Ultrasonically Assisted Conversion of Lignocellulosic Biomass to Ethanol

In order to improve the bioconversion of lignocellulosic materials in an environmentally friendly manner, we choose to use the ultrasonic energy. Since enzymatic hydrolysis of cellulose represents a significant part of production costs, it is economically advantageous to develop a technique for reducing the cellulolitic enzyme loading. Thus, the sonication was employed in a two steps concept process: ultrasonically pretreatment and ultrasonically assisted hydrolysis of biomass. Our results show that a direct exposure of the lignocellulosic substrate to the 20 kHz high power ultrasound is in the main benefit of both pretreatment and ongoing hydrolysis. Introduction Hydrolysis of cellulose can be carried out by numerous factors of a chemical or physical nature based on the action of cellulolytic enzyme, various kind of chemicals or even steam. Enzymatic saccharification of cellulose has been employed since long time to product sugar and other products. Lignocellulosic materials are particularly attractive in this context because of their relatively low cost and plentiful supply. The main impediment to bioconversion of this important resource is the recalcitrance of cellulosic biomass. Current pretreatment processes are mostly chemically catalyzed [1].The chemically heterogeneous lignocellulosic biomass consist of high molecular weight structural components (cellulose, polyoses or hemicelluloses, and lignin), which are the major cell wall components; and of low molecular weight nonstructural components (extractives and inorganic compounds). Accessibility of the substrate to cellulase is a primary factor influencing the efficiency of enzymatic degradation of cellulose [2]. The efficiency of ultrasound in the processing of vegetal materials has been already proved [3]. The known ultrasounds benefits, such as swelling of vegetal cells and fragmentation due to the cavitational effect associated to the ultrasonic treatment, act by increasing the yield and by shortening of the extraction time. The effect of ultrasound on lignocellulosic biomass has been employed in order to improve the extractability of hemicelluloses [4], cellulose [5], lignin [6,7] or to get clean cellulosic fiber from used paper [8,9] but only few attempts to improve the susceptibility of lignocellulosic materials to biodegradation by using ultrasound power have been described. It was find out that ultrasound have a beneficial effect on saccharification processes [10]. Sonication have been reported to decrease cellulase requirements by 1/3 to 1/2 [11] and to increase ethanol production from mixed waste office paper by approximately 20% [12]. It was notice that the effect of ultrasound fragmentation of Avicel microcrystalline cellulose is similar to that of the enzymes for short incubation intervals [13].The time needed for ultrasonic treatment could be reduced when increasing the irradiation power [14]. In this work, the availability of the two approaches to enhance the cellulose hydrolysis, i.e. the pretreatment by ultrasonic irradiation of the substrate and ultrasonically assisted hydrolysis of the substrate are discussed. Results and Discussions Enzymatic bioconversion of cellulose from biomass involves the transfer of enzyme molecules from the processing liquid across the surface of the substrate and the mass transfer of the products to the bulk solution. When cavitation bubbles collapse near the surface of the solid substrate, they generate powerful shock waves. Two specific features of cavitation phenomena are especially important for bioconversion of lignocellulosic biomass, i.e. mass transfer intensification and an effective fine grinding together of deaglomeration. This is of great advantage since it has been reported that particle size distribution has significant implications for the rate of hydrolysis [15] Optimization of lignocellulosic substrate sonication. It is well known that the sonication processes are influenced by exposure parameters such as ultrasonic frequency and intensity, exposure time and temperature. In the case of vegetal materials this process is tremendously affected also by the shape of vessel and the ultrasonic device.