Concentrated Acid Hydrolysis
Among all cellulose hydrolysis methods, the use of concentrated acid for producing sugars is the oldest chemical approach known. The dissolution of pure cotton, which is mainly cellulose, in concentrated sulfuric acid, and then hydrolysis after dilution with water was reported in the literature as early as 1883 [3]. Concentrated sulfuric or hydrochloric acid was the acid used in many of these early experiments and cellulosic ethanol plants. Some of the earliest commercial cellulosic ethanol plants using concentrated acid were built in Germany around 1937. They were based on the use and recovery of hydrochloric acid, and several such facilities were successfully operated in this period. During World War II, researchers at the USDA’s Northern Regional Research Laboratory in Peoria, Illinois, further refined the concentrated acid process, where they used sulfuric acid and abundant agricultural waste corncobs as the biomass. Hydrolysis of corncobs produced a large fraction of C5 sugars for which they developed a continuous process that yielded a 15%-20% xylose sugar stream and a 10%-12% glucose sugar stream, with the lignin residue remaining as a byproduct. The glucose was readily fermented to ethanol at 85%-90% of theoretical yield. In 1948 Japanese scientists also developed a concentrated sulfuric acid process that went into large-scale commercial production. The exceptional feature of the Japanese acid process was the use of membrane technology for separation of sugars and acid in the product stream, where they could recover 80% of the acid for reuse [4]. After a period of lukewarm interest, cellulosic ethanol research picked up in the mid 1970s. During this period the old American process known as the "Peoria Process" was further developed in the United States by researchers at Purdue University in Indiana [5] and at the Tennessee Valley Authority (TVA) [6]. Among the improvements added by these researchers were recycling of dilute acid from the hydrolysis step and minimizing the use of sulfuric acid, which are critical factors in the development of an economically feasible strong acid process.