Kinetic Modelling of Dilute Acid Hydrolysis of Lignocellulosic Biomass

Abstract

Dilute Acid hydrolysis refers to the hydrolysis of hemicellulosic material by acids (typically sulphuric, hydrochloric or phosphoric acid) at concentrations of 1-10% using a moderate temperature (in the range of 100-1500C). But in these relatively moderate operational conditions, it proves less effective in the formation of hexoses1. This is mainly due to the decomposition of the monosaccharides into less desirable compounds during hydrolysis. These compounds include furfural, a product of dehydration of pentoses and hydroxymethilfurfural-HMF, a product of the dehydration of hexoses. These compounds along with acetic acid which forms during initial decomposition of the hemicelluloses, as a result of hydrolysis of acetyl groups linked to the sugar, inhibit the later fermentation, leading to reduced ethanol yields2. The production of these inhibitors increases when hydrolysis takes place at higher temperatures and higher acid concentrations3. Sulphuric and Hydrochloric acids are the most commonly used catalysts for hydrolysis of lignocellulosic residues. In contrast to these acids, phosphoric acid can be more advantageous for hydrolysis. Phosphoric acid is less aggressive than other acids which give solutions with higher concentrations of growth inhibitors of microorganisms such as furfural or acetic acid2. Dilute Phosphoric acid, on hydrolysates from sugar cane bagasse, has shown fermentable sugars with 21.4 g of sugar L-1 with less than 4 g L-1 of inhibitors at operating conditions of 6% acid concentration at 1000C for 300 mins4. Similarly on hydrolysates from olive tree pruning, have shown hemicelluloses conversion rates of 77% with glucose and reducing sugar concentrations being observed as 89% of the hemicellulosic sugars contained in the raw material at conditions of 8% acid concentration at 900C for 240 mins2. These hydrolysates obtained after the acid hydrolysis need to be processed if they are going to be used as fermentation media. In general the following operations are needed (in this sequence): concentration, detoxification, neutralization and supplementation with nutrients. This process is illustrated in Figure 1. The concentration of hydrolysates by evaporation is usual to increase the sugar concentration. In this operation, besides water, small amounts of growth inhibitors such as acetic acid, furfural and HMF are removed1. A detoxification operation by adsorption on active carbon in the form of charcoal can remove the growth inhibitors cited. In this operation, phenolic compounds proceeding from lignin can also be removed1. In the operation of neutralization, it is usual to add chemicals that neutralize the acids of the hydrolysates, forming salts5. These salts have low solubility and are normally removed by filtration. For example, hydrolysates containing sulphuric acid are neutralized with calcium carbonate, forming calcium sulphate1. Finally, the processed hydrolysates are supplemented with several nutrients to be a favorable fermentation medium. These nutrients contribute the nitrogen and micronutrients needed for the growth of the microorganisms6