Dilute Acid Hydrolysis
As dilute acids can be used for pretreatment of biomass, higher temperatures and longer exposure times to dilute acid can cause hydrolysis of glycosidic linkages producing monosaccharides. Sulfuric acid is the most commonly used acid and concentration below 4% is generally employed as it is comparatively inexpensive and helps to achieve high reaction rates. Dilute acid hydrolysis of biomass is by far the oldest technique used for converting biomass to fermentable sugars in the ethanol process. The first attempt at commercializing a process for ethanol production from lignocellulosic biomass was carried out in Germany in 1898, where they used wood chips. This process involved the use of dilute acid to hydrolyze the cellulose to glucose, and was able to produce 7.6 liters of ethanol per 100 kg of wood waste (18 gal per ton). Later they were able to optimize the process to yield 50 gallons per ton of biomass. During the period of World War I, two commercial plants were built in the United States based on a similar dilute acid hydrolysis technology. These plants used what was called "The American Process," which is a single-stage dilute sulfuric acid hydrolysis technology. Though the yields were lower than the original German process, the throughput of the American process was much higher. However, shortly after the end of World War I these plants were forced to close due to the drop in lumber production [20]. In the meantime, a small, but steady amount of research on dilute acid hydrolysis continued at the USDA’s Forest Products Laboratory. In the 1930s, an improved dilute sulfuric acid percolation process was developed in Germany, and this process is commonly known as the "Scholler process." In this technique, a solution of hot dilute sulfuric acid is pumped through a bed of wood chips. Later, a Scholler process-based plant was constructed in Springfield, Oregon, in the United States as well. During this time, Forest Products Laboratory in the United States put forward some improvements to the Scholler process [21], and their work resulted in the so-called "Madison wood sugar process," which showed substantial enhancement in productivity and yield over the Scholler process [22]. The Madison wood sugar process was further improved by the Tennessee Valley Authority’s (TVA) Wilson Dam facility. The TVA’s pilot plant studies led to a further refined process; producing a higher yield and simplifying mechanical aspects of the process, including the mechanics of dilute acid hydrolysis percolation reactor, culminating in the design developed in 1952, which is still one of the simplest means of producing sugars from biomass [23]. This 1952 design is considered the benchmark against which new biomass saccharification processes are often compared [23].