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Water for Energy and Fuel Production
of reactions followed by a slow set of reactions. Both steps generate a large number
of intermediates. These intermediates produce tars via the second-order condensation reactions along with the main products (hydroxymethylfurfural [HMF] in the
first stage and LA and formic acid in the second stage). Short reactor residence time
is important for the first rapid kinetic step and good mixing is important for the
kinetically controlled second reaction.
The detailed flow diagram for the entire process is illustrated in Figure 7.4 [12].
The process can use a variety of biomass such as wood and crops; cellulosic wastes
such as waste paper, cellulosic sludges, and agriculture residues; and carbohydrates
such as starch and molasses. The solid feedstock is shredded to reduce its particle
size to 0.5–1.0 cm. This ensures an efficient hydrolysis and an optimum yield of the
desired products. This shredded feedstock is mixed with fresh and recycled dilute
sulfuric acid (1.5–3.0 wt%) solution. The acid concentration can be adjusted depending on the requirement based on the nature of the feedstock and rest of the process
conditions. Sulfuric acid acts as a catalyst for the hydrolysis process. The hydrolysis
process differs from other similar processes in that free monomeric sugars are not
the products. Instead, six- and five-carbon monosaccharides undergo multiple acidcatalyzed reactions to give the platform chemicals furfural, LA, and formic acid
along with ligneous char.
As shown in Figure 7.4, the hydrolysis process is carried out in two distinct acidcatalyzed stages that are operated to give optimum yields with a minimum degradation of products and tar formation. The first fast reaction produces HMF and is
carried out in a plug flow reactor, whereas the second slow reaction of HMF hydration
to form LA is carried out in a back-mixed reactor. The first stage is carried out at
210°C–220°C and pressure of 25 bar, and the reaction lasts only for 12 s. The reaction
is the first-order acid hydrolysis of carbohydrate polysaccharides to their soluble intermediates. The second reactor is operated at 190°C–200°C and pressure of 14 bar, and
the reaction takes about 20 min [8]. As mentioned earlier, along with the desired products, the intermediates in both reactors and HMF produce tars by the second-order
condensation reactions. After the second stage, furfural and other volatile products
are removed, and levulinic and formic acids are separated from water by the dehydration unit. The separated acid solution is recycled back to the feed unit. LA (4-oxopentanoic acid) is recovered by boiling under reduced pressure and further purified in
a product refining stage. Ligneous char is bone-dried and recovered separately both
from product separation and product refining stage. The maximum theoretical yield
of LA from a hexose is 71.6 wt% and the remainder is formic acid [15].
HMF is an intermediate product formed in the first stage of the process. A series
of consecutive reactions that occur to produce HMF have been established by numerous studies that aimed at the identification of intermediate products and analyses
of pathways for their further transformation [9]. These reactions involve dehydrations of six-carbon compounds such as d-glucose, d-mannose, and d-fructose to
form enediol, which undergoes a series of further dehydration reactions to form
3,4-dideoxyglucosulosene. This substance is readily converted to dienediol that
eventually forms HMF [16,17].
The five-carbon sugar, namely, xylose and pentose, is produced by substituting
CH 2 OH group of the hexoses by hydrogen. The hydrolysis of xylose and pentose