(239c) Thermodynamic and Economic Assessment of the Production of Ethylene and Propylene from Bioethanol

Authors: 
Becerra, J., Universidad de La Sabana

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.

b)a)

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).

Topics: