Environmental impact assessment of soybean oil production: Extruding-expelling process, hexane extraction and aqueous extraction

https://doi.org/10.1016/j.fbp.2018.01.001Get rights and content

Highlights

  • EIA is conducted based on input, output components and energy consumption.

  • EAEP has similar environmental impact to expelling due to water is used as solvent.

  • The GHG emission from EAEP is between expelling and hexane extraction.

  • Higher energy consumption is required for EAEP pretreatment to increase oil recovery.

Abstract

Expelling and hexane extraction are two typical processes for soybean oil production used in industry. The main issues for these two processes are the low efficiency and hazardous chemical problems respectively. Enzyme assisted aqueous extraction process (EAEP) was proposed to increase the efficiency without using organic solvent, which is replaced by water. The environmental impact analysis of these three processes are based on their mass flows, energy consumption and global warming potential. For mass flows, the environmental impact indices were calculated based on material flow of input and output components. Energy consumption was used to evaluate the carbon dioxide, other greenhouse gas (GHG), and criteria pollutants emissions by GREET models. According to our results, hexane extraction has the highest environmental impact due to the application of organic solvent. Expelling has the highest GHG and criteria pollutants emissions because of the high energy requirement for heat pressing processes. EAEP has similar environmental impacts to the expelling process, but it also lowers GHG and criteria pollutants emissions. EAEP has the potential to be a green process adopted by industry although a high energy intense pretreatment to produce finer soybean flakes for increasing oil recovery is still a challenge.

Introduction

The US is the largest soybean producer in the world; around 34% of soybean production takes place in the US (Soystats, 2016). Due to its high oil content (Bernardini, 1983), soybean is the main oilseed used in edible oil production. In industry, the mechanical pressing-expelling, and hexane extraction are two typically used processes. However, lower oil recovery from expelling, and safety and environmental issues (Li et al., 2004, Oliveira et al., 2013) resulted from hexane extraction are the main flaws in the soybean oil industry. For improving the oil yield and mitigating the safety and environment related problems caused by expelling and hexane extraction, the enzyme assisted aqueous extraction process (EAEP) has been developed and might be a proper method for industrial application (Rosenthal et al., 1996).

Before pressing and extraction in mechanical expelling process, a series of pretreatment including cleaning, cracking, dehulling, and conditioning is required (Fig. 1). These treatments are mainly used to clean crops and reduce particle size to increase the oil recovery (Lamsal et al., 2006). During the extraction step (Fig. 1), heat and pressure are applied in the expelling process to denature the oleosins and to break the structure of oil body to release oil. The solubility of hexane and oil is the principle for the solvent extraction to extract oil from crushed soybean, and the desolvenization is applied to recover free oil and soybean meal. Further degumming and refining processes are needed for both expelling and hexane extraction to remove phospholipids and other impurities.

As to aqueous extraction (Fig. 1), contrary to solvent hexane extraction, water is used as the solvent and the insolubility of water oil is applied. During the process, the oil in water emulsion is formed. Consequently, the demulsification is conducted to separate the oil from the emulsion. The protein is extracted and dissolved in the aqueous fraction as well, therefore the further degumming process can be exempted (Johnson and Lucas, 1983, Jung et al., 2009, Sekhon et al., 2015). Thus, the safety and environmental related problems derived from chemical usages can be avoided. Additionally, this leads to a higher oil recovery than the mechanical expelling process.

In addition to technical improvement and feasibility, the environmental sustainability is another critical factor to evaluate the feasibility of the process. As to oil extraction, electricity is the main energy used in facility operations; steam is the heating resource which is mainly generated from a natural gas boiler built in the plant (Li et al., 2006). Besides energy consumption of the operation, the fossil-derived chemical addition is another critical issue for environmental impact evaluation, especially for hexane extraction. For EAEP, water is used as the solvent which could mitigate the environmental impact when compared to solvent extraction. However, the demulsification has been regarded as a critical step for oil recovery in aqueous extraction due to its high energy requirement, especially on physical (Hagenmaier et al., 1972, Harada and Yokomizi, 2000, McClements, 2005) and chemical methods (Menon and Wasan, 1985).

Based on the characteristic of different extraction processes, the environmental impact assessment (EIA) has been used to investigate the potential environmental impact resulted from the process. The mass balance, mass flow, and energy consumption are the main objectives used to evaluate the energy efficiency, greenhouse gas (GHG) and pollutants emissions from the processes (Salomone and Ioppolo, 2012). Heinzle et al. (1998) proposed the quantifying approach to evaluate the environmental impacts derived from chemical processing by calculating all input and output components. Also, the Organization for Economic Co-operating and Development (OECD) proposed the environmental indicator to assess the sustainability of industrial processing in 2001.

There are many computation models which can be used for GHG and air pollutant emission estimation such as Aspen Plus (Morais et al., 2010) and Simapro (Kiwjaroun et al., 2009). The GREET model (the greenhouse gases, regulated emissions, and energy use in transportation model, Argonne National Laboratory) was introduced to evaluate the GHG and criteria air pollutants emissions. Although the GREET model has the restriction for only investigating biofuels used in transportation sector, the soybean oil has been regarded as a critical resource for biodiesel production. Therefore, the GHG and air pollutants emissions of soybean oil production can be extracted from the soy-based biodiesel GREET model. However, there were few studies mainly focused on soybean oil production, especially comparing different processes and the alternative extraction methodology.

This study mainly focuses on the comparison among these three extraction processes. The EIA is divided into two sections including environmental impacts derived from material flows of the process and the GHG and air pollutants emissions from the oil extraction processes. The environmental impacts will be quantified based on material balance of the whole process, especially from input and output components. The total energy consumption, heating agent, and mass flow were used to build up an oil extraction pathway via the GREET model. The GHG and criteria air pollutants emissions were investigated by the GREET model. According to these criteria, the environmental feasibility of these three processes could be obtained and compared.

Section snippets

Boundary definition

The assessment boundary of soybean oil extraction includes oilseed pretreatment, extracting processes, oil degumming, and coproducts handling. The transportation, however, was not considered (Fig. 2). Additionally, the land use and the generations of primary energies were not considered in this EIA. Steam (assumed to be produced by the natural gas boiler within the plant) and natural gas were used as the primary source of heat energy. Therefore, the whole boundary can be defined as the

Classification of impact groups and categories

The results according to the classification of the impact groups and categories for input components are shown in Table 4. In the resource group, these three processes are allocated to class B due to the addition of chemicals, namely the hexane which is used in solvent extraction, phosphoric acid (H3PO4) is used for degumming process for mechanical expelling and solvent extraction, and sodium hydroxide is used for pH adjustment in EAEP. Though only small amounts of phosphoric acid are used in

Conclusions

From the results of environmental impacts, energy consumptions, GHG and criteria pollutants emissions, these prove that expelling is a clean approach for oil extraction with the lowest environmental impacts, but it generates the highest GHG and criteria pollutants emissions due to the energy intense heat pressing process. The hexane extraction is the most energy efficient and has the lowest GHG and criteria pollutants emissions, however, it has the highest environmental impact potential due to

Acknowledgements

The authors would like to thank USDA-NIFA for providing funding for this project. They would also like to thank Iowa State University for use of facilities and equipment.

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