In chemical hydrolysis, pretreatment and hydrolysis may be carried out in a single step. There are two basic types of acid hydrolysis processes commonly used: dilute acid and concentrated acid, each with variations.

Acid Hydrolysis

Acid-catalyzed process can be divided into two general approaches, based on concentrate acid/low temperature and dilute-acid/high temperature hydrolysis. Sulfuric acid is the common acid employed although, however, hydrochloric, nitric and trifluoracetic acids, phosphoric acid, weak organic acids have also been used.

Concentrated-Acid Hydrolysis

Concentrate acid processes enable the hydrolysis of both hemicelluloses and cellulose. The solubilization of polysaccharides is reached using different acid concentrations, like 72% H2SO4, 41% HCl or 100% TFA [45]. Concentrate-acid — based processes have the advantage to allow operating at low/medium tempera­tures leading to the reduction in the operational costs. Hydrolysis of cellulosic materials by concentrated sulphuric or hydrochloric acids is a relatively old pro­cess. The concentrated acid process uses relatively mild temperatures, and the only pressures involved are those created by pumping materials from vessel to vessel. Reaction times are typically much longer than for dilute acid. This method gen­erally uses concentrated sulphuric acid followed by a dilution with water to dis­solve and hydrolyze or convert the substrate into sugar and provides a complete and rapid conversion of cellulose into glucose and hemicelluloses into 5-carbon sugars with little degradation. The critical factors needed to make this process economically viable are to optimize sugar recovery and cost-effectively recovery of the acid for recycling. The solid residue from the first stage is dewatered and soaked in a 30-40% concentration of sulphuric acid for 1-4 h as a pre-cellulose hydrolysis step. The solution is again dewatered and dried, increasing the acid concentration to about 70%. After reacting in another vessel for 1-4 h at low temperatures, the contents are separated to recover the sugar and acid. The sugar/ acid solution from the second stage is recycled to the first stage to provide the acid for the first-stage hydrolysis. The primary advantage of the concentrated acid process is the potential for high sugar recovery efficiency. The acid and sugar are separated via ion exchange and then, acid is re-concentrated via multiple effect evaporators. The low temperatures and pressures employed allow the use of rel­atively low cost materials such as fiberglass tanks and piping. The low tempera­tures and pressures also minimize the degradation of sugars. Unfortunately, it is a relatively slow process and cost — effective acid recovery systems have been dif­ficult to develop. Without acid recovery, large quantities of lime must be used to neutralize the acid in the sugar solution. This neutralization forms large quantities of calcium sulfate, which requires disposal and creates additional expense. Moreover, the equipment corrosion is an additional disadvantage. Nevertheless, there seems to be a renewed interest in these processes [209] owing to the mod­erate operation temperatures and because no enzymes are required.