Elsevier

Fuel Processing Technology

Volume 138, October 2015, Pages 344-354
Fuel Processing Technology

Research article
Optimization of DIC technology as a pretreatment stage for enzymatic saccharification of Retama raetam

https://doi.org/10.1016/j.fuproc.2015.06.017Get rights and content

Highlights

  • DIC pretreatment is based on a specific thermomechanical effect on lignocellulose.

  • DIC causes higher amount of expanding gas resulting in more porous solid material.

  • The highest hexose yields were obtained with lower pressure and lower water.

  • DIC maximizes the sugar releasing and reduce its degradation into unwanted compounds.

Abstract

In this work, we propose a new technology for the pretreatment and the conversion of Retama raetam biomass into oligosaccharides and glucose based on Instant Controlled Pressure Drop (DIC). DIC technology is a thermo-mechanical method based on short-time/high-temperature and pressure pretreatment that avoids the thermal degradation of sugars.

The main objective of this work is to prove the feasibility of DIC as a pretreatment method of R. raetam biomass. Response Surface Methodology was used through a design of experiments in order to optimize the processing parameters, maximize the generation of sugars and minimize their degradation yields. The obtained results confirm that R. raetam pretreatment using DIC has a significantly positive effect on the generated monomeric sugar yields. Our experiments show that the highest hexose yields were obtained with lower pressure and water content. Moreover, these conditions minimized the degradation of sugars into ethanol conversion inhibitors. Hereby, the maximum glucose yield obtained after the enzymatic hydrolysis of the DIC treated R. raetam was 18 mg/g dry matter.

Introduction

Due to the increasing fuel demand, fossil fuel depletion and environmental pollution by greenhouse gas emissions, several research efforts focus on proposing alternative ways to produce transportation fuels [1], [2]. Lignocellulosic biomass is gaining increasing interest in the industrial and research fields. In fact, it can be used via integral biorefinery as a renewable feedstock for the coproduction of transportation fuels, materials, energy and chemicals [3]. The integral biorefinery concept consists in the fractionation of the lignocellulosic biomass into its three main compounds (i.e. cellulose, hemicelluloses and lignin) that can be converted into a variety of energies and chemical products [4], [5]. The cost effectiveness of lignocellulose biorefinery may be improved by developing the applications of lignin as phenolics, fuel additives or in electricity and heat co-generation [4], [5].

The conversion process of lignocellulosic biomass into cellulosic ethanol involves four main steps which are (1) pre-treatment, (2) acid or enzymatic hydrolysis, (3) fermentation and (4) distillation [6]. However, due to the resistance of the lignocellulosic structure, several feedstock pretreatment methods are proposed in order to break down ultracellular components and cell wall structures and make biomass polysaccharides (cellulose and hemicelluloses) available for enzymatic attack [7].

An efficient lignocellulosic biomass pretreatment should be able to [8]:

  • Remove or redistribute hemicellulose and lignin.

  • Disrupt the ultrastructure of cellulose.

  • Open the lignin and hemicellulose matrix encapsulating cellulose while increasing the proportion of enzyme accessible surface area.

  • Limit sugar and lignin degradation into undesirable molecules (furfural, hydroxymethyl furfural, aliphatic acids) that inhibit microorganism fermentation into ethanol.

  • Reduce both energy consumption and costs [8].

Several physical, biological, chemical and physical–chemical methods have been proposed and studied as pretreatments capable of disrupting the cell wall structure of lignocellulosic biomass.

Physical pretreatments (e.g. grinding, milling, or chipping) aim at breaking cellulose's crystallinity and reducing its polymerization degree [9] in order to improve the enzymatic hydrolysis of lignocellulosic feedstocks. However, this type of pretreatment becomes economically unfeasible in large scale productions [9].

Biological pretreatment processes use micro-organisms in order to degrade lignin via extracellular enzymes (e.g. peroxidases and lactases). These pretreatment methods are inexpensive, but their hydrolysis yields are low compared to other technologies [10].

Chemical pretreatments utilize acidic, alkaline, ozone, organosolv and ionic liquids. Concentrated acid and dilute acid pretreatments are less attractive because they can produce inhibitory molecules (e.g. furfural, HMF and phenolic acids) which can limit both enzymatic hydrolysis and fermentation and cause corrosion problems on the equipment [11].

Alkaline pretreatments have shown a great potential in dissolving lignin and increasing cellulose digestibility using alkaline catalysts (e.g. sodium, potassium, calcium and ammonium hydroxides) [12]. However, in these processes, the alkali is converted into irrecoverable salts in the biomass during the pretreatment reactions [13].

The organosolv pretreatment process is performed with organic solvents (e.g. ethanol, methanol, acetone and ethylene glycol) in order to extract lignin from lignocellulosic biomass. Although the produced lignin is of a relatively high quality, this pretreatment requires the removal and the recovery of solvent in order to reduce its costs and environmental impact [14].

The physical–chemical pretreatment processes include several methods such as steam explosion, liquid hot water, ammonia fiber explosion, CO2 explosion, and oxidation delignification.

Steam explosion (SE) is a thermo-hydro-mechanical method considered as a promising pretreatment capable of de-lignifying cellulose materials [15]. The objective of steam explosion (SE), as a pretreatment of lignocellulosic biomass, is to intensify cellulose hydrolysis into fermentable sugars. However, this highly severe treatment leads to sugar degradation into specific molecules inhibiting ethanol conversion (e.g. acetic acid and 5-hydroxymethylfurfural). It implies subjecting the biomass to high pressure and temperature for a short time (i.e. dozens of seconds or minutes), followed by a rapid release toward atmospheric pressure. In the presence of water, this process can contribute to some transformation of polysaccharides into low-molecular-weight and water-soluble products [16], [17]. The vapor generated from the decompression allows the texturing of biomass structure (i.e. substantial breakdown of the lignocellulosic structure), possibly leading to cell wall breaking, hydrolysis of the hemicellulosic fraction and depolymerization of the lignin components which results in higher glucose and xylose yields after enzymatic hydrolysis of pretreated biomass [18].

Steam explosion is a high energy consuming process since it uses high temperature (~ 200 °C) and pressure (up to 3 MPa) [19]. Moreover, it leads to an important thermal degradation of sugars that inhibits microorganism growth and ethanol production.

In this work, we propose a new pretreatment method, named Instant Controlled Pressure Drop (DIC), that has been developed by Allaf and collaborators at the University of Technology of Compiegne in 1987 [20]. The DIC method is comparable to SE since they are mainly thermo-mechanical pretreatments. In fact, DIC is based on subjecting raw material to steam pressure (up to 0.6 MPa) at high temperature (up to 160 °C) for a short time (a few seconds to 1 min) followed by an abrupt pressure drop [21]. It has been very effective in several industrial fields such as microbiological decontamination, drying and texturing of biological products [22], [23] and the extraction of volatile compounds and essential oils [24], [25].

The DIC process involves first a heat treatment of materials inside the DIC reactor. Then, the pressure is abruptly released (DP/Dt > 1 MPa/s) toward the vacuum (about 5 kPa). The sudden pressure drop causes a rapid cooling of the treated material, an auto-evaporation of the water it contains causing the stopping of sugars' thermal degradation. The resulting biomass expansion and texture change help in breaking cells' walls and make its compounds more accessible [26], [27].

In this work, our main goal is to prove the feasibility, the relevancy and the effectiveness of DIC pretreatment of Retama raetam. DIC pretreatment was carried out according to an adequate experimental design in order to infer the operating parameters that maximize sugar yields while minimizing degradation product yields. We believe that this technology is promising thanks to its lower energy consumption, greater processing kinetics and lower operation cost.

Section snippets

Raw material

Spontaneous R. raetam was harvested in its native ecosystem from the sandy coastal zone of Borj Cedria (N 36 42 34, E 10 25 33) in Tunisia. The raw material was air dried by exposing it to sunlight for two days and then milled in a hammer mill in order to obtain a particle size of 3 mm. Afterwards, it was post-dried at 45 °C for 12 h in an oven and stored in a plastic container at room conditions. The material's initial moisture content was between 10% and 25%. The utilized R. raetam contained

Results

The determination of the yields of the two families of fermentable sugar molecules and fermentation inhibitor molecules was carried out in both liquid and solid fractions. The sugar molecules we considered were arabinose, glucose, fructose and mannose, while the degradation products were furfural, HMF, levulinic acid, acetic acid and formic acid (Table 3).

Discussion

Based on the experimental design results, the maximum glucose and fructose yields in the liquid fraction were respectively 1.68 and 2.18 mg/g db. Moreover, sugars' total yield was 4.07 mg/g db. These results were obtained at 0.5 M of acid content, 0.3 MPa of steam pressure and low level of water content (100%, 138.5%, 123% db) for 9 min. The results showed that the maximum sugar yields are obtained with low steam pressure during DIC pretreatment. These results can be explained by the fact that steam

Conclusion

This research proposed, on the one hand, a new lignocellulosic biomass from R. raetam that can compete favorably with other conventional sources for bioethanol production. On the other hand, we proposed a new technology for the pretreatment of lignocellulosic biomass, namely “Instant Controlled Pressure Drop”, capable of breaking down the cell walls in order to enhance the accessibility of their polysaccharides to enzymatic attack. A Response Surface Methodology was applied following an

Acknowledgments

The authors would like to thank the financial support provided by the Engineering Procurement & Project Management (EPPM).

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