A novel process intensification strategy for second-generation ethanol production from sugarcane bagasse in fluidized bed reactor
Graphical abstract
Introduction
Regarding the environmental and political problems related to the imminent scarcity of fossil fuels, biofuels has received great attention in the global scenario. Several agro-industrial residues such as bagasses, grains, husks, straws and others are used aiming ethanol production [1]. Amongst them, sugarcane bagasse (SCB) has shown higher advantages due to its large availability in countries such as Brazil, India, China and others [2]. However, SCB, like any other lignocellulosic materials presents recalcitrant structure, difficulting the access of enzymes and microorganisms to the carbohydrate fractions of plant cell wall. Thus, for second generation (2G) ethanol process, from lignocellulosic materials, the biomass pretreatment in order to release fermentable sugars is commonly the first step, being considered as one of main bottleneck along the total process [3]. Several studies dealing with pretreatment such as chemical (alkaline, acid, organosolv, ionic liquid), physical (steam explosion, ultrasound, extrusion, microwave, supercritical fluids, hydrodynamic cavitation), and biological methods (enzymatic) have already been studied [4]. However, process feasibility, easily scale-up and less production costs are major challenges for sustainable 2G ethanol production. Among different pre-treatments, alkaline hydrolysis is considered a promising method due to efficient lignin removal, changes in the structure of biomass and hence, improving the accessibility of enzymes by increasing the porosity of substrates in addition to the reduction in the crystallinity of biomass [5]. However, new strategies to enhance this pre-treatment efficiency are required to be investigated, e.g., approaches exploring novel process configurations, such as column reactor in fluidized bed reactor operation, as well as reuse of remaining solubilized liquid black liquor (alkaline lignin solution). Literally, the use and study of column reactors for lignocelluloses alkaline pretreatment process is scarcely reported in scientific reports. Indeed, fluidized bed reactors (FBR) are an interesting and feasible approach for chemical and biological process that requires high homogenization. FBR when compared with the stirred tank reactor (STR) offers better mass transfer for more effective pretreatment and subsequently combining hydrolysis and fermentation process in one reaction, without causing problems with undesired shear stresses, as FBR does not use stirrers [6]. The most conventional type of reactor used in chemical as well as in biological processes is the stirred tank reactor (STR) [7]. In STR, agitation is usually provided mechanically by stirrers in addition to insertion of oxygen from outside. Although, this kind of reactor is largely applied in different process, however it presents limitations such as complex and costly construction, mainly due to the requirement to move an agitator shaft. Also, in some process composed by solid and liquid phases as well as in highly non-Newtonian process fluids, high running costs due to continuous mechanical agitation [8] are required. These factors pose a big concern while developing scale-up operation. In FBR, hydrodynamic parameters can be efficiently kept under larger scale (compared to the use with some other reactors configurations) due to the easily implementation of similar conditions, e.g., volume of air per volume of medium, solid load relation, homogenization, and others [6], [8].
Besides chemical pretreatment, FBR can also be used in enzymatic and fermentative process such as Separated Hydrolysis and Fermentation (SHF), Simultaneous Saccharification and Fermentation (SSF). The SHF is conducted in two steps under different conditions first via hydrolysis, aimed to release fermentable sugars, followed by fermentation process of sugar conversion into bioproduct [9]. On the other hand, SSF or Simultaneous Saccharification and co-Fermentation (SSCF) operation is driven through one single step, as in this method hydrolysis and fermentation occur under the same conditions in one reactor, with addition of enzymes and microorganism (s) in the same reaction medium. Important advantages have been described for SSF process, such as higher yields due to low residual sugar relieves inhibition on some enzymes [10]. Also, consumption of glucose and the presence of ethanol in the culture medium could reduce the risk of undesired contamination by glucose-dependent organisms [11]. To the best of our knowledge, SSF process in fluidized bed reactor have not already been reported yet. Taking this into account, sequential development of all three steps i.e. pretreatment (chemical), hydrolysis (enzymatic) and fermentation were performed in a same reactor so called process intensification. The term “process intensification” is defined based on productive unit operations, which can be performed in smaller production plants, achieving a certain production objective, with a reduction in capital costs, due to the reduction of the number of reactors and the integration of different stages of production [12]. Process intensification could be an interesting and promising approach for cost effective ethanol production in FBR.
Within this context, this work was aimed using FBR as a strategy to process intensification of ethanol 2G production, involving sequentially chemical pretreatment of biomass, enzymatic hydrolysis and fermentation in the same reactor.
Section snippets
Sugarcane bagasse
Sugarcane bagasse was kindly donated by Usina São Francisco (Sertãozinho-SP, Brazil), previously characterized in other work which showed 40.0% of cellulose, 26.0% of hemicellulose and 24.1% of lignin [13]. The particle size used in this study was between 1.18 and 1.70 mm, and the classification of this fraction was using a standard Tyler sieves.
Fluidized bed reactor
A column reactor (Bioengineering AG, PID Fermenter AWS, Wald, Switzerland) with 2 L of volume work (540 mm × 55 mm column, with central vertical tube
Alkaline pretreatment of sugarcane bagasse in FBR reactor
Fluidized bed reactors (FBRs) offer the possibility to achieve high homogenization in process with solid/liquid contents, without use of mechanical impellers, that require high energy demand and thus increasing the operations costs. FBRs also provide the unique possibilities of scale-up due to the simple geometric configuration to perform different process applications in this reactor. Conceptualizing this fact, we performed here biomass pre-treatment, hydrolysis and fermentation in sequential
Conclusions
In this study, we evaluated first time the use of fluidized bed reactor to develop an intensified process for second generation ethanol production from sugarcane bagasse. For the alkaline delignification of bagasse, pretreatment In the alkaline process, a 22 full factorial design of experiments showed that 0.5 M of NaOH concentration, and 4 h were major influential parameters for the maximum glucose and xylose yield of around 87% and 44%, respectively, in saccharification step. Aiming to
Acknowledgement
The authors would like to thank the Research Council for the State of São Paulo (FAPESP) (Award Number 2014/27055-2), Brazilian National Council for Scientific and Technological Development and Brazilian Federal Agency for the Support and Evaluation of Graduate Education for financial support.
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