Theoretical Acid Pretreatment Analysis

Physical pretreatment involves comminution, pyrolysis or irradiation of the biomass. Comminution aims at mechanical size reduction by chipping, shredding, grinding or milling. These treatments increase the available specific surface area and reduce both the degree of polymerization and cellulose crystallinity. Milling and grinding are more effective in reduction of cellulose crystallinity and biomass size (0.2-2 mm) than chipping (10-30 mm) (Alvira et al 2010). Vibratory ball milling is more efficient than the ordinary milling process (Sun and Cheng, 2002). The energy requirements during comminution depend on the biomass characteristics and final desired particle size. However, higher energy consumptions in comminution compared to theoretical

It can be simply categorized into two types of pretreatments, i.e., dilute and concentrated. In concentrated acid hydrolysis method, biomass is treated with high concentrations of inorganic acids such as sulphuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3) at ambient temperatures (Taherzadeh and Karimi, 2008; Broduer et al., 2011). Concentrated acids are powerful carbohydrate hydrolyzing agents. But this method has several difficulties due to toxic, corrosive and hazardous nature of acids (Sun and Cheng, 2002; Kumar et al., 2009; Chaturvedi and Verma, 2013). The requirement of specialized corrosion resistant reactors increases the cost of pretreatment, which can be minimized by recovering acid after hydrolysis to make the process economical (Sun and Cheng, 2002; Kumar et al.,2009). Dilute acid pretreatment is preferred over concentrated acid treatment. It has been successfully utilized for pretreatment of a wide variety of lignocellulosic biomass. It results in quick hydrolysis of amorphous hemicellulose through the disruption of xylosidic bonds and cleavage of acetyl ester groups. The lignin seal is degraded through substitution reactions and broken links accompanied by condensation reactions that prevent dissolution (Bensah and Mensah, 2013). The cellulose is left intact in solid residue which becomes accessible to hydrolytic enzymes for their saccharification to glucose or it may undergo degradation in amorphous regions leading to enlarged cellulose fibrils and increased crystallinity index of the pretreated material. In dilute acid hydrolysis, biomass is subjected to acid treatment at concentrations lower than 4% (usually between 0.2 to 2.5%), at high temperature and pressure (Kumar et al., 2009). The reaction time may vary from minutes to hours depending on biomass composition as well as conditions used during treatment.