Ethanol dehydration by absorption and biodiesel production by reactive distillation: An innovative integrated process

Highlights

• An innovative ethylic biodiesel production process was simulated in Aspen Plus.

• Glycerol was used as entrainer for ethanol dehydration in the absorption section.

• Simulated results of ethanol dehydration were validated with experimental data.

• Absorption and reactive distillation were optimized based on multivariate analysis.

• High oil conversion, biodiesel yield and ethanol purity were achieved.

Abstract

An innovative integrated process of ethanol dehydration and ethylic biodiesel production based on absorption and reactive distillation processes was demonstrated by experimental validated simulation and optimization using Aspen Plus® and Statistica® software, respectively. Glycerol, a byproduct of the transesterification of triacylglycerols, was adopted as solvent for ethanol purification. Experimental and simulated results of the absorption process step were compared and showed no significant difference from t-test with 95% of confidence level. The optimal conditions of the processes were obtained using a multivariate analysis. For the absorption process, the optimized parameters were solvent feed stream temperature, $T$ = 27 °C; number of stages, $N$ = 6; and solvent-to-feed ratio, $S/F$ = 1.5, 1.4 and 1.0 for ethanol mass fractions on feed stream of 0.8800, 0.9380 and 0.9910. The optimal parameters of the reactive distillation column (RDC) for biodiesel production were ethanol-to-oil molar ratio, $MR$ = 12; $N$ = 6; total reflux ratio; and residence time to number of stages ratio, $t/N$ = 8 min/stage. The integrated process produced anhydrous ethanol with mass purity above 99.9%, oil conversion ($X_{oil}$) above 99%, and biodiesel yield of 98%. Moreover, $X_{oil}$ > 96.5% was also obtained for a lower value of $MR$ = 7 adopting $t/N$ = 8 min/stage. Finally, the integrated process proved to be a potential economically attractive technology for ethylic biodiesel production, since reactive distillation and absorption columns were coupled based on the concept of process intensification.

Introduction

Among the alternative energy sources, biofuels play a key role in reducing fossil greenhouse gas emissions [[1], [2], [3], [4]]. Biodiesel is a biofuel that can be added to fossil diesel with many advantages. It is biodegradable, an excellent lubricant, non-toxic, free of sulfur and aromatics and a clean-burning fuel that produces low particulate emissions [[5], [6], [7], [8]]. The mandatory content of biodiesel to diesel has been 10% (volume percent) in Brazil since 2020 and is expected an increase to 15% in 2023 [9].

Biodiesel is still mainly produced via the methanolysis route in most industrial plants due to the higher yield, lower alcohol price and easier purification steps compared to the use of ethanol based on the ethanolysis route [10]. However, both routes are hindered by water presence. According to Lin and Ma [11], water mass purity must be kept below 0.06% before a transesterification reaction using methanol, in order to reduce undesirable effects of saponification and hydrolysis reactions.

Consequently, the absence of water in the biodiesel production system during the transesterification reaction is extremely important, as water is a catalyst for parallel reactions of saponification, compromising the yield and the quality of the esters [12,13]. For this reason, biodiesel production by ethanolysis route is challenging, since the ethanol used in the transesterification reaction has residual water, usually accounting for 0.5 wt%, and recovery of anhydrous ethanol is not possible by simple distillation [14].

There are several advantages, however, of using anhydrous ethanol instead of methanol for biodiesel production, including lower toxicity, lower rates of particulate matter and lower greenhouse gas emissions. Moreover, ethylic biodiesel is more easily biodegradable in water, presenting higher cetane number and higher lubricity than methyl esters. Brazil, in particular, is the second largest producer of bioethanol in the world, which leads to a greener biodiesel since sugarcane is used as the raw material, while methanol is mainly produced from petroleum industry [15].

Although the main technologies used for anhydrous ethanol production include azeotropic distillation, extractive distillation [[16], [17], [18]] and membrane distillation [19,20], absorption can be used for ethanol dehydration, as first proposed in 1924 by Mariller [21,22]. This process was used in Europe until the 1960s. However, the glycerol shortage, associated with its high cost, were the main reasons for the discontinuation of this technology [23]. Currently, the worldwide availability of glycerol is significantly higher than its demand and the reuse of glycerol, as part of biodiesel production process, is seen to be more economically attractive [22,24]. These aspects constituted the main factors for revisiting the absorption process with glycerol as describe in the present work.

Currently, the biodiesel industry is the major source of glycerol, contributing to the generation of approximately 4 billion liters of this byproduct worldwide in 2019 [25]. The Brazilian market, for example, exported 406,400 tons of glycerol in 2019. The consumption, however, was significantly below the total available commercial production of approximately 623,000 tons [26]. Accordingly, a possible alternative for glycerol reuse is proposed as entrainer for anhydrous ethanol production [27,28].

In contrast, biodiesel cost is still higher than diesel cost based on the conventional homogeneous alkali-catalyzed (HAC) route [29]. For this reason, the process intensification concept has been encouraged in biodiesel industry in order to increase productivity and selectivity, as well as to decrease operation and capital costs [[30], [31], [32]]. For instance, the use of a reactive distillation column (RDC) applied to biodiesel production significantly reduces the volumetric flow of alcohol, which is fed into the bottom of the column. This is possible because the alcohol is continuously recycled at the top of the RDC increasing reactive stage holdup [[30], [31], [32]]. In addition, as the vapor moves upward, it partially condenses in each stage, supplying additional alcohol to the RDC stages [31]. As a result, biodiesel is continuously generated along the reactive section with a countercurrent flow between the oil moving downwards and the alcohol moving in the opposite direction [33]. Moreover, lower residence time is required inside the RDC for biodiesel production compared to the use of batch reactors due to continuous product removal, which contributes to shift the reaction equilibrium towards produce more products [34,35].

For the development of more feasible processes, generally, process design and simulation of these potential alternatives are investigated by adopting process simulators such as Aspen Plus®, which is commonly used by chemical industries and presents an extensive databank of components, properties and thermodynamic models [36]. In biodiesel production studies, for instance, the Universal Functional-Group Activity Coefficient Dortmund (UNIFAC-DMD) model is reported to be suitable due to low deviations in temperature and vapor compositions [37]. Carmo et al. [38] also reported a liquid-liquid equilibrium (LLE) study of 34 different biodiesel systems, where the UNIFAC-DMD model best represented the LLE for biodiesel mixtures [24]. Zhang et al. [39], Souza et al. [28] and Pla Franco et al. [40] performed a vapor-liquid equilibrium (VLE) modeling of the water-ethanol-glycerol systems using experimental and literature data. According to the authors, results from Non-Random Two-Liquid (NRTL) model agreed with VLE data [28,39,40].

Little research has been identified in terms of ethylic biodiesel production using a RDC [23,35,41,42]. Moreover, no report has been found of either experimental or computational level studies of an integrated process of ethanol purification using glycerol as entrainer by absorption and biodiesel production by reactive distillation, with reuse of glycerol taking place in a single piece of equipment. Furthermore, this integrated process can be considered as a technological innovation and can encourage the use of hydrated bioethanol for biodiesel production instead of the more expensive anhydrous bioethanol.

Within this context, the main goal of this work was to provide a computational analysis of the innovative and integrated process of ethanol purification by absorption and biodiesel production by reactive distillation. Aspen Plus® was used as simulation software. Experimental data were obtained in the absorption study to validate the simulated results. The optimization of the absorption and reactive distillation processes were carried out with a multivariate analysis, by selecting the significant variables and using the desirability function technique.