Thermodynamic and Economic Assessment of the Production of Ethylene and Propylene from Bioethanol

Introduction

The increasing worldwide pollution and the environmental problems associated with the petroleum extraction and use, have led to the development of new energy resources and sustainable raw materials [1]. The production of bioethanol from biomass offers a sustainable biotechnology solution at competitive costs with high availability. In Colombia (South America), bioethanol production has increased because the local policies encourage its production, being the third producer in America [2]. However, the focus on bioethanol production has been related to its use as biofuel. Alternatively, there are some high-commercial-value compounds —mostly produced from petroleum— that can also be obtained from renewable sources [3]. Recently, bioethanol has been proposed as a source for producing commodities such as light olefins in the framework of the biorefinery concept [4]. Among these biomaterials, ethylene stands out as one of the most produced chemicals worldwide, having a projected production of 400 million ton in 2020 [1]. In addition, the demand for propylene is growing faster, having a projected production of 100 million ton in 2021[5]. Therefore, this study aims to propose a conceptual design for producing ethylene and propylene from bioethanol samples procured in Colombia, integrating the reaction and separation stages and assessing its technical and economic feasibility. Aspen Plus and Aspen Hysys were used as process simulators.

Methodology

Aspen Plus V7.3 and Aspen Hysys V7.3 (Bedford, USA) were used as process simulators throughout the study. In both, the Soave-Redlich-Kwong (SRK) equation of state was used as thermodynamic model. First, the bioethanol dehydration reaction was analyzed in the RGibbs reactor module in Aspen Plus. Water/ethanol (S/E) feed ratio and reaction temperature were varied in the module in order to assess their effect on products selectivity. Ethylene, propylene and n-butene were selected as possible products, and a selectivity response surface was obtained in Matlab R 2014 (Salt Lake City, USA). Aspen Hysys was used to perform the sizing of the process units, heat integration, and process yields. Products purification was modeled in the Distillation Column module and the energetic analysis was performed using Heat Exchanger and Heaters/Coolers modules in Aspen Hysys. Finally, a class-V-order  economic evaluation was done to the final flowsheets by determining the capital expenses for all units in the processes [6].

Results and analysis

Figure 1a shows the surface response for ethylene selectivity during bioethanol dehydration simulation, when varying S/E ratio and temperature. The higher the temperature and S/E ratio, the higher the ethylene yield, with a selectivity exceeding 60% beyond S/E = 5:1 and 600 °C. On the other hand, Figure 1b shows a maximum peak of 45% selectivity for propylene at 400 °C, and a slightly positive effect of the S/E ratio. n-butene formation also presented a similar trend (Figure 1c), showing a maximum selectivity of 14% at 300 °C. Heavier olefins are formed by ethylene condensation, which is thermodynamically favored at lower temperatures [7]. However, the absence of ethylene at these lower temperatures reduces the amount of this compounds.

Figure 1. Matlab response surface from Aspen Plus simulated data for the production of (a) ethylene, (b) propylene, and (c) n-butene using different water:ethanol ratios (S/E) in the temperature range of 200–800 °C; and (d) the effect of the S/E ratio on the maximum ethylene and propylene yields.

Figure 1d shows the effect of the S/E on the maximum both ethylene and propylene selectivity. The excess of water is slightly positive for the ethylene formation because its acts as a quasi-inert diluent from a thermodynamic point of view [7]. In this way, a bioethanol dehydration reactor feed with a stream of S/E = 5 and operating at 615 °C was selected as the proper design to produce ethylene. In addition, a dehydration reactor operating at 400 °C was selected to produce propylene. These reactors were included in two different processes to analyze the conceptual design of two plants, one producing ethylene and another producing propylene from bioethanol.

Figure 2 shows the designed processes for producing (a) ethylene and (b) propylene from bioethanol. The processes begin with the feed stream of 300,000 ton/year of raw, fermented bioethanol (5 mol% ethanol, S1), which corresponds to the 10% of the national bioethanol production. This stream is heated to 40 °C before its further introduction to a flash separator V-100, which operates at 95 °C and 1 atm. These conditions were established according to the ethanol-water equilibrium to obtain an S/E = 5:1 (16.4 mol% ethanol). After V-100, the product stream rich in ethanol (S3) is heated to either 615 °C or 400 °C (reaction temperatures for ethylene or propylene production, respectively) by a recycle of the flash bottoms stream (S4). S4 is rich in water and is used to the process heat integration. The product streams (S7 in Figure 2a and S6 in Figure 2b) are pressurized to 19 atm in the compressor K-100. The pressurized streams are cooled to 18 °C by fresh water streams. These streams would deliver the energy duty for heating streams (S20 in Figure 2a and S19 in Figure 2b). Subsequently, the cooled streams are used as an inlet in a three-phases flash separator V-101 with the purpose of removing the residual water after the reaction. Finally, the product streams are introduced into distillation columns to separate the olefins.

The ethylene process delivered a stream of 99.7 mol% ethylene with an almost 100% recovery (S16 in Figure 2a), producing 0.60 mol ethylene/mol ethanol feed. In addition, propylene process delivers a stream with 92.0 mol% propylene in S15 in Figure 2b. This corresponds to the 70% of the propylene entering to the distillation tower, producing 0.43 mol propylene/mol ethanol feed.

A class-V economic assessment delivered investments of USD 2 million for producing ethylene and USD 2.23 million for propylene. These costs represent 159 USD/ton-year of ethylene and 327 USD/ton-year for propylene. True [8] reported investment costs of 1300 USD/ton-year for ethylene and 1450 USD/ton-year for propyylene produced from petroleum for a plant constructed in 2016. However, these costs include lumpsum, engineering, procurement, construction and commissioning turnkey contract; which not were included in the current analysis. Nevertheless, this class-V economic assessment shows that olefins production from bioethanol could be a profitable and sustainable alternative to petrochemical processes.

Figure 2. Process flowsheets designed in Aspen Hysys for producing (a) ethylene and (b) propylene from bioethanol. Blue lines represent mass streams and red lines represent energy streams. Equipment notation: E: Heat exchanger, V: Flash separator, K: Compressor, MIX: Mixer, T: Distillation tower, TEE: Splitter, and GBR: Gibbs reactor.

 

Conclusion

This study evaluated a conceptual design for producing light olefins from bioethanol base on thermodynamic predictions and using commercial software. To the best of our knowledge, it is the first time that reaction and separation stages are integrated and economically evaluated in two different processes, one to produce ethylene and another to produce propylene from bioethanol. Due to water showed a positive effect on the ethanol dehydration reaction, both processes start with a simple flash separator operating at 95 °C and 1 atm to reach the ethanol initial concentration needed in the reactors (S/E=5, 17.6 mol% ethanol). Two different reactors were designed, one operating at 615 °C for producing ethylene and another at 450 °C for producing propylene. The designed ethylene process delivered a stream of 99.7 mol% ethylene with a 60% yield and an investment cost of USD 2 million, equivalent to 159 USD/ton-year of ethylene. On the other hand, propylene production from bioethanol delivers a stream with 92 mol% propylene and 43% yield. Propylene process has an investment cost of USD 2.23 million (327 USD/ton-year propylene). These conceptual analysis show the technical, environmental and economic feasibility of the biorefinery concept development.

References

[1]      H. Xin, X. Li, Y. Fang, X. Yi, W. Hu, Y. Chu, et al., J. Catal. 312 (2014) 204–215.

[2]      G.P. Ortegón, F.M. Arboleda, L. Candela, K. Tamoh, J. Valdes-Abellan, Sci. Total Environ. 539 (2016) 410–9.

[3]      O.J. Sánchez, C.A. Cardona,  Bioresour. Technol. 99 (2008) 5270–95.

[4]      R. Le Van Mao, T.M. Nguyen, G.P. McLaughlin,  Appl. Catal. 48 (1989) 265–277.

[5]      Ceresana, Market study: Propylene, (2014). http://www.ceresana.com/en (accessed May 1, 2016).

[6]      American Association of Cost Engineers, (2015). http://www.aacei.org/ (accessed November 15, 2015).

[7]      T. Lehmann, A. Seidel-Morgenstern, Chem. Eng. J. 242 (2014) 422–432.

[8]      W. True,  Oil Gas J. 110 (2012).