Integrated production of biodiesel and bioethanol from sweet potato

Highlights

• An integrated process for bioethanol and biodiesel production was studied.

• Bioethanol yield at the start-up of the pilot scale plant (3000 L/day) was monitored.

• Average yield of sweet potato ethanol was 161.4 L/t (149.2Min - 184.0Max).

• Around 75% biodiesel yield and purity was obtained integrating 99.4 °GL bioethanol.

• Absolute ethanol is required to reach a high quality ethylic biodiesel.

Abstract

In the present study, the production of sweet potato bioethanol was conducted at a small industrial plant in Brazil and integrated in biodiesel production through ethanolysis. The specific objectives were: (i) monitor bioethanol production at a small industrial plant, considering production yield; (ii) study the ethanolysis of sunflower oil using sweet potato bioethanol with different grades (98, 99 and 99.4 °GL), with focus on biodiesel yield and purity. Bioethanol was produced under the following conditions: Sweet potato dilution in water (1:1 in wt.%); enzymatic hydrolysis with alpha-amylase (90 °C) and glucoamylase (60 °C) enzymes (700 mL/t); and, fermentation (30 °C) with saccharomyces cerevisiae (3.3 kg/t). Ethanolysis was performed as follows: 6:1 ethanol:oil molar ratio; 1.0 wt% NaOH; 45 °C; 1 h. The average yield of bioethanol was 161.4 L/t, corresponding to 10 598 L/ha. Biodiesel synthesis using bioethanol of different grades showed that both the yield and purity of the product increased with the grade of the ethanol used, with values ranging from 63 to 83 wt%, and 50 to 94 wt% for yield and purity, respectively. Results show that a high alcohol grade is required to make viable integrated production of biodiesel and bioethanol.

Introduction

World population growth has practically doubled in the last five decades, increasing energy consumption due to the development of new technologies and processes, which, having brought great benefits to mankind, are based on a rather insecure and limited energy matrix, derived from fossil fuels (coal, oil and natural gas), which are mainly responsible for greenhouse gases (GHG). Such an energy model makes the planet progressively less healthy, so it is extremely important that the next generations encourage the use of renewable energies such as solar, wind, thermal, hydroelectric and biomass, among others. The use of biomass in the production of biofuels (bioethanol and biodiesel) can meet the energy needs in a sustainable manner, conserving biodiversity and becoming an alternative for small farming communities, which should be adapted to regional constraints and characteristics, namely through absence of competition with food sources and reduced land use impacts [1], [2], [3].

In 2015, for the first time in history, some developing countries (such as China, India and Brazil) invested more in renewable energy and biofuels than other developed economies, and 19% more than in 2014 [4]. The International Energy Agency (IEA) proposes to increase investments in renewable energy to reach US $ 400 billion by 2030 [5] as it is recognized that the promotion of renewable energy contributes decisively to an environmentally sustainable future [6]. Biofuels, including biodiesel and bioethanol, already play an important role in the current supply of renewable energy, and it is estimated that there will be a continued growth in the use of these biomass sources; in agreement, the demand for bioethanol may increase by 108% between 2015 and 2050 [3].

In 2015, world bioethanol production was approximately 98.3 Mm³, corresponding to about 74% of all biofuels produced, showing the potential and relevance of the production of this renewable energy source [4]. The bioethanol currently used is mainly produced from sugarcane in Brazil, corn in the United States and sugar beet in Europe. Recent studies indicate that sweet potato is a highly promising complementary alternative raw material for the production of this biofuel because of high yields of ethanol, between 5859 and 10 467 L/ha [7], [8], [9], [10], when compared to sugarcane, between 6281 and 7224 L/ha [11], [12], [13], and corn, between 1598 and 4250 L/ha [14], [15].

By 2014, the International Potato Center [16] refers to sweet potato [Ipomoea batatas (L.) Lam.] as the sixth most cultivated food crop on the planet, after rice, wheat, potatoes, maize and cassava. In 2016, the world production was 105 million tons, about 95% of which was originated in developing countries and Brazil ranked the 18th place, with approximately 0.5% of world production [17]; the northern region produced only around 1%, despite the high potential [18]. Sweet potato is a perennial crop of wide adaptation, easy growth and high resistance to drought, with low production cost when compared to other conventional crops, being resistant to pests and also contributing to soil erosion prevention. Taking into account the harvesting practices, it is very versatile, allowing mechanical processes adapted from existing technologies, or manual harvesting, creating local jobs, with high relevance for the country [10], [19].

The energy source contained in the sweet potato is in the form of starch; bioethanol production from starch involves 3 steps: hydrolysis (acid or enzymatic), fermentation and distillation [20]. The hydrolysis process causes the total unfolding of the amylose molecules, which break down and transform into dextrins, and finally into glucose. At the fermentation phase, the simple sugars are transformed into ethanol. To obtain high yields it is extremely relevant to optimize pre-treatments and fermentation conditions [21], [22], [23]. The bioethanol produced is separated from the wet residue by the distillation process to obtain the final ethanol product [10]; this step is responsible for the highest energy consumptions in the bioethanol production process [22] and dictates the final grade of the product.

The overall energy consumption and production in ethanol production depends upon raw material production and transport, bioethanol production (sugars obtention, fermentation and distillation) as well as final product energy content and associated yields [22]. The use of locally available raw materials has the potential to reduce transport costs, also affecting positively the energy balance.

Currently, almost all biodiesel produced worldwide is based on the transesterification process using the methylic route (methanolysis), due to low production costs and operational simplicity [24]. The kinetics of the methanolysis reaction is also usually referred to as faster compared to ethanolysis [25]. However, the use of methanol for the production of biodiesel is environmentally less favorable because it is mostly obtained from fossil resources. On the opposite side, ethanol has low toxicity and becomes more environmentally sustainable as it is easily obtained from renewable biomass sources [26].

In 2015, biodiesel was the second most produced biofuel in the world, with 30.1 Mm³, corresponding to 22% of the total biofuels produced [4]. Biodiesel is a mixture of esters that can be produced by various processes [27]. The most commonly used is transesterification, whereby triglyceride molecules react with an alcohol (methanol or ethanol), generating esters of fatty acids (biodiesel) and glycerol, usually using a homogeneous alkaline catalyst (NaOH, KOH and respective methoxides, between others). As already mentioned, the biodiesel production industries generally follow the methanolic route, due to the lower cost and operational simplicity [28]. Amongst the problems arising from the use of ethanol, the difficulty in products separation and purification is considered the most relevant [29] since it can lead to lower biodiesel yield and purity.

Several studies indicate ethanol as a promising alternative for the production of biodiesel [29], [30], [31], [32], [33], mainly from an environmental point of view, due to the replacement of methanol (from fossil derived resources) by a renewable energy source. Sweet potato is also indicated as a good substrate for the production of a more sustainable bioethanol [7], [9], [10], [21], [34], [35], [36]. Most of the existing studies focus on the laboratory production o biofuels, with little emphasis on the pilot and industrial scale productions, with related difficulties and developments. The development of small scale production plants has high relevance in remote regions of several countries, including Brazil (North, with large territorial area and rich biodiversity), adding value to regional raw materials and making processes more economically-viable, contributing to local sustainable development. In particular, for the production of bioethanol at a small scale, the purity of the product is a key factor, that strongly conditions the production cost. The integrated production of biodiesel and bioethanol is not a new topic [29], but in this study, the startup of a bioethanol small industrial plant using sweet potato was followed and the integration of bioethanol for biodiesel production was evaluated, considering different bioethanol grades, to assess technical integration constrains. Such approach has without doubt novelty, without similar studies being found in the literature.

The specific objectives were established as: (i) bioethanol production in the start-up phase of the process at a small industrial plant in Palmas (Brazil), with focus on production yield; and, (ii) evaluation of the ethanolysis of sunflower oil using sweet potato bioethanol with different grades (98, 99 and 99.4 °GL), with focus on biodiesel yield and purity.