Production of biolubricants from soybean oil: Studies for an integrated process with the current biodiesel industry

https://doi.org/10.1016/j.cherd.2020.11.012Get rights and content

Highlights

  • Potential biolubricants were synthesized from soybean fatty methyl esters.

  • Water, 2-ethylhexanol and their mixtures were studied as nucleophilic agent.

  • Bio-based lubricants were obtained with viscosities at 40 °C ranging from 26.6 to 99.6 cSt.

  • Process integration between a biodiesel plant and biolubricant production was evaluated.

  • An industrial process was proposed to obtain both biodiesel and biolubricants.

Abstract

The competitiveness of the biodiesel industry may be improved by adding value through co-products and integration with the oleochemical industry, especially for novel products such as biolubricants, a product of increasing world demand. In this study the synthesis of biolubricants from soybean oil was evaluated using transesterification, epoxidation and oxirane ring opening reactions. Water, 2-ethylhexanol and their mixtures were used to obtain hydroxyl-rich and/or ether-type branched molecules. All chemical modifications were monitored by Nuclear Magnetic Resonance (1H NMR) and evaluated through the physicochemical properties of the products. Several potential biolubricant samples were synthesized with viscosities at 40 °C ranging from 26.6 to 99.6 cSt, viscosity index from 26 to 139, densities at 20 °C from 0.925 to 0.964 g/cm3, and pour points from −3 to −12 °C. From these results, a proposal of a feasible industrial process for the production of biolubricants from soybean oil is presented, consisting of 16 units, of which 15 may be integrated with an existing biodiesel plant.

Introduction

The global biodiesel production exceeded 33 million tones, with hundreds of thousands of tons of vegetable oil, non-edible oil and animal fat used in this industry as raw materials, generating more than US $ 15 billion per year (ANP – Agência Nacional do Petróleo, Gás Natural e Biocombustíveis, 2019). Studies have shown that the biodiesel value chain emits 70% less greenhouse gases into the same energy unit compared to the fossil gasoil productive chain. It is equivalent to planting hundreds of millions of trees (MAPA, 2015; UBRABIO, 2014). Also, it has been reported a direct relationship between the average life expectancy of an urban population and the content of particulate matter in the air, especially those emitted by the burning of fossil fuels (Saldiva, 2015).

However, even after more than 15 years of development of the biodiesel industry in several countries, the production cost of this biofuel is still higher than mineral diesel, with quality specifications that reduce the possibility of differentiation in value. Also the biodiesel plants have high idleness and the glycerol co-product has depreciated prices due to oversupply. These characteristics result in the permanent need to support the existence of this sector with mandatory public policies for biodiesel blending in mineral diesel oil, as well as tax incentives. The motivation of this study was based on the fact that the competitiveness of the biodiesel industry and the enhancement of its positive socio-environmental externalities will be achieved not only by the development of low-cost raw materials such as moringa, jatropha, carmelina and others (Ntaribi and Paul, 2018; Rashid et al., 2011), but also especially by the value aggregation of oleochemical co-products that may be obtained in an integrated way with the biodiesel production (Oliveira et al., 2020; Dimian et al., 2019; Interrante et al., 2018; Casas et al., 2011; Brehmer et al., 2009).

Among potential co-products of the oleochemical industry, bio-based lubricants are highly added-value products with increasing demand. Commonly, the basestock oils used in lubricants formulations are hydrocarbons obtained from crude oil processing, predominantly containing molecules of high molecular weight (between 20 and 50 carbon atoms), providing high viscosity, high lubricity and low volatility, important properties that are directly related to the formation of a film that will protect metallic surfaces (Borugadda and Goud, 2016).

The performance of formulated lubricants is evaluated by their ability to reduce friction, resist oxidation, minimize the formation of deposits, and prevent corrosion and wear. Frequently the use of chemical additives is carefully formulated to improve their physicochemical and tribological properties (Koh et al., 2014). Additives, in addition to being more expensive than the basestock oil, generally contain sulfur, chlorine, phosphorus and other undesired elements. Their use should therefore be minimized by developing better performing lubricating basestock oil (da Silva et al., 2015d).

On the other hand, mineral lubricants can negatively impact the environment and human health when handled or improperly disposed. The current lubricant market is in the order of 37 million tons annually, of which about one third are inappropriately discarded to the environment (Kerman et al., 2011; Luna et al., 2016). Salimon et al. (2012a) suppose that about 50% of all the lubricant sold in the world ends up in the environment because of loss by volatility, leaks, accidents or full loss applications. These findings, allied to increasing population awareness, government guidelines and regulations, and economic incentives for more responsible activities, have been motivating studies for the development of new synthetic and bio-based lubricants (Salih et al., 2017; Koh et al., 2014; da Silva et al., 2015d; Borugadda and Goud, 2016).

The use of vegetable oils and animal fats in natura (soybean, castor bean, palm, rapeseed, sunflower, beef tallow) as lubricants is not recent (Soufi et al., 2015). They have good lubricating properties, such as high viscosity index, flash point and lubricity, low volatility and toxicity combined with their high biodegradability (Cavalcante et al., 2018; Luna et al., 2015, 2011). However, vegetable oils usually have low oxidative or hydrolytic stability, being very susceptible to degradation by the presence of oxygen and the formation of insoluble deposits and acidic substances. Another disadvantage of vegetable oils for lubricating applications is generally related to the presence of unsaturations in their molecules and to the presence of the β-CH group in its combined glycerol present in the triglyceride molecules. Double bonds are especially reactive with air oxygen, leading to the break of the ester molecules into olefins and acids (Salimon et al., 2012a, b; Koh et al., 2014; Borugadda and Goud, 2016; da Silva et al., 2015d; Soufi et al., 2015; Salih et al., 2017; Luna et al., 2019).

Previous studies have also reported an inadequate behavior of vegetable oils in cold applications due to their higher propensity to crystallization and freezing compared to mineral oils. In this way, chemical modifications in the molecular structure of vegetable oils have been extensively studied in order to potentiate their good properties and correct these weaknesses of their application as lubricants (Salimon et al., 2012a, b; da Silva et al., 2015d; Borugadda and Goud, 2016; Silva et al., 2015; Soufi et al., 2015; Saboya et al., 2017a, b; Salih et al., 2017).

Bio-based products from vegetable oil may be synthesized using branched alcohol molecules in order to remove the β-hydrogen and to reduce the polarity of the carboxylic group, with effect both on the oxidative stability increase and pour point reduction, respectively (Oliveira et al., 2020). In addition, the insertion of hydroxyl groups and molecular branches in polyunsaturated fatty acids causes an increase of the viscosity index and lubricity of the final products (Salimon et al., 2012a; Oh et al., 2013 and Salih et al., 2017). A possible physicochemical mechanism to further increase the viscosity is a combined effect of increasing its molecular weight and polarity in order to increase intermolecular forces. Considering that the double bonds of the soybean fatty acids may be converted into epoxy groups, epoxidation reactions of these double bonds may be carried out to obtain oxirane rings. After this, the oxirane rings may be opened by different types of nucleophiles forming branches and hydroxyl groups (Leveneur et al., 2014; Sinadinović-Fišer et al., 2012).

The use of soybean oil as the feedstock of this study was motivated by two main reasons. First, soybean oil is among the main raw materials of the world biodiesel industry (especially in the US, Brazil and Argentine) and this study has the intent to propose an integrated processing scheme for both biodiesel and biolubricants. Besides, soybean oil fatty acids composition is rich in unsaturated molecules (>80 % wt. of unsaturated fatty acids, Galão et al., 2014), which increases significantly the possible combinations of obtaining distinct molecules by varying the nucleophilic agent in the step of the oxirane ring opening, compared to other conventional vegetable oils.

In this way, this study proposes to develop several potential biolubricant samples from soybean oil that could be obtained in an integrated way with the biodiesel processing industry, through transesterification, epoxidation and oxirane ring opening reactions of methyl esters. Water, 2-ethylhexanol and their mixtures were evaluated as nucleophilic agents in the oxirane ring opening reaction for the purpose of producing hydroxyl-rich molecules or ether-type branching, respectively. The effect of these chemical modifications on the physicochemical properties of the obtained biolubricant samples was evaluated. Finally, this study also proposes a possible industrial process for biolubricant production and evaluates its integration with the existing biodiesel processing plants, seeking to improve the competitiveness of this industrial sector.

Section snippets

Materials

The raw materials used to obtain biolubricant samples were fatty acid methyl esters from refined soybean oil (Soya, Brazil). The composition of refined soybean oil is reported in Table 1. Methanol (99.8 % wt.) and anhydrous sodium sulfate (>99.5 % wt.) were supplied by Vetec (Brazil), hydrogen peroxide 30 %(aq) was kindly provided by Peróxidos do Brasil (Brazil), sodium methylate (>99.9 % wt.) was supplied by Evonik Degussa (Brazil) and 2-ethylhexanol (99.5 % wt.) was provided by Elekeiroz

Strategies for Process Development

A Process Flowchart was proposed for each stage of the biolubricant production process used in this study: transesterification of soybean FAME, epoxidation, and opening of the oxirane ring using water, 2-ethylhexanol or a mixture of both nucleophilic agents (Fig. 1).

The proposal of an industrial process for the production of biolubricants followed some considerations:

  • configuration of processes, equipment and units that allow integration with the existing industrial process of biodiesel;

Synthesis of new biolubricant samples

The properties of the first proposed biolubricant samples (BL1 - BL4) are shown in Table 3. As described in the experimental section, all these samples were obtained following the same first steps (methanolysis, transesterification with 2-ethylhexanol, epoxidation). The different samples were obtained by varying the oxirane ring opening step. It may be seen that the sample obtained with water as the oxirane ring opening agent (BL2) showed higher viscosities and higher pour point than the sample

Conclusions

The synthesis of bio-based lubricants from the soybean oil was evaluated by a chemical route via epoxidation and oxirane ring opening reactions using different nucleophilic agents. Water, 2-ethylhexanol and their mixtures were tested for the purpose of producing hydroxyl-rich molecules or ether-type branching. Thus, by varying the water / 2-ethylhexanol concentration at the opening of the oxirane ring and its stoichiometric excesses, several biolubricant samples from soybean oil were

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

The authors wish to thank financial support provided by CAPES (Coordenação de Aperfeiçoamento do Pessoal do Ensino Superior),FUNCAP (Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico). Coremal S.A. and Peróxidos do Brasil S.A. are also acknowledged for the donation of hydrogen peroxide volumes. The authors are also thankful to CENAUREMN (Centro Nordestino de Aplicação e Uso da Ressonância Magnética

References (35)

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