Acid hydrolysis pretreatment method

Cellulase enzyme complexes consist of three major types of proteins that synergistically catalyze the breakdown of a cellulosic substrate. Because the enzymes are strictly substrate-specific in their action, any change in the structure or accessibility of the substrate can have a considerable influence on the course of the hydrolysis reaction. A pretreatment method based on exposing cellulosic substrate to phosphoric acid solution [9] and addition of the nonionic... 

In general, the high specificity of enzymes limits the application of this method, because for the treatment of each wastewater sample it is necessary to use a special consortium of various types of enzymes. Therefore, for practical reasons the acid hydrolysis of wastewater is the preferred laboratory method for the pretreatment of polymers. By the use of acid hydrolysis for one hour at 148°C with 6 mol/1 HCl ( acid hydrolyse kit from Dr. Lange GmbH Berlin 1996 [48]), the polymers are decomposed to their monomers such as monosaccharides, glycerol, fatty acids, or amino acids, respectively. The hydrolysis of wastewater caused in most cases an increase of the BOD measured (Table 6), resulting in an improved concurrence of sensorBOD and BOD5 [53,68]. 

Figure 1 shows the experimental procedure used to evaluate the hydrolysis methods of SFF. First, the SFF material was subjected to saccharification with amyloglucosidase to determine the amount of starch available in the material. Second, the material was subjected to pretreatment in either a steam pretreatment unit or a microwave oven, followed by enzymatic hydrolysis. Pretreatment in a microwave oven was performed both with and without the addition of acid. Direct enzymatic hydrolysis was also performed. 

Besides two different hydrolysis methods (i.e., acid hydrolysis and pretreatment without the addition of acid), two different pieces of pretreatment equipment were used to perform the experiments (Fig. 4). Acid hydrolysis was conducted in a microwave oven, while pretreatment was performed in a steam pretreatment unit. The microwave oven provides a closed system where the amount of water added is fixed and there is no loss of material during the process (17-18). On the other hand, the sampleshave to be rather diluted for the microwave oven to be efficient. Another disadvantage is that the microwaves penetrate the material only a few center-meters, and therefore this method is not feasible on a large scale. The microwave oven may, however, still be of interest in the laboratory as a screening method to analyze the composition of feedstock as well as to determine a range of optimal conditions for steam pretreatment. 

In this project, we will use acid hydrolysis as the pretreatment method. The milled wood chips are heated to about 100 °C using low-pressure steam. Then, steam and sulfuric acid are added to the mixture in a pretreatment reactor. The reactor temperature is 190 °C, while the pressure is 12.2 atm. The concentration of acid is 0.5% (weight), while the concentration of solids in the outlet stream is 22%. 

Enzymatic Hydrolysis. Saccharification of wood polysaccharides to sugars can be accomplished by enzymatic techniques instead of acid hydrolysis. The U.S. Army Natick Laboratories developed a method for conversion of cellulose to glucose with a cellulose enzyme from an active strain of the fungus Trichoderma viride. However, extensive pretreatment of wood is necessary before sufficient enzymatic hydrolysis will take place. 

The hydrolysis of polysaccharides into monomeric sugars is a well-studied process its mechanism is still believed to be accurately described by the work of Saeman in 1945.432 Its significance is linked to the production of fermentable sugars for ethanol manufacture. Many of these processes are based on acid catalysis and overlap closely the pretreatment methods described earlier.363,364,373 Treatments with both concentrated and dilute acids are well known as methods for the hydrolysis of cellulose and hemicellulose, as is the use of organic dicarboxylic acids as alternative catalysts to mineral acids.433... 

The effectiveness of dilute acid hydrolysis as a pretreatment has been verified experimentally. Researchers at NREL have characterized the susceptibility of a variety of short rotation woody and herbaceous crops and agricultural residues upon dilute acid pretreatment [17,38-41]. What is more interesting is that the maximum digestibility usually coincides with complete hemicellulose removal. The dilute acid treatment of biomass aimed at hemicellulose hydrolysis has since become a widely accepted pretreatment method for enzymatic hydrolysis [16,42-44]. 

Com stover used for this study was harvested in 2003 at the Kramer Farm in Wray, Colorado. The stover was pretreated either in-house at the National Renewable Energy Laboratory or received via subcontract fiom the CAFI [12] pretreatment group members. The samples selected for this study were pietreated by alkaline peroxide (NREL), sulfite steam explosion (UBC), ammonia fiber explosion (MSU), and dilute sulfuric acid (NREL) methods. The composition of the pretreated stover was determined by a two-stage sulfuric acid hydrolysis treatment according to the NREL Laboratory Analytical Procedure titled Determination of Stmctural Carbohydrates and Lignin in Biomass [13]. The pretreatment conditions and compositional information for each substrate are listed in Table 2. 

However, not all of these methods are desired for pretreatment of the lignocellulosic materials because of technical or economic feasibility. In some cases, a mediod is used to increase the efficiency of another method. For instance, milling could be applied to create a better steam explosion by reducing the chip size (8). Furthermore, it should be noticed that the selection of pretreatment method should be compatible widi the selection of hydrolysis. For example, if acid hydrolysis is to be applied, a pretreatment with alkali may not be beneficial. The most commonly applied pretreatment methods will be discussed here. 

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