The process involves two-stage sulfuric acid hydrolysis, relatively low tem­perature, and a cellulose prehydrolysis treatment with concentrated acid. Figure 4.6 is a schematic flow diagram of the TVA process. Corn stover is ground and mixed with the dilute sulfuric acid (about 10% by weight). The

hemicellulose fraction of the stover is converted to pentose (xylose) sugars by heating the solution to 100°C for two hours in the first hydrolysis reactor. Raw corn stover contains, on a dry basis, about 40% cellulose, 25% hemicel­lulose, and 25% lignin. Sulfuric acid for the hydrolysis reaction is provided by recycling the product stream from the second hydrolysis step, which con­tains the sulfuric acid and hexose sugars. The pentose and hexose sugars, which are primarily xylose and glucose, respectively, are leached from the reactor with warm water. The sugar-rich leachate is then neutralized with lime (calcined limestone, CaO or calcined dolomite, CaO • MgO), filtered to remove precipitated materials, and fermented to produce ethanol [43].

Residue stover from the first hydrolysis step (hemicellulose conversion) is dewatered and prepared for the second hydrolysis step (cellulose con­version) by soaking (prehydrolysis treatment step) in sulfuric acid (about 20-30% concentration) from one to two hours. The residue is then screened, mechanically dewatered, and vacuum dried to increase the acid concentra­tion to 75-80% in the liquid phase before entering the cellulose reactor. The second hydrolysis reactor operates at 100°C and requires a time of four hours. The reactor product is filtered to remove solids (primarily lignin and unre­acted cellulose). Because the second hydrolysis reactor product stream con­tains about 10% acid, it is used in the first hydrolysis step to supply the acid required for hemicellulose hydrolysis. Residue from the reactor is washed to recover the remaining sulfuric acid and the sugar not removed in the filtra­tion step.

Lignin is the unreacted fraction of the feedstock that can be burned as a boiler fuel. It has a heating value of about 5,270 kcal/kg (or, 9,486 BTU/ lb), which is comparable to that of subbituminous coal. Other products such as surfactants, concrete plasticizers, and adhesives can also be made from lignin. Stillage can be used to produce several products, including methane. Preliminary research showed that 30 liters of biogas containing 60% meth­ane gas was produced from a liter of corn stover stillage. For each liter of ethanol produced, 10 liters of stillage were produced [42].

All process piping, vessels, and reactors in contact with corrosive sul­furic acid were made of fiberglass-reinforced vinyl ester resin. The dryer was made of carbon steel and lined with Kynar®, which is a trademark of Arkema Inc. (formerly Atofina) for poly(vinylidene fluoride), or PVDF. Conveyor belts were also made of acid-resistant material. Mild steel agi­tator shafts were coated with Kynar or Teflon®, which is DuPont’s trade­mark for polytetrafluoroethylene, or PTFE. Heat exchangers were made with CPVC (chlorinated poly(vinyl chloride)) pipe shells and Carpenter 20 stainless steel coils. Carpenter 20, also known as Alloy 20, is a nickel-iron — chromium austenitic alloy that was developed for maximum corrosion resistance to acid attack, in particular sulfuric acid attack. Pumps were made with nonmetallic compound Teflon lining, or Carpenter 20 stain­less steel. The two filter press units had plates made of polypropylene (PP) [42].