Pretreatment by using dilute-acid processes for the hydrolysis of hemicellulose renders the cellulose fraction more amenable for a further enzymatic treatment, but in this case a two-step-hydrolysis is required. The dilute acid process is conducted under high temperature and pressure, and has a reaction time in the range of seconds or minutes, which facilitates continuous processing. The difference between these two steps is mainly the operational temperature, which is high in the second step (generally around 230-240°C) [108, 196, 197]. Example cited by using a dilute acid process with 1% sulfuric acid in a continuous flow reactor at a residence time of 0.22 min and a temperature of 510 K with pure cellulose pro­vided a yield of over 50% sugars. In this case, 1,000 kg of dry wood would yield about 164 kg of pure ethanol. The biggest advantage of dilute acid processes is their fast rate of reaction, which facilitates continuous processing.

Compared to the concentrate acid hydrolysis, one of the advantages of dilute — acid hydrolysis is the relatively low acid consumption, limited problem associated with equipment corrosion, and less energy demanding for acid recovery. Under controlled conditions, the levels of the degradation compounds generated can also be low. As an alternative to the conventional dilute-acid processes, the addition of CO2 to aqueous solutions, taking advantage of the carbonic acid formation has been described [190], but the results obtained were not interesting enough to consider application.