The Bergius-Rheinau wood saccharification process has its origin in Willstatter’s

[13] discovery that the cellulose of wood is easily hydrolyzed by highly concentrated HCl at low temperature into dextrose. Karl Goldschmidt, one of the leading men in the German chemical industry of that time, who had an open mind for its great future problems, devoted himself to the industrial application of this process. For this the work of Hagglund [14] was decisive, who found that the obtained solution of dextrose in HCl can dissolve fresh cellulose several times over and, besides, that the HCl can be evaporated in a vacuum from the sugar and then recycled. However, from the beginning in the first half of the twentieth century, the technical execution of this process was considered extremely difficult [15].

During the first 20 years, a manufacturing process was developed in the Rheinau pilot plant in Germany under Friedrich Bergius. It represented in these days an ex­cellent technical achievement; especially the problem of the technical manipulation of the highly concentrated HCl was well solved.

In 1927, Bergius was able to conclude his own work on the liquefaction of coal, after the practical possibilities had been proved on a large scale. The I. G. Farbenindustrie and Imperial Chemical Industries then took up the work on an industrial scale. From that time onward Bergius devoted himself to a process of obtaining sugar from cellulose in wood, on which he had already worked during the First World War. He succeeded after 15-year work and an industrial plant was set up, also in the Rheinau works. It is amazing with what intensity Bergius took up the second part of his life’s work, namely this hydrolysis of cellulose in wood and similar substances to sugar. It seems as if the well-known difficulties of working with highly concentrated HCl had presented a special challenge to Bergius. Initially the process was taken up only in England and only during the 1930s of the twentieth century did Bergius manage to continue these experiments in Germany; his main concern was to rationalize the process and to ensure complete recovery of the HCl used by constructing intricate devices.

The capacity of the Rheinau plant was raised to 400 tons of raw sugar a month, and at Regensburg a manufacturing plant of 1,600 t of raw sugar a month was erected for food-yeast production. However, a profit with all other wood saccharification processes was only reached as long as the products were protected by the government [16]. At the end of the war, both plants had to be closed down. In Russia, a plant using HCl technology was active for 20 years in Siberia and in the US Dow Chemicals erected a pilot plant in the 1980s. All plants were closed; the main problem was the inability to recycle the HCl efficiently, thus causing the process to be uneconomical.

Concentrated (fuming) HCl-driven hydrolysis provides the most powerful and industrially proven technology for converting all cellulosic wastes—wood, solids from city sewage plants, bagasse, grasses, etc. into sugars that can be fermented to ethanol or other biofuels as well as a large variety of chemicals and bio-products and food and feed. HCl permeates the wood more easily than H2SO4. HCl makes the cellulose more susceptible to hydrolysis and it is a volatile compound, which assists in the crucial acid recovery steps.

The Virdia process begins by steam expansion of debarked chipped wood, which undergoes a pre-extraction stage to remove all extractives, for example, tall oil and ash. The pre-extracted wood continues into hydrolysis stage performed using highly concentrated HCl (42 %) at low-temperature (10-15 °C), thus affording sugars hy — drolyzate with minimum degradation products (e. g., furfurals), while simultaneously separating the solid lignin. Approximately 98 % of the theoretically available sugars, composing ca. 65 % of the dry weight of the wood chips for pine wood are converted into sugars, which are dissolved in the hydrolyzate. The sugars hydrolyzate is further treated by extracting of the acid for recycling. The soluble oligo-saccharides formed to some extent are converted into the more desired mono-saccharides mixture of glu­cose, mannose, galactose, xylose, and arabinose, thus removing any impurities that may remain or may have been created during the course of the hydrolysis process.

The hydrolysis catalyst—HCl, forms hydrates, for example: HCl ■ 2H2O; HCl ■ 3H2O; HCl ■ 4H2O (fuming HCl). It is assumed these species are responsi­ble to the efficient hydrolysis of cellulose. These hydrates are formed mostly at high-HCl concentration in water, that is, 40-42 %; below this concentration, the uniqueness of the HCl hydrates as dispersants of lignocellulose presumably drops sharply.

Another view [17] describes the following structures for hydrated HCl:

• For the dihydrate, (H2O-H+-OH2)(Cl-);

• For the trihydrate, (H2O-H+-OH2)(H2O)(Cl-);

• For the hexahydrate, (H3O+)(H2O)5(Cl-).

Fig. 7.3 Model of wood penetration by 42 % HCl aq

Or as shown by Botti et al. [18] where these species are so-called Eigen and Zundel — type complexes:

• Eigen [H9O4]+

• Zundel [H5O2]+

In the above descriptions are not shown any chloride ions that might be in the vicinity of the complex or water molecules that might be bonded to the other end of the Zundel ion. Not shown are also any chloride ions that might be in the vicinity of the Eigen ion complex.

Figure 7.3 shows a model of wood, into which the 42 % HCl succeeds to go through and separate between the lignin and the saccharides and probably penetrates the crystalline cellulose structure.

A key limitation to any concentrated acid hydrolysis is the difficulty in recovering the acid. In particular, HCl solution forms an azeotrope at between 21 and 25 % depending on the pressure; simple distillation cannot concentrate a dilute solution beyond the azeotropic point. The efficiency of acid recovery is a key condition to making acid hydrolysis of lignocellulosic materials an economically viable source of fermentable sugars.

It is important in minimizing the need for make-up HCl, for neutralizing chemi­cals, and for costly disposal and negative impact on the environment. Full recovery of HCl at high acid concentration and its reuse yields very minor waste stream, no complicating air emissions, and favorable life cycle analysis. As most of the HCl

Fig. 7.4 The extractant roles: removal of HCl and obtaining highly concentrated sugars aqueous solution

recovery happens at relatively low temperatures, this also permits highly efficient energy integration. In addition to the HCl removal from the sugars hydrolyzate, it is also removed from the remaining lignin through a proprietary de-acidification pro­cess at low temperatures, thus being almost fully recycled into the process, leaving a very pure lignin. The main innovation in Virdia process is based on the problem the Germans had—recovering the acid, that is, evaporating water from the dilute acid at azeotrope concentration means breaking the bond between the acid and water at ~23 %. This is performed by using a medium, that is, an extractant composition that can perform two contradictory roles at two different circumstances (Fig. 7.4):

1. Taking the HCl out of the water at a relatively low temperature, thus yielding highly concentrated sugars solution;

2. Recovering the acid at high concentration.

Virdia developed several processes for the recovery of HCl from a dilute solution [19]. The following process describes the extraction of the acid by bringing a dilute aqueous HCl solution into contact with a substantially immiscible extractant, thatis, comprising of tris-2-ethylhexyl amine (TEHA; Fig. 7.5), which is substantially water insoluble in both free and salt forms, an oil soluble weak organic acid, for example,

Fig. 7.6 Carboxylic acid O

enhancer to a secondary amine extractant

octanoic acid which is substantially water insoluble, in both free and salt forms; and a solvent for the amine and organic acid, for example, dodecane, as a result of which HCl selectively transfers to the extractant to form an HCl-carrying extractant. See the following scheme:

The role of the carboxylic acid, which is used an enhancer, is to form a stable complex with the amine and the HCl, as seen in the following (Fig. 7.6):

Table 7.2 shows equilibrium data of HCI extraction with TEHA/octanoic acid in dodecane extractant:

This HCl-loaded extractant is further treated to obtain gaseous HCl.

Stripping of HCl is performed by passing, for example, xylene vapors stream through the HCl-loaded extractant. The recovered HCl during the stripping is shown in Table 7.3:

The whole extraction/stripping process is presented by the following scheme (Fig. 7.7):

The final product consists of the following sugars (Figs. 7.8 and 7.9):

The HPLC of a typical final sugar product is seen in Fig. 7.10 and a typical final product in Fig. 7.11:

The full Virdia process is presented in the following scheme (Fig. 7.12):

As can be seen in the above scheme, in addition to the sugars produced, two more products are obtained, that is, lignin and tall oil.

HCl in aqueous phase (mol/kg)	HCl in extractant (mol/kg)	HCl in aqueous phase (g/1,000gH2O)	HCl in extractant (g/1,000 g extractant)
0.039	0.019	1.4	0.7
0.064	0.050	2.4	1.8
0.154	0.069	5.7	2.5
0.31	0.24	11.4	8.7
0.42	0.35	15.8	13.0
0.62	0.56	23.2	20.9
0.78	0.68	29.5	25.3
1.18	0.87	45.0	32.7

Table 7.2 Equilibrium data of HCl extraction with TEHA/octanoic acid 1:0.25 mol/kg in dodecane at 27 °C

Time (min)	Recovered HCl (%)
0	0
10	31.0
20	46.7
30	88.0
50	98.1

Table 7.3 The HCl recovered by xylene stripping

The lignin (Fig. 7.13) is separated as solid from the process, the HCl is stripped and recycled, and the lignin is dried.

Lignin is used for binders, activated carbon, carbon fibers, fire-retardants, motor fuel, dispersants, sorbents, surfactants, and as starting material for vanillin.

An additional by-product of the Virdia process is tall oil—a generic name for a group of compounds which consist of resin acids, fatty acids, fatty alcohols, some sterols, and other alkyl hydrocarbon derivates. Resin acids occur in pine in a number of isomeric forms having the molecular formula of C20H30O2 and some related structures. The most prevalent are abietic-type acids, such as levopimaric, palustric, abietic, and neoabietic acids; and pimaric-type acids, such as pimaric and isopimaric acids (Fig. 7.14).

The fatty acids include more than 10 different acids: both saturated and unsat­urated. The most common are palmitic and stearic acids, which are saturated, and

oleic and linoleic, which are unsaturated. The unsaponiflables present in tall oil in­clude higher fatty alcohols, esters, plant sterols, and some hydrocarbons. The most common sterol present is в-sitosterol (Fig. 7.15) [20].

The resin acids part of the tall oil is used for inks, adhesives, paper-making, road­marking, and tyres. The fatty acids are applied to paints and coatings, bio-lubricants, fuel-additives, and performance polymers. Sterols are used as health-enhancing food additives and for pharmaceuticals.