies focused on reducing emissions, capturing, utilizing, and consuming these gases, specifically CO2. In order to reduce the use of environmentally harmful compounds and replace them with biodegradable and low-toxicity alternatives, various methods of producing diethyl carbonate (DEC) are being studied. In the fuel market, DEC stands out due to its higher gasoline/water distribution coefficient and lower volatility compared to other organic carbonates such as dimethyl carbonate (DMC). This makes DEC an excellent additive for fuel, replacing compounds like MTBE and ETBE, which pose risks of soil, river, sea, and groundwater pollution.
The production of DEC has recently been studied by Yu et al. (2020), who focused on economic viability and compound reuse. Meanwhile, Ramos et al. (2022)  This work aims to evaluate the best operational conditions by testing thermodynamic models for each phase (liquid-vapor) and varying temperature and pressure. The objective is to achieve the highest DEC production possible from ethanol and carbon dioxide through simulation using Aspen Plus, version 12.

METODOLOGY
The simulation was developed using a computer with 8GB of RAM and an Intel(R) Core™ i7 processor. The thermodynamic models employed were NRTL for the liquid phase and Peng-Robinson for the vapor phase of the system. These two models were chosen because they were suitable for the operational conditions involved. The reactor utilized in the simulation was a plug flow reactor (PFR), and the catalyst chosen, following the literature for higher reaction yield, was Cerium Dioxide (CeO2) with a porosity of 0.4 and a catalytic density of 7130 kg/m 3 .
The simulations were conducted based on the reactions and kinetic parameters described by Yu et al. (2020) and Giram et al. (2018). With these data inputted into the simulator, the temperature and pressure of the system were varied in the range of 25-500°C and 1-300 bar, respectively.

RESULTS AND DISCUSSION
To evaluate the behavior of ethanol conversion as a function of temperature and pressure, we have Figure 1 In order to determine the optimal reactor design and analyze ethanol conversion, a comparison was made with established studies using a 200-meter-long PFR reactor with a single one-inch diameter tube, as illustrated in Figure 2. There is good agreement between the results obtained in this work and the literature data, as reported by Ramos et al. (2022) and Yu et al. (2020). Figure 3 illustrates the simulation of DEC production under the same reactor conditions as the one designed by Ramos et al. (2022), which consists of a 5-meter reactor with 20 oneinch diameter tubes and a pressure of 50 bar. The results shown in Figure 3 demonstrate that high reaction rates of DEC production occur up to 175°C in this configuration. It is also observed that the maximum production of DEC occurs at 150°C and 50 bar. The behavior of the phases along the reactor can be analyzed in Figure 4, which summarizes the bubble and dew curves for mixtures of the reaction system at different reactor sizes. It is evident that before the reaction starts, the phase diagram differs significantly compared to the reaction occurring from the first half meter of the reactor. This occurs due to the drastic change in system properties and an increase in the number of compounds present inside the reactor. For clarity, Figure 5 illustrates each phase diagram separately.
Based on Figures 4 and 5, it is noticeable that as the pressure varies in the system, the properties of the analyzed phases change very little. Due to the high stability of CO2, its use as a reactant tends to result in low yields due to unfavorable thermodynamic equilibrium. In this context, the employed route requires the application of 2-cyanopyridine as a water suppressant. 2-cyanopyridine is hydrolyzed and forms 2-picolinamide as a byproduct. 2-picolinamide, in turn, reacts with the present ethanol, forming ethyl picolinimidate, which also interacts with other compounds. The final reaction system consists of eight additional reactions, besides the main reaction. The reactions occur in the liquid phase on the catalyst surface. High pressures favor the solubilization of CO2 in the liquid phase, contributing to the reaction rate. However, they also promote the hydrolysis of DEC, thereby reducing the product yield. This explains why ethanol conversion approaches constancy as a function of pressure, as illustrated in Figure  1.

CONCLUSION
The production of DEC was analyzed through the direct carbonylation of ethanol using CO2 and a metal catalyst in a PFR reactor. It was observed that the conversion of ethanol into DEC has significant sensitivity to temperature and low sensitivity to pressure. The conditions for higher rates of DEC production per studied pressure are: 50bar and 150ºC generating 4.207kmol/h; in addition, a rate of 4.267kmol/h at 100bar and 4.268kmol/h for the other pressures (150, 200, 250, 300 bar) was observed at 165ºC.